January 2018 - ghc-tickets

[GHC] #9980: TcS monad is too heavy
by GHC 24 May '18

24 May '18

#9980: TcS monad is too heavy -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.4 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- The typehcecker's constraint-solving monad, `TcS`, is simply built on top of `TcM`, but it doesn't use most of `TcM`'s facilities. That was fine while `TcS` was only called from the typechecker, but now (as part of [wiki:PatternMatchCheck fixing pattern-match overlap checking]) we invoke it from the desugarer. It seems quite wrong to construct a vast, and unnecessary `TcM` context just to invoke `TcS`. Better: make `TcS` into its own monad, with its own `TcSLclEnv`, built on `IOEnv` like the others. Main objection: the plugins mechanism exposes `unsafeTcPluginTcM`, which would become unavailable. But it it used? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9980> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11968: Document AMP in the deviations from the standard
by GHC 24 May '18

24 May '18

#11968: Document AMP in the deviations from the standard -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Documentation | Version: 8.1 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: -------------------------------------+------------------------------------- The manual includes a section describing deviations from the Haskell2010 standard [https://downloads.haskell.org/~ghc/master/users-guide/bugs.html here]. But this page makes no mention of `Applicative` being a superclass of `Monad`. We should fix this omission. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11968> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11319: ImpredicativeTypes cause trouble (affects deriving of Traversable)
by GHC 24 May '18

24 May '18

#11319: ImpredicativeTypes cause trouble (affects deriving of Traversable) -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.11 (Type checker) | Keywords: | Operating System: Linux ImpredicativeTypes | Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I don't have the latest version of GHC, trying to derive `Functor A` and `Foldable A` is fine but when I derive `Traversable A` in the attachment Error.hs: {{{#!hs {-# LANGUAGE DeriveFunctor, DeriveFoldable, DeriveTraversable, ImpredicativeTypes #-} import Data.Functor (Functor) import Data.Foldable (Foldable) import Data.Traversable (Traversable) data A a = A deriving (Functor, Foldable, Traversable) }}} GHC barks at me (verbose log attached): {{{#!hs /tmp/Error.hs:8:32: error: • Couldn't match type ‘forall a1. A a1’ with ‘A b’ Expected type: f (A b) Actual type: f (forall a. A a) • In the expression: pure A In an equation for ‘traverse’: traverse f A = pure A When typechecking the code for ‘traverse’ in a derived instance for ‘Traversable A’: To see the code I am typechecking, use -ddump-deriv In the instance declaration for ‘Traversable A’ • Relevant bindings include f :: a -> f b (bound at /tmp/Error.hs:8:32) traverse :: (a -> f b) -> A a -> f (A b) (bound at /tmp/Error.hs:8:32) }}} With `-ddump-deriv` we get this (unqualified) instance: {{{#!hs instance Traversable A where traverse f_a2Le A = pure A }}} which by itself causes the same problem in the attachment Error2.hs: {{{#!hs {-# LANGUAGE DeriveFunctor, DeriveFoldable, ImpredicativeTypes #-} import Data.Functor (Functor) import Data.Foldable (Foldable) import Data.Traversable (Traversable) data A a = A deriving (Functor, Foldable) instance Traversable A where traverse f A = pure A }}} Works fine in GHC-7.10.2 and GHC-7.10.0.20150316 and GHC-7.4 (with some additional imports), is this an `ImpredicativeTypes` regression? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11319> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13275: ghci ignores -fprint-explicit-runtime-reps
by GHC 23 May '18

23 May '18

#13275: ghci ignores -fprint-explicit-runtime-reps -------------------------------------+------------------------------------- Reporter: Konrad Gądek | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 8.0.2 Keywords: levity | Operating System: Unknown/Multiple Architecture: | Type of failure: Other Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The docs [https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/using.html #ghc-flag--fprint-explicit-runtime-reps here] mentions the following use- case: {{{ ghci> :t ($) ($) :: (a -> b) -> a -> b ghci> :set -fprint-explicit-runtime-reps ghci> :t ($) ($) :: forall (r :: GHC.Types.RuntimeRep) a (b :: TYPE r). (a -> b) -> a -> b }}} However, I get this: {{{ GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help Prelude> :t ($) ($) :: (a -> b) -> a -> b Prelude> :i ($) ($) :: forall (r :: GHC.Types.RuntimeRep) a (b :: TYPE r). (a -> b) -> a -> b -- Defined in ‘GHC.Base’ infixr 0 $ Prelude> :set -fprint-explicit-runtime-reps Prelude> :t ($) ($) :: (a -> b) -> a -> b Prelude> :i ($) ($) :: forall (r :: GHC.Types.RuntimeRep) a (b :: TYPE r). (a -> b) -> a -> b -- Defined in ‘GHC.Base’ infixr 0 $ }}} I'm not sure if the docs are wrong or GHCi is erroneously ignoring the flag when using `:t`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13275> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10089: feature: warn about unused data definitions (with typeclass instances)
by GHC 23 May '18

