+
+
+<sect3 id="class-default-signatures">
+<title>Default signatures</title>
+
+<para>
+Haskell 98 allows you to define a default implementation when declaring a class:
+<programlisting>
+ class Enum a where
+ enum :: [a]
+ enum = []
+</programlisting>
+The type of the <literal>enum</literal> method is <literal>[a]</literal>, and
+this is also the type of the default method. You can lift this restriction
+and give another type to the default method using the flag
+<option>-XDefaultSignatures</option>. For instance, if you have written a
+generic implementation of enumeration in a class <literal>GEnum</literal>
+with method <literal>genum</literal> in terms of <literal>GHC.Generics</literal>,
+you can specify a default method that uses that generic implementation:
+<programlisting>
+ class Enum a where
+ enum :: [a]
+ default enum :: (Generic a, GEnum (Rep a)) => [a]
+ enum = map to genum
+</programlisting>
+We reuse the keyword <literal>default</literal> to signal that a signature
+applies to the default method only; when defining instances of the
+<literal>Enum</literal> class, the original type <literal>[a]</literal> of
+<literal>enum</literal> still applies. When giving an empty instance, however,
+the default implementation <literal>map to0 genum</literal> is filled-in,
+and type-checked with the type
+<literal>(Generic a, GEnum (Rep a)) => [a]</literal>.
+</para>
+
+<para>
+We use default signatures to simplify generic programming in GHC
+(<xref linkend="generic-programming"/>).
+</para>
+
+
+</sect3>