1 <?xml version="1.0" encoding="iso-8859-1"?>
3 <indexterm><primary>language, GHC</primary></indexterm>
4 <indexterm><primary>extensions, GHC</primary></indexterm>
5 As with all known Haskell systems, GHC implements some extensions to
6 the language. They are all enabled by options; by default GHC
7 understands only plain Haskell 98.
11 Some of the Glasgow extensions serve to give you access to the
12 underlying facilities with which we implement Haskell. Thus, you can
13 get at the Raw Iron, if you are willing to write some non-portable
14 code at a more primitive level. You need not be “stuck”
15 on performance because of the implementation costs of Haskell's
16 “high-level” features—you can always code
17 “under” them. In an extreme case, you can write all your
18 time-critical code in C, and then just glue it together with Haskell!
22 Before you get too carried away working at the lowest level (e.g.,
23 sloshing <literal>MutableByteArray#</literal>s around your
24 program), you may wish to check if there are libraries that provide a
25 “Haskellised veneer” over the features you want. The
26 separate <ulink url="../libraries/index.html">libraries
27 documentation</ulink> describes all the libraries that come with GHC.
30 <!-- LANGUAGE OPTIONS -->
31 <sect1 id="options-language">
32 <title>Language options</title>
34 <indexterm><primary>language</primary><secondary>option</secondary>
36 <indexterm><primary>options</primary><secondary>language</secondary>
38 <indexterm><primary>extensions</primary><secondary>options controlling</secondary>
41 <para>The language option flag control what variation of the language are
42 permitted. Leaving out all of them gives you standard Haskell
45 <para>Generally speaking, all the language options are introduced by "<option>-X</option>",
46 e.g. <option>-XTemplateHaskell</option>.
49 <para> All the language options can be turned off by using the prefix "<option>No</option>";
50 e.g. "<option>-XNoTemplateHaskell</option>".</para>
52 <para> Language options recognised by Cabal can also be enabled using the <literal>LANGUAGE</literal> pragma,
53 thus <literal>{-# LANGUAGE TemplateHaskell #-}</literal> (see <xref linkend="language-pragma"/>>). </para>
55 <para>The flag <option>-fglasgow-exts</option>
56 <indexterm><primary><option>-fglasgow-exts</option></primary></indexterm>
57 is equivalent to enabling the following extensions:
58 <option>-XPrintExplicitForalls</option>,
59 <option>-XForeignFunctionInterface</option>,
60 <option>-XUnliftedFFITypes</option>,
61 <option>-XGADTs</option>,
62 <option>-XImplicitParams</option>,
63 <option>-XScopedTypeVariables</option>,
64 <option>-XUnboxedTuples</option>,
65 <option>-XTypeSynonymInstances</option>,
66 <option>-XStandaloneDeriving</option>,
67 <option>-XDeriveDataTypeable</option>,
68 <option>-XFlexibleContexts</option>,
69 <option>-XFlexibleInstances</option>,
70 <option>-XConstrainedClassMethods</option>,
71 <option>-XMultiParamTypeClasses</option>,
72 <option>-XFunctionalDependencies</option>,
73 <option>-XMagicHash</option>,
74 <option>-XPolymorphicComponents</option>,
75 <option>-XExistentialQuantification</option>,
76 <option>-XUnicodeSyntax</option>,
77 <option>-XPostfixOperators</option>,
78 <option>-XPatternGuards</option>,
79 <option>-XLiberalTypeSynonyms</option>,
80 <option>-XRankNTypes</option>,
81 <option>-XImpredicativeTypes</option>,
82 <option>-XTypeOperators</option>,
83 <option>-XRecursiveDo</option>,
84 <option>-XParallelListComp</option>,
85 <option>-XEmptyDataDecls</option>,
86 <option>-XKindSignatures</option>,
87 <option>-XGeneralizedNewtypeDeriving</option>,
88 <option>-XTypeFamilies</option>.
89 Enabling these options is the <emphasis>only</emphasis>
90 effect of <option>-fglasgow-exts</option>.
91 We are trying to move away from this portmanteau flag,
92 and towards enabling features individually.</para>
96 <!-- UNBOXED TYPES AND PRIMITIVE OPERATIONS -->
97 <sect1 id="primitives">
98 <title>Unboxed types and primitive operations</title>
100 <para>GHC is built on a raft of primitive data types and operations;
101 "primitive" in the sense that they cannot be defined in Haskell itself.
102 While you really can use this stuff to write fast code,
103 we generally find it a lot less painful, and more satisfying in the
104 long run, to use higher-level language features and libraries. With
105 any luck, the code you write will be optimised to the efficient
106 unboxed version in any case. And if it isn't, we'd like to know
109 <para>All these primitive data types and operations are exported by the
110 library <literal>GHC.Prim</literal>, for which there is
111 <ulink url="../libraries/base/GHC.Prim.html">detailed online documentation</ulink>.
112 (This documentation is generated from the file <filename>compiler/prelude/primops.txt.pp</filename>.)
115 If you want to mention any of the primitive data types or operations in your
116 program, you must first import <literal>GHC.Prim</literal> to bring them
117 into scope. Many of them have names ending in "#", and to mention such
118 names you need the <option>-XMagicHash</option> extension (<xref linkend="magic-hash"/>).
121 <para>The primops make extensive use of <link linkend="glasgow-unboxed">unboxed types</link>
122 and <link linkend="unboxed-tuples">unboxed tuples</link>, which
123 we briefly summarise here. </para>
125 <sect2 id="glasgow-unboxed">
130 <indexterm><primary>Unboxed types (Glasgow extension)</primary></indexterm>
133 <para>Most types in GHC are <firstterm>boxed</firstterm>, which means
134 that values of that type are represented by a pointer to a heap
135 object. The representation of a Haskell <literal>Int</literal>, for
136 example, is a two-word heap object. An <firstterm>unboxed</firstterm>
137 type, however, is represented by the value itself, no pointers or heap
138 allocation are involved.
142 Unboxed types correspond to the “raw machine” types you
143 would use in C: <literal>Int#</literal> (long int),
144 <literal>Double#</literal> (double), <literal>Addr#</literal>
145 (void *), etc. The <emphasis>primitive operations</emphasis>
146 (PrimOps) on these types are what you might expect; e.g.,
147 <literal>(+#)</literal> is addition on
148 <literal>Int#</literal>s, and is the machine-addition that we all
149 know and love—usually one instruction.
153 Primitive (unboxed) types cannot be defined in Haskell, and are
154 therefore built into the language and compiler. Primitive types are
155 always unlifted; that is, a value of a primitive type cannot be
156 bottom. We use the convention (but it is only a convention)
157 that primitive types, values, and
158 operations have a <literal>#</literal> suffix (see <xref linkend="magic-hash"/>).
159 For some primitive types we have special syntax for literals, also
160 described in the <link linkend="magic-hash">same section</link>.
164 Primitive values are often represented by a simple bit-pattern, such
165 as <literal>Int#</literal>, <literal>Float#</literal>,
166 <literal>Double#</literal>. But this is not necessarily the case:
167 a primitive value might be represented by a pointer to a
168 heap-allocated object. Examples include
169 <literal>Array#</literal>, the type of primitive arrays. A
170 primitive array is heap-allocated because it is too big a value to fit
171 in a register, and would be too expensive to copy around; in a sense,
172 it is accidental that it is represented by a pointer. If a pointer
173 represents a primitive value, then it really does point to that value:
174 no unevaluated thunks, no indirections…nothing can be at the
175 other end of the pointer than the primitive value.
176 A numerically-intensive program using unboxed types can
177 go a <emphasis>lot</emphasis> faster than its “standard”
178 counterpart—we saw a threefold speedup on one example.
182 There are some restrictions on the use of primitive types:
184 <listitem><para>The main restriction
185 is that you can't pass a primitive value to a polymorphic
186 function or store one in a polymorphic data type. This rules out
187 things like <literal>[Int#]</literal> (i.e. lists of primitive
188 integers). The reason for this restriction is that polymorphic
189 arguments and constructor fields are assumed to be pointers: if an
190 unboxed integer is stored in one of these, the garbage collector would
191 attempt to follow it, leading to unpredictable space leaks. Or a
192 <function>seq</function> operation on the polymorphic component may
193 attempt to dereference the pointer, with disastrous results. Even
194 worse, the unboxed value might be larger than a pointer
195 (<literal>Double#</literal> for instance).
198 <listitem><para> You cannot define a newtype whose representation type
199 (the argument type of the data constructor) is an unboxed type. Thus,
205 <listitem><para> You cannot bind a variable with an unboxed type
206 in a <emphasis>top-level</emphasis> binding.
208 <listitem><para> You cannot bind a variable with an unboxed type
209 in a <emphasis>recursive</emphasis> binding.
211 <listitem><para> You may bind unboxed variables in a (non-recursive,
212 non-top-level) pattern binding, but any such variable causes the entire
214 to become strict. For example:
216 data Foo = Foo Int Int#
218 f x = let (Foo a b, w) = ..rhs.. in ..body..
220 Since <literal>b</literal> has type <literal>Int#</literal>, the entire pattern
222 is strict, and the program behaves as if you had written
224 data Foo = Foo Int Int#
226 f x = case ..rhs.. of { (Foo a b, w) -> ..body.. }
235 <sect2 id="unboxed-tuples">
236 <title>Unboxed Tuples
240 Unboxed tuples aren't really exported by <literal>GHC.Exts</literal>,
241 they're available by default with <option>-fglasgow-exts</option>. An
242 unboxed tuple looks like this:
254 where <literal>e_1..e_n</literal> are expressions of any
255 type (primitive or non-primitive). The type of an unboxed tuple looks
260 Unboxed tuples are used for functions that need to return multiple
261 values, but they avoid the heap allocation normally associated with
262 using fully-fledged tuples. When an unboxed tuple is returned, the
263 components are put directly into registers or on the stack; the
264 unboxed tuple itself does not have a composite representation. Many
265 of the primitive operations listed in <literal>primops.txt.pp</literal> return unboxed
267 In particular, the <literal>IO</literal> and <literal>ST</literal> monads use unboxed
268 tuples to avoid unnecessary allocation during sequences of operations.
272 There are some pretty stringent restrictions on the use of unboxed tuples:
277 Values of unboxed tuple types are subject to the same restrictions as
278 other unboxed types; i.e. they may not be stored in polymorphic data
279 structures or passed to polymorphic functions.
286 No variable can have an unboxed tuple type, nor may a constructor or function
287 argument have an unboxed tuple type. The following are all illegal:
291 data Foo = Foo (# Int, Int #)
293 f :: (# Int, Int #) -> (# Int, Int #)
296 g :: (# Int, Int #) -> Int
299 h x = let y = (# x,x #) in ...
306 The typical use of unboxed tuples is simply to return multiple values,
307 binding those multiple results with a <literal>case</literal> expression, thus:
309 f x y = (# x+1, y-1 #)
310 g x = case f x x of { (# a, b #) -> a + b }
312 You can have an unboxed tuple in a pattern binding, thus
314 f x = let (# p,q #) = h x in ..body..
316 If the types of <literal>p</literal> and <literal>q</literal> are not unboxed,
317 the resulting binding is lazy like any other Haskell pattern binding. The
318 above example desugars like this:
320 f x = let t = case h x o f{ (# p,q #) -> (p,q)
325 Indeed, the bindings can even be recursive.
332 <!-- ====================== SYNTACTIC EXTENSIONS ======================= -->
334 <sect1 id="syntax-extns">
335 <title>Syntactic extensions</title>
337 <sect2 id="magic-hash">
338 <title>The magic hash</title>
339 <para>The language extension <option>-XMagicHash</option> allows "#" as a
340 postfix modifier to identifiers. Thus, "x#" is a valid variable, and "T#" is
341 a valid type constructor or data constructor.</para>
343 <para>The hash sign does not change sematics at all. We tend to use variable
344 names ending in "#" for unboxed values or types (e.g. <literal>Int#</literal>),
345 but there is no requirement to do so; they are just plain ordinary variables.
346 Nor does the <option>-XMagicHash</option> extension bring anything into scope.
347 For example, to bring <literal>Int#</literal> into scope you must
348 import <literal>GHC.Prim</literal> (see <xref linkend="primitives"/>);
349 the <option>-XMagicHash</option> extension
350 then allows you to <emphasis>refer</emphasis> to the <literal>Int#</literal>
351 that is now in scope.</para>
352 <para> The <option>-XMagicHash</option> also enables some new forms of literals (see <xref linkend="glasgow-unboxed"/>):
354 <listitem><para> <literal>'x'#</literal> has type <literal>Char#</literal></para> </listitem>
355 <listitem><para> <literal>"foo"#</literal> has type <literal>Addr#</literal></para> </listitem>
356 <listitem><para> <literal>3#</literal> has type <literal>Int#</literal>. In general,
357 any Haskell 98 integer lexeme followed by a <literal>#</literal> is an <literal>Int#</literal> literal, e.g.
358 <literal>-0x3A#</literal> as well as <literal>32#</literal></para>.</listitem>
359 <listitem><para> <literal>3##</literal> has type <literal>Word#</literal>. In general,
360 any non-negative Haskell 98 integer lexeme followed by <literal>##</literal>
361 is a <literal>Word#</literal>. </para> </listitem>
362 <listitem><para> <literal>3.2#</literal> has type <literal>Float#</literal>.</para> </listitem>
363 <listitem><para> <literal>3.2##</literal> has type <literal>Double#</literal></para> </listitem>
368 <sect2 id="new-qualified-operators">
369 <title>New qualified operator syntax</title>
371 <para>A new syntax for referencing qualified operators is
372 planned to be introduced by Haskell', and is enabled in GHC
374 the <option>-XNewQualifiedOperators</option><indexterm><primary><option>-XNewQualifiedOperators</option></primary></indexterm>
375 option. In the new syntax, the prefix form of a qualified
377 written <literal><replaceable>module</replaceable>.(<replaceable>symbol</replaceable>)</literal>
378 (in Haskell 98 this would
379 be <literal>(<replaceable>module</replaceable>.<replaceable>symbol</replaceable>)</literal>),
380 and the infix form is
381 written <literal>`<replaceable>module</replaceable>.(<replaceable>symbol</replaceable>)`</literal>
382 (in Haskell 98 this would
383 be <literal>`<replaceable>module</replaceable>.<replaceable>symbol</replaceable>`</literal>.
386 add x y = Prelude.(+) x y
387 subtract y = (`Prelude.(-)` y)
389 The new form of qualified operators is intended to regularise
390 the syntax by eliminating odd cases
391 like <literal>Prelude..</literal>. For example,
392 when <literal>NewQualifiedOperators</literal> is on, it is possible to
393 write the enerated sequence <literal>[Monday..]</literal>
394 without spaces, whereas in Haskell 98 this would be a
395 reference to the operator ‘<literal>.</literal>‘
396 from module <literal>Monday</literal>.</para>
398 <para>When <option>-XNewQualifiedOperators</option> is on, the old Haskell
399 98 syntax for qualified operators is not accepted, so this
400 option may cause existing Haskell 98 code to break.</para>
405 <!-- ====================== HIERARCHICAL MODULES ======================= -->
408 <sect2 id="hierarchical-modules">
409 <title>Hierarchical Modules</title>
411 <para>GHC supports a small extension to the syntax of module
412 names: a module name is allowed to contain a dot
413 <literal>‘.’</literal>. This is also known as the
414 “hierarchical module namespace” extension, because
415 it extends the normally flat Haskell module namespace into a
416 more flexible hierarchy of modules.</para>
418 <para>This extension has very little impact on the language
419 itself; modules names are <emphasis>always</emphasis> fully
420 qualified, so you can just think of the fully qualified module
421 name as <quote>the module name</quote>. In particular, this
422 means that the full module name must be given after the
423 <literal>module</literal> keyword at the beginning of the
424 module; for example, the module <literal>A.B.C</literal> must
427 <programlisting>module A.B.C</programlisting>
430 <para>It is a common strategy to use the <literal>as</literal>
431 keyword to save some typing when using qualified names with
432 hierarchical modules. For example:</para>
435 import qualified Control.Monad.ST.Strict as ST
438 <para>For details on how GHC searches for source and interface
439 files in the presence of hierarchical modules, see <xref
440 linkend="search-path"/>.</para>
442 <para>GHC comes with a large collection of libraries arranged
443 hierarchically; see the accompanying <ulink
444 url="../libraries/index.html">library
445 documentation</ulink>. More libraries to install are available
447 url="http://hackage.haskell.org/packages/hackage.html">HackageDB</ulink>.</para>
450 <!-- ====================== PATTERN GUARDS ======================= -->
452 <sect2 id="pattern-guards">
453 <title>Pattern guards</title>
456 <indexterm><primary>Pattern guards (Glasgow extension)</primary></indexterm>
457 The discussion that follows is an abbreviated version of Simon Peyton Jones's original <ulink url="http://research.microsoft.com/~simonpj/Haskell/guards.html">proposal</ulink>. (Note that the proposal was written before pattern guards were implemented, so refers to them as unimplemented.)
461 Suppose we have an abstract data type of finite maps, with a
465 lookup :: FiniteMap -> Int -> Maybe Int
468 The lookup returns <function>Nothing</function> if the supplied key is not in the domain of the mapping, and <function>(Just v)</function> otherwise,
469 where <varname>v</varname> is the value that the key maps to. Now consider the following definition:
473 clunky env var1 var2 | ok1 && ok2 = val1 + val2
474 | otherwise = var1 + var2
485 The auxiliary functions are
489 maybeToBool :: Maybe a -> Bool
490 maybeToBool (Just x) = True
491 maybeToBool Nothing = False
493 expectJust :: Maybe a -> a
494 expectJust (Just x) = x
495 expectJust Nothing = error "Unexpected Nothing"
499 What is <function>clunky</function> doing? The guard <literal>ok1 &&
500 ok2</literal> checks that both lookups succeed, using
501 <function>maybeToBool</function> to convert the <function>Maybe</function>
502 types to booleans. The (lazily evaluated) <function>expectJust</function>
503 calls extract the values from the results of the lookups, and binds the
504 returned values to <varname>val1</varname> and <varname>val2</varname>
505 respectively. If either lookup fails, then clunky takes the
506 <literal>otherwise</literal> case and returns the sum of its arguments.
510 This is certainly legal Haskell, but it is a tremendously verbose and
511 un-obvious way to achieve the desired effect. Arguably, a more direct way
512 to write clunky would be to use case expressions:
516 clunky env var1 var2 = case lookup env var1 of
518 Just val1 -> case lookup env var2 of
520 Just val2 -> val1 + val2
526 This is a bit shorter, but hardly better. Of course, we can rewrite any set
527 of pattern-matching, guarded equations as case expressions; that is
528 precisely what the compiler does when compiling equations! The reason that
529 Haskell provides guarded equations is because they allow us to write down
530 the cases we want to consider, one at a time, independently of each other.
531 This structure is hidden in the case version. Two of the right-hand sides
532 are really the same (<function>fail</function>), and the whole expression
533 tends to become more and more indented.
537 Here is how I would write clunky:
542 | Just val1 <- lookup env var1
543 , Just val2 <- lookup env var2
545 ...other equations for clunky...
549 The semantics should be clear enough. The qualifiers are matched in order.
550 For a <literal><-</literal> qualifier, which I call a pattern guard, the
551 right hand side is evaluated and matched against the pattern on the left.
552 If the match fails then the whole guard fails and the next equation is
553 tried. If it succeeds, then the appropriate binding takes place, and the
554 next qualifier is matched, in the augmented environment. Unlike list
555 comprehensions, however, the type of the expression to the right of the
556 <literal><-</literal> is the same as the type of the pattern to its
557 left. The bindings introduced by pattern guards scope over all the
558 remaining guard qualifiers, and over the right hand side of the equation.
562 Just as with list comprehensions, boolean expressions can be freely mixed
563 with among the pattern guards. For example:
574 Haskell's current guards therefore emerge as a special case, in which the
575 qualifier list has just one element, a boolean expression.
579 <!-- ===================== View patterns =================== -->
581 <sect2 id="view-patterns">
586 View patterns are enabled by the flag <literal>-XViewPatterns</literal>.
587 More information and examples of view patterns can be found on the
588 <ulink url="http://hackage.haskell.org/trac/ghc/wiki/ViewPatterns">Wiki
593 View patterns are somewhat like pattern guards that can be nested inside
594 of other patterns. They are a convenient way of pattern-matching
595 against values of abstract types. For example, in a programming language
596 implementation, we might represent the syntax of the types of the
605 view :: Type -> TypeView
607 -- additional operations for constructing Typ's ...
610 The representation of Typ is held abstract, permitting implementations
611 to use a fancy representation (e.g., hash-consing to manage sharing).
613 Without view patterns, using this signature a little inconvenient:
615 size :: Typ -> Integer
616 size t = case view t of
618 Arrow t1 t2 -> size t1 + size t2
621 It is necessary to iterate the case, rather than using an equational
622 function definition. And the situation is even worse when the matching
623 against <literal>t</literal> is buried deep inside another pattern.
627 View patterns permit calling the view function inside the pattern and
628 matching against the result:
630 size (view -> Unit) = 1
631 size (view -> Arrow t1 t2) = size t1 + size t2
634 That is, we add a new form of pattern, written
635 <replaceable>expression</replaceable> <literal>-></literal>
636 <replaceable>pattern</replaceable> that means "apply the expression to
637 whatever we're trying to match against, and then match the result of
638 that application against the pattern". The expression can be any Haskell
639 expression of function type, and view patterns can be used wherever
644 The semantics of a pattern <literal>(</literal>
645 <replaceable>exp</replaceable> <literal>-></literal>
646 <replaceable>pat</replaceable> <literal>)</literal> are as follows:
652 <para>The variables bound by the view pattern are the variables bound by
653 <replaceable>pat</replaceable>.
657 Any variables in <replaceable>exp</replaceable> are bound occurrences,
658 but variables bound "to the left" in a pattern are in scope. This
659 feature permits, for example, one argument to a function to be used in
660 the view of another argument. For example, the function
661 <literal>clunky</literal> from <xref linkend="pattern-guards" /> can be
662 written using view patterns as follows:
665 clunky env (lookup env -> Just val1) (lookup env -> Just val2) = val1 + val2
666 ...other equations for clunky...
671 More precisely, the scoping rules are:
675 In a single pattern, variables bound by patterns to the left of a view
676 pattern expression are in scope. For example:
678 example :: Maybe ((String -> Integer,Integer), String) -> Bool
679 example Just ((f,_), f -> 4) = True
682 Additionally, in function definitions, variables bound by matching earlier curried
683 arguments may be used in view pattern expressions in later arguments:
685 example :: (String -> Integer) -> String -> Bool
686 example f (f -> 4) = True
688 That is, the scoping is the same as it would be if the curried arguments
689 were collected into a tuple.
695 In mutually recursive bindings, such as <literal>let</literal>,
696 <literal>where</literal>, or the top level, view patterns in one
697 declaration may not mention variables bound by other declarations. That
698 is, each declaration must be self-contained. For example, the following
699 program is not allowed:
706 restriction in the future; the only cost is that type checking patterns
707 would get a little more complicated.)
717 <listitem><para> Typing: If <replaceable>exp</replaceable> has type
718 <replaceable>T1</replaceable> <literal>-></literal>
719 <replaceable>T2</replaceable> and <replaceable>pat</replaceable> matches
720 a <replaceable>T2</replaceable>, then the whole view pattern matches a
721 <replaceable>T1</replaceable>.
724 <listitem><para> Matching: To the equations in Section 3.17.3 of the
725 <ulink url="http://www.haskell.org/onlinereport/">Haskell 98
726 Report</ulink>, add the following:
728 case v of { (e -> p) -> e1 ; _ -> e2 }
730 case (e v) of { p -> e1 ; _ -> e2 }
732 That is, to match a variable <replaceable>v</replaceable> against a pattern
733 <literal>(</literal> <replaceable>exp</replaceable>
734 <literal>-></literal> <replaceable>pat</replaceable>
735 <literal>)</literal>, evaluate <literal>(</literal>
736 <replaceable>exp</replaceable> <replaceable> v</replaceable>
737 <literal>)</literal> and match the result against
738 <replaceable>pat</replaceable>.
741 <listitem><para> Efficiency: When the same view function is applied in
742 multiple branches of a function definition or a case expression (e.g.,
743 in <literal>size</literal> above), GHC makes an attempt to collect these
744 applications into a single nested case expression, so that the view
745 function is only applied once. Pattern compilation in GHC follows the
746 matrix algorithm described in Chapter 4 of <ulink
747 url="http://research.microsoft.com/~simonpj/Papers/slpj-book-1987/">The
748 Implementation of Functional Programming Languages</ulink>. When the
749 top rows of the first column of a matrix are all view patterns with the
750 "same" expression, these patterns are transformed into a single nested
751 case. This includes, for example, adjacent view patterns that line up
754 f ((view -> A, p1), p2) = e1
755 f ((view -> B, p3), p4) = e2
759 <para> The current notion of when two view pattern expressions are "the
760 same" is very restricted: it is not even full syntactic equality.
761 However, it does include variables, literals, applications, and tuples;
762 e.g., two instances of <literal>view ("hi", "there")</literal> will be
763 collected. However, the current implementation does not compare up to
764 alpha-equivalence, so two instances of <literal>(x, view x ->
765 y)</literal> will not be coalesced.
775 <!-- ===================== Recursive do-notation =================== -->
777 <sect2 id="mdo-notation">
778 <title>The recursive do-notation
781 <para> The recursive do-notation (also known as mdo-notation) is implemented as described in
782 <ulink url="http://citeseer.ist.psu.edu/erk02recursive.html">A recursive do for Haskell</ulink>,
783 by Levent Erkok, John Launchbury,
784 Haskell Workshop 2002, pages: 29-37. Pittsburgh, Pennsylvania.
785 This paper is essential reading for anyone making non-trivial use of mdo-notation,
786 and we do not repeat it here.
789 The do-notation of Haskell does not allow <emphasis>recursive bindings</emphasis>,
790 that is, the variables bound in a do-expression are visible only in the textually following
791 code block. Compare this to a let-expression, where bound variables are visible in the entire binding
792 group. It turns out that several applications can benefit from recursive bindings in
793 the do-notation, and this extension provides the necessary syntactic support.
796 Here is a simple (yet contrived) example:
799 import Control.Monad.Fix
801 justOnes = mdo xs <- Just (1:xs)
805 As you can guess <literal>justOnes</literal> will evaluate to <literal>Just [1,1,1,...</literal>.
809 The Control.Monad.Fix library introduces the <literal>MonadFix</literal> class. It's definition is:
812 class Monad m => MonadFix m where
813 mfix :: (a -> m a) -> m a
816 The function <literal>mfix</literal>
817 dictates how the required recursion operation should be performed. For example,
818 <literal>justOnes</literal> desugars as follows:
820 justOnes = mfix (\xs' -> do { xs <- Just (1:xs'); return xs }
822 For full details of the way in which mdo is typechecked and desugared, see
823 the paper <ulink url="http://citeseer.ist.psu.edu/erk02recursive.html">A recursive do for Haskell</ulink>.
824 In particular, GHC implements the segmentation technique described in Section 3.2 of the paper.
827 If recursive bindings are required for a monad,
828 then that monad must be declared an instance of the <literal>MonadFix</literal> class.
829 The following instances of <literal>MonadFix</literal> are automatically provided: List, Maybe, IO.
830 Furthermore, the Control.Monad.ST and Control.Monad.ST.Lazy modules provide the instances of the MonadFix class
831 for Haskell's internal state monad (strict and lazy, respectively).
834 Here are some important points in using the recursive-do notation:
837 The recursive version of the do-notation uses the keyword <literal>mdo</literal> (rather
838 than <literal>do</literal>).
842 It is enabled with the flag <literal>-XRecursiveDo</literal>, which is in turn implied by
843 <literal>-fglasgow-exts</literal>.
847 Unlike ordinary do-notation, but like <literal>let</literal> and <literal>where</literal> bindings,
848 name shadowing is not allowed; that is, all the names bound in a single <literal>mdo</literal> must
849 be distinct (Section 3.3 of the paper).
853 Variables bound by a <literal>let</literal> statement in an <literal>mdo</literal>
854 are monomorphic in the <literal>mdo</literal> (Section 3.1 of the paper). However
855 GHC breaks the <literal>mdo</literal> into segments to enhance polymorphism,
856 and improve termination (Section 3.2 of the paper).
862 Historical note: The old implementation of the mdo-notation (and most
863 of the existing documents) used the name
864 <literal>MonadRec</literal> for the class and the corresponding library.
865 This name is not supported by GHC.
871 <!-- ===================== PARALLEL LIST COMPREHENSIONS =================== -->
873 <sect2 id="parallel-list-comprehensions">
874 <title>Parallel List Comprehensions</title>
875 <indexterm><primary>list comprehensions</primary><secondary>parallel</secondary>
877 <indexterm><primary>parallel list comprehensions</primary>
880 <para>Parallel list comprehensions are a natural extension to list
881 comprehensions. List comprehensions can be thought of as a nice
882 syntax for writing maps and filters. Parallel comprehensions
883 extend this to include the zipWith family.</para>
885 <para>A parallel list comprehension has multiple independent
886 branches of qualifier lists, each separated by a `|' symbol. For
887 example, the following zips together two lists:</para>
890 [ (x, y) | x <- xs | y <- ys ]
893 <para>The behavior of parallel list comprehensions follows that of
894 zip, in that the resulting list will have the same length as the
895 shortest branch.</para>
897 <para>We can define parallel list comprehensions by translation to
898 regular comprehensions. Here's the basic idea:</para>
900 <para>Given a parallel comprehension of the form: </para>
903 [ e | p1 <- e11, p2 <- e12, ...
904 | q1 <- e21, q2 <- e22, ...
909 <para>This will be translated to: </para>
912 [ e | ((p1,p2), (q1,q2), ...) <- zipN [(p1,p2) | p1 <- e11, p2 <- e12, ...]
913 [(q1,q2) | q1 <- e21, q2 <- e22, ...]
918 <para>where `zipN' is the appropriate zip for the given number of
923 <!-- ===================== TRANSFORM LIST COMPREHENSIONS =================== -->
925 <sect2 id="generalised-list-comprehensions">
926 <title>Generalised (SQL-Like) List Comprehensions</title>
927 <indexterm><primary>list comprehensions</primary><secondary>generalised</secondary>
929 <indexterm><primary>extended list comprehensions</primary>
931 <indexterm><primary>group</primary></indexterm>
932 <indexterm><primary>sql</primary></indexterm>
935 <para>Generalised list comprehensions are a further enhancement to the
936 list comprehension syntatic sugar to allow operations such as sorting
937 and grouping which are familiar from SQL. They are fully described in the
938 paper <ulink url="http://research.microsoft.com/~simonpj/papers/list-comp">
939 Comprehensive comprehensions: comprehensions with "order by" and "group by"</ulink>,
940 except that the syntax we use differs slightly from the paper.</para>
941 <para>Here is an example:
943 employees = [ ("Simon", "MS", 80)
944 , ("Erik", "MS", 100)
946 , ("Gordon", "Ed", 45)
947 , ("Paul", "Yale", 60)]
949 output = [ (the dept, sum salary)
950 | (name, dept, salary) <- employees
952 , then sortWith by (sum salary)
955 In this example, the list <literal>output</literal> would take on
959 [("Yale", 60), ("Ed", 85), ("MS", 180)]
962 <para>There are three new keywords: <literal>group</literal>, <literal>by</literal>, and <literal>using</literal>.
963 (The function <literal>sortWith</literal> is not a keyword; it is an ordinary
964 function that is exported by <literal>GHC.Exts</literal>.)</para>
966 <para>There are five new forms of comprehension qualifier,
967 all introduced by the (existing) keyword <literal>then</literal>:
975 This statement requires that <literal>f</literal> have the type <literal>
976 forall a. [a] -> [a]</literal>. You can see an example of it's use in the
977 motivating example, as this form is used to apply <literal>take 5</literal>.
988 This form is similar to the previous one, but allows you to create a function
989 which will be passed as the first argument to f. As a consequence f must have
990 the type <literal>forall a. (a -> t) -> [a] -> [a]</literal>. As you can see
991 from the type, this function lets f "project out" some information
992 from the elements of the list it is transforming.</para>
994 <para>An example is shown in the opening example, where <literal>sortWith</literal>
995 is supplied with a function that lets it find out the <literal>sum salary</literal>
996 for any item in the list comprehension it transforms.</para>
1004 then group by e using f
1007 <para>This is the most general of the grouping-type statements. In this form,
1008 f is required to have type <literal>forall a. (a -> t) -> [a] -> [[a]]</literal>.
