#12834: GHC panic: while printing Non type-variable argument -------------------------------------+------------------------------------- Reporter: phadej | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 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: -------------------------------------+------------------------------------- {{{ {-# LANGUAGE GADTs, TypeFamilies, DataKinds, TypeOperators, MultiParamTypeClasses, UndecidableInstances, UndecidableSuperClasses, FlexibleInstances, PolyKinds, KindSignatures #-} import GHC.Exts (Constraint) newtype I a = I a data NP :: (k -> *) -> [k] -> * where Nil :: NP f '[] (:*) :: f x -> NP f xs -> NP f (x ': xs) infixr 5 :* class (AllF f xs, SListI xs) => All (f :: k -> Constraint) (xs :: [k]) instance (AllF f xs, SListI xs) => All f xs data SList :: [k] -> * where SNil :: SList '[] SCons :: SListI xs => SList (x ': xs) class SListI (xs :: [k]) where -- | Get hold of the explicit singleton (that one can then -- pattern match on). sList :: SList xs instance SListI '[] where sList = SNil instance SListI xs => SListI (x ': xs) where sList = SCons -- | Type family used to implement 'All'. -- type family AllF (c :: k -> Constraint) (xs :: [k]) :: Constraint type instance AllF _c '[] = () type instance AllF c (x ': xs) = (c x, All c xs) semigroup :: All ((~) (Maybe Int)) xs => NP I xs -> NP I xs -> NP I xs semigroup = undefined }}} Causes {{{ ghc-failure-all.hs:37:14: error: • Non type-variable argumentghc: panic! (the 'impossible' happened) (GHC version 8.0.1 for x86_64-apple-darwin): print_equality ~ }}} If `AllF` is used directly in the definition of `sappend`, there is no error whatsoever. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12834 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler