Sean Leather wrote:

I have a module A that re-exports module B, and module B contains only

class
instances. Since I'm using -Wall, I get this warning about module B
exporting nothing. Is there a way to disable this particular warning,
since
it is unnecessary in this case? No mailing list messages or GHC
documentation has told me what to do.

Well, the warning is right that you don't need to re-export module B:
instances are implicitly exported.  So you could just remove the export of
"module B", unless there's a reason to export it (such as, you might add
some exported functions or data types to it later)

Hmm, the disappointing result of removing module B from the export list is
that now it doesn't show up in the Haddock-generated documentation for
module A. Not that there was anything specific in B to document, because
it's only instances. But I do want the user to know that importing A also
imports B. Sigh... I suppose there's CPP if I really want it.

You could put a link to the module in the intro-documentation that comes before the export list, possibly in a sentence saying e.g. "deliberately exports instances from <at> module B <at> " (except I've forgotten Haddock syntax and might have used it wrong there

It's an interesting use-case though.  I wonder if Haddock should automatically provide links to all instances exported from the module that are either (1) instances for datatypes or classes exported by the module, or (2) orphan (i.e. not defined in the same module as either the class or the datatype).  (It would be too tedious to list *all* exported instances, but luckily it's not necessary for completeness.)

On Thu, Aug 14, 2008 at 08:00:25PM +0200, Sean Leather wrote:
>     Well, the warning is right that you don't need to re-export module B:
>     instances are implicitly exported.  So you could just remove the export of
>     "module B", unless there's a reason to export it (such as, you might add
>     some exported functions or data types to it later)
>
> Hmm, the disappointing result of removing module B from the export list
> is that now it doesn't show up in the Haddock-generated documentation
> for module A. Not that there was anything specific in B to document,
> because it's only instances.  But I do want the user to know that
> importing A also imports B. Sigh... I suppose there's CPP if I really
> want it.

That shouldn't be necessary.  Because instances are implicitly exported
and imported, you'll really want to avoid orphan instances, so that the
user can be sure that if both the class and the data type are in scope,
so is the instance.  But then the instance will be listed under both
the class and the type in the Haddock documentation.  It won't matter
where the instance was defined, just as it doesn't matter where other
stuff is defined, just where it is in scope.

Unfortunately Haddock does not allow doc comments to be attached to
instances, but that's independent of whether the defining module is shown.