1009 As with the <literal>then f by e</literal> case above, the first argument
1010 is a function supplied to f by the compiler which lets it compute e on every
1011 element of the list being transformed. However, unlike the non-grouping case,
1012 f additionally partitions the list into a number of sublists: this means that
1013 at every point after this statement, binders occurring before it in the comprehension
1014 refer to <emphasis>lists</emphasis> of possible values, not single values. To help understand
1015 this, let's look at an example:</para>
1018 -- This works similarly to groupWith in GHC.Exts, but doesn't sort its input first
1019 groupRuns :: Eq b => (a -> b) -> [a] -> [[a]]
1020 groupRuns f = groupBy (\x y -> f x == f y)
1022 output = [ (the x, y)
1023 | x <- ([1..3] ++ [1..2])
1025 , then group by x using groupRuns ]
1028 <para>This results in the variable <literal>output</literal> taking on the value below:</para>
1031 [(1, [4, 5, 6]), (2, [4, 5, 6]), (3, [4, 5, 6]), (1, [4, 5, 6]), (2, [4, 5, 6])]
1034 <para>Note that we have used the <literal>the</literal> function to change the type
1035 of x from a list to its original numeric type. The variable y, in contrast, is left
1036 unchanged from the list form introduced by the grouping.</para>
1046 <para>This form of grouping is essentially the same as the one described above. However,
1047 since no function to use for the grouping has been supplied it will fall back on the
1048 <literal>groupWith</literal> function defined in
1049 <ulink url="../libraries/base/GHC-Exts.html"><literal>GHC.Exts</literal></ulink>. This
1050 is the form of the group statement that we made use of in the opening example.</para>
1061 <para>With this form of the group statement, f is required to simply have the type
1062 <literal>forall a. [a] -> [[a]]</literal>, which will be used to group up the
1063 comprehension so far directly. An example of this form is as follows:</para>
1069 , then group using inits]
1072 <para>This will yield a list containing every prefix of the word "hello" written out 5 times:</para>
1075 ["","h","he","hel","hell","hello","helloh","hellohe","hellohel","hellohell","hellohello","hellohelloh",...]
1083 <!-- ===================== REBINDABLE SYNTAX =================== -->
1085 <sect2 id="rebindable-syntax">
1086 <title>Rebindable syntax and the implicit Prelude import</title>
1088 <para><indexterm><primary>-XNoImplicitPrelude
1089 option</primary></indexterm> GHC normally imports
1090 <filename>Prelude.hi</filename> files for you. If you'd
1091 rather it didn't, then give it a
1092 <option>-XNoImplicitPrelude</option> option. The idea is
1093 that you can then import a Prelude of your own. (But don't
1094 call it <literal>Prelude</literal>; the Haskell module
1095 namespace is flat, and you must not conflict with any
1096 Prelude module.)</para>
1098 <para>Suppose you are importing a Prelude of your own
1099 in order to define your own numeric class
1100 hierarchy. It completely defeats that purpose if the
1101 literal "1" means "<literal>Prelude.fromInteger
1102 1</literal>", which is what the Haskell Report specifies.
1103 So the <option>-XNoImplicitPrelude</option>
1104 flag <emphasis>also</emphasis> causes
1105 the following pieces of built-in syntax to refer to
1106 <emphasis>whatever is in scope</emphasis>, not the Prelude
1110 <para>An integer literal <literal>368</literal> means
1111 "<literal>fromInteger (368::Integer)</literal>", rather than
1112 "<literal>Prelude.fromInteger (368::Integer)</literal>".
1115 <listitem><para>Fractional literals are handed in just the same way,
1116 except that the translation is
1117 <literal>fromRational (3.68::Rational)</literal>.
1120 <listitem><para>The equality test in an overloaded numeric pattern
1121 uses whatever <literal>(==)</literal> is in scope.
1124 <listitem><para>The subtraction operation, and the
1125 greater-than-or-equal test, in <literal>n+k</literal> patterns
1126 use whatever <literal>(-)</literal> and <literal>(>=)</literal> are in scope.
1130 <para>Negation (e.g. "<literal>- (f x)</literal>")
1131 means "<literal>negate (f x)</literal>", both in numeric
1132 patterns, and expressions.
1136 <para>"Do" notation is translated using whatever
1137 functions <literal>(>>=)</literal>,
1138 <literal>(>>)</literal>, and <literal>fail</literal>,
1139 are in scope (not the Prelude
1140 versions). List comprehensions, mdo (<xref linkend="mdo-notation"/>), and parallel array
1141 comprehensions, are unaffected. </para></listitem>
1145 notation (see <xref linkend="arrow-notation"/>)
1146 uses whatever <literal>arr</literal>,
1147 <literal>(>>>)</literal>, <literal>first</literal>,
1148 <literal>app</literal>, <literal>(|||)</literal> and
1149 <literal>loop</literal> functions are in scope. But unlike the
1150 other constructs, the types of these functions must match the
1151 Prelude types very closely. Details are in flux; if you want
1155 In all cases (apart from arrow notation), the static semantics should be that of the desugared form,
1156 even if that is a little unexpected. For example, the
1157 static semantics of the literal <literal>368</literal>
1158 is exactly that of <literal>fromInteger (368::Integer)</literal>; it's fine for
1159 <literal>fromInteger</literal> to have any of the types:
1161 fromInteger :: Integer -> Integer
1162 fromInteger :: forall a. Foo a => Integer -> a
1163 fromInteger :: Num a => a -> Integer
1164 fromInteger :: Integer -> Bool -> Bool
1168 <para>Be warned: this is an experimental facility, with
1169 fewer checks than usual. Use <literal>-dcore-lint</literal>
1170 to typecheck the desugared program. If Core Lint is happy
1171 you should be all right.</para>
1175 <sect2 id="postfix-operators">
1176 <title>Postfix operators</title>
1179 The <option>-XPostfixOperators</option> flag enables a small
1180 extension to the syntax of left operator sections, which allows you to
1181 define postfix operators. The extension is this: the left section
1185 is equivalent (from the point of view of both type checking and execution) to the expression
1189 (for any expression <literal>e</literal> and operator <literal>(!)</literal>.
1190 The strict Haskell 98 interpretation is that the section is equivalent to
1194 That is, the operator must be a function of two arguments. GHC allows it to
1195 take only one argument, and that in turn allows you to write the function
1198 <para>The extension does not extend to the left-hand side of function
1199 definitions; you must define such a function in prefix form.</para>
1203 <sect2 id="disambiguate-fields">
1204 <title>Record field disambiguation</title>
1206 In record construction and record pattern matching
1207 it is entirely unambiguous which field is referred to, even if there are two different
1208 data types in scope with a common field name. For example:
1211 data S = MkS { x :: Int, y :: Bool }
1216 data T = MkT { x :: Int }
1218 ok1 (MkS { x = n }) = n+1 -- Unambiguous
1220 ok2 n = MkT { x = n+1 } -- Unambiguous
1222 bad1 k = k { x = 3 } -- Ambiguous
1223 bad2 k = x k -- Ambiguous
1225 Even though there are two <literal>x</literal>'s in scope,
1226 it is clear that the <literal>x</literal> in the pattern in the
1227 definition of <literal>ok1</literal> can only mean the field
1228 <literal>x</literal> from type <literal>S</literal>. Similarly for
1229 the function <literal>ok2</literal>. However, in the record update
1230 in <literal>bad1</literal> and the record selection in <literal>bad2</literal>
1231 it is not clear which of the two types is intended.
1234 Haskell 98 regards all four as ambiguous, but with the
1235 <option>-fdisambiguate-record-fields</option> flag, GHC will accept
1236 the former two. The rules are precisely the same as those for instance
1237 declarations in Haskell 98, where the method names on the left-hand side
1238 of the method bindings in an instance declaration refer unambiguously
1239 to the method of that class (provided they are in scope at all), even
1240 if there are other variables in scope with the same name.
1241 This reduces the clutter of qualified names when you import two
1242 records from different modules that use the same field name.
1246 <!-- ===================== Record puns =================== -->
1248 <sect2 id="record-puns">
1253 Record puns are enabled by the flag <literal>-XNamedFieldPuns</literal>.
1257 When using records, it is common to write a pattern that binds a
1258 variable with the same name as a record field, such as:
1261 data C = C {a :: Int}
1267 Record punning permits the variable name to be elided, so one can simply
1274 to mean the same pattern as above. That is, in a record pattern, the
1275 pattern <literal>a</literal> expands into the pattern <literal>a =
1276 a</literal> for the same name <literal>a</literal>.
1280 Note that puns and other patterns can be mixed in the same record:
1282 data C = C {a :: Int, b :: Int}
1283 f (C {a, b = 4}) = a
1285 and that puns can be used wherever record patterns occur (e.g. in
1286 <literal>let</literal> bindings or at the top-level).
1290 Record punning can also be used in an expression, writing, for example,
1296 let a = 1 in C {a = a}
1299 Note that this expansion is purely syntactic, so the record pun
1300 expression refers to the nearest enclosing variable that is spelled the
1301 same as the field name.
1306 <!-- ===================== Record wildcards =================== -->
1308 <sect2 id="record-wildcards">
1309 <title>Record wildcards
1313 Record wildcards are enabled by the flag <literal>-XRecordWildCards</literal>.
1317 For records with many fields, it can be tiresome to write out each field
1318 individually in a record pattern, as in
1320 data C = C {a :: Int, b :: Int, c :: Int, d :: Int}
1321 f (C {a = 1, b = b, c = c, d = d}) = b + c + d
1326 Record wildcard syntax permits a (<literal>..</literal>) in a record
1327 pattern, where each elided field <literal>f</literal> is replaced by the
1328 pattern <literal>f = f</literal>. For example, the above pattern can be
1331 f (C {a = 1, ..}) = b + c + d
1336 Note that wildcards can be mixed with other patterns, including puns
1337 (<xref linkend="record-puns"/>); for example, in a pattern <literal>C {a
1338 = 1, b, ..})</literal>. Additionally, record wildcards can be used
1339 wherever record patterns occur, including in <literal>let</literal>
1340 bindings and at the top-level. For example, the top-level binding
1344 defines <literal>b</literal>, <literal>c</literal>, and
1345 <literal>d</literal>.
1349 Record wildcards can also be used in expressions, writing, for example,
1352 let {a = 1; b = 2; c = 3; d = 4} in C {..}
1358 let {a = 1; b = 2; c = 3; d = 4} in C {a=a, b=b, c=c, d=d}
1361 Note that this expansion is purely syntactic, so the record wildcard
1362 expression refers to the nearest enclosing variables that are spelled
1363 the same as the omitted field names.
1368 <!-- ===================== Local fixity declarations =================== -->
1370 <sect2 id="local-fixity-declarations">
1371 <title>Local Fixity Declarations
1374 <para>A careful reading of the Haskell 98 Report reveals that fixity
1375 declarations (<literal>infix</literal>, <literal>infixl</literal>, and
1376 <literal>infixr</literal>) are permitted to appear inside local bindings
1377 such those introduced by <literal>let</literal> and
1378 <literal>where</literal>. However, the Haskell Report does not specify
1379 the semantics of such bindings very precisely.
1382 <para>In GHC, a fixity declaration may accompany a local binding:
1389 and the fixity declaration applies wherever the binding is in scope.
1390 For example, in a <literal>let</literal>, it applies in the right-hand
1391 sides of other <literal>let</literal>-bindings and the body of the
1392 <literal>let</literal>C. Or, in recursive <literal>do</literal>
1393 expressions (<xref linkend="mdo-notation"/>), the local fixity
1394 declarations of a <literal>let</literal> statement scope over other
1395 statements in the group, just as the bound name does.
1399 Moreover, a local fixity declaration *must* accompany a local binding of
1400 that name: it is not possible to revise the fixity of name bound
1403 let infixr 9 $ in ...
1406 Because local fixity declarations are technically Haskell 98, no flag is
1407 necessary to enable them.
1411 <sect2 id="package-imports">
1412 <title>Package-qualified imports</title>
1414 <para>With the <option>-XPackageImports</option> flag, GHC allows
1415 import declarations to be qualified by the package name that the
1416 module is intended to be imported from. For example:</para>
1419 import "network" Network.Socket
1422 <para>would import the module <literal>Network.Socket</literal> from
1423 the package <literal>network</literal> (any version). This may
1424 be used to disambiguate an import when the same module is
1425 available from multiple packages, or is present in both the
1426 current package being built and an external package.</para>
1428 <para>Note: you probably don't need to use this feature, it was
1429 added mainly so that we can build backwards-compatible versions of
1430 packages when APIs change. It can lead to fragile dependencies in
1431 the common case: modules occasionally move from one package to
1432 another, rendering any package-qualified imports broken.</para>
1435 <sect2 id="syntax-stolen">
1436 <title>Summary of stolen syntax</title>
1438 <para>Turning on an option that enables special syntax
1439 <emphasis>might</emphasis> cause working Haskell 98 code to fail
1440 to compile, perhaps because it uses a variable name which has
1441 become a reserved word. This section lists the syntax that is
1442 "stolen" by language extensions.
1444 notation and nonterminal names from the Haskell 98 lexical syntax
1445 (see the Haskell 98 Report).
1446 We only list syntax changes here that might affect
1447 existing working programs (i.e. "stolen" syntax). Many of these
1448 extensions will also enable new context-free syntax, but in all
1449 cases programs written to use the new syntax would not be
1450 compilable without the option enabled.</para>
1452 <para>There are two classes of special
1457 <para>New reserved words and symbols: character sequences
1458 which are no longer available for use as identifiers in the
1462 <para>Other special syntax: sequences of characters that have
1463 a different meaning when this particular option is turned
1468 The following syntax is stolen:
1473 <literal>forall</literal>
1474 <indexterm><primary><literal>forall</literal></primary></indexterm>
1477 Stolen (in types) by: <option>-XScopedTypeVariables</option>,
1478 <option>-XLiberalTypeSynonyms</option>,
1479 <option>-XRank2Types</option>,
1480 <option>-XRankNTypes</option>,
1481 <option>-XPolymorphicComponents</option>,
1482 <option>-XExistentialQuantification</option>
1488 <literal>mdo</literal>
1489 <indexterm><primary><literal>mdo</literal></primary></indexterm>
1492 Stolen by: <option>-XRecursiveDo</option>,
1498 <literal>foreign</literal>
1499 <indexterm><primary><literal>foreign</literal></primary></indexterm>
1502 Stolen by: <option>-XForeignFunctionInterface</option>,
1508 <literal>rec</literal>,
1509 <literal>proc</literal>, <literal>-<</literal>,
1510 <literal>>-</literal>, <literal>-<<</literal>,
1511 <literal>>>-</literal>, and <literal>(|</literal>,
1512 <literal>|)</literal> brackets
1513 <indexterm><primary><literal>proc</literal></primary></indexterm>
1516 Stolen by: <option>-XArrows</option>,
1522 <literal>?<replaceable>varid</replaceable></literal>,
1523 <literal>%<replaceable>varid</replaceable></literal>
1524 <indexterm><primary>implicit parameters</primary></indexterm>
1527 Stolen by: <option>-XImplicitParams</option>,
1533 <literal>[|</literal>,
1534 <literal>[e|</literal>, <literal>[p|</literal>,
1535 <literal>[d|</literal>, <literal>[t|</literal>,
1536 <literal>$(</literal>,
1537 <literal>$<replaceable>varid</replaceable></literal>
1538 <indexterm><primary>Template Haskell</primary></indexterm>
1541 Stolen by: <option>-XTemplateHaskell</option>,
1547 <literal>[:<replaceable>varid</replaceable>|</literal>
1548 <indexterm><primary>quasi-quotation</primary></indexterm>
1551 Stolen by: <option>-XQuasiQuotes</option>,
1557 <replaceable>varid</replaceable>{<literal>#</literal>},
1558 <replaceable>char</replaceable><literal>#</literal>,
1559 <replaceable>string</replaceable><literal>#</literal>,
1560 <replaceable>integer</replaceable><literal>#</literal>,
1561 <replaceable>float</replaceable><literal>#</literal>,
1562 <replaceable>float</replaceable><literal>##</literal>,
1563 <literal>(#</literal>, <literal>#)</literal>,
1566 Stolen by: <option>-XMagicHash</option>,
1575 <!-- TYPE SYSTEM EXTENSIONS -->
1576 <sect1 id="data-type-extensions">
1577 <title>Extensions to data types and type synonyms</title>
1579 <sect2 id="nullary-types">
1580 <title>Data types with no constructors</title>
1582 <para>With the <option>-fglasgow-exts</option> flag, GHC lets you declare
1583 a data type with no constructors. For example:</para>
1587 data T a -- T :: * -> *
1590 <para>Syntactically, the declaration lacks the "= constrs" part. The
1591 type can be parameterised over types of any kind, but if the kind is
1592 not <literal>*</literal> then an explicit kind annotation must be used
1593 (see <xref linkend="kinding"/>).</para>
1595 <para>Such data types have only one value, namely bottom.
1596 Nevertheless, they can be useful when defining "phantom types".</para>
1599 <sect2 id="infix-tycons">
1600 <title>Infix type constructors, classes, and type variables</title>
1603 GHC allows type constructors, classes, and type variables to be operators, and
1604 to be written infix, very much like expressions. More specifically:
1607 A type constructor or class can be an operator, beginning with a colon; e.g. <literal>:*:</literal>.
1608 The lexical syntax is the same as that for data constructors.
1611 Data type and type-synonym declarations can be written infix, parenthesised
1612 if you want further arguments. E.g.
1614 data a :*: b = Foo a b
1615 type a :+: b = Either a b
1616 class a :=: b where ...
1618 data (a :**: b) x = Baz a b x
1619 type (a :++: b) y = Either (a,b) y
1623 Types, and class constraints, can be written infix. For example
1626 f :: (a :=: b) => a -> b
1630 A type variable can be an (unqualified) operator e.g. <literal>+</literal>.
1631 The lexical syntax is the same as that for variable operators, excluding "(.)",
1632 "(!)", and "(*)". In a binding position, the operator must be
1633 parenthesised. For example:
1635 type T (+) = Int + Int
1639 liftA2 :: Arrow (~>)
1640 => (a -> b -> c) -> (e ~> a) -> (e ~> b) -> (e ~> c)
1646 as for expressions, both for type constructors and type variables; e.g. <literal>Int `Either` Bool</literal>, or
1647 <literal>Int `a` Bool</literal>. Similarly, parentheses work the same; e.g. <literal>(:*:) Int Bool</literal>.
1650 Fixities may be declared for type constructors, or classes, just as for data constructors. However,
1651 one cannot distinguish between the two in a fixity declaration; a fixity declaration
1652 sets the fixity for a data constructor and the corresponding type constructor. For example:
1656 sets the fixity for both type constructor <literal>T</literal> and data constructor <literal>T</literal>,
1657 and similarly for <literal>:*:</literal>.
1658 <literal>Int `a` Bool</literal>.
1661 Function arrow is <literal>infixr</literal> with fixity 0. (This might change; I'm not sure what it should be.)
1668 <sect2 id="type-synonyms">
1669 <title>Liberalised type synonyms</title>
1672 Type synonyms are like macros at the type level, but Haskell 98 imposes many rules
1673 on individual synonym declarations.
1674 With the <option>-XLiberalTypeSynonyms</option> extension,
1675 GHC does validity checking on types <emphasis>only after expanding type synonyms</emphasis>.
1676 That means that GHC can be very much more liberal about type synonyms than Haskell 98.
1679 <listitem> <para>You can write a <literal>forall</literal> (including overloading)
1680 in a type synonym, thus:
1682 type Discard a = forall b. Show b => a -> b -> (a, String)
1687 g :: Discard Int -> (Int,String) -- A rank-2 type
1694 If you also use <option>-XUnboxedTuples</option>,
1695 you can write an unboxed tuple in a type synonym:
1697 type Pr = (# Int, Int #)
1705 You can apply a type synonym to a forall type:
1707 type Foo a = a -> a -> Bool
1709 f :: Foo (forall b. b->b)
1711 After expanding the synonym, <literal>f</literal> has the legal (in GHC) type:
1713 f :: (forall b. b->b) -> (forall b. b->b) -> Bool
1718 You can apply a type synonym to a partially applied type synonym:
1720 type Generic i o = forall x. i x -> o x
1723 foo :: Generic Id []
1725 After expanding the synonym, <literal>foo</literal> has the legal (in GHC) type:
1727 foo :: forall x. x -> [x]
1735 GHC currently does kind checking before expanding synonyms (though even that
1739 After expanding type synonyms, GHC does validity checking on types, looking for
1740 the following mal-formedness which isn't detected simply by kind checking:
1743 Type constructor applied to a type involving for-alls.
1746 Unboxed tuple on left of an arrow.
1749 Partially-applied type synonym.
1753 this will be rejected:
1755 type Pr = (# Int, Int #)
1760 because GHC does not allow unboxed tuples on the left of a function arrow.
1765 <sect2 id="existential-quantification">
1766 <title>Existentially quantified data constructors
1770 The idea of using existential quantification in data type declarations
1771 was suggested by Perry, and implemented in Hope+ (Nigel Perry, <emphasis>The Implementation
1772 of Practical Functional Programming Languages</emphasis>, PhD Thesis, University of
1773 London, 1991). It was later formalised by Laufer and Odersky
1774 (<emphasis>Polymorphic type inference and abstract data types</emphasis>,
1775 TOPLAS, 16(5), pp1411-1430, 1994).
1776 It's been in Lennart
1777 Augustsson's <command>hbc</command> Haskell compiler for several years, and
1778 proved very useful. Here's the idea. Consider the declaration:
1784 data Foo = forall a. MkFoo a (a -> Bool)
1791 The data type <literal>Foo</literal> has two constructors with types:
1797 MkFoo :: forall a. a -> (a -> Bool) -> Foo
1804 Notice that the type variable <literal>a</literal> in the type of <function>MkFoo</function>
1805 does not appear in the data type itself, which is plain <literal>Foo</literal>.
1806 For example, the following expression is fine:
1812 [MkFoo 3 even, MkFoo 'c' isUpper] :: [Foo]
1818 Here, <literal>(MkFoo 3 even)</literal> packages an integer with a function
1819 <function>even</function> that maps an integer to <literal>Bool</literal>; and <function>MkFoo 'c'
1820 isUpper</function> packages a character with a compatible function. These
1821 two things are each of type <literal>Foo</literal> and can be put in a list.
1825 What can we do with a value of type <literal>Foo</literal>?. In particular,
1826 what happens when we pattern-match on <function>MkFoo</function>?
1832 f (MkFoo val fn) = ???
1838 Since all we know about <literal>val</literal> and <function>fn</function> is that they
1839 are compatible, the only (useful) thing we can do with them is to
1840 apply <function>fn</function> to <literal>val</literal> to get a boolean. For example:
1847 f (MkFoo val fn) = fn val
1853 What this allows us to do is to package heterogeneous values
1854 together with a bunch of functions that manipulate them, and then treat
1855 that collection of packages in a uniform manner. You can express
1856 quite a bit of object-oriented-like programming this way.
1859 <sect3 id="existential">
1860 <title>Why existential?
1864 What has this to do with <emphasis>existential</emphasis> quantification?
1865 Simply that <function>MkFoo</function> has the (nearly) isomorphic type
1871 MkFoo :: (exists a . (a, a -> Bool)) -> Foo
1877 But Haskell programmers can safely think of the ordinary
1878 <emphasis>universally</emphasis> quantified type given above, thereby avoiding
1879 adding a new existential quantification construct.
1884 <sect3 id="existential-with-context">
1885 <title>Existentials and type classes</title>
1888 An easy extension is to allow
1889 arbitrary contexts before the constructor. For example:
1895 data Baz = forall a. Eq a => Baz1 a a
1896 | forall b. Show b => Baz2 b (b -> b)
1902 The two constructors have the types you'd expect:
1908 Baz1 :: forall a. Eq a => a -> a -> Baz
1909 Baz2 :: forall b. Show b => b -> (b -> b) -> Baz
1915 But when pattern matching on <function>Baz1</function> the matched values can be compared
1916 for equality, and when pattern matching on <function>Baz2</function> the first matched
1917 value can be converted to a string (as well as applying the function to it).
1918 So this program is legal:
1925 f (Baz1 p q) | p == q = "Yes"
1927 f (Baz2 v fn) = show (fn v)
1933 Operationally, in a dictionary-passing implementation, the
1934 constructors <function>Baz1</function> and <function>Baz2</function> must store the
1935 dictionaries for <literal>Eq</literal> and <literal>Show</literal> respectively, and
1936 extract it on pattern matching.
1941 <sect3 id="existential-records">
1942 <title>Record Constructors</title>
1945 GHC allows existentials to be used with records syntax as well. For example:
1948 data Counter a = forall self. NewCounter
1950 , _inc :: self -> self
1951 , _display :: self -> IO ()
1955 Here <literal>tag</literal> is a public field, with a well-typed selector
1956 function <literal>tag :: Counter a -> a</literal>. The <literal>self</literal>
1957 type is hidden from the outside; any attempt to apply <literal>_this</literal>,
1958 <literal>_inc</literal> or <literal>_display</literal> as functions will raise a
1959 compile-time error. In other words, <emphasis>GHC defines a record selector function
1960 only for fields whose type does not mention the existentially-quantified variables</emphasis>.
1961 (This example used an underscore in the fields for which record selectors
1962 will not be defined, but that is only programming style; GHC ignores them.)
1966 To make use of these hidden fields, we need to create some helper functions:
1969 inc :: Counter a -> Counter a
1970 inc (NewCounter x i d t) = NewCounter
1971 { _this = i x, _inc = i, _display = d, tag = t }
1973 display :: Counter a -> IO ()
1974 display NewCounter{ _this = x, _display = d } = d x
1977 Now we can define counters with different underlying implementations:
1980 counterA :: Counter String
1981 counterA = NewCounter
1982 { _this = 0, _inc = (1+), _display = print, tag = "A" }
1984 counterB :: Counter String
1985 counterB = NewCounter
1986 { _this = "", _inc = ('#':), _display = putStrLn, tag = "B" }
1989 display (inc counterA) -- prints "1"
1990 display (inc (inc counterB)) -- prints "##"
1993 At the moment, record update syntax is only supported for Haskell 98 data types,
1994 so the following function does <emphasis>not</emphasis> work:
1997 -- This is invalid; use explicit NewCounter instead for now
1998 setTag :: Counter a -> a -> Counter a
1999 setTag obj t = obj{ tag = t }
2008 <title>Restrictions</title>
2011 There are several restrictions on the ways in which existentially-quantified
2012 constructors can be use.
2021 When pattern matching, each pattern match introduces a new,
2022 distinct, type for each existential type variable. These types cannot
2023 be unified with any other type, nor can they escape from the scope of
2024 the pattern match. For example, these fragments are incorrect:
2032 Here, the type bound by <function>MkFoo</function> "escapes", because <literal>a</literal>
2033 is the result of <function>f1</function>. One way to see why this is wrong is to
2034 ask what type <function>f1</function> has:
2038 f1 :: Foo -> a -- Weird!
2042 What is this "<literal>a</literal>" in the result type? Clearly we don't mean
2047 f1 :: forall a. Foo -> a -- Wrong!
2051 The original program is just plain wrong. Here's another sort of error
2055 f2 (Baz1 a b) (Baz1 p q) = a==q
2059 It's ok to say <literal>a==b</literal> or <literal>p==q</literal>, but
2060 <literal>a==q</literal> is wrong because it equates the two distinct types arising
2061 from the two <function>Baz1</function> constructors.
2069 You can't pattern-match on an existentially quantified
2070 constructor in a <literal>let</literal> or <literal>where</literal> group of
2071 bindings. So this is illegal:
2075 f3 x = a==b where { Baz1 a b = x }
2078 Instead, use a <literal>case</literal> expression:
2081 f3 x = case x of Baz1 a b -> a==b
2084 In general, you can only pattern-match
2085 on an existentially-quantified constructor in a <literal>case</literal> expression or
2086 in the patterns of a function definition.
2088 The reason for this restriction is really an implementation one.
2089 Type-checking binding groups is already a nightmare without
2090 existentials complicating the picture. Also an existential pattern
2091 binding at the top level of a module doesn't make sense, because it's
2092 not clear how to prevent the existentially-quantified type "escaping".
2093 So for now, there's a simple-to-state restriction. We'll see how
2101 You can't use existential quantification for <literal>newtype</literal>
2102 declarations. So this is illegal:
2106 newtype T = forall a. Ord a => MkT a
2110 Reason: a value of type <literal>T</literal> must be represented as a
2111 pair of a dictionary for <literal>Ord t</literal> and a value of type
2112 <literal>t</literal>. That contradicts the idea that
2113 <literal>newtype</literal> should have no concrete representation.
2114 You can get just the same efficiency and effect by using
2115 <literal>data</literal> instead of <literal>newtype</literal>. If
2116 there is no overloading involved, then there is more of a case for
2117 allowing an existentially-quantified <literal>newtype</literal>,
2118 because the <literal>data</literal> version does carry an
2119 implementation cost, but single-field existentially quantified
2120 constructors aren't much use. So the simple restriction (no
2121 existential stuff on <literal>newtype</literal>) stands, unless there
2122 are convincing reasons to change it.
2130 You can't use <literal>deriving</literal> to define instances of a
2131 data type with existentially quantified data constructors.
2133 Reason: in most cases it would not make sense. For example:;
2136 data T = forall a. MkT [a] deriving( Eq )
2139 To derive <literal>Eq</literal> in the standard way we would need to have equality
2140 between the single component of two <function>MkT</function> constructors:
2144 (MkT a) == (MkT b) = ???
2147 But <varname>a</varname> and <varname>b</varname> have distinct types, and so can't be compared.
2148 It's just about possible to imagine examples in which the derived instance
2149 would make sense, but it seems altogether simpler simply to prohibit such
2150 declarations. Define your own instances!
2161 <!-- ====================== Generalised algebraic data types ======================= -->
2163 <sect2 id="gadt-style">
2164 <title>Declaring data types with explicit constructor signatures</title>
2166 <para>GHC allows you to declare an algebraic data type by
2167 giving the type signatures of constructors explicitly. For example:
2171 Just :: a -> Maybe a
2173 The form is called a "GADT-style declaration"
2174 because Generalised Algebraic Data Types, described in <xref linkend="gadt"/>,
2175 can only be declared using this form.</para>
2176 <para>Notice that GADT-style syntax generalises existential types (<xref linkend="existential-quantification"/>).
2177 For example, these two declarations are equivalent:
2179 data Foo = forall a. MkFoo a (a -> Bool)
2180 data Foo' where { MKFoo :: a -> (a->Bool) -> Foo' }
2183 <para>Any data type that can be declared in standard Haskell-98 syntax
2184 can also be declared using GADT-style syntax.
2185 The choice is largely stylistic, but GADT-style declarations differ in one important respect:
2186 they treat class constraints on the data constructors differently.
2187 Specifically, if the constructor is given a type-class context, that
2188 context is made available by pattern matching. For example:
2191 MkSet :: Eq a => [a] -> Set a
2193 makeSet :: Eq a => [a] -> Set a
2194 makeSet xs = MkSet (nub xs)
2196 insert :: a -> Set a -> Set a
2197 insert a (MkSet as) | a `elem` as = MkSet as
2198 | otherwise = MkSet (a:as)
2200 A use of <literal>MkSet</literal> as a constructor (e.g. in the definition of <literal>makeSet</literal>)
2201 gives rise to a <literal>(Eq a)</literal>
2202 constraint, as you would expect. The new feature is that pattern-matching on <literal>MkSet</literal>
2203 (as in the definition of <literal>insert</literal>) makes <emphasis>available</emphasis> an <literal>(Eq a)</literal>
2204 context. In implementation terms, the <literal>MkSet</literal> constructor has a hidden field that stores
2205 the <literal>(Eq a)</literal> dictionary that is passed to <literal>MkSet</literal>; so
2206 when pattern-matching that dictionary becomes available for the right-hand side of the match.
2207 In the example, the equality dictionary is used to satisfy the equality constraint
2208 generated by the call to <literal>elem</literal>, so that the type of
2209 <literal>insert</literal> itself has no <literal>Eq</literal> constraint.
