March 2018 - ghc-tickets

[GHC] #14783: Initializing record with similarly named field from a different record results in warning rather than error
by GHC 08 Jun '18

08 Jun '18

#14783: Initializing record with similarly named field from a different record results in warning rather than error -------------------------------------+------------------------------------- Reporter: | Owner: (none) ulrikrasmussen | Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: records | Operating System: Unknown/Multiple DuplicateRecordFields | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- When a record is initialized using a similarly named field from another record, the compiler ignores the initialization and generates a warning instead of throwing an error. I believe this is a bug introduced in GHC 8.0.1 (and probably related to `DuplicateRecordFields`), since earlier versions of GHC would refuse to compile the code. Consider the following example: {{{ -- A.hs module A where data A = A { a :: (), b :: () } -- B.hs module B where data B = B { a :: (), b :: () } -- Main.hs module Main where import A hiding (a) import B x = A { a = (), b = () } main = case x of A () () -> return () }}} On GHC 8.0.1, this compiles (with a warning), and the program throws an exception when run: {{{ $ stack ghc --compiler ghc-8.0.1 -- --make Main.hs [1 of 3] Compiling B ( B.hs, B.o ) [2 of 3] Compiling A ( A.hs, A.o ) [3 of 3] Compiling Main ( Main.hs, Main.o ) Main.hs:5:7: warning: [-Wmissing-fields] • Fields of ‘A’ not initialised: a • In the expression: A {a = (), b = ()} In an equation for ‘x’: x = A {a = (), b = ()} Linking Main ... $ ./Main Main: Main.hs:5:7-26: Missing field in record construction a }}} On GHC 7.10.3, the program will not compile: {{{ $ stack ghc --compiler ghc-7.10.3 -- --make Main.hs [1 of 3] Compiling B ( B.hs, B.o ) [2 of 3] Compiling A ( A.hs, A.o ) [3 of 3] Compiling Main ( Main.hs, Main.o ) Main.hs:5:7: Constructor ‘A’ does not have field ‘a’ In the expression: A {a = (), b = ()} In an equation for ‘x’: x = A {a = (), b = ()} }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14783> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12023: Problems getting information and kind from GHC.Prim ~#, ~R#, ...
by GHC 08 Jun '18

08 Jun '18

#12023: Problems getting information and kind from GHC.Prim ~#, ~R#, ... -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 7.10.3 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: -------------------------------------+------------------------------------- {{{ >>> import GHC.Prim >>> :bro GHC.Prim [...] type role (~#) nominal nominal data (~#) (t3 :: a) (t4 :: b) type role ~P# phantom phantom data ~P# (t3 :: a) (t4 :: b) type role (~R#) representational representational data (~R#) (t3 :: a) (t4 :: b) }}} getting the kind of `~`, `~#`, `~~` works fine: {{{ >>> import GHC.Prim >>> import GHC.Types >>> :kind (~) (~) :: k -> k -> Constraint >>> :kind (~#) (~#) :: a -> b -> TYPE 'VoidRep >>> :kind (~~) (~~) :: j -> k -> Constraint }}} but `#` gets incorrectly treated as an operator in `~R#` and `~P#`: {{{ >>> :kind (~R#) <interactive>:1:3: error: Illegal operator ‘#’ in type ‘~R #’ Use TypeOperators to allow operators in types <interactive>:1:3: error: Operator applied to too few arguments: ~R # <interactive>:1:4: error: Not in scope: type constructor or class ‘R’ }}} Furthermore while running `:info` on `~~` and `~#` works fine {{{ >>> :info (~~) class a ~# b => (~~) (a :: j) (b :: k) -- Defined in ‘GHC.Types’ >>> :info (~#) type role (~#) nominal nominal data (~#) (t3 :: a) (t4 :: b) -- Defined in ‘GHC.Prim’ }}} the others don't parse {{{ >>> :info (~) <interactive>:1:2: error: parse error on input ‘~’ >>> :info (~R#) <interactive>:1:2: error: parse error on input ‘~’ >>> :info (~P#) <interactive>:1:2: error: parse error on input ‘~’ }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12023> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14381: Consider making ghc-pkg fill in abi-depends based on depends
by GHC 07 Jun '18

