</sect1>
<!-- UNBOXED TYPES AND PRIMITIVE OPERATIONS -->
-<!-- included from primitives.sgml -->
-<!-- &primitives; -->
<sect1 id="primitives">
<title>Unboxed types and primitive operations</title>
(<literal>Double#</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>
</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 ================= -->