[Haskell-cafe] Impredicativity confusion

Jeff Polakow jeff.polakow at db.com
Wed Aug 22 10:11:17 EDT 2007


> {-# 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.



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