07 Jun '18

#14381: Consider making ghc-pkg fill in abi-depends based on depends -------------------------------------+------------------------------------- Reporter: ezyang | Owner: (none) Type: bug | Status: new Priority: high | Milestone: Component: ghc-pkg | Version: 8.2.1 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: -------------------------------------+------------------------------------- In GHC 8.2, we introduced `abi-depends` to solve #12485. Following the same pattern as all ghc-pkg fields, this field is to be fllled by whoever is performing the registration. I now suspect designing it this way was a mistake. In https://github.com/haskell/cabal/issues/4728 we have a bug where Cabal is writing nonsense data for `abi-depends`, `ghc-pkg` isn't noticing it, and GHC is rejecting the package (with a "shadow" warning) when it gets to the end. The problem is the Cabal aggressively caches the contents of the package database (ostensibly because it is expensive to query `ghc-pkg`); this means that it is easy to get into a situation where its understanding of the ABIs of its dependencies is out-of-date (because it is not re- reading the database in order to get newer information). The insult to the injury is, in most cases, ghc-pkg already knows what the ABIs are supposed to be: they're whatever the ABIs of the packages pointed at by 'depends' already in the database are. So ghc-pkg could have computed the abi dependency itself, and prevented this stale data situation from ever happening. This sounds quite attractive to me. What do people think? Here is one possible proposal (but it is just one in the space): * `ghc-pkg` will be modified to ignore the `abi-depends` field (perhaps with a warning), to prevent itself from being poisoned by buggy versions of Cabal which are giving bad ABI information * Instead, `ghc-pkg` generates the `abi-depends` field by looking up dependency IDs from the database. If an ID is not found, it omits the dep from `abi-depends` (this is equivalent to suspending ABI checking in GHC, so this won't break anything; it will just make ABI checking less robust) * Possibly, introduce a new "virtual" field, which can be used to override `ghc-pkg` default -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14381> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13438: ghci :browse does not work with DuplicateRecordFields
by GHC 07 Jun '18

07 Jun '18

#13438: ghci :browse does not work with DuplicateRecordFields -------------------------------------+------------------------------------- Reporter: rik | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 8.0.2 Keywords: | Operating System: Unknown/Multiple duplicaterecordfields ghci | Architecture: | Type of failure: Other Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Using `:browse` on module which has enabled DuplicateRecordFields and datatypes are exported, does not return the field accessor functions. So for example: {{{#!hs data Login = Login { code :: Text , password :: Text } )}}} `browse` for module does not return `code` and `password` accessor functions. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13438> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13471: segmentation fault by compiling GHC
by GHC 07 Jun '18

07 Jun '18

#13471: segmentation fault by compiling GHC -------------------------------------+------------------------------------- Reporter: vanto | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Build System | Version: 8.0.2 Keywords: | Operating System: Linux Architecture: x86_64 | Type of failure: Building GHC (amd64) | failed Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Hi,\\ I tried to compile GHC on my computer.\\ by compiling GHC under Linux I got this error message.\\ {{{ compiler/ghc.mk:588: recipe for target 'compiler/stage2/build/DynFlags.o' failed make[1]: *** [compiler/stage2/build/DynFlags.o] segmentation fault Makefile:122: recipe for target 'install' failed make: *** [install] Error 2 }}} on my computer Linux runs on VirtualBox.\\ the problem is probably with my hardware but as I know nothing I tell you this issue.\\ {{{ ~/ghc/inplace/bin$ /ghc-stage1 --version The Glorious Glasgow Haskell Compilation System, version 8.3.20170322 }}} Hope this help\\ -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13471> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14713: GHCi doesn't load project.
by GHC 06 Jun '18

06 Jun '18

#14713: GHCi doesn't load project. -------------------------------------+------------------------------------- Reporter: recursion- | Owner: (none) ninja | Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 8.2.2 Keywords: | Operating System: Unknown/Multiple Architecture: x86_64 | Type of failure: GHCi crash (amd64) | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I got the following output when trying to load my Haskell project into GHCi using stack: {{{ GHCi, version 8.2.2: http://www.haskell.org/ghc/ :? for help ghc: panic! (the 'impossible' happened) (GHC version 8.2.2 for x86_64-unknown-linux): Loading temp shared object failed: /tmp/ghc3372_0/libghc_13.so: undefined symbol: numStates_g Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} No other informative output was displayed. Output says report the defect, so you get a "bug report." -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14713> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12075: Fails to build on powerpcspe because of inline assembly
by GHC 05 Jun '18

