diff options
| -rw-r--r-- | docs/users_guide/glasgow_exts.xml | 42 |
1 files changed, 36 insertions, 6 deletions
diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml index 43e84394c0..c0feb5bff0 100644 --- a/docs/users_guide/glasgow_exts.xml +++ b/docs/users_guide/glasgow_exts.xml @@ -2729,16 +2729,22 @@ GHC now allows stand-alone <literal>deriving</literal> declarations, enabled by </programlisting> The syntax is identical to that of an ordinary instance declaration apart from (a) the keyword <literal>deriving</literal>, and (b) the absence of the <literal>where</literal> part. -You must supply a context (in the example the context is <literal>(Eq a)</literal>), +Note the following points: +<itemizedlist> +<listitem><para> +You must supply an explicit context (in the example the context is <literal>(Eq a)</literal>), exactly as you would in an ordinary instance declaration. -(In contrast the context is inferred in a <literal>deriving</literal> clause -attached to a data type declaration.) +(In contrast, in a <literal>deriving</literal> clause +attached to a data type declaration, the context is inferred.) +</para></listitem> +<listitem><para> A <literal>deriving instance</literal> declaration must obey the same rules concerning form and termination as ordinary instance declarations, controlled by the same flags; see <xref linkend="instance-decls"/>. -</para> -<para> +</para></listitem> + +<listitem><para> Unlike a <literal>deriving</literal> declaration attached to a <literal>data</literal> declaration, the instance can be more specific than the data type (assuming you also use @@ -2752,8 +2758,31 @@ for example </programlisting> This will generate a derived instance for <literal>(Foo [a])</literal> and <literal>(Foo (Maybe a))</literal>, but other types such as <literal>(Foo (Int,Bool))</literal> will not be an instance of <literal>Eq</literal>. +</para></listitem> + +<listitem><para> +Unlike a <literal>deriving</literal> +declaration attached to a <literal>data</literal> declaration, +GHC does not restrict the form of the data type. Instead, GHC simply generates the appropriate +boilerplate code for the specified class, and typechecks it. If there is a type error, it is +your problem. (GHC will show you the offending code if it has a type error.) +The merit of this is that you can derive instances for GADTs and other exotic +data types, providing only that the boilerplate code does indeed typecheck. For example: +<programlisting> + data T a where + T1 :: T Int + T2 :: T Bool + + deriving instance Show (T a) +</programlisting> +In this example, you cannot say <literal>... deriving( Show )</literal> on the +data type declaration for <literal>T</literal>, +because <literal>T</literal> is a GADT, but you <emphasis>can</emphasis> generate +the instance declaration using stand-alone deriving. </para> +</listitem> +<listitem> <para>The stand-alone syntax is generalised for newtypes in exactly the same way that ordinary <literal>deriving</literal> clauses are generalised (<xref linkend="newtype-deriving"/>). For example: @@ -2764,7 +2793,8 @@ For example: </programlisting> GHC always treats the <emphasis>last</emphasis> parameter of the instance (<literal>Foo</literal> in this example) as the type whose instance is being derived. -</para> +</para></listitem> +</itemizedlist></para> </sect2> |