<indexterm><primary>extensions</primary><secondary>options controlling</secondary>
</indexterm>
- <para>These flags control what variation of the language are
+ <para>The language option flag control what variation of the language are
permitted. Leaving out all of them gives you standard Haskell
98.</para>
- <para>NB. turning on an option that enables special syntax
+ <para>Generally speaking, all the language options are introduced by "<option>-X</option>",
+ e.g. <option>-XTemplateHaskell</option>.
+ </para>
+
+ <para> All the language options can be turned off by using the prefix "<option>No</option>";
+ e.g. "<option>-XNoTemplateHaskell</option>".</para>
+
+ <para> Language options recognised by Cabal can also be enabled using the <literal>LANGUAGE</literal> pragma,
+ thus <literal>{-# LANGUAGE TemplateHaskell #-}</literal> (see <xref linkend="language-pragma"/>>). </para>
+
+ <para>Turning on an option that enables special syntax
<emphasis>might</emphasis> cause working Haskell 98 code to fail
to compile, perhaps because it uses a variable name which has
become a reserved word. So, together with each option below, we
<para>This simultaneously enables all of the extensions to
Haskell 98 described in <xref
linkend="ghc-language-features"/>, except where otherwise
- noted. </para>
+ noted. We are trying to move away from this portmanteau flag,
+ and towards enabling features individaully.</para>
<para>New reserved words: <literal>forall</literal> (only in
types), <literal>mdo</literal>.</para>
<replaceable>float</replaceable><literal>##</literal>,
<literal>(#</literal>, <literal>#)</literal>,
<literal>|)</literal>, <literal>{|</literal>.</para>
+
+ <para>Implies these specific language options:
+ <option>-XForeignFunctionInterface</option>,
+ <option>-XImplicitParams</option>,
+ <option>-XScopedTypeVariables</option>,
+ <option>-XGADTs</option>,
+ <option>-XTypeFamilies</option>. </para>
</listitem>
</varlistentry>
<varlistentry>
<term>
- <option>-ffi</option> and <option>-fffi</option>:
- <indexterm><primary><option>-ffi</option></primary></indexterm>
- <indexterm><primary><option>-fffi</option></primary></indexterm>
+ <option>-XForeignFunctionInterface</option>:
+ <indexterm><primary><option>-XForeignFunctionInterface</option></primary></indexterm>
</term>
<listitem>
<para>This option enables the language extension defined in the
<varlistentry>
<term>
- <option>-fno-monomorphism-restriction</option>,<option>-fno-mono-pat-binds</option>:
+ <option>-XMonomorphismRestriction</option>,<option>-XMonoPatBinds</option>:
</term>
<listitem>
<para> These two flags control how generalisation is done.
<varlistentry>
<term>
- <option>-fextended-default-rules</option>:
- <indexterm><primary><option>-fextended-default-rules</option></primary></indexterm>
+ <option>-XExtendedDefaultRules</option>:
+ <indexterm><primary><option>-XExtendedDefaultRules</option></primary></indexterm>
</term>
<listitem>
<para> Use GHCi's extended default rules in a regular module (<xref linkend="extended-default-rules"/>).
<varlistentry>
<term>
- <option>-fallow-overlapping-instances</option>
- <indexterm><primary><option>-fallow-overlapping-instances</option></primary></indexterm>
+ <option>-XOverlappingInstances</option>
+ <indexterm><primary><option>-XOverlappingInstances</option></primary></indexterm>
</term>
<term>
- <option>-fallow-undecidable-instances</option>
- <indexterm><primary><option>-fallow-undecidable-instances</option></primary></indexterm>
+ <option>-XUndecidableInstances</option>
+ <indexterm><primary><option>-XUndecidableInstances</option></primary></indexterm>
</term>
<term>
- <option>-fallow-incoherent-instances</option>
- <indexterm><primary><option>-fallow-incoherent-instances</option></primary></indexterm>
+ <option>-XIncoherentInstances</option>
+ <indexterm><primary><option>-XIncoherentInstances</option></primary></indexterm>
</term>
<term>
<option>-fcontext-stack=N</option>
<varlistentry>
<term>
- <option>-farrows</option>
- <indexterm><primary><option>-farrows</option></primary></indexterm>
+ <option>-XArrows</option>
+ <indexterm><primary><option>-XArrows</option></primary></indexterm>
</term>
<listitem>
<para>See <xref linkend="arrow-notation"/>. Independent of
<varlistentry>
<term>
- <option>-fgenerics</option>
- <indexterm><primary><option>-fgenerics</option></primary></indexterm>
+ <option>-XGenerics</option>
+ <indexterm><primary><option>-XGenerics</option></primary></indexterm>
</term>
<listitem>
<para>See <xref linkend="generic-classes"/>. Independent of
</varlistentry>
<varlistentry>
- <term><option>-fno-implicit-prelude</option></term>
+ <term><option>-XNoImplicitPrelude</option></term>
<listitem>
- <para><indexterm><primary>-fno-implicit-prelude
+ <para><indexterm><primary>-XNoImplicitPrelude
option</primary></indexterm> GHC normally imports
<filename>Prelude.hi</filename> files for you. If you'd
rather it didn't, then give it a
- <option>-fno-implicit-prelude</option> option. The idea is
+ <option>-XNoImplicitPrelude</option> option. The idea is
that you can then import a Prelude of your own. (But don't
call it <literal>Prelude</literal>; the Haskell module
namespace is flat, and you must not conflict with any
translation for list comprehensions continues to use
<literal>Prelude.map</literal> etc.</para>
- <para>However, <option>-fno-implicit-prelude</option> does
+ <para>However, <option>-XNoImplicitPrelude</option> does
change the handling of certain built-in syntax: see <xref
linkend="rebindable-syntax"/>.</para>
</listitem>
</varlistentry>
<varlistentry>
- <term><option>-fimplicit-params</option></term>
+ <term><option>-XImplicitParams</option></term>
<listitem>
<para>Enables implicit parameters (see <xref
linkend="implicit-parameters"/>). Currently also implied by
</varlistentry>
<varlistentry>
- <term><option>-foverloaded-strings</option></term>
+ <term><option>-XOverloadedStrings</option></term>
<listitem>
<para>Enables overloaded string literals (see <xref
linkend="overloaded-strings"/>).</para>
</varlistentry>
<varlistentry>
- <term><option>-fscoped-type-variables</option></term>
+ <term><option>-XScopedTypeVariables</option></term>
<listitem>
<para>Enables lexically-scoped type variables (see <xref
linkend="scoped-type-variables"/>). Implied by
</varlistentry>
<varlistentry>
- <term><option>-fth</option></term>
+ <term><option>-XTemplateHaskell</option></term>
<listitem>
<para>Enables Template Haskell (see <xref
linkend="template-haskell"/>). This flag must
</sect1>
<!-- UNBOXED TYPES AND PRIMITIVE OPERATIONS -->
-<!-- included from primitives.sgml -->
-<!-- &primitives; -->
<sect1 id="primitives">
<title>Unboxed types and primitive operations</title>
(<literal>Double#</literal> for instance).
