Derive Typable for promoted data constructors (Trac #8950)

I got sucked into a significant refactoring of the way that
Typeable instances are derived.  This makes it simpler and
more uniform.

I also improved the documentation in the user manual.  Typeable
really is different to other classes, and now gets its own subsection.

2 files changed, 62 insertions, 33 deletions

diff --git a/docs/users_guide/flags.xml b/docs/users_guide/flags.xml
index 6acd28dc0a..0ad6fc2f6c 100644
--- a/docs/users_guide/flags.xml
+++ b/docs/users_guide/flags.xml
@@ -745,7 +745,7 @@
           </row>
           <row>
             <entry><option>-XAutoDeriveTypeable</option></entry>
-            <entry>Automatically <link linkend="auto-derive-typeable">derive Typeable instances for every datatype and type class declaration</link>.
+            <entry>Automatically <link linkend="deriving-typeable">derive Typeable instances for every datatype and type class declaration</link>.
             Implies <option>-XDeriveDataTypeable</option>.</entry>
             <entry>dynamic</entry>
             <entry><option>-XNoAutoDeriveTypeable</option></entry>
diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml
index 33124122ef..44f0d98ab2 100644
--- a/docs/users_guide/glasgow_exts.xml
+++ b/docs/users_guide/glasgow_exts.xml
@@ -3842,9 +3842,8 @@ GHC always treats the <emphasis>last</emphasis> parameter of the instance
 
 </sect2>
 
-
-<sect2 id="deriving-typeable">
-<title>Deriving clause for extra classes (<literal>Typeable</literal>, <literal>Data</literal>, etc)</title>
+<sect2 id="deriving-extra">
+<title>Deriving instances of extra classes (<literal>Data</literal>, etc)</title>
 
 <para>
 Haskell 98 allows the programmer to add "<literal>deriving( Eq, Ord )</literal>" to a data type
@@ -3856,27 +3855,6 @@ classes <literal>Eq</literal>, <literal>Ord</literal>,
 <para>
 GHC extends this list with several more classes that may be automatically derived:
 <itemizedlist>
-<listitem><para> With <option>-XDeriveDataTypeable</option>, you can derive instances of the classes
-<literal>Typeable</literal>, and <literal>Data</literal>, defined in the library
-modules <literal>Data.Typeable</literal> and <literal>Data.Data</literal> respectively.
-</para>
-<para>Since GHC 7.8.1, <literal>Typeable</literal> is kind-polymorphic (see
-<xref linkend="kind-polymorphism"/>) and can be derived for any datatype and
-type class. Instances for datatypes can be derived by attaching a
-<literal>deriving Typeable</literal> clause to the datatype declaration, or by
-using standalone deriving (see <xref linkend="stand-alone-deriving"/>).
-Instances for type classes can only be derived using standalone deriving.
-For data families, <literal>Typeable</literal> should only be derived for the
-uninstantiated family type; each instance will then automatically have a
-<literal>Typeable</literal> instance too.
-See also <xref linkend="auto-derive-typeable"/>.
-</para>
-<para>
-Also since GHC 7.8.1, handwritten (ie. not derived) instances of
-<literal>Typeable</literal> are forbidden, and will result in an error.
-</para>
-</listitem>
-
 <listitem><para> With <option>-XDeriveGeneric</option>, you can derive
 instances of the classes <literal>Generic</literal> and
 <literal>Generic1</literal>, defined in <literal>GHC.Generics</literal>.
@@ -3889,6 +3867,12 @@ the class <literal>Functor</literal>,
 defined in <literal>GHC.Base</literal>.
 </para></listitem>
 
+<listitem><para> With <option>-XDeriveDataTypeable</option>, you can derive instances of
+the class <literal>Data</literal>,
+defined in <literal>Data.Data</literal>.  See <xref linkend="deriving-typeable"/> for 
+deriving <literal>Typeable</literal>.
+</para></listitem>
+
 <listitem><para> With <option>-XDeriveFoldable</option>, you can derive instances of
 the class <literal>Foldable</literal>,
 defined in <literal>Data.Foldable</literal>.
@@ -3899,21 +3883,66 @@ the class <literal>Traversable</literal>,
 defined in <literal>Data.Traversable</literal>.
 </para></listitem>
 </itemizedlist>
+You can also use a standalone deriving declaration instead
+(see <xref linkend="stand-alone-deriving"/>).
+</para>
+<para>
 In each case the appropriate class must be in scope before it
 can be mentioned in the <literal>deriving</literal> clause.
 </para>
 </sect2>
 
-<sect2 id="auto-derive-typeable">
-<title>Automatically deriving <literal>Typeable</literal> instances</title>
+<sect2 id="deriving-typeable">
+<title>Deriving <literal>Typeable</literal> instances</title>
 
-<para>
+<para>The class <literal>Typeable</literal> is very special:
+<itemizedlist>
+<listitem><para>
+<literal>Typeable</literal> is kind-polymorphic (see
+<xref linkend="kind-polymorphism"/>).
+</para></listitem>
+
+<listitem><para>
+Only derived instances of <literal>Typeable</literal> are allowed;
+i.e. handwritten instances are forbidden.  This ensures that the
+programmer cannot subert the type system by writing bogus instances.
+</para></listitem>
+
+<listitem><para>
+With <option>-XDeriveDataTypeable</option> 
+GHC allows you to derive instances of <literal>Typeable</literal> for data types or newtypes,
+using a <literal>deriving</literal> clause, or using 
+a standalone deriving declaration (<xref linkend="stand-alone-deriving"/>).
+</para></listitem>
+
+<listitem><para>
+With <option>-XDataKinds</option>, deriving <literal>Typeable</literal> for a data
+type (whether via a deriving clause or standalone deriving)
+also derives <literal>Typeable</literal> for the promoted data constructors (<xref linkend="promotion"/>).
+</para></listitem>
+
+<listitem><para>
+However, using standalone deriving, you can <emphasis>also</emphasis> derive
+a <literal>Typeable</literal> instance for a data family.
+You may not add a <literal>deriving(Typeable)</literal> clause to a
+<literal>data instance</literal> declaration; instead you must use a
+standalone deriving declaration for the data family.
+</para></listitem>
+
+<listitem><para>
+Using standalone deriving, you can <emphasis>also</emphasis> derive
+a <literal>Typeable</literal> instance for a type class.
+</para></listitem>
+
+<listitem><para>
 The flag <option>-XAutoDeriveTypeable</option> triggers the generation
-of derived <literal>Typeable</literal> instances for every datatype and type
-class declaration in the module it is used. It will also generate
-<literal>Typeable</literal> instances for any promoted data constructors
-(<xref linkend="promotion"/>). This flag implies
-<option>-XDeriveDataTypeable</option> (<xref linkend="deriving-typeable"/>).
+of derived <literal>Typeable</literal> instances for every datatype, data family, 
+and type class declaration in the module it is used, unless a manually-specified one is
+already provided.
+This flag implies <option>-XDeriveDataTypeable</option>.
+</para></listitem>
+</itemizedlist>
+
 </para>
 
 </sect2>