class Wibble a where
visit :: (Foo -> b) -> (Bar -> b) -> a -> b
The type `forall b. (X -> b) -> (Y -> b) -> b` is isomorphic to `Either X Y`. If you rearrange the signature of `visit` a little bit, visit_ :: a -> forall b . (Foo -> b) -> (Bar -> b) -> b you can see that you're really just defining a function `a -> Either Foo Bar`. So the class can be simplified to: class Wibble' a where visit' :: a -> Either Foo Bar It follows that `WibblePackage'` is equivalent to type WibblePackage' = forall a . (WibbleInstance a, a) where type WibbleInstance a = a -> Either Foo Bar Because of the `forall`, there's nothing you can do with the second component of the tuple (`a`) except to apply it to the first (`a -> Either Foo Bar`), so it's just a long-winded way of saying type WibblePackage'' = Either Foo Bar In a non-strict language like Haskell, there's no reason to use such a complicated type signature to delay the application of `a -> Either Foo Bar` to `a`.