</para>
</listitem>
+<listitem><para> You cannot define a newtype whose representation type
+(the argument type of the data constructor) is an unboxed type. Thus,
+this is illegal:
+<programlisting>
+ newtype A = MkA Int#
+</programlisting>
+</para></listitem>
<listitem><para> You cannot bind a variable with an unboxed type
in a <emphasis>top-level</emphasis> binding.
</para></listitem>
linkend="search-path"/>.</para>
<para>GHC comes with a large collection of libraries arranged
- hierarchically; see the accompanying library documentation.
- There is an ongoing project to create and maintain a stable set
- of <quote>core</quote> libraries used by several Haskell
- compilers, and the libraries that GHC comes with represent the
- current status of that project. For more details, see <ulink
- url="http://www.haskell.org/~simonmar/libraries/libraries.html">Haskell
- Libraries</ulink>.</para>
-
+ hierarchically; see the accompanying <ulink
+ url="../libraries/index.html">library
+ documentation</ulink>. More libraries to install are available
+ from <ulink
+ url="http://hackage.haskell.org/packages/hackage.html">HackageDB</ulink>.</para>
</sect2>
<!-- ====================== PATTERN GUARDS ======================= -->
</title>
<para> The recursive do-notation (also known as mdo-notation) is implemented as described in
-"A recursive do for Haskell",
-Levent Erkok, John Launchbury",
+<ulink url="http://citeseer.ist.psu.edu/erk02recursive.html">A recursive do for Haskell</ulink>,
+by Levent Erkok, John Launchbury,
Haskell Workshop 2002, pages: 29-37. Pittsburgh, Pennsylvania.
+This paper is essential reading for anyone making non-trivial use of mdo-notation,
+and we do not repeat it here.
</para>
<para>
The do-notation of Haskell does not allow <emphasis>recursive bindings</emphasis>,
</programlisting>
<para>
The function <literal>mfix</literal>
-dictates how the required recursion operation should be performed. If recursive bindings are required for a monad,
-then that monad must be declared an instance of the <literal>MonadFix</literal> class.
-For details, see the above mentioned reference.
+dictates how the required recursion operation should be performed. For example,
+<literal>justOnes</literal> desugars as follows:
+<programlisting>
+justOnes = mfix (\xs' -> do { xs <- Just (1:xs'); return xs }
+</programlisting>
+For full details of the way in which mdo is typechecked and desugared, see
+the paper <ulink url="http://citeseer.ist.psu.edu/erk02recursive.html">A recursive do for Haskell</ulink>.
+In particular, GHC implements the segmentation technique described in Section 3.2 of the paper.
</para>
<para>
+If recursive bindings are required for a monad,
+then that monad must be declared an instance of the <literal>MonadFix</literal> class.
The following instances of <literal>MonadFix</literal> are automatically provided: List, Maybe, IO.
Furthermore, the Control.Monad.ST and Control.Monad.ST.Lazy modules provide the instances of the MonadFix class
for Haskell's internal state monad (strict and lazy, respectively).
</para>
<para>
-There are three important points in using the recursive-do notation:
+Here are some important points in using the recursive-do notation:
<itemizedlist>
<listitem><para>
The recursive version of the do-notation uses the keyword <literal>mdo</literal> (rather
</para></listitem>
<listitem><para>
-You should <literal>import Control.Monad.Fix</literal>.
-(Note: Strictly speaking, this import is required only when you need to refer to the name
-<literal>MonadFix</literal> in your program, but the import is always safe, and the programmers
-are encouraged to always import this module when using the mdo-notation.)
+It is enabled with the flag <literal>-XRecursiveDo</literal>, which is in turn implied by
+<literal>-fglasgow-exts</literal>.
+</para></listitem>
+
+<listitem><para>
+Unlike ordinary do-notation, but like <literal>let</literal> and <literal>where</literal> bindings,
+name shadowing is not allowed; that is, all the names bound in a single <literal>mdo</literal> must
+be distinct (Section 3.3 of the paper).
</para></listitem>
<listitem><para>
-As with other extensions, ghc should be given the flag <literal>-fglasgow-exts</literal>
+Variables bound by a <literal>let</literal> statement in an <literal>mdo</literal>
+are monomorphic in the <literal>mdo</literal> (Section 3.1 of the paper). However
+GHC breaks the <literal>mdo</literal> into segments to enhance polymorphism,
+and improve termination (Section 3.2 of the paper).
