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Diffstat (limited to 'docs')
| -rw-r--r-- | docs/users_guide/glasgow_exts.xml | 14 |
1 files changed, 14 insertions, 0 deletions
diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml index 7443abfb23..e3c0e73dba 100644 --- a/docs/users_guide/glasgow_exts.xml +++ b/docs/users_guide/glasgow_exts.xml @@ -2944,6 +2944,20 @@ data Counter a where As before, only one selector function is generated here, that for <literal>tag</literal>. Nevertheless, you can still use all the field names in pattern matching and record construction. </para></listitem> + +<listitem><para> +In a GADT-style data type declaration there is no obvious way to specify that a data constructor +should be infix, which makes a difference if you derive <literal>Show</literal> for the type. +(Data constructors declared infix are displayed infix by the derived <literal>show</literal>.) +So GHC implements the following design: a data constructor declared in a GADT-style data type +declaration is displayed infix by <literal>Show</literal> iff (a) it is an operator symbol, +(b) it has two arguments, (c) it has a programmer-supplied fixity declaration. For example +<programlisting> + infix 6 (:--:) + data T a where + (:--:) :: Int -> Bool -> T Int +</programlisting> +</para></listitem> </itemizedlist></para> </sect2> |