First documentation on stand-alone instance deriving.

[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 09fd3f5..b46bf30 100644 (file)
--- a/docs/users_guide/glasgow_exts.xml
+++ b/docs/users_guide/glasgow_exts.xml
@@ -3301,6 +3301,7 @@ changing the program.</para></listitem>
 A <emphasis>lexically scoped type variable</emphasis> can be bound by:
 <itemizedlist>
 <listitem><para>A declaration type signature (<xref linkend="decl-type-sigs"/>)</para></listitem>
+<listitem><para>An expression type signature (<xref linkend="exp-type-sigs"/>)</para></listitem>
 <listitem><para>A pattern type signature (<xref linkend="pattern-type-sigs"/>)</para></listitem>
 <listitem><para>Class and instance declarations (<xref linkend="cls-inst-scoped-tyvars"/>)</para></listitem>
 </itemizedlist>
@@ -3352,6 +3353,23 @@ quantification rules.
 </para>
 </sect3>
 
+<sect3 id="exp-type-sigs">
+<title>Expression type signatures</title>
+
+<para>An expression type signature that has <emphasis>explicit</emphasis>
+quantification (using <literal>forall</literal>) brings into scope the
+explicitly-quantified
+type variables, in the annotated expression.  For example:
+<programlisting>
+  f = runST ( (op >>= \(x :: STRef s Int) -> g x) :: forall s. ST s Bool )
+</programlisting>
+Here, the type signature <literal>forall a. ST s Bool</literal> brings the 
+type variable <literal>s</literal> into scope, in the annotated expression 
+<literal>(op >>= \(x :: STRef s Int) -> g x)</literal>.
+</para>
+
+</sect3>
+
 <sect3 id="pattern-type-sigs">
 <title>Pattern type signatures</title>
 <para>
@@ -3709,6 +3727,33 @@ the standard method is used or the one described here.)
 
 </sect2>
 
+<sect2 id="stand-alone-deriving">
+<title>Stand-alone deriving declarations</title>
+
+<para>
+GHC now allows stand-alone <literal>deriving</literal> declarations:
+</para>
+
+<programlisting>
+  data Foo = Bar Int | Baz String
+
+  deriving Eq for Foo
+</programlisting>
+
+<para>Deriving instances of multi-parameter type classes for newtypes is
+also allowed:</para>
+
+<programlisting>
+  newtype Foo a = MkFoo (State Int a)
+
+  deriving (MonadState Int) for Foo
+</programlisting>
+
+<para>
+</para>
+
+</sect2>
+
 <sect2 id="typing-binds">
 <title>Generalised typing of mutually recursive bindings</title>