</para></listitem>
</itemizedlist>
</para>
hierarchy. It completely defeats that purpose if the
literal "1" means "<literal>Prelude.fromInteger
1</literal>", which is what the Haskell Report specifies.
- So the <option>-fno-implicit-prelude</option> flag causes
+ So the <option>-XNoImplicitPrelude</option> flag causes
the following pieces of built-in syntax to refer to
<emphasis>whatever is in scope</emphasis>, not the Prelude
versions:
</sect2>
+<sect2 id="disambiguate-fields">
+<title>Record field disambiguation</title>
+<para>
+In record construction and record pattern matching
+it is entirely unambiguous which field is referred to, even if there are two different
+data types in scope with a common field name. For example:
+<programlisting>
+module M where
+ data S = MkS { x :: Int, y :: Bool }
+
+module Foo where
+ import M
+
+ data T = MkT { x :: Int }
+
+ ok1 (MkS { x = n }) = n+1 -- Unambiguous
+
+ ok2 n = MkT { x = n+1 } -- Unambiguous
+
+ bad1 k = k { x = 3 } -- Ambiguous
+ bad2 k = x k -- Ambiguous
+</programlisting>
+Even though there are two <literal>x</literal>'s in scope,
+it is clear that the <literal>x</literal> in the pattern in the
+definition of <literal>ok1</literal> can only mean the field
+<literal>x</literal> from type <literal>S</literal>. Similarly for
+the function <literal>ok2</literal>. However, in the record update
+in <literal>bad1</literal> and the record selection in <literal>bad2</literal>
+it is not clear which of the two types is intended.
+</para>
+<para>
+Haskell 98 regards all four as ambiguous, but with the
+<option>-fdisambiguate-record-fields</option> flag, GHC will accept
+the former two. The rules are precisely the same as those for instance
+declarations in Haskell 98, where the method names on the left-hand side
+of the method bindings in an instance declaration refer unambiguously
+to the method of that class (provided they are in scope at all), even
+if there are other variables in scope with the same name.
+This reduces the clutter of qualified names when you import two
+records from different modules that use the same field name.
+</para>
+</sect2>
</sect1>
may use different notation to that implemented in GHC.
</para>
<para>
-The rest of this section outlines the extensions to GHC that support GADTs.
+The rest of this section outlines the extensions to GHC that support GADTs. The extension is enabled with
+<option>-XGADTs</option>.
<itemizedlist>
<listitem><para>
A GADT can only be declared using GADT-style syntax (<xref linkend="gadt-style"/>);
</para>
</sect2>
+</sect1>
<!-- ====================== End of Generalised algebraic data types ======================= -->
+<sect1 id="deriving">
+<title>Extensions to the "deriving" mechanism</title>
+
+<sect2 id="deriving-inferred">
+<title>Inferred context for deriving clauses</title>
+
+<para>
+The Haskell Report is vague about exactly when a <literal>deriving</literal> clause is
+legal. For example:
+<programlisting>
+ data T0 f a = MkT0 a deriving( Eq )
+ data T1 f a = MkT1 (f a) deriving( Eq )
+ data T2 f a = MkT2 (f (f a)) deriving( Eq )
+</programlisting>
+The natural generated <literal>Eq</literal> code would result in these instance declarations:
+<programlisting>
+ instance Eq a => Eq (T0 f a) where ...
+ instance Eq (f a) => Eq (T1 f a) where ...
+ instance Eq (f (f a)) => Eq (T2 f a) where ...
+</programlisting>
+The first of these is obviously fine. The second is still fine, although less obviously.
+The third is not Haskell 98, and risks losing termination of instances.
+</para>
+<para>
+GHC takes a conservative position: it accepts the first two, but not the third. The rule is this:
+each constraint in the inferred instance context must consist only of type variables,
+with no repititions.
+</para>
+<para>
+This rule is applied regardless of flags. If you want a more exotic context, you can write
+it yourself, using the <link linkend="stand-alone-deriving">standalone deriving mechanism</link>.
+</para>
+</sect2>
+
+<sect2 id="stand-alone-deriving">
+<title>Stand-alone deriving declarations</title>
+
+<para>
+GHC now allows stand-alone <literal>deriving</literal> declarations, enabled by <literal>-XStandaloneDeriving</literal>:
+<programlisting>
+ data Foo a = Bar a | Baz String
+
+ deriving instance Eq a => Eq (Foo a)
+</programlisting>
+The syntax is identical to that of an ordinary instance declaration apart from (a) the keyword
+<literal>deriving</literal>, and (b) the absence of the <literal>where</literal> part.
+You must supply a context (in the example the context is <literal>(Eq a)</literal>),
+exactly as you would in an ordinary instance declaration.
+(In contrast the context is inferred in a <literal>deriving</literal> clause
+attached to a data type declaration.) These <literal>deriving instance</literal>
+rules obey the same rules concerning form and termination as ordinary instance declarations,
+controlled by the same flags; see <xref linkend="instance-decls"/>. </para>
+
+<para>The stand-alone syntax is generalised for newtypes in exactly the same
+way that ordinary <literal>deriving</literal> clauses are generalised (<xref linkend="newtype-deriving"/>).
+For example:
+<programlisting>
+ newtype Foo a = MkFoo (State Int a)
+
+ deriving instance MonadState Int Foo
+</programlisting>
+GHC always treats the <emphasis>last</emphasis> parameter of the instance
+(<literal>Foo</literal> in this exmample) as the type whose instance is being derived.
