September 2014 - ghc-tickets

[GHC] #8988: Documentation build fails if GHCi is unavailable
by GHC 15 Dec '14

15 Dec '14

#8988: Documentation build fails if GHCi is unavailable -------------------------------------+------------------------------------- Reporter: cjwatson | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.8.1 checker) | Operating System: Linux Keywords: | Type of failure: Building GHC Architecture: arm | failed Difficulty: Easy (less than 1 | Test Case: hour) | Blocking: Blocked By: | Related Tickets: | -------------------------------------+------------------------------------- Joachim Breitner [http://www.haskell.org/pipermail/glasgow-haskell- users/2014-March/024730.html reported] that the documentation fails to build on Debian armhf: {{{ 0% ( 0 / 5) in 'WwLib' 0% ( 0 / 2) in 'DmdAnal' 0% ( 0 / 2) in 'WorkWrap' compiler/typecheck/TcSplice.lhs-boot:29:1: TcSplice.tcTopSpliceExpr is exported by the hs-boot file, but not exported by the module compiler/typecheck/TcSplice.lhs-boot:37:1: TcSplice.runMetaE is exported by the hs-boot file, but not exported by the module compiler/typecheck/TcSplice.lhs-boot:38:1: TcSplice.runMetaP is exported by the hs-boot file, but not exported by the module compiler/typecheck/TcSplice.lhs-boot:39:1: TcSplice.runMetaT is exported by the hs-boot file, but not exported by the module compiler/typecheck/TcSplice.lhs-boot:40:1: TcSplice.runMetaD is exported by the hs-boot file, but not exported by the module 67% ( 2 / 3) in 'CmmPipeline' 0% ( 0 / 3) in 'StgCmmHpc' 0% ( 0 / 13) in 'PrelInfo' 0% ( 0 / 4) in 'StgCmmCon' 0% ( 0 / 2) in 'StgCmmExpr' 0% ( 0 / 6) in 'StgCmmBind' 0% ( 0 / 2) in 'CmmParse' 0% ( 0 / 2) in 'StgCmm' 5% ( 9 /175) in 'TcRnMonad' make[2]: *** [compiler/stage2/doc/html/ghc/ghc.haddock] Error 1 }}} The attached patch fixes this. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8988> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9552: powerpc64 little endian: dll-split: Reachable modules from DynFlags out of date
by GHC 15 Dec '14

15 Dec '14

#9552: powerpc64 little endian: dll-split: Reachable modules from DynFlags out of date ----------------------------+---------------------------------------------- Reporter: trommler | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Keywords: | Operating System: Linux Architecture: Other | Type of failure: Building GHC failed Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: ----------------------------+---------------------------------------------- Building ghc for powerpc64 little endian (powerpc64le) on Linux fails: {{{ inplace/bin/dll-split compiler/stage2/build/.depend-v-dyn.haskell "DynFlags" "Annotations Avail Bag BasicTypes BinIface Binary Bitmap BlockId BooleanFormula BreakArray BufWrite BuildTyCl ByteCodeAsm ByteCodeInstr ByteCodeItbls CLabel Class CmdLineParser Cmm CmmCallConv CmmExpr CmmInfo CmmMachOp CmmNode CmmType CmmUtils CoAxiom ConLike CodeGen.Platform CodeGen.Platform.ARM CodeGen.Platform.NoRegs CodeGen.Platform.PPC CodeGen.Platform.PPC_Darwin CodeGen.Platform.SPARC CodeGen.Platform.X86 CodeGen.Platform.X86_64 Coercion Config Constants CoreArity CoreFVs CoreLint CoreSubst CoreSyn CoreTidy CoreUnfold CoreUtils CostCentre DataCon Demand Digraph DriverPhases DsMonad DynFlags Encoding ErrUtils Exception ExtsCompat46 FamInstEnv FastBool FastFunctions FastMutInt FastString FastTypes Finder Fingerprint FiniteMap ForeignCall Hooks Hoopl Hoopl.Dataflow HsBinds HsDecls HsDoc HsExpr HsImpExp HsLit HsPat HsSyn HsTypes HsUtils HscTypes IOEnv Id IdInfo IfaceEnv IfaceSyn IfaceType InstEnv InteractiveEvalTypes Kind ListSetOps Literal LoadIface Maybes MkCore MkGraph MkId Module MonadUtils Name NameEnv NameSet OccName OccurAnal OptCoercion OrdList Outputable PackageConfig Packages Pair Panic PatSyn PipelineMonad Platform PlatformConstants PprCmm PprCmmDecl PprCmmExpr PprCore PrelInfo PrelNames PrelRules Pretty PrimOp RdrName Reg RegClass Rules SMRep Serialized SrcLoc StaticFlags StgCmmArgRep StgCmmClosure StgCmmEnv StgCmmLayout StgCmmMonad StgCmmProf StgCmmTicky StgCmmUtils StgSyn Stream StringBuffer TcEvidence TcIface TcRnMonad TcRnTypes TcType TcTypeNats TrieMap TyCon Type TypeRep TysPrim TysWiredIn Unify UniqFM UniqSet UniqSupply Unique Util Var VarEnv VarSet" Reachable modules from DynFlags out of date Please fix compiler/ghc.mk, or building DLLs on Windows may break (#7780) Redundant modules: Bitmap BlockId ByteCodeAsm ByteCodeInstr ByteCodeItbls CLabel Cmm CmmCallConv CmmExpr CmmInfo CmmMachOp CmmNode CmmUtils CodeGen.Platform CodeGen.Platform.ARM CodeGen.Platform.NoRegs CodeGen.Platform.PPC CodeGen.Platform.PPC_Darwin CodeGen.Platform.SPARC CodeGen.Platform.X86 CodeGen.Platform.X86_64 FastBool Hoopl Hoopl.Dataflow InteractiveEvalTypes MkGraph PprCmm PprCmmDecl PprCmmExpr Reg RegClass SMRep StgCmmArgRep StgCmmClosure StgCmmEnv StgCmmLayout StgCmmMonad StgCmmProf StgCmmTicky StgCmmUtils StgSyn Stream compiler/ghc.mk:640: recipe for target 'compiler/stage2/dll-split.stamp' failed }}} The same build, however, works for big endian powerpc64 which is also an unregisterised build via C code. Both builds were done with `GHC_DYNAMIC_PROGRAMS=YES` in `mk/build.mk`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9552> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9336: binutils gold linker detection does not work when called via gcc and selected by commandline parameters
by GHC 15 Dec '14

