[project @ 2002-03-08 16:03:13 by simonpj]

Documentation for more liberal class-method types

ghc/docs/users_guide/glasgow_exts.sgml

@@ -38,6 +38,13 @@ Executive summary of our extensions:
         type system.  Specifically:</para>
 
 	<variablelist>
+	  <varlistentry>
+	    <term>Class method types:</term>
+	    <listitem>
+	      <para><xref LinkEnd="classs-method-types"></para>
+	    </listitem>
+	  </varlistentry>
+
 	  <varlistentry>
 	    <term>Multi-parameter type classes:</term>
 	    <listitem>
@@ -628,6 +635,27 @@ qualifier list has just one element, a boolean expression.
 
   </sect1>
 
+<sect1 id="class-method-types">
+<title>Class method types
+</title>
+<para>
+Haskell 98 prohibits class method types to mention constraints on the
+class type variable, thus:
+<programlisting>
+  class Seq s a where
+    fromList :: [a] -> s a
+    elem     :: Eq a => a -> s a -> Bool
+</programlisting>
+The type of <literal>elem</literal> is illegal in Haskell 98, because it
+contains the constraint <literal>Eq a</literal>, constrains only the 
+class type variable (in this case <literal>a</literal>).
+</para>
+<para>
+With the <option>-fglasgow-exts</option> GHC lifts this restriction.
+</para>
+
+</sect1>
+
 <sect1 id="multi-param-type-classes">
 <title>Multi-parameter type classes
 </title>