Untouchable type variable (regression from 7.6)

Oleg says: what used to type check in GHC 7.4.1 (and I think in 7.6.2, although I no longer have access to that version) fails in GHC 7.8.2.

The following program type-checks with GHC 7.4.1 and GHC 7.8.2:

{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeFamilies #-}

module T where

foo :: (forall f g. (Functor f) => f a -> f b) -> [a] -> [b]
-- foo :: (forall f g. (Functor f, g ~ f) => g a -> g b) -> [a] -> [b]
foo tr x = tr x

t = foo (fmap not) [True]

The following code (which differs only in the signature of foo)

{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeFamilies #-}

module T where

-- foo :: (forall f g. (Functor f) => f a -> f b) -> [a] -> [b]
foo :: (forall f g. (Functor f, g ~ f) => g a -> g b) -> [a] -> [b]
foo tr x = tr x

t = foo (fmap not) [True]

type-checks with 7.4.1 but not with 7.8.2. The latter reports the error

    Couldn't match type `b' with `Bool'
      `b' is untouchable
        inside the constraints (Functor f, g ~ f)
        bound by a type expected by the context:
                   (Functor f, g ~ f) => g Bool -> g b
        at /tmp/t.hs:12:5-25
      `b' is a rigid type variable bound by
          the inferred type of t :: [b] at /tmp/t.hs:12:1
    Expected type: Bool -> b
      Actual type: Bool -> Bool
    Relevant bindings include t :: [b] (bound at /tmp/t.hs:12:1)
    In the first argument of `fmap', namely `not'
    In the first argument of `foo', namely `(fmap not)'

Giving t the type signature [Bool] fixes the problem. Alas, I come across the similar untouchable error in situations where giving the type signature is quite difficult (in local bindings, with quite large types).

Trac metadata

Trac field	Value
Version	7.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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