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| -rw-r--r-- | docs/users_guide/glasgow_exts.xml | 35 |
1 files changed, 20 insertions, 15 deletions
diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml index 1adcdac7dc..a19bc8b629 100644 --- a/docs/users_guide/glasgow_exts.xml +++ b/docs/users_guide/glasgow_exts.xml @@ -4070,47 +4070,52 @@ the newtype and its representation. <sect3> <title> A more precise specification </title> <para> -Derived instance declarations are constructed as follows. Consider the -declaration (after expansion of any type synonyms) +A derived instance is derived only for declarations of these forms (after expansion of any type synonyms) <programlisting> - newtype T v1...vn = T' (t vk+1...vn) deriving (c1...cm) + newtype T v1..vn = MkT (t vk+1..vn) deriving (C t1..tj) + newtype instance T s1..sk vk+1..vn = MkT (t vk+1..vn) deriving (C t1..tj) </programlisting> - where <itemizedlist> <listitem><para> - The <literal>ci</literal> are partial applications of - classes of the form <literal>C t1'...tj'</literal>, where the arity of <literal>C</literal> +<literal>v1..vn</literal> are type variables, and <literal>t</literal>, +<literal>s1..sk</literal>, <literal>t1..tj</literal> are types. +</para></listitem> +<listitem><para> + The <literal>(C t1..tj)</literal> is a partial applications of the class <literal>C</literal>, + where the arity of <literal>C</literal> is exactly <literal>j+1</literal>. That is, <literal>C</literal> lacks exactly one type argument. </para></listitem> <listitem><para> - The <literal>k</literal> is chosen so that <literal>ci (T v1...vk)</literal> is well-kinded. + <literal>k</literal> is chosen so that <literal>C t1..tj (T v1...vk)</literal> is well-kinded. +(Or, in the case of a <literal>data instance</literal>, so that <literal>C t1..tj (T s1..sk)</literal> is +well kinded.) </para></listitem> <listitem><para> The type <literal>t</literal> is an arbitrary type. </para></listitem> <listitem><para> - The type variables <literal>vk+1...vn</literal> do not occur in <literal>t</literal>, - nor in the <literal>ci</literal>, and + The type variables <literal>vk+1...vn</literal> do not occur in the types <literal>t</literal>, + <literal>s1..sk</literal>, or <literal>t1..tj</literal>. </para></listitem> <listitem><para> - None of the <literal>ci</literal> is <literal>Read</literal>, <literal>Show</literal>, + <literal>C</literal> is not <literal>Read</literal>, <literal>Show</literal>, <literal>Typeable</literal>, or <literal>Data</literal>. These classes should not "look through" the type or its constructor. You can still derive these classes for a newtype, but it happens in the usual way, not via this new mechanism. </para></listitem> <listitem><para> - It is safe to coerce each of the methods of <literal>ci</literal>. That is, - the missing last argument to each of the <literal>ci</literal> is not used - at a nominal role in any of the <literal>ci</literal>'s methods. + It is safe to coerce each of the methods of <literal>C</literal>. That is, + the missing last argument to <literal>C</literal> is not used + at a nominal role in any of the <literal>C</literal>'s methods. (See <xref linkend="roles"/>.)</para></listitem> </itemizedlist> -Then, for each <literal>ci</literal>, the derived instance +Then the derived instance is of form declaration is: <programlisting> - instance ci t => ci (T v1...vk) + instance C t1..tj t => C t1..tj (T v1...vk) </programlisting> As an example which does <emphasis>not</emphasis> work, consider <programlisting> |