Opened 11 months ago
Closed 9 months ago
#9106 closed bug (fixed)
GHC Panic related to functional dependencies - kindFunResult
Reported by: | yuriy | Owned by: | |
---|---|---|---|
Priority: | normal | Milestone: | 7.8.3 |
Component: | Compiler | Version: | 7.8.2 |
Keywords: | Cc: | ||
Operating System: | Windows | Architecture: | x86 |
Type of failure: | Compile-time crash | Test Case: | polykinds/T9106 |
Blocked By: | Blocking: | ||
Related Tickets: | Differential Revisions: |
Description
Minimal example:
{-# LANGUAGE MultiParamTypeClasses, DataKinds, FunctionalDependencies, TypeOperators, KindSignatures, PolyKinds, FlexibleInstances, FlexibleContexts, UndecidableInstances #-} import GHC.TypeLits data Proxy (a :: k) = Proxy class FunctorN n f a fa | n f a -> fa where fmapn :: Proxy n -> Proxy f -> (a -> a) -> fa -> fa instance FunctorN 0 f a a where fmapn _ _ a = a instance (Functor f, FunctorN (n - 1) f a fa) => FunctorN n f a (f fa) where fmapn _ pf f = fmap (fmapn (Proxy :: Proxy (n-1)) pf f)
Crashes with ghc and ghci:
>ghc test [1 of 1] Compiling Main ( test.hs, test.o ) ghc.exe: panic! (the 'impossible' happened) (GHC version 7.8.20140130 for x86_64-unknown-mingw32): kindFunResult k{tv azb} [sk] >ghci test GHCi, version 7.8.20140130: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( test.hs, interpreted ) ghc.exe: panic! (the 'impossible' happened) (GHC version 7.8.20140130 for x86_64-unknown-mingw32): kindFunResult k{tv aPm} [sk]
Removing the functional dependency makes the code compile.
Attachments (3)
Change History (11)
Changed 11 months ago by yuriy
comment:1 Changed 11 months ago by yuriy
The same code without GHC.TypeLits works:
{-# LANGUAGE MultiParamTypeClasses, DataKinds, FunctionalDependencies, TypeOperators, KindSignatures, PolyKinds, FlexibleInstances, FlexibleContexts, UndecidableInstances, ScopedTypeVariables #-} data Proxy (a :: k) = Proxy data Nat = Z | P Nat class FunctorN n f a fa | n f a -> fa where fmapn :: Proxy n -> Proxy f -> (a -> a) -> fa -> fa instance FunctorN Z f a a where fmapn _ _ a = a instance (Functor f, FunctorN n f a fa) => FunctorN (P n) f a (f fa) where fmapn _ pf f = fmap (fmapn (Proxy :: Proxy n) pf f)
comment:2 Changed 11 months ago by darchon
Also annotating the Z instance with a monomorphic kind, (f :: *), makes the GHC.TypeLits version work:
{-# LANGUAGE MultiParamTypeClasses, DataKinds, FunctionalDependencies, TypeOperators, KindSignatures, PolyKinds, FlexibleInstances, FlexibleContexts, UndecidableInstances, ScopedTypeVariables #-} module FunctorN where import GHC.TypeLits data Proxy (a :: k) = Proxy class FunctorN n f a fa | n f a -> fa where fmapn :: Proxy n -> Proxy f -> (a -> a) -> fa -> fa instance FunctorN 0 (f :: *) a a where fmapn _ _ a = a instance (Functor f, FunctorN (n - 1) f a fa) => FunctorN n f a (f fa) where fmapn _ pf f = fmap (fmapn (Proxy :: Proxy (n-1)) pf f)
Without the annotation, f is poly-kinded in the first FunctorN instance (it is * -> * in the second instance), and the kindFunResult error seems to be related to this kind-variable. That is when I annotate the f in the first FunctorN instance as (f :: p) I get:
ghc: panic! (the 'impossible' happened) (GHC version 7.8.2 for x86_64-apple-darwin): kindFunResult p{tv a1FO} [sk]
comment:3 Changed 10 months ago by darchon
The type family version also works:
{-# LANGUAGE DataKinds, FlexibleInstances, MultiParamTypeClasses, PolyKinds, ScopedTypeVariables, TypeFamilies, TypeOperators, UndecidableInstances #-} module FunctorN where import GHC.TypeLits data Proxy (a :: k) = Proxy type family F n f a where F 0 f a = a F n f a = f (F (n-1) f a) class F n f a ~ fa => FunctorN n f a fa where fmapn :: Proxy n -> Proxy f -> (a -> a) -> fa -> fa instance FunctorN 0 f a a where fmapn _ _ a = a instance ( Functor f, FunctorN (n-1) f a fa, F (n - 1) f a ~ fa , F n f a ~ f fa) => FunctorN n f a (f fa) where fmapn _ pf f = fmap (fmapn (Proxy :: Proxy (n-1)) pf f) test :: Maybe (Maybe (Maybe Int)) test = fmapn (Proxy :: Proxy 3) (Proxy :: Proxy Maybe) (+1) (Just (Just (Just 3)))
comment:4 Changed 10 months ago by simonpj
I'm working on this... I know more or less what's happening. Thanks for the examples
comment:5 Changed 10 months ago by Simon Peyton Jones <simonpj@…>
comment:6 Changed 10 months ago by simonpj
- Status changed from new to merge
- Test Case set to polykinds/T9106
Thank you. I've fixed the crash. Now we get
T9106.hs:13:10: Illegal instance declaration for ‘FunctorN n f a (f fa)’ The liberal coverage condition fails in class ‘FunctorN’ for functional dependency: ‘n f a -> fa’ Reason: lhs types ‘n’, ‘f’, ‘a’ do not jointly determine rhs type ‘f fa’ In the instance declaration for ‘FunctorN n f a (f fa)’
which looks right to me.
Simon
comment:7 Changed 10 months ago by thoughtpolice
- Milestone set to 7.8.3
comment:8 Changed 9 months ago by thoughtpolice
- Resolution set to fixed
- Status changed from merge to closed
File containing the source which causes the crash.