15 Dec '14

#9336: binutils gold linker detection does not work when called via gcc and selected by commandline parameters -------------------------------------+------------------------------------- Reporter: slomo | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Keywords: | Differential Revisions: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: -------------------------------------+------------------------------------- The binutils gold detection does not work if it is called via gcc and only selected via commandline parameters (i.e. -fuse-ld=gold). Reason is that the "linker" (actually compiler) is called without its commandline parameters. Attached patch fixes that. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9336> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9620: libffi.a is put in the wrong folder
by GHC 15 Dec '14

15 Dec '14

#9620: libffi.a is put in the wrong folder -------------------------------------+------------------------------------- Reporter: rezb1t | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.10.1 Component: Build System | Version: 7.9 Keywords: libffi lib64 | Operating System: Linux nixos | Type of failure: Building Architecture: Unknown/Multiple | GHC failed Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- This is dfeuer's error message, but it is equivalent to mine on NixOS x86_64 {{{ "/home/dfeuer/GHC/7.8.3.bin/bin/ghc" -o utils/genapply/dist/build/tmp/genapply -hisuf hi -osuf o -hcsuf hc -static -O -H64m -package pretty -package-db libraries/bootstrapping.conf -i -iutils/genapply/. -iutils/genapply/dist/build -iutils/genapply/dist/build/autogen -Iutils/genapply/dist/build -Iutils/genapply/dist/build/autogen -no-user-package-db -rtsopts -odir utils/genapply/dist/build -hidir utils/genapply/dist/build -stubdir utils/genapply/dist/build -static -O -H64m -package pretty -package-db libraries/bootstrapping.conf -i -iutils/genapply/. -iutils/genapply/dist/build -iutils/genapply/dist/build/autogen -Iutils/genapply/dist/build -Iutils/genapply/dist/build/autogen -no-user-package-db -rtsopts utils/genapply/dist/build/GenApply.o libffi/stamp.ffi.static-shared.install exists, but libffi/build/inst/lib/libffi.a does not. Suggest removing libffi/stamp.ffi.static-shared.install. make[1]: *** [libffi/build/inst/lib/libffi.a] Error 1 make[1]: *** Waiting for unfinished jobs.... make: *** [all] Error 2 }}} libffi.a is build and linked into libffi/build/inst/lib64 instead of libffi/build/inst/lib, but the includes and pkg-config specific stuff is still put into libffi/build/inst/lib. GHC looks in lib for libffi.a and fails, the current workaround is to copy the contents of libffi/build/inst/lib64 into libffi/build/inst/lib and type make again. Symlinking won't work because libffi seems to sanitize the build directory before building the library I'd like to note that when compiling via the Debian source snapshot, libffi is put in the correct place if I run ./configure without using perl boot. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9620> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8951: genSym uses atomic_inc but doesn't link arm_atomic_spin_lock
by GHC 15 Dec '14

