Commit c2c84b71 authored by simonpj's avatar simonpj
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[project @ 2005-03-08 08:48:35 by simonpj]

Add notes about newtype deriving
parent f97dcf9f
......@@ -3226,7 +3226,7 @@ class to the new type:
<para>
As a result of this extension, all derived instances in newtype
declarations are treated uniformly (and implemented just by reusing
declarations are treated uniformly (and implemented just by reusing
the dictionary for the representation type), <emphasis>except</emphasis>
<literal>Show</literal> and <literal>Read</literal>, which really behave differently for
the newtype and its representation.
......@@ -3307,6 +3307,14 @@ then we would not have been able to derive an instance for the
classes usually have one "main" parameter for which deriving new
instances is most interesting.
</para>
<para>Lastly, all of this applies only for classes other than
<literal>Read</literal>, <literal>Show</literal>, <literal>Typeable</literal>,
and <literal>Data</literal>, for which the built-in derivation applies (section
4.3.3. of the Haskell Report).
(For the standard classes <literal>Eq</literal>, <literal>Ord</literal>,
<literal>Ix</literal>, and <literal>Bounded</literal> it is immaterial whether
the standard method is used or the one described here.)
</para>
</sect3>
</sect2>
......
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