</varlistentry>
<varlistentry>
+ <term><option>-farrows</option></term>
+ <indexterm><primary><option>-farrows</option></primary></indexterm>
+ <listitem>
+ <para>See <xref LinkEnd="arrow-notation">. Independent of
+ <option>-fglasgow-exts</option>.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
<term><option>-fgenerics</option></term>
<indexterm><primary><option>-fgenerics</option></primary></indexterm>
<listitem>
<!-- UNBOXED TYPES AND PRIMITIVE OPERATIONS -->
<!-- included from primitives.sgml -->
-&primitives;
+<!-- &primitives; -->
+<sect1 id="primitives">
+ <title>Unboxed types and primitive operations</title>
+
+<para>GHC is built on a raft of primitive data types and operations.
+While you really can use this stuff to write fast code,
+ we generally find it a lot less painful, and more satisfying in the
+ long run, to use higher-level language features and libraries. With
+ any luck, the code you write will be optimised to the efficient
+ unboxed version in any case. And if it isn't, we'd like to know
+ about it.</para>
+
+<para>We do not currently have good, up-to-date documentation about the
+primitives, perhaps because they are mainly intended for internal use.
+There used to be a long section about them here in the User Guide, but it
+became out of date, and wrong information is worse than none.</para>
+
+<para>The Real Truth about what primitive types there are, and what operations
+work over those types, is held in the file
+<filename>fptools/ghc/compiler/prelude/primops.txt</filename>.
+This file is used directly to generate GHC's primitive-operation definitions, so
+it is always correct! It is also intended for processing into text.</para>
+
+<para> Indeed,
+the result of such processing is part of the description of the
+ <ulink
+ url="http://haskell.cs.yale.edu/ghc/docs/papers/core.ps.gz">External
+ Core language</ulink>.
+So that document is a good place to look for a type-set version.
+We would be very happy if someone wanted to volunteer to produce an SGML
+back end to the program that processes <filename>primops.txt</filename> so that
+we could include the results here in the User Guide.</para>
+
+<para>What follows here is a brief summary of some main points.</para>
+
+<sect2 id="glasgow-unboxed">
+<title>Unboxed types
+</title>
+
+<para>
+<indexterm><primary>Unboxed types (Glasgow extension)</primary></indexterm>
+</para>
+
+<para>Most types in GHC are <firstterm>boxed</firstterm>, which means
+that values of that type are represented by a pointer to a heap
+object. The representation of a Haskell <literal>Int</literal>, for
+example, is a two-word heap object. An <firstterm>unboxed</firstterm>
+type, however, is represented by the value itself, no pointers or heap
+allocation are involved.
+</para>
+
+<para>
+Unboxed types correspond to the “raw machine” types you
+would use in C: <literal>Int#</literal> (long int),
+<literal>Double#</literal> (double), <literal>Addr#</literal>
+(void *), etc. The <emphasis>primitive operations</emphasis>
+(PrimOps) on these types are what you might expect; e.g.,
+<literal>(+#)</literal> is addition on
+<literal>Int#</literal>s, and is the machine-addition that we all
+know and love—usually one instruction.
+</para>
+
+<para>
+Primitive (unboxed) types cannot be defined in Haskell, and are
+therefore built into the language and compiler. Primitive types are
+always unlifted; that is, a value of a primitive type cannot be
+bottom. We use the convention that primitive types, values, and
+operations have a <literal>#</literal> suffix.
+</para>
+
+<para>
+Primitive values are often represented by a simple bit-pattern, such
+as <literal>Int#</literal>, <literal>Float#</literal>,
+<literal>Double#</literal>. But this is not necessarily the case:
+a primitive value might be represented by a pointer to a
+heap-allocated object. Examples include
+<literal>Array#</literal>, the type of primitive arrays. A
+primitive array is heap-allocated because it is too big a value to fit
+in a register, and would be too expensive to copy around; in a sense,
+it is accidental that it is represented by a pointer. If a pointer
+represents a primitive value, then it really does point to that value:
+no unevaluated thunks, no indirections…nothing can be at the
+other end of the pointer than the primitive value.
+</para>
+
+<para>
+There are some restrictions on the use of primitive types, the main
+one being that you can't pass a primitive value to a polymorphic
+function or store one in a polymorphic data type. This rules out
+things like <literal>[Int#]</literal> (i.e. lists of primitive
+integers). The reason for this restriction is that polymorphic
+arguments and constructor fields are assumed to be pointers: if an
+unboxed integer is stored in one of these, the garbage collector would
+attempt to follow it, leading to unpredictable space leaks. Or a
+<function>seq</function> operation on the polymorphic component may
+attempt to dereference the pointer, with disastrous results. Even
+worse, the unboxed value might be larger than a pointer
+(<literal>Double#</literal> for instance).
+</para>
+
+<para>
+Nevertheless, A numerically-intensive program using unboxed types can
+go a <emphasis>lot</emphasis> faster than its “standard”
+counterpart—we saw a threefold speedup on one example.
+</para>
+
+</sect2>
+
+<sect2 id="unboxed-tuples">
+<title>Unboxed Tuples
+</title>
+
+<para>
+Unboxed tuples aren't really exported by <literal>GHC.Exts</literal>,
+they're available by default with <option>-fglasgow-exts</option>. An
+unboxed tuple looks like this:
+</para>
+
+<para>
+
+<programlisting>
+(# e_1, ..., e_n #)
+</programlisting>
+
+</para>
+
+<para>
+where <literal>e_1..e_n</literal> are expressions of any
+type (primitive or non-primitive). The type of an unboxed tuple looks
+the same.
+</para>
+
+<para>
+Unboxed tuples are used for functions that need to return multiple
+values, but they avoid the heap allocation normally associated with
+using fully-fledged tuples. When an unboxed tuple is returned, the
+components are put directly into registers or on the stack; the
+unboxed tuple itself does not have a composite representation. Many
+of the primitive operations listed in this section return unboxed
+tuples.
+</para>
+
+<para>
+There are some pretty stringent restrictions on the use of unboxed tuples:
+</para>
+
+<para>
+
+<itemizedlist>
+<listitem>
+
+<para>
+ Unboxed tuple types are subject to the same restrictions as
+other unboxed types; i.e. they may not be stored in polymorphic data
+structures or passed to polymorphic functions.
+
+</para>
+</listitem>
+<listitem>
+
+<para>
+ Unboxed tuples may only be constructed as the direct result of
+a function, and may only be deconstructed with a <literal>case</literal> expression.
+eg. the following are valid:
+
+
+<programlisting>
+f x y = (# x+1, y-1 #)
+g x = case f x x of { (# a, b #) -> a + b }
+</programlisting>
+
+
+but the following are invalid:
+
+
+<programlisting>
+f x y = g (# x, y #)
+g (# x, y #) = x + y
+</programlisting>
+
+
+</para>
+</listitem>
+<listitem>
+
+<para>
+ No variable can have an unboxed tuple type. This is illegal:
+
+
+<programlisting>
+f :: (# Int, Int #) -> (# Int, Int #)
+f x = x
+</programlisting>
+
+
+because <literal>x</literal> has an unboxed tuple type.
+
+</para>
+</listitem>
+
+</itemizedlist>
+
+</para>
+
+<para>
+Note: we may relax some of these restrictions in the future.
+</para>
+
+<para>
+The <literal>IO</literal> and <literal>ST</literal> monads use unboxed
+tuples to avoid unnecessary allocation during sequences of operations.
+</para>
+
+</sect2>
+</sect1>
+
<!-- ====================== SYNTACTIC EXTENSIONS ======================= -->
</sect2>
-<sect2> <title> Infix type constructors </title>
-
-<para>GHC supports infix type constructors, much as it supports infix data constructors. For example:
-<programlisting>
- infixl 5 :+:
-
- data a :+: b = Inl a | Inr b
-
- f :: a `Either` b -> a :+: b
- f (Left x) = Inl x
-</programlisting>
-</para>
-<para>The lexical
-syntax of an infix type constructor is just like that of an infix data constructor: either
-it's an operator beginning with ":", or it is an ordinary (alphabetic) type constructor enclosed in
-back-quotes.</para>
-
-<para>
-When you give a fixity declaration, the fixity applies to both the data constructor and the
-type constructor with the specified name. You cannot give different fixities to the type constructor T
-and the data constructor T.
-</para>
-
-
-</sect2>
-
<!-- ===================== PARALLEL LIST COMPREHENSIONS =================== -->
<sect2 id="parallel-list-comprehensions">
Jones, Erik Meijer).
</para>
-<para>
-I'd like to thank people who reported shorcomings in the GHC 3.02
-implementation. Our default decisions were all conservative ones, and
-the experience of these heroic pioneers has given useful concrete
-examples to support several generalisations. (These appear below as
-design choices not implemented in 3.02.)
-</para>
-
-<para>
-I've discussed these notes with Mark Jones, and I believe that Hugs
-will migrate towards the same design choices as I outline here.
-Thanks to him, and to many others who have offered very useful
-feedback.
