<indexterm><primary>extensions</primary><secondary>options controlling</secondary>
</indexterm>
- <para>The language option flag control what variation of the language are
+ <para>The language option flags control what variation of the language are
permitted. Leaving out all of them gives you standard Haskell
98.</para>
- <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>Language options can be controlled in two ways:
+ <itemizedlist>
+ <listitem><para>Every language option can switched on by a command-line flag "<option>-X...</option>"
+ (e.g. <option>-XTemplateHaskell</option>), and switched off by the flag "<option>-XNo...</option>";
+ (e.g. <option>-XNoTemplateHaskell</option>).</para></listitem>
+ <listitem><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>
+ </listitem>
+ </itemizedlist></para>
<para>The flag <option>-fglasgow-exts</option>
<indexterm><primary><option>-fglasgow-exts</option></primary></indexterm>
can be any number.
</para>
<para>
- Data and newtype instance declarations are only legit when an
- appropriate family declaration is in scope - just like class instances
- require the class declaration to be visible. Moreover, each instance
+ Data and newtype instance declarations are only permitted when an
+ appropriate family declaration is in scope - just as a class instance declaratoin
+ requires the class declaration to be visible. Moreover, each instance
declaration has to conform to the kind determined by its family
declaration. This implies that the number of parameters of an instance
declaration matches the arity determined by the kind of the family.
- Although, all data families are declared with
- the <literal>data</literal> keyword, instances can be
- either <literal>data</literal> or <literal>newtype</literal>s, or a mix
- of both.
</para>
<para>
+ A data family instance declaration can use the full exprssiveness of
+ ordinary <literal>data</literal> or <literal>newtype</literal> declarations:
+ <itemizedlist>
+ <listitem><para> Although, a data family is <emphasis>introduced</emphasis> with
+ the keyword "<literal>data</literal>", a data family <emphasis>instance</emphasis> can
+ use either <literal>data</literal> or <literal>newtype</literal>. For example:
+<programlisting>
+data family T a
+data instance T Int = T1 Int | T2 Bool
+newtype instance T Char = TC Bool
+</programlisting>
+ </para></listitem>
+ <listitem><para> A <literal>data instance</literal> can use GADT syntax for the data constructors,
+ and indeed can define a GADT. For example:
+<programlisting>
+data family G a b
+data instance G [a] b where
+ G1 :: c -> G [Int] b
+ G2 :: G [a] Bool
+</programlisting>
+ </para></listitem>
+ <listitem><para> You can use a <literal>deriving</literal> clause on a
+ <literal>data instance</literal> or <literal>newtype instance</literal>
+ declaration.
+ </para></listitem>
+ </itemizedlist>
+ </para>
+
+ <para>
Even if type families are defined as toplevel declarations, functions
- that perform different computations for different family instances still
+ that perform different computations for different family instances may still
need to be defined as methods of type classes. In particular, the
following is not possible:
<programlisting>
data family T a
data instance T Int = A
data instance T Char = B
-nonsence :: T a -> Int
-nonsence A = 1 -- WRONG: These two equations together...
-nonsence B = 2 -- ...will produce a type error.
+foo :: T a -> Int
+foo A = 1 -- WRONG: These two equations together...
+foo B = 2 -- ...will produce a type error.
+</programlisting>
+Instead, you would have to write <literal>foo</literal> as a class operation, thus:
+<programlisting>
+class C a where
+ foo :: T a -> Int
+instance Foo Int where
+ foo A = 1
+instance Foo Char where
+ foo B = 2
</programlisting>
- Given the functionality provided by GADTs (Generalised Algebraic Data
+ (Given the functionality provided by GADTs (Generalised Algebraic Data
Types), it might seem as if a definition, such as the above, should be
feasible. However, type families are - in contrast to GADTs - are
<emphasis>open;</emphasis> i.e., new instances can always be added,
possibly in other
modules. Supporting pattern matching across different data instances
- would require a form of extensible case construct.
+ would require a form of extensible case construct.)
</para>
<sect4 id="assoc-data-inst">
module Expr where
import qualified Language.Haskell.TH as TH
-import Language.Haskell.TH.Quasi
+import Language.Haskell.TH.Quote
data Expr = IntExpr Integer
| AntiIntExpr String
ignored. The layout rule applies in pragmas, so the closing <literal>#-}</literal>
should start in a column to the right of the opening <literal>{-#</literal>. </para>
- <para>Certain pragmas are <emphasis>file-header pragmas</emphasis>. A file-header
- pragma must precede the <literal>module</literal> keyword in the file.
+ <para>Certain pragmas are <emphasis>file-header pragmas</emphasis>:
+ <itemizedlist>
+ <listitem><para>
+ A file-header
+ pragma must precede the <literal>module</literal> keyword in the file.
+ </para></listitem>
+ <listitem><para>
There can be as many file-header pragmas as you please, and they can be
- preceded or followed by comments.</para>
+ preceded or followed by comments.
+ </para></listitem>
+ <listitem><para>
+ File-header pragmas are read once only, before
+ pre-processing the file (e.g. with cpp).
+ </para></listitem>
+ <listitem><para>
+ The file-header pragmas are: <literal>{-# LANGUAGE #-}</literal>,
+ <literal>{-# OPTIONS_GHC #-}</literal>, and
+ <literal>{-# INCLUDE #-}</literal>.
+ </para></listitem>
+ </itemizedlist>
+ </para>
<sect2 id="language-pragma">
<title>LANGUAGE pragma</title>
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