2212 For example, one possible application is to reify dictionaries:
2214 data NumInst a where
2215 MkNumInst :: Num a => NumInst a
2217 intInst :: NumInst Int
2220 plus :: NumInst a -> a -> a -> a
2221 plus MkNumInst p q = p + q
2223 Here, a value of type <literal>NumInst a</literal> is equivalent
2224 to an explicit <literal>(Num a)</literal> dictionary.
2227 All this applies to constructors declared using the syntax of <xref linkend="existential-with-context"/>.
2228 For example, the <literal>NumInst</literal> data type above could equivalently be declared
2232 = Num a => MkNumInst (NumInst a)
2234 Notice that, unlike the situation when declaring an existential, there is
2235 no <literal>forall</literal>, because the <literal>Num</literal> constrains the
2236 data type's universally quantified type variable <literal>a</literal>.
2237 A constructor may have both universal and existential type variables: for example,
2238 the following two declarations are equivalent:
2241 = forall b. (Num a, Eq b) => MkT1 a b
2243 MkT2 :: (Num a, Eq b) => a -> b -> T2 a
2246 <para>All this behaviour contrasts with Haskell 98's peculiar treatment of
2247 contexts on a data type declaration (Section 4.2.1 of the Haskell 98 Report).
2248 In Haskell 98 the definition
2250 data Eq a => Set' a = MkSet' [a]
2252 gives <literal>MkSet'</literal> the same type as <literal>MkSet</literal> above. But instead of
2253 <emphasis>making available</emphasis> an <literal>(Eq a)</literal> constraint, pattern-matching
2254 on <literal>MkSet'</literal> <emphasis>requires</emphasis> an <literal>(Eq a)</literal> constraint!
2255 GHC faithfully implements this behaviour, odd though it is. But for GADT-style declarations,
2256 GHC's behaviour is much more useful, as well as much more intuitive.
2260 The rest of this section gives further details about GADT-style data
2265 The result type of each data constructor must begin with the type constructor being defined.
2266 If the result type of all constructors
2267 has the form <literal>T a1 ... an</literal>, where <literal>a1 ... an</literal>
2268 are distinct type variables, then the data type is <emphasis>ordinary</emphasis>;
2269 otherwise is a <emphasis>generalised</emphasis> data type (<xref linkend="gadt"/>).
2273 The type signature of
2274 each constructor is independent, and is implicitly universally quantified as usual.
2275 Different constructors may have different universally-quantified type variables
2276 and different type-class constraints.
2277 For example, this is fine:
2280 T1 :: Eq b => b -> T b
2281 T2 :: (Show c, Ix c) => c -> [c] -> T c
2286 Unlike a Haskell-98-style
2287 data type declaration, the type variable(s) in the "<literal>data Set a where</literal>" header
2288 have no scope. Indeed, one can write a kind signature instead:
2290 data Set :: * -> * where ...
2292 or even a mixture of the two:
2294 data Foo a :: (* -> *) -> * where ...
2296 The type variables (if given) may be explicitly kinded, so we could also write the header for <literal>Foo</literal>
2299 data Foo a (b :: * -> *) where ...
2305 You can use strictness annotations, in the obvious places
2306 in the constructor type:
2309 Lit :: !Int -> Term Int
2310 If :: Term Bool -> !(Term a) -> !(Term a) -> Term a
2311 Pair :: Term a -> Term b -> Term (a,b)
2316 You can use a <literal>deriving</literal> clause on a GADT-style data type
2317 declaration. For example, these two declarations are equivalent
2319 data Maybe1 a where {
2320 Nothing1 :: Maybe1 a ;
2321 Just1 :: a -> Maybe1 a
2322 } deriving( Eq, Ord )
2324 data Maybe2 a = Nothing2 | Just2 a
2330 You can use record syntax on a GADT-style data type declaration:
2334 Adult { name :: String, children :: [Person] } :: Person
2335 Child { name :: String } :: Person
2337 As usual, for every constructor that has a field <literal>f</literal>, the type of
2338 field <literal>f</literal> must be the same (modulo alpha conversion).
2341 At the moment, record updates are not yet possible with GADT-style declarations,
2342 so support is limited to record construction, selection and pattern matching.
2345 aPerson = Adult { name = "Fred", children = [] }
2347 shortName :: Person -> Bool
2348 hasChildren (Adult { children = kids }) = not (null kids)
2349 hasChildren (Child {}) = False
2354 As in the case of existentials declared using the Haskell-98-like record syntax
2355 (<xref linkend="existential-records"/>),
2356 record-selector functions are generated only for those fields that have well-typed
2358 Here is the example of that section, in GADT-style syntax:
2360 data Counter a where
2361 NewCounter { _this :: self
2362 , _inc :: self -> self
2363 , _display :: self -> IO ()
2368 As before, only one selector function is generated here, that for <literal>tag</literal>.
2369 Nevertheless, you can still use all the field names in pattern matching and record construction.
2371 </itemizedlist></para>
2375 <title>Generalised Algebraic Data Types (GADTs)</title>
2377 <para>Generalised Algebraic Data Types generalise ordinary algebraic data types
2378 by allowing constructors to have richer return types. Here is an example:
2381 Lit :: Int -> Term Int
2382 Succ :: Term Int -> Term Int
2383 IsZero :: Term Int -> Term Bool
2384 If :: Term Bool -> Term a -> Term a -> Term a
2385 Pair :: Term a -> Term b -> Term (a,b)
2387 Notice that the return type of the constructors is not always <literal>Term a</literal>, as is the
2388 case with ordinary data types. This generality allows us to
2389 write a well-typed <literal>eval</literal> function
2390 for these <literal>Terms</literal>:
2394 eval (Succ t) = 1 + eval t
2395 eval (IsZero t) = eval t == 0
2396 eval (If b e1 e2) = if eval b then eval e1 else eval e2
2397 eval (Pair e1 e2) = (eval e1, eval e2)
2399 The key point about GADTs is that <emphasis>pattern matching causes type refinement</emphasis>.
2400 For example, in the right hand side of the equation
2405 the type <literal>a</literal> is refined to <literal>Int</literal>. That's the whole point!
2406 A precise specification of the type rules is beyond what this user manual aspires to,
2407 but the design closely follows that described in
2409 url="http://research.microsoft.com/%7Esimonpj/papers/gadt/">Simple
2410 unification-based type inference for GADTs</ulink>,
2412 The general principle is this: <emphasis>type refinement is only carried out
2413 based on user-supplied type annotations</emphasis>.
2414 So if no type signature is supplied for <literal>eval</literal>, no type refinement happens,
2415 and lots of obscure error messages will
2416 occur. However, the refinement is quite general. For example, if we had:
2418 eval :: Term a -> a -> a
2419 eval (Lit i) j = i+j
2421 the pattern match causes the type <literal>a</literal> to be refined to <literal>Int</literal> (because of the type
2422 of the constructor <literal>Lit</literal>), and that refinement also applies to the type of <literal>j</literal>, and
2423 the result type of the <literal>case</literal> expression. Hence the addition <literal>i+j</literal> is legal.
2426 These and many other examples are given in papers by Hongwei Xi, and
2427 Tim Sheard. There is a longer introduction
2428 <ulink url="http://www.haskell.org/haskellwiki/GADT">on the wiki</ulink>,
2430 <ulink url="http://www.informatik.uni-bonn.de/~ralf/publications/With.pdf">Fun with phantom types</ulink> also has a number of examples. Note that papers
2431 may use different notation to that implemented in GHC.
2434 The rest of this section outlines the extensions to GHC that support GADTs. The extension is enabled with
2435 <option>-XGADTs</option>. The <option>-XGADTs</option> flag also sets <option>-XRelaxedPolyRec</option>.
2438 A GADT can only be declared using GADT-style syntax (<xref linkend="gadt-style"/>);
2439 the old Haskell-98 syntax for data declarations always declares an ordinary data type.
2440 The result type of each constructor must begin with the type constructor being defined,
2441 but for a GADT the arguments to the type constructor can be arbitrary monotypes.
2442 For example, in the <literal>Term</literal> data
2443 type above, the type of each constructor must end with <literal>Term ty</literal>, but
2444 the <literal>ty</literal> need not be a type variable (e.g. the <literal>Lit</literal>
2449 It's is permitted to declare an ordinary algebraic data type using GADT-style syntax.
2450 What makes a GADT into a GADT is not the syntax, but rather the presence of data constructors
2451 whose result type is not just <literal>T a b</literal>.
2455 You cannot use a <literal>deriving</literal> clause for a GADT; only for
2456 an ordinary data type.
2460 As mentioned in <xref linkend="gadt-style"/>, record syntax is supported.
2464 Lit { val :: Int } :: Term Int
2465 Succ { num :: Term Int } :: Term Int
2466 Pred { num :: Term Int } :: Term Int
2467 IsZero { arg :: Term Int } :: Term Bool
2468 Pair { arg1 :: Term a
2471 If { cnd :: Term Bool
2476 However, for GADTs there is the following additional constraint:
2477 every constructor that has a field <literal>f</literal> must have
2478 the same result type (modulo alpha conversion)
2479 Hence, in the above example, we cannot merge the <literal>num</literal>
2480 and <literal>arg</literal> fields above into a
2481 single name. Although their field types are both <literal>Term Int</literal>,
2482 their selector functions actually have different types:
2485 num :: Term Int -> Term Int
2486 arg :: Term Bool -> Term Int
2491 When pattern-matching against data constructors drawn from a GADT,
2492 for example in a <literal>case</literal> expression, the following rules apply:
2494 <listitem><para>The type of the scrutinee must be rigid.</para></listitem>
2495 <listitem><para>The type of the entire <literal>case</literal> expression must be rigid.</para></listitem>
2496 <listitem><para>The type of any free variable mentioned in any of
2497 the <literal>case</literal> alternatives must be rigid.</para></listitem>
2499 A type is "rigid" if it is completely known to the compiler at its binding site. The easiest
2500 way to ensure that a variable a rigid type is to give it a type signature.
2501 For more precise details see <ulink url="http://research.microsoft.com/%7Esimonpj/papers/gadt">
2502 Simple unification-based type inference for GADTs
2503 </ulink>. The criteria implemented by GHC are given in the Appendix.
2513 <!-- ====================== End of Generalised algebraic data types ======================= -->
2515 <sect1 id="deriving">
2516 <title>Extensions to the "deriving" mechanism</title>
2518 <sect2 id="deriving-inferred">
2519 <title>Inferred context for deriving clauses</title>
2522 The Haskell Report is vague about exactly when a <literal>deriving</literal> clause is
2525 data T0 f a = MkT0 a deriving( Eq )
2526 data T1 f a = MkT1 (f a) deriving( Eq )
2527 data T2 f a = MkT2 (f (f a)) deriving( Eq )
2529 The natural generated <literal>Eq</literal> code would result in these instance declarations:
2531 instance Eq a => Eq (T0 f a) where ...
2532 instance Eq (f a) => Eq (T1 f a) where ...
2533 instance Eq (f (f a)) => Eq (T2 f a) where ...
2535 The first of these is obviously fine. The second is still fine, although less obviously.
2536 The third is not Haskell 98, and risks losing termination of instances.
2539 GHC takes a conservative position: it accepts the first two, but not the third. The rule is this:
2540 each constraint in the inferred instance context must consist only of type variables,
2541 with no repetitions.
2544 This rule is applied regardless of flags. If you want a more exotic context, you can write
2545 it yourself, using the <link linkend="stand-alone-deriving">standalone deriving mechanism</link>.
2549 <sect2 id="stand-alone-deriving">
2550 <title>Stand-alone deriving declarations</title>
2553 GHC now allows stand-alone <literal>deriving</literal> declarations, enabled by <literal>-XStandaloneDeriving</literal>:
2555 data Foo a = Bar a | Baz String
2557 deriving instance Eq a => Eq (Foo a)
2559 The syntax is identical to that of an ordinary instance declaration apart from (a) the keyword
2560 <literal>deriving</literal>, and (b) the absence of the <literal>where</literal> part.
2561 You must supply a context (in the example the context is <literal>(Eq a)</literal>),
2562 exactly as you would in an ordinary instance declaration.
2563 (In contrast the context is inferred in a <literal>deriving</literal> clause
2564 attached to a data type declaration.)
2566 A <literal>deriving instance</literal> declaration
2567 must obey the same rules concerning form and termination as ordinary instance declarations,
2568 controlled by the same flags; see <xref linkend="instance-decls"/>.
2571 Unlike a <literal>deriving</literal>
2572 declaration attached to a <literal>data</literal> declaration, the instance can be more specific
2573 than the data type (assuming you also use
2574 <literal>-XFlexibleInstances</literal>, <xref linkend="instance-rules"/>). Consider
2577 data Foo a = Bar a | Baz String
2579 deriving instance Eq a => Eq (Foo [a])
2580 deriving instance Eq a => Eq (Foo (Maybe a))
2582 This will generate a derived instance for <literal>(Foo [a])</literal> and <literal>(Foo (Maybe a))</literal>,
2583 but other types such as <literal>(Foo (Int,Bool))</literal> will not be an instance of <literal>Eq</literal>.
2586 <para>The stand-alone syntax is generalised for newtypes in exactly the same
2587 way that ordinary <literal>deriving</literal> clauses are generalised (<xref linkend="newtype-deriving"/>).
2590 newtype Foo a = MkFoo (State Int a)
2592 deriving instance MonadState Int Foo
2594 GHC always treats the <emphasis>last</emphasis> parameter of the instance
2595 (<literal>Foo</literal> in this example) as the type whose instance is being derived.
2601 <sect2 id="deriving-typeable">
2602 <title>Deriving clause for classes <literal>Typeable</literal> and <literal>Data</literal></title>
2605 Haskell 98 allows the programmer to add "<literal>deriving( Eq, Ord )</literal>" to a data type
2606 declaration, to generate a standard instance declaration for classes specified in the <literal>deriving</literal> clause.
2607 In Haskell 98, the only classes that may appear in the <literal>deriving</literal> clause are the standard
2608 classes <literal>Eq</literal>, <literal>Ord</literal>,
2609 <literal>Enum</literal>, <literal>Ix</literal>, <literal>Bounded</literal>, <literal>Read</literal>, and <literal>Show</literal>.
2612 GHC extends this list with two more classes that may be automatically derived
2613 (provided the <option>-XDeriveDataTypeable</option> flag is specified):
2614 <literal>Typeable</literal>, and <literal>Data</literal>. These classes are defined in the library
2615 modules <literal>Data.Typeable</literal> and <literal>Data.Generics</literal> respectively, and the
2616 appropriate class must be in scope before it can be mentioned in the <literal>deriving</literal> clause.
2618 <para>An instance of <literal>Typeable</literal> can only be derived if the
2619 data type has seven or fewer type parameters, all of kind <literal>*</literal>.
2620 The reason for this is that the <literal>Typeable</literal> class is derived using the scheme
2622 <ulink url="http://research.microsoft.com/%7Esimonpj/papers/hmap/gmap2.ps">
2623 Scrap More Boilerplate: Reflection, Zips, and Generalised Casts
2625 (Section 7.4 of the paper describes the multiple <literal>Typeable</literal> classes that
2626 are used, and only <literal>Typeable1</literal> up to
2627 <literal>Typeable7</literal> are provided in the library.)
2628 In other cases, there is nothing to stop the programmer writing a <literal>TypableX</literal>
2629 class, whose kind suits that of the data type constructor, and
2630 then writing the data type instance by hand.
2634 <sect2 id="newtype-deriving">
2635 <title>Generalised derived instances for newtypes</title>
2638 When you define an abstract type using <literal>newtype</literal>, you may want
2639 the new type to inherit some instances from its representation. In
2640 Haskell 98, you can inherit instances of <literal>Eq</literal>, <literal>Ord</literal>,
2641 <literal>Enum</literal> and <literal>Bounded</literal> by deriving them, but for any
2642 other classes you have to write an explicit instance declaration. For
2643 example, if you define
2646 newtype Dollars = Dollars Int
2649 and you want to use arithmetic on <literal>Dollars</literal>, you have to
2650 explicitly define an instance of <literal>Num</literal>:
2653 instance Num Dollars where
2654 Dollars a + Dollars b = Dollars (a+b)
2657 All the instance does is apply and remove the <literal>newtype</literal>
2658 constructor. It is particularly galling that, since the constructor
2659 doesn't appear at run-time, this instance declaration defines a
2660 dictionary which is <emphasis>wholly equivalent</emphasis> to the <literal>Int</literal>
2661 dictionary, only slower!
2665 <sect3> <title> Generalising the deriving clause </title>
2667 GHC now permits such instances to be derived instead,
2668 using the flag <option>-XGeneralizedNewtypeDeriving</option>,
2671 newtype Dollars = Dollars Int deriving (Eq,Show,Num)
2674 and the implementation uses the <emphasis>same</emphasis> <literal>Num</literal> dictionary
2675 for <literal>Dollars</literal> as for <literal>Int</literal>. Notionally, the compiler
2676 derives an instance declaration of the form
2679 instance Num Int => Num Dollars
2682 which just adds or removes the <literal>newtype</literal> constructor according to the type.
2686 We can also derive instances of constructor classes in a similar
2687 way. For example, suppose we have implemented state and failure monad
2688 transformers, such that
2691 instance Monad m => Monad (State s m)
2692 instance Monad m => Monad (Failure m)
2694 In Haskell 98, we can define a parsing monad by
2696 type Parser tok m a = State [tok] (Failure m) a
2699 which is automatically a monad thanks to the instance declarations
2700 above. With the extension, we can make the parser type abstract,
2701 without needing to write an instance of class <literal>Monad</literal>, via
2704 newtype Parser tok m a = Parser (State [tok] (Failure m) a)
2707 In this case the derived instance declaration is of the form
2709 instance Monad (State [tok] (Failure m)) => Monad (Parser tok m)
2712 Notice that, since <literal>Monad</literal> is a constructor class, the
2713 instance is a <emphasis>partial application</emphasis> of the new type, not the
2714 entire left hand side. We can imagine that the type declaration is
2715 "eta-converted" to generate the context of the instance
2720 We can even derive instances of multi-parameter classes, provided the
2721 newtype is the last class parameter. In this case, a ``partial
2722 application'' of the class appears in the <literal>deriving</literal>
2723 clause. For example, given the class
2726 class StateMonad s m | m -> s where ...
2727 instance Monad m => StateMonad s (State s m) where ...
2729 then we can derive an instance of <literal>StateMonad</literal> for <literal>Parser</literal>s by
2731 newtype Parser tok m a = Parser (State [tok] (Failure m) a)
2732 deriving (Monad, StateMonad [tok])
2735 The derived instance is obtained by completing the application of the
2736 class to the new type:
2739 instance StateMonad [tok] (State [tok] (Failure m)) =>
2740 StateMonad [tok] (Parser tok m)
2745 As a result of this extension, all derived instances in newtype
2746 declarations are treated uniformly (and implemented just by reusing
2747 the dictionary for the representation type), <emphasis>except</emphasis>
2748 <literal>Show</literal> and <literal>Read</literal>, which really behave differently for
2749 the newtype and its representation.
2753 <sect3> <title> A more precise specification </title>
2755 Derived instance declarations are constructed as follows. Consider the
2756 declaration (after expansion of any type synonyms)
2759 newtype T v1...vn = T' (t vk+1...vn) deriving (c1...cm)
2765 The <literal>ci</literal> are partial applications of
2766 classes of the form <literal>C t1'...tj'</literal>, where the arity of <literal>C</literal>
2767 is exactly <literal>j+1</literal>. That is, <literal>C</literal> lacks exactly one type argument.
2770 The <literal>k</literal> is chosen so that <literal>ci (T v1...vk)</literal> is well-kinded.
2773 The type <literal>t</literal> is an arbitrary type.
2776 The type variables <literal>vk+1...vn</literal> do not occur in <literal>t</literal>,
2777 nor in the <literal>ci</literal>, and
2780 None of the <literal>ci</literal> is <literal>Read</literal>, <literal>Show</literal>,
2781 <literal>Typeable</literal>, or <literal>Data</literal>. These classes
2782 should not "look through" the type or its constructor. You can still
2783 derive these classes for a newtype, but it happens in the usual way, not
2784 via this new mechanism.
2787 Then, for each <literal>ci</literal>, the derived instance
2790 instance ci t => ci (T v1...vk)
2792 As an example which does <emphasis>not</emphasis> work, consider
2794 newtype NonMonad m s = NonMonad (State s m s) deriving Monad
2796 Here we cannot derive the instance
2798 instance Monad (State s m) => Monad (NonMonad m)
2801 because the type variable <literal>s</literal> occurs in <literal>State s m</literal>,
2802 and so cannot be "eta-converted" away. It is a good thing that this
2803 <literal>deriving</literal> clause is rejected, because <literal>NonMonad m</literal> is
2804 not, in fact, a monad --- for the same reason. Try defining
2805 <literal>>>=</literal> with the correct type: you won't be able to.
2809 Notice also that the <emphasis>order</emphasis> of class parameters becomes
2810 important, since we can only derive instances for the last one. If the
2811 <literal>StateMonad</literal> class above were instead defined as
2814 class StateMonad m s | m -> s where ...
2817 then we would not have been able to derive an instance for the
2818 <literal>Parser</literal> type above. We hypothesise that multi-parameter
2819 classes usually have one "main" parameter for which deriving new
2820 instances is most interesting.
2822 <para>Lastly, all of this applies only for classes other than
2823 <literal>Read</literal>, <literal>Show</literal>, <literal>Typeable</literal>,
2824 and <literal>Data</literal>, for which the built-in derivation applies (section
2825 4.3.3. of the Haskell Report).
2826 (For the standard classes <literal>Eq</literal>, <literal>Ord</literal>,
2827 <literal>Ix</literal>, and <literal>Bounded</literal> it is immaterial whether
2828 the standard method is used or the one described here.)
2835 <!-- TYPE SYSTEM EXTENSIONS -->
2836 <sect1 id="type-class-extensions">
2837 <title>Class and instances declarations</title>
2839 <sect2 id="multi-param-type-classes">
2840 <title>Class declarations</title>
2843 This section, and the next one, documents GHC's type-class extensions.
2844 There's lots of background in the paper <ulink
2845 url="http://research.microsoft.com/~simonpj/Papers/type-class-design-space/">Type
2846 classes: exploring the design space</ulink> (Simon Peyton Jones, Mark
2847 Jones, Erik Meijer).
2850 All the extensions are enabled by the <option>-fglasgow-exts</option> flag.
2854 <title>Multi-parameter type classes</title>
2856 Multi-parameter type classes are permitted. For example:
2860 class Collection c a where
2861 union :: c a -> c a -> c a
2869 <title>The superclasses of a class declaration</title>
2872 There are no restrictions on the context in a class declaration
2873 (which introduces superclasses), except that the class hierarchy must
2874 be acyclic. So these class declarations are OK:
2878 class Functor (m k) => FiniteMap m k where
2881 class (Monad m, Monad (t m)) => Transform t m where
2882 lift :: m a -> (t m) a
2888 As in Haskell 98, The class hierarchy must be acyclic. However, the definition
2889 of "acyclic" involves only the superclass relationships. For example,
2895 op :: D b => a -> b -> b
2898 class C a => D a where { ... }
2902 Here, <literal>C</literal> is a superclass of <literal>D</literal>, but it's OK for a
2903 class operation <literal>op</literal> of <literal>C</literal> to mention <literal>D</literal>. (It
2904 would not be OK for <literal>D</literal> to be a superclass of <literal>C</literal>.)
2911 <sect3 id="class-method-types">
2912 <title>Class method types</title>
2915 Haskell 98 prohibits class method types to mention constraints on the
2916 class type variable, thus:
2919 fromList :: [a] -> s a
2920 elem :: Eq a => a -> s a -> Bool
2922 The type of <literal>elem</literal> is illegal in Haskell 98, because it
2923 contains the constraint <literal>Eq a</literal>, constrains only the
2924 class type variable (in this case <literal>a</literal>).
2925 GHC lifts this restriction (flag <option>-XConstrainedClassMethods</option>).
2932 <sect2 id="functional-dependencies">
2933 <title>Functional dependencies
2936 <para> Functional dependencies are implemented as described by Mark Jones
2937 in “<ulink url="http://citeseer.ist.psu.edu/jones00type.html">Type Classes with Functional Dependencies</ulink>”, Mark P. Jones,
2938 In Proceedings of the 9th European Symposium on Programming,
2939 ESOP 2000, Berlin, Germany, March 2000, Springer-Verlag LNCS 1782,
2943 Functional dependencies are introduced by a vertical bar in the syntax of a
2944 class declaration; e.g.
2946 class (Monad m) => MonadState s m | m -> s where ...
2948 class Foo a b c | a b -> c where ...
2950 There should be more documentation, but there isn't (yet). Yell if you need it.
2953 <sect3><title>Rules for functional dependencies </title>
2955 In a class declaration, all of the class type variables must be reachable (in the sense
2956 mentioned in <xref linkend="type-restrictions"/>)
2957 from the free variables of each method type.
2961 class Coll s a where
2963 insert :: s -> a -> s
2966 is not OK, because the type of <literal>empty</literal> doesn't mention
2967 <literal>a</literal>. Functional dependencies can make the type variable
2970 class Coll s a | s -> a where
2972 insert :: s -> a -> s
2975 Alternatively <literal>Coll</literal> might be rewritten
2978 class Coll s a where
2980 insert :: s a -> a -> s a
2984 which makes the connection between the type of a collection of
2985 <literal>a</literal>'s (namely <literal>(s a)</literal>) and the element type <literal>a</literal>.
2986 Occasionally this really doesn't work, in which case you can split the
2994 class CollE s => Coll s a where
2995 insert :: s -> a -> s
3002 <title>Background on functional dependencies</title>
3004 <para>The following description of the motivation and use of functional dependencies is taken
3005 from the Hugs user manual, reproduced here (with minor changes) by kind
3006 permission of Mark Jones.
3009 Consider the following class, intended as part of a
3010 library for collection types:
3012 class Collects e ce where
3014 insert :: e -> ce -> ce
3015 member :: e -> ce -> Bool
3017 The type variable e used here represents the element type, while ce is the type
3018 of the container itself. Within this framework, we might want to define
3019 instances of this class for lists or characteristic functions (both of which
3020 can be used to represent collections of any equality type), bit sets (which can
3021 be used to represent collections of characters), or hash tables (which can be
3022 used to represent any collection whose elements have a hash function). Omitting
3023 standard implementation details, this would lead to the following declarations:
3025 instance Eq e => Collects e [e] where ...
3026 instance Eq e => Collects e (e -> Bool) where ...
3027 instance Collects Char BitSet where ...
3028 instance (Hashable e, Collects a ce)
3029 => Collects e (Array Int ce) where ...
3031 All this looks quite promising; we have a class and a range of interesting
3032 implementations. Unfortunately, there are some serious problems with the class
3033 declaration. First, the empty function has an ambiguous type:
3035 empty :: Collects e ce => ce
3037 By "ambiguous" we mean that there is a type variable e that appears on the left
3038 of the <literal>=></literal> symbol, but not on the right. The problem with
3039 this is that, according to the theoretical foundations of Haskell overloading,
3040 we cannot guarantee a well-defined semantics for any term with an ambiguous
3044 We can sidestep this specific problem by removing the empty member from the
3045 class declaration. However, although the remaining members, insert and member,
3046 do not have ambiguous types, we still run into problems when we try to use
3047 them. For example, consider the following two functions:
3049 f x y = insert x . insert y
3052 for which GHC infers the following types:
3054 f :: (Collects a c, Collects b c) => a -> b -> c -> c
3055 g :: (Collects Bool c, Collects Char c) => c -> c
3057 Notice that the type for f allows the two parameters x and y to be assigned
3058 different types, even though it attempts to insert each of the two values, one
3059 after the other, into the same collection. If we're trying to model collections
3060 that contain only one type of value, then this is clearly an inaccurate
3061 type. Worse still, the definition for g is accepted, without causing a type
3062 error. As a result, the error in this code will not be flagged at the point
3063 where it appears. Instead, it will show up only when we try to use g, which
3064 might even be in a different module.
3067 <sect4><title>An attempt to use constructor classes</title>
3070 Faced with the problems described above, some Haskell programmers might be
3071 tempted to use something like the following version of the class declaration:
3073 class Collects e c where
3075 insert :: e -> c e -> c e
3076 member :: e -> c e -> Bool
3078 The key difference here is that we abstract over the type constructor c that is
3079 used to form the collection type c e, and not over that collection type itself,
3080 represented by ce in the original class declaration. This avoids the immediate
3081 problems that we mentioned above: empty has type <literal>Collects e c => c
3082 e</literal>, which is not ambiguous.
3085 The function f from the previous section has a more accurate type:
3087 f :: (Collects e c) => e -> e -> c e -> c e
3089 The function g from the previous section is now rejected with a type error as
3090 we would hope because the type of f does not allow the two arguments to have
3092 This, then, is an example of a multiple parameter class that does actually work
3093 quite well in practice, without ambiguity problems.
3094 There is, however, a catch. This version of the Collects class is nowhere near
3095 as general as the original class seemed to be: only one of the four instances
3096 for <literal>Collects</literal>
3097 given above can be used with this version of Collects because only one of
3098 them---the instance for lists---has a collection type that can be written in
3099 the form c e, for some type constructor c, and element type e.
3103 <sect4><title>Adding functional dependencies</title>
3106 To get a more useful version of the Collects class, Hugs provides a mechanism
3107 that allows programmers to specify dependencies between the parameters of a
3108 multiple parameter class (For readers with an interest in theoretical
3109 foundations and previous work: The use of dependency information can be seen
3110 both as a generalization of the proposal for `parametric type classes' that was
3111 put forward by Chen, Hudak, and Odersky, or as a special case of Mark Jones's
3112 later framework for "improvement" of qualified types. The
3113 underlying ideas are also discussed in a more theoretical and abstract setting
3114 in a manuscript [implparam], where they are identified as one point in a
3115 general design space for systems of implicit parameterization.).
3117 To start with an abstract example, consider a declaration such as:
3119 class C a b where ...
3121 which tells us simply that C can be thought of as a binary relation on types
3122 (or type constructors, depending on the kinds of a and b). Extra clauses can be
3123 included in the definition of classes to add information about dependencies
3124 between parameters, as in the following examples:
3126 class D a b | a -> b where ...
3127 class E a b | a -> b, b -> a where ...
3129 The notation <literal>a -> b</literal> used here between the | and where
3130 symbols --- not to be
3131 confused with a function type --- indicates that the a parameter uniquely
3132 determines the b parameter, and might be read as "a determines b." Thus D is
3133 not just a relation, but actually a (partial) function. Similarly, from the two
3134 dependencies that are included in the definition of E, we can see that E
3135 represents a (partial) one-one mapping between types.
3138 More generally, dependencies take the form <literal>x1 ... xn -> y1 ... ym</literal>,
3139 where x1, ..., xn, and y1, ..., yn are type variables with n>0 and
3140 m>=0, meaning that the y parameters are uniquely determined by the x
3141 parameters. Spaces can be used as separators if more than one variable appears
3142 on any single side of a dependency, as in <literal>t -> a b</literal>. Note that a class may be
3143 annotated with multiple dependencies using commas as separators, as in the
3144 definition of E above. Some dependencies that we can write in this notation are
3145 redundant, and will be rejected because they don't serve any useful
3146 purpose, and may instead indicate an error in the program. Examples of
3147 dependencies like this include <literal>a -> a </literal>,
3148 <literal>a -> a a </literal>,
3149 <literal>a -> </literal>, etc. There can also be
3150 some redundancy if multiple dependencies are given, as in
3151 <literal>a->b</literal>,
3152 <literal>b->c </literal>, <literal>a->c </literal>, and
3153 in which some subset implies the remaining dependencies. Examples like this are
3154 not treated as errors. Note that dependencies appear only in class
3155 declarations, and not in any other part of the language. In particular, the
3156 syntax for instance declarations, class constraints, and types is completely
3160 By including dependencies in a class declaration, we provide a mechanism for
3161 the programmer to specify each multiple parameter class more precisely. The
3162 compiler, on the other hand, is responsible for ensuring that the set of
3163 instances that are in scope at any given point in the program is consistent
3164 with any declared dependencies. For example, the following pair of instance
3165 declarations cannot appear together in the same scope because they violate the
3166 dependency for D, even though either one on its own would be acceptable:
3168 instance D Bool Int where ...
