GHC panic: No skolem info: k2
The following code causes a GHC panic on GHC 8.0.1, 8.0.2, 8.2.1, and HEAD:
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE Trustworthy #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeOperators #-}
module Eliminator where
import Data.Kind
data family Sing (a :: k)
data instance Sing (z :: [a]) where
SNil :: Sing '[]
SCons :: Sing x -> Sing xs -> Sing (x:xs)
data TyFun :: * -> * -> *
type a ~> b = TyFun a b -> *
infixr 0 ~>
type family Apply (f :: k1 ~> k2) (x :: k1) :: k2
type a @@ b = Apply a b
infixl 9 @@
data FunArrow = (:->) | (:~>)
class FunType (arr :: FunArrow) where
type Fun (k1 :: Type) arr (k2 :: Type) :: Type
class FunType arr => AppType (arr :: FunArrow) where
type App k1 arr k2 (f :: Fun k1 arr k2) (x :: k1) :: k2
type FunApp arr = (FunType arr, AppType arr)
instance FunType (:->) where
type Fun k1 (:->) k2 = k1 -> k2
$(return []) -- This is only necessary for GHC 8.0 -- GHC 8.2 is smarter
instance AppType (:->) where
type App k1 (:->) k2 (f :: k1 -> k2) x = f x
instance FunType (:~>) where
type Fun k1 (:~>) k2 = k1 ~> k2
$(return [])
instance AppType (:~>) where
type App k1 (:~>) k2 (f :: k1 ~> k2) x = f @@ x
infixr 0 -?>
type (-?>) (k1 :: Type) (k2 :: Type) (arr :: FunArrow) = Fun k1 arr k2
listElim :: forall (a :: Type) (p :: [a] -> Type) (l :: [a]).
Sing l
-> p '[]
-> (forall (x :: a) (xs :: [a]). Sing x -> Sing xs -> p xs -> p (x:xs))
-> p l
listElim = listElimPoly @(:->) @a @p @l
listElimTyFun :: forall (a :: Type) (p :: [a] ~> Type) (l :: [a]).
Sing l
-> p @@ '[]
-> (forall (x :: a) (xs :: [a]). Sing x -> Sing xs -> p @@ xs -> p @@ (x:xs))
-> p @@ l
-- The line below causes a GHC panic. It should not typecheck;
-- uncomment the line below it for the correct version
listElimTyFun = listElimPoly @(:->) @a @p @l
-- listElimTyFun = listElimPoly @(:~>) @a @p @l
listElimPoly :: forall (arr :: FunArrow) (a :: Type) (p :: ([a] -?> Type) arr) (l :: [a]).
FunApp arr
=> Sing l
-> App [a] arr Type p '[]
-> (forall (x :: a) (xs :: [a]). Sing x -> Sing xs -> App [a] arr Type p xs -> App [a] arr Type p (x:xs))
-> App [a] arr Type p l
listElimPoly SNil pNil _ = pNil
listElimPoly (SCons x (xs :: Sing xs)) pNil pCons = pCons x xs (listElimPoly @arr @a @p @xs xs pNil pCons)
$ /opt/ghc/8.2.1/bin/ghci Eliminator.hs
GHCi, version 8.2.0.20170623: http://www.haskell.org/ghc/ :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Eliminator ( Eliminator.hs, interpreted )
Eliminator.hs:72:17: error:ghc: panic! (the 'impossible' happened)
(GHC version 8.2.0.20170623 for x86_64-unknown-linux):
No skolem info:
k2_a5cr
Call stack:
CallStack (from HasCallStack):
prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in ghc:Outputable
pprPanic, called at compiler/typecheck/TcErrors.hs:2653:5 in ghc:TcErrors
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
Trac metadata
Trac field | Value |
---|---|
Version | 8.0.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |