Right. I know there was some argument a while back, but I thought
that position that instances are global period was pretty "official".
At least it made sense to me. The more libraries you import the less
control you have over the extent of what they may import. But I guess
it wouldn't be haskell if every third person didn't have an idea for a
better way to implement the mtl... I just want an exception with a
message though!

A few days ago I was trying to refactor some code to use transformers + monads-tf instead of mtl. Eventually I gave up after getting error messages about Either having conflicting Monad instances. A few of the libraries I'm using depend on mtl.

if I understand you correctly, all libraries that software I write depends on, directly or indirectly, must be free of namespace conflicts. Is that correct?