May 2016 - ghc-tickets

[GHC] #11547: Accessing shadowed definitions in GHCi
by GHC 04 Jul '18

04 Jul '18

#11547: Accessing shadowed definitions in GHCi -------------------------------------+------------------------------------- Reporter: mniip | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 8.0.1-rc1 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: -------------------------------------+------------------------------------- Apparently GHCi creates a qualified namespace for each line it interprets, as can be observed with: {{{#!hs > data A = A > data A = B > :t A A :: Ghci3.A }}} Apparently accessing shadowed definitions in this way sometimes breaks GHCi: {{{#!hs > let a = a > let a = a > :t Ghci7.a <interactive>:1:1: error: • GHC internal error: ‘Ghci7.a’ is not in scope during type checking, but it passed the renamer tcl_env of environment: [] • In the expression: Ghci7.a }}} More specifically, complete reproduction instructions: {{{#!hs GHCi, version 8.1.20160205: http://www.haskell.org/ghc/ :? for help > let foo = foo > :t Ghci1.foo Ghci1.foo :: t > let foo = foo > :t Ghci1.foo <interactive>:1:1: error: • GHC internal error: ‘Ghci1.foo’ is not in scope during type checking, but it passed the renamer tcl_env of environment: [] • In the expression: Ghci1.foo }}} 7.10.2 (and 7.8.3 as verified over IRC by monochrom) appears to have a slightly different behavior. It seems it only creates namespaces for data/newtype/class declarations. Yet still, it fails with an error the first time but works completely fine thereafter: {{{#!hs > data A = A > data A = A > :t Ghci1.A Failed to load interface for ‘Ghci1’ no package key matching ‘interactive’ was found > :t Ghci1.A Ghci1.A :: Ghci1.A }}} 7.6.3 uses a different approach to displaying out of scope names so the behavior doesn't apply: {{{#!hs > data A = A > data A = B > :t A A :: main::Interactive.A }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11547> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8263: allow duplicate deriving / standalone deriving
by GHC 04 Jul '18

04 Jul '18

#8263: allow duplicate deriving / standalone deriving ------------------------------------+------------------------------------- Reporter: aavogt | Owner: Type: feature request | 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: | ------------------------------------+------------------------------------- I would like to avoid conditional compilation. Currently I need to write: {{{ #if MIN_VERSION_Cabal(1,18,0) -- cabal 1.18 has these instances #else deriving instance Typeable Flag #endif }}} Would it be sane to have the deriving instance do nothing if the imported instance that clashes was also derived? reifyInstances together with a fixed #8100 could allow faking this feature. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8263> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10391: Ability to get contents of TH reified module
by GHC 02 Jul '18

02 Jul '18

#10391: Ability to get contents of TH reified module -------------------------------------+------------------------------------- Reporter: dmcclean | Owner: Type: feature | Status: new request | Milestone: Priority: low | Version: Component: Template | Operating System: Unknown/Multiple Haskell | Type of failure: None/Unknown Keywords: | Blocked By: Architecture: | Related Tickets: Unknown/Multiple | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- After using reifyModule, I'd like to be able to inspect the list of exports from that module. At first I thought that this could perhaps be done by changing from {{{#!hs data ModuleInfo = -- | Contains the import list of the module. ModuleInfo [Module] }}} to {{{#!hs data ModuleInfo = -- | Contains the import list and export list of the module. ModuleInfo [Module] [Name] }}} but I now wonder if this is the best plan, because `thisModule >>= reifyModule` might be expected to function as a time machine, allowing us to see declarations that may not have been made yet (in the case where this module's export list is implicit). Can anyone propose a type that would allow us to perform this inspection for modules that have already been baked, but not for "this" module? An intended use case would be in our dimensional types library. We have several modules that define a whole bunch of units. It's useful for the unit name parser to have access to a dictionary with all of those units in it, but maintaining the definition of such a dictionary is redundant. If this feature existed, a module in the parser could import the modules where units are defined, examine the export lists, reify the names in the export lists, determine which represent unit definitions, and emit a declaration of a dictionary that contains them all. (This is in some ways similar to #9699, though perhaps more difficult?) -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10391> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8489: clean up dependency and usages handling in interface files
by GHC 01 Jul '18

01 Jul '18

#8489: clean up dependency and usages handling in interface files ----------------------------------------------+---------------------------- Reporter: errge | Owner: errge Type: task | Status: new Priority: normal | Milestone: 7.10.1 Component: Template Haskell | Version: 7.7 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Moderate (less than a day) | Type of failure: Blocked By: 1480 | None/Unknown Related Tickets: | Test Case: | Blocking: ----------------------------------------------+---------------------------- While reviewing #1480, it was found that there are corner cases that are not easy to handle. This task is about fixing TH module reification to reify direct import list, not usages. For this we may need to extend the current `ModIface`. This task also includes cleaning up the comments and code in related areas, to quote simonpj: "we need more precise commentary on the fields of `HscTypes.Dependencies`, `TcRnTypes.ImportAvails`, and `mi_usages` of `ModIface`. For example, I wonder whether the `mi_usages` field could be part of the `Dependencies`". -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8489> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[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

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[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

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[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

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[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

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[GHC] #9046: Panic in GHCi when using :print
by GHC 28 Jun '18

28 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

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[GHC] #10789: Notify user when a kind mismatch holds up a type family reduction
by GHC 27 Jun '18

27 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

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