Jeff Polakow wrote:
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.
I'm sorry, I don't understand this. GHC manual [1] says that "you can call a polymorphic function at a polymorphic type, and parameterise data structures over polymorphic types". That's what I do in the code I posted. What make GHC to treat 'a' monomorphically when explicit type signature is given? [1]http://haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html#imp...
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
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