-</para>
-<sect3>
+<sect3 id="type-restrictions">
<title>Types</title>
<para>
-There are the following restrictions on the form of a qualified
-type:
-</para>
-
-<para>
+GHC imposes the following restrictions on the form of a qualified
+type, whether declared in a type signature
+or inferred. Consider the type:
<programlisting>
forall tv1..tvn (c1, ...,cn) => type
</programlisting>
-</para>
-
-<para>
(Here, I write the "foralls" explicitly, although the Haskell source
language omits them; in Haskell 1.4, all the free type variables of an
explicit source-language type signature are universally quantified,
<para>
<emphasis>Each universally quantified type variable
-<literal>tvi</literal> must be mentioned (i.e. appear free) in <literal>type</literal></emphasis>.
+<literal>tvi</literal> must be reachable from <literal>type</literal></emphasis>.
+A type variable is "reachable" if it it is functionally dependent
+(see <xref linkend="functional-dependencies">)
+on the type variables free in <literal>type</literal>.
The reason for this is that a value with a type that does not obey
this restriction could not be used without introducing
-ambiguity. Here, for example, is an illegal type:
+ambiguity.
+Here, for example, is an illegal type:
<programlisting>
</para>
-<para>
-These restrictions apply to all types, whether declared in a type signature
-or inferred.
-</para>
<para>
Unlike Haskell 1.4, constraints in types do <emphasis>not</emphasis> have to be of
</para>
</listitem>
-<listitem>
-
-<para>
- <emphasis>In the signature of a class operation, every constraint
-must mention at least one type variable that is not a class type
-variable</emphasis>.
-
-Thus:
-
-
-<programlisting>
- class Collection c a where
- mapC :: Collection c b => (a->b) -> c a -> c b
-</programlisting>
-
-
-is OK because the constraint <literal>(Collection a b)</literal> mentions
-<literal>b</literal>, even though it also mentions the class variable
-<literal>a</literal>. On the other hand:
-
-<programlisting>
- class C a where
- op :: Eq a => (a,b) -> (a,b)
-</programlisting>
-
-
-is not OK because the constraint <literal>(Eq a)</literal> mentions on the class
-type variable <literal>a</literal>, but not <literal>b</literal>. However, any such
-example is easily fixed by moving the offending context up to the
-superclass context:
-
-
-<programlisting>
- class Eq a => C a where
- op ::(a,b) -> (a,b)
-</programlisting>
-
-
-A yet more relaxed rule would allow the context of a class-op signature
-to mention only class type variables. However, that conflicts with
-Rule 1(b) for types above.
-
-</para>
-</listitem>
<listitem>
<para>
- <emphasis>The type of each class operation must mention <emphasis>all</emphasis> of
-the class type variables</emphasis>. For example:
+ <emphasis>All of the class type variables must be reachable (in the sense
+mentioned in <xref linkend="type-restrictions">)
+from the free varibles of each method type
+</emphasis>. For example:
<programlisting>
instance Stateful (ST s) (MutVar s) where ...
</programlisting>
-
-The "at least one not a type variable" restriction is to ensure that
-context reduction terminates: each reduction step removes one type
-constructor. For example, the following would make the type checker
-loop if it wasn't excluded:
-
-
-<programlisting>
- instance C a => C a where ...
-</programlisting>
-
-
-There are two situations in which the rule is a bit of a pain. First,
-if one allows overlapping instance declarations then it's quite
-convenient to have a "default instance" declaration that applies if
-something more specific does not:
-
-
-<programlisting>
- instance C a where
- op = ... -- Default
-</programlisting>
-
-
-Second, sometimes you might want to use the following to get the
-effect of a "class synonym":
-
-
-<programlisting>
- class (C1 a, C2 a, C3 a) => C a where { }
-
- instance (C1 a, C2 a, C3 a) => C a where { }
-</programlisting>
-
-
-This allows you to write shorter signatures:
-
-
-<programlisting>
- f :: C a => ...
-</programlisting>
-
-
-instead of
-
-
-<programlisting>
- f :: (C1 a, C2 a, C3 a) => ...
-</programlisting>
-
-
-I'm on the lookout for a simple rule that preserves decidability while
-allowing these idioms. The experimental flag
-<option>-fallow-undecidable-instances</option><indexterm><primary>-fallow-undecidable-instances
-option</primary></indexterm> lifts this restriction, allowing all the types in an
-instance head to be type variables.
-
+See <xref linkend="undecidable-instances"> for an experimental
+extension to lift this restriction.
</para>
</listitem>
<listitem>
</programlisting>
-is not OK. Again, the intent here is to make sure that context
-reduction terminates.
+is not OK. See <xref linkend="undecidable-instances"> for an experimental
+extension to lift this restriction.
+
-Voluminous correspondence on the Haskell mailing list has convinced me
-that it's worth experimenting with a more liberal rule. If you use
-the flag <option>-fallow-undecidable-instances</option> can use arbitrary
-types in an instance context. Termination is ensured by having a
-fixed-depth recursion stack. If you exceed the stack depth you get a
-sort of backtrace, and the opportunity to increase the stack depth
-with <option>-fcontext-stack</option><emphasis>N</emphasis>.
</para>
</listitem>
</sect2>
-<sect2 id="implicit-parameters">
-<title>Implicit parameters
-</title>
+<sect2 id="undecidable-instances">
+<title>Undecidable instances</title>
-<para> Implicit paramters are implemented as described in
-"Implicit parameters: dynamic scoping with static types",
-J Lewis, MB Shields, E Meijer, J Launchbury,
-27th ACM Symposium on Principles of Programming Languages (POPL'00),
-Boston, Jan 2000.
-</para>
-<para>(Most of the following, stil rather incomplete, documentation is due to Jeff Lewis.)</para>
-<para>
-A variable is called <emphasis>dynamically bound</emphasis> when it is bound by the calling
-context of a function and <emphasis>statically bound</emphasis> when bound by the callee's
-context. In Haskell, all variables are statically bound. Dynamic
-binding of variables is a notion that goes back to Lisp, but was later
-discarded in more modern incarnations, such as Scheme. Dynamic binding
-can be very confusing in an untyped language, and unfortunately, typed
-languages, in particular Hindley-Milner typed languages like Haskell,
-only support static scoping of variables.
-</para>
-<para>
-However, by a simple extension to the type class system of Haskell, we
-can support dynamic binding. Basically, we express the use of a
-dynamically bound variable as a constraint on the type. These
-constraints lead to types of the form <literal>(?x::t') => t</literal>, which says "this
-function uses a dynamically-bound variable <literal>?x</literal>
-of type <literal>t'</literal>". For
-example, the following expresses the type of a sort function,
-implicitly parameterized by a comparison function named <literal>cmp</literal>.
+<para>The rules for instance declarations state that:
+<itemizedlist>
+<listitem><para>At least one of the types in the <emphasis>head</emphasis> of
+an instance declaration <emphasis>must not</emphasis> be a type variable.
+</para></listitem>
+<listitem><para>All of the types in the <emphasis>context</emphasis> of
+an instance declaration <emphasis>must</emphasis> be type variables.
+</para></listitem>
+</itemizedlist>
+These restrictions ensure that
+context reduction terminates: each reduction step removes one type
+constructor. For example, the following would make the type checker
+loop if it wasn't excluded:
<programlisting>
- sort :: (?cmp :: a -> a -> Bool) => [a] -> [a]
+ instance C a => C a where ...
</programlisting>
-The dynamic binding constraints are just a new form of predicate in the type class system.
-</para>
-<para>
-An implicit parameter is introduced by the special form <literal>?x</literal>,
-where <literal>x</literal> is
-any valid identifier. Use if this construct also introduces new
-dynamic binding constraints. For example, the following definition
+There are two situations in which the rule is a bit of a pain. First,
+if one allows overlapping instance declarations then it's quite
+convenient to have a "default instance" declaration that applies if
+something more specific does not:
+
+
+<programlisting>
+ instance C a where
+ op = ... -- Default
+</programlisting>
+
+
+Second, sometimes you might want to use the following to get the
+effect of a "class synonym":
+
+
+<programlisting>
+ class (C1 a, C2 a, C3 a) => C a where { }
+
+ instance (C1 a, C2 a, C3 a) => C a where { }
+</programlisting>
+
+
+This allows you to write shorter signatures:
+
+
+<programlisting>
+ f :: C a => ...
+</programlisting>
+
+
+instead of
+
+
+<programlisting>
+ f :: (C1 a, C2 a, C3 a) => ...
+</programlisting>
+
+
+Voluminous correspondence on the Haskell mailing list has convinced me
+that it's worth experimenting with more liberal rules. If you use
+the experimental flag <option>-fallow-undecidable-instances</option>
+<indexterm><primary>-fallow-undecidable-instances
+option</primary></indexterm>, you can use arbitrary
+types in both an instance context and instance head. Termination is ensured by having a
+fixed-depth recursion stack. If you exceed the stack depth you get a
+sort of backtrace, and the opportunity to increase the stack depth
+with <option>-fcontext-stack</option><emphasis>N</emphasis>.
