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.