#14151: Invisible kind variable referenced in typeclass instance error message -------------------------------------+------------------------------------- Reporter: andrewthad | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Here is a minimal example: {{{ {-# LANGUAGE DataKinds #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE RankNTypes #-} newtype HFix h a = HFix (h (HFix h) a) class EqForall f where eqForall :: f a -> f a -> Bool class EqHetero h where eqHetero :: (forall x. f x -> f x -> Bool) -> h f a -> h f a -> Bool instance EqHetero h => EqForall (HFix h) where eqForall (HFix a) (HFix b) = eqHetero eqForall a b instance EqHetero h => Eq (HFix h a) where (==) = eqForall }}} When we build this with GHC 8.2.1, we get the following error message: {{{ instance_head.hs:18:10: error: • Variable ‘k’ occurs more often in the constraint ‘EqHetero h’ than in the instance head (Use UndecidableInstances to permit this) • In the instance declaration for ‘Eq (HFix h a)’ | 18 | instance EqHetero h => Eq (HFix h a) where | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ }}} The error message is bad, although most people trying to do this kind of thing can probably reason that there is an invisible `k` and that `h :: (k -> Type) -> k -> Type` and that `a :: k`. I don't really see a way to improve it, but I thought I would bring it up. Also, should the typechecker even reject this? I don't have a great understanding of how GHC's restrictions for keep things decidable work, but a repeated invisible variable seems like it's a somewhat different thing than a repeated visible variable. I'm likely wrong about this, but I just wanted to raise the question. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14151 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler