Sigh. I'm not having much luck convincing you, so let me try a
And there I thought the goal of this discussion was to reach a common understanding, not to convince anyone of anything..
different tack. Here's a scenario I'd like your opinion on:
We have a test in GHC's test suite called instance-leak, which tests that none of the Haskell 98 libraries exports the Functor instance for (->). I broke this test recently by accident. How did I break it? By using Data.Map internally in System.Process. The Functor instance for (->) is exported from Control.Monad.Instances, makes its way into Data.Map, then into System.Process and thereby into System, which is a Haskell 98 module.
In your opinion, who or what is at fault for this test failure?
You, of course:-p But seriously: I can't see easily how the instance even gets into Data.Map, but since Data.Map doesn't seem to be using either Functor or fmap (other than defining an instance itself), it has no business importing (and thus re-exporting) the instance. If it is actually using any of the other instances (is it?), then perhaps Control.Monad.Instances needs to be split up. That is the approach taken for Data.Generics.Instances, which also has no business being imported in Data.Map. Btw, it would be useful to have a tool for tracing instances (not just final export and original definition, but the import path in between). And it would be useful to see exported instances for each module in Haddock (not just alongside class and type).
Think carefully about the ramifications of your answer.
That was my quick answer. Do you think it will change?-) Claus