+</para>
+<para>
+I'm on the lookout for a less brutal solution: a simple rule that preserves decidability while
+allowing these idioms interesting idioms.
+</para>
+</sect2>
+
+<sect2 id="implicit-parameters">
+<title>Implicit parameters
+</title>
+
+<para> Implicit paramters are implemented as described in
+"Implicit parameters: dynamic scoping with static types",
+J Lewis, MB Shields, E Meijer, J Launchbury,
+27th ACM Symposium on Principles of Programming Languages (POPL'00),
+Boston, Jan 2000.
+</para>
+<para>(Most of the following, stil rather incomplete, documentation is due to Jeff Lewis.)</para>
+<para>
+A variable is called <emphasis>dynamically bound</emphasis> when it is bound by the calling
+context of a function and <emphasis>statically bound</emphasis> when bound by the callee's
+context. In Haskell, all variables are statically bound. Dynamic
+binding of variables is a notion that goes back to Lisp, but was later
+discarded in more modern incarnations, such as Scheme. Dynamic binding
+can be very confusing in an untyped language, and unfortunately, typed
+languages, in particular Hindley-Milner typed languages like Haskell,
+only support static scoping of variables.
+</para>
+<para>
+However, by a simple extension to the type class system of Haskell, we
+can support dynamic binding. Basically, we express the use of a
+dynamically bound variable as a constraint on the type. These
+constraints lead to types of the form <literal>(?x::t') => t</literal>, which says "this
+function uses a dynamically-bound variable <literal>?x</literal>
+of type <literal>t'</literal>". For
+example, the following expresses the type of a sort function,
+implicitly parameterized by a comparison function named <literal>cmp</literal>.
+<programlisting>
+ sort :: (?cmp :: a -> a -> Bool) => [a] -> [a]
+</programlisting>
+The dynamic binding constraints are just a new form of predicate in the type class system.
+</para>
+<para>
+An implicit parameter occurs in an expression using the special form <literal>?x</literal>,
+where <literal>x</literal> is
+any valid identifier (e.g. <literal>ord ?x</literal> is a valid expression).
+Use of this construct also introduces a new
+dynamic-binding constraint in the type of the expression.
+For example, the following definition
shows how we can define an implicitly parameterized sort function in
terms of an explicitly parameterized <literal>sortBy</literal> function:
<programlisting>
sort :: (?cmp :: a -> a -> Bool) => [a] -> [a]
sort = sortBy ?cmp
</programlisting>
+</para>
+
+<sect3>
+<title>Implicit-parameter type constraints</title>
+<para>
Dynamic binding constraints behave just like other type class
constraints in that they are automatically propagated. Thus, when a
function is used, its implicit parameters are inherited by the
propagate them.
</para>
<para>
-An implicit parameter differs from other type class constraints in the
+An implicit-parameter type constraint differs from other type class constraints in the
following way: All uses of a particular implicit parameter must have
the same type. This means that the type of <literal>(?x, ?x)</literal>
is <literal>(?x::a) => (a,a)</literal>, and not
<literal>(?x::a, ?x::b) => (a, b)</literal>, as would be the case for type
class constraints.
</para>
+
+<para> You can't have an implicit parameter in the context of a class or instance
+declaration. For example, both these declarations are illegal:
+<programlisting>
+ class (?x::Int) => C a where ...
+ instance (?x::a) => Foo [a] where ...
+</programlisting>
+Reason: exactly which implicit parameter you pick up depends on exactly where
+you invoke a function. But the ``invocation'' of instance declarations is done
+behind the scenes by the compiler, so it's hard to figure out exactly where it is done.
+Easiest thing is to outlaw the offending types.</para>
+<para>
+Implicit-parameter constraints do not cause ambiguity. For example, consider:
+<programlisting>
+ f :: (?x :: [a]) => Int -> Int
+ f n = n + length ?x
+
+ g :: (Read a, Show a) => String -> String
+ g s = show (read s)
+</programlisting>
+Here, <literal>g</literal> has an ambiguous type, and is rejected, but <literal>f</literal>
+is fine. The binding for <literal>?x</literal> at <literal>f</literal>'s call site is
+quite unambiguous, and fixes the type <literal>a</literal>.
+</para>
+</sect3>
+
+<sect3>
+<title>Implicit-parameter bindings</title>
+
<para>
-An implicit parameter is bound using the standard
-<literal>let</literal> binding form, where the bindings must be a
-collection of simple bindings to implicit-style variables (no
-function-style bindings, and no type signatures); these bindings are
-neither polymorphic or recursive. This form binds the implicit
-parameters arising in the body, not the free variables as a
-<literal>let</literal> or <literal>where</literal> would do. For
-example, we define the <literal>min</literal> function by binding
-<literal>cmp</literal>.</para>
+An implicit parameter is <emphasis>bound</emphasis> using the standard
+<literal>let</literal> or <literal>where</literal> binding forms.
+For example, we define the <literal>min</literal> function by binding
+<literal>cmp</literal>.
<programlisting>
min :: [a] -> a
min = let ?cmp = (<=) in least
</programlisting>
+</para>
<para>
+A group of implicit-parameter bindings may occur anywhere a normal group of Haskell
+bindings can occur, except at top level. That is, they can occur in a <literal>let</literal>
+(including in a list comprehension, or do-notation, or pattern guards),
+or a <literal>where</literal> clause.
Note the following points:
<itemizedlist>
<listitem><para>
+An implicit-parameter binding group must be a
+collection of simple bindings to implicit-style variables (no
+function-style bindings, and no type signatures); these bindings are
+neither polymorphic or recursive.
+</para></listitem>
+<listitem><para>
You may not mix implicit-parameter bindings with ordinary bindings in a
single <literal>let</literal>
expression; use two nested <literal>let</literal>s instead.
+(In the case of <literal>where</literal> you are stuck, since you can't nest <literal>where</literal> clauses.)
</para></listitem>
<listitem><para>
You may put multiple implicit-parameter bindings in a
-single <literal>let</literal> expression; they are <emphasis>not</emphasis> treated
+single binding group; but they are <emphasis>not</emphasis> treated
as a mutually recursive group (as ordinary <literal>let</literal> bindings are).
-Instead they are treated as a non-recursive group, each scoping over the bindings that
-follow. For example, consider:
+Instead they are treated as a non-recursive group, simultaneously binding all the implicit
+parameter. The bindings are not nested, and may be re-ordered without changing
+the meaning of the program.
+For example, consider:
<programlisting>
- f y = let { ?x = y; ?x = ?x+1 } in ?x
+ f t = let { ?x = t; ?y = ?x+(1::Int) } in ?x + ?y
</programlisting>
-This function adds one to its argument.
-</para></listitem>
-
-<listitem><para>
-You may not have an implicit-parameter binding in a <literal>where</literal> clause,
-only in a <literal>let</literal> binding.
-</para></listitem>
-
-<listitem>
-<para> You can't have an implicit parameter in the context of a class or instance
-declaration. For example, both these declarations are illegal:
+The use of <literal>?x</literal> in the binding for <literal>?y</literal> does not "see"
+the binding for <literal>?x</literal>, so the type of <literal>f</literal> is
<programlisting>
- class (?x::Int) => C a where ...
- instance (?x::a) => Foo [a] where ...
+ f :: (?x::Int) => Int -> Int
</programlisting>
-Reason: exactly which implicit parameter you pick up depends on exactly where
-you invoke a function. But the ``invocation'' of instance declarations is done
-behind the scenes by the compiler, so it's hard to figure out exactly where it is done.
-Easiest thing is to outlaw the offending types.</para>
-</listitem>
+</para></listitem>
</itemizedlist>
</para>
+</sect3>
</sect2>
<sect2 id="linear-implicit-parameters">
ESOP 2000, Berlin, Germany, March 2000, Springer-Verlag LNCS 1782,
.
</para>
-
<para>
+Functional dependencies are introduced by a vertical bar in the syntax of a
+class declaration; e.g.
+<programlisting>
+ class (Monad m) => MonadState s m | m -> s where ...
+
+ class Foo a b c | a b -> c where ...
+</programlisting>
There should be more documentation, but there isn't (yet). Yell if you need it.
</para>
</sect2>
</sect3>
<sect3>
-<title>Result type signatures</title>
-
-<para>
-
-<itemizedlist>
-<listitem>
-
-<para>
- The result type of a function can be given a signature,
-thus:
-
-
-<programlisting>
- f (x::a) :: [a] = [x,x,x]
-</programlisting>
-
-
-The final <literal>:: [a]</literal> after all the patterns gives a signature to the
-result type. Sometimes this is the only way of naming the type variable
-you want:
-
-
-<programlisting>
- f :: Int -> [a] -> [a]
- f n :: ([a] -> [a]) = let g (x::a, y::a) = (y,x)
- in \xs -> map g (reverse xs `zip` xs)
-</programlisting>
-
-
-</para>
-</listitem>
-
-</itemizedlist>
-
-</para>
-
-<para>
-Result type signatures are not yet implemented in Hugs.
