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{-# LANGUAGE RankNTypes, TypeFamilies #-}
-- Unification yielding a coercion under a forall
module Data.Vector.Unboxed where
import Control.Monad.ST ( ST )
import Data.Kind (Type)
data MVector s a = MV
data Vector a = V
type family Mutable (v :: Type -> Type) :: Type -> Type -> Type
type instance Mutable Vector = MVector
create :: (forall s. MVector s a) -> Int
create = create1
-- Here we get Couldn't match expected type `forall s. MVector s a'
-- with actual type `forall s. Mutable Vector s a1'
-- Reason: when unifying under a for-all we don't solve type
-- equalities. Think more about this.
create1 :: (forall s. Mutable Vector s a) -> Int
create1 _ = error "urk"
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