Added -ftype-families to the user's manual

[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 4ed8c2a..ac10e21 100644 (file)
--- a/docs/users_guide/glasgow_exts.xml
+++ b/docs/users_guide/glasgow_exts.xml
@@ -381,6 +381,13 @@ worse, the unboxed value might be larger than a pointer
 (<literal>Double&num;</literal> for instance).
 </para>
 </listitem>
+<listitem><para> You cannot define a newtype whose representation type
+(the argument type of the data constructor) is an unboxed type.  Thus,
+this is illegal:
+<programlisting>
+  newtype A = MkA Int#
+</programlisting>
+</para></listitem>
 <listitem><para> You cannot bind a variable with an unboxed type
 in a <emphasis>top-level</emphasis> binding.
 </para></listitem>
@@ -4132,6 +4139,33 @@ to work since it gets translated into an equality comparison.
 </para>
 </sect2>
 
+<sect2 id="type-families">
+<title>Type families
+</title>
+
+<para>
+GHC supports the definition of type families indexed by types.  They may be
+seen as an extension of Haskell 98's class-based overloading of values to
+types.  When type families are declared in classes, they are also known as
+associated types.
+</para>
+<para>
+There are two forms of type families: data families and type synonym families.
+Currently, only the former are fully implemented, while we are still working
+on the latter.  As a result, the specification of the language extension is
+also still to some degree in flux.  Hence, a more detailed description of
+the language extension and its use is currently available
+from <ulink url="http://haskell.org/haskellwiki/GHC/Indexed_types">the Haskell
+wiki page on type families</ulink>.  The material will be moved to this user's
+guide when it has stabilised.
+</para>
+<para>
+Type families are enabled by the flag <option>-ftype-families</option>.
+</para>
+
+
+</sect2>
+
 </sect1>
 <!-- ==================== End of type system extensions =================  -->