Warning for missing export lists

[tim]

It occurred to me that it would be nice to have a warning for modules that lack an export list. Obviously not everybody prefers to write export lists for all their modules, but that's why it would be an optional warning. (It would also be handy to use in conjunction with -fwarn-unused-binds.)

If there is already such a flag, then the task is to add it to the users' guide, since I don't see it there :-)

[igloo]

I guess we wouldn't warn about

module Foo (module Foo) where ...

?

[morabbin]

Methinks this sort of warning is better suited for the likes of hlint