#12021: Type variable escapes its scope -------------------------------------+------------------------------------- Reporter: ttuegel | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1-rc3 Keywords: | Operating System: Linux Architecture: x86_64 | Type of failure: GHC rejects (amd64) | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- In the following example, I believe a type variable is incorrectly escaping its scope. The example produces the error: {{{ Bug.hs:20:20: error: • Expected kind ‘Cat k’, but ‘(:-)’ has kind ‘Constraint -> Constraint -> *’ • In the first argument of ‘Iso’, namely ‘(:-)’ In the type signature: functor :: forall a b. Iso (:-) (:-) (Ob (Dom f) a) (Ob (Dom f) b) (Ob (Cod f) (f a)) (Ob (Cod f) (f b)) In the class declaration for ‘Functor’ }}} If the definition of `Iso` is changed from {{{#!hs type Iso (c :: Cat k) (d :: Cat k) s t a b = forall p. (Cod p ~ Nat d (->)) => p a b -> p s t }}} to {{{#!hs type Iso (c :: Cat k) (d :: Cat k) s t a b = forall p. p a b -> p s t }}} then this error does not occur. (The example still will not compile because I have omitted almost the entire implementation, but it should not fail here.) I am not certain what is really happening here, but it seems to me that when the RHS of `Iso` is constrained, then the type variable `k` introduced on the LHS of `Iso` is being unified incorrectly with the type variable `k` introduced in the definition of the `Functor` class. When the constraint is removed, GHC seems to recognize (correctly!) that the type variables are distinct. (This bug actually occurs with GHC 8.0.1-rc4, but the "Version" menu doesn't give me that option.) {{{#!hs {-# LANGUAGE ConstraintKinds #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE NoImplicitPrelude #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeInType #-} {-# LANGUAGE TypeOperators #-} import GHC.Base ( Constraint, Type ) type Cat k = k -> k -> Type class Category (p :: Cat k) where type Ob p :: k -> Constraint class (Category (Dom f), Category (Cod f)) => Functor (f :: j -> k) where type Dom f :: Cat j type Cod f :: Cat k functor :: forall a b. Iso (:-) (:-) (Ob (Dom f) a) (Ob (Dom f) b) (Ob (Cod f) (f a)) (Ob (Cod f) (f b)) class (Functor f , Dom f ~ p, Cod f ~ q) => Fun (p :: Cat j) (q :: Cat k) (f :: j -> k) | f -> p q instance (Functor f , Dom f ~ p, Cod f ~ q) => Fun (p :: Cat j) (q :: Cat k) (f :: j -> k) data Nat (p :: Cat j) (q :: Cat k) (f :: j -> k) (g :: j -> k) type Iso (c :: Cat k) (d :: Cat k) s t a b = forall p. (Cod p ~ Nat d (->)) => p a b -> p s t data (p :: Constraint) :- (q :: Constraint) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12021 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler