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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