05 Jun '18

#12075: Fails to build on powerpcspe because of inline assembly --------------------------------+---------------------------------------- Reporter: glaubitz | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Linux Architecture: powerpc | Type of failure: Building GHC failed Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: --------------------------------+---------------------------------------- Hello! I'm currently bootstrapping GHC on powerpcspe (e500v2) which is a PowerPC architecture without an FPU but with signal processing extensions (hence SPE). When building GHC on this architecture, it detects the host architecture as powerpc-linux-gnuspe or short "powerpc" which works most of the time. However, GHC contains some inline assembly in rts/StgCRunAsm.s which uses FPU instructions not available on e500v2 CPUs resulting in assembler error, here with GHC 7.10.3: {{{ "inplace/bin/ghc-stage1" -optc-fno-stack-protector -optc-Wall -optc-Wextra -optc-Wstrict-prototypes -optc-Wmissing-prototypes -optc-Wmissing- declarations -optc-Winline -optc-Waggregate-return -optc-Wpointer-arith -optc-Wmissing-noreturn -optc-Wnested-externs -optc-Wredundant-decls -optc-Iincludes -optc-Iincludes/dist -optc-Iincludes/dist- derivedconstants/header -optc-Iincludes/dist-ghcconstants/header -optc- Irts -optc-Irts/dist/build -optc-DCOMPILING_RTS -optc- DUSE_LIBFFI_FOR_ADJUSTORS -optc-fno-strict-aliasing -optc-fno-common -optc-Irts/dist/build/autogen -optc-O2 -optc-fomit-frame-pointer -optc-g -optc-DRtsWay=\"rts_v\" -optc-w -static -H32m -O -lffi -optl-pthread -lffi -optl-pthread -Iincludes -Iincludes/dist -Iincludes/dist- derivedconstants/header -Iincludes/dist-ghcconstants/header -Irts -Irts/dist/build -DCOMPILING_RTS -this-package-key rts -dcmm-lint -i -irts -irts/dist/build -irts/dist/build/autogen -Irts/dist/build -Irts/dist/build/autogen -O2 -c rts/StgCRun.c -o rts/dist/build/StgCRun.o /bin/bash: warning: setlocale: LC_ALL: cannot change locale (en_US.UTF-8) /bin/bash: warning: setlocale: LC_ALL: cannot change locale (en_US.UTF-8) /tmp/ghcf20d_0/ghc_1.s: Assembler messages: /tmp/ghcf20d_0/ghc_1.s:24:0: Error: unrecognized opcode: `stfd' (...) /tmp/ghcf20d_0/ghc_1.s:67:0: Error: unrecognized opcode: `lfd' /tmp/ghcf20d_0/ghc_1.s:68:0: Error: unrecognized opcode: `lfd' /tmp/ghcf20d_0/ghc_1.s:69:0: Error: unrecognized opcode: `lfd' rts/ghc.mk:236: recipe for target 'rts/dist/build/StgCRun.o' failed make[3]: *** [rts/dist/build/StgCRun.o] Error 1 Makefile:71: recipe for target 'all' failed make[2]: *** [all] Error 2 make[2]: Leaving directory '/usr/src/ghc-7.10.3' dh_auto_build: make -j1 returned exit code 2 debian/rules:109: recipe for target 'override_dh_auto_build' failed make[1]: *** [override_dh_auto_build] Error 2 make[1]: Leaving directory '/usr/src/ghc-7.10.3' debian/rules:47: recipe for target 'binary-arch' failed make: *** [binary-arch] Error 2 (sid2-powerpcspe-sbuild)root@atlantis:/usr/src/ghc-7.10.3# }}} This can be verified by commenting out the assembly which will make the above compile command execute without any problems. Trying to continue compiling GHC will still fail with: {{{ "inplace/bin/ghc-stage1" -static -H32m -O -lffi -optl-pthread -lffi -optl-pthread -Iincludes -Iincludes/dist -Iincludes/dist- derivedconstants/header -Iincludes/dist-ghcconstants/header -Irts -Irts/dist/build -DCOMPILING_RTS -this-package-key rts -dcmm-lint -i -irts -irts/dist/build -irts/dist/build/autogen -Irts/dist/build -Irts/dist/build/autogen -O2 -c rts/HeapStackCheck.cmm -o rts/dist/build/HeapStackCheck.o /bin/bash: warning: setlocale: LC_ALL: cannot change locale (en_US.UTF-8) /bin/bash: warning: setlocale: LC_ALL: cannot change locale (en_US.UTF-8) /tmp/ghcca51_0/ghc_3.s: Assembler messages: /tmp/ghcca51_0/ghc_3.s:525:0: Error: unrecognized opcode: `lfs' /tmp/ghcca51_0/ghc_3.s:538:0: Error: unrecognized opcode: `lfd' /tmp/ghcca51_0/ghc_3.s:592:0: Error: unrecognized opcode: `stfs' /tmp/ghcca51_0/ghc_3.s:604:0: Error: unrecognized opcode: `stfd' rts/ghc.mk:236: recipe for target 'rts/dist/build/HeapStackCheck.o' failed make[3]: *** [rts/dist/build/HeapStackCheck.o] Error 1 Makefile:71: recipe for target 'all' failed make[2]: *** [all] Error 2 make[2]: Leaving directory '/usr/src/ghc-7.10.3' dh_auto_build: make -j1 returned exit code 2 debian/rules:109: recipe for target 'override_dh_auto_build' failed make[1]: *** [override_dh_auto_build] Error 2 make[1]: Leaving directory '/usr/src/ghc-7.10.3' debian/rules:47: recipe for target 'binary-arch' failed make: *** [binary-arch] Error 2 (sid2-powerpcspe-sbuild)root@atlantis:/usr/src/ghc-7.10.3# }}} So, there is obviously more assembly code that needs to be disabled on powerpcspe. For that, I suggest testing whether the compiler sets __NO_FPRS__which is only set on powerpc hosts without FP registers: {{{ root@atlantis:~# echo | gcc -E -dM -|grep -i FP #define __NO_FPRS__ 1 root@atlantis:~# dpkg --print-architecture powerpcspe root@atlantis:~# uname -m ppc root@atlantis:~# }}} For reference, on a real powerpc host, the output is as follows: {{{ (sid_powerpc-dchroot)glaubitz@partch:~$ echo | gcc -E -dM -|grep -i FP #define __FP_FAST_FMAF 1 #define __FP_FAST_FMA 1 (sid_powerpc-dchroot)glaubitz@partch:~$ dpkg --print-architecture powerpc (sid_powerpc-dchroot)glaubitz@partch:~$ uname -m ppc (sid_powerpc-dchroot)glaubitz@partch:~$ }}} If you need any more input, please let me know. Adrian -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12075> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14735: GHC Panic with QuantifiedConstraints
by GHC 05 Jun '18