-</para>
-
-</sect3>
-
-<sect3>
<title>Where a pattern type signature can occur</title>
<para>
</para>
</sect3>
+
+<sect3>
+<title>Result type signatures</title>
+
+<para>
+The result type of a function can be given a signature, thus:
+
+
+<programlisting>
+ f (x::a) :: [a] = [x,x,x]
+</programlisting>
+
+
+The final <literal>:: [a]</literal> after all the patterns gives a signature to the
+result type. Sometimes this is the only way of naming the type variable
+you want:
+
+
+<programlisting>
+ f :: Int -> [a] -> [a]
+ f n :: ([a] -> [a]) = let g (x::a, y::a) = (y,x)
+ in \xs -> map g (reverse xs `zip` xs)
+</programlisting>
+
+</para>
+<para>
+The type variables bound in a result type signature scope over the right hand side
+of the definition. However, consider this corner-case:
+<programlisting>
+ rev1 :: [a] -> [a] = \xs -> reverse xs
+
+ foo ys = rev (ys::[a])
+</programlisting>
+The signature on <literal>rev1</literal> is considered a pattern type signature, not a result
+type signature, and the type variables it binds have the same scope as <literal>rev1</literal>
+itself (i.e. the right-hand side of <literal>rev1</literal> and the rest of the module too).
+In particular, the expression <literal>(ys::[a])</literal> is OK, because the type variable <literal>a</literal>
+is in scope (otherwise it would mean <literal>(ys::forall a.[a])</literal>, which would be rejected).
+</para>
+<para>
+As mentioned above, <literal>rev1</literal> is made monomorphic by this scoping rule.
+For example, the following program would be rejected, because it claims that <literal>rev1</literal>
+is polymorphic:
+<programlisting>
+ rev1 :: [b] -> [b]
+ rev1 :: [a] -> [a] = \xs -> reverse xs
+</programlisting>
+</para>
+
+<para>
+Result type signatures are not yet implemented in Hugs.
+</para>
+
+</sect3>
+
</sect2>
<sect2 id="newtype-deriving">
newtype T v1...vn = T' (S t1...tk vk+1...vn) deriving (c1...cm)
</programlisting>
-where <literal>S</literal> is a type constructor, <literal>t1...tk</literal> are
-types,
-<literal>vk+1...vn</literal> are type variables which do not occur in any of
-the <literal>ti</literal>, and the <literal>ci</literal> are partial applications of
-classes of the form <literal>C t1'...tj'</literal>. The derived instance
-declarations are, for each <literal>ci</literal>,
-
+where
+ <itemizedlist>
+<listitem><para>
+ <literal>S</literal> is a type constructor,
+</para></listitem>
+<listitem><para>
+ <literal>t1...tk</literal> are types,
+</para></listitem>
+<listitem><para>
+ <literal>vk+1...vn</literal> are type variables which do not occur in any of
+ the <literal>ti</literal>, and
+</para></listitem>
+<listitem><para>
+ the <literal>ci</literal> are partial applications of
+ classes of the form <literal>C t1'...tj'</literal>, where the arity of <literal>C</literal>
+ is exactly <literal>j+1</literal>. That is, <literal>C</literal> lacks exactly one type argument.
+</para></listitem>
+</itemizedlist>
+Then, for each <literal>ci</literal>, the derived instance
+declaration is:
<programlisting>
instance ci (S t1...tk vk+1...v) => ci (T v1...vp)
</programlisting>
<sect1 id="template-haskell">
<title>Template Haskell</title>
-<para>Template Haskell allows you to do compile-time meta-programming in Haskell. The background
-the main technical innovations are discussed in "<ulink
+<para>Template Haskell allows you to do compile-time meta-programming in Haskell. There is a "home page" for
+Template Haskell at <ulink url="http://www.haskell.org/th/">
+http://www.haskell.org/th/</ulink>, while
+the background to
+the main technical innovations is discussed in "<ulink
url="http://research.microsoft.com/~simonpj/papers/meta-haskell">
-Template Meta-programming for Haskell</ulink>", in
-Proc Haskell Workshop 2002.
+Template Meta-programming for Haskell</ulink>" (Proc Haskell Workshop 2002).
+</para>
+
+<para> The first example from that paper is set out below as a worked example to help get you started.
</para>
<para>
</para>
<para> A splice can occur in place of
<itemizedlist>
- <listitem><para> an expression;</para></listitem>
- <listitem><para> a list of top-level declarations;</para></listitem>
- <listitem><para> a pattern;</para></listitem>
- <listitem><para> a type;</para></listitem>
+ <listitem><para> an expression; the spliced expression must have type <literal>Expr</literal></para></listitem>
+ <listitem><para> a list of top-level declarations; ; the spliced expression must have type <literal>Q [Dec]</literal></para></listitem>
+ <listitem><para> a type; the spliced expression must have type <literal>Type</literal>.</para></listitem>
</itemizedlist>
+ (Note that the syntax for a declaration splice uses "<literal>$</literal>" not "<literal>splice</literal>" as in
+ the paper. Also the type of the enclosed expression must be <literal>Q [Dec]</literal>, not <literal>[Q Dec]</literal>
+ as in the paper.)
</para></listitem>
<listitem><para>
A expression quotation is written in Oxford brackets, thus:
<itemizedlist>
- <listitem><para> <literal>[| ... |]</literal>, where the "..." is an expression;</para></listitem>
- <listitem><para> <literal>[d| ... |]</literal>, where the "..." is a list of top-level declarations;</para></listitem>
- <listitem><para> <literal>[p| ... |]</literal>, where the "..." is a pattern;</para></listitem>
- <listitem><para> <literal>[t| ... |]</literal>, where the "..." is a type;</para></listitem>
+ <listitem><para> <literal>[| ... |]</literal>, where the "..." is an expression;
+ the quotation has type <literal>Expr</literal>.</para></listitem>
+ <listitem><para> <literal>[d| ... |]</literal>, where the "..." is a list of top-level declarations;
+ the quotation has type <literal>Q [Dec]</literal>.</para></listitem>
+ <listitem><para> <literal>[t| ... |]</literal>, where the "..." is a type;
+ the quotation has type <literal>Type</literal>.</para></listitem>
</itemizedlist></para></listitem>
<listitem><para>
</para></listitem>
<listitem><para>
- If the module contains any top-level splices that must be run, you must use GHC with
- <literal>--make</literal> or <literal>--interactive</literal> flags. (Reason: that
- means it walks the dependency tree and knows what modules must be linked etc.)
- </para></listitem>
-
- <listitem><para>
You can only run a function at compile time if it is imported from another module. That is,
you can't define a function in a module, and call it from within a splice in the same module.
(It would make sense to do so, but it's hard to implement.)
<listitem><para>
The flag <literal>-ddump-splices</literal> shows the expansion of all top-level splices as they happen.
</para></listitem>
+ <listitem><para>
+ If you are building GHC from source, you need at least a stage-2 bootstrap compiler to
+ run Template Haskell. A stage-1 compiler will reject the TH constructs. Reason: TH
+ compiles and runs a program, and then looks at the result. So it's important that
+ the program it compiles produces results whose representations are identical to
+ those of the compiler itself.
+ </para></listitem>
</itemizedlist>
</para>
+<para> Template Haskell works in any mode (<literal>--make</literal>, <literal>--interactive</literal>,
+ or file-at-a-time). There used to be a restriction to the former two, but that restriction
+ has been lifted.
+</para>
</sect2>
-</sect1>
-
-<!-- ==================== ASSERTIONS ================= -->
+<sect2> <title> A Template Haskell Worked Example </title>
+<para>To help you get over the confidence barrier, try out this skeletal worked example.
+ First cut and paste the two modules below into "Main.hs" and "Printf.hs":</para>
-<sect1 id="sec-assertions">
-<title>Assertions
-<indexterm><primary>Assertions</primary></indexterm>
-</title>
+<programlisting>
+{- Main.hs -}
+module Main where
-<para>
-If you want to make use of assertions in your standard Haskell code, you
-could define a function like the following:
-</para>
+-- Import our template "pr"
+import Printf ( pr )
-<para>
+-- The splice operator $ takes the Haskell source code
+-- generated at compile time by "pr" and splices it into
+-- the argument of "putStrLn".
+main = putStrLn ( $(pr "Hello") )
+</programlisting>
<programlisting>
-assert :: Bool -> a -> a
-assert False x = error "assertion failed!"
-assert _ x = x
-</programlisting>
+{- Printf.hs -}
+module Printf where
-</para>
+-- Skeletal printf from the paper.
+-- It needs to be in a separate module to the one where
+-- you intend to use it.
+
+-- Import some Template Haskell syntax
+import Language.Haskell.THSyntax
+
+-- Describe a format string
+data Format = D | S | L String
+
+-- Parse a format string. This is left largely to you
+-- as we are here interested in building our first ever
+-- Template Haskell program and not in building printf.
+parse :: String -> [Format]
+parse s = [ L s ]
+
+-- Generate Haskell source code from a parsed representation
+-- of the format string. This code will be spliced into
+-- the module which calls "pr", at compile time.
+gen :: [Format] -> Expr
+gen [D] = [| \n -> show n |]
+gen [S] = [| \s -> s |]
+gen [L s] = string s
+
+-- Here we generate the Haskell code for the splice
+-- from an input format string.
+pr :: String -> Expr
+pr s = gen (parse s)
+</programlisting>
+
+<para>Now run the compiler (here we are using a "stage three" build of GHC, at a Cygwin prompt on Windows):
+</para>
+<programlisting>
+ghc/compiler/stage3/ghc-inplace --make -fglasgow-exts -package haskell-src main.hs -o main.exe
+</programlisting>
+
+<para>Run "main.exe" and here is your output:
+</para>
+
+<programlisting>
+$ ./main
+Hello
+</programlisting>
+
+</sect2>
+
+</sect1>
+
+<!-- ===================== Arrow notation =================== -->
+
+<sect1 id="arrow-notation">
+<title>Arrow notation
+</title>
+
+<para>Arrows are a generalization of monads introduced by John Hughes.
+For more details, see
+<itemizedlist>
+
+<listitem>
+<para>
+“Generalising Monads to Arrows”,
+John Hughes, in <citetitle>Science of Computer Programming</citetitle> 37,
+pp67–111, May 2000.
+</para>
+</listitem>
+
+<listitem>
+<para>
+“<ulink url="http://www.soi.city.ac.uk/~ross/papers/notation.html">A New Notation for Arrows</ulink>”,
+Ross Paterson, in <citetitle>ICFP</citetitle>, Sep 2001.
+</para>
+</listitem>
+
+<listitem>
+<para>
+“<ulink url="http://www.soi.city.ac.uk/~ross/papers/fop.html">Arrows and Computation</ulink>”,
+Ross Paterson, in <citetitle>The Fun of Programming</citetitle>,
+Palgrave, 2003.
+</para>
+</listitem>
+
+</itemizedlist>
+and the arrows web page at
+<ulink url="http://www.haskell.org/arrows/"><literal>http://www.haskell.org/arrows/</literal></ulink>.
+With the <option>-farrows</option> flag, GHC supports the arrow
+notation described in the second of these papers.
+What follows is a brief introduction to the notation;
+it won't make much sense unless you've read Hughes's paper.
+This notation is translated to ordinary Haskell,
+using combinators from the
+<ulink url="../base/Control.Arrow.html"><literal>Control.Arrow</literal></ulink>
+module.
+</para>
+
+<para>The extension adds a new kind of expression for defining arrows,
+of the form <literal>proc pat -> cmd</literal>,
+where <literal>proc</literal> is a new keyword.
+The variables of the pattern are bound in the body of the
+<literal>proc</literal>-expression,
+which is a new sort of thing called a <firstterm>command</firstterm>.
+The syntax of commands is as follows:
+<screen>
+cmd ::= exp1 -< exp2
+ | exp1 -<< exp2
+ | do { cstmt1 .. cstmtn ; cmd }
+ | let decls in cmd
+ | if exp then cmd1 else cmd2
+ | case exp of { calts }
+ | cmd1 qop cmd2
+ | (| aexp cmd1 .. cmdn |)
+ | \ pat1 .. patn -> cmd
+ | cmd aexp
+ | ( cmd )
+
+cstmt ::= let decls
+ | pat <- cmd
+ | rec { cstmt1 .. cstmtn }
+ | cmd
+</screen>
+Commands produce values, but (like monadic computations)
+may yield more than one value,
+or none, and may do other things as well.
+For the most part, familiarity with monadic notation is a good guide to
+using commands.
+However the values of expressions, even monadic ones,
+are determined by the values of the variables they contain;
+this is not necessarily the case for commands.
+</para>
+
+<para>
+A simple example of the new notation is the expression
+<screen>
+proc x -> f -< x+1
+</screen>
+We call this a <firstterm>procedure</firstterm> or
+<firstterm>arrow abstraction</firstterm>.
+As with a lambda expression, the variable <literal>x</literal>
+is a new variable bound within the <literal>proc</literal>-expression.
+It refers to the input to the arrow.
+In the above example, <literal>-<</literal> is not an identifier but an
+new reserved symbol used for building commands from an expression of arrow
+type and an expression to be fed as input to that arrow.
+(The weird look will make more sense later.)
+It may be read as analogue of application for arrows.
+The above example is equivalent to the Haskell expression
+<screen>
+arr (\ x -> x+1) >>> f
+</screen>
+That would make no sense if the expression to the left of
+<literal>-<</literal> involves the bound variable <literal>x</literal>.
+More generally, the expression to the left of <literal>-<</literal>
+may not involve any <firstterm>local variable</firstterm>,
+i.e. a variable bound in the current arrow abstraction.
+For such a situation there is a variant <literal>-<<</literal>, as in
+<screen>
+proc x -> f x -<< x+1
+</screen>
+which is equivalent to
+<screen>
+arr (\ x -> (f, x+1)) >>> app
+</screen>
+so in this case the arrow must belong to the <literal>ArrowApply</literal>
+class.
+Such an arrow is equivalent to a monad, so if you're using this form
+you may find a monadic formulation more convenient.
+</para>
+
+<sect2>
+<title>do-notation for commands</title>
+
+<para>
+Another form of command is a form of <literal>do</literal>-notation.
+For example, you can write
+<screen>
+proc x -> do
+ y <- f -< x+1
+ g -< 2*y
+ let z = x+y
+ t <- h -< x*z
+ returnA -< t+z
+</screen>
+You can read this much like ordinary <literal>do</literal>-notation,
+but with commands in place of monadic expressions.
+The first line sends the value of <literal>x+1</literal> as an input to
+the arrow <literal>f</literal>, and matches its output against
+<literal>y</literal>.
+In the next line, the output is discarded.
+The arrow <literal>returnA</literal> is defined in the
+<ulink url="../base/Control.Arrow.html"><literal>Control.Arrow</literal></ulink>
+module as <literal>arr id</literal>.
+The above example is treated as an abbreviation for
+<screen>
+arr (\ x -> (x, x)) >>>
+ first (arr (\ x -> x+1) >>> f) >>>
+ arr (\ (y, x) -> (y, (x, y))) >>>
+ first (arr (\ y -> 2*y) >>> g) >>>
+ arr snd >>>
+ arr (\ (x, y) -> let z = x+y in ((x, z), z)) >>>
+ first (arr (\ (x, z) -> x*z) >>> h) >>>
+ arr (\ (t, z) -> t+z) >>>
+ returnA
+</screen>
+Note that variables not used later in the composition are projected out.
+After simplification using rewrite rules (see <xref linkEnd="rewrite-rules">)
+defined in the
+<ulink url="../base/Control.Arrow.html"><literal>Control.Arrow</literal></ulink>
+module, this reduces to
+<screen>
+arr (\ x -> (x+1, x)) >>>
+ first f >>>
+ arr (\ (y, x) -> (2*y, (x, y))) >>>
+ first g >>>
+ arr (\ (_, (x, y)) -> let z = x+y in (x*z, z)) >>>
+ first h >>>
+ arr (\ (t, z) -> t+z)
+</screen>
+which is what you might have written by hand.
+With arrow notation, GHC keeps track of all those tuples of variables for you.
+</para>
+
+<para>
+Note that although the above translation suggests that
+<literal>let</literal>-bound variables like <literal>z</literal> must be
+monomorphic, the actual translation produces Core,
+so polymorphic variables are allowed.
+</para>
+
+<para>
+It's also possible to have mutually recursive bindings,
+using the new <literal>rec</literal> keyword, as in the following example:
+<screen>
+counter :: ArrowCircuit a => a Bool Int
+counter = proc reset -> do
+ rec output <- returnA -< if reset then 0 else next
+ next <- delay 0 -< output+1
+ returnA -< output
+</screen>
+The translation of such forms uses the <literal>loop</literal> combinator,
+so the arrow concerned must belong to the <literal>ArrowLoop</literal> class.
+</para>
+
+</sect2>
+
+<sect2>
+<title>Conditional commands</title>
+
+<para>
+In the previous example, we used a conditional expression to construct the
+input for an arrow.
+Sometimes we want to conditionally execute different commands, as in
+<screen>
+proc (x,y) ->
+ if f x y
+ then g -< x+1
+ else h -< y+2
+</screen>
+which is translated to
+<screen>
+arr (\ (x,y) -> if f x y then Left x else Right y) >>>
+ (arr (\x -> x+1) >>> f) ||| (arr (\y -> y+2) >>> g)
+</screen>
+Since the translation uses <literal>|||</literal>,
+the arrow concerned must belong to the <literal>ArrowChoice</literal> class.
+</para>
+
+<para>
+There are also <literal>case</literal> commands, like
+<screen>
+case input of
+ [] -> f -< ()
+ [x] -> g -< x+1
+ x1:x2:xs -> do
+ y <- h -< (x1, x2)
+ ys <- k -< xs
+ returnA -< y:ys
+</screen>
+The syntax is the same as for <literal>case</literal> expressions,
+except that the bodies of the alternatives are commands rather than expressions.
+The translation is similar to that of <literal>if</literal> commands.
+</para>
+
+</sect2>
+
+<sect2>
+<title>Defining your own control structures</title>
+
+<para>
+As we're seen, arrow notation provides constructs,
+modelled on those for expressions,
+for sequencing, value recursion and conditionals.
+But suitable combinators,
+which you can define in ordinary Haskell,
+may also be used to build new commands out of existing ones.
+The basic idea is that a command defines an arrow from environments to values.
+These environments assign values to the free local variables of the command.
+Thus combinators that produce arrows from arrows
+may also be used to build commands from commands.
+For example, the <literal>ArrowChoice</literal> class includes a combinator
+<programlisting>
+ArrowChoice a => (<+>) :: a e c -> a e c -> a e c
+</programlisting>
+so we can use it to build commands:
+<programlisting>
+expr' = proc x ->
+ returnA -< x
+ <+> do
+ symbol Plus -< ()
+ y <- term -< ()
+ expr' -< x + y
+ <+> do
+ symbol Minus -< ()
+ y <- term -< ()
+ expr' -< x - y
+</programlisting>
+This is equivalent to
+<programlisting>
+expr' = (proc x -> returnA -< x)
+ <+> (proc x -> do
+ symbol Plus -< ()
+ y <- term -< ()
+ expr' -< x + y)
+ <+> (proc x -> do
+ symbol Minus -< ()
+ y <- term -< ()
+ expr' -< x - y)
+</programlisting>
+It is essential that this operator be polymorphic in <literal>e</literal>
+(representing the environment input to the command
+and thence to its subcommands)
+and satisfy the corresponding naturality property
+<screen>
+arr k >>> (f <+> g) = (arr k >>> f) <+> (arr k >>> g)
+</screen>
+at least for strict <literal>k</literal>.
+(This should be automatic if you're not using <literal>seq</literal>.)
+This ensures that environments seen by the subcommands are environments
+of the whole command,
+and also allows the translation to safely trim these environments.
+The operator must also not use any variable defined within the current
+arrow abstraction.
+</para>
+
+<para>
+We could define our own operator
+<programlisting>
+untilA :: ArrowChoice a => a e () -> a e Bool -> a e ()
+untilA body cond = proc x ->
+ if cond x then returnA -< ()
+ else do
+ body -< x
+ untilA body cond -< x
+</programlisting>
+and use it in the same way.
+Of course this infix syntax only makes sense for binary operators;
+there is also a more general syntax involving special brackets:
+<screen>
+proc x -> do
+ y <- f -< x+1
+ (|untilA (increment -< x+y) (within 0.5 -< x)|)
+</screen>
+</para>
+
+</sect2>
+
+<sect2>
+<title>Primitive constructs</title>
+
+<para>
+Some operators will need to pass additional inputs to their subcommands.
+For example, in an arrow type supporting exceptions,
+the operator that attaches an exception handler will wish to pass the
+exception that occurred to the handler.
+Such an operator might have a type
+<screen>
+handleA :: ... => a e c -> a (e,Ex) c -> a e c
+</screen>
+where <literal>Ex</literal> is the type of exceptions handled.
+You could then use this with arrow notation by writing a command
+<screen>
+body `handleA` \ ex -> handler
+</screen>
+so that if an exception is raised in the command <literal>body</literal>,
+the variable <literal>ex</literal> is bound to the value of the exception
+and the command <literal>handler</literal>,
+which typically refers to <literal>ex</literal>, is entered.
+Though the syntax here looks like a functional lambda,
+we are talking about commands, and something different is going on.
+The input to the arrow represented by a command consists of values for
+the free local variables in the command, plus a stack of anonymous values.
+In all the prior examples, this stack was empty.
+In the second argument to <literal>handleA</literal>,
+this stack consists of one value, the value of the exception.
+The command form of lambda merely gives this value a name.
+</para>
+
+<para>
+More concretely,
+the values on the stack are paired to the right of the environment.
+So when designing operators like <literal>handleA</literal> that pass
+extra inputs to their subcommands,
+More precisely, the type of each argument of the operator (and its result)
+should have the form
+<screen>
+a (...(e,t1), ... tn) t
+</screen>
+where <replaceable>e</replaceable> is a polymorphic variable
+(representing the environment)
+and <replaceable>ti</replaceable> are the types of the values on the stack,
+with <replaceable>t1</replaceable> being the <quote>top</quote>.
+The polymorphic variable <replaceable>e</replaceable> must not occur in
+<replaceable>a</replaceable>, <replaceable>ti</replaceable> or
+<replaceable>t</replaceable>.
+However the arrows involved need not be the same.
+Here are some more examples of suitable operators:
+<screen>
+bracketA :: ... => a e b -> a (e,b) c -> a (e,c) d -> a e d
+runReader :: ... => a e c -> a' (e,State) c
+runState :: ... => a e c -> a' (e,State) (c,State)
+</screen>
+We can supply the extra input required by commands built with the last two
+by applying them to ordinary expressions, as in
+<screen>
+proc x -> do
+ s <- ...
+ (|runReader (do { ... })|) s
+</screen>
+which adds <literal>s</literal> to the stack of inputs to the command
+built using <literal>runReader</literal>.
+</para>
+
+<para>
+The command versions of lambda abstraction and application are analogous to
+the expression versions.
+In particular, the beta and eta rules describe equivalences of commands.
+These three features (operators, lambda abstraction and application)
+are the core of the notation; everything else can be built using them,
+though the results would be somewhat clumsy.
+For example, we could simulate <literal>do</literal>-notation by defining
+<programlisting>
+bind :: Arrow a => a e b -> a (e,b) c -> a e c
+u `bind` f = returnA &&& u >>> f
+
+bind_ :: Arrow a => a e b -> a e c -> a e c
+u `bind_` f = u `bind` (arr fst >>> f)
+</programlisting>
+We could simulate <literal>do</literal> by defining
+<programlisting>
+cond :: ArrowChoice a => a e b -> a e b -> a (e,Bool) b
+cond f g = arr (\ (e,b) -> if b then Left e else Right e) >>> f ||| g
+</programlisting>
+</para>
+
+</sect2>
+
+<sect2>
+<title>Differences with the paper</title>
+
+<itemizedlist>
+
+<listitem>
+<para>Instead of a single form of arrow application (arrow tail) with two
+translations, the implementation provides two forms
+<quote><literal>-<</literal></quote> (first-order)
+and <quote><literal>-<<</literal></quote> (higher-order).
+</para>
+</listitem>
+
+<listitem>
+<para>User-defined operators are flagged with banana brackets instead of
+a new <literal>form</literal> keyword.
+</para>
+</listitem>
+
+</itemizedlist>
+
+</sect2>
+
+<sect2>
+<title>Portability</title>
+
+<para>
+Although only GHC implements arrow notation directly,
+there is also a preprocessor
+(available from the
+<ulink url="http://www.haskell.org/arrows/">arrows web page></ulink>)
+that translates arrow notation into Haskell 98
+for use with other Haskell systems.
+You would still want to check arrow programs with GHC;
+tracing type errors in the preprocessor output is not easy.
+Modules intended for both GHC and the preprocessor must observe some
+additional restrictions:
+<itemizedlist>
+
+<listitem>
+<para>
+The module must import
+<ulink url="../base/Control.Arrow.html"><literal>Control.Arrow</literal></ulink>.
+</para>
+</listitem>
+
+<listitem>
+<para>
+The preprocessor cannot cope with other Haskell extensions.
+These would have to go in separate modules.
+</para>
+</listitem>
+
+<listitem>
+<para>
+Because the preprocessor targets Haskell (rather than Core),
+<literal>let</literal>-bound variables are monomorphic.
+</para>
+</listitem>
+
+</itemizedlist>
+</para>
+
+</sect2>
+
+</sect1>
+
+<!-- ==================== ASSERTIONS ================= -->
+
+<sect1 id="sec-assertions">
+<title>Assertions
+<indexterm><primary>Assertions</primary></indexterm>
+</title>
+
+<para>
+If you want to make use of assertions in your standard Haskell code, you
+could define a function like the following:
+</para>
+
+<para>
+
+<programlisting>
+assert :: Bool -> a -> a
+assert False x = error "assertion failed!"
+assert _ x = x
+</programlisting>
+
+</para>
<para>
which works, but gives you back a less than useful error message --
unrecognised <replaceable>word</replaceable> is (silently)
ignored.</para>
-<sect2 id="inline-pragma">
-<title>INLINE pragma
+ <sect2 id="deprecated-pragma">
+ <title>DEPRECATED pragma</title>
+ <indexterm><primary>DEPRECATED</primary>
+ </indexterm>
-<indexterm><primary>INLINE pragma</primary></indexterm>
-<indexterm><primary>pragma, INLINE</primary></indexterm></title>
+ <para>The DEPRECATED pragma lets you specify that a particular
+ function, class, or type, is deprecated. There are two
+ forms.</para>
-<para>
-GHC (with <option>-O</option>, as always) tries to inline (or “unfold”)
-functions/values that are “small enough,” thus avoiding the call
-overhead and possibly exposing other more-wonderful optimisations.
