#15625: GHC panic, with QuantifiedConstraints -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.1-beta1 Resolution: | Keywords: | QuantifiedConstraints Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by Iceland_jack: Old description:

I got a GHC Panic (I made some minor changes to GHC so it may have been added by me) but I think it's caused by the quality constraint

{{{ $ ~/code/latestghc/inplace/bin/ghc-stage2 --interactive -ignore-dot-ghci ~/hs/390.hs GHCi, version 8.7.20180828: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( /home/baldur/hs/390.hs, interpreted ) ghc-stage2: panic! (the 'impossible' happened) (GHC version 8.7.20180828 for x86_64-unknown-linux): ASSERT failed! co_a2DG df_a2DS @ Any Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable pprPanic, called at compiler/utils/Outputable.hs:1219:5 in ghc:Outputable assertPprPanic, called at compiler/coreSyn/CoreSubst.hs:189:49 in ghc:CoreSubst

Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug

...

}}}

given this code

{{{#!hs {-# Language RankNTypes, TypeInType, DataKinds, PolyKinds, TypeOperators, GADTs, FlexibleInstances, MultiParamTypeClasses, ConstraintKinds, CPP, UndecidableSuperClasses, QuantifiedConstraints, FlexibleContexts #-}

import Data.Kind

type Cat ob = ob -> ob -> Type

data KLEISLI (m :: Type -> Type) :: Cat (KL_kind m) where MkKLEISLI :: (a -> m b) -> KLEISLI(m) (KL a) (KL b)

data KL_kind (m :: Type -> Type) = KL Type

class (a ~ KL xx) => AsKL a xx instance (a ~ KL xx) => AsKL a xx

ekki__ :: Monad m => (forall xx. AsKL a xx) => KLEISLI m a a ekki__ = MkKLEISLI undefined }}}

New description: I got a GHC Panic (I made some minor changes to GHC so it may have been added by me) but I think it's caused by the equality constraint {{{ $ ~/code/latestghc/inplace/bin/ghc-stage2 --interactive -ignore-dot-ghci ~/hs/390.hs GHCi, version 8.7.20180828: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( /home/baldur/hs/390.hs, interpreted ) ghc-stage2: panic! (the 'impossible' happened) (GHC version 8.7.20180828 for x86_64-unknown-linux): ASSERT failed! co_a2DG df_a2DS @ Any Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable pprPanic, called at compiler/utils/Outputable.hs:1219:5 in ghc:Outputable assertPprPanic, called at compiler/coreSyn/CoreSubst.hs:189:49 in ghc:CoreSubst Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug

}}} given this code {{{#!hs {-# Language RankNTypes, TypeInType, DataKinds, PolyKinds, TypeOperators, GADTs, FlexibleInstances, MultiParamTypeClasses, ConstraintKinds, CPP, UndecidableSuperClasses, QuantifiedConstraints, FlexibleContexts #-} import Data.Kind type Cat ob = ob -> ob -> Type data KLEISLI (m :: Type -> Type) :: Cat (KL_kind m) where MkKLEISLI :: (a -> m b) -> KLEISLI(m) (KL a) (KL b) data KL_kind (m :: Type -> Type) = KL Type class (a ~ KL xx) => AsKL a xx instance (a ~ KL xx) => AsKL a xx ekki__ :: Monad m => (forall xx. AsKL a xx) => KLEISLI m a a ekki__ = MkKLEISLI undefined }}} -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15625#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#15625: GHC panic, with QuantifiedConstraints -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.1-beta1 Resolution: | Keywords: | QuantifiedConstraints Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Iceland_jack):

''[narrator] it was the equality constraint''

thanks monoidal :] the purpose for this is to be able to write `Control.Category.id` of the form {{{#!hs id :: KLEISLI m (KL a) (KL a) id = MkKLEISLI pure }}} but when we define {{{#!hs instance Category (KLEISLI m) where id :: KLEISLI m kl_a kl_a id = .. }}} GHC doesn't know that `kl_a` is of the form `KL a` (same as #7259) so I want to constraint the type signature of `id` by saying `(forall a. kl_a ~ KL a)`. But that is uncouth since it's "quantifying over constraints headed by `(~)`" (ticket:15593#comment:1) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15625#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#15625: GHC panic, with QuantifiedConstraints -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.1-beta1 Resolution: | Keywords: | QuantifiedConstraints Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Simon Peyton Jones ): In [changeset:"bd76875ae6ad0cdd734564dddfb9ab88a6de9579/ghc" bd76875a/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="bd76875ae6ad0cdd734564dddfb9ab88a6de9579" Allow (~) in the head of a quantified constraints Since the introduction of quantified constraints, GHC has rejected a quantified constraint with (~) in the head, thus f :: (forall a. blah => a ~ ty) => stuff I am frankly dubious that this is ever useful. But /is/ necessary for Coercible (representation equality version of (~)) and it does no harm to allow it for (~) as well. Plus, our users are asking for it (Trac #15359, #15625). It was really only excluded by accident, so this patch lifts the restriction. See TcCanonical Note [Equality superclasses in quantified constraints] There are a number of wrinkles: * If the context of the quantified constraint is empty, we can get trouble when we get down to unboxed equality (a ~# b) or (a ~R# b), as Trac #15625 showed. This is even more of a corner case, but it produced an outright crash, so I elaborated the superclass machinery in TcCanonical.makeStrictSuperClasses to add a void argument in this case. See Note [Equality superclasses in quantified constraints] * The restriction on (~) was in TcValidity.checkValidInstHead. In lifting the restriction I discovered an old special case for (~), namely | clas_nm `elem` [ heqTyConName, eqTyConName] , nameModule clas_nm /= this_mod This was (solely) to support the strange instance instance a ~~ b => a ~ b in Data.Type.Equality. But happily that is no longer with us, since commit f265008fb6f70830e7e92ce563f6d83833cef071 Refactor (~) to reduce the suerpclass stack So I removed the special case. * I found that the Core invariants on when we could have co = <expr> were entirely not written down. (Getting this wrong ws the proximate source of the crash in Trac #15625. So - Documented them better in CoreSyn Note [CoreSyn type and coercion invariant], - Modified CoreOpt and CoreLint to match - Modified CoreUtils.bindNonRec to match - Made MkCore.mkCoreLet use bindNonRec, rather than duplicate its logic - Made Simplify.rebuildCase case-to-let respect Note [CoreSyn type and coercion invariant], }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15625#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler