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| author | Max Bolingbroke <batterseapower@hotmail.com> | 2011-09-06 15:14:06 +0100 |
|---|---|---|
| committer | Max Bolingbroke <batterseapower@hotmail.com> | 2011-09-09 08:27:10 +0100 |
| commit | 4c353372c32276f179b96d6964a5c33c3211d295 (patch) | |
| tree | 4430bd7fd52086781cdb76f9d454217d3dfc7ca4 | |
| parent | de8c8d68cabb5f24304fad2f03caa41fdf182b4f (diff) | |
| download | haskell-4c353372c32276f179b96d6964a5c33c3211d295.tar.gz | |
Documentation for associated type instance defaults
| -rw-r--r-- | docs/users_guide/glasgow_exts.xml | 16 |
1 files changed, 15 insertions, 1 deletions
diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml index 1988f74746..3e9f21b584 100644 --- a/docs/users_guide/glasgow_exts.xml +++ b/docs/users_guide/glasgow_exts.xml @@ -4878,7 +4878,21 @@ instance GMapKey Flob where the free indexed parameter is of a kind with a finite number of alternatives (unlike <literal>*</literal>). </para> - </sect3> + <para> + It is possible for the class defining the associated type to specify a default for + associated type instances. So for example, this is OK: +<programlisting> +class IsBoolMap v where + type Key v + type Key v = Int + + lookupKey :: Key v -> v -> Maybe Bool + +instance IsBoolMap [(Int, Bool)] where + lookupKey = lookup +</programlisting> + </para> + </sect3> <sect3 id="scoping-class-params"> <title>Scoping of class parameters</title> |