[GHC] #8066: Compiler Panic When Using Instance Derivation With Singletons
#8066: Compiler Panic When Using Instance Derivation With Singletons ------------------------------------+------------------------------------- Reporter: tvynr | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- The following code causes a compiler panic: {{{ {-# LANGUAGE MultiParamTypeClasses, TypeSynonymInstances, FlexibleInstances, TemplateHaskell, GADTs, TypeFamilies, DataKinds, KindSignatures #-} module Example ( UnqualifiedTVar(..) , QualifiedTVar(..) , TVar(..) , QVar , UVar , TQual(..) ) where import GHC.TypeLits import qualified Data.Set as Set type Identifier = String type Span = (Int,Int) data QualForm = UnqualifiedTVar | QualifiedTVar deriving (Eq, Ord, Read, Show) data instance Sing (f :: QualForm) where UnqualifiedTVarS :: Sing UnqualifiedTVar QualifiedTVarS :: Sing QualifiedTVar instance Eq (Sing (a::QualForm)) where (==) = undefined instance Ord (Sing (a::QualForm)) where compare = undefined instance SingI UnqualifiedTVar where sing = UnqualifiedTVarS instance SingI QualifiedTVar where sing = QualifiedTVarS data TVar (a :: QualForm) = TVar (Sing a) Int [Span] deriving (Eq, Ord, Show) type QVar = TVar QualifiedTVar type UVar = TVar UnqualifiedTVar data AnyVar = SomeQVar QVar | SomeUVar UVar anyVar :: TVar (a :: QualForm) -> AnyVar anyVar a = let (TVar s _ _) = a in case fromSing s of UnqualifiedTVarS -> SomeUVar a QualifiedTVarS -> SomeQVar a }}} This has been tested on GHC 7.6.1 and GHC 7.6.3; both give an error message such as {{{ ghc: panic! (the 'impossible' happened) (GHC version 7.6.1.20121207 for i386-unknown-linux): find_thing APromotionErr FamDataConPE }}} This problem seems to have something to do with instance derivation; removing the instances for Eq and Ord causes the problem to go away. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8066 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#8066: Compiler Panic When Using Instance Derivation With Singletons ---------------------------------------+----------------------------------- Reporter: tvynr | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time crash | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: ---------------------------------------+----------------------------------- Changes (by tvynr): * failure: None/Unknown => Compile-time crash -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8066#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#8066: Compiler Panic When Using Instance Derivation With Singletons ---------------------------------------+----------------------------------- Reporter: tvynr | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time crash | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: ---------------------------------------+----------------------------------- Comment (by simonpj): The HEAD gives {{{ T8066.hs:42:22: No instance for (SingE QualForm (KindOf QualForm a)) arising from the 'deriving' clause of a data type declaration Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself When deriving the instance for (Show (TVar a)) }}} Does that seem more reasonable? We've fixed lots of kind-related bugs in 7.6. If the error looks ok, can we close the ticket? Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8066#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#8066: Compiler Panic When Using Instance Derivation With Singletons ---------------------------------------+----------------------------------- Reporter: tvynr | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time crash | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: ---------------------------------------+----------------------------------- Comment (by tvynr): That looks great, thanks. :) And for anyone else who encounters this problem or is perhaps about as confused by the TypeLits stuff as I am, adding the following fixes the panic in GHC 7.6.1 (and leads to revealing mistakes in the anyVar function): {{{ instance SingE (Kind :: QualForm) QualForm where fromSing x = case x of UnqualifiedTVarS -> UnqualifiedTVar QualifiedTVarS -> QualifiedTVar }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8066#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#8066: Compiler Panic When Using Instance Derivation With Singletons ---------------------------------------+----------------------------------- Reporter: tvynr | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time crash | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: ---------------------------------------+----------------------------------- Changes (by simonpj): * status: new => closed * resolution: => fixed -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8066#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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