15 Dec '14

#8951: genSym uses atomic_inc but doesn't link arm_atomic_spin_lock --------------------------------+---------------------------------------- Reporter: cjwatson | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Build System | Version: 7.8.1-rc1 Keywords: | Operating System: Linux Architecture: arm | Type of failure: Building GHC failed Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | --------------------------------+---------------------------------------- On Debian armel, a snapshot of GHC 7.8 from 20140228 fails to build as follows: https://buildd.debian.org/status/fetch.php?pkg=ghc&arch=armel&ver=7.8.20140… {{{ "inplace/bin/ghc-stage2" -o utils/haddock/dist/build/tmp/haddock -hisuf hi -osuf o -hcsuf hc -static -H32m -O -lffi -optl-pthread -optc-mlong-calls -hide-all-packages -i -iutils/haddock/driver -iutils/haddock/src -iutils/haddock/vendor/attoparsec-0.10.4.0 -iutils/haddock/dist/build -iutils/haddock/dist/build/autogen -Iutils/haddock/dist/build -Iutils/haddock/dist/build/autogen -optP-DIN_GHC_TREE -optP-include -optPutils/haddock/dist/build/autogen/cabal_macros.h -package Cabal-1.18.1.3 -package array-0.5.0.0 -package base-4.7.0.0 -package bytestring-0.10.4.0 -package containers-0.5.4.0 -package deepseq-1.3.0.2 -package directory-1.2.0.2 -package filepath-1.3.0.2 -package ghc-7.8.0.20140228 -package xhtml-3000.2.1 -funbox-strict-fields -Wall -fwarn-tabs -O2 -XHaskell2010 -no-user-package-db -rtsopts -odir utils/haddock/dist/build -hidir utils/haddock/dist/build -stubdir utils/haddock/dist/build utils/haddock/dist/build/Main.o utils/haddock/dist/build/Documentation/Haddock.o utils/haddock/dist/build/Data/Attoparsec.o utils/haddock/dist/build/Data/Attoparsec/ByteString.o utils/haddock/dist/build/Data/Attoparsec/ByteString/Char8.o utils/haddock/dist/build/Data/Attoparsec/Combinator.o utils/haddock/dist/build/Data/Attoparsec/Number.o utils/haddock/dist/build/Data/Attoparsec/ByteString/FastSet.o utils/haddock/dist/build/Data/Attoparsec/ByteString/Internal.o utils/haddock/dist/build/Data/Attoparsec/Internal.o utils/haddock/dist/build/Data/Attoparsec/Internal/Types.o utils/haddock/dist/build/Haddock.o utils/haddock/dist/build/Haddock/Interface.o utils/haddock/dist/build/Haddock/Interface/Rename.o utils/haddock/dist/build/Haddock/Interface/Create.o utils/haddock/dist/build/Haddock/Interface/AttachInstances.o utils/haddock/dist/build/Haddock/Interface/LexParseRn.o utils/haddock/dist/build/Haddock/Interface/ParseModuleHeader.o utils/haddock/dist/build/Haddock/Parser.o utils/haddock/dist/build/Haddock/Parser/Util.o utils/haddock/dist/build/Haddock/Utf8.o utils/haddock/dist/build/Haddock/Utils.o utils/haddock/dist/build/Haddock/Backends/Xhtml.o utils/haddock/dist/build/Haddock/Backends/Xhtml/Decl.o utils/haddock/dist/build/Haddock/Backends/Xhtml/DocMarkup.o utils/haddock/dist/build/Haddock/Backends/Xhtml/Layout.o utils/haddock/dist/build/Haddock/Backends/Xhtml/Names.o utils/haddock/dist/build/Haddock/Backends/Xhtml/Themes.o utils/haddock/dist/build/Haddock/Backends/Xhtml/Types.o utils/haddock/dist/build/Haddock/Backends/Xhtml/Utils.o utils/haddock/dist/build/Haddock/Backends/LaTeX.o utils/haddock/dist/build/Haddock/Backends/HaddockDB.o utils/haddock/dist/build/Haddock/Backends/Hoogle.o utils/haddock/dist/build/Haddock/ModuleTree.o utils/haddock/dist/build/Haddock/Types.o utils/haddock/dist/build/Haddock/Doc.o utils/haddock/dist/build/Haddock/Version.o utils/haddock/dist/build/Haddock/InterfaceFile.o utils/haddock/dist/build/Haddock/Options.o utils/haddock/dist/build/Haddock/GhcUtils.o utils/haddock/dist/build/Haddock/Convert.o utils/haddock/dist/build/Paths_haddock.o /«PKGBUILDDIR»/compiler/stage2/build/libHSghc-7.8.0.20140228.a(genSym.o): In function `genSym': genSym.c:(.text+0x84): undefined reference to `arm_atomic_spin_lock' genSym.c:(.text+0x88): undefined reference to `arm_atomic_spin_unlock' collect2: error: ld returned 1 exit status make[2]: *** [utils/haddock/dist/build/tmp/haddock] Error 1 }}} Our armel architecture satisfies "arm_HOST_ARCH && defined(arm_HOST_ARCH_PRE_ARMv6)", so cas() in includes/stg/SMP.h will (uniquely among architectures) not be entirely inline: it calls arm_atomic_spin_lock and arm_atomic_spin_unlock, which are in libHSrts. 7.8 introduces compiler/cbits/genSym.c which uses atomic_inc. How should the linkage requirements be satisfied here? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8951> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9372: dll-split during stage 2 compiling ghc v7.8.3 for arm_linux
by GHC 14 Dec '14

