[Add docs for DatatypeContexts extension
Ian Lynagh <igloo@earth.li>**20100707230907
 Ignore-this: 8158f03b35a2d7442a75fe85d6f1b1c7
] hunk ./docs/users_guide/glasgow_exts.xml 1897
+<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>
+