class Foo p where instance Foo Double where foo :: Double -> (forall q . Foo q => q) foo p = p
From my humble (lack of) knowledge, there seems to be nothing wrong here, but ghc (5.03) complains about unifying q with Double.
Well, of course! The result has to have type (forall q . Foo q => q), but p actually has type Double. Of course GHC complains.
Ah yes, silly me. What I had in mind, I suppose, was something more along the lines of: foo :: Double -> (exists q . Foo q => q) Basically, I want to be able to write a function that takes a value and produces any value so long as the type of that returned value is an instance of Foo. So, for instance, a trimmed down real example of how I want to use this: class Foo p e where foo :: exists q . Foo q e => p -> e -> q bar :: p -> e -> Double instance Foo Double e where foo p _ = p bar p _ = p instance Foo [(e,Double)] e where foo l e = case lookup e l of { Nothing -> 0 ; _ -> 1 } bar l e = case lookup e l of { Nothing -> 0 ; Just x -> x } Of course this latter Foo definition isn't really the instance I would define, but that instance is much more complex and the complexity isn't needed to illustrate the point. The idea is that you take an instance of "Foo" together with an "e" (an event) and apply those to "foo" and you get a new instance of Foo. OTOH you can also take an instance of Foo together with an event and get a double out. Sadly I'm not aware of any such "exists" predicate. I'm hoping I'm simply not aware :). - Hal