14 Dec '14

#9372: dll-split during stage 2 compiling ghc v7.8.3 for arm_linux ----------------------------+---------------------------------------------- Reporter: chrisfgl | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Keywords: | Operating System: Linux Architecture: arm | Type of failure: Building GHC failed Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: ----------------------------+---------------------------------------------- Trying to compile GHC 7.8.3 for ubuntu/arm. Was in Stage 1 for 8 hours before exiting with following message: dll-split: internal error: evacuate(static): strange closure type 0 (GHC version 7.8.3 for arm_unknown_linux) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug make[1]: *** [compiler/stage2/dll-split.stamp] Aborted (core dumped) make: *** [all] Error 2 -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9372> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9583: Simplifier ticks exhausted while compiling Cabal HEAD
by GHC 11 Dec '14

11 Dec '14

#9583: Simplifier ticks exhausted while compiling Cabal HEAD -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: bug | Status: new Priority: highest | Milestone: Component: Compiler | Version: 7.9 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Compile- Blocked By: | time crash Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Steps to reproduce: 1. Clone Cabal HEAD 2. `cabal configure --with-ghc=/path/to/ghc-head` 3. `dist/setup/setup build` Actual results: {{{ [ 8 of 78] Compiling Distribution.Text ( Distribution/Text.hs, dist/build/Distribution/Text.o ) [ 9 of 78] Compiling Distribution.Version ( Distribution/Version.hs, dist/build/Distribution/Version.o ) ghc-stage2: panic! (the 'impossible' happened) (GHC version 7.9.20140911 for x86_64-unknown-linux): Simplifier ticks exhausted When trying UnfoldingDone GHC.Word.$fNumWord8_$c+ To increase the limit, use -fsimpl-tick-factor=N (default 100) If you need to do this, let GHC HQ know, and what factor you needed To see detailed counts use -ddump-simpl-stats Total ticks: 813720 Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} I know that it still fails with tick factor 4096; I haven't found a tick amount which makes it succeed. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9583> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9566: T7891 fails as '*** Core Lint errors : in result of Simplifier ***' in WAY=optasm
by GHC 11 Dec '14

