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| author | Richard Eisenberg <eir@cis.upenn.edu> | 2014-01-10 15:26:02 -0500 |
|---|---|---|
| committer | Richard Eisenberg <eir@cis.upenn.edu> | 2014-01-10 16:12:33 -0500 |
| commit | 566ba6fa10ade0af159a7b86f4df706cbd6b4163 (patch) | |
| tree | f50e6f0f8ae4abe9b1d83c99c5c662658a4d396d /docs | |
| parent | 0369c9745e2ce66f4c2d817a3aa9f2487395e0c7 (diff) | |
| download | haskell-566ba6fa10ade0af159a7b86f4df706cbd6b4163.tar.gz | |
Fix #8599.
This change is just some documentation around ignoring the context
of an enclosing instance when processing `deriving` clauses of an
associated data instance.
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/users_guide/glasgow_exts.xml | 19 |
1 files changed, 18 insertions, 1 deletions
diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml index d62f9953af..70a38768f9 100644 --- a/docs/users_guide/glasgow_exts.xml +++ b/docs/users_guide/glasgow_exts.xml @@ -5679,6 +5679,23 @@ instance C [c] d where type safety. </para> </sect3> + + <sect3><title>Instance contexts and associated type and data instances</title> + <para>Associated type and data instance declarations do not inherit any + context specified on the enclosing instance. For type instance declarations, + it is unclear what the context would mean. For data instance declarations, + it is unlikely a user would want the context repeated for every data constructor. + The only place where the context might likely be useful is in a + <literal>deriving</literal> clause of an associated data instance. However, + even here, the role of the outer instance context is murky. So, for + clarity, we just stick to the rule above: the enclosing instance context + is ignored. If you need to use + a non-trivial context on a derived instance, + use a <link linkend="stand-alone-deriving">standalone + deriving</link> clause (at the top level). + </para> + </sect3> + </sect2> <sect2 id="data-family-import-export"> @@ -6087,7 +6104,7 @@ instance C b where Yet, in the instance declaration, the right-hand side of the associated type instance <literal>b -> b</literal> says that <literal>b</literal> must be of kind <literal>*</literal>. GHC could theoretically propagate this information back into the instance head, and make that instance declaration apply only to type of kind <literal>*</literal>, as opposed -to types of any kind. <emphasis>However, GHC does not do this.</emphasis></para> +to types of any kind. However, GHC does <emphasis>not</emphasis> do this.</para> <para>In short: GHC does <emphasis>not</emphasis> propagate kind information from the members of a class instance declaration into the instance declaration head.</para> |