3169 instance D Bool Char where ...
3171 Note also that the following declaration is not allowed, even by itself:
3173 instance D [a] b where ...
3175 The problem here is that this instance would allow one particular choice of [a]
3176 to be associated with more than one choice for b, which contradicts the
3177 dependency specified in the definition of D. More generally, this means that,
3178 in any instance of the form:
3180 instance D t s where ...
3182 for some particular types t and s, the only variables that can appear in s are
3183 the ones that appear in t, and hence, if the type t is known, then s will be
3184 uniquely determined.
3187 The benefit of including dependency information is that it allows us to define
3188 more general multiple parameter classes, without ambiguity problems, and with
3189 the benefit of more accurate types. To illustrate this, we return to the
3190 collection class example, and annotate the original definition of <literal>Collects</literal>
3191 with a simple dependency:
3193 class Collects e ce | ce -> e where
3195 insert :: e -> ce -> ce
3196 member :: e -> ce -> Bool
3198 The dependency <literal>ce -> e</literal> here specifies that the type e of elements is uniquely
3199 determined by the type of the collection ce. Note that both parameters of
3200 Collects are of kind *; there are no constructor classes here. Note too that
3201 all of the instances of Collects that we gave earlier can be used
3202 together with this new definition.
3205 What about the ambiguity problems that we encountered with the original
3206 definition? The empty function still has type Collects e ce => ce, but it is no
3207 longer necessary to regard that as an ambiguous type: Although the variable e
3208 does not appear on the right of the => symbol, the dependency for class
3209 Collects tells us that it is uniquely determined by ce, which does appear on
3210 the right of the => symbol. Hence the context in which empty is used can still
3211 give enough information to determine types for both ce and e, without
3212 ambiguity. More generally, we need only regard a type as ambiguous if it
3213 contains a variable on the left of the => that is not uniquely determined
3214 (either directly or indirectly) by the variables on the right.
3217 Dependencies also help to produce more accurate types for user defined
3218 functions, and hence to provide earlier detection of errors, and less cluttered
3219 types for programmers to work with. Recall the previous definition for a
3222 f x y = insert x y = insert x . insert y
3224 for which we originally obtained a type:
3226 f :: (Collects a c, Collects b c) => a -> b -> c -> c
3228 Given the dependency information that we have for Collects, however, we can
3229 deduce that a and b must be equal because they both appear as the second
3230 parameter in a Collects constraint with the same first parameter c. Hence we
3231 can infer a shorter and more accurate type for f:
3233 f :: (Collects a c) => a -> a -> c -> c
3235 In a similar way, the earlier definition of g will now be flagged as a type error.
3238 Although we have given only a few examples here, it should be clear that the
3239 addition of dependency information can help to make multiple parameter classes
3240 more useful in practice, avoiding ambiguity problems, and allowing more general
3241 sets of instance declarations.
3247 <sect2 id="instance-decls">
3248 <title>Instance declarations</title>
3250 <sect3 id="instance-rules">
3251 <title>Relaxed rules for instance declarations</title>
3253 <para>An instance declaration has the form
3255 instance ( <replaceable>assertion</replaceable><subscript>1</subscript>, ..., <replaceable>assertion</replaceable><subscript>n</subscript>) => <replaceable>class</replaceable> <replaceable>type</replaceable><subscript>1</subscript> ... <replaceable>type</replaceable><subscript>m</subscript> where ...
3257 The part before the "<literal>=></literal>" is the
3258 <emphasis>context</emphasis>, while the part after the
3259 "<literal>=></literal>" is the <emphasis>head</emphasis> of the instance declaration.
3263 In Haskell 98 the head of an instance declaration
3264 must be of the form <literal>C (T a1 ... an)</literal>, where
3265 <literal>C</literal> is the class, <literal>T</literal> is a type constructor,
3266 and the <literal>a1 ... an</literal> are distinct type variables.
3267 Furthermore, the assertions in the context of the instance declaration
3268 must be of the form <literal>C a</literal> where <literal>a</literal>
3269 is a type variable that occurs in the head.
3272 The <option>-XFlexibleInstances</option> flag loosens these restrictions
3273 considerably. Firstly, multi-parameter type classes are permitted. Secondly,
3274 the context and head of the instance declaration can each consist of arbitrary
3275 (well-kinded) assertions <literal>(C t1 ... tn)</literal> subject only to the
3279 The Paterson Conditions: for each assertion in the context
3281 <listitem><para>No type variable has more occurrences in the assertion than in the head</para></listitem>
3282 <listitem><para>The assertion has fewer constructors and variables (taken together
3283 and counting repetitions) than the head</para></listitem>
3287 <listitem><para>The Coverage Condition. For each functional dependency,
3288 <replaceable>tvs</replaceable><subscript>left</subscript> <literal>-></literal>
3289 <replaceable>tvs</replaceable><subscript>right</subscript>, of the class,
3290 every type variable in
3291 S(<replaceable>tvs</replaceable><subscript>right</subscript>) must appear in
3292 S(<replaceable>tvs</replaceable><subscript>left</subscript>), where S is the
3293 substitution mapping each type variable in the class declaration to the
3294 corresponding type in the instance declaration.
3297 These restrictions ensure that context reduction terminates: each reduction
3298 step makes the problem smaller by at least one
3299 constructor. Both the Paterson Conditions and the Coverage Condition are lifted
3300 if you give the <option>-XUndecidableInstances</option>
3301 flag (<xref linkend="undecidable-instances"/>).
3302 You can find lots of background material about the reason for these
3303 restrictions in the paper <ulink
3304 url="http://research.microsoft.com/%7Esimonpj/papers/fd%2Dchr/">
3305 Understanding functional dependencies via Constraint Handling Rules</ulink>.
3308 For example, these are OK:
3310 instance C Int [a] -- Multiple parameters
3311 instance Eq (S [a]) -- Structured type in head
3313 -- Repeated type variable in head
3314 instance C4 a a => C4 [a] [a]
3315 instance Stateful (ST s) (MutVar s)
3317 -- Head can consist of type variables only
3319 instance (Eq a, Show b) => C2 a b
3321 -- Non-type variables in context
3322 instance Show (s a) => Show (Sized s a)
3323 instance C2 Int a => C3 Bool [a]
3324 instance C2 Int a => C3 [a] b
3328 -- Context assertion no smaller than head
3329 instance C a => C a where ...
3330 -- (C b b) has more more occurrences of b than the head
3331 instance C b b => Foo [b] where ...
3336 The same restrictions apply to instances generated by
3337 <literal>deriving</literal> clauses. Thus the following is accepted:
3339 data MinHeap h a = H a (h a)
3342 because the derived instance
3344 instance (Show a, Show (h a)) => Show (MinHeap h a)
3346 conforms to the above rules.
3350 A useful idiom permitted by the above rules is as follows.
3351 If one allows overlapping instance declarations then it's quite
3352 convenient to have a "default instance" declaration that applies if
3353 something more specific does not:
3361 <sect3 id="undecidable-instances">
3362 <title>Undecidable instances</title>
3365 Sometimes even the rules of <xref linkend="instance-rules"/> are too onerous.
3366 For example, sometimes you might want to use the following to get the
3367 effect of a "class synonym":
3369 class (C1 a, C2 a, C3 a) => C a where { }
3371 instance (C1 a, C2 a, C3 a) => C a where { }
3373 This allows you to write shorter signatures:
3379 f :: (C1 a, C2 a, C3 a) => ...
3381 The restrictions on functional dependencies (<xref
3382 linkend="functional-dependencies"/>) are particularly troublesome.
3383 It is tempting to introduce type variables in the context that do not appear in
3384 the head, something that is excluded by the normal rules. For example:
3386 class HasConverter a b | a -> b where
3389 data Foo a = MkFoo a
3391 instance (HasConverter a b,Show b) => Show (Foo a) where
3392 show (MkFoo value) = show (convert value)
3394 This is dangerous territory, however. Here, for example, is a program that would make the
3399 instance F [a] [[a]]
3400 instance (D c, F a c) => D [a] -- 'c' is not mentioned in the head
3402 Similarly, it can be tempting to lift the coverage condition:
3404 class Mul a b c | a b -> c where
3405 (.*.) :: a -> b -> c
3407 instance Mul Int Int Int where (.*.) = (*)
3408 instance Mul Int Float Float where x .*. y = fromIntegral x * y
3409 instance Mul a b c => Mul a [b] [c] where x .*. v = map (x.*.) v
3411 The third instance declaration does not obey the coverage condition;
3412 and indeed the (somewhat strange) definition:
3414 f = \ b x y -> if b then x .*. [y] else y
3416 makes instance inference go into a loop, because it requires the constraint
3417 <literal>(Mul a [b] b)</literal>.
3420 Nevertheless, GHC allows you to experiment with more liberal rules. If you use
3421 the experimental flag <option>-XUndecidableInstances</option>
3422 <indexterm><primary>-XUndecidableInstances</primary></indexterm>,
3423 both the Paterson Conditions and the Coverage Condition
3424 (described in <xref linkend="instance-rules"/>) are lifted. Termination is ensured by having a
3425 fixed-depth recursion stack. If you exceed the stack depth you get a
3426 sort of backtrace, and the opportunity to increase the stack depth
3427 with <option>-fcontext-stack=</option><emphasis>N</emphasis>.
3433 <sect3 id="instance-overlap">
3434 <title>Overlapping instances</title>
3436 In general, <emphasis>GHC requires that that it be unambiguous which instance
3438 should be used to resolve a type-class constraint</emphasis>. This behaviour
3439 can be modified by two flags: <option>-XOverlappingInstances</option>
3440 <indexterm><primary>-XOverlappingInstances
3441 </primary></indexterm>
3442 and <option>-XIncoherentInstances</option>
3443 <indexterm><primary>-XIncoherentInstances
3444 </primary></indexterm>, as this section discusses. Both these
3445 flags are dynamic flags, and can be set on a per-module basis, using
3446 an <literal>OPTIONS_GHC</literal> pragma if desired (<xref linkend="source-file-options"/>).</para>
3448 When GHC tries to resolve, say, the constraint <literal>C Int Bool</literal>,
3449 it tries to match every instance declaration against the
3451 by instantiating the head of the instance declaration. For example, consider
3454 instance context1 => C Int a where ... -- (A)
3455 instance context2 => C a Bool where ... -- (B)
3456 instance context3 => C Int [a] where ... -- (C)
3457 instance context4 => C Int [Int] where ... -- (D)
3459 The instances (A) and (B) match the constraint <literal>C Int Bool</literal>,
3460 but (C) and (D) do not. When matching, GHC takes
3461 no account of the context of the instance declaration
3462 (<literal>context1</literal> etc).
3463 GHC's default behaviour is that <emphasis>exactly one instance must match the
3464 constraint it is trying to resolve</emphasis>.
3465 It is fine for there to be a <emphasis>potential</emphasis> of overlap (by
3466 including both declarations (A) and (B), say); an error is only reported if a
3467 particular constraint matches more than one.
3471 The <option>-XOverlappingInstances</option> flag instructs GHC to allow
3472 more than one instance to match, provided there is a most specific one. For
3473 example, the constraint <literal>C Int [Int]</literal> matches instances (A),
3474 (C) and (D), but the last is more specific, and hence is chosen. If there is no
3475 most-specific match, the program is rejected.
3478 However, GHC is conservative about committing to an overlapping instance. For example:
3483 Suppose that from the RHS of <literal>f</literal> we get the constraint
3484 <literal>C Int [b]</literal>. But
3485 GHC does not commit to instance (C), because in a particular
3486 call of <literal>f</literal>, <literal>b</literal> might be instantiate
3487 to <literal>Int</literal>, in which case instance (D) would be more specific still.
3488 So GHC rejects the program.
3489 (If you add the flag <option>-XIncoherentInstances</option>,
3490 GHC will instead pick (C), without complaining about
3491 the problem of subsequent instantiations.)
3494 Notice that we gave a type signature to <literal>f</literal>, so GHC had to
3495 <emphasis>check</emphasis> that <literal>f</literal> has the specified type.
3496 Suppose instead we do not give a type signature, asking GHC to <emphasis>infer</emphasis>
3497 it instead. In this case, GHC will refrain from
3498 simplifying the constraint <literal>C Int [b]</literal> (for the same reason
3499 as before) but, rather than rejecting the program, it will infer the type
3501 f :: C Int [b] => [b] -> [b]
3503 That postpones the question of which instance to pick to the
3504 call site for <literal>f</literal>
3505 by which time more is known about the type <literal>b</literal>.
3506 You can write this type signature yourself if you use the
3507 <link linkend="flexible-contexts"><option>-XFlexibleContexts</option></link>
3511 Exactly the same situation can arise in instance declarations themselves. Suppose we have
3515 instance Foo [b] where
3518 and, as before, the constraint <literal>C Int [b]</literal> arises from <literal>f</literal>'s
3519 right hand side. GHC will reject the instance, complaining as before that it does not know how to resolve
3520 the constraint <literal>C Int [b]</literal>, because it matches more than one instance
3521 declaration. The solution is to postpone the choice by adding the constraint to the context
3522 of the instance declaration, thus:
3524 instance C Int [b] => Foo [b] where
3527 (You need <link linkend="instance-rules"><option>-XFlexibleInstances</option></link> to do this.)
3530 The willingness to be overlapped or incoherent is a property of
3531 the <emphasis>instance declaration</emphasis> itself, controlled by the
3532 presence or otherwise of the <option>-XOverlappingInstances</option>
3533 and <option>-XIncoherentInstances</option> flags when that module is
3534 being defined. Neither flag is required in a module that imports and uses the
3535 instance declaration. Specifically, during the lookup process:
3538 An instance declaration is ignored during the lookup process if (a) a more specific
3539 match is found, and (b) the instance declaration was compiled with
3540 <option>-XOverlappingInstances</option>. The flag setting for the
3541 more-specific instance does not matter.
3544 Suppose an instance declaration does not match the constraint being looked up, but
3545 does unify with it, so that it might match when the constraint is further
3546 instantiated. Usually GHC will regard this as a reason for not committing to
3547 some other constraint. But if the instance declaration was compiled with
3548 <option>-XIncoherentInstances</option>, GHC will skip the "does-it-unify?"
3549 check for that declaration.
3552 These rules make it possible for a library author to design a library that relies on
3553 overlapping instances without the library client having to know.
3556 If an instance declaration is compiled without
3557 <option>-XOverlappingInstances</option>,
3558 then that instance can never be overlapped. This could perhaps be
3559 inconvenient. Perhaps the rule should instead say that the
3560 <emphasis>overlapping</emphasis> instance declaration should be compiled in
3561 this way, rather than the <emphasis>overlapped</emphasis> one. Perhaps overlap
3562 at a usage site should be permitted regardless of how the instance declarations
3563 are compiled, if the <option>-XOverlappingInstances</option> flag is
3564 used at the usage site. (Mind you, the exact usage site can occasionally be
3565 hard to pin down.) We are interested to receive feedback on these points.
3567 <para>The <option>-XIncoherentInstances</option> flag implies the
3568 <option>-XOverlappingInstances</option> flag, but not vice versa.
3573 <title>Type synonyms in the instance head</title>
3576 <emphasis>Unlike Haskell 98, instance heads may use type
3577 synonyms</emphasis>. (The instance "head" is the bit after the "=>" in an instance decl.)
3578 As always, using a type synonym is just shorthand for
3579 writing the RHS of the type synonym definition. For example:
3583 type Point = (Int,Int)
3584 instance C Point where ...
3585 instance C [Point] where ...
3589 is legal. However, if you added
3593 instance C (Int,Int) where ...
3597 as well, then the compiler will complain about the overlapping
3598 (actually, identical) instance declarations. As always, type synonyms
3599 must be fully applied. You cannot, for example, write:
3604 instance Monad P where ...
3608 This design decision is independent of all the others, and easily
3609 reversed, but it makes sense to me.
3617 <sect2 id="overloaded-strings">
3618 <title>Overloaded string literals
3622 GHC supports <emphasis>overloaded string literals</emphasis>. Normally a
3623 string literal has type <literal>String</literal>, but with overloaded string
3624 literals enabled (with <literal>-XOverloadedStrings</literal>)
3625 a string literal has type <literal>(IsString a) => a</literal>.
3628 This means that the usual string syntax can be used, e.g., for packed strings
3629 and other variations of string like types. String literals behave very much
3630 like integer literals, i.e., they can be used in both expressions and patterns.
3631 If used in a pattern the literal with be replaced by an equality test, in the same
3632 way as an integer literal is.
3635 The class <literal>IsString</literal> is defined as:
3637 class IsString a where
3638 fromString :: String -> a
3640 The only predefined instance is the obvious one to make strings work as usual:
3642 instance IsString [Char] where
3645 The class <literal>IsString</literal> is not in scope by default. If you want to mention
3646 it explicitly (for example, to give an instance declaration for it), you can import it
3647 from module <literal>GHC.Exts</literal>.
3650 Haskell's defaulting mechanism is extended to cover string literals, when <option>-XOverloadedStrings</option> is specified.
3654 Each type in a default declaration must be an
3655 instance of <literal>Num</literal> <emphasis>or</emphasis> of <literal>IsString</literal>.
3659 The standard defaulting rule (<ulink url="http://www.haskell.org/onlinereport/decls.html#sect4.3.4">Haskell Report, Section 4.3.4</ulink>)
3660 is extended thus: defaulting applies when all the unresolved constraints involve standard classes
3661 <emphasis>or</emphasis> <literal>IsString</literal>; and at least one is a numeric class
3662 <emphasis>or</emphasis> <literal>IsString</literal>.
3671 import GHC.Exts( IsString(..) )
3673 newtype MyString = MyString String deriving (Eq, Show)
3674 instance IsString MyString where
3675 fromString = MyString
3677 greet :: MyString -> MyString
3678 greet "hello" = "world"
3682 print $ greet "hello"
3683 print $ greet "fool"
3687 Note that deriving <literal>Eq</literal> is necessary for the pattern matching
3688 to work since it gets translated into an equality comparison.
3694 <sect1 id="type-families">
3695 <title>Type families</title>
3698 <firstterm>Indexed type families</firstterm> are a new GHC extension to
3699 facilitate type-level
3700 programming. Type families are a generalisation of <firstterm>associated
3701 data types</firstterm>
3702 (“<ulink url="http://www.cse.unsw.edu.au/~chak/papers/CKPM05.html">Associated
3703 Types with Class</ulink>”, M. Chakravarty, G. Keller, S. Peyton Jones,
3704 and S. Marlow. In Proceedings of “The 32nd Annual ACM SIGPLAN-SIGACT
3705 Symposium on Principles of Programming Languages (POPL'05)”, pages
3706 1-13, ACM Press, 2005) and <firstterm>associated type synonyms</firstterm>
3707 (“<ulink url="http://www.cse.unsw.edu.au/~chak/papers/CKP05.html">Type
3708 Associated Type Synonyms</ulink>”. M. Chakravarty, G. Keller, and
3710 In Proceedings of “The Tenth ACM SIGPLAN International Conference on
3711 Functional Programming”, ACM Press, pages 241-253, 2005). Type families
3712 themselves are described in the paper “<ulink
3713 url="http://www.cse.unsw.edu.au/~chak/papers/SPCS08.html">Type
3714 Checking with Open Type Functions</ulink>”, T. Schrijvers,
3716 M. Chakravarty, and M. Sulzmann, in Proceedings of “ICFP 2008: The
3717 13th ACM SIGPLAN International Conference on Functional
3718 Programming”, ACM Press, pages 51-62, 2008. Type families
3719 essentially provide type-indexed data types and named functions on types,
3720 which are useful for generic programming and highly parameterised library
3721 interfaces as well as interfaces with enhanced static information, much like
3722 dependent types. They might also be regarded as an alternative to functional
3723 dependencies, but provide a more functional style of type-level programming
3724 than the relational style of functional dependencies.
3727 Indexed type families, or type families for short, are type constructors that
3728 represent sets of types. Set members are denoted by supplying the type family
3729 constructor with type parameters, which are called <firstterm>type
3730 indices</firstterm>. The
3731 difference between vanilla parametrised type constructors and family
3732 constructors is much like between parametrically polymorphic functions and
3733 (ad-hoc polymorphic) methods of type classes. Parametric polymorphic functions
3734 behave the same at all type instances, whereas class methods can change their
3735 behaviour in dependence on the class type parameters. Similarly, vanilla type
3736 constructors imply the same data representation for all type instances, but
3737 family constructors can have varying representation types for varying type
3741 Indexed type families come in two flavours: <firstterm>data
3742 families</firstterm> and <firstterm>type synonym
3743 families</firstterm>. They are the indexed family variants of algebraic
3744 data types and type synonyms, respectively. The instances of data families
3745 can be data types and newtypes.
3748 Type families are enabled by the flag <option>-XTypeFamilies</option>.
3749 Additional information on the use of type families in GHC is available on
3750 <ulink url="http://www.haskell.org/haskellwiki/GHC/Indexed_types">the
3751 Haskell wiki page on type families</ulink>.
3754 <sect2 id="data-families">
3755 <title>Data families</title>
3758 Data families appear in two flavours: (1) they can be defined on the
3760 or (2) they can appear inside type classes (in which case they are known as
3761 associated types). The former is the more general variant, as it lacks the
3762 requirement for the type-indexes to coincide with the class
3763 parameters. However, the latter can lead to more clearly structured code and
3764 compiler warnings if some type instances were - possibly accidentally -
3765 omitted. In the following, we always discuss the general toplevel form first
3766 and then cover the additional constraints placed on associated types.
3769 <sect3 id="data-family-declarations">
3770 <title>Data family declarations</title>
3773 Indexed data families are introduced by a signature, such as
3775 data family GMap k :: * -> *
3777 The special <literal>family</literal> distinguishes family from standard
3778 data declarations. The result kind annotation is optional and, as
3779 usual, defaults to <literal>*</literal> if omitted. An example is
3783 Named arguments can also be given explicit kind signatures if needed.
3785 [http://www.haskell.org/ghc/docs/latest/html/users_guide/gadt.html GADT
3786 declarations] named arguments are entirely optional, so that we can
3787 declare <literal>Array</literal> alternatively with
3789 data family Array :: * -> *
3793 <sect4 id="assoc-data-family-decl">
3794 <title>Associated data family declarations</title>
3796 When a data family is declared as part of a type class, we drop
3797 the <literal>family</literal> special. The <literal>GMap</literal>
3798 declaration takes the following form
3800 class GMapKey k where
3801 data GMap k :: * -> *
3804 In contrast to toplevel declarations, named arguments must be used for
3805 all type parameters that are to be used as type-indexes. Moreover,
3806 the argument names must be class parameters. Each class parameter may
3807 only be used at most once per associated type, but some may be omitted
3808 and they may be in an order other than in the class head. Hence, the
3809 following contrived example is admissible:
3818 <sect3 id="data-instance-declarations">
3819 <title>Data instance declarations</title>
3822 Instance declarations of data and newtype families are very similar to
3823 standard data and newtype declarations. The only two differences are
3824 that the keyword <literal>data</literal> or <literal>newtype</literal>
3825 is followed by <literal>instance</literal> and that some or all of the
3826 type arguments can be non-variable types, but may not contain forall
3827 types or type synonym families. However, data families are generally
3828 allowed in type parameters, and type synonyms are allowed as long as
3829 they are fully applied and expand to a type that is itself admissible -
3830 exactly as this is required for occurrences of type synonyms in class
3831 instance parameters. For example, the <literal>Either</literal>
3832 instance for <literal>GMap</literal> is
3834 data instance GMap (Either a b) v = GMapEither (GMap a v) (GMap b v)
3836 In this example, the declaration has only one variant. In general, it
3840 Data and newtype instance declarations are only legit when an
3841 appropriate family declaration is in scope - just like class instances
3842 require the class declaration to be visible. Moreover, each instance
3843 declaration has to conform to the kind determined by its family
3844 declaration. This implies that the number of parameters of an instance
3845 declaration matches the arity determined by the kind of the family.
3846 Although, all data families are declared with
3847 the <literal>data</literal> keyword, instances can be
3848 either <literal>data</literal> or <literal>newtype</literal>s, or a mix
3852 Even if type families are defined as toplevel declarations, functions
3853 that perform different computations for different family instances still
3854 need to be defined as methods of type classes. In particular, the
3855 following is not possible:
3858 data instance T Int = A
3859 data instance T Char = B
3860 nonsence :: T a -> Int
3861 nonsence A = 1 -- WRONG: These two equations together...
3862 nonsence B = 2 -- ...will produce a type error.
3864 Given the functionality provided by GADTs (Generalised Algebraic Data
3865 Types), it might seem as if a definition, such as the above, should be
3866 feasible. However, type families are - in contrast to GADTs - are
3867 <emphasis>open;</emphasis> i.e., new instances can always be added,
3869 modules. Supporting pattern matching across different data instances
3870 would require a form of extensible case construct.
3873 <sect4 id="assoc-data-inst">
3874 <title>Associated data instances</title>
3876 When an associated data family instance is declared within a type
3877 class instance, we drop the <literal>instance</literal> keyword in the
3878 family instance. So, the <literal>Either</literal> instance
3879 for <literal>GMap</literal> becomes:
3881 instance (GMapKey a, GMapKey b) => GMapKey (Either a b) where
3882 data GMap (Either a b) v = GMapEither (GMap a v) (GMap b v)
3885 The most important point about associated family instances is that the
3886 type indexes corresponding to class parameters must be identical to
3887 the type given in the instance head; here this is the first argument
3888 of <literal>GMap</literal>, namely <literal>Either a b</literal>,
3889 which coincides with the only class parameter. Any parameters to the
3890 family constructor that do not correspond to class parameters, need to
3891 be variables in every instance; here this is the
3892 variable <literal>v</literal>.
3895 Instances for an associated family can only appear as part of
3896 instances declarations of the class in which the family was declared -
3897 just as with the equations of the methods of a class. Also in
3898 correspondence to how methods are handled, declarations of associated
3899 types can be omitted in class instances. If an associated family
3900 instance is omitted, the corresponding instance type is not inhabited;
3901 i.e., only diverging expressions, such
3902 as <literal>undefined</literal>, can assume the type.
3906 <sect4 id="scoping-class-params">
3907 <title>Scoping of class parameters</title>
3909 In the case of multi-parameter type classes, the visibility of class
3910 parameters in the right-hand side of associated family instances
3911 depends <emphasis>solely</emphasis> on the parameters of the data
3912 family. As an example, consider the simple class declaration
3917 Only one of the two class parameters is a parameter to the data
3918 family. Hence, the following instance declaration is invalid:
3920 instance C [c] d where
3921 data T [c] = MkT (c, d) -- WRONG!! 'd' is not in scope
3923 Here, the right-hand side of the data instance mentions the type
3924 variable <literal>d</literal> that does not occur in its left-hand
3925 side. We cannot admit such data instances as they would compromise
3930 <sect4 id="family-class-inst">
3931 <title>Type class instances of family instances</title>
3933 Type class instances of instances of data families can be defined as
3934 usual, and in particular data instance declarations can
3935 have <literal>deriving</literal> clauses. For example, we can write
3937 data GMap () v = GMapUnit (Maybe v)
3940 which implicitly defines an instance of the form
3942 instance Show v => Show (GMap () v) where ...
3946 Note that class instances are always for
3947 particular <emphasis>instances</emphasis> of a data family and never
3948 for an entire family as a whole. This is for essentially the same
3949 reasons that we cannot define a toplevel function that performs
3950 pattern matching on the data constructors
3951 of <emphasis>different</emphasis> instances of a single type family.
3952 It would require a form of extensible case construct.
3956 <sect4 id="data-family-overlap">
3957 <title>Overlap of data instances</title>
3959 The instance declarations of a data family used in a single program
3960 may not overlap at all, independent of whether they are associated or
3961 not. In contrast to type class instances, this is not only a matter
3962 of consistency, but one of type safety.
3968 <sect3 id="data-family-import-export">
3969 <title>Import and export</title>
3972 The association of data constructors with type families is more dynamic
3973 than that is the case with standard data and newtype declarations. In
3974 the standard case, the notation <literal>T(..)</literal> in an import or
3975 export list denotes the type constructor and all the data constructors
3976 introduced in its declaration. However, a family declaration never
3977 introduces any data constructors; instead, data constructors are
3978 introduced by family instances. As a result, which data constructors
3979 are associated with a type family depends on the currently visible
3980 instance declarations for that family. Consequently, an import or
3981 export item of the form <literal>T(..)</literal> denotes the family
3982 constructor and all currently visible data constructors - in the case of
3983 an export item, these may be either imported or defined in the current
3984 module. The treatment of import and export items that explicitly list
3985 data constructors, such as <literal>GMap(GMapEither)</literal>, is
3989 <sect4 id="data-family-impexp-assoc">
3990 <title>Associated families</title>
3992 As expected, an import or export item of the
3993 form <literal>C(..)</literal> denotes all of the class' methods and
3994 associated types. However, when associated types are explicitly
3995 listed as subitems of a class, we need some new syntax, as uppercase
3996 identifiers as subitems are usually data constructors, not type
3997 constructors. To clarify that we denote types here, each associated
3998 type name needs to be prefixed by the keyword <literal>type</literal>.
3999 So for example, when explicitly listing the components of
4000 the <literal>GMapKey</literal> class, we write <literal>GMapKey(type
4001 GMap, empty, lookup, insert)</literal>.
4005 <sect4 id="data-family-impexp-examples">
4006 <title>Examples</title>
4008 Assuming our running <literal>GMapKey</literal> class example, let us
4009 look at some export lists and their meaning:
4012 <para><literal>module GMap (GMapKey) where...</literal>: Exports
4013 just the class name.</para>
4016 <para><literal>module GMap (GMapKey(..)) where...</literal>:
4017 Exports the class, the associated type <literal>GMap</literal>
4019 functions <literal>empty</literal>, <literal>lookup</literal>,
4020 and <literal>insert</literal>. None of the data constructors is
4024 <para><literal>module GMap (GMapKey(..), GMap(..))
4025 where...</literal>: As before, but also exports all the data
4026 constructors <literal>GMapInt</literal>,
4027 <literal>GMapChar</literal>,
4028 <literal>GMapUnit</literal>, <literal>GMapPair</literal>,
4029 and <literal>GMapUnit</literal>.</para>
4032 <para><literal>module GMap (GMapKey(empty, lookup, insert),
4033 GMap(..)) where...</literal>: As before.</para>
4036 <para><literal>module GMap (GMapKey, empty, lookup, insert, GMap(..))
4037 where...</literal>: As before.</para>
4042 Finally, you can write <literal>GMapKey(type GMap)</literal> to denote
4043 both the class <literal>GMapKey</literal> as well as its associated
4044 type <literal>GMap</literal>. However, you cannot
4045 write <literal>GMapKey(type GMap(..))</literal> — i.e.,
4046 sub-component specifications cannot be nested. To
4047 specify <literal>GMap</literal>'s data constructors, you have to list
4052 <sect4 id="data-family-impexp-instances">
4053 <title>Instances</title>
4055 Family instances are implicitly exported, just like class instances.
4056 However, this applies only to the heads of instances, not to the data
4057 constructors an instance defines.
4065 <sect2 id="synonym-families">
4066 <title>Synonym families</title>
4069 Type families appear in two flavours: (1) they can be defined on the
4070 toplevel or (2) they can appear inside type classes (in which case they
4071 are known as associated type synonyms). The former is the more general
4072 variant, as it lacks the requirement for the type-indexes to coincide with
4073 the class parameters. However, the latter can lead to more clearly
4074 structured code and compiler warnings if some type instances were -
4075 possibly accidentally - omitted. In the following, we always discuss the
4076 general toplevel form first and then cover the additional constraints
4077 placed on associated types.
