September 2017 - ghc-tickets

[GHC] #11812: Template Haskell can induce non-unique Uniques
by GHC 01 Jul '18

01 Jul '18

#11812: Template Haskell can induce non-unique Uniques -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Template | Version: 8.1 Haskell | 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: -------------------------------------+------------------------------------- When quoting a Template Haskell expression (or type), you can get your hands on renamed variables. These variables have assigned Uniques. If you then use the same variable locally in ''different'' top-level expressions, chaos can ensue. It's certainly expected that something bizarre would happen if you used the same Unique twice within the same scope, but it surprised me that using the same Unique twice in different scopes would cause a problem. Below is the rather hard-won reduced test case: {{{ {-# LANGUAGE TemplateHaskell, PolyKinds, RankNTypes, TypeFamilies #-} module Bug where import Language.Haskell.TH import Data.Type.Equality type Const a b = a $( do ForallT [PlainTV n] _ _ <- [t| forall n. n |] let noBang = Bang NoSourceUnpackedness NoSourceStrictness return [ClosedTypeFamilyD (TypeFamilyHead (mkName "F1") [ KindedTV (mkName "a") (VarT n) , PlainTV (mkName "b") ] (KindSig (VarT n)) Nothing) [TySynEqn [VarT (mkName "a"), VarT (mkName "b")] (ConT ''Const `AppT` VarT (mkName "a") `AppT` (ConT (mkName "T1") `AppT` VarT (mkName "a") `AppT` VarT (mkName "b")))] ,DataD [] (mkName "T1") [ KindedTV (mkName "a") (VarT n) , PlainTV (mkName "b") , PlainTV (mkName "c")] Nothing [NormalC (mkName "K1") [(noBang, ConT ''(:~:) `AppT` VarT (mkName "c") `AppT` (ConT (mkName "F1") `AppT` VarT (mkName "a") `AppT` VarT (mkName "b")))]] []]) }}} This blob produces {{{ type family F1 (a :: n_auRf) b :: n_auRf where F1 a b = Const a (T1 a b) data T1 (a :: n_auRf) b c = K1 ((:~:) c (F1 a b)) }}} which compiles fine when typed in directly. Note that this hinges on the `SigTv` behavior of kind variables in non-CUSK declarations, but I don't think that's the nub of the problem. What happens is this: during renaming, the `n`s are renamed to have the same `Unique`, because `n` is `Exact`. Type-checking invents `SigTv`s for each `n`. Naturally, both `n`s have ''different'' `IORef`s stored in their `TcTyVarDetails`. When the two `n`s are compared during checking, though, their `Unique`s are the same, and so nothing happens -- even though they should be unified. The upshot is that we get one logical variable `n` with different `IORef`s in different occurrences, causing chaos. It might be reasonable to punt on this, but we should document in the manual what the problem is. It puzzled me for quite a while when the problem came up in real code! -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11812> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 5

[GHC] #8695: Arithmetic overflow from (minBound :: Int) `quot` (-1)
by GHC 30 Jun '18

30 Jun '18

#8695: Arithmetic overflow from (minBound :: Int) `quot` (-1) -------------------------------------+------------------------------------- Reporter: rleslie | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: | Version: 7.6.3 libraries/haskell2010 | Operating System: Unknown/Multiple Keywords: | Type of failure: Incorrect result Architecture: Unknown/Multiple | at runtime Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | -------------------------------------+------------------------------------- According to the documentation for Data.Int, “All arithmetic is performed modulo 2!^n, where n is the number of bits in the type.” However, this promise is broken by the following exception: {{{ Prelude> (minBound :: Int) `quot` (-1) *** Exception: arithmetic overflow }}} (This also occurs with Int8, Int16, Int32, and Int64, and likewise for `div`.) If all arithmetic is performed modulo 2!^n, I would rather expect the above to evaluate to `minBound`, since after all `minBound * (-1) == minBound`. The fact that an exception is raised adds an unnecessary burden in pure code to ensure `quot` (or `div`) is never called with these specific arguments. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8695> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 39

