[project @ 2002-05-27 15:28:07 by simonpj]

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

diff --git a/ghc/docs/users_guide/glasgow_exts.sgml b/ghc/docs/users_guide/glasgow_exts.sgml
index 05b299a..528e13f 100644 (file)
--- a/ghc/docs/users_guide/glasgow_exts.sgml
+++ b/ghc/docs/users_guide/glasgow_exts.sgml
@@ -218,6 +218,59 @@ types.</para>
 Nevertheless, they can be useful when defining "phantom types".</para>
 </sect2>
 
+<sect2 id="infix-tycons">
+<title>Infix type constructors</title>
+
+<para>
+GHC allows type constructors to be operators, and to be written infix, very much 
+like expressions.  More specifically:
+<itemizedlist>
+<listitem><para>
+  A type constructor can be an operator, beginning with a colon; e.g. <literal>:*:</literal>.
+  The lexical syntax is the same as that for data constructors.
+  </para></listitem>
+<listitem><para>
+  Types can be written infix.  For example <literal>Int :*: Bool</literal>.  
+  </para></listitem>
+<listitem><para>
+  Back-quotes work
+  as for expressions, both for type constructors and type variables;  e.g. <literal>Int `Either` Bool</literal>, or
+  <literal>Int `a` Bool</literal>.  Similarly, parentheses work the same; e.g.  <literal>(:*:) Int Bool</literal>.
+  </para></listitem>
+<listitem><para>
+  Fixities may be declared for type constructors just as for data constructors.  However,
+  one cannot distinguish between the two in a fixity declaration; a fixity declaration
+  sets the fixity for a data constructor and the corresponding type constructor.  For example:
+<screen>
+  infixl 7 T, :*:
+</screen>
+  sets the fixity for both type constructor <literal>T</literal> and data constructor <literal>T</literal>,
+  and similarly for <literal>:*:</literal>.
+  <literal>Int `a` Bool</literal>.
+  </para></listitem>
+<listitem><para>
+  Function arrow is <literal>infixr</literal> with fixity 0.  (This might change; I'm not sure what it should be.)
+  </para></listitem>
+<listitem><para>
+  Data type and type-synonym declarations can be written infix.  E.g.
+<screen>
+  data a :*: b = Foo a b
+  type a :+: b = Either a b
+</screen>
+  </para></listitem>
+<listitem><para>
+  The only thing that differs between operators in types and operators in expressions is that
+  ordinary non-constructor operators, such as <literal>+</literal> and <literal>*</literal>
+  are not allowed in types. Reason: the uniform thing to do would be to make them type
+  variables, but that's not very useful.  A less uniform but more useful thing would be to
+  allow them to be type <emphasis>constructors</emphasis>.  But that gives trouble in export
+  lists.  So for now we just exclude them.
+  </para></listitem>
+
+</itemizedlist>
+</para>
+</sect2>
+
 <sect2 id="class-method-types">
 <title>Class method types
 </title>
@@ -1825,7 +1878,7 @@ declarations.  Define your own instances!
 </sect2>
 
 <sect2 id="scoped-type-variables">
-<title>Scoped Type Variables
+<title>Scoped type variables
 </title>
 
 <para>