4080 <sect3 id="type-family-declarations">
4081 <title>Type family declarations</title>
4084 Indexed type families are introduced by a signature, such as
4086 type family Elem c :: *
4088 The special <literal>family</literal> distinguishes family from standard
4089 type declarations. The result kind annotation is optional and, as
4090 usual, defaults to <literal>*</literal> if omitted. An example is
4094 Parameters can also be given explicit kind signatures if needed. We
4095 call the number of parameters in a type family declaration, the family's
4096 arity, and all applications of a type family must be fully saturated
4097 w.r.t. to that arity. This requirement is unlike ordinary type synonyms
4098 and it implies that the kind of a type family is not sufficient to
4099 determine a family's arity, and hence in general, also insufficient to
4100 determine whether a type family application is well formed. As an
4101 example, consider the following declaration:
4103 type family F a b :: * -> * -- F's arity is 2,
4104 -- although it's overall kind is * -> * -> * -> *
4106 Given this declaration the following are examples of well-formed and
4109 F Char [Int] -- OK! Kind: * -> *
4110 F Char [Int] Bool -- OK! Kind: *
4111 F IO Bool -- WRONG: kind mismatch in the first argument
4112 F Bool -- WRONG: unsaturated application
4116 <sect4 id="assoc-type-family-decl">
4117 <title>Associated type family declarations</title>
4119 When a type family is declared as part of a type class, we drop
4120 the <literal>family</literal> special. The <literal>Elem</literal>
4121 declaration takes the following form
4123 class Collects ce where
4127 The argument names of the type family must be class parameters. Each
4128 class parameter may only be used at most once per associated type, but
4129 some may be omitted and they may be in an order other than in the
4130 class head. Hence, the following contrived example is admissible:
4135 These rules are exactly as for associated data families.
4140 <sect3 id="type-instance-declarations">
4141 <title>Type instance declarations</title>
4143 Instance declarations of type families are very similar to standard type
4144 synonym declarations. The only two differences are that the
4145 keyword <literal>type</literal> is followed
4146 by <literal>instance</literal> and that some or all of the type
4147 arguments can be non-variable types, but may not contain forall types or
4148 type synonym families. However, data families are generally allowed, and
4149 type synonyms are allowed as long as they are fully applied and expand
4150 to a type that is admissible - these are the exact same requirements as
4151 for data instances. For example, the <literal>[e]</literal> instance
4152 for <literal>Elem</literal> is
4154 type instance Elem [e] = e
4158 Type family instance declarations are only legitimate when an
4159 appropriate family declaration is in scope - just like class instances
4160 require the class declaration to be visible. Moreover, each instance
4161 declaration has to conform to the kind determined by its family
4162 declaration, and the number of type parameters in an instance
4163 declaration must match the number of type parameters in the family
4164 declaration. Finally, the right-hand side of a type instance must be a
4165 monotype (i.e., it may not include foralls) and after the expansion of
4166 all saturated vanilla type synonyms, no synonyms, except family synonyms
4167 may remain. Here are some examples of admissible and illegal type
4170 type family F a :: *
4171 type instance F [Int] = Int -- OK!
4172 type instance F String = Char -- OK!
4173 type instance F (F a) = a -- WRONG: type parameter mentions a type family
4174 type instance F (forall a. (a, b)) = b -- WRONG: a forall type appears in a type parameter
4175 type instance F Float = forall a.a -- WRONG: right-hand side may not be a forall type
4177 type family G a b :: * -> *
4178 type instance G Int = (,) -- WRONG: must be two type parameters
4179 type instance G Int Char Float = Double -- WRONG: must be two type parameters
4183 <sect4 id="assoc-type-instance">
4184 <title>Associated type instance declarations</title>
4186 When an associated family instance is declared within a type class
4187 instance, we drop the <literal>instance</literal> keyword in the family
4188 instance. So, the <literal>[e]</literal> instance
4189 for <literal>Elem</literal> becomes:
4191 instance (Eq (Elem [e])) => Collects ([e]) where
4195 The most important point about associated family instances is that the
4196 type indexes corresponding to class parameters must be identical to the
4197 type given in the instance head; here this is <literal>[e]</literal>,
4198 which coincides with the only class parameter.
4201 Instances for an associated family can only appear as part of instances
4202 declarations of the class in which the family was declared - just as
4203 with the equations of the methods of a class. Also in correspondence to
4204 how methods are handled, declarations of associated types can be omitted
4205 in class instances. If an associated family instance is omitted, the
4206 corresponding instance type is not inhabited; i.e., only diverging
4207 expressions, such as <literal>undefined</literal>, can assume the type.
4211 <sect4 id="type-family-overlap">
4212 <title>Overlap of type synonym instances</title>
4214 The instance declarations of a type family used in a single program
4215 may only overlap if the right-hand sides of the overlapping instances
4216 coincide for the overlapping types. More formally, two instance
4217 declarations overlap if there is a substitution that makes the
4218 left-hand sides of the instances syntactically the same. Whenever
4219 that is the case, the right-hand sides of the instances must also be
4220 syntactically equal under the same substitution. This condition is
4221 independent of whether the type family is associated or not, and it is
4222 not only a matter of consistency, but one of type safety.
4225 Here are two example to illustrate the condition under which overlap
4228 type instance F (a, Int) = [a]
4229 type instance F (Int, b) = [b] -- overlap permitted
4231 type instance G (a, Int) = [a]
4232 type instance G (Char, a) = [a] -- ILLEGAL overlap, as [Char] /= [Int]
4237 <sect4 id="type-family-decidability">
4238 <title>Decidability of type synonym instances</title>
4240 In order to guarantee that type inference in the presence of type
4241 families decidable, we need to place a number of additional
4242 restrictions on the formation of type instance declarations (c.f.,
4243 Definition 5 (Relaxed Conditions) of “<ulink
4244 url="http://www.cse.unsw.edu.au/~chak/papers/SPCS08.html">Type
4245 Checking with Open Type Functions</ulink>”). Instance
4246 declarations have the general form
4248 type instance F t1 .. tn = t
4250 where we require that for every type family application <literal>(G s1
4251 .. sm)</literal> in <literal>t</literal>,
4254 <para><literal>s1 .. sm</literal> do not contain any type family
4255 constructors,</para>
4258 <para>the total number of symbols (data type constructors and type
4259 variables) in <literal>s1 .. sm</literal> is strictly smaller than
4260 in <literal>t1 .. tn</literal>, and</para>
4263 <para>for every type
4264 variable <literal>a</literal>, <literal>a</literal> occurs
4265 in <literal>s1 .. sm</literal> at most as often as in <literal>t1
4266 .. tn</literal>.</para>
4269 These restrictions are easily verified and ensure termination of type
4270 inference. However, they are not sufficient to guarantee completeness
4271 of type inference in the presence of, so called, ''loopy equalities'',
4272 such as <literal>a ~ [F a]</literal>, where a recursive occurrence of
4273 a type variable is underneath a family application and data
4274 constructor application - see the above mentioned paper for details.
4277 If the option <option>-XUndecidableInstances</option> is passed to the
4278 compiler, the above restrictions are not enforced and it is on the
4279 programmer to ensure termination of the normalisation of type families
4280 during type inference.
4285 <sect3 id-="equality-constraints">
4286 <title>Equality constraints</title>
4288 Type context can include equality constraints of the form <literal>t1 ~
4289 t2</literal>, which denote that the types <literal>t1</literal>
4290 and <literal>t2</literal> need to be the same. In the presence of type
4291 families, whether two types are equal cannot generally be decided
4292 locally. Hence, the contexts of function signatures may include
4293 equality constraints, as in the following example:
4295 sumCollects :: (Collects c1, Collects c2, Elem c1 ~ Elem c2) => c1 -> c2 -> c2
4297 where we require that the element type of <literal>c1</literal>
4298 and <literal>c2</literal> are the same. In general, the
4299 types <literal>t1</literal> and <literal>t2</literal> of an equality
4300 constraint may be arbitrary monotypes; i.e., they may not contain any
4301 quantifiers, independent of whether higher-rank types are otherwise
4305 Equality constraints can also appear in class and instance contexts.
4306 The former enable a simple translation of programs using functional
4307 dependencies into programs using family synonyms instead. The general
4308 idea is to rewrite a class declaration of the form
4310 class C a b | a -> b
4314 class (F a ~ b) => C a b where
4317 That is, we represent every functional dependency (FD) <literal>a1 .. an
4318 -> b</literal> by an FD type family <literal>F a1 .. an</literal> and a
4319 superclass context equality <literal>F a1 .. an ~ b</literal>,
4320 essentially giving a name to the functional dependency. In class
4321 instances, we define the type instances of FD families in accordance
4322 with the class head. Method signatures are not affected by that
4326 NB: Equalities in superclass contexts are not fully implemented in
4335 <sect1 id="other-type-extensions">
4336 <title>Other type system extensions</title>
4338 <sect2 id="type-restrictions">
4339 <title>Type signatures</title>
4341 <sect3 id="flexible-contexts"><title>The context of a type signature</title>
4343 The <option>-XFlexibleContexts</option> flag lifts the Haskell 98 restriction
4344 that the type-class constraints in a type signature must have the
4345 form <emphasis>(class type-variable)</emphasis> or
4346 <emphasis>(class (type-variable type-variable ...))</emphasis>.
4347 With <option>-XFlexibleContexts</option>
4348 these type signatures are perfectly OK
4351 g :: Ord (T a ()) => ...
4355 GHC imposes the following restrictions on the constraints in a type signature.
4359 forall tv1..tvn (c1, ...,cn) => type
4362 (Here, we write the "foralls" explicitly, although the Haskell source
4363 language omits them; in Haskell 98, all the free type variables of an
4364 explicit source-language type signature are universally quantified,
4365 except for the class type variables in a class declaration. However,
4366 in GHC, you can give the foralls if you want. See <xref linkend="universal-quantification"/>).
4375 <emphasis>Each universally quantified type variable
4376 <literal>tvi</literal> must be reachable from <literal>type</literal></emphasis>.
4378 A type variable <literal>a</literal> is "reachable" if it appears
4379 in the same constraint as either a type variable free in
4380 <literal>type</literal>, or another reachable type variable.
4381 A value with a type that does not obey
4382 this reachability restriction cannot be used without introducing
4383 ambiguity; that is why the type is rejected.
4384 Here, for example, is an illegal type:
4388 forall a. Eq a => Int
4392 When a value with this type was used, the constraint <literal>Eq tv</literal>
4393 would be introduced where <literal>tv</literal> is a fresh type variable, and
4394 (in the dictionary-translation implementation) the value would be
4395 applied to a dictionary for <literal>Eq tv</literal>. The difficulty is that we
4396 can never know which instance of <literal>Eq</literal> to use because we never
4397 get any more information about <literal>tv</literal>.
4401 that the reachability condition is weaker than saying that <literal>a</literal> is
4402 functionally dependent on a type variable free in
4403 <literal>type</literal> (see <xref
4404 linkend="functional-dependencies"/>). The reason for this is there
4405 might be a "hidden" dependency, in a superclass perhaps. So
4406 "reachable" is a conservative approximation to "functionally dependent".
4407 For example, consider:
4409 class C a b | a -> b where ...
4410 class C a b => D a b where ...
4411 f :: forall a b. D a b => a -> a
4413 This is fine, because in fact <literal>a</literal> does functionally determine <literal>b</literal>
4414 but that is not immediately apparent from <literal>f</literal>'s type.
4420 <emphasis>Every constraint <literal>ci</literal> must mention at least one of the
4421 universally quantified type variables <literal>tvi</literal></emphasis>.
4423 For example, this type is OK because <literal>C a b</literal> mentions the
4424 universally quantified type variable <literal>b</literal>:
4428 forall a. C a b => burble
4432 The next type is illegal because the constraint <literal>Eq b</literal> does not
4433 mention <literal>a</literal>:
4437 forall a. Eq b => burble
4441 The reason for this restriction is milder than the other one. The
4442 excluded types are never useful or necessary (because the offending
4443 context doesn't need to be witnessed at this point; it can be floated
4444 out). Furthermore, floating them out increases sharing. Lastly,
4445 excluding them is a conservative choice; it leaves a patch of
4446 territory free in case we need it later.
4460 <sect2 id="implicit-parameters">
4461 <title>Implicit parameters</title>
4463 <para> Implicit parameters are implemented as described in
4464 "Implicit parameters: dynamic scoping with static types",
4465 J Lewis, MB Shields, E Meijer, J Launchbury,
4466 27th ACM Symposium on Principles of Programming Languages (POPL'00),
4470 <para>(Most of the following, still rather incomplete, documentation is
4471 due to Jeff Lewis.)</para>
4473 <para>Implicit parameter support is enabled with the option
4474 <option>-XImplicitParams</option>.</para>
4477 A variable is called <emphasis>dynamically bound</emphasis> when it is bound by the calling
4478 context of a function and <emphasis>statically bound</emphasis> when bound by the callee's
4479 context. In Haskell, all variables are statically bound. Dynamic
4480 binding of variables is a notion that goes back to Lisp, but was later
4481 discarded in more modern incarnations, such as Scheme. Dynamic binding
4482 can be very confusing in an untyped language, and unfortunately, typed
4483 languages, in particular Hindley-Milner typed languages like Haskell,
4484 only support static scoping of variables.
4487 However, by a simple extension to the type class system of Haskell, we
4488 can support dynamic binding. Basically, we express the use of a
4489 dynamically bound variable as a constraint on the type. These
4490 constraints lead to types of the form <literal>(?x::t') => t</literal>, which says "this
4491 function uses a dynamically-bound variable <literal>?x</literal>
4492 of type <literal>t'</literal>". For
4493 example, the following expresses the type of a sort function,
4494 implicitly parameterized by a comparison function named <literal>cmp</literal>.
4496 sort :: (?cmp :: a -> a -> Bool) => [a] -> [a]
4498 The dynamic binding constraints are just a new form of predicate in the type class system.
4501 An implicit parameter occurs in an expression using the special form <literal>?x</literal>,
4502 where <literal>x</literal> is
4503 any valid identifier (e.g. <literal>ord ?x</literal> is a valid expression).
4504 Use of this construct also introduces a new
4505 dynamic-binding constraint in the type of the expression.
4506 For example, the following definition
4507 shows how we can define an implicitly parameterized sort function in
4508 terms of an explicitly parameterized <literal>sortBy</literal> function:
4510 sortBy :: (a -> a -> Bool) -> [a] -> [a]
4512 sort :: (?cmp :: a -> a -> Bool) => [a] -> [a]
4518 <title>Implicit-parameter type constraints</title>
4520 Dynamic binding constraints behave just like other type class
4521 constraints in that they are automatically propagated. Thus, when a
4522 function is used, its implicit parameters are inherited by the
4523 function that called it. For example, our <literal>sort</literal> function might be used
4524 to pick out the least value in a list:
4526 least :: (?cmp :: a -> a -> Bool) => [a] -> a
4527 least xs = head (sort xs)
4529 Without lifting a finger, the <literal>?cmp</literal> parameter is
4530 propagated to become a parameter of <literal>least</literal> as well. With explicit
4531 parameters, the default is that parameters must always be explicit
4532 propagated. With implicit parameters, the default is to always
4536 An implicit-parameter type constraint differs from other type class constraints in the
4537 following way: All uses of a particular implicit parameter must have
4538 the same type. This means that the type of <literal>(?x, ?x)</literal>
4539 is <literal>(?x::a) => (a,a)</literal>, and not
4540 <literal>(?x::a, ?x::b) => (a, b)</literal>, as would be the case for type
4544 <para> You can't have an implicit parameter in the context of a class or instance
4545 declaration. For example, both these declarations are illegal:
4547 class (?x::Int) => C a where ...
4548 instance (?x::a) => Foo [a] where ...
4550 Reason: exactly which implicit parameter you pick up depends on exactly where
4551 you invoke a function. But the ``invocation'' of instance declarations is done
4552 behind the scenes by the compiler, so it's hard to figure out exactly where it is done.
4553 Easiest thing is to outlaw the offending types.</para>
4555 Implicit-parameter constraints do not cause ambiguity. For example, consider:
4557 f :: (?x :: [a]) => Int -> Int
4560 g :: (Read a, Show a) => String -> String
4563 Here, <literal>g</literal> has an ambiguous type, and is rejected, but <literal>f</literal>
4564 is fine. The binding for <literal>?x</literal> at <literal>f</literal>'s call site is
4565 quite unambiguous, and fixes the type <literal>a</literal>.
4570 <title>Implicit-parameter bindings</title>
4573 An implicit parameter is <emphasis>bound</emphasis> using the standard
4574 <literal>let</literal> or <literal>where</literal> binding forms.
4575 For example, we define the <literal>min</literal> function by binding
4576 <literal>cmp</literal>.
4579 min = let ?cmp = (<=) in least
4583 A group of implicit-parameter bindings may occur anywhere a normal group of Haskell
4584 bindings can occur, except at top level. That is, they can occur in a <literal>let</literal>
4585 (including in a list comprehension, or do-notation, or pattern guards),
4586 or a <literal>where</literal> clause.
4587 Note the following points:
4590 An implicit-parameter binding group must be a
4591 collection of simple bindings to implicit-style variables (no
4592 function-style bindings, and no type signatures); these bindings are
4593 neither polymorphic or recursive.
4596 You may not mix implicit-parameter bindings with ordinary bindings in a
4597 single <literal>let</literal>
4598 expression; use two nested <literal>let</literal>s instead.
4599 (In the case of <literal>where</literal> you are stuck, since you can't nest <literal>where</literal> clauses.)
4603 You may put multiple implicit-parameter bindings in a
4604 single binding group; but they are <emphasis>not</emphasis> treated
4605 as a mutually recursive group (as ordinary <literal>let</literal> bindings are).
4606 Instead they are treated as a non-recursive group, simultaneously binding all the implicit
4607 parameter. The bindings are not nested, and may be re-ordered without changing
4608 the meaning of the program.
4609 For example, consider:
4611 f t = let { ?x = t; ?y = ?x+(1::Int) } in ?x + ?y
4613 The use of <literal>?x</literal> in the binding for <literal>?y</literal> does not "see"
4614 the binding for <literal>?x</literal>, so the type of <literal>f</literal> is
4616 f :: (?x::Int) => Int -> Int
4624 <sect3><title>Implicit parameters and polymorphic recursion</title>
4627 Consider these two definitions:
4630 len1 xs = let ?acc = 0 in len_acc1 xs
4633 len_acc1 (x:xs) = let ?acc = ?acc + (1::Int) in len_acc1 xs
4638 len2 xs = let ?acc = 0 in len_acc2 xs
4640 len_acc2 :: (?acc :: Int) => [a] -> Int
4642 len_acc2 (x:xs) = let ?acc = ?acc + (1::Int) in len_acc2 xs
4644 The only difference between the two groups is that in the second group
4645 <literal>len_acc</literal> is given a type signature.
4646 In the former case, <literal>len_acc1</literal> is monomorphic in its own
4647 right-hand side, so the implicit parameter <literal>?acc</literal> is not
4648 passed to the recursive call. In the latter case, because <literal>len_acc2</literal>
4649 has a type signature, the recursive call is made to the
4650 <emphasis>polymorphic</emphasis> version, which takes <literal>?acc</literal>
4651 as an implicit parameter. So we get the following results in GHCi:
4658 Adding a type signature dramatically changes the result! This is a rather
4659 counter-intuitive phenomenon, worth watching out for.
4663 <sect3><title>Implicit parameters and monomorphism</title>
4665 <para>GHC applies the dreaded Monomorphism Restriction (section 4.5.5 of the
4666 Haskell Report) to implicit parameters. For example, consider:
4674 Since the binding for <literal>y</literal> falls under the Monomorphism
4675 Restriction it is not generalised, so the type of <literal>y</literal> is
4676 simply <literal>Int</literal>, not <literal>(?x::Int) => Int</literal>.
4677 Hence, <literal>(f 9)</literal> returns result <literal>9</literal>.
4678 If you add a type signature for <literal>y</literal>, then <literal>y</literal>
4679 will get type <literal>(?x::Int) => Int</literal>, so the occurrence of
4680 <literal>y</literal> in the body of the <literal>let</literal> will see the
4681 inner binding of <literal>?x</literal>, so <literal>(f 9)</literal> will return
4682 <literal>14</literal>.
4687 <!-- ======================= COMMENTED OUT ========================
4689 We intend to remove linear implicit parameters, so I'm at least removing
4690 them from the 6.6 user manual
4692 <sect2 id="linear-implicit-parameters">
4693 <title>Linear implicit parameters</title>
4695 Linear implicit parameters are an idea developed by Koen Claessen,
4696 Mark Shields, and Simon PJ. They address the long-standing
4697 problem that monads seem over-kill for certain sorts of problem, notably:
4700 <listitem> <para> distributing a supply of unique names </para> </listitem>
4701 <listitem> <para> distributing a supply of random numbers </para> </listitem>
4702 <listitem> <para> distributing an oracle (as in QuickCheck) </para> </listitem>
4706 Linear implicit parameters are just like ordinary implicit parameters,
4707 except that they are "linear"; that is, they cannot be copied, and
4708 must be explicitly "split" instead. Linear implicit parameters are
4709 written '<literal>%x</literal>' instead of '<literal>?x</literal>'.
4710 (The '/' in the '%' suggests the split!)
4715 import GHC.Exts( Splittable )
4717 data NameSupply = ...
4719 splitNS :: NameSupply -> (NameSupply, NameSupply)
4720 newName :: NameSupply -> Name
4722 instance Splittable NameSupply where
4726 f :: (%ns :: NameSupply) => Env -> Expr -> Expr
4727 f env (Lam x e) = Lam x' (f env e)
4730 env' = extend env x x'
4731 ...more equations for f...
4733 Notice that the implicit parameter %ns is consumed
4735 <listitem> <para> once by the call to <literal>newName</literal> </para> </listitem>
4736 <listitem> <para> once by the recursive call to <literal>f</literal> </para></listitem>
4740 So the translation done by the type checker makes
4741 the parameter explicit:
4743 f :: NameSupply -> Env -> Expr -> Expr
4744 f ns env (Lam x e) = Lam x' (f ns1 env e)
4746 (ns1,ns2) = splitNS ns
4748 env = extend env x x'
4750 Notice the call to 'split' introduced by the type checker.
4751 How did it know to use 'splitNS'? Because what it really did
4752 was to introduce a call to the overloaded function 'split',
4753 defined by the class <literal>Splittable</literal>:
4755 class Splittable a where
4758 The instance for <literal>Splittable NameSupply</literal> tells GHC how to implement
4759 split for name supplies. But we can simply write
4765 g :: (Splittable a, %ns :: a) => b -> (b,a,a)
4767 The <literal>Splittable</literal> class is built into GHC. It's exported by module
4768 <literal>GHC.Exts</literal>.
4773 <listitem> <para> '<literal>?x</literal>' and '<literal>%x</literal>'
4774 are entirely distinct implicit parameters: you
4775 can use them together and they won't interfere with each other. </para>
4778 <listitem> <para> You can bind linear implicit parameters in 'with' clauses. </para> </listitem>
4780 <listitem> <para>You cannot have implicit parameters (whether linear or not)
4781 in the context of a class or instance declaration. </para></listitem>
4785 <sect3><title>Warnings</title>
4788 The monomorphism restriction is even more important than usual.
4789 Consider the example above:
4791 f :: (%ns :: NameSupply) => Env -> Expr -> Expr
4792 f env (Lam x e) = Lam x' (f env e)
4795 env' = extend env x x'
4797 If we replaced the two occurrences of x' by (newName %ns), which is
4798 usually a harmless thing to do, we get:
4800 f :: (%ns :: NameSupply) => Env -> Expr -> Expr
4801 f env (Lam x e) = Lam (newName %ns) (f env e)
4803 env' = extend env x (newName %ns)
4805 But now the name supply is consumed in <emphasis>three</emphasis> places
4806 (the two calls to newName,and the recursive call to f), so
4807 the result is utterly different. Urk! We don't even have
4811 Well, this is an experimental change. With implicit
4812 parameters we have already lost beta reduction anyway, and
4813 (as John Launchbury puts it) we can't sensibly reason about
4814 Haskell programs without knowing their typing.
4819 <sect3><title>Recursive functions</title>
4820 <para>Linear implicit parameters can be particularly tricky when you have a recursive function
4823 foo :: %x::T => Int -> [Int]
4825 foo n = %x : foo (n-1)
4827 where T is some type in class Splittable.</para>
4829 Do you get a list of all the same T's or all different T's
4830 (assuming that split gives two distinct T's back)?
4832 If you supply the type signature, taking advantage of polymorphic
4833 recursion, you get what you'd probably expect. Here's the
4834 translated term, where the implicit param is made explicit:
4837 foo x n = let (x1,x2) = split x
4838 in x1 : foo x2 (n-1)
4840 But if you don't supply a type signature, GHC uses the Hindley
4841 Milner trick of using a single monomorphic instance of the function
4842 for the recursive calls. That is what makes Hindley Milner type inference
4843 work. So the translation becomes
4847 foom n = x : foom (n-1)
4851 Result: 'x' is not split, and you get a list of identical T's. So the
4852 semantics of the program depends on whether or not foo has a type signature.
4855 You may say that this is a good reason to dislike linear implicit parameters
4856 and you'd be right. That is why they are an experimental feature.
4862 ================ END OF Linear Implicit Parameters commented out -->
4864 <sect2 id="kinding">
4865 <title>Explicitly-kinded quantification</title>
4868 Haskell infers the kind of each type variable. Sometimes it is nice to be able
4869 to give the kind explicitly as (machine-checked) documentation,
4870 just as it is nice to give a type signature for a function. On some occasions,
4871 it is essential to do so. For example, in his paper "Restricted Data Types in Haskell" (Haskell Workshop 1999)
4872 John Hughes had to define the data type:
4874 data Set cxt a = Set [a]
4875 | Unused (cxt a -> ())
4877 The only use for the <literal>Unused</literal> constructor was to force the correct
4878 kind for the type variable <literal>cxt</literal>.
4881 GHC now instead allows you to specify the kind of a type variable directly, wherever
4882 a type variable is explicitly bound, with the flag <option>-XKindSignatures</option>.
4885 This flag enables kind signatures in the following places:
4887 <listitem><para><literal>data</literal> declarations:
4889 data Set (cxt :: * -> *) a = Set [a]
4890 </screen></para></listitem>
4891 <listitem><para><literal>type</literal> declarations:
4893 type T (f :: * -> *) = f Int
4894 </screen></para></listitem>
4895 <listitem><para><literal>class</literal> declarations:
4897 class (Eq a) => C (f :: * -> *) a where ...
4898 </screen></para></listitem>
4899 <listitem><para><literal>forall</literal>'s in type signatures:
4901 f :: forall (cxt :: * -> *). Set cxt Int
4902 </screen></para></listitem>
4907 The parentheses are required. Some of the spaces are required too, to
4908 separate the lexemes. If you write <literal>(f::*->*)</literal> you
4909 will get a parse error, because "<literal>::*->*</literal>" is a
4910 single lexeme in Haskell.
4914 As part of the same extension, you can put kind annotations in types
4917 f :: (Int :: *) -> Int
4918 g :: forall a. a -> (a :: *)
4922 atype ::= '(' ctype '::' kind ')
4924 The parentheses are required.
4929 <sect2 id="universal-quantification">
4930 <title>Arbitrary-rank polymorphism
4934 Haskell type signatures are implicitly quantified. The new keyword <literal>forall</literal>
4935 allows us to say exactly what this means. For example:
4943 g :: forall b. (b -> b)
4945 The two are treated identically.
4949 However, GHC's type system supports <emphasis>arbitrary-rank</emphasis>
4950 explicit universal quantification in
4952 For example, all the following types are legal:
4954 f1 :: forall a b. a -> b -> a
4955 g1 :: forall a b. (Ord a, Eq b) => a -> b -> a
4957 f2 :: (forall a. a->a) -> Int -> Int
4958 g2 :: (forall a. Eq a => [a] -> a -> Bool) -> Int -> Int
4960 f3 :: ((forall a. a->a) -> Int) -> Bool -> Bool
4962 f4 :: Int -> (forall a. a -> a)
4964 Here, <literal>f1</literal> and <literal>g1</literal> are rank-1 types, and
4965 can be written in standard Haskell (e.g. <literal>f1 :: a->b->a</literal>).
4966 The <literal>forall</literal> makes explicit the universal quantification that
4967 is implicitly added by Haskell.
4970 The functions <literal>f2</literal> and <literal>g2</literal> have rank-2 types;
4971 the <literal>forall</literal> is on the left of a function arrow. As <literal>g2</literal>
4972 shows, the polymorphic type on the left of the function arrow can be overloaded.
4975 The function <literal>f3</literal> has a rank-3 type;
4976 it has rank-2 types on the left of a function arrow.
4979 GHC has three flags to control higher-rank types:
4982 <option>-XPolymorphicComponents</option>: data constructors (only) can have polymorphic argument types.
4985 <option>-XRank2Types</option>: any function (including data constructors) can have a rank-2 type.
4988 <option>-XRankNTypes</option>: any function (including data constructors) can have an arbitrary-rank type.
4989 That is, you can nest <literal>forall</literal>s
4990 arbitrarily deep in function arrows.
4991 In particular, a forall-type (also called a "type scheme"),
4992 including an operational type class context, is legal:
4994 <listitem> <para> On the left or right (see <literal>f4</literal>, for example)
4995 of a function arrow </para> </listitem>
4996 <listitem> <para> As the argument of a constructor, or type of a field, in a data type declaration. For
4997 example, any of the <literal>f1,f2,f3,g1,g2</literal> above would be valid
4998 field type signatures.</para> </listitem>
4999 <listitem> <para> As the type of an implicit parameter </para> </listitem>
5000 <listitem> <para> In a pattern type signature (see <xref linkend="scoped-type-variables"/>) </para> </listitem>
5004 Of course <literal>forall</literal> becomes a keyword; you can't use <literal>forall</literal> as
5005 a type variable any more!
5014 In a <literal>data</literal> or <literal>newtype</literal> declaration one can quantify
5015 the types of the constructor arguments. Here are several examples:
5021 data T a = T1 (forall b. b -> b -> b) a
5023 data MonadT m = MkMonad { return :: forall a. a -> m a,
5024 bind :: forall a b. m a -> (a -> m b) -> m b
5027 newtype Swizzle = MkSwizzle (Ord a => [a] -> [a])
5033 The constructors have rank-2 types:
5039 T1 :: forall a. (forall b. b -> b -> b) -> a -> T a
5040 MkMonad :: forall m. (forall a. a -> m a)
5041 -> (forall a b. m a -> (a -> m b) -> m b)
5043 MkSwizzle :: (Ord a => [a] -> [a]) -> Swizzle
5049 Notice that you don't need to use a <literal>forall</literal> if there's an
5050 explicit context. For example in the first argument of the
5051 constructor <function>MkSwizzle</function>, an implicit "<literal>forall a.</literal>" is
5052 prefixed to the argument type. The implicit <literal>forall</literal>
5053 quantifies all type variables that are not already in scope, and are
5054 mentioned in the type quantified over.
5058 As for type signatures, implicit quantification happens for non-overloaded
5059 types too. So if you write this:
5062 data T a = MkT (Either a b) (b -> b)
5065 it's just as if you had written this:
5068 data T a = MkT (forall b. Either a b) (forall b. b -> b)
5071 That is, since the type variable <literal>b</literal> isn't in scope, it's
5072 implicitly universally quantified. (Arguably, it would be better
5073 to <emphasis>require</emphasis> explicit quantification on constructor arguments
5074 where that is what is wanted. Feedback welcomed.)
5078 You construct values of types <literal>T1, MonadT, Swizzle</literal> by applying
5079 the constructor to suitable values, just as usual. For example,
5090 a3 = MkSwizzle reverse
5093 a4 = let r x = Just x
5100 mkTs :: (forall b. b -> b -> b) -> a -> [T a]
5101 mkTs f x y = [T1 f x, T1 f y]
5107 The type of the argument can, as usual, be more general than the type
5108 required, as <literal>(MkSwizzle reverse)</literal> shows. (<function>reverse</function>
5109 does not need the <literal>Ord</literal> constraint.)
5113 When you use pattern matching, the bound variables may now have
5114 polymorphic types. For example:
5120 f :: T a -> a -> (a, Char)
5121 f (T1 w k) x = (w k x, w 'c' 'd')
5123 g :: (Ord a, Ord b) => Swizzle -> [a] -> (a -> b) -> [b]
5124 g (MkSwizzle s) xs f = s (map f (s xs))
5126 h :: MonadT m -> [m a] -> m [a]
5127 h m [] = return m []
5128 h m (x:xs) = bind m x $ \y ->
5129 bind m (h m xs) $ \ys ->
5136 In the function <function>h</function> we use the record selectors <literal>return</literal>
5137 and <literal>bind</literal> to extract the polymorphic bind and return functions
5138 from the <literal>MonadT</literal> data structure, rather than using pattern
5144 <title>Type inference</title>
5147 In general, type inference for arbitrary-rank types is undecidable.
5148 GHC uses an algorithm proposed by Odersky and Laufer ("Putting type annotations to work", POPL'96)
5149 to get a decidable algorithm by requiring some help from the programmer.