[GHC] #11024: Fix strange parsing of BooleanFormula
by GHC 29 Jun '18

29 Jun '18

#11024: Fix strange parsing of BooleanFormula -------------------------------------+------------------------------------- Reporter: mpickering | Owner: Type: task | Status: new Priority: low | Milestone: Component: Compiler | Version: 7.10.2 (Parser) | Keywords: newcomer | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider the following program. {{{ module Foo where class Meh a where foo :: a -> String foo1 :: a -> String foo2 :: a -> String {-# MINIMAL foo, foo1, foo2 #-} }}} The minimal pragma is parsed quite weirdly. {{{ (MinimalSig "{-# MINIMAL" (And [ (Var ({ test.hs:8:15-17 } (Unqual {OccName: foo}))), (And [ (Var ({ test.hs:8:20-23 } (Unqual {OccName: foo1}))), (Var ({ test.hs:8:26-29 } (Unqual {OccName: foo2})))])])))] {Bag(Located (HsBind RdrName)):}}} To be clear, it is parsed as `And [foo, [And [foo1, foo2]]` rather than `And [foo, foo1, foo2]`. Either the datatype should be change to be more like a binary tree or the parser should be changed to handle this properly. I think the second option would be easiest. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11024> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 11

[GHC] #11764: ghc internal error building llvm-general-3.5.1.2
by GHC 29 Jun '18

29 Jun '18

#11764: ghc internal error building llvm-general-3.5.1.2 -------------------------------------+------------------------------------- Reporter: andrew.wja | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 Keywords: | Operating System: Linux Architecture: x86_64 | Type of failure: None/Unknown (amd64) | Test Case: cabal install | Blocked By: llvm-general | Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- $ cabal install llvm-general Resolving dependencies... Configuring llvm-general-3.5.1.2... Building llvm-general-3.5.1.2... Failed to install llvm-general-3.5.1.2 Build log ( /home/andrew/.cabal/logs/llvm-general-3.5.1.2.log ): [1 of 1] Compiling Main ( /tmp/cabal-tmp-12630/llvm- general-3.5.1.2/dist/setup/setup.hs, /tmp/cabal-tmp-12630/llvm- general-3.5.1.2/dist/setup/Main.o ) Linking /tmp/cabal-tmp-12630/llvm-general-3.5.1.2/dist/setup/setup ... Configuring llvm-general-3.5.1.2... Building llvm-general-3.5.1.2... Preprocessing library llvm-general-3.5.1.2... [ 1 of 93] Compiling LLVM.General.Internal.FFI.ByteRangeCallback ( src/LLVM/General/Internal/FFI/ByteRangeCallback.hs, dist/build/LLVM/General/Internal/FFI/ByteRangeCallback.o ) [ 2 of 93] Compiling LLVM.General.Internal.FFI.Transforms ( src/LLVM/General/Internal/FFI/Transforms.hs, dist/build/LLVM/General/Internal/FFI/Transforms.o ) [ 3 of 93] Compiling LLVM.General.Internal.Inject ( src/LLVM/General/Internal/Inject.hs, dist/build/LLVM/General/Internal/Inject.o ) [ 4 of 93] Compiling LLVM.General.Internal.FFI.Context ( src/LLVM/General/Internal/FFI/Context.hs, dist/build/LLVM/General/Internal/FFI/Context.o ) [ 5 of 93] Compiling LLVM.General.Internal.FFI.CommandLine ( src/LLVM/General/Internal/FFI/CommandLine.hs, dist/build/LLVM/General/Internal/FFI/CommandLine.o ) [ 6 of 93] Compiling LLVM.General.Internal.FFI.Iterate ( src/LLVM/General/Internal/FFI/Iterate.hs, dist/build/LLVM/General/Internal/FFI/Iterate.o ) [ 7 of 93] Compiling LLVM.General.Internal.FFI.PtrHierarchy ( src/LLVM/General/Internal/FFI/PtrHierarchy.hs, dist/build/LLVM/General/Internal/FFI/PtrHierarchy.o ) [ 8 of 93] Compiling LLVM.General.Internal.FFI.BasicBlock ( src/LLVM/General/Internal/FFI/BasicBlock.hs, dist/build/LLVM/General/Internal/FFI/BasicBlock.o ) [ 9 of 93] Compiling LLVM.General.Internal.FFI.User ( src/LLVM/General/Internal/FFI/User.hs, dist/build/LLVM/General/Internal/FFI/User.o ) [10 of 93] Compiling LLVM.General.Internal.FFI.GlobalAlias ( src/LLVM/General/Internal/FFI/GlobalAlias.hs, dist/build/LLVM/General/Internal/FFI/GlobalAlias.o ) [11 of 93] Compiling LLVM.General.Internal.FFI.LLVMCTypes ( dist/build/LLVM/General/Internal/FFI/LLVMCTypes.hs, dist/build/LLVM/General/Internal/FFI/LLVMCTypes.o ) [12 of 93] Compiling LLVM.General.Internal.FFI.Attribute ( src/LLVM/General/Internal/FFI/Attribute.hs, dist/build/LLVM/General/Internal/FFI/Attribute.o ) [13 of 93] Compiling LLVM.General.Internal.FFI.GlobalValue ( src/LLVM/General/Internal/FFI/GlobalValue.hs, dist/build/LLVM/General/Internal/FFI/GlobalValue.o ) [14 of 93] Compiling LLVM.General.Internal.FFI.Instruction ( src/LLVM/General/Internal/FFI/Instruction.hs, dist/build/LLVM/General/Internal/FFI/Instruction.o ) [15 of 93] Compiling LLVM.General.Internal.FFI.Type ( src/LLVM/General/Internal/FFI/Type.hs, dist/build/LLVM/General/Internal/FFI/Type.o ) [16 of 93] Compiling LLVM.General.Internal.FFI.Value ( src/LLVM/General/Internal/FFI/Value.hs, dist/build/LLVM/General/Internal/FFI/Value.o ) [17 of 93] Compiling LLVM.General.Internal.FFI.BinaryOperator ( src/LLVM/General/Internal/FFI/BinaryOperator.hs, dist/build/LLVM/General/Internal/FFI/BinaryOperator.o ) [18 of 93] Compiling LLVM.General.Internal.FFI.DataLayout ( src/LLVM/General/Internal/FFI/DataLayout.hs, dist/build/LLVM/General/Internal/FFI/DataLayout.o ) [19 of 93] Compiling LLVM.General.Internal.FFI.SMDiagnostic ( src/LLVM/General/Internal/FFI/SMDiagnostic.hs, dist/build/LLVM/General/Internal/FFI/SMDiagnostic.o ) [20 of 93] Compiling LLVM.General.Internal.FFI.Module ( src/LLVM/General/Internal/FFI/Module.hs, dist/build/LLVM/General/Internal/FFI/Module.o ) [21 of 93] Compiling LLVM.General.Internal.FFI.ExecutionEngine ( src/LLVM/General/Internal/FFI/ExecutionEngine.hs, dist/build/LLVM/General/Internal/FFI/ExecutionEngine.o ) [22 of 93] Compiling LLVM.General.Internal.FFI.Function ( src/LLVM/General/Internal/FFI/Function.hs, dist/build/LLVM/General/Internal/FFI/Function.o ) [23 of 93] Compiling LLVM.General.Internal.FFI.InlineAssembly ( src/LLVM/General/Internal/FFI/InlineAssembly.hs, dist/build/LLVM/General/Internal/FFI/InlineAssembly.o ) [24 of 93] Compiling LLVM.General.Internal.FFI.InstructionDefs ( dist/build/LLVM/General/Internal/FFI/InstructionDefs.hs, dist/build/LLVM/General/Internal/FFI/InstructionDefs.o ) [25 of 93] Compiling LLVM.General.Internal.InstructionDefs ( src/LLVM/General/Internal/InstructionDefs.hs, dist/build/LLVM/General/Internal/InstructionDefs.o ) [26 of 93] Compiling LLVM.General.Internal.FFI.MemoryBuffer ( src/LLVM/General/Internal/FFI/MemoryBuffer.hs, dist/build/LLVM/General/Internal/FFI/MemoryBuffer.o ) [27 of 93] Compiling LLVM.General.Internal.FFI.Metadata ( src/LLVM/General/Internal/FFI/Metadata.hs, dist/build/LLVM/General/Internal/FFI/Metadata.o ) [28 of 93] Compiling LLVM.General.Internal.FFI.GlobalVariable ( src/LLVM/General/Internal/FFI/GlobalVariable.hs, dist/build/LLVM/General/Internal/FFI/GlobalVariable.o ) [29 of 93] Compiling LLVM.General.Internal.FFI.RawOStream ( src/LLVM/General/Internal/FFI/RawOStream.hs, dist/build/LLVM/General/Internal/FFI/RawOStream.o ) [30 of 93] Compiling LLVM.General.Internal.FFI.Assembly ( src/LLVM/General/Internal/FFI/Assembly.hs, dist/build/LLVM/General/Internal/FFI/Assembly.o ) [31 of 93] Compiling LLVM.General.Internal.FFI.Bitcode ( src/LLVM/General/Internal/FFI/Bitcode.hs, dist/build/LLVM/General/Internal/FFI/Bitcode.o ) [32 of 93] Compiling LLVM.General.Internal.FFI.Target ( src/LLVM/General/Internal/FFI/Target.hs, dist/build/LLVM/General/Internal/FFI/Target.o ) [33 of 93] Compiling LLVM.General.Internal.FFI.Threading ( src/LLVM/General/Internal/FFI/Threading.hs, dist/build/LLVM/General/Internal/FFI/Threading.o ) [34 of 93] Compiling LLVM.General.Internal.FFI.Cleanup ( src/LLVM/General/Internal/FFI/Cleanup.hs, dist/build/LLVM/General/Internal/FFI/Cleanup.o ) [35 of 93] Compiling LLVM.General.Internal.FFI.Constant ( src/LLVM/General/Internal/FFI/Constant.hs, dist/build/LLVM/General/Internal/FFI/Constant.o ) ghc: internal error: stg_ap_pv_ret (GHC version 7.10.3 for x86_64_unknown_linux) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug cabal: Error: some packages failed to install: llvm-general-3.5.1.2 failed during the building phase. The exception was: ExitFailure (-6) -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11764> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 9

