Re: Overlapping, undecidable, incoherent -- or worse?

I think you should read the GHC manual (assuming it is up to date and undecidable instances means what it says it does, the difference is... Without undecidable instances at least one type in the instance must not be a type variable. With undecidable instances you can have a default-instance (all type variables) or a class synonym (one instance - all type variables) Thats it... Neither GHC nor Hugs pay any attention to the dependancies when choosing which instance to use. The dependancies are only considered after the decision has been irrevocably made. If the dependancies don't hold, the only option the compiler has is to bail out with a compile time error. Keean.