5150 We do not yet have a formal specification of "some help" but the rule is this:
5153 <emphasis>For a lambda-bound or case-bound variable, x, either the programmer
5154 provides an explicit polymorphic type for x, or GHC's type inference will assume
5155 that x's type has no foralls in it</emphasis>.
5158 What does it mean to "provide" an explicit type for x? You can do that by
5159 giving a type signature for x directly, using a pattern type signature
5160 (<xref linkend="scoped-type-variables"/>), thus:
5162 \ f :: (forall a. a->a) -> (f True, f 'c')
5164 Alternatively, you can give a type signature to the enclosing
5165 context, which GHC can "push down" to find the type for the variable:
5167 (\ f -> (f True, f 'c')) :: (forall a. a->a) -> (Bool,Char)
5169 Here the type signature on the expression can be pushed inwards
5170 to give a type signature for f. Similarly, and more commonly,
5171 one can give a type signature for the function itself:
5173 h :: (forall a. a->a) -> (Bool,Char)
5174 h f = (f True, f 'c')
5176 You don't need to give a type signature if the lambda bound variable
5177 is a constructor argument. Here is an example we saw earlier:
5179 f :: T a -> a -> (a, Char)
5180 f (T1 w k) x = (w k x, w 'c' 'd')
5182 Here we do not need to give a type signature to <literal>w</literal>, because
5183 it is an argument of constructor <literal>T1</literal> and that tells GHC all
5190 <sect3 id="implicit-quant">
5191 <title>Implicit quantification</title>
5194 GHC performs implicit quantification as follows. <emphasis>At the top level (only) of
5195 user-written types, if and only if there is no explicit <literal>forall</literal>,
5196 GHC finds all the type variables mentioned in the type that are not already
5197 in scope, and universally quantifies them.</emphasis> For example, the following pairs are
5201 f :: forall a. a -> a
5208 h :: forall b. a -> b -> b
5214 Notice that GHC does <emphasis>not</emphasis> find the innermost possible quantification
5217 f :: (a -> a) -> Int
5219 f :: forall a. (a -> a) -> Int
5221 f :: (forall a. a -> a) -> Int
5224 g :: (Ord a => a -> a) -> Int
5225 -- MEANS the illegal type
5226 g :: forall a. (Ord a => a -> a) -> Int
5228 g :: (forall a. Ord a => a -> a) -> Int
5230 The latter produces an illegal type, which you might think is silly,
5231 but at least the rule is simple. If you want the latter type, you
5232 can write your for-alls explicitly. Indeed, doing so is strongly advised
5239 <sect2 id="impredicative-polymorphism">
5240 <title>Impredicative polymorphism
5242 <para>GHC supports <emphasis>impredicative polymorphism</emphasis>,
5243 enabled with <option>-XImpredicativeTypes</option>.
5245 that you can call a polymorphic function at a polymorphic type, and
5246 parameterise data structures over polymorphic types. For example:
5248 f :: Maybe (forall a. [a] -> [a]) -> Maybe ([Int], [Char])
5249 f (Just g) = Just (g [3], g "hello")
5252 Notice here that the <literal>Maybe</literal> type is parameterised by the
5253 <emphasis>polymorphic</emphasis> type <literal>(forall a. [a] ->
5256 <para>The technical details of this extension are described in the paper
5257 <ulink url="http://research.microsoft.com/%7Esimonpj/papers/boxy/">Boxy types:
5258 type inference for higher-rank types and impredicativity</ulink>,
5259 which appeared at ICFP 2006.
5263 <sect2 id="scoped-type-variables">
5264 <title>Lexically scoped type variables
5268 GHC supports <emphasis>lexically scoped type variables</emphasis>, without
5269 which some type signatures are simply impossible to write. For example:
5271 f :: forall a. [a] -> [a]
5277 The type signature for <literal>f</literal> brings the type variable <literal>a</literal> into scope; it scopes over
5278 the entire definition of <literal>f</literal>.
5279 In particular, it is in scope at the type signature for <varname>ys</varname>.
5280 In Haskell 98 it is not possible to declare
5281 a type for <varname>ys</varname>; a major benefit of scoped type variables is that
5282 it becomes possible to do so.
5284 <para>Lexically-scoped type variables are enabled by
5285 <option>-XScopedTypeVariables</option>. This flag implies <option>-XRelaxedPolyRec</option>.
5287 <para>Note: GHC 6.6 contains substantial changes to the way that scoped type
5288 variables work, compared to earlier releases. Read this section
5292 <title>Overview</title>
5294 <para>The design follows the following principles
5296 <listitem><para>A scoped type variable stands for a type <emphasis>variable</emphasis>, and not for
5297 a <emphasis>type</emphasis>. (This is a change from GHC's earlier
5298 design.)</para></listitem>
5299 <listitem><para>Furthermore, distinct lexical type variables stand for distinct
5300 type variables. This means that every programmer-written type signature
5301 (including one that contains free scoped type variables) denotes a
5302 <emphasis>rigid</emphasis> type; that is, the type is fully known to the type
5303 checker, and no inference is involved.</para></listitem>
5304 <listitem><para>Lexical type variables may be alpha-renamed freely, without
5305 changing the program.</para></listitem>
5309 A <emphasis>lexically scoped type variable</emphasis> can be bound by:
5311 <listitem><para>A declaration type signature (<xref linkend="decl-type-sigs"/>)</para></listitem>
5312 <listitem><para>An expression type signature (<xref linkend="exp-type-sigs"/>)</para></listitem>
5313 <listitem><para>A pattern type signature (<xref linkend="pattern-type-sigs"/>)</para></listitem>
5314 <listitem><para>Class and instance declarations (<xref linkend="cls-inst-scoped-tyvars"/>)</para></listitem>
5318 In Haskell, a programmer-written type signature is implicitly quantified over
5319 its free type variables (<ulink
5320 url="http://www.haskell.org/onlinereport/decls.html#sect4.1.2">Section
5322 of the Haskell Report).
5323 Lexically scoped type variables affect this implicit quantification rules
5324 as follows: any type variable that is in scope is <emphasis>not</emphasis> universally
5325 quantified. For example, if type variable <literal>a</literal> is in scope,
5328 (e :: a -> a) means (e :: a -> a)
5329 (e :: b -> b) means (e :: forall b. b->b)
5330 (e :: a -> b) means (e :: forall b. a->b)
5338 <sect3 id="decl-type-sigs">
5339 <title>Declaration type signatures</title>
5340 <para>A declaration type signature that has <emphasis>explicit</emphasis>
5341 quantification (using <literal>forall</literal>) brings into scope the
5342 explicitly-quantified
5343 type variables, in the definition of the named function. For example:
5345 f :: forall a. [a] -> [a]
5346 f (x:xs) = xs ++ [ x :: a ]
5348 The "<literal>forall a</literal>" brings "<literal>a</literal>" into scope in
5349 the definition of "<literal>f</literal>".
5351 <para>This only happens if:
5353 <listitem><para> The quantification in <literal>f</literal>'s type
5354 signature is explicit. For example:
5357 g (x:xs) = xs ++ [ x :: a ]
5359 This program will be rejected, because "<literal>a</literal>" does not scope
5360 over the definition of "<literal>f</literal>", so "<literal>x::a</literal>"
5361 means "<literal>x::forall a. a</literal>" by Haskell's usual implicit
5362 quantification rules.
5364 <listitem><para> The signature gives a type for a function binding or a bare variable binding,
5365 not a pattern binding.
5368 f1 :: forall a. [a] -> [a]
5369 f1 (x:xs) = xs ++ [ x :: a ] -- OK
5371 f2 :: forall a. [a] -> [a]
5372 f2 = \(x:xs) -> xs ++ [ x :: a ] -- OK
5374 f3 :: forall a. [a] -> [a]
5375 Just f3 = Just (\(x:xs) -> xs ++ [ x :: a ]) -- Not OK!
5377 The binding for <literal>f3</literal> is a pattern binding, and so its type signature
5378 does not bring <literal>a</literal> into scope. However <literal>f1</literal> is a
5379 function binding, and <literal>f2</literal> binds a bare variable; in both cases
5380 the type signature brings <literal>a</literal> into scope.
5386 <sect3 id="exp-type-sigs">
5387 <title>Expression type signatures</title>
5389 <para>An expression type signature that has <emphasis>explicit</emphasis>
5390 quantification (using <literal>forall</literal>) brings into scope the
5391 explicitly-quantified
5392 type variables, in the annotated expression. For example:
5394 f = runST ( (op >>= \(x :: STRef s Int) -> g x) :: forall s. ST s Bool )
5396 Here, the type signature <literal>forall a. ST s Bool</literal> brings the
5397 type variable <literal>s</literal> into scope, in the annotated expression
5398 <literal>(op >>= \(x :: STRef s Int) -> g x)</literal>.
5403 <sect3 id="pattern-type-sigs">
5404 <title>Pattern type signatures</title>
5406 A type signature may occur in any pattern; this is a <emphasis>pattern type
5407 signature</emphasis>.
5410 -- f and g assume that 'a' is already in scope
5411 f = \(x::Int, y::a) -> x
5413 h ((x,y) :: (Int,Bool)) = (y,x)
5415 In the case where all the type variables in the pattern type signature are
5416 already in scope (i.e. bound by the enclosing context), matters are simple: the
5417 signature simply constrains the type of the pattern in the obvious way.
5420 Unlike expression and declaration type signatures, pattern type signatures are not implicitly generalised.
5421 The pattern in a <emphasis>pattern binding</emphasis> may only mention type variables
5422 that are already in scope. For example:
5424 f :: forall a. [a] -> (Int, [a])
5427 (ys::[a], n) = (reverse xs, length xs) -- OK
5428 zs::[a] = xs ++ ys -- OK
5430 Just (v::b) = ... -- Not OK; b is not in scope
5432 Here, the pattern signatures for <literal>ys</literal> and <literal>zs</literal>
5433 are fine, but the one for <literal>v</literal> is not because <literal>b</literal> is
5437 However, in all patterns <emphasis>other</emphasis> than pattern bindings, a pattern
5438 type signature may mention a type variable that is not in scope; in this case,
5439 <emphasis>the signature brings that type variable into scope</emphasis>.
5440 This is particularly important for existential data constructors. For example:
5442 data T = forall a. MkT [a]
5445 k (MkT [t::a]) = MkT t3
5449 Here, the pattern type signature <literal>(t::a)</literal> mentions a lexical type
5450 variable that is not already in scope. Indeed, it <emphasis>cannot</emphasis> already be in scope,
5451 because it is bound by the pattern match. GHC's rule is that in this situation
5452 (and only then), a pattern type signature can mention a type variable that is
5453 not already in scope; the effect is to bring it into scope, standing for the
5454 existentially-bound type variable.
5457 When a pattern type signature binds a type variable in this way, GHC insists that the
5458 type variable is bound to a <emphasis>rigid</emphasis>, or fully-known, type variable.
5459 This means that any user-written type signature always stands for a completely known type.
5462 If all this seems a little odd, we think so too. But we must have
5463 <emphasis>some</emphasis> way to bring such type variables into scope, else we
5464 could not name existentially-bound type variables in subsequent type signatures.
5467 This is (now) the <emphasis>only</emphasis> situation in which a pattern type
5468 signature is allowed to mention a lexical variable that is not already in
5470 For example, both <literal>f</literal> and <literal>g</literal> would be
5471 illegal if <literal>a</literal> was not already in scope.
5477 <!-- ==================== Commented out part about result type signatures
5479 <sect3 id="result-type-sigs">
5480 <title>Result type signatures</title>
5483 The result type of a function, lambda, or case expression alternative can be given a signature, thus:
5486 {- f assumes that 'a' is already in scope -}
5487 f x y :: [a] = [x,y,x]
5489 g = \ x :: [Int] -> [3,4]
5491 h :: forall a. [a] -> a
5495 The final <literal>:: [a]</literal> after the patterns of <literal>f</literal> gives the type of
5496 the result of the function. Similarly, the body of the lambda in the RHS of
5497 <literal>g</literal> is <literal>[Int]</literal>, and the RHS of the case
5498 alternative in <literal>h</literal> is <literal>a</literal>.
5500 <para> A result type signature never brings new type variables into scope.</para>
5502 There are a couple of syntactic wrinkles. First, notice that all three
5503 examples would parse quite differently with parentheses:
5505 {- f assumes that 'a' is already in scope -}
5506 f x (y :: [a]) = [x,y,x]
5508 g = \ (x :: [Int]) -> [3,4]
5510 h :: forall a. [a] -> a
5514 Now the signature is on the <emphasis>pattern</emphasis>; and
5515 <literal>h</literal> would certainly be ill-typed (since the pattern
5516 <literal>(y:ys)</literal> cannot have the type <literal>a</literal>.
5518 Second, to avoid ambiguity, the type after the “<literal>::</literal>” in a result
5519 pattern signature on a lambda or <literal>case</literal> must be atomic (i.e. a single
5520 token or a parenthesised type of some sort). To see why,
5521 consider how one would parse this:
5530 <sect3 id="cls-inst-scoped-tyvars">
5531 <title>Class and instance declarations</title>
5534 The type variables in the head of a <literal>class</literal> or <literal>instance</literal> declaration
5535 scope over the methods defined in the <literal>where</literal> part. For example:
5553 <sect2 id="typing-binds">
5554 <title>Generalised typing of mutually recursive bindings</title>
5557 The Haskell Report specifies that a group of bindings (at top level, or in a
5558 <literal>let</literal> or <literal>where</literal>) should be sorted into
5559 strongly-connected components, and then type-checked in dependency order
5560 (<ulink url="http://www.haskell.org/onlinereport/decls.html#sect4.5.1">Haskell
5561 Report, Section 4.5.1</ulink>).
5562 As each group is type-checked, any binders of the group that
5564 an explicit type signature are put in the type environment with the specified
5566 and all others are monomorphic until the group is generalised
5567 (<ulink url="http://www.haskell.org/onlinereport/decls.html#sect4.5.2">Haskell Report, Section 4.5.2</ulink>).
5570 <para>Following a suggestion of Mark Jones, in his paper
5571 <ulink url="http://citeseer.ist.psu.edu/424440.html">Typing Haskell in
5573 GHC implements a more general scheme. If <option>-XRelaxedPolyRec</option> is
5575 <emphasis>the dependency analysis ignores references to variables that have an explicit
5576 type signature</emphasis>.
5577 As a result of this refined dependency analysis, the dependency groups are smaller, and more bindings will
5578 typecheck. For example, consider:
5580 f :: Eq a => a -> Bool
5581 f x = (x == x) || g True || g "Yes"
5583 g y = (y <= y) || f True
5585 This is rejected by Haskell 98, but under Jones's scheme the definition for
5586 <literal>g</literal> is typechecked first, separately from that for
5587 <literal>f</literal>,
5588 because the reference to <literal>f</literal> in <literal>g</literal>'s right
5589 hand side is ignored by the dependency analysis. Then <literal>g</literal>'s
5590 type is generalised, to get
5592 g :: Ord a => a -> Bool
5594 Now, the definition for <literal>f</literal> is typechecked, with this type for
5595 <literal>g</literal> in the type environment.
5599 The same refined dependency analysis also allows the type signatures of
5600 mutually-recursive functions to have different contexts, something that is illegal in
5601 Haskell 98 (Section 4.5.2, last sentence). With
5602 <option>-XRelaxedPolyRec</option>
5603 GHC only insists that the type signatures of a <emphasis>refined</emphasis> group have identical
5604 type signatures; in practice this means that only variables bound by the same
5605 pattern binding must have the same context. For example, this is fine:
5607 f :: Eq a => a -> Bool
5608 f x = (x == x) || g True
5610 g :: Ord a => a -> Bool
5611 g y = (y <= y) || f True
5617 <!-- ==================== End of type system extensions ================= -->
5619 <!-- ====================== TEMPLATE HASKELL ======================= -->
5621 <sect1 id="template-haskell">
5622 <title>Template Haskell</title>
5624 <para>Template Haskell allows you to do compile-time meta-programming in
5627 the main technical innovations is discussed in "<ulink
5628 url="http://research.microsoft.com/~simonpj/papers/meta-haskell/">
5629 Template Meta-programming for Haskell</ulink>" (Proc Haskell Workshop 2002).
5632 There is a Wiki page about
5633 Template Haskell at <ulink url="http://www.haskell.org/haskellwiki/Template_Haskell">
5634 http://www.haskell.org/haskellwiki/Template_Haskell</ulink>, and that is the best place to look for
5638 url="http://www.haskell.org/ghc/docs/latest/html/libraries/index.html">online
5639 Haskell library reference material</ulink>
5640 (look for module <literal>Language.Haskell.TH</literal>).
5641 Many changes to the original design are described in
5642 <ulink url="http://research.microsoft.com/~simonpj/papers/meta-haskell/notes2.ps">
5643 Notes on Template Haskell version 2</ulink>.
5644 Not all of these changes are in GHC, however.
5647 <para> The first example from that paper is set out below (<xref linkend="th-example"/>)
5648 as a worked example to help get you started.
5652 The documentation here describes the realisation of Template Haskell in GHC. It is not detailed enough to
5653 understand Template Haskell; see the <ulink url="http://haskell.org/haskellwiki/Template_Haskell">
5658 <title>Syntax</title>
5660 <para> Template Haskell has the following new syntactic
5661 constructions. You need to use the flag
5662 <option>-XTemplateHaskell</option>
5663 <indexterm><primary><option>-XTemplateHaskell</option></primary>
5664 </indexterm>to switch these syntactic extensions on
5665 (<option>-XTemplateHaskell</option> is no longer implied by
5666 <option>-fglasgow-exts</option>).</para>
5670 A splice is written <literal>$x</literal>, where <literal>x</literal> is an
5671 identifier, or <literal>$(...)</literal>, where the "..." is an arbitrary expression.
5672 There must be no space between the "$" and the identifier or parenthesis. This use
5673 of "$" overrides its meaning as an infix operator, just as "M.x" overrides the meaning
5674 of "." as an infix operator. If you want the infix operator, put spaces around it.
5676 <para> A splice can occur in place of
5678 <listitem><para> an expression; the spliced expression must
5679 have type <literal>Q Exp</literal></para></listitem>
5680 <listitem><para> a list of top-level declarations; the spliced expression must have type <literal>Q [Dec]</literal></para></listitem>
5683 Inside a splice you can can only call functions defined in imported modules,
5684 not functions defined elsewhere in the same module.</listitem>
5688 A expression quotation is written in Oxford brackets, thus:
5690 <listitem><para> <literal>[| ... |]</literal>, where the "..." is an expression;
5691 the quotation has type <literal>Q Exp</literal>.</para></listitem>
5692 <listitem><para> <literal>[d| ... |]</literal>, where the "..." is a list of top-level declarations;
5693 the quotation has type <literal>Q [Dec]</literal>.</para></listitem>
5694 <listitem><para> <literal>[t| ... |]</literal>, where the "..." is a type;
5695 the quotation has type <literal>Q Typ</literal>.</para></listitem>
5696 </itemizedlist></para></listitem>
5699 A quasi-quotation can appear in either a pattern context or an
5700 expression context and is also written in Oxford brackets:
5702 <listitem><para> <literal>[:<replaceable>varid</replaceable>| ... |]</literal>,
5703 where the "..." is an arbitrary string; a full description of the
5704 quasi-quotation facility is given in <xref linkend="th-quasiquotation"/>.</para></listitem>
5705 </itemizedlist></para></listitem>
5708 A name can be quoted with either one or two prefix single quotes:
5710 <listitem><para> <literal>'f</literal> has type <literal>Name</literal>, and names the function <literal>f</literal>.
5711 Similarly <literal>'C</literal> has type <literal>Name</literal> and names the data constructor <literal>C</literal>.
5712 In general <literal>'</literal><replaceable>thing</replaceable> interprets <replaceable>thing</replaceable> in an expression context.
5714 <listitem><para> <literal>''T</literal> has type <literal>Name</literal>, and names the type constructor <literal>T</literal>.
5715 That is, <literal>''</literal><replaceable>thing</replaceable> interprets <replaceable>thing</replaceable> in a type context.
5718 These <literal>Names</literal> can be used to construct Template Haskell expressions, patterns, declarations etc. They
5719 may also be given as an argument to the <literal>reify</literal> function.
5725 (Compared to the original paper, there are many differences of detail.
5726 The syntax for a declaration splice uses "<literal>$</literal>" not "<literal>splice</literal>".
5727 The type of the enclosed expression must be <literal>Q [Dec]</literal>, not <literal>[Q Dec]</literal>.
5728 Type splices are not implemented, and neither are pattern splices or quotations.
5732 <sect2> <title> Using Template Haskell </title>
5736 The data types and monadic constructor functions for Template Haskell are in the library
5737 <literal>Language.Haskell.THSyntax</literal>.
5741 You can only run a function at compile time if it is imported from another module. That is,
5742 you can't define a function in a module, and call it from within a splice in the same module.
5743 (It would make sense to do so, but it's hard to implement.)
5747 You can only run a function at compile time if it is imported
5748 from another module <emphasis>that is not part of a mutually-recursive group of modules
5749 that includes the module currently being compiled</emphasis>. Furthermore, all of the modules of
5750 the mutually-recursive group must be reachable by non-SOURCE imports from the module where the
5751 splice is to be run.</para>
5753 For example, when compiling module A,
5754 you can only run Template Haskell functions imported from B if B does not import A (directly or indirectly).
5755 The reason should be clear: to run B we must compile and run A, but we are currently type-checking A.
5759 The flag <literal>-ddump-splices</literal> shows the expansion of all top-level splices as they happen.
5762 If you are building GHC from source, you need at least a stage-2 bootstrap compiler to
5763 run Template Haskell. A stage-1 compiler will reject the TH constructs. Reason: TH
5764 compiles and runs a program, and then looks at the result. So it's important that
5765 the program it compiles produces results whose representations are identical to
5766 those of the compiler itself.
5770 <para> Template Haskell works in any mode (<literal>--make</literal>, <literal>--interactive</literal>,
5771 or file-at-a-time). There used to be a restriction to the former two, but that restriction
5776 <sect2 id="th-example"> <title> A Template Haskell Worked Example </title>
5777 <para>To help you get over the confidence barrier, try out this skeletal worked example.
5778 First cut and paste the two modules below into "Main.hs" and "Printf.hs":</para>
5785 -- Import our template "pr"
5786 import Printf ( pr )
5788 -- The splice operator $ takes the Haskell source code
5789 -- generated at compile time by "pr" and splices it into
5790 -- the argument of "putStrLn".
5791 main = putStrLn ( $(pr "Hello") )
5797 -- Skeletal printf from the paper.
5798 -- It needs to be in a separate module to the one where
5799 -- you intend to use it.
5801 -- Import some Template Haskell syntax
5802 import Language.Haskell.TH
5804 -- Describe a format string
5805 data Format = D | S | L String
5807 -- Parse a format string. This is left largely to you
5808 -- as we are here interested in building our first ever
5809 -- Template Haskell program and not in building printf.
5810 parse :: String -> [Format]
5813 -- Generate Haskell source code from a parsed representation
5814 -- of the format string. This code will be spliced into
5815 -- the module which calls "pr", at compile time.
5816 gen :: [Format] -> Q Exp
5817 gen [D] = [| \n -> show n |]
5818 gen [S] = [| \s -> s |]
5819 gen [L s] = stringE s
5821 -- Here we generate the Haskell code for the splice
5822 -- from an input format string.
5823 pr :: String -> Q Exp
5824 pr s = gen (parse s)
5827 <para>Now run the compiler (here we are a Cygwin prompt on Windows):
5830 $ ghc --make -XTemplateHaskell main.hs -o main.exe
5833 <para>Run "main.exe" and here is your output:</para>
5843 <title>Using Template Haskell with Profiling</title>
5844 <indexterm><primary>profiling</primary><secondary>with Template Haskell</secondary></indexterm>
5846 <para>Template Haskell relies on GHC's built-in bytecode compiler and
5847 interpreter to run the splice expressions. The bytecode interpreter
5848 runs the compiled expression on top of the same runtime on which GHC
5849 itself is running; this means that the compiled code referred to by
5850 the interpreted expression must be compatible with this runtime, and
5851 in particular this means that object code that is compiled for
5852 profiling <emphasis>cannot</emphasis> be loaded and used by a splice
5853 expression, because profiled object code is only compatible with the
5854 profiling version of the runtime.</para>
5856 <para>This causes difficulties if you have a multi-module program
5857 containing Template Haskell code and you need to compile it for
5858 profiling, because GHC cannot load the profiled object code and use it
5859 when executing the splices. Fortunately GHC provides a workaround.
5860 The basic idea is to compile the program twice:</para>
5864 <para>Compile the program or library first the normal way, without
5865 <option>-prof</option><indexterm><primary><option>-prof</option></primary></indexterm>.</para>
5868 <para>Then compile it again with <option>-prof</option>, and
5869 additionally use <option>-osuf
5870 p_o</option><indexterm><primary><option>-osuf</option></primary></indexterm>
5871 to name the object files differently (you can choose any suffix
5872 that isn't the normal object suffix here). GHC will automatically
5873 load the object files built in the first step when executing splice
5874 expressions. If you omit the <option>-osuf</option> flag when
5875 building with <option>-prof</option> and Template Haskell is used,
5876 GHC will emit an error message. </para>
5881 <sect2 id="th-quasiquotation"> <title> Template Haskell Quasi-quotation </title>
5882 <para>Quasi-quotation allows patterns and expressions to be written using
5883 programmer-defined concrete syntax; the motivation behind the extension and
5884 several examples are documented in
5885 "<ulink url="http://www.eecs.harvard.edu/~mainland/ghc-quasiquoting/">Why It's
5886 Nice to be Quoted: Quasiquoting for Haskell</ulink>" (Proc Haskell Workshop
5887 2007). The example below shows how to write a quasiquoter for a simple
5888 expression language.</para>
5891 In the example, the quasiquoter <literal>expr</literal> is bound to a value of
5892 type <literal>Language.Haskell.TH.Quote.QuasiQuoter</literal> which contains two
5893 functions for quoting expressions and patterns, respectively. The first argument
5894 to each quoter is the (arbitrary) string enclosed in the Oxford brackets. The
5895 context of the quasi-quotation statement determines which of the two parsers is
5896 called: if the quasi-quotation occurs in an expression context, the expression
5897 parser is called, and if it occurs in a pattern context, the pattern parser is
5901 Note that in the example we make use of an antiquoted
5902 variable <literal>n</literal>, indicated by the syntax <literal>'int:n</literal>
5903 (this syntax for anti-quotation was defined by the parser's
5904 author, <emphasis>not</emphasis> by GHC). This binds <literal>n</literal> to the
5905 integer value argument of the constructor <literal>IntExpr</literal> when
5906 pattern matching. Please see the referenced paper for further details regarding
5907 anti-quotation as well as the description of a technique that uses SYB to
5908 leverage a single parser of type <literal>String -> a</literal> to generate both
5909 an expression parser that returns a value of type <literal>Q Exp</literal> and a
5910 pattern parser that returns a value of type <literal>Q Pat</literal>.
5913 <para>In general, a quasi-quote has the form
5914 <literal>[$<replaceable>quoter</replaceable>| <replaceable>string</replaceable> |]</literal>.
5915 The <replaceable>quoter</replaceable> must be the name of an imported quoter; it
5916 cannot be an arbitrary expression. The quoted <replaceable>string</replaceable>
5917 can be arbitrary, and may contain newlines.
5920 Quasiquoters must obey the same stage restrictions as Template Haskell, e.g., in
5921 the example, <literal>expr</literal> cannot be defined
5922 in <literal>Main.hs</literal> where it is used, but must be imported.
5933 main = do { print $ eval [$expr|1 + 2|]
5935 { [$expr|'int:n|] -> print n
5944 import qualified Language.Haskell.TH as TH
5945 import Language.Haskell.TH.Quasi
5947 data Expr = IntExpr Integer
5948 | AntiIntExpr String
5949 | BinopExpr BinOp Expr Expr
5951 deriving(Show, Typeable, Data)
5957 deriving(Show, Typeable, Data)
5959 eval :: Expr -> Integer
5960 eval (IntExpr n) = n
5961 eval (BinopExpr op x y) = (opToFun op) (eval x) (eval y)
5968 expr = QuasiQuoter parseExprExp parseExprPat
5970 -- Parse an Expr, returning its representation as
5971 -- either a Q Exp or a Q Pat. See the referenced paper
5972 -- for how to use SYB to do this by writing a single
5973 -- parser of type String -> Expr instead of two
5974 -- separate parsers.
5976 parseExprExp :: String -> Q Exp
5979 parseExprPat :: String -> Q Pat
5983 <para>Now run the compiler:
5986 $ ghc --make -XQuasiQuotes Main.hs -o main
5989 <para>Run "main" and here is your output:</para>
6001 <!-- ===================== Arrow notation =================== -->
6003 <sect1 id="arrow-notation">
6004 <title>Arrow notation
6007 <para>Arrows are a generalization of monads introduced by John Hughes.
6008 For more details, see
6013 “Generalising Monads to Arrows”,
6014 John Hughes, in <citetitle>Science of Computer Programming</citetitle> 37,
6015 pp67–111, May 2000.
6016 The paper that introduced arrows: a friendly introduction, motivated with
6017 programming examples.
6023 “<ulink url="http://www.soi.city.ac.uk/~ross/papers/notation.html">A New Notation for Arrows</ulink>”,
6024 Ross Paterson, in <citetitle>ICFP</citetitle>, Sep 2001.
6025 Introduced the notation described here.
6031 “<ulink url="http://www.soi.city.ac.uk/~ross/papers/fop.html">Arrows and Computation</ulink>”,
6032 Ross Paterson, in <citetitle>The Fun of Programming</citetitle>,
6039 “<ulink url="http://www.cs.chalmers.se/~rjmh/afp-arrows.pdf">Programming with Arrows</ulink>”,
6040 John Hughes, in <citetitle>5th International Summer School on
6041 Advanced Functional Programming</citetitle>,
6042 <citetitle>Lecture Notes in Computer Science</citetitle> vol. 3622,
6044 This paper includes another introduction to the notation,
6045 with practical examples.
6051 “<ulink url="http://www.haskell.org/ghc/docs/papers/arrow-rules.pdf">Type and Translation Rules for Arrow Notation in GHC</ulink>”,
6052 Ross Paterson and Simon Peyton Jones, September 16, 2004.
6053 A terse enumeration of the formal rules used
6054 (extracted from comments in the source code).
6060 The arrows web page at
6061 <ulink url="http://www.haskell.org/arrows/"><literal>http://www.haskell.org/arrows/</literal></ulink>.
6066 With the <option>-XArrows</option> flag, GHC supports the arrow
6067 notation described in the second of these papers,
6068 translating it using combinators from the
6069 <ulink url="../libraries/base/Control-Arrow.html"><literal>Control.Arrow</literal></ulink>
6071 What follows is a brief introduction to the notation;
6072 it won't make much sense unless you've read Hughes's paper.
6075 <para>The extension adds a new kind of expression for defining arrows:
6077 <replaceable>exp</replaceable><superscript>10</superscript> ::= ...
6078 | proc <replaceable>apat</replaceable> -> <replaceable>cmd</replaceable>
6080 where <literal>proc</literal> is a new keyword.
6081 The variables of the pattern are bound in the body of the
6082 <literal>proc</literal>-expression,
6083 which is a new sort of thing called a <firstterm>command</firstterm>.