-</para>
+ <itemizedlist>
+ <listitem>
+ <para>You can deprecate an entire module thus:</para>
+<programlisting>
+ module Wibble {-# DEPRECATED "Use Wobble instead" #-} where
+ ...
+</programlisting>
+ <para>When you compile any module that import
+ <literal>Wibble</literal>, GHC will print the specified
+ message.</para>
+ </listitem>
-<para>
-You will probably see these unfoldings (in Core syntax) in your
-interface files.
-</para>
+ <listitem>
+ <para>You can deprecate a function, class, or type, with the
+ following top-level declaration:</para>
+<programlisting>
+ {-# DEPRECATED f, C, T "Don't use these" #-}
+</programlisting>
+ <para>When you compile any module that imports and uses any
+ of the specifed entities, GHC will print the specified
+ message.</para>
+ </listitem>
+ </itemizedlist>
-<para>
-Normally, if GHC decides a function is “too expensive” to inline, it
-will not do so, nor will it export that unfolding for other modules to
-use.
-</para>
+ <para>You can suppress the warnings with the flag
+ <option>-fno-warn-deprecations</option>.</para>
+ </sect2>
-<para>
-The sledgehammer you can bring to bear is the
-<literal>INLINE</literal><indexterm><primary>INLINE pragma</primary></indexterm> pragma, used thusly:
+ <sect2 id="inline-noinline-pragma">
+ <title>INLINE and NOINLINE pragmas</title>
+
+ <para>These pragmas control the inlining of function
+ definitions.</para>
+
+ <sect3 id="inline-pragma">
+ <title>INLINE pragma</title>
+ <indexterm><primary>INLINE</primary></indexterm>
+
+ <para>GHC (with <option>-O</option>, as always) tries to
+ inline (or “unfold”) functions/values that are
+ “small enough,” thus avoiding the call overhead
+ and possibly exposing other more-wonderful optimisations.
+ Normally, if GHC decides a function is “too
+ expensive” to inline, it will not do so, nor will it
+ export that unfolding for other modules to use.</para>
+
+ <para>The sledgehammer you can bring to bear is the
+ <literal>INLINE</literal><indexterm><primary>INLINE
+ pragma</primary></indexterm> pragma, used thusly:</para>
<programlisting>
key_function :: Int -> String -> (Bool, Double)
#endif
</programlisting>
-(You don't need to do the C pre-processor carry-on unless you're going
-to stick the code through HBC—it doesn't like <literal>INLINE</literal> pragmas.)
-</para>
+ <para>(You don't need to do the C pre-processor carry-on
+ unless you're going to stick the code through HBC—it
+ doesn't like <literal>INLINE</literal> pragmas.)</para>
-<para>
-The major effect of an <literal>INLINE</literal> pragma is to declare a function's
-“cost” to be very low. The normal unfolding machinery will then be
-very keen to inline it.
-</para>
+ <para>The major effect of an <literal>INLINE</literal> pragma
+ is to declare a function's “cost” to be very low.
+ The normal unfolding machinery will then be very keen to
+ inline it.</para>
-<para>
-An <literal>INLINE</literal> pragma for a function can be put anywhere its type
-signature could be put.
-</para>
+ <para>Syntactially, an <literal>INLINE</literal> pragma for a
+ function can be put anywhere its type signature could be
+ put.</para>
-<para>
-<literal>INLINE</literal> pragmas are a particularly good idea for the
-<literal>then</literal>/<literal>return</literal> (or <literal>bind</literal>/<literal>unit</literal>) functions in a monad.
-For example, in GHC's own <literal>UniqueSupply</literal> monad code, we have:
+ <para><literal>INLINE</literal> pragmas are a particularly
+ good idea for the
+ <literal>then</literal>/<literal>return</literal> (or
+ <literal>bind</literal>/<literal>unit</literal>) functions in
+ a monad. For example, in GHC's own
+ <literal>UniqueSupply</literal> monad code, we have:</para>
<programlisting>
#ifdef __GLASGOW_HASKELL__
#endif
</programlisting>
-</para>
+ <para>See also the <literal>NOINLINE</literal> pragma (<xref
+ linkend="noinline-pragma">).</para>
+ </sect3>
+
+ <sect3 id="noinline-pragma">
+ <title>NOINLINE pragma</title>
+
+ <indexterm><primary>NOINLINE</primary></indexterm>
+ <indexterm><primary>NOTINLINE</primary></indexterm>
+
+ <para>The <literal>NOINLINE</literal> pragma does exactly what
+ you'd expect: it stops the named function from being inlined
+ by the compiler. You shouldn't ever need to do this, unless
+ you're very cautious about code size.</para>
+
+ <para><literal>NOTINLINE</literal> is a synonym for
+ <literal>NOINLINE</literal> (<literal>NOTINLINE</literal> is
+ specified by Haskell 98 as the standard way to disable
+ inlining, so it should be used if you want your code to be
+ portable).</para>
+ </sect3>
+
+ <sect3 id="phase-control">
+ <title>Phase control</title>
+
+ <para> Sometimes you want to control exactly when in GHC's
+ pipeline the INLINE pragma is switched on. Inlining happens
+ only during runs of the <emphasis>simplifier</emphasis>. Each
+ run of the simplifier has a different <emphasis>phase
+ number</emphasis>; the phase number decreases towards zero.
+ If you use <option>-dverbose-core2core</option> you'll see the
+ sequence of phase numbers for successive runs of the
+ simpifier. In an INLINE pragma you can optionally specify a
+ phase number, thus:</para>
-</sect2>
+ <itemizedlist>
+ <listitem>
+ <para>You can say "inline <literal>f</literal> in Phase 2
+ and all subsequent phases":
+<programlisting>
+ {-# INLINE [2] f #-}
+</programlisting>
+ </para>
+ </listitem>
-<sect2 id="noinline-pragma">
-<title>NOINLINE pragma
-</title>
+ <listitem>
+ <para>You can say "inline <literal>g</literal> in all
+ phases up to, but not including, Phase 3":
+<programlisting>
+ {-# INLINE [~3] g #-}
+</programlisting>
+ </para>
+ </listitem>
-<indexterm><primary>NOINLINE pragma</primary></indexterm>
-<indexterm><primary>pragma</primary><secondary>NOINLINE</secondary></indexterm>
-<indexterm><primary>NOTINLINE pragma</primary></indexterm>
-<indexterm><primary>pragma</primary><secondary>NOTINLINE</secondary></indexterm>
+ <listitem>
+ <para>If you omit the phase indicator, you mean "inline in
+ all phases".</para>
+ </listitem>
+ </itemizedlist>
-<para>
-The <literal>NOINLINE</literal> pragma does exactly what you'd expect:
-it stops the named function from being inlined by the compiler. You
-shouldn't ever need to do this, unless you're very cautious about code
-size.
