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The instantiation of a rank 2 type with a type containing type synonyms that rearrange or omit type variables fails.

For example, consider the following function:

mapR :: Functor f => (forall a. g a -> b) -> f (g a) -> f b
mapR f t = fmap f t

Now we want to instantiate the type variable g in the above type:

data F a = F a
type G a = F a

mapG :: Functor f =>  (forall a. G a -> b) -> f (G a) -> f b
mapG f t = mapR f t

This works fine. But if we change the definition of the type synonym G to

type G a = F ()

we get the following error message:

/home/paba/code/bug.hs:11:17:
    Could not deduce (a1 ~ ())
    from the context (Functor f)
      bound by the type signature for
                 mapG :: Functor f => (forall a1. G a1 -> b) -> f (G a) -> f b
      at /home/paba/code/bug.hs:11:1-19
      `a1' is a rigid type variable bound by
           a type expected by the context: F a1 -> b
           at /home/paba/code/bug.hs:11:12
    Expected type: F a -> b
      Actual type: G a0 -> b
    In the first argument of `mapR', namely `f'
    In the expression: mapR f t

This only happens for rank 2 types. If we change the type of mapR by removing the quantifier, the instantiation of the type of mapR succeeds.

Trac metadata

Trac field	Value
Version	7.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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