Claus Reinke wrote:
most of us would be happy if instance contexts would be required to uniquely determine the instance to be chosen, a rather conservative extension of current practice.
How would that work? There's no way to prove that instance (Num a) => Foo a where ... instance Foo Char where ... don't overlap, because there's no way to know what instances of Num might be declared in other modules. If you reason based on what's in scope, you end up with the same problems that Simon detailed for overlapping instances. I can't think of any situation where the context could safely be used to resolve overlap except for a class private to the current module whose instances are locally constrained, but that doesn't seem like it would be very useful. -- Ben