Ticket #5600 (closed bug: invalid)
Instantiation of a rank 2 type fails for type synonyms
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.