Hugo Pacheco:
Just something I have been wondering.
I would like to implement somehting like:
type family F a :: * -> * ... class C a b where ... instance (F a ~ F b) => C a b where ...
But apparently type equality coercions can not be used as a single context. If I enable -fallow-undecidable-instances, whenever the equality does not hold, the instance returns a compile error, what does make sense.
Is there any way I could overcome this?
I think I don't understand your question fully. This class instance requires both -XFlexibleInstances and -fallow-undecidable-instances to compile. If the equality does not hold, you should get a type error because your program is not type correct. So, what is it that you would like different? Manuel