+</para>
+
+</sect2>
+
<sect2 id="deriving-typeable">
<title>Deriving clause for classes <literal>Typeable</literal> and <literal>Data</literal></title>
</para>
<para>
GHC extends this list with two more classes that may be automatically derived
-(provided the <option>-fglasgow-exts</option> flag is specified):
+(provided the <option>-XDeriveDataTypeable</option> flag is specified):
<literal>Typeable</literal>, and <literal>Data</literal>. These classes are defined in the library
modules <literal>Data.Typeable</literal> and <literal>Data.Generics</literal> respectively, and the
appropriate class must be in scope before it can be mentioned in the <literal>deriving</literal> clause.
<sect3> <title> Generalising the deriving clause </title>
<para>
-GHC now permits such instances to be derived instead, so one can write
+GHC now permits such instances to be derived instead,
+using the flag <option>-XGeneralizedNewtypeDeriving</option>,
+so one can write
<programlisting>
newtype Dollars = Dollars Int deriving (Eq,Show,Num)
</programlisting>
Notice that, since <literal>Monad</literal> is a constructor class, the
instance is a <emphasis>partial application</emphasis> of the new type, not the
entire left hand side. We can imagine that the type declaration is
-``eta-converted'' to generate the context of the instance
+"eta-converted" to generate the context of the instance
declaration.
</para>
<para>
the standard method is used or the one described here.)
</para>
</sect3>
-
-</sect2>
-
-<sect2 id="stand-alone-deriving">
-<title>Stand-alone deriving declarations</title>
-
-<para>
-GHC now allows stand-alone <literal>deriving</literal> declarations, enabled by <literal>-fglasgow-exts</literal>:
-<programlisting>
- data Foo a = Bar a | Baz String
-
- derive instance Eq (Foo a)
-</programlisting>
-The token "<literal>derive</literal>" is a keyword only when followed by "<literal>instance</literal>";
-you can use it as a variable name elsewhere.</para>
-<para>The stand-alone syntax is generalised for newtypes in exactly the same
-way that ordinary <literal>deriving</literal> clauses are generalised (<xref linkend="newtype-deriving"/>).
-For example:
-<programlisting>
- newtype Foo a = MkFoo (State Int a)
-
- derive instance MonadState Int Foo
-</programlisting>
-GHC always treats the <emphasis>last</emphasis> parameter of the instance
-(<literal>Foo</literal> in this exmample) as the type whose instance is being derived.
-</para>
-
</sect2>
-
</sect1>
<!-- TYPE SYSTEM EXTENSIONS -->
-<sect1 id="other-type-extensions">
-<title>Other type system extensions</title>
+<sect1 id="type-class-extensions">
+<title>Class and instances declarations</title>
<sect2 id="multi-param-type-classes">
<title>Class declarations</title>
</para>
<para>
Nevertheless, GHC allows you to experiment with more liberal rules. If you use
-the experimental flag <option>-fallow-undecidable-instances</option>
-<indexterm><primary>-fallow-undecidable-instances
-option</primary></indexterm>, both the Paterson Conditions and the Coverage Condition
+the experimental flag <option>-XUndecidableInstances</option>
+<indexterm><primary>-XUndecidableInstances</primary></indexterm>,
+both the Paterson Conditions and the Coverage Condition
(described in <xref linkend="instance-rules"/>) are lifted. 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
In general, <emphasis>GHC requires that that it be unambiguous which instance
declaration
should be used to resolve a type-class constraint</emphasis>. This behaviour
-can be modified by two flags: <option>-fallow-overlapping-instances</option>
-<indexterm><primary>-fallow-overlapping-instances
+can be modified by two flags: <option>-XOverlappingInstances</option>
+<indexterm><primary>-XOverlappingInstances
</primary></indexterm>
-and <option>-fallow-incoherent-instances</option>
-<indexterm><primary>-fallow-incoherent-instances
+and <option>-XIncoherentInstances</option>
+<indexterm><primary>-XIncoherentInstances
</primary></indexterm>, as this section discusses. Both these
flags are dynamic flags, and can be set on a per-module basis, using
an <literal>OPTIONS_GHC</literal> pragma if desired (<xref linkend="source-file-options"/>).</para>
</para>
<para>
-The <option>-fallow-overlapping-instances</option> flag instructs GHC to allow
+The <option>-XOverlappingInstances</option> flag instructs GHC to allow
more than one instance to match, provided there is a most specific one. For
example, the constraint <literal>C Int [Int]</literal> matches instances (A),
(C) and (D), but the last is more specific, and hence is chosen. If there is no
GHC does not commit to instance (C), because in a particular
call of <literal>f</literal>, <literal>b</literal> might be instantiate
to <literal>Int</literal>, in which case instance (D) would be more specific still.
-So GHC rejects the program. If you add the flag <option>-fallow-incoherent-instances</option>,
+So GHC rejects the program.
+(If you add the flag <option>-XIncoherentInstances</option>,
GHC will instead pick (C), without complaining about
-the problem of subsequent instantiations.
+the problem of subsequent instantiations.)
+</para>
+<para>
+Notice that we gave a type signature to <literal>f</literal>, so GHC had to
+<emphasis>check</emphasis> that <literal>f</literal> has the specified type.
+Suppose instead we do not give a type signature, asking GHC to <emphasis>infer</emphasis>
+it instead. In this case, GHC will refrain from
+simplifying the constraint <literal>C Int [Int]</literal> (for the same reason
+as before) but, rather than rejecting the program, it will infer the type
+<programlisting>
+ f :: C Int b => [b] -> [b]
+</programlisting>
+That postpones the question of which instance to pick to the
+call site for <literal>f</literal>
+by which time more is known about the type <literal>b</literal>.
