The problem is that GHC's type system is (almost entirely)
predicative. I couldn't tell you just what that means, but to a first
approximation, it means that type variables cannot be instantiated to
polymorphic types. You write
trip = Wrap . extract
which means
(.) Wrap extract
(.)::(b->c)->(a->b)->a->c
Wrap :: (forall f. Functor f => f Int) -> Wrap
The trouble here is that the type variable b in the type of (.) isn't
allowed to be polymorphic, but Wrap's argument must be.
Note that there's a weird exception: ($) actually has an impredicative
type, because it's a special case in the type checker. This is largely
for historical reasons.
On Tue, Feb 10, 2015 at 3:38 PM, Tyson Whitehead <twhiteh...@gmail.com> wrote:
> I came across something that seems a bit strange to me. Here is a simplified
> version (the original was trying to move from a lens ReifiedFold to a
> lens-action ReifiedMonadicFold)
>
> {-# LANGUAGE RankNTypes #-}
>
> import Control.Applicative
>
> newtype Wrap = Wrap { extract :: forall f. Functor f => f Int }
>
> trip :: Wrap -> Wrap
> trip a = Wrap (extract a)
>
> The compiler is okay with this. It chokes on this alternative though
>
> trip :: Wrap -> Wrap
> trip = Wrap . extract
>
> giving (GHC 7.8.2)
>
> Couldn't match type ‘f0 Int’
> with ‘forall (f :: * -> *). Functor f => f Int’
> Expected type: f0 Int -> Wrap
> Actual type: (forall (f :: * -> *). Functor f => f Int) -> Wrap
> In the first argument of ‘(.)’, namely ‘Wrap’
> In the expression: Wrap . extract
>
> I'm guessing this is because the compiler fancy footwork to handle the
> implicit parameters, something like
>
> trip a = Wrap (\f fDict -> extract a f fDict)
>
> where f is the Functor type and fDict is the associated dictionary, isn't
> compatible with the (.) definition of
>
> f . g = \x -> f (g x)
>
> Is this correct? I would appreciate anyone insight here. Is there a way
> combine these (.) style?
>
> Thanks! -Tyson
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