Re: [Haskell-cafe] Rigid skolem type variable escaping scope

Quoting "Matthew Steele" :

{-# LANGUAGE Rank2Types #-}

class FooClass a where ...

foo :: (forall a. (FooClass a) => a -> Int) -> Bool foo fn = ...

newtype IntFn a = IntFn (a -> Int)

bar :: (forall a. (FooClass a) => IntFn a) -> Bool bar (IntFn fn) = foo fn

In case you hadn't yet discovered it, the solution here is to unpack the IntFn a bit later in a context where the required type argument is known: bar ifn = foo (case ifn of IntFn fn -> fn) Hope this helps. Lauri