</para>
<para>
The willingness to be overlapped or incoherent is a property of
the <emphasis>instance declaration</emphasis> itself, controlled by the
-presence or otherwise of the <option>-fallow-overlapping-instances</option>
-and <option>-fallow-incoherent-instances</option> flags when that mdodule is
+presence or otherwise of the <option>-XOverlappingInstances</option>
+and <option>-XIncoherentInstances</option> flags when that mdodule is
being defined. Neither flag is required in a module that imports and uses the
instance declaration. Specifically, during the lookup process:
<itemizedlist>
<listitem><para>
An instance declaration is ignored during the lookup process if (a) a more specific
match is found, and (b) the instance declaration was compiled with
-<option>-fallow-overlapping-instances</option>. The flag setting for the
+<option>-XOverlappingInstances</option>. The flag setting for the
more-specific instance does not matter.
</para></listitem>
<listitem><para>
-Suppose an instance declaration does not matche the constraint being looked up, but
+Suppose an instance declaration does not match the constraint being looked up, but
does unify with it, so that it might match when the constraint is further
instantiated. Usually GHC will regard this as a reason for not committing to
some other constraint. But if the instance declaration was compiled with
-<option>-fallow-incoherent-instances</option>, GHC will skip the "does-it-unify?"
+<option>-XIncoherentInstances</option>, GHC will skip the "does-it-unify?"
check for that declaration.
</para></listitem>
</itemizedlist>
</para>
<para>
If an instance declaration is compiled without
-<option>-fallow-overlapping-instances</option>,
+<option>-XOverlappingInstances</option>,
then that instance can never be overlapped. This could perhaps be
inconvenient. Perhaps the rule should instead say that the
<emphasis>overlapping</emphasis> instance declaration should be compiled in
this way, rather than the <emphasis>overlapped</emphasis> one. Perhaps overlap
at a usage site should be permitted regardless of how the instance declarations
-are compiled, if the <option>-fallow-overlapping-instances</option> flag is
+are compiled, if the <option>-XOverlappingInstances</option> flag is
used at the usage site. (Mind you, the exact usage site can occasionally be
hard to pin down.) We are interested to receive feedback on these points.
</para>
-<para>The <option>-fallow-incoherent-instances</option> flag implies the
-<option>-fallow-overlapping-instances</option> flag, but not vice versa.
+<para>The <option>-XIncoherentInstances</option> flag implies the
+<option>-XOverlappingInstances</option> flag, but not vice versa.
</para>
</sect3>
</sect2>
+<sect2 id="overloaded-strings">
+<title>Overloaded string literals
+</title>
+
+<para>
+GHC supports <emphasis>overloaded string literals</emphasis>. Normally a
+string literal has type <literal>String</literal>, but with overloaded string
+literals enabled (with <literal>-XOverloadedStrings</literal>)
+ a string literal has type <literal>(IsString a) => a</literal>.
+</para>
+<para>
+This means that the usual string syntax can be used, e.g., for packed strings
+and other variations of string like types. String literals behave very much
+like integer literals, i.e., they can be used in both expressions and patterns.
+If used in a pattern the literal with be replaced by an equality test, in the same
+way as an integer literal is.
+</para>
+<para>
+The class <literal>IsString</literal> is defined as:
+<programlisting>
+class IsString a where
+ fromString :: String -> a
+</programlisting>
+The only predefined instance is the obvious one to make strings work as usual:
+<programlisting>
+instance IsString [Char] where
+ fromString cs = cs
+</programlisting>
+The class <literal>IsString</literal> is not in scope by default. If you want to mention
+it explicitly (for exmaple, to give an instance declaration for it), you can import it
+from module <literal>GHC.Exts</literal>.
+</para>
+<para>
+Haskell's defaulting mechanism is extended to cover string literals, when <option>-XOverloadedStrings</option> is specified.
+Specifically:
+<itemizedlist>
+<listitem><para>
+Each type in a default declaration must be an
+instance of <literal>Num</literal> <emphasis>or</emphasis> of <literal>IsString</literal>.
+</para></listitem>
+
+<listitem><para>
+The standard defaulting rule (<ulink url="http://haskell.org/onlinereport/decls.html#sect4.3.4">Haskell Report, Section 4.3.4</ulink>)
+is extended thus: defaulting applies when all the unresolved constraints involve standard classes
+<emphasis>or</emphasis> <literal>IsString</literal>; and at least one is a numeric class
+<emphasis>or</emphasis> <literal>IsString</literal>.
+</para></listitem>
+</itemizedlist>
+</para>
+<para>
+A small example:
+<programlisting>
+module Main where
+
+import GHC.Exts( IsString(..) )
+
+newtype MyString = MyString String deriving (Eq, Show)
+instance IsString MyString where
+ fromString = MyString
+
+greet :: MyString -> MyString
+greet "hello" = "world"
+greet other = other
+
+main = do
+ print $ greet "hello"
+ print $ greet "fool"
+</programlisting>
+</para>
+<para>
+Note that deriving <literal>Eq</literal> is necessary for the pattern matching
+to work since it gets translated into an equality comparison.
+</para>
+</sect2>
+
+</sect1>
+
+<sect1 id="other-type-extensions">
+<title>Other type system extensions</title>
+
<sect2 id="type-restrictions">
<title>Type signatures</title>
due to Jeff Lewis.)</para>
<para>Implicit parameter support is enabled with the option
-<option>-fimplicit-params</option>.</para>
+<option>-XImplicitParams</option>.</para>
<para>
A variable is called <emphasis>dynamically bound</emphasis> when it is bound by the calling
<para>Following a suggestion of Mark Jones, in his paper
<ulink url="http://www.cse.ogi.edu/~mpj/thih/">Typing Haskell in
Haskell</ulink>,
-GHC implements a more general scheme. If <option>-fglasgow-exts</option> is
+GHC implements a more general scheme. If <option>-XRelaxedPolyRec</option> is
specified:
<emphasis>the dependency analysis ignores references to variables that have an explicit
type signature</emphasis>.
