Perhaps Niklas can say whether #1148 is the bug he is describing, and if not file a new one?
Niklas's example:
module A where foo x = x
module B(foo) where import A
module C where import A import B c = foo 'a'
ghc -W says
C.hs:2:0: Warning: Module `A' is imported, but nothing from it is used, except perhaps instances visible in `A' To suppress this warning, use: import A()
It's true that the import can be deleted, but not that "nothing from it is used".
This is actually not what I was hinting at, I'll see if I can reproduce the bug I experienced. The basic idea behind it was that module A in package a defines some type class, with member function foo. Modules B and C in packages b and c separately import A and create instances of that class, and reexports foo. Then module D in package D cannot import both B and C and use foo, it will give an "Ambiguous occurence" error. I tried reproducing this in a simpler setting, with ABCD all in one package, but the error wouldn't appear. I hope I wasn't imagining things (or rather, it would be good if I was :-)). There may have been some qualified imports involved too... But there are a few other inconvenient behaviors related only to warnings, for instance module A where foo x = x module B(foo) where import A module C (module A, module B) where import A import B %> ghc --make C.hs -W: C.hs:1:20: Warning: `foo' is exported by `module B' and `module A' That's certainly true, but it's the same 'foo'. Just a warning of course, and I'm not so sure I would argue that it shouldn't be there. It's the same kind of warning as with any other multiple export of a symbol I guess. But this pattern is not uncommon (in my code at least), so the warning is inconvenient. Then again, that particular warning can be turned off. Another, somewhat stranger, behavior: module A where foo x = x module B(foo) where import A module C(foo) where import A module D () where import B import C bar x = foo x %> ghc --make D.hs -W D.hs:2:0: Warning: Module `B' is imported, but nothing from it is used, except perhaps instances visible in `B' To suppress this warning, use: import B() D.hs:3:0: Warning: Module `C' is imported, but nothing from it is used, except perhaps instances visible in `C' To suppress this warning, use: import C() D.hs:4:0: Warning: Defined but not used: `bar' This might be said to be a bug, although it's a bug that would rarely bite you, since it requires that 'bar' is not exported (or used in something that is exported). If you export bar, then the warning talks only of B - which is a rather arbitrary choice. I'll see what I can do about that more serious bug. As regards to this particular discussion though - my main point is that we shouldn't make decisions on libraries based on what tools make of them, rather the other way around. Obvious perhaps (I hope), but bears repeating. :-) Cheers, /Niklas