Dear all, I have been curious about the ability to detect the presence of a certain instance (ClassFoo TypeBar) in the type system. Specifically, is it possible to "dispatch" a type on the existence (or not) of such an instance. For example given two functions: withInstance :: (ClassFoo TypeBar) => TypeIfInstanceExists withoutInstance :: TypeIfInstanceDoesNotExists I would be able to consolidate them into something like this: withOrWithoutInstance :: (r ~ InstanceExists ClassFoo TypeBar, a ~ If r TypeIfInstanceExists TypeIfInstanceDoesNotExists) => a I guess what I need is: 1) A type-level "if". 2) The possibility of "converting" a constraint into a type-level bool. I am sure (1) is possible but have no idea about (2). Anyone? Best regards, Hans