The same refined dependency analysis also allows the type signatures of
mutually-recursive functions to have different contexts, something that is illegal in
Haskell 98 (Section 4.5.2, last sentence). With
-<option>-fglasgow-exts</option>
+<option>-XRelaxedPolyRec</option>
GHC only insists that the type signatures of a <emphasis>refined</emphasis> group have identical
type signatures; in practice this means that only variables bound by the same
pattern binding must have the same context. For example, this is fine:
</para>
</sect2>
-<sect2 id="overloaded-strings">
-<title>Overloaded string literals
+<sect2 id="type-families">
+<title>Type families
</title>
<para>
-GHC supports <emphasis>overloaded string literals</emphasis>. Normally a
-string literal has type <literal>String</literal>, but with overloaded string
-literals enabled (with <literal>-foverloaded-strings</literal>)
- a string literal has type <literal>(IsString a) => a</literal>.
+GHC supports the definition of type families indexed by types. They may be
+seen as an extension of Haskell 98's class-based overloading of values to
+types. When type families are declared in classes, they are also known as
+associated types.
</para>
<para>
-This means that the usual string syntax can be used, e.g., for packed strings
-and other variations of string like types. String literals behave very much
-like integer literals, i.e., they can be used in both expressions and patterns.
-If used in a pattern the literal with be replaced by an equality test, in the same
-way as an integer literal is.
+There are two forms of type families: data families and type synonym families.
+Currently, only the former are fully implemented, while we are still working
+on the latter. As a result, the specification of the language extension is
+also still to some degree in flux. Hence, a more detailed description of
+the language extension and its use is currently available
+from <ulink url="http://haskell.org/haskellwiki/GHC/Indexed_types">the Haskell
+wiki page on type families</ulink>. The material will be moved to this user's
+guide when it has stabilised.
</para>
<para>
-The class <literal>IsString</literal> is defined as:
-<programlisting>
-class IsString a where
- fromString :: String -> a
-</programlisting>
-The only predefined instance is the obvious one to make strings work as usual:
-<programlisting>
-instance IsString [Char] where
- fromString cs = cs
-</programlisting>
-The class <literal>IsString</literal> is not in scope by default. If you want to mention
-it explicitly (for exmaple, to give an instance declaration for it), you can import it
-from module <literal>GHC.Exts</literal>.
+Type families are enabled by the flag <option>-XTypeFamilies</option>.
</para>
-<para>
-Haskell's defaulting mechanism is extended to cover string literals, when <option>-foverloaded-strings</option> is specified.
-Specifically:
-<itemizedlist>
-<listitem><para>
-Each type in a default declaration must be an
-instance of <literal>Num</literal> <emphasis>or</emphasis> of <literal>IsString</literal>.
-</para></listitem>
-<listitem><para>
-The standard defaulting rule (<ulink url="http://haskell.org/onlinereport/decls.html#sect4.3.4">Haskell Report, Section 4.3.4</ulink>)
-is extended thus: defaulting applies when all the unresolved constraints involve standard classes
-<emphasis>or</emphasis> <literal>IsString</literal>; and at least one is a numeric class
-<emphasis>or</emphasis> <literal>IsString</literal>.
-</para></listitem>
-</itemizedlist>
-</para>
-<para>
-A small example:
-<programlisting>
-module Main where
-
-import GHC.Exts( IsString(..) )
-
-newtype MyString = MyString String deriving (Eq, Show)
-instance IsString MyString where
- fromString = MyString
-
-greet :: MyString -> MyString
-greet "hello" = "world"
-greet other = other
-main = do
- print $ greet "hello"
- print $ greet "fool"
-</programlisting>
-</para>
-<para>
-Note that deriving <literal>Eq</literal> is necessary for the pattern matching
-to work since it gets translated into an equality comparison.
-</para>
</sect2>
</sect1>
<para>
There is a Wiki page about
Template Haskell at <ulink url="http://haskell.org/haskellwiki/Template_Haskell">
-http://www.haskell.org/th/</ulink>, and that is the best place to look for
+http://www.haskell.org/haskellwiki/Template_Haskell</ulink>, and that is the best place to look for
further details.
You may also
consult the <ulink
url="http://www.haskell.org/ghc/docs/latest/html/libraries/index.html">online
Haskell library reference material</ulink>
-(search for the type ExpQ).
-[Temporary: many changes to the original design are described in
- <ulink url="http://research.microsoft.com/~simonpj/tmp/notes2.ps">"http://research.microsoft.com/~simonpj/tmp/notes2.ps"</ulink>.
-Not all of these changes are in GHC 6.6.]
+(look for module <literal>Language.Haskell.TH</literal>).
+Many changes to the original design are described in
+ <ulink url="http://research.microsoft.com/~simonpj/papers/meta-haskell/notes2.ps">
+Notes on Template Haskell version 2</ulink>.
+Not all of these changes are in GHC, however.
</para>
-<para> The first example from that paper is set out below as a worked example to help get you started.
+<para> The first example from that paper is set out below (<xref linkend="th-example"/>)
+as a worked example to help get you started.
</para>
<para>
-The documentation here describes the realisation in GHC. (It's rather sketchy just now;
-Tim Sheard is going to expand it.)
