Type application with ambiguous types

问题

I recently discovered -XTypeApplications, which allows me to write abbreviate my code. Consider the following:

{-# LANGUAGE AllowAmbiguousTypes, DataKinds, KindSignatures, RankNTypes, 
             ScopedTypeVariables, TypeApplications #-}

import Data.Tagged
import Data.Proxy

class Foo (a :: Bool) where
  foo :: Tagged a Int

instance Foo 'True where
  foo = 3

instance Foo 'False where
  foo = 4

instead of writing proxy foo (Proxy::Proxy 'True), I can now write untag $ foo @'True, which saves me a lot of Proxy boilerplate. That's okay, but we can do better with ambiguous types:

foo' :: forall (a :: Bool) . (Foo a) => Int
foo' = untag $ foo @a

Now I can write foo' @'True! Note that although the type was ambiguous prior to -XTypeApplications, it is no longer since I can specify the type a. I think this is pretty great, but others might not. So I was hoping to introduce a wrapper

wrapAmbiguous :: forall (a :: Bool) b . (Foo a) => (forall (t :: Bool) . (Foo t) => b) -> Tagged a b
wrapAmbiguous f = Tagged $ f @a

(even more general in my actual use case, but this gets the point across), so that users don't need -XTypeApplications. However, when I try to use wrapAmbiguous like proxy (wrapAmbiguous foo') (Proxy::Proxy 'True), I get the error

• Could not deduce (Foo a0) arising from a use of ‘foo'’
  from the context: Foo t
    bound by a type expected by the context:
               Foo t => Int
    at Foo.hs:26:18-35
  The type variable ‘a0’ is ambiguous
  These potential instances exist:
    instance Foo 'False -- Defined at Foo.hs:12:10
    instance Foo 'True -- Defined at Foo.hs:9:10
• In the first argument of ‘wrapAmbiguous’, namely ‘foo'’
  In the first argument of ‘proxy’, namely ‘(wrapAmbiguous foo')’
  In the second argument of ‘($)’, namely
    ‘proxy (wrapAmbiguous foo') (Proxy :: Proxy True)’

It seems to me that this should be legal, but GHC apparently can't unify the type of foo' with forall (t :: Bool) (Foo t) => b. Is there some way to make my wrapper work?

来源：https://stackoverflow.com/questions/39772531/type-application-with-ambiguous-types

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