05 Jun '18

#14735: GHC Panic with QuantifiedConstraints -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Keywords: | Operating System: Unknown/Multiple QuantifiedContexts | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- From branch [https://ghc.haskell.org/trac/ghc/ticket/2893#comment:28 wip/T2893] {{{#!hs {-# Language QuantifiedConstraints #-} {-# Language StandaloneDeriving #-} {-# Language DataKinds #-} {-# Language TypeOperators #-} {-# Language GADTs #-} {-# Language KindSignatures #-} {-# Language FlexibleInstances #-} {-# Language UndecidableInstances #-} {-# Language MultiParamTypeClasses #-} {-# Language RankNTypes #-} {-# Language ConstraintKinds #-} import Data.Kind data D c where D :: c => D c newtype a :- b = S (a => D b) class C1 a b class C2 a b instance C1 a b => C2 a b class (forall xx. f xx) => Limit f instance (forall xx. f xx) => Limit f -- impl :: Limit (C1 a) :- Limit (C2 a) -- impl = S D infixr 5 :< data Sig a = N a | a :< Sig a data AST :: (Sig Type -> Type) -> (Sig Type -> Type) where Sym :: dom a -> AST dom a (:$) :: AST dom (xx :< a) -> AST dom (N xx) -> AST dom a deriving instance (forall xx. Show (dom xx)) => Show (AST dom a) data Arith a where Plus :: Arith (Int :< Int :< N Int) deriving instance Show (Arith a) }}} loading this program and evaluating `Sym Plus` works fine: {{{ $ ghc-stage2 --interactive hs/175-bug.hs GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( hs/175-bug.hs, interpreted ) Ok, one module loaded. *Main> Sym Plus Sym Plus *Main> }}} but we uncomment `impl` we get a panic! {{{ GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( hs/175-bug.hs, interpreted ) Ok, one module loaded. *Main> Sym Plus ghc-stage2: panic! (the 'impossible' happened) (GHC version 8.5.20180128 for x86_64-unknown-linux): nameModule system df_a2VB Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in ghc:Outputable pprPanic, called at compiler/basicTypes/Name.hs:241:3 in ghc:Name Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug *Main> }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14735> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14961: QuantifiedConstraints: class name introduced via an equality constraint does not reduce
by GHC 05 Jun '18