+The documentation here describes the realisation of Template Haskell in GHC. It is not detailed enough to
+understand Template Haskell; see the <ulink url="http://haskell.org/haskellwiki/Template_Haskell">
+Wiki page</ulink>.
</para>
<sect2>
<para> Template Haskell has the following new syntactic
constructions. You need to use the flag
- <option>-fth</option><indexterm><primary><option>-fth</option></primary>
+ <option>-XTemplateHaskell</option>
+ <indexterm><primary><option>-XTemplateHaskell</option></primary>
</indexterm>to switch these syntactic extensions on
- (<option>-fth</option> is no longer implied by
+ (<option>-XTemplateHaskell</option> is no longer implied by
<option>-fglasgow-exts</option>).</para>
<itemizedlist>
<itemizedlist>
<listitem><para> an expression; the spliced expression must
have type <literal>Q Exp</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> [Planned, but not implemented yet.] a
- type; the spliced expression must have type <literal>Q Typ</literal>.</para></listitem>
+ <listitem><para> a list of top-level declarations; the spliced expression must have type <literal>Q [Dec]</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>
+ </para>
+ Inside a splice you can can only call functions defined in imported modules,
+ not functions defined elsewhere in the same module.</listitem>
<listitem><para>
A expression quotation is written in Oxford brackets, thus:
<itemizedlist>
<listitem><para> <literal>[| ... |]</literal>, where the "..." is an expression;
- the quotation has type <literal>Expr</literal>.</para></listitem>
+ the quotation has type <literal>Q Exp</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> [Planned, but not implemented yet.] <literal>[t| ... |]</literal>, where the "..." is a type;
- the quotation has type <literal>Type</literal>.</para></listitem>
+ <listitem><para> <literal>[t| ... |]</literal>, where the "..." is a type;
+ the quotation has type <literal>Q Typ</literal>.</para></listitem>
</itemizedlist></para></listitem>
<listitem><para>
- Reification is written thus:
+ A name can be quoted with either one or two prefix single quotes:
<itemizedlist>
- <listitem><para> <literal>reifyDecl T</literal>, where <literal>T</literal> is a type constructor; this expression
- has type <literal>Dec</literal>. </para></listitem>
- <listitem><para> <literal>reifyDecl C</literal>, where <literal>C</literal> is a class; has type <literal>Dec</literal>.</para></listitem>
- <listitem><para> <literal>reifyType f</literal>, where <literal>f</literal> is an identifier; has type <literal>Typ</literal>.</para></listitem>
- <listitem><para> Still to come: fixities </para></listitem>
-
- </itemizedlist></para>
+ <listitem><para> <literal>'f</literal> has type <literal>Name</literal>, and names the function <literal>f</literal>.
+ Similarly <literal>'C</literal> has type <literal>Name</literal> and names the data constructor <literal>C</literal>.
+ In general <literal>'</literal><replaceable>thing</replaceable> interprets <replaceable>thing</replaceable> in an expression context.
+ </para></listitem>
+ <listitem><para> <literal>''T</literal> has type <literal>Name</literal>, and names the type constructor <literal>T</literal>.
+ That is, <literal>''</literal><replaceable>thing</replaceable> interprets <replaceable>thing</replaceable> in a type context.
+ </para></listitem>
+ </itemizedlist>
+ These <literal>Names</literal> can be used to construct Template Haskell expressions, patterns, delarations etc. They
+ may also be given as an argument to the <literal>reify</literal> function.
+ </para>
</listitem>
</itemizedlist>
+(Compared to the original paper, there are many differnces of detail.
+The syntax for a declaration splice uses "<literal>$</literal>" not "<literal>splice</literal>".
+The type of the enclosed expression must be <literal>Q [Dec]</literal>, not <literal>[Q Dec]</literal>.
+Type splices are not implemented, and neither are pattern splices or quotations.
+
</sect2>
<sect2> <title> Using Template Haskell </title>
</para>
</sect2>
-<sect2> <title> A Template Haskell Worked Example </title>
+<sect2 id="th-example"> <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>
-- 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] -> ExpQ
+gen :: [Format] -> Q Exp
gen [D] = [| \n -> show n |]
gen [S] = [| \s -> s |]
gen [L s] = stringE s
-- Here we generate the Haskell code for the splice
-- from an input format string.
-pr :: String -> ExpQ
-pr s = gen (parse s)
+pr :: String -> Q Exp
+pr s = gen (parse s)
</programlisting>
<para>Now run the compiler (here we are a Cygwin prompt on Windows):
</para>
<programlisting>
-$ ghc --make -fth main.hs -o main.exe
+$ ghc --make -XTemplateHaskell main.hs -o main.exe
</programlisting>
<para>Run "main.exe" and here is your output:</para>
</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
+With the <option>-XArrows</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.
than the material below.
</para>
<para>
-Bang patterns are enabled by the flag <option>-fbang-patterns</option>.
+Bang patterns are enabled by the flag <option>-XBangPatterns</option>.
</para>
<sect2 id="bang-patterns-informal">
<sect1 id="special-ids">
<title>Special built-in functions</title>
-<para>GHC has a few built-in funcions with special behaviour,
-described in this section. All are exported by
-<literal>GHC.Exts</literal>.</para>
-
-<sect2> <title>The <literal>seq</literal> function </title>
-<para>
-The function <literal>seq</literal> is as described in the Haskell98 Report.
-<programlisting>
- seq :: a -> b -> b
-</programlisting>
-It evaluates its first argument to head normal form, and then returns its
-second argument as the result. The reason that it is documented here is
-that, despite <literal>seq</literal>'s polymorphism, its
-second argument can have an unboxed type, or
-can be an unboxed tuple; for example <literal>(seq x 4#)</literal>
-or <literal>(seq x (# p,q #))</literal>. This requires <literal>b</literal>
-to be instantiated to an unboxed type, which is not usually allowed.