6084 The syntax of commands is as follows:
6086 <replaceable>cmd</replaceable> ::= <replaceable>exp</replaceable><superscript>10</superscript> -< <replaceable>exp</replaceable>
6087 | <replaceable>exp</replaceable><superscript>10</superscript> -<< <replaceable>exp</replaceable>
6088 | <replaceable>cmd</replaceable><superscript>0</superscript>
6090 with <replaceable>cmd</replaceable><superscript>0</superscript> up to
6091 <replaceable>cmd</replaceable><superscript>9</superscript> defined using
6092 infix operators as for expressions, and
6094 <replaceable>cmd</replaceable><superscript>10</superscript> ::= \ <replaceable>apat</replaceable> ... <replaceable>apat</replaceable> -> <replaceable>cmd</replaceable>
6095 | let <replaceable>decls</replaceable> in <replaceable>cmd</replaceable>
6096 | if <replaceable>exp</replaceable> then <replaceable>cmd</replaceable> else <replaceable>cmd</replaceable>
6097 | case <replaceable>exp</replaceable> of { <replaceable>calts</replaceable> }
6098 | do { <replaceable>cstmt</replaceable> ; ... <replaceable>cstmt</replaceable> ; <replaceable>cmd</replaceable> }
6099 | <replaceable>fcmd</replaceable>
6101 <replaceable>fcmd</replaceable> ::= <replaceable>fcmd</replaceable> <replaceable>aexp</replaceable>
6102 | ( <replaceable>cmd</replaceable> )
6103 | (| <replaceable>aexp</replaceable> <replaceable>cmd</replaceable> ... <replaceable>cmd</replaceable> |)
6105 <replaceable>cstmt</replaceable> ::= let <replaceable>decls</replaceable>
6106 | <replaceable>pat</replaceable> <- <replaceable>cmd</replaceable>
6107 | rec { <replaceable>cstmt</replaceable> ; ... <replaceable>cstmt</replaceable> [;] }
6108 | <replaceable>cmd</replaceable>
6110 where <replaceable>calts</replaceable> are like <replaceable>alts</replaceable>
6111 except that the bodies are commands instead of expressions.
6115 Commands produce values, but (like monadic computations)
6116 may yield more than one value,
6117 or none, and may do other things as well.
6118 For the most part, familiarity with monadic notation is a good guide to
6120 However the values of expressions, even monadic ones,
6121 are determined by the values of the variables they contain;
6122 this is not necessarily the case for commands.
6126 A simple example of the new notation is the expression
6128 proc x -> f -< x+1
6130 We call this a <firstterm>procedure</firstterm> or
6131 <firstterm>arrow abstraction</firstterm>.
6132 As with a lambda expression, the variable <literal>x</literal>
6133 is a new variable bound within the <literal>proc</literal>-expression.
6134 It refers to the input to the arrow.
6135 In the above example, <literal>-<</literal> is not an identifier but an
6136 new reserved symbol used for building commands from an expression of arrow
6137 type and an expression to be fed as input to that arrow.
6138 (The weird look will make more sense later.)
6139 It may be read as analogue of application for arrows.
6140 The above example is equivalent to the Haskell expression
6142 arr (\ x -> x+1) >>> f
6144 That would make no sense if the expression to the left of
6145 <literal>-<</literal> involves the bound variable <literal>x</literal>.
6146 More generally, the expression to the left of <literal>-<</literal>
6147 may not involve any <firstterm>local variable</firstterm>,
6148 i.e. a variable bound in the current arrow abstraction.
6149 For such a situation there is a variant <literal>-<<</literal>, as in
6151 proc x -> f x -<< x+1
6153 which is equivalent to
6155 arr (\ x -> (f x, x+1)) >>> app
6157 so in this case the arrow must belong to the <literal>ArrowApply</literal>
6159 Such an arrow is equivalent to a monad, so if you're using this form
6160 you may find a monadic formulation more convenient.
6164 <title>do-notation for commands</title>
6167 Another form of command is a form of <literal>do</literal>-notation.
6168 For example, you can write
6177 You can read this much like ordinary <literal>do</literal>-notation,
6178 but with commands in place of monadic expressions.
6179 The first line sends the value of <literal>x+1</literal> as an input to
6180 the arrow <literal>f</literal>, and matches its output against
6181 <literal>y</literal>.
6182 In the next line, the output is discarded.
6183 The arrow <function>returnA</function> is defined in the
6184 <ulink url="../libraries/base/Control-Arrow.html"><literal>Control.Arrow</literal></ulink>
6185 module as <literal>arr id</literal>.
6186 The above example is treated as an abbreviation for
6188 arr (\ x -> (x, x)) >>>
6189 first (arr (\ x -> x+1) >>> f) >>>
6190 arr (\ (y, x) -> (y, (x, y))) >>>
6191 first (arr (\ y -> 2*y) >>> g) >>>
6193 arr (\ (x, y) -> let z = x+y in ((x, z), z)) >>>
6194 first (arr (\ (x, z) -> x*z) >>> h) >>>
6195 arr (\ (t, z) -> t+z) >>>
6198 Note that variables not used later in the composition are projected out.
6199 After simplification using rewrite rules (see <xref linkend="rewrite-rules"/>)
6201 <ulink url="../libraries/base/Control-Arrow.html"><literal>Control.Arrow</literal></ulink>
6202 module, this reduces to
6204 arr (\ x -> (x+1, x)) >>>
6206 arr (\ (y, x) -> (2*y, (x, y))) >>>
6208 arr (\ (_, (x, y)) -> let z = x+y in (x*z, z)) >>>
6210 arr (\ (t, z) -> t+z)
6212 which is what you might have written by hand.
6213 With arrow notation, GHC keeps track of all those tuples of variables for you.
6217 Note that although the above translation suggests that
6218 <literal>let</literal>-bound variables like <literal>z</literal> must be
6219 monomorphic, the actual translation produces Core,
6220 so polymorphic variables are allowed.
6224 It's also possible to have mutually recursive bindings,
6225 using the new <literal>rec</literal> keyword, as in the following example:
6227 counter :: ArrowCircuit a => a Bool Int
6228 counter = proc reset -> do
6229 rec output <- returnA -< if reset then 0 else next
6230 next <- delay 0 -< output+1
6231 returnA -< output
6233 The translation of such forms uses the <function>loop</function> combinator,
6234 so the arrow concerned must belong to the <literal>ArrowLoop</literal> class.
6240 <title>Conditional commands</title>
6243 In the previous example, we used a conditional expression to construct the
6245 Sometimes we want to conditionally execute different commands, as in
6252 which is translated to
6254 arr (\ (x,y) -> if f x y then Left x else Right y) >>>
6255 (arr (\x -> x+1) >>> f) ||| (arr (\y -> y+2) >>> g)
6257 Since the translation uses <function>|||</function>,
6258 the arrow concerned must belong to the <literal>ArrowChoice</literal> class.
6262 There are also <literal>case</literal> commands, like
6268 y <- h -< (x1, x2)
6272 The syntax is the same as for <literal>case</literal> expressions,
6273 except that the bodies of the alternatives are commands rather than expressions.
6274 The translation is similar to that of <literal>if</literal> commands.
6280 <title>Defining your own control structures</title>
6283 As we're seen, arrow notation provides constructs,
6284 modelled on those for expressions,
6285 for sequencing, value recursion and conditionals.
6286 But suitable combinators,
6287 which you can define in ordinary Haskell,
6288 may also be used to build new commands out of existing ones.
6289 The basic idea is that a command defines an arrow from environments to values.
6290 These environments assign values to the free local variables of the command.
6291 Thus combinators that produce arrows from arrows
6292 may also be used to build commands from commands.
6293 For example, the <literal>ArrowChoice</literal> class includes a combinator
6295 ArrowChoice a => (<+>) :: a e c -> a e c -> a e c
6297 so we can use it to build commands:
6299 expr' = proc x -> do
6302 symbol Plus -< ()
6303 y <- term -< ()
6306 symbol Minus -< ()
6307 y <- term -< ()
6310 (The <literal>do</literal> on the first line is needed to prevent the first
6311 <literal><+> ...</literal> from being interpreted as part of the
6312 expression on the previous line.)
6313 This is equivalent to
6315 expr' = (proc x -> returnA -< x)
6316 <+> (proc x -> do
6317 symbol Plus -< ()
6318 y <- term -< ()
6320 <+> (proc x -> do
6321 symbol Minus -< ()
6322 y <- term -< ()
6325 It is essential that this operator be polymorphic in <literal>e</literal>
6326 (representing the environment input to the command
6327 and thence to its subcommands)
6328 and satisfy the corresponding naturality property
6330 arr k >>> (f <+> g) = (arr k >>> f) <+> (arr k >>> g)
6332 at least for strict <literal>k</literal>.
6333 (This should be automatic if you're not using <function>seq</function>.)
6334 This ensures that environments seen by the subcommands are environments
6335 of the whole command,
6336 and also allows the translation to safely trim these environments.
6337 The operator must also not use any variable defined within the current
6342 We could define our own operator
6344 untilA :: ArrowChoice a => a e () -> a e Bool -> a e ()
6345 untilA body cond = proc x ->
6346 b <- cond -< x
6347 if b then returnA -< ()
6350 untilA body cond -< x
6352 and use it in the same way.
6353 Of course this infix syntax only makes sense for binary operators;
6354 there is also a more general syntax involving special brackets:
6358 (|untilA (increment -< x+y) (within 0.5 -< x)|)
6365 <title>Primitive constructs</title>
6368 Some operators will need to pass additional inputs to their subcommands.
6369 For example, in an arrow type supporting exceptions,
6370 the operator that attaches an exception handler will wish to pass the
6371 exception that occurred to the handler.
6372 Such an operator might have a type
6374 handleA :: ... => a e c -> a (e,Ex) c -> a e c
6376 where <literal>Ex</literal> is the type of exceptions handled.
6377 You could then use this with arrow notation by writing a command
6379 body `handleA` \ ex -> handler
6381 so that if an exception is raised in the command <literal>body</literal>,
6382 the variable <literal>ex</literal> is bound to the value of the exception
6383 and the command <literal>handler</literal>,
6384 which typically refers to <literal>ex</literal>, is entered.
6385 Though the syntax here looks like a functional lambda,
6386 we are talking about commands, and something different is going on.
6387 The input to the arrow represented by a command consists of values for
6388 the free local variables in the command, plus a stack of anonymous values.
6389 In all the prior examples, this stack was empty.
6390 In the second argument to <function>handleA</function>,
6391 this stack consists of one value, the value of the exception.
6392 The command form of lambda merely gives this value a name.
6397 the values on the stack are paired to the right of the environment.
6398 So operators like <function>handleA</function> that pass
6399 extra inputs to their subcommands can be designed for use with the notation
6400 by pairing the values with the environment in this way.
6401 More precisely, the type of each argument of the operator (and its result)
6402 should have the form
6404 a (...(e,t1), ... tn) t
6406 where <replaceable>e</replaceable> is a polymorphic variable
6407 (representing the environment)
6408 and <replaceable>ti</replaceable> are the types of the values on the stack,
6409 with <replaceable>t1</replaceable> being the <quote>top</quote>.
6410 The polymorphic variable <replaceable>e</replaceable> must not occur in
6411 <replaceable>a</replaceable>, <replaceable>ti</replaceable> or
6412 <replaceable>t</replaceable>.
6413 However the arrows involved need not be the same.
6414 Here are some more examples of suitable operators:
6416 bracketA :: ... => a e b -> a (e,b) c -> a (e,c) d -> a e d
6417 runReader :: ... => a e c -> a' (e,State) c
6418 runState :: ... => a e c -> a' (e,State) (c,State)
6420 We can supply the extra input required by commands built with the last two
6421 by applying them to ordinary expressions, as in
6425 (|runReader (do { ... })|) s
6427 which adds <literal>s</literal> to the stack of inputs to the command
6428 built using <function>runReader</function>.
6432 The command versions of lambda abstraction and application are analogous to
6433 the expression versions.
6434 In particular, the beta and eta rules describe equivalences of commands.
6435 These three features (operators, lambda abstraction and application)
6436 are the core of the notation; everything else can be built using them,
6437 though the results would be somewhat clumsy.
6438 For example, we could simulate <literal>do</literal>-notation by defining
6440 bind :: Arrow a => a e b -> a (e,b) c -> a e c
6441 u `bind` f = returnA &&& u >>> f
6443 bind_ :: Arrow a => a e b -> a e c -> a e c
6444 u `bind_` f = u `bind` (arr fst >>> f)
6446 We could simulate <literal>if</literal> by defining
6448 cond :: ArrowChoice a => a e b -> a e b -> a (e,Bool) b
6449 cond f g = arr (\ (e,b) -> if b then Left e else Right e) >>> f ||| g
6456 <title>Differences with the paper</title>
6461 <para>Instead of a single form of arrow application (arrow tail) with two
6462 translations, the implementation provides two forms
6463 <quote><literal>-<</literal></quote> (first-order)
6464 and <quote><literal>-<<</literal></quote> (higher-order).
6469 <para>User-defined operators are flagged with banana brackets instead of
6470 a new <literal>form</literal> keyword.
6479 <title>Portability</title>
6482 Although only GHC implements arrow notation directly,
6483 there is also a preprocessor
6485 <ulink url="http://www.haskell.org/arrows/">arrows web page</ulink>)
6486 that translates arrow notation into Haskell 98
6487 for use with other Haskell systems.
6488 You would still want to check arrow programs with GHC;
6489 tracing type errors in the preprocessor output is not easy.
6490 Modules intended for both GHC and the preprocessor must observe some
6491 additional restrictions:
6496 The module must import
6497 <ulink url="../libraries/base/Control-Arrow.html"><literal>Control.Arrow</literal></ulink>.
6503 The preprocessor cannot cope with other Haskell extensions.
6504 These would have to go in separate modules.
6510 Because the preprocessor targets Haskell (rather than Core),
6511 <literal>let</literal>-bound variables are monomorphic.
6522 <!-- ==================== BANG PATTERNS ================= -->
6524 <sect1 id="bang-patterns">
6525 <title>Bang patterns
6526 <indexterm><primary>Bang patterns</primary></indexterm>
6528 <para>GHC supports an extension of pattern matching called <emphasis>bang
6529 patterns</emphasis>. Bang patterns are under consideration for Haskell Prime.
6531 url="http://hackage.haskell.org/trac/haskell-prime/wiki/BangPatterns">Haskell
6532 prime feature description</ulink> contains more discussion and examples
6533 than the material below.
6536 Bang patterns are enabled by the flag <option>-XBangPatterns</option>.
6539 <sect2 id="bang-patterns-informal">
6540 <title>Informal description of bang patterns
6543 The main idea is to add a single new production to the syntax of patterns:
6547 Matching an expression <literal>e</literal> against a pattern <literal>!p</literal> is done by first
6548 evaluating <literal>e</literal> (to WHNF) and then matching the result against <literal>p</literal>.
6553 This definition makes <literal>f1</literal> is strict in <literal>x</literal>,
6554 whereas without the bang it would be lazy.
6555 Bang patterns can be nested of course:
6559 Here, <literal>f2</literal> is strict in <literal>x</literal> but not in
6560 <literal>y</literal>.
6561 A bang only really has an effect if it precedes a variable or wild-card pattern:
6566 Here, <literal>f3</literal> and <literal>f4</literal> are identical; putting a bang before a pattern that
6567 forces evaluation anyway does nothing.
6569 Bang patterns work in <literal>case</literal> expressions too, of course:
6571 g5 x = let y = f x in body
6572 g6 x = case f x of { y -> body }
6573 g7 x = case f x of { !y -> body }
6575 The functions <literal>g5</literal> and <literal>g6</literal> mean exactly the same thing.
6576 But <literal>g7</literal> evaluates <literal>(f x)</literal>, binds <literal>y</literal> to the
6577 result, and then evaluates <literal>body</literal>.
6579 Bang patterns work in <literal>let</literal> and <literal>where</literal>
6580 definitions too. For example:
6584 is a strict pattern: operationally, it evaluates <literal>e</literal>, matches
6585 it against the pattern <literal>[x,y]</literal>, and then evaluates <literal>b</literal>
6586 The "<literal>!</literal>" should not be regarded as part of the pattern; after all,
6587 in a function argument <literal>![x,y]</literal> means the
6588 same as <literal>[x,y]</literal>. Rather, the "<literal>!</literal>"
6589 is part of the syntax of <literal>let</literal> bindings.
6594 <sect2 id="bang-patterns-sem">
6595 <title>Syntax and semantics
6599 We add a single new production to the syntax of patterns:
6603 There is one problem with syntactic ambiguity. Consider:
6607 Is this a definition of the infix function "<literal>(!)</literal>",
6608 or of the "<literal>f</literal>" with a bang pattern? GHC resolves this
6609 ambiguity in favour of the latter. If you want to define
6610 <literal>(!)</literal> with bang-patterns enabled, you have to do so using
6615 The semantics of Haskell pattern matching is described in <ulink
6616 url="http://www.haskell.org/onlinereport/exps.html#sect3.17.2">
6617 Section 3.17.2</ulink> of the Haskell Report. To this description add
6618 one extra item 10, saying:
6619 <itemizedlist><listitem><para>Matching
6620 the pattern <literal>!pat</literal> against a value <literal>v</literal> behaves as follows:
6621 <itemizedlist><listitem><para>if <literal>v</literal> is bottom, the match diverges</para></listitem>
6622 <listitem><para>otherwise, <literal>pat</literal> is matched against
6623 <literal>v</literal></para></listitem>
6625 </para></listitem></itemizedlist>
6626 Similarly, in Figure 4 of <ulink url="http://www.haskell.org/onlinereport/exps.html#sect3.17.3">
6627 Section 3.17.3</ulink>, add a new case (t):
6629 case v of { !pat -> e; _ -> e' }
6630 = v `seq` case v of { pat -> e; _ -> e' }
6633 That leaves let expressions, whose translation is given in
6634 <ulink url="http://www.haskell.org/onlinereport/exps.html#sect3.12">Section
6636 of the Haskell Report.
6637 In the translation box, first apply
6638 the following transformation: for each pattern <literal>pi</literal> that is of
6639 form <literal>!qi = ei</literal>, transform it to <literal>(xi,!qi) = ((),ei)</literal>, and and replace <literal>e0</literal>
6640 by <literal>(xi `seq` e0)</literal>. Then, when none of the left-hand-side patterns
6641 have a bang at the top, apply the rules in the existing box.
6643 <para>The effect of the let rule is to force complete matching of the pattern
6644 <literal>qi</literal> before evaluation of the body is begun. The bang is
6645 retained in the translated form in case <literal>qi</literal> is a variable,
6653 The let-binding can be recursive. However, it is much more common for
6654 the let-binding to be non-recursive, in which case the following law holds:
6655 <literal>(let !p = rhs in body)</literal>
6657 <literal>(case rhs of !p -> body)</literal>
6660 A pattern with a bang at the outermost level is not allowed at the top level of
6666 <!-- ==================== ASSERTIONS ================= -->
6668 <sect1 id="assertions">
6670 <indexterm><primary>Assertions</primary></indexterm>
6674 If you want to make use of assertions in your standard Haskell code, you
6675 could define a function like the following:
6681 assert :: Bool -> a -> a
6682 assert False x = error "assertion failed!"
6689 which works, but gives you back a less than useful error message --
6690 an assertion failed, but which and where?
6694 One way out is to define an extended <function>assert</function> function which also
6695 takes a descriptive string to include in the error message and
6696 perhaps combine this with the use of a pre-processor which inserts
6697 the source location where <function>assert</function> was used.
6701 Ghc offers a helping hand here, doing all of this for you. For every
6702 use of <function>assert</function> in the user's source:
6708 kelvinToC :: Double -> Double
6709 kelvinToC k = assert (k >= 0.0) (k+273.15)
6715 Ghc will rewrite this to also include the source location where the
6722 assert pred val ==> assertError "Main.hs|15" pred val
6728 The rewrite is only performed by the compiler when it spots
6729 applications of <function>Control.Exception.assert</function>, so you
6730 can still define and use your own versions of
6731 <function>assert</function>, should you so wish. If not, import
6732 <literal>Control.Exception</literal> to make use
6733 <function>assert</function> in your code.
6737 GHC ignores assertions when optimisation is turned on with the
6738 <option>-O</option><indexterm><primary><option>-O</option></primary></indexterm> flag. That is, expressions of the form
6739 <literal>assert pred e</literal> will be rewritten to
6740 <literal>e</literal>. You can also disable assertions using the
6741 <option>-fignore-asserts</option>
6742 option<indexterm><primary><option>-fignore-asserts</option></primary>
6743 </indexterm>.</para>
6746 Assertion failures can be caught, see the documentation for the
6747 <literal>Control.Exception</literal> library for the details.
6753 <!-- =============================== PRAGMAS =========================== -->
6755 <sect1 id="pragmas">
6756 <title>Pragmas</title>
6758 <indexterm><primary>pragma</primary></indexterm>
6760 <para>GHC supports several pragmas, or instructions to the
6761 compiler placed in the source code. Pragmas don't normally affect
6762 the meaning of the program, but they might affect the efficiency
6763 of the generated code.</para>
6765 <para>Pragmas all take the form
6767 <literal>{-# <replaceable>word</replaceable> ... #-}</literal>
6769 where <replaceable>word</replaceable> indicates the type of
6770 pragma, and is followed optionally by information specific to that
6771 type of pragma. Case is ignored in
6772 <replaceable>word</replaceable>. The various values for
6773 <replaceable>word</replaceable> that GHC understands are described
6774 in the following sections; any pragma encountered with an
6775 unrecognised <replaceable>word</replaceable> is (silently)
6776 ignored. The layout rule applies in pragmas, so the closing <literal>#-}</literal>
6777 should start in a column to the right of the opening <literal>{-#</literal>. </para>
6779 <para>Certain pragmas are <emphasis>file-header pragmas</emphasis>. A file-header
6780 pragma must precede the <literal>module</literal> keyword in the file.
6781 There can be as many file-header pragmas as you please, and they can be
6782 preceded or followed by comments.</para>
6784 <sect2 id="language-pragma">
6785 <title>LANGUAGE pragma</title>
6787 <indexterm><primary>LANGUAGE</primary><secondary>pragma</secondary></indexterm>
6788 <indexterm><primary>pragma</primary><secondary>LANGUAGE</secondary></indexterm>
6790 <para>The <literal>LANGUAGE</literal> pragma allows language extensions to be enabled
6792 It is the intention that all Haskell compilers support the
6793 <literal>LANGUAGE</literal> pragma with the same syntax, although not
6794 all extensions are supported by all compilers, of
6795 course. The <literal>LANGUAGE</literal> pragma should be used instead
6796 of <literal>OPTIONS_GHC</literal>, if possible.</para>
6798 <para>For example, to enable the FFI and preprocessing with CPP:</para>
6800 <programlisting>{-# LANGUAGE ForeignFunctionInterface, CPP #-}</programlisting>
6802 <para><literal>LANGUAGE</literal> is a file-header pragma (see <xref linkend="pragmas"/>).</para>
6804 <para>Every language extension can also be turned into a command-line flag
6805 by prefixing it with "<literal>-X</literal>"; for example <option>-XForeignFunctionInterface</option>.
6806 (Similarly, all "<literal>-X</literal>" flags can be written as <literal>LANGUAGE</literal> pragmas.
6809 <para>A list of all supported language extensions can be obtained by invoking
6810 <literal>ghc --supported-languages</literal> (see <xref linkend="modes"/>).</para>
6812 <para>Any extension from the <literal>Extension</literal> type defined in
6814 url="../libraries/Cabal/Language-Haskell-Extension.html"><literal>Language.Haskell.Extension</literal></ulink>
6815 may be used. GHC will report an error if any of the requested extensions are not supported.</para>
6819 <sect2 id="options-pragma">
6820 <title>OPTIONS_GHC pragma</title>
6821 <indexterm><primary>OPTIONS_GHC</primary>
6823 <indexterm><primary>pragma</primary><secondary>OPTIONS_GHC</secondary>
6826 <para>The <literal>OPTIONS_GHC</literal> pragma is used to specify
6827 additional options that are given to the compiler when compiling
6828 this source file. See <xref linkend="source-file-options"/> for
6831 <para>Previous versions of GHC accepted <literal>OPTIONS</literal> rather
6832 than <literal>OPTIONS_GHC</literal>, but that is now deprecated.</para>
6835 <para><literal>OPTIONS_GHC</literal> is a file-header pragma (see <xref linkend="pragmas"/>).</para>
6837 <sect2 id="include-pragma">
6838 <title>INCLUDE pragma</title>
6840 <para>The <literal>INCLUDE</literal> pragma is for specifying the names
6841 of C header files that should be <literal>#include</literal>'d into
6842 the C source code generated by the compiler for the current module (if
6843 compiling via C). For example:</para>
6846 {-# INCLUDE "foo.h" #-}
6847 {-# INCLUDE <stdio.h> #-}</programlisting>
6849 <para><literal>INCLUDE</literal> is a file-header pragma (see <xref linkend="pragmas"/>).</para>
6851 <para>An <literal>INCLUDE</literal> pragma is the preferred alternative
6852 to the <option>-#include</option> option (<xref
6853 linkend="options-C-compiler" />), because the
6854 <literal>INCLUDE</literal> pragma is understood by other
6855 compilers. Yet another alternative is to add the include file to each
6856 <literal>foreign import</literal> declaration in your code, but we
6857 don't recommend using this approach with GHC.</para>
6860 <sect2 id="warning-deprecated-pragma">
6861 <title>WARNING and DEPRECATED pragmas</title>
6862 <indexterm><primary>WARNING</primary></indexterm>
6863 <indexterm><primary>DEPRECATED</primary></indexterm>
6865 <para>The WARNING pragma allows you to attach an arbitrary warning
6866 to a particular function, class, or type.
6867 A DEPRECATED pragma lets you specify that
6868 a particular function, class, or type is deprecated.
6869 There are two ways of using these pragmas.
6873 <para>You can work on an entire module thus:</para>
6875 module Wibble {-# DEPRECATED "Use Wobble instead" #-} where
6880 module Wibble {-# WARNING "This is an unstable interface." #-} where
6883 <para>When you compile any module that import
6884 <literal>Wibble</literal>, GHC will print the specified
6889 <para>You can attach a warning to a function, class, type, or data constructor, with the
6890 following top-level declarations:</para>
6892 {-# DEPRECATED f, C, T "Don't use these" #-}
6893 {-# WARNING unsafePerformIO "This is unsafe; I hope you know what you're doing" #-}
6895 <para>When you compile any module that imports and uses any
6896 of the specified entities, GHC will print the specified
6898 <para> You can only attach to entities declared at top level in the module
6899 being compiled, and you can only use unqualified names in the list of
6900 entities. A capitalised name, such as <literal>T</literal>
6901 refers to <emphasis>either</emphasis> the type constructor <literal>T</literal>
6902 <emphasis>or</emphasis> the data constructor <literal>T</literal>, or both if
6903 both are in scope. If both are in scope, there is currently no way to
6904 specify one without the other (c.f. fixities
6905 <xref linkend="infix-tycons"/>).</para>
6908 Warnings and deprecations are not reported for
6909 (a) uses within the defining module, and
6910 (b) uses in an export list.
6911 The latter reduces spurious complaints within a library
6912 in which one module gathers together and re-exports
6913 the exports of several others.
6915 <para>You can suppress the warnings with the flag
6916 <option>-fno-warn-warnings-deprecations</option>.</para>
6919 <sect2 id="inline-noinline-pragma">
6920 <title>INLINE and NOINLINE pragmas</title>
6922 <para>These pragmas control the inlining of function
6925 <sect3 id="inline-pragma">
6926 <title>INLINE pragma</title>
6927 <indexterm><primary>INLINE</primary></indexterm>
6929 <para>GHC (with <option>-O</option>, as always) tries to
6930 inline (or “unfold”) functions/values that are
6931 “small enough,” thus avoiding the call overhead
6932 and possibly exposing other more-wonderful optimisations.
6933 Normally, if GHC decides a function is “too
6934 expensive” to inline, it will not do so, nor will it
6935 export that unfolding for other modules to use.</para>
6937 <para>The sledgehammer you can bring to bear is the
6938 <literal>INLINE</literal><indexterm><primary>INLINE
6939 pragma</primary></indexterm> pragma, used thusly:</para>
6942 key_function :: Int -> String -> (Bool, Double)
6943 {-# INLINE key_function #-}
6946 <para>The major effect of an <literal>INLINE</literal> pragma
6947 is to declare a function's “cost” to be very low.
6948 The normal unfolding machinery will then be very keen to
6949 inline it. However, an <literal>INLINE</literal> pragma for a
6950 function "<literal>f</literal>" has a number of other effects:
6953 No functions are inlined into <literal>f</literal>. Otherwise
6954 GHC might inline a big function into <literal>f</literal>'s right hand side,
6955 making <literal>f</literal> big; and then inline <literal>f</literal> blindly.
6958 The float-in, float-out, and common-sub-expression transformations are not
6959 applied to the body of <literal>f</literal>.
6962 An INLINE function is not worker/wrappered by strictness analysis.
6963 It's going to be inlined wholesale instead.
6966 All of these effects are aimed at ensuring that what gets inlined is
6967 exactly what you asked for, no more and no less.
6969 <para>GHC ensures that inlining cannot go on forever: every mutually-recursive
6970 group is cut by one or more <emphasis>loop breakers</emphasis> that is never inlined
6971 (see <ulink url="http://research.microsoft.com/%7Esimonpj/Papers/inlining/index.htm">
6972 Secrets of the GHC inliner, JFP 12(4) July 2002</ulink>).
6973 GHC tries not to select a function with an INLINE pragma as a loop breaker, but
6974 when there is no choice even an INLINE function can be selected, in which case
6975 the INLINE pragma is ignored.
6976 For example, for a self-recursive function, the loop breaker can only be the function
6977 itself, so an INLINE pragma is always ignored.</para>
6979 <para>Syntactically, an <literal>INLINE</literal> pragma for a
6980 function can be put anywhere its type signature could be
6983 <para><literal>INLINE</literal> pragmas are a particularly
6985 <literal>then</literal>/<literal>return</literal> (or
6986 <literal>bind</literal>/<literal>unit</literal>) functions in
6987 a monad. For example, in GHC's own
6988 <literal>UniqueSupply</literal> monad code, we have:</para>
6991 {-# INLINE thenUs #-}
6992 {-# INLINE returnUs #-}
6995 <para>See also the <literal>NOINLINE</literal> pragma (<xref
6996 linkend="noinline-pragma"/>).</para>
6998 <para>Note: the HBC compiler doesn't like <literal>INLINE</literal> pragmas,
6999 so if you want your code to be HBC-compatible you'll have to surround
7000 the pragma with C pre-processor directives
7001 <literal>#ifdef __GLASGOW_HASKELL__</literal>...<literal>#endif</literal>.</para>
7005 <sect3 id="noinline-pragma">
7006 <title>NOINLINE pragma</title>
7008 <indexterm><primary>NOINLINE</primary></indexterm>
7009 <indexterm><primary>NOTINLINE</primary></indexterm>
7011 <para>The <literal>NOINLINE</literal> pragma does exactly what
7012 you'd expect: it stops the named function from being inlined
7013 by the compiler. You shouldn't ever need to do this, unless
7014 you're very cautious about code size.</para>
7016 <para><literal>NOTINLINE</literal> is a synonym for
7017 <literal>NOINLINE</literal> (<literal>NOINLINE</literal> is
7018 specified by Haskell 98 as the standard way to disable
7019 inlining, so it should be used if you want your code to be
7023 <sect3 id="phase-control">
7024 <title>Phase control</title>
7026 <para> Sometimes you want to control exactly when in GHC's
7027 pipeline the INLINE pragma is switched on. Inlining happens
7028 only during runs of the <emphasis>simplifier</emphasis>. Each
7029 run of the simplifier has a different <emphasis>phase
7030 number</emphasis>; the phase number decreases towards zero.
7031 If you use <option>-dverbose-core2core</option> you'll see the
7032 sequence of phase numbers for successive runs of the
7033 simplifier. In an INLINE pragma you can optionally specify a
7037 <para>"<literal>INLINE[k] f</literal>" means: do not inline
7038 <literal>f</literal>
7039 until phase <literal>k</literal>, but from phase
7040 <literal>k</literal> onwards be very keen to inline it.
7043 <para>"<literal>INLINE[~k] f</literal>" means: be very keen to inline
7044 <literal>f</literal>
7045 until phase <literal>k</literal>, but from phase
7046 <literal>k</literal> onwards do not inline it.
7049 <para>"<literal>NOINLINE[k] f</literal>" means: do not inline
7050 <literal>f</literal>
7051 until phase <literal>k</literal>, but from phase
7052 <literal>k</literal> onwards be willing to inline it (as if
7053 there was no pragma).
7056 <para>"<literal>NOINLINE[~k] f</literal>" means: be willing to inline
7057 <literal>f</literal>
7058 until phase <literal>k</literal>, but from phase
7059 <literal>k</literal> onwards do not inline it.
