Ian Lynagh
* You want to use a different instance for the same type.
Instances are global in Haskell, so if one library needs Ratio instance 1, and another library needs Ratio instance 2, then those libraries cannot be used in the same program.
For me it's not so much about the libraries. I've run into cases (not in SYB) where I want my program to use a different definition of an instance than the one provided by a library, while still using the other stuff provided by that library. The pervasive globality of instances does seem to me to be a Haskell bug; it seems to me like they ought to be controlled using the same module export control rules as anything else. But I imagine there's some reason I don't understand that makes this hard or impossible...