-</para>
-</sect2>
-
-<sect2> <title>The <literal>inline</literal> function </title>
-<para>
-The <literal>inline</literal> function is somewhat experimental.
-<programlisting>
- inline :: a -> a
-</programlisting>
-The call <literal>(inline f)</literal> arranges that <literal>f</literal>
-is inlined, regardless of its size. More precisely, the call
-<literal>(inline f)</literal> rewrites to the right-hand side of <literal>f</literal>'s
-definition.
-This allows the programmer to control inlining from
-a particular <emphasis>call site</emphasis>
-rather than the <emphasis>definition site</emphasis> of the function
-(c.f. <literal>INLINE</literal> pragmas <xref linkend="inline-noinline-pragma"/>).
-</para>
-<para>
-This inlining occurs regardless of the argument to the call
-or the size of <literal>f</literal>'s definition; it is unconditional.
-The main caveat is that <literal>f</literal>'s definition must be
-visible to the compiler. That is, <literal>f</literal> must be
-let-bound in the current scope.
-If no inlining takes place, the <literal>inline</literal> function
-expands to the identity function in Phase zero; so its use imposes
-no overhead.</para>
-
-<para> If the function is defined in another
-module, GHC only exposes its inlining in the interface file if the
-function is sufficiently small that it <emphasis>might</emphasis> be
-inlined by the automatic mechanism. There is currently no way to tell
-GHC to expose arbitrarily-large functions in the interface file. (This
-shortcoming is something that could be fixed, with some kind of pragma.)
-</para>
-</sect2>
-
-<sect2> <title>The <literal>lazy</literal> function </title>
-<para>
-The <literal>lazy</literal> function restrains strictness analysis a little:
-<programlisting>
- lazy :: a -> a
-</programlisting>
-The call <literal>(lazy e)</literal> means the same as <literal>e</literal>,
-but <literal>lazy</literal> has a magical property so far as strictness
-analysis is concerned: it is lazy in its first argument,
-even though its semantics is strict. After strictness analysis has run,
-calls to <literal>lazy</literal> are inlined to be the identity function.
-</para>
-<para>
-This behaviour is occasionally useful when controlling evaluation order.
-Notably, <literal>lazy</literal> is used in the library definition of
-<literal>Control.Parallel.par</literal>:
-<programlisting>
- par :: a -> b -> b
- par x y = case (par# x) of { _ -> lazy y }
-</programlisting>
-If <literal>lazy</literal> were not lazy, <literal>par</literal> would
-look strict in <literal>y</literal> which would defeat the whole
-purpose of <literal>par</literal>.
-</para>
-<para>
-Like <literal>seq</literal>, the argument of <literal>lazy</literal> can have
-an unboxed type.
-</para>
-
-</sect2>
-
-<sect2> <title>The <literal>unsafeCoerce#</literal> function </title>
-<para>
-The function <literal>unsafeCoerce#</literal> allows you to side-step the
-typechecker entirely. It has type
-<programlisting>
- unsafeCoerce# :: a -> b
-</programlisting>
-That is, it allows you to coerce any type into any other type. If you use this
-function, you had better get it right, otherwise segmentation faults await.
-It is generally used when you want to write a program that you know is
-well-typed, but where Haskell's type system is not expressive enough to prove
-that it is well typed.
-</para>
-<para>
-The argument to <literal>unsafeCoerce#</literal> can have unboxed types,
-although extremely bad things will happen if you coerce a boxed type
-to an unboxed type.
-</para>
-
-</sect2>
-
+<para>GHC has a few built-in funcions with special behaviour. These
+are now described in the module <ulink
+url="../libraries/base/GHC-Prim.html"><literal>GHC.Prim</literal></ulink>
+in the library documentation.</para>
</sect1>
<itemizedlist>
<listitem>
<para>Use the flags <option>-fglasgow-exts</option> (to enable the extra syntax),
- <option>-fgenerics</option> (to generate extra per-data-type code),
+ <option>-XGenerics</option> (to generate extra per-data-type code),
and <option>-package lang</option> (to make the <literal>Generics</literal> library
available. </para>
</listitem>
<sect2>
<title>Switching off the dreaded Monomorphism Restriction</title>
- <indexterm><primary><option>-fno-monomorphism-restriction</option></primary></indexterm>
+ <indexterm><primary><option>-XNoMonomorphismRestriction</option></primary></indexterm>
<para>Haskell's monomorphism restriction (see
<ulink url="http://haskell.org/onlinereport/decls.html#sect4.5.5">Section
4.5.5</ulink>
of the Haskell Report)
can be completely switched off by
-<option>-fno-monomorphism-restriction</option>.
+<option>-XNoMonomorphismRestriction</option>.
</para>
</sect2>
<sect2>
<title>Monomorphic pattern bindings</title>
- <indexterm><primary><option>-fno-mono-pat-binds</option></primary></indexterm>
- <indexterm><primary><option>-fmono-pat-binds</option></primary></indexterm>
+ <indexterm><primary><option>-XNoMonoPatBinds</option></primary></indexterm>
+ <indexterm><primary><option>-XMonoPatBinds</option></primary></indexterm>
<para> As an experimental change, we are exploring the possibility of
making pattern bindings monomorphic; that is, not generalised at all.
[x] = e -- A pattern binding
</programlisting>
Experimentally, GHC now makes pattern bindings monomorphic <emphasis>by
-default</emphasis>. Use <option>-fno-mono-pat-binds</option> to recover the
+default</emphasis>. Use <option>-XMonoPatBinds</option> to recover the
standard behaviour.
</para>
</sect2>
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