7062 The same information is summarised here:
7064 -- Before phase 2 Phase 2 and later
7065 {-# INLINE [2] f #-} -- No Yes
7066 {-# INLINE [~2] f #-} -- Yes No
7067 {-# NOINLINE [2] f #-} -- No Maybe
7068 {-# NOINLINE [~2] f #-} -- Maybe No
7070 {-# INLINE f #-} -- Yes Yes
7071 {-# NOINLINE f #-} -- No No
7073 By "Maybe" we mean that the usual heuristic inlining rules apply (if the
7074 function body is small, or it is applied to interesting-looking arguments etc).
7075 Another way to understand the semantics is this:
7077 <listitem><para>For both INLINE and NOINLINE, the phase number says
7078 when inlining is allowed at all.</para></listitem>
7079 <listitem><para>The INLINE pragma has the additional effect of making the
7080 function body look small, so that when inlining is allowed it is very likely to
7085 <para>The same phase-numbering control is available for RULES
7086 (<xref linkend="rewrite-rules"/>).</para>
7090 <sect2 id="line-pragma">
7091 <title>LINE pragma</title>
7093 <indexterm><primary>LINE</primary><secondary>pragma</secondary></indexterm>
7094 <indexterm><primary>pragma</primary><secondary>LINE</secondary></indexterm>
7095 <para>This pragma is similar to C's <literal>#line</literal>
7096 pragma, and is mainly for use in automatically generated Haskell
7097 code. It lets you specify the line number and filename of the
7098 original code; for example</para>
7100 <programlisting>{-# LINE 42 "Foo.vhs" #-}</programlisting>
7102 <para>if you'd generated the current file from something called
7103 <filename>Foo.vhs</filename> and this line corresponds to line
7104 42 in the original. GHC will adjust its error messages to refer
7105 to the line/file named in the <literal>LINE</literal>
7110 <title>RULES pragma</title>
7112 <para>The RULES pragma lets you specify rewrite rules. It is
7113 described in <xref linkend="rewrite-rules"/>.</para>
7116 <sect2 id="specialize-pragma">
7117 <title>SPECIALIZE pragma</title>
7119 <indexterm><primary>SPECIALIZE pragma</primary></indexterm>
7120 <indexterm><primary>pragma, SPECIALIZE</primary></indexterm>
7121 <indexterm><primary>overloading, death to</primary></indexterm>
7123 <para>(UK spelling also accepted.) For key overloaded
7124 functions, you can create extra versions (NB: more code space)
7125 specialised to particular types. Thus, if you have an
7126 overloaded function:</para>
7129 hammeredLookup :: Ord key => [(key, value)] -> key -> value
7132 <para>If it is heavily used on lists with
7133 <literal>Widget</literal> keys, you could specialise it as
7137 {-# SPECIALIZE hammeredLookup :: [(Widget, value)] -> Widget -> value #-}
7140 <para>A <literal>SPECIALIZE</literal> pragma for a function can
7141 be put anywhere its type signature could be put.</para>
7143 <para>A <literal>SPECIALIZE</literal> has the effect of generating
7144 (a) a specialised version of the function and (b) a rewrite rule
7145 (see <xref linkend="rewrite-rules"/>) that rewrites a call to the
7146 un-specialised function into a call to the specialised one.</para>
7148 <para>The type in a SPECIALIZE pragma can be any type that is less
7149 polymorphic than the type of the original function. In concrete terms,
7150 if the original function is <literal>f</literal> then the pragma
7152 {-# SPECIALIZE f :: <type> #-}
7154 is valid if and only if the definition
7156 f_spec :: <type>
7159 is valid. Here are some examples (where we only give the type signature
7160 for the original function, not its code):
7162 f :: Eq a => a -> b -> b
7163 {-# SPECIALISE f :: Int -> b -> b #-}
7165 g :: (Eq a, Ix b) => a -> b -> b
7166 {-# SPECIALISE g :: (Eq a) => a -> Int -> Int #-}
7168 h :: Eq a => a -> a -> a
7169 {-# SPECIALISE h :: (Eq a) => [a] -> [a] -> [a] #-}
7171 The last of these examples will generate a
7172 RULE with a somewhat-complex left-hand side (try it yourself), so it might not fire very
7173 well. If you use this kind of specialisation, let us know how well it works.
7176 <para>A <literal>SPECIALIZE</literal> pragma can optionally be followed with a
7177 <literal>INLINE</literal> or <literal>NOINLINE</literal> pragma, optionally
7178 followed by a phase, as described in <xref linkend="inline-noinline-pragma"/>.
7179 The <literal>INLINE</literal> pragma affects the specialised version of the
7180 function (only), and applies even if the function is recursive. The motivating
7183 -- A GADT for arrays with type-indexed representation
7185 ArrInt :: !Int -> ByteArray# -> Arr Int
7186 ArrPair :: !Int -> Arr e1 -> Arr e2 -> Arr (e1, e2)
7188 (!:) :: Arr e -> Int -> e
7189 {-# SPECIALISE INLINE (!:) :: Arr Int -> Int -> Int #-}
7190 {-# SPECIALISE INLINE (!:) :: Arr (a, b) -> Int -> (a, b) #-}
7191 (ArrInt _ ba) !: (I# i) = I# (indexIntArray# ba i)
7192 (ArrPair _ a1 a2) !: i = (a1 !: i, a2 !: i)
7194 Here, <literal>(!:)</literal> is a recursive function that indexes arrays
7195 of type <literal>Arr e</literal>. Consider a call to <literal>(!:)</literal>
7196 at type <literal>(Int,Int)</literal>. The second specialisation will fire, and
7197 the specialised function will be inlined. It has two calls to
7198 <literal>(!:)</literal>,
7199 both at type <literal>Int</literal>. Both these calls fire the first
7200 specialisation, whose body is also inlined. The result is a type-based
7201 unrolling of the indexing function.</para>
7202 <para>Warning: you can make GHC diverge by using <literal>SPECIALISE INLINE</literal>
7203 on an ordinarily-recursive function.</para>
7205 <para>Note: In earlier versions of GHC, it was possible to provide your own
7206 specialised function for a given type:
7209 {-# SPECIALIZE hammeredLookup :: [(Int, value)] -> Int -> value = intLookup #-}
7212 This feature has been removed, as it is now subsumed by the
7213 <literal>RULES</literal> pragma (see <xref linkend="rule-spec"/>).</para>
7217 <sect2 id="specialize-instance-pragma">
7218 <title>SPECIALIZE instance pragma
7222 <indexterm><primary>SPECIALIZE pragma</primary></indexterm>
7223 <indexterm><primary>overloading, death to</primary></indexterm>
7224 Same idea, except for instance declarations. For example:
7227 instance (Eq a) => Eq (Foo a) where {
7228 {-# SPECIALIZE instance Eq (Foo [(Int, Bar)]) #-}
7232 The pragma must occur inside the <literal>where</literal> part
7233 of the instance declaration.
7236 Compatible with HBC, by the way, except perhaps in the placement
7242 <sect2 id="unpack-pragma">
7243 <title>UNPACK pragma</title>
7245 <indexterm><primary>UNPACK</primary></indexterm>
7247 <para>The <literal>UNPACK</literal> indicates to the compiler
7248 that it should unpack the contents of a constructor field into
7249 the constructor itself, removing a level of indirection. For
7253 data T = T {-# UNPACK #-} !Float
7254 {-# UNPACK #-} !Float
7257 <para>will create a constructor <literal>T</literal> containing
7258 two unboxed floats. This may not always be an optimisation: if
7259 the <function>T</function> constructor is scrutinised and the
7260 floats passed to a non-strict function for example, they will
7261 have to be reboxed (this is done automatically by the
7264 <para>Unpacking constructor fields should only be used in
7265 conjunction with <option>-O</option>, in order to expose
7266 unfoldings to the compiler so the reboxing can be removed as
7267 often as possible. For example:</para>
7271 f (T f1 f2) = f1 + f2
7274 <para>The compiler will avoid reboxing <function>f1</function>
7275 and <function>f2</function> by inlining <function>+</function>
7276 on floats, but only when <option>-O</option> is on.</para>
7278 <para>Any single-constructor data is eligible for unpacking; for
7282 data T = T {-# UNPACK #-} !(Int,Int)
7285 <para>will store the two <literal>Int</literal>s directly in the
7286 <function>T</function> constructor, by flattening the pair.
7287 Multi-level unpacking is also supported:
7290 data T = T {-# UNPACK #-} !S
7291 data S = S {-# UNPACK #-} !Int {-# UNPACK #-} !Int
7294 will store two unboxed <literal>Int#</literal>s
7295 directly in the <function>T</function> constructor. The
7296 unpacker can see through newtypes, too.</para>
7298 <para>If a field cannot be unpacked, you will not get a warning,
7299 so it might be an idea to check the generated code with
7300 <option>-ddump-simpl</option>.</para>
7302 <para>See also the <option>-funbox-strict-fields</option> flag,
7303 which essentially has the effect of adding
7304 <literal>{-# UNPACK #-}</literal> to every strict
7305 constructor field.</para>
7308 <sect2 id="source-pragma">
7309 <title>SOURCE pragma</title>
7311 <indexterm><primary>SOURCE</primary></indexterm>
7312 <para>The <literal>{-# SOURCE #-}</literal> pragma is used only in <literal>import</literal> declarations,
7313 to break a module loop. It is described in detail in <xref linkend="mutual-recursion"/>.
7319 <!-- ======================= REWRITE RULES ======================== -->
7321 <sect1 id="rewrite-rules">
7322 <title>Rewrite rules
7324 <indexterm><primary>RULES pragma</primary></indexterm>
7325 <indexterm><primary>pragma, RULES</primary></indexterm>
7326 <indexterm><primary>rewrite rules</primary></indexterm></title>
7329 The programmer can specify rewrite rules as part of the source program
7335 "map/map" forall f g xs. map f (map g xs) = map (f.g) xs
7340 Use the debug flag <option>-ddump-simpl-stats</option> to see what rules fired.
7341 If you need more information, then <option>-ddump-rule-firings</option> shows you
7342 each individual rule firing in detail.
7346 <title>Syntax</title>
7349 From a syntactic point of view:
7355 There may be zero or more rules in a <literal>RULES</literal> pragma, separated by semicolons (which
7356 may be generated by the layout rule).
7362 The layout rule applies in a pragma.
7363 Currently no new indentation level
7364 is set, so if you put several rules in single RULES pragma and wish to use layout to separate them,
7365 you must lay out the starting in the same column as the enclosing definitions.
7368 "map/map" forall f g xs. map f (map g xs) = map (f.g) xs
7369 "map/append" forall f xs ys. map f (xs ++ ys) = map f xs ++ map f ys
7372 Furthermore, the closing <literal>#-}</literal>
7373 should start in a column to the right of the opening <literal>{-#</literal>.
7379 Each rule has a name, enclosed in double quotes. The name itself has
7380 no significance at all. It is only used when reporting how many times the rule fired.
7386 A rule may optionally have a phase-control number (see <xref linkend="phase-control"/>),
7387 immediately after the name of the rule. Thus:
7390 "map/map" [2] forall f g xs. map f (map g xs) = map (f.g) xs
7393 The "[2]" means that the rule is active in Phase 2 and subsequent phases. The inverse
7394 notation "[~2]" is also accepted, meaning that the rule is active up to, but not including,
7403 Each variable mentioned in a rule must either be in scope (e.g. <function>map</function>),
7404 or bound by the <literal>forall</literal> (e.g. <function>f</function>, <function>g</function>, <function>xs</function>). The variables bound by
7405 the <literal>forall</literal> are called the <emphasis>pattern</emphasis> variables. They are separated
7406 by spaces, just like in a type <literal>forall</literal>.
7412 A pattern variable may optionally have a type signature.
7413 If the type of the pattern variable is polymorphic, it <emphasis>must</emphasis> have a type signature.
7414 For example, here is the <literal>foldr/build</literal> rule:
7417 "fold/build" forall k z (g::forall b. (a->b->b) -> b -> b) .
7418 foldr k z (build g) = g k z
7421 Since <function>g</function> has a polymorphic type, it must have a type signature.
7428 The left hand side of a rule must consist of a top-level variable applied
7429 to arbitrary expressions. For example, this is <emphasis>not</emphasis> OK:
7432 "wrong1" forall e1 e2. case True of { True -> e1; False -> e2 } = e1
7433 "wrong2" forall f. f True = True
7436 In <literal>"wrong1"</literal>, the LHS is not an application; in <literal>"wrong2"</literal>, the LHS has a pattern variable
7443 A rule does not need to be in the same module as (any of) the
7444 variables it mentions, though of course they need to be in scope.
7450 All rules are implicitly exported from the module, and are therefore
7451 in force in any module that imports the module that defined the rule, directly
7452 or indirectly. (That is, if A imports B, which imports C, then C's rules are
7453 in force when compiling A.) The situation is very similar to that for instance
7461 Inside a RULE "<literal>forall</literal>" is treated as a keyword, regardless of
7462 any other flag settings. Furthermore, inside a RULE, the language extension
7463 <option>-XScopedTypeVariables</option> is automatically enabled; see
7464 <xref linkend="scoped-type-variables"/>.
7470 Like other pragmas, RULE pragmas are always checked for scope errors, and
7471 are typechecked. Typechecking means that the LHS and RHS of a rule are typechecked,
7472 and must have the same type. However, rules are only <emphasis>enabled</emphasis>
7473 if the <option>-fenable-rewrite-rules</option> flag is
7474 on (see <xref linkend="rule-semantics"/>).
7483 <sect2 id="rule-semantics">
7484 <title>Semantics</title>
7487 From a semantic point of view:
7492 Rules are enabled (that is, used during optimisation)
7493 by the <option>-fenable-rewrite-rules</option> flag.
7494 This flag is implied by <option>-O</option>, and may be switched
7495 off (as usual) by <option>-fno-enable-rewrite-rules</option>.
7496 (NB: enabling <option>-fenable-rewrite-rules</option> without <option>-O</option>
7497 may not do what you expect, though, because without <option>-O</option> GHC
7498 ignores all optimisation information in interface files;
7499 see <option>-fignore-interface-pragmas</option>, <xref linkend="options-f"/>.)
7500 Note that <option>-fenable-rewrite-rules</option> is an <emphasis>optimisation</emphasis> flag, and
7501 has no effect on parsing or typechecking.
7507 Rules are regarded as left-to-right rewrite rules.
7508 When GHC finds an expression that is a substitution instance of the LHS
7509 of a rule, it replaces the expression by the (appropriately-substituted) RHS.
7510 By "a substitution instance" we mean that the LHS can be made equal to the
7511 expression by substituting for the pattern variables.
7518 GHC makes absolutely no attempt to verify that the LHS and RHS
7519 of a rule have the same meaning. That is undecidable in general, and
7520 infeasible in most interesting cases. The responsibility is entirely the programmer's!
7527 GHC makes no attempt to make sure that the rules are confluent or
7528 terminating. For example:
7531 "loop" forall x y. f x y = f y x
7534 This rule will cause the compiler to go into an infinite loop.
7541 If more than one rule matches a call, GHC will choose one arbitrarily to apply.
7547 GHC currently uses a very simple, syntactic, matching algorithm
7548 for matching a rule LHS with an expression. It seeks a substitution
7549 which makes the LHS and expression syntactically equal modulo alpha
7550 conversion. The pattern (rule), but not the expression, is eta-expanded if
7551 necessary. (Eta-expanding the expression can lead to laziness bugs.)
7552 But not beta conversion (that's called higher-order matching).
7556 Matching is carried out on GHC's intermediate language, which includes
7557 type abstractions and applications. So a rule only matches if the
7558 types match too. See <xref linkend="rule-spec"/> below.
7564 GHC keeps trying to apply the rules as it optimises the program.
7565 For example, consider:
7574 The expression <literal>s (t xs)</literal> does not match the rule <literal>"map/map"</literal>, but GHC
7575 will substitute for <varname>s</varname> and <varname>t</varname>, giving an expression which does match.
7576 If <varname>s</varname> or <varname>t</varname> was (a) used more than once, and (b) large or a redex, then it would
7577 not be substituted, and the rule would not fire.
7584 Ordinary inlining happens at the same time as rule rewriting, which may lead to unexpected
7585 results. Consider this (artificial) example
7588 {-# RULES "f" f True = False #-}
7594 Since <literal>f</literal>'s right-hand side is small, it is inlined into <literal>g</literal>,
7599 Now <literal>g</literal> is inlined into <literal>h</literal>, but <literal>f</literal>'s RULE has
7601 If instead GHC had first inlined <literal>g</literal> into <literal>h</literal> then there
7602 would have been a better chance that <literal>f</literal>'s RULE might fire.
7605 The way to get predictable behaviour is to use a NOINLINE
7606 pragma on <literal>f</literal>, to ensure
7607 that it is not inlined until its RULEs have had a chance to fire.
7617 <title>List fusion</title>
7620 The RULES mechanism is used to implement fusion (deforestation) of common list functions.
7621 If a "good consumer" consumes an intermediate list constructed by a "good producer", the
7622 intermediate list should be eliminated entirely.
7626 The following are good producers:
7638 Enumerations of <literal>Int</literal> and <literal>Char</literal> (e.g. <literal>['a'..'z']</literal>).
7644 Explicit lists (e.g. <literal>[True, False]</literal>)
7650 The cons constructor (e.g <literal>3:4:[]</literal>)
7656 <function>++</function>
7662 <function>map</function>
7668 <function>take</function>, <function>filter</function>
7674 <function>iterate</function>, <function>repeat</function>
7680 <function>zip</function>, <function>zipWith</function>
7689 The following are good consumers:
7701 <function>array</function> (on its second argument)
7707 <function>++</function> (on its first argument)
7713 <function>foldr</function>
7719 <function>map</function>
7725 <function>take</function>, <function>filter</function>
7731 <function>concat</function>
7737 <function>unzip</function>, <function>unzip2</function>, <function>unzip3</function>, <function>unzip4</function>
7743 <function>zip</function>, <function>zipWith</function> (but on one argument only; if both are good producers, <function>zip</function>
7744 will fuse with one but not the other)
7750 <function>partition</function>
7756 <function>head</function>
7762 <function>and</function>, <function>or</function>, <function>any</function>, <function>all</function>
7768 <function>sequence_</function>
7774 <function>msum</function>
7780 <function>sortBy</function>
7789 So, for example, the following should generate no intermediate lists:
7792 array (1,10) [(i,i*i) | i <- map (+ 1) [0..9]]
7798 This list could readily be extended; if there are Prelude functions that you use
7799 a lot which are not included, please tell us.
7803 If you want to write your own good consumers or producers, look at the
7804 Prelude definitions of the above functions to see how to do so.
7809 <sect2 id="rule-spec">
7810 <title>Specialisation
7814 Rewrite rules can be used to get the same effect as a feature
7815 present in earlier versions of GHC.
7816 For example, suppose that:
7819 genericLookup :: Ord a => Table a b -> a -> b
7820 intLookup :: Table Int b -> Int -> b
7823 where <function>intLookup</function> is an implementation of
7824 <function>genericLookup</function> that works very fast for
7825 keys of type <literal>Int</literal>. You might wish
7826 to tell GHC to use <function>intLookup</function> instead of
7827 <function>genericLookup</function> whenever the latter was called with
7828 type <literal>Table Int b -> Int -> b</literal>.
7829 It used to be possible to write
7832 {-# SPECIALIZE genericLookup :: Table Int b -> Int -> b = intLookup #-}
7835 This feature is no longer in GHC, but rewrite rules let you do the same thing:
7838 {-# RULES "genericLookup/Int" genericLookup = intLookup #-}
7841 This slightly odd-looking rule instructs GHC to replace
7842 <function>genericLookup</function> by <function>intLookup</function>
7843 <emphasis>whenever the types match</emphasis>.
7844 What is more, this rule does not need to be in the same
7845 file as <function>genericLookup</function>, unlike the
7846 <literal>SPECIALIZE</literal> pragmas which currently do (so that they
7847 have an original definition available to specialise).
7850 <para>It is <emphasis>Your Responsibility</emphasis> to make sure that
7851 <function>intLookup</function> really behaves as a specialised version
7852 of <function>genericLookup</function>!!!</para>
7854 <para>An example in which using <literal>RULES</literal> for
7855 specialisation will Win Big:
7858 toDouble :: Real a => a -> Double
7859 toDouble = fromRational . toRational
7861 {-# RULES "toDouble/Int" toDouble = i2d #-}
7862 i2d (I# i) = D# (int2Double# i) -- uses Glasgow prim-op directly
7865 The <function>i2d</function> function is virtually one machine
7866 instruction; the default conversion—via an intermediate
7867 <literal>Rational</literal>—is obscenely expensive by
7874 <title>Controlling what's going on</title>
7882 Use <option>-ddump-rules</option> to see what transformation rules GHC is using.
7888 Use <option>-ddump-simpl-stats</option> to see what rules are being fired.
7889 If you add <option>-dppr-debug</option> you get a more detailed listing.
7895 The definition of (say) <function>build</function> in <filename>GHC/Base.lhs</filename> looks like this:
7898 build :: forall a. (forall b. (a -> b -> b) -> b -> b) -> [a]
7899 {-# INLINE build #-}
7903 Notice the <literal>INLINE</literal>! That prevents <literal>(:)</literal> from being inlined when compiling
7904 <literal>PrelBase</literal>, so that an importing module will “see” the <literal>(:)</literal>, and can
7905 match it on the LHS of a rule. <literal>INLINE</literal> prevents any inlining happening
7906 in the RHS of the <literal>INLINE</literal> thing. I regret the delicacy of this.
7913 In <filename>libraries/base/GHC/Base.lhs</filename> look at the rules for <function>map</function> to
7914 see how to write rules that will do fusion and yet give an efficient
7915 program even if fusion doesn't happen. More rules in <filename>GHC/List.lhs</filename>.
7925 <sect2 id="core-pragma">
7926 <title>CORE pragma</title>
7928 <indexterm><primary>CORE pragma</primary></indexterm>
7929 <indexterm><primary>pragma, CORE</primary></indexterm>
7930 <indexterm><primary>core, annotation</primary></indexterm>
7933 The external core format supports <quote>Note</quote> annotations;
7934 the <literal>CORE</literal> pragma gives a way to specify what these
7935 should be in your Haskell source code. Syntactically, core
7936 annotations are attached to expressions and take a Haskell string
7937 literal as an argument. The following function definition shows an
7941 f x = ({-# CORE "foo" #-} show) ({-# CORE "bar" #-} x)
7944 Semantically, this is equivalent to:
7952 However, when external core is generated (via
7953 <option>-fext-core</option>), there will be Notes attached to the
7954 expressions <function>show</function> and <varname>x</varname>.
7955 The core function declaration for <function>f</function> is:
7959 f :: %forall a . GHCziShow.ZCTShow a ->
7960 a -> GHCziBase.ZMZN GHCziBase.Char =
7961 \ @ a (zddShow::GHCziShow.ZCTShow a) (eta::a) ->
7963 %case zddShow %of (tpl::GHCziShow.ZCTShow a)
7965 (tpl1::GHCziBase.Int ->
7967 GHCziBase.ZMZN GHCziBase.Char -> GHCziBase.ZMZN GHCziBase.Cha
7969 (tpl2::a -> GHCziBase.ZMZN GHCziBase.Char)
7970 (tpl3::GHCziBase.ZMZN a ->
7971 GHCziBase.ZMZN GHCziBase.Char -> GHCziBase.ZMZN GHCziBase.Cha
7979 Here, we can see that the function <function>show</function> (which
7980 has been expanded out to a case expression over the Show dictionary)
7981 has a <literal>%note</literal> attached to it, as does the
7982 expression <varname>eta</varname> (which used to be called
7983 <varname>x</varname>).
7990 <sect1 id="special-ids">
7991 <title>Special built-in functions</title>
7992 <para>GHC has a few built-in functions with special behaviour. These
7993 are now described in the module <ulink
7994 url="../libraries/base/GHC-Prim.html"><literal>GHC.Prim</literal></ulink>
7995 in the library documentation.</para>
7999 <sect1 id="generic-classes">
8000 <title>Generic classes</title>
8003 The ideas behind this extension are described in detail in "Derivable type classes",
8004 Ralf Hinze and Simon Peyton Jones, Haskell Workshop, Montreal Sept 2000, pp94-105.
8005 An example will give the idea:
8013 fromBin :: [Int] -> (a, [Int])
8015 toBin {| Unit |} Unit = []
8016 toBin {| a :+: b |} (Inl x) = 0 : toBin x
8017 toBin {| a :+: b |} (Inr y) = 1 : toBin y
8018 toBin {| a :*: b |} (x :*: y) = toBin x ++ toBin y
8020 fromBin {| Unit |} bs = (Unit, bs)
8021 fromBin {| a :+: b |} (0:bs) = (Inl x, bs') where (x,bs') = fromBin bs
8022 fromBin {| a :+: b |} (1:bs) = (Inr y, bs') where (y,bs') = fromBin bs
8023 fromBin {| a :*: b |} bs = (x :*: y, bs'') where (x,bs' ) = fromBin bs
8024 (y,bs'') = fromBin bs'
8027 This class declaration explains how <literal>toBin</literal> and <literal>fromBin</literal>
8028 work for arbitrary data types. They do so by giving cases for unit, product, and sum,
8029 which are defined thus in the library module <literal>Generics</literal>:
8033 data a :+: b = Inl a | Inr b
8034 data a :*: b = a :*: b
8037 Now you can make a data type into an instance of Bin like this:
8039 instance (Bin a, Bin b) => Bin (a,b)
8040 instance Bin a => Bin [a]
8042 That is, just leave off the "where" clause. Of course, you can put in the
8043 where clause and over-ride whichever methods you please.
8047 <title> Using generics </title>
8048 <para>To use generics you need to</para>
8051 <para>Use the flags <option>-fglasgow-exts</option> (to enable the extra syntax),
8052 <option>-XGenerics</option> (to generate extra per-data-type code),
8053 and <option>-package lang</option> (to make the <literal>Generics</literal> library
8057 <para>Import the module <literal>Generics</literal> from the
8058 <literal>lang</literal> package. This import brings into
8059 scope the data types <literal>Unit</literal>,
8060 <literal>:*:</literal>, and <literal>:+:</literal>. (You
8061 don't need this import if you don't mention these types
8062 explicitly; for example, if you are simply giving instance
8063 declarations.)</para>
8068 <sect2> <title> Changes wrt the paper </title>
8070 Note that the type constructors <literal>:+:</literal> and <literal>:*:</literal>
8071 can be written infix (indeed, you can now use
8072 any operator starting in a colon as an infix type constructor). Also note that
8073 the type constructors are not exactly as in the paper (Unit instead of 1, etc).
8074 Finally, note that the syntax of the type patterns in the class declaration
8075 uses "<literal>{|</literal>" and "<literal>|}</literal>" brackets; curly braces
8076 alone would ambiguous when they appear on right hand sides (an extension we
8077 anticipate wanting).
8081 <sect2> <title>Terminology and restrictions</title>
8083 Terminology. A "generic default method" in a class declaration
8084 is one that is defined using type patterns as above.
8085 A "polymorphic default method" is a default method defined as in Haskell 98.
8086 A "generic class declaration" is a class declaration with at least one
8087 generic default method.
8095 Alas, we do not yet implement the stuff about constructor names and
8102 A generic class can have only one parameter; you can't have a generic
8103 multi-parameter class.
8109 A default method must be defined entirely using type patterns, or entirely
8110 without. So this is illegal:
8113 op :: a -> (a, Bool)
8114 op {| Unit |} Unit = (Unit, True)
8117 However it is perfectly OK for some methods of a generic class to have
8118 generic default methods and others to have polymorphic default methods.
8124 The type variable(s) in the type pattern for a generic method declaration
8125 scope over the right hand side. So this is legal (note the use of the type variable ``p'' in a type signature on the right hand side:
8129 op {| p :*: q |} (x :*: y) = op (x :: p)
8137 The type patterns in a generic default method must take one of the forms:
8143 where "a" and "b" are type variables. Furthermore, all the type patterns for
8144 a single type constructor (<literal>:*:</literal>, say) must be identical; they
8145 must use the same type variables. So this is illegal:
8149 op {| a :+: b |} (Inl x) = True
8150 op {| p :+: q |} (Inr y) = False
8152 The type patterns must be identical, even in equations for different methods of the class.
8153 So this too is illegal:
8157 op1 {| a :*: b |} (x :*: y) = True
8160 op2 {| p :*: q |} (x :*: y) = False
8162 (The reason for this restriction is that we gather all the equations for a particular type constructor
8163 into a single generic instance declaration.)
8169 A generic method declaration must give a case for each of the three type constructors.
8175 The type for a generic method can be built only from:
8177 <listitem> <para> Function arrows </para> </listitem>
8178 <listitem> <para> Type variables </para> </listitem>
8179 <listitem> <para> Tuples </para> </listitem>
8180 <listitem> <para> Arbitrary types not involving type variables </para> </listitem>
8182 Here are some example type signatures for generic methods:
8185 op2 :: Bool -> (a,Bool)
8186 op3 :: [Int] -> a -> a
8189 Here, op1, op2, op3 are OK, but op4 is rejected, because it has a type variable
8193 This restriction is an implementation restriction: we just haven't got around to
8194 implementing the necessary bidirectional maps over arbitrary type constructors.
8195 It would be relatively easy to add specific type constructors, such as Maybe and list,
8196 to the ones that are allowed.</para>
8201 In an instance declaration for a generic class, the idea is that the compiler
8202 will fill in the methods for you, based on the generic templates. However it can only
8207 The instance type is simple (a type constructor applied to type variables, as in Haskell 98).
8212 No constructor of the instance type has unboxed fields.
8216 (Of course, these things can only arise if you are already using GHC extensions.)
8217 However, you can still give an instance declarations for types which break these rules,
8218 provided you give explicit code to override any generic default methods.
8226 The option <option>-ddump-deriv</option> dumps incomprehensible stuff giving details of
8227 what the compiler does with generic declarations.
8232 <sect2> <title> Another example </title>
8234 Just to finish with, here's another example I rather like:
8238 nCons {| Unit |} _ = 1
8239 nCons {| a :*: b |} _ = 1
8240 nCons {| a :+: b |} _ = nCons (bot::a) + nCons (bot::b)
8243 tag {| Unit |} _ = 1
8244 tag {| a :*: b |} _ = 1
8245 tag {| a :+: b |} (Inl x) = tag x
8246 tag {| a :+: b |} (Inr y) = nCons (bot::a) + tag y
8252 <sect1 id="monomorphism">
8253 <title>Control over monomorphism</title>
8255 <para>GHC supports two flags that control the way in which generalisation is
8256 carried out at let and where bindings.
8260 <title>Switching off the dreaded Monomorphism Restriction</title>
8261 <indexterm><primary><option>-XNoMonomorphismRestriction</option></primary></indexterm>
8263 <para>Haskell's monomorphism restriction (see
8264 <ulink url="http://www.haskell.org/onlinereport/decls.html#sect4.5.5">Section
8266 of the Haskell Report)
8267 can be completely switched off by
8268 <option>-XNoMonomorphismRestriction</option>.
8273 <title>Monomorphic pattern bindings</title>
8274 <indexterm><primary><option>-XNoMonoPatBinds</option></primary></indexterm>
8275 <indexterm><primary><option>-XMonoPatBinds</option></primary></indexterm>
8277 <para> As an experimental change, we are exploring the possibility of
8278 making pattern bindings monomorphic; that is, not generalised at all.
8279 A pattern binding is a binding whose LHS has no function arguments,
8280 and is not a simple variable. For example:
8282 f x = x -- Not a pattern binding
8283 f = \x -> x -- Not a pattern binding
8284 f :: Int -> Int = \x -> x -- Not a pattern binding
8286 (g,h) = e -- A pattern binding
8287 (f) = e -- A pattern binding
8288 [x] = e -- A pattern binding
8290 Experimentally, GHC now makes pattern bindings monomorphic <emphasis>by
8291 default</emphasis>. Use <option>-XNoMonoPatBinds</option> to recover the
8300 ;;; Local Variables: ***
8302 ;;; sgml-parent-document: ("users_guide.xml" "book" "chapter" "sect1") ***
8303 ;;; ispell-local-dictionary: "british" ***
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