05 Jun '18

#14961: QuantifiedConstraints: class name introduced via an equality constraint does not reduce -------------------------------------+------------------------------------- Reporter: mrkgnao | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Keywords: | Operating System: Unknown/Multiple QuantifiedConstraints wipT2893 | Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: #14860 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following doesn't typecheck with the `wip/T2893` branch: {{{#!hs {-# LANGUAGE QuantifiedConstraints #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE ConstraintKinds #-} {-# LANGUAGE TypeFamilies #-} module Subst where class (forall x. c x => d x) => c ~=> d instance (forall x. c x => d x) => c ~=> d foo :: forall c a. c ~=> Monoid => (c a => a) -- ok foo = mempty bar :: forall c a m. (m ~ Monoid, c ~=> m) => (c a => a) -- ok bar = mempty baz :: forall c a. (forall m. m ~ Monoid => c ~=> m) => (c a => a) -- fails baz = mempty }}} {{{ Prelude> :reload [1 of 1] Compiling Subst ( src/Subst.hs, interpreted ) src/Subst.hs:21:7: error: • Could not deduce (Monoid a) arising from a use of ‘mempty’ from the context: (forall (m :: * -> Constraint). (m ~ Monoid) => c ~=> m, c a) bound by the type signature for: baz :: forall (c :: * -> Constraint) a. (forall (m :: * -> Constraint). (m ~ Monoid) => c ~=> m, c a) => a at src/Subst.hs:20:1-66 Possible fix: add (Monoid a) to the context of the type signature for: baz :: forall (c :: * -> Constraint) a. (forall (m :: * -> Constraint). (m ~ Monoid) => c ~=> m, c a) => a • In the expression: mempty In an equation for ‘baz’: baz = mempty | 21 | baz = mempty | ^^^^^^ Failed, no modules loaded. }}} Shouldn't the equality constraint be "substituted in"? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14961> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14863: QuantifiedConstraints: Can't deduce `c' from `(a, b)' and `a |- b |- c'
by GHC 05 Jun '18

05 Jun '18

#14863: QuantifiedConstraints: Can't deduce `c' from `(a, b)' and `a |- b |- c' -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Keywords: | Operating System: Unknown/Multiple QuantifiedConstraints wipT2893 | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- #14833 fixes #14835: we can witness `curryC :: ((a, b) |- c) :- (a |- b |- c)`. Now for the entirely predictable, how about **un**`curryC` {{{#!hs {-# Language QuantifiedConstraints, RankNTypes, PolyKinds, ConstraintKinds, UndecidableInstances, GADTs, LiberalTypeSynonyms, TypeOperators, MultiParamTypeClasses, FlexibleInstances #-} data Dict c where Dict :: c => Dict c infixr |- class (a => b) => (a |- b) instance (a => b) => (a |- b) type a :- b = Dict (a |- b) uncurryC :: forall a b c. (a |- b |- c) :- ((a, b) |- c) uncurryC = Dict }}} {{{ $ ghci -ignore-dot-ghci Z.hs GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( Z.hs, interpreted ) Z.hs:13:12: error: • Could not deduce: c arising from a use of ‘Dict’ from the context: a |- (b |- c) bound by a quantified context at Z.hs:1:1 or from: (a, b) bound by a quantified context at Z.hs:1:1 • In the expression: Dict In an equation for ‘uncurryC’: uncurryC = Dict • Relevant bindings include uncurryC :: (a |- (b |- c)) :- ((a, b) |- c) (bound at Z.hs:13:1) | 13 | uncurryC = Dict | ^^^^ Failed, no modules loaded. Prelude> }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14863> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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