Claus Reinke wrote:
.. I'm not trying to say that Haskell 98 only supports global instances, rather I'm saying we should think of it that way and design our libraries with global instances in mind, because otherwise we have serious problems with abstraction.
Thinking of it that way still doesn't make it so. If you want instances to be global, you have to change Haskell 98, by propagating instances through the whole program.
Sigh. I'm not having much luck convincing you, so let me try a 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? Think carefully about the ramifications of your answer. Oh, and "Haskell 98" is not a valid answer, because we can't change that. Cheers, Simon