-</para>
+ <para>You can use a phase number on a NOINLINE pragma too:</para>
-<para><literal>NOTINLINE</literal> is a synonym for
-<literal>NOINLINE</literal> (<literal>NOTINLINE</literal> is specified
-by Haskell 98 as the standard way to disable inlining, so it should be
-used if you want your code to be portable).</para>
+ <itemizedlist>
+ <listitem>
+ <para>You can say "do not inline <literal>f</literal>
+ until Phase 2; in Phase 2 and subsequently behave as if
+ there was no pragma at all":
+<programlisting>
+ {-# NOINLINE [2] f #-}
+</programlisting>
+ </para>
+ </listitem>
-</sect2>
+ <listitem>
+ <para>You can say "do not inline <literal>g</literal> in
+ Phase 3 or any subsequent phase; before that, behave as if
+ there was no pragma":
+<programlisting>
+ {-# NOINLINE [~3] g #-}
+</programlisting>
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>If you omit the phase indicator, you mean "never
+ inline this function".</para>
+ </listitem>
+ </itemizedlist>
+
+ <para>The same phase-numbering control is available for RULES
+ (<xref LinkEnd="rewrite-rules">).</para>
+ </sect3>
+ </sect2>
+
+ <sect2 id="line-pragma">
+ <title>LINE pragma</title>
+
+ <indexterm><primary>LINE</primary><secondary>pragma</secondary></indexterm>
+ <indexterm><primary>pragma</primary><secondary>LINE</secondary></indexterm>
+ <para>This pragma is similar to C's <literal>#line</literal>
+ pragma, and is mainly for use in automatically generated Haskell
+ code. It lets you specify the line number and filename of the
+ original code; for example</para>
+
+<programlisting>
+{-# LINE 42 "Foo.vhs" #-}
+</programlisting>
+
+ <para>if you'd generated the current file from something called
+ <filename>Foo.vhs</filename> and this line corresponds to line
+ 42 in the original. GHC will adjust its error messages to refer
+ to the line/file named in the <literal>LINE</literal>
+ pragma.</para>
+ </sect2>
+
+ <sect2 id="options-pragma">
+ <title>OPTIONS pragma</title>
+ <indexterm><primary>OPTIONS</primary>
+ </indexterm>
+ <indexterm><primary>pragma</primary><secondary>OPTIONS</secondary>
+ </indexterm>
+
+ <para>The <literal>OPTIONS</literal> pragma is used to specify
+ additional options that are given to the compiler when compiling
+ this source file. See <xref linkend="source-file-options"> for
+ details.</para>
+ </sect2>
+
+ <sect2 id="rules">
+ <title>RULES pragma</title>
+
+ <para>The RULES pragma lets you specify rewrite rules. It is
+ described in <xref LinkEnd="rewrite-rules">.</para>
+ </sect2>
<sect2 id="specialize-pragma">
<title>SPECIALIZE pragma</title>
{-# SPECIALIZE hammeredLookup :: [(Widget, value)] -> Widget -> value #-}
</programlisting>
+ <para>A <literal>SPECIALIZE</literal> pragma for a function can
+ be put anywhere its type signature could be put.</para>
+
<para>To get very fancy, you can also specify a named function
to use for the specialised value, as in:</para>
<programlisting>
-{-# RULES hammeredLookup = blah #-}
+{-# RULES "hammeredLookup" hammeredLookup = blah #-}
</programlisting>
<para>where <literal>blah</literal> is an implementation of
toDouble :: Real a => a -> Double
toDouble = fromRational . toRational
-{-# SPECIALIZE toDouble :: Int -> Double = i2d #-}
+{-# RULES "toDouble/Int" toDouble = i2d #-}
i2d (I# i) = D# (int2Double# i) -- uses Glasgow prim-op directly
</programlisting>
<literal>Rational</literal>—is obscenely expensive by
comparison.</para>
- <para>A <literal>SPECIALIZE</literal> pragma for a function can
- be put anywhere its type signature could be put.</para>
-
</sect2>
<sect2 id="specialize-instance-pragma">
</sect2>
-<sect2 id="line-pragma">
-<title>LINE pragma
-</title>
-
-<para>
-<indexterm><primary>LINE pragma</primary></indexterm>
-<indexterm><primary>pragma, LINE</primary></indexterm>
-</para>
-
-<para>
-This pragma is similar to C's <literal>#line</literal> pragma, and is mainly for use in
-automatically generated Haskell code. It lets you specify the line
-number and filename of the original code; for example
-</para>
-
-<para>
-
-<programlisting>
-{-# LINE 42 "Foo.vhs" #-}
-</programlisting>
-
-</para>
-
-<para>
-if you'd generated the current file from something called <filename>Foo.vhs</filename>
-and this line corresponds to line 42 in the original. GHC will adjust
-its error messages to refer to the line/file named in the <literal>LINE</literal>
-pragma.
-</para>
-
-</sect2>
-
-<sect2 id="rules">
-<title>RULES pragma</title>
-
-<para>
-The RULES pragma lets you specify rewrite rules. It is described in
-<xref LinkEnd="rewrite-rules">.
-</para>
-
-</sect2>
-
-<sect2 id="deprecated-pragma">
-<title>DEPRECATED pragma</title>
-<para>
-The DEPRECATED pragma lets you specify that a particular function, class, or type, is deprecated.
-There are two forms.
-</para>
-<itemizedlist>
-<listitem><para>
-You can deprecate an entire module thus:</para>
-<programlisting>
- module Wibble {-# DEPRECATED "Use Wobble instead" #-} where
- ...
-</programlisting>
-<para>
-When you compile any module that import <literal>Wibble</literal>, GHC will print
-the specified message.</para>
-</listitem>
-
-<listitem>
-<para>
-You can deprecate a function, class, or type, with the following top-level declaration:
-</para>
-<programlisting>
- {-# DEPRECATED f, C, T "Don't use these" #-}
-</programlisting>
-<para>
-When you compile any module that imports and uses any of the specifed entities,
-GHC will print the specified message.
-</para>
-</listitem>
-</itemizedlist>
-<para>You can suppress the warnings with the flag <option>-fno-warn-deprecations</option>.</para>
-
-</sect2>
</sect1>
<listitem>
<para>
+ There may be zero or more rules in a <literal>RULES</literal> pragma.
+</para>
+</listitem>
+
+<listitem>
+
+<para>
Each rule has a name, enclosed in double quotes. The name itself has
no significance at all. It is only used when reporting how many times the rule fired.
</para>
</listitem>
-<listitem>
+<listitem>
<para>
- There may be zero or more rules in a <literal>RULES</literal> pragma.
+A rule may optionally have a phase-control number (see <xref LinkEnd="phase-control">),
+immediately after the name of the rule. Thus:
+<programlisting>
+ {-# RULES
+ "map/map" [2] forall f g xs. map f (map g xs) = map (f.g) xs
+ #-}
+</programlisting>
+The "[2]" means that the rule is active in Phase 2 and subsequent phases. The inverse
+notation "[~2]" is also accepted, meaning that the rule is active up to, but not including,
+Phase 2.
</para>
</listitem>
+
+
<listitem>
<para>
enclosing definitions.
</para>
</listitem>
+
<listitem>
<para>
<listitem>
<para>
- The defintion of (say) <function>build</function> in <FileName>PrelBase.lhs</FileName> looks llike this:
+ The defintion of (say) <function>build</function> in <FileName>GHC/Base.lhs</FileName> looks llike this:
<programlisting>
build :: forall a. (forall b. (a -> b -> b) -> b -> b) -> [a]
<listitem>
<para>
- In <filename>ghc/lib/std/PrelBase.lhs</filename> look at the rules for <function>map</function> to
+ In <filename>libraries/base/GHC/Base.lhs</filename> look at the rules for <function>map</function> to
see how to write rules that will do fusion and yet give an efficient
-program even if fusion doesn't happen. More rules in <filename>PrelList.lhs</filename>.
+program even if fusion doesn't happen. More rules in <filename>GHC/List.lhs</filename>.
</para>
</listitem>
</sect2>
+<sect2 id="core-pragma">
+ <title>CORE pragma</title>
+
+ <indexterm><primary>CORE pragma</primary></indexterm>
+ <indexterm><primary>pragma, CORE</primary></indexterm>
+ <indexterm><primary>core, annotation</primary></indexterm>
+
+<para>
+ The external core format supports <quote>Note</quote> annotations;
+ the <literal>CORE</literal> pragma gives a way to specify what these
+ should be in your Haskell source code. Syntactically, core
+ annotations are attached to expressions and take a Haskell string
+ literal as an argument. The following function definition shows an
+ example:
+
+<programlisting>
+f x = ({-# CORE "foo" #-} show) ({-# CORE "bar" #-} x)
+</programlisting>
+
+ Sematically, this is equivalent to:
+
+<programlisting>
+g x = show x
+</programlisting>
+</para>
+
+<para>
+ However, when external for is generated (via
+ <option>-fext-core</option>), there will be Notes attached to the
+ expressions <function>show</function> and <VarName>x</VarName>.
+ The core function declaration for <function>f</function> is:
+</para>
+
+<programlisting>
+ f :: %forall a . GHCziShow.ZCTShow a ->
+ a -> GHCziBase.ZMZN GHCziBase.Char =
+ \ @ a (zddShow::GHCziShow.ZCTShow a) (eta::a) ->
+ (%note "foo"
+ %case zddShow %of (tpl::GHCziShow.ZCTShow a)
+ {GHCziShow.ZCDShow
+ (tpl1::GHCziBase.Int ->
+ a ->
+ GHCziBase.ZMZN GHCziBase.Char -> GHCziBase.ZMZN GHCziBase.Cha
+r)
+ (tpl2::a -> GHCziBase.ZMZN GHCziBase.Char)
+ (tpl3::GHCziBase.ZMZN a ->
+ GHCziBase.ZMZN GHCziBase.Char -> GHCziBase.ZMZN GHCziBase.Cha
+r) ->
+ tpl2})
+ (%note "foo"
+ eta);
+</programlisting>
+
+<para>
+ Here, we can see that the function <function>show</function> (which
+ has been expanded out to a case expression over the Show dictionary)
+ has a <literal>%note</literal> attached to it, as does the
+ expression <VarName>eta</VarName> (which used to be called
+ <VarName>x</VarName>).
+</para>
+
+</sect2>
+
</sect1>
<sect1 id="generic-classes">
instance (Bin a, Bin b) => Bin (a,b)
instance Bin a => Bin [a]
</programlisting>
-That is, just leave off the "where" clasuse. Of course, you can put in the
+That is, just leave off the "where" clause. Of course, you can put in the
where clause and over-ride whichever methods you please.
</para>
projects / ghc-hetmet.git / blobdiff