remove 'mode: xml' emacs settings (#2208)

[ghc-hetmet.git] / docs / users_guide / glasgow_exts.xml

diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml
index b7d43c9..9fea7a4 100644 (file)
--- a/docs/users_guide/glasgow_exts.xml
+++ b/docs/users_guide/glasgow_exts.xml
@@ -818,10 +818,8 @@ let {(x -> y) = e1 ;
      (y -> x) = e2 } in x
 </programlisting>
 
-(We may lift this
-restriction in the future; the only cost is that type checking patterns
-would get a little more complicated.)  
-
+(For some amplification on this design choice see 
+<ulink url="http://hackage.haskell.org/trac/ghc/ticket/4061">Trac #4061</ulink>.)
 
 </para>
 </listitem>
@@ -1896,6 +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>
 
@@ -9079,7 +9097,6 @@ standard behaviour.
 
 <!-- Emacs stuff:
      ;;; Local Variables: ***
-     ;;; mode: xml ***
      ;;; sgml-parent-document: ("users_guide.xml" "book" "chapter" "sect1") ***
      ;;; ispell-local-dictionary: "british" ***
      ;;; End: ***