[GHC] #9046: Panic in GHCi when using :print
by GHC 29 Jun '18

29 Jun '18

#9046: Panic in GHCi when using :print ------------------------------------+------------------------------------- Reporter: quchen | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.8.3 Component: GHCi | Version: 7.8.2 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: GHCi crash Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- When using names generated by :print, GHCi panics sometimes. According to #ghc, this is reproducible on ''x86_64-apple-darwin'' as well, but is not an issue in ''7.6.3 x86_64 linux''. {{{ > let a = [1] > :print a a = (_t1::Num t => [t]) > :t _t1 ghc: panic! (the 'impossible' happened) (GHC version 7.8.2 for x86_64-unknown-linux): tcTyVarDetails t{tv avD} [tv] > _t1 <same panic> }}} From playing around with it a bit, this seems to have something to do with polymorphic values. {{{ -- No panic > let a = [1 :: Int] > let a = 1 :: Int > let a = "hello" -- Panic > let a = [1] > let a = 1 > let a = "hello" -- With -XOverloadedStrings }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9046> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 14

[GHC] #10789: Notify user when a kind mismatch holds up a type family reduction
by GHC 28 Jun '18

28 Jun '18

#10789: Notify user when a kind mismatch holds up a type family reduction -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- Consider this contrived example: {{{ {-# LANGUAGE TypeFamilies, PolyKinds, UndecidableInstances #-} module Bug where import Data.Proxy type family F (a :: k) :: k type instance F a = G a type family G a type instance G a = a foo :: Proxy (F Maybe) -> Proxy Maybe foo = id }}} This (correctly) fails to compile. The error message is {{{ Bug.hs:14:7: Couldn't match type ‘F Maybe’ with ‘Maybe’ Expected type: Proxy (F Maybe) -> Proxy Maybe Actual type: Proxy Maybe -> Proxy Maybe In the expression: id In an equation for ‘foo’: foo = id Failed, modules loaded: none. }}} But this is peculiar, but it surely looks like `F` should be a type-level identity function! Of course, upon further inspection, we see that `F` is partial. It reduces only at kind `*`. This is quite hard to figure out, though, especially given that we're using the "default to `*`" behavior of open type families to arrange for this kind restriction. Thus, I propose: figure out when a type family reduction is held up due to a kind mismatch, and inform the user. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10789> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 15

[GHC] #12922: Kind classes compile with PolyKinds
by GHC 27 Jun '18

27 Jun '18

#12922: Kind classes compile with PolyKinds -------------------------------------+------------------------------------- Reporter: Tritlo | 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: -------------------------------------+------------------------------------- I was asking around on #haskell to better understand the new language extension, -XTypeInType, and how it is different from -XPolyKinds. To study it, I was working with the following small example: {{{ {-# LANGUAGE TypeFamilies, GADTs, TypeOperators, DataKinds, PolyKinds #-} module Main where import Data.Kind (Type) -- Define a Type for the natural numbers, Zero and a successor data Nat = Z | S Nat class Addable k where type (a :: k) + (b :: k) :: k instance Addable Nat where type (Z + a) = a }}} (for more context, see https://gist.github.com/Tritlo/ce5510e80935ac398a33934ee47c7930) Since according to a responder on #haskell, the ability to have kind classes required TypeInType, and should not work with PolyKinds. As the documentation mentions that there could be leakage between PolyKinds and TypeInType and to report such leaks, I felt that I should report it. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12922> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 11

