Hello,
{-# OPTIONS_GHC -fglasgow-exts #-}
data Foo a
foo :: Foo a -> a -> Bool foo = undefined
newtype A = A (forall a. a->a)
ok = foo f (A id) where f = undefined :: Foo A
type B = forall a. a->a
boom = foo f (id :: B) where f = undefined :: Foo B
boom doesn't typecheck because foo's second argument is of type a which will cause GHC to treat it monomorphically (at least from the top-level of the type-- excuse my ignorance of the correct terminology for this). ok typechecks because the forall is hidden under the A. To better illustrate, the following will typecheck: foo :: Foo (forall a.a -> a) -> (forall a.a -> a) -> Bool foo = undefined type B = forall a.a -> a boom = foo f (id :: B) where f = undefined :: Foo B but this type for foo will prevent ok from typechecking. -Jeff --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.