11 Dec '14

#9566: T7891 fails as '*** Core Lint errors : in result of Simplifier ***' in WAY=optasm -------------------------------------+------------------------------------- Reporter: slyfox | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.9 Keywords: | Operating System: Architecture: x86_64 (amd64) | Unknown/Multiple Difficulty: Unknown | Type of failure: GHC Blocked By: | rejects valid program Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- ghc-HEAD: {{{ $ make fulltest TEST="T7891" WAY=optasm cd ./typecheck/should_compile && '/home/slyfox/dev/git/ghc- validate/inplace/bin/ghc-stage2' -fforce-recomp -dcore-lint -dcmm-lint -dno-debug-output -no-user-package-db -rtsopts -fno-ghci-history -c T7891.hs -O -fasm -fno-warn-incomplete-patterns >T7891.comp.stderr 2>&1 Compile failed (status 256) errors were: *** Core Lint errors : in result of Simplifier *** <no location info>: Warning: In the type ‘a_12 -> t_aiF -> t_aiF’ @ a_12 is out of scope *** Offending Program *** T7891.tf :: T7891.T [LclIdX, Arity=1, Str=DmdType, Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True, WorkFree=True, Expandable=True, Guidance=ALWAYS_IF(arity=0,unsat_ok=True,boring_ok=True)}] T7891.tf = GHC.Base.id `cast` (Sym T7891.NTCo:T[0] :: (forall t_ahj. t_ahj -> t_ahj) ~R# T7891.T) T7891.f :: forall t_ahw. t_ahw -> t_ahw [LclIdX, Arity=1, Str=DmdType, Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True, WorkFree=True, Expandable=True, Guidance=ALWAYS_IF(arity=0,unsat_ok=True,boring_ok=True)}] T7891.f = T7891.tf `cast` (T7891.NTCo:T[0] :: T7891.T ~R# (forall t_ahj. t_ahj -> t_ahj)) ... }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9566> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9567: tc124 fails as '*** Core Lint errors : in result of Simplifier ***' in WAY=optasm
by GHC 11 Dec '14

11 Dec '14

#9567: tc124 fails as '*** Core Lint errors : in result of Simplifier ***' in WAY=optasm -------------------------------------+------------------------------------- Reporter: slyfox | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.9 Keywords: | Operating System: Architecture: x86_64 (amd64) | Unknown/Multiple Difficulty: Unknown | Type of failure: GHC Blocked By: | rejects valid program Related Tickets: | Test Case: tc124 | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- ghc-HEAD: {{{ $ make fulltest TEST="tc124" WAY=optasm =====> tc124(optasm) 3136 of 4101 [0, 0, 0] cd ./typecheck/should_compile && '/home/slyfox/dev/git/ghc- validate/inplace/bin/ghc-stage2' -fforce-recomp -dcore-lint -dcmm-lint -dno-debug-output -no-user-package-db -rtsopts -fno-ghci-history -c tc124.hs -O -fasm -fno-warn-incomplete-patterns >tc124.comp.stderr 2>&1 Compile failed (status 256) errors were: *** Core Lint errors : in result of Simplifier *** <no location info>: Warning: In the type ‘a_12 -> a_aia -> a_aia’ @ a_12 is out of scope *** Offending Program *** Foo.g :: Foo.T -> Foo.T [LclIdX, Arity=1, Str=DmdType, Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True, WorkFree=True, Expandable=True, Guidance=IF_ARGS [20] 40 30}] Foo.g = \ (t_ahr :: Foo.T) -> case t_ahr of _ [Occ=Dead] { Foo.T ds_dlB ds_dlC -> Foo.T ds_dlB (\ (@ a_ai4) (@ b_ai5) _ [Occ=Dead] (y_aht :: b_ai5) -> y_aht) } ... }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9567> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8240: Better error messages for type family constraints
by GHC 11 Dec '14

11 Dec '14

#8240: Better error messages for type family constraints ------------------------------------+------------------------------------- Reporter: simonpj | 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: | ------------------------------------+------------------------------------- Consider this constraint {{{ F Int ~ Bool -- Arising (say) on line 5 F Int ~ Char -- Arising (say) on line 10 }}} At the moment we'll combine the two to {{{ F Int ~ Bool -- Arising on line 5 Bool ~ Char -- Arising on line 10 }}} The second of these errors is "insoluble" and is reported first. But it's totally misleading: nowhere in the program did we directly need `Int~Char`. Another variant of this came up in Trac #8227. There we get the constraint {{{ Scalar (V a) ~ t0 Scalar (V a) ~ t0 -> t0 }}} which leads to {{{ Scalar (V a) ~ t0 -> t0 t0 ~ t0 -> t0 -- Occurs check }}} which gives the confusing error {{{ T8227.hs:16:44: Occurs check: cannot construct the infinite type: t0 ~ t0 -> t0 Expected type: Scalar (V (t0 -> t0)) Actual type: Scalar (V a) }}} I think the solution is * Do not combine two Wanteds {{{ [W] F tys ~ t1 [W] F tys ~ t2 }}} unless t1 and t2 are both touchable unification variables. * Instead (perhaps) generate a Derived constraint {{{ [D] t1 ~ t2 }}} to express the fact that t1 and t2 had better end up equal. * To do this the inert set would need to contain many `CTyFunEq` constraints with the same LHS. I'm not very sure about this, and no time now, so I'm just making this ticket to record the issue. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8240> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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