[GHC] #9383: GHCi parser doesn't properly handle -fobject-code
by GHC 27 Jun '18

27 Jun '18

#9383: GHCi parser doesn't properly handle -fobject-code -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 7.8.2 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Other Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- My complete Haskell source: module Foo where f x = x Shell: $ ghci Foo.hs -fobject-code GHCi, version 7.8.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. target ‘-fobject-code’ is not a module name or a source file Ok, modules loaded: Linker. Prelude Linker> Despite the wanring/error, GHCi *does* generate the object code. This error also prints when I *load* the compiled Foo object file. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9383> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 2

[GHC] #10965: GHC Panic on import with 'OPTIONS_GHC -fobject-code -O'
by GHC 27 Jun '18

27 Jun '18

#10965: GHC Panic on import with 'OPTIONS_GHC -fobject-code -O' --------------------------------------+--------------------------------- Reporter: Orome | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Keywords: | Operating System: MacOS X Architecture: x86_64 (amd64) | Type of failure: None/Unknown Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: --------------------------------------+--------------------------------- I get {{{ > :l Test.hs [1 of 1] Compiling Main ( Test.hs, interpreted ) ghc: panic! (the 'impossible' happened) (GHC version 7.10.2 for x86_64-apple-darwin): floatExpr tick break<11>() Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} with Test.hs: {{{ {-# OPTIONS_GHC -fobject-code -O #-} import Control.Exception (assert) import Data.Char (ord) f :: String -> String f s = assert (all (`elem` letters) s) $ (letters!!) <$> (ix <$> s) where ix ch = (ord ch - ord 'A') letters = ['A'..'Z'] }}} OSX 10.11, GHC 7.10.2; uad-core 64-bit haswell; Homebrew GHC -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10965> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 6

[GHC] #13790: GHC doesn't reduce type family in kind signature unless its arm is twisted
by GHC 26 Jun '18

26 Jun '18

#13790: GHC doesn't reduce type family in kind signature unless its arm is twisted -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 (Type checker) | Keywords: TypeInType, | Operating System: Unknown/Multiple TypeFamilies | Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Here's some code (inspired by Richard's musings [https://github.com/goldfirere/singletons/issues/150#issuecomment-306088297 here]) which typechecks with GHC 8.2.1 or HEAD: {{{#!hs {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE TemplateHaskell #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeInType #-} module Bug where import Data.Kind data family Sing (a :: k) data SomeSing (k :: Type) where SomeSing :: Sing (a :: k) -> SomeSing k type family Promote k :: Type class (Promote (Demote k) ~ k) => SingKind (k :: Type) where type Demote k :: Type fromSing :: Sing (a :: k) -> Demote k toSing :: Demote k -> SomeSing k type family DemoteX (a :: k) :: Demote k type instance DemoteX (a :: Type) = Demote a type instance Promote Type = Type instance SingKind Type where type Demote Type = Type fromSing = error "fromSing Type" toSing = error "toSing Type" ----- data N = Z | S N data instance Sing (z :: N) where SZ :: Sing Z SS :: Sing n -> Sing (S n) type instance Promote N = N instance SingKind N where type Demote N = N fromSing SZ = Z fromSing (SS n) = S (fromSing n) toSing Z = SomeSing SZ toSing (S n) = case toSing n of SomeSing sn -> SomeSing (SS sn) }}} Things get more interesting if you try to add this type instance at the end of this file: {{{#!hs type instance DemoteX (n :: N) = n }}} Now GHC will complain: {{{ GHCi, version 8.2.0.20170522: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Bug.hs:49:34: error: • Expected kind ‘Demote N’, but ‘n’ has kind ‘N’ • In the type ‘n’ In the type instance declaration for ‘DemoteX’ | 49 | type instance DemoteX (n :: N) = n | ^ }}} That error message smells funny, since we do have a type family instance that says `Demote N = N`! In fact, if you use Template Haskell to split up the declarations manually: {{{#!hs ... instance SingKind N where type Demote N = N fromSing SZ = Z fromSing (SS n) = S (fromSing n) toSing Z = SomeSing SZ toSing (S n) = case toSing n of SomeSing sn -> SomeSing (SS sn) $(return []) type instance DemoteX (n :: N) = n }}} Then the file typechecks without issue. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13790> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 3