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| -rw-r--r-- | docs/users_guide/glasgow_exts.xml | 20 |
1 files changed, 20 insertions, 0 deletions
diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml index f8cd504a84..bf825455e1 100644 --- a/docs/users_guide/glasgow_exts.xml +++ b/docs/users_guide/glasgow_exts.xml @@ -1894,6 +1894,26 @@ not <literal>*</literal> then an explicit kind annotation must be used Nevertheless, they can be useful when defining "phantom types".</para> </sect2> +<sect2 id="datatype-contexts"> +<title>Data type contexts</title> + +<para>Haskell allows datatypes to be given contexts, e.g.</para> + +<programlisting> +data Eq a => Set a = NilSet | ConsSet a (Set a) +</programlisting> + +<para>give constructors with types:</para> + +<programlisting> +NilSet :: Set a +ConsSet :: Eq a => a -> Set a -> Set a +</programlisting> + +<para>In GHC this feature is an extension called +<literal>DatatypeContexts</literal>, and on by default.</para> +</sect2> + <sect2 id="infix-tycons"> <title>Infix type constructors, classes, and type variables</title> |