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)
Ah, got it. Thanks. I didn't realize instances of an imported module were also implicitly exported. I should have looked at the report, however. For the future reference of others: "Instance declarations cannot be explicitly named on import or export lists. All instances in scope within a module are *always *exported and any import brings *all* instances in from the imported module. Thus, an instance declaration is in scope if and only if a chain of import declarations leads to the module containing the instance declaration." -- 5.4 Importing and Exporting Instance Declarations at http://www.haskell.org/onlinereport/modules.html It would definitely be nice to have a bit more control over importing/exporting instances. I noticed this idea has been discussed a few times already. Sean