23 May '18

#10089: feature: warn about unused data definitions (with typeclass instances) -------------------------------------+------------------------------------- Reporter: slyfox | Owner: Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.8.4 Component: Compiler | Operating System: Unknown/Multiple Keywords: | Type of failure: None/Unknown Architecture: | Blocked By: Unknown/Multiple | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Let's consider an example: {{{#!hs -- M.hs: module M () where -- ghc knows about uselessness of those data D = D -- but doesn't about this: data E = E deriving (Show, Read) }}} '''ghc''' warns about useless '''D''', but not '''E''': {{{ $ ghc-7.8.4 -Wall -c M.hs -fforce-recomp M.hs:5:1: Warning: Defined but not used: type constructor or class ‘D’ M.hs:5:10: Warning: Defined but not used: data constructor ‘D’ }}} There is no way to refer to '''E''' type outside or call it's unstances, correct? I would expect no code should be generated for this module. '''ghc''' could also warn about defined, but not used instances for types, that are not exported. Some background on where those stray types come from: 1. programmer usually adds a set of data types into their program (say, 10-15 types to represent an AST) 2. then annotates them with 'deriving (Read, Show)' convenience instances 3. (months later) amends AST representation a bit and leaves leftover data types not used anywhere Does that make sense? Thanks! -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10089> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12201: Wrong instance selection with overlapping instances and local bindings
by GHC 23 May '18

23 May '18

#12201: Wrong instance selection with overlapping instances and local bindings -------------------------------------+------------------------------------- Reporter: kanetw | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 (Type checker) | 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: -------------------------------------+------------------------------------- test and test2 should have the same type, but it seems like the {-# OVERLAPPING #-} instance is chosen prematurely. {{{#!hs {-# LANGUAGE NoMonomorphismRestriction, FlexibleContexts, FlexibleInstances #-} module Bug where class A a where class A a => B a where data Foo s a instance {-# OVERLAPPING #-} A (Foo Int Bool) instance A (Foo s a) instance B (Foo s a) bar :: (A a, B a) => a bar = undefined helper :: Foo s Bool -> s -> Int helper = undefined {- *Bug> :t test test :: A (Foo s Bool) => s -> Int -} test = helper bar {- *Bug> :t test2 test2 :: s -> Int -} test2 = let foo = bar in helper foo }}} Compare with 7.10.3: {{{ *Bug> :t test test :: A (Foo s Bool) => s -> Int *Bug> :t test2 test2 :: A (Foo s Bool) => s -> Int }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12201> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9918: GHC chooses an instance between two overlapping, but cannot resolve a clause within the similar closed type family
by GHC 23 May '18

23 May '18

#9918: GHC chooses an instance between two overlapping, but cannot resolve a clause within the similar closed type family -------------------------------------+------------------------------------- Reporter: qnikst | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- We hoped that closed type families could replace overlapping instances. It is not currently the case: GHC can resolve overlapping instances but cannot resolve the similar closed type family. I'm attaching a minimized example of program where it can be observed. This is reproducible on ghc-7.8.3 and HEAD. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9918> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13547: ghc: panic! StgCmmEnv: variable not found
by GHC 22 May '18

22 May '18

#13547: ghc: panic! StgCmmEnv: variable not found -------------------------------------+------------------------------------- Reporter: cipher1024 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 (CodeGen) | Keywords: | Operating System: Unknown/Multiple Architecture: x86_64 | Type of failure: Compile-time (amd64) | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: 10158 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- When building with `stack build --resolver lts-7.20` and `stack build --resolver lts-8.8`, i.e. with ghc-8.0.1 and ghc-8.0.2 and {{{ [37 of 60] Compiling Document.Phase.Proofs ( src/Document/Phase/Proofs.hs, .stack- work/dist/x86_64-osx/Cabal-1.24.0.0/build/Document/Phase/Proofs.o ) <no location info>: error: ghc: panic! (the 'impossible' happened) (GHC version 8.0.1 for x86_64-apple-darwin): StgCmmEnv: variable not found $dTypeable_aZSM local binds for: $sunionWith_$sunionWithKey $sfromList1 $sfromList3 $sfromList $s$fOrdEither $s$fMonadReaderrReaderT $s$fIsTupleconstrIdentity $s$fIsTupleconstr(,,,,) $s$fIsTupleconstr(,,,) $s$fIsTupleconstr(,,) $s$fIsTupleconstr(,,)2 $s$fIsTupleconstr(,)1 $s$fIsTupleconstr(,) $s$fIsTupleconstr(,)2 $s$fHasMachineP2p $fNFDataEventRefA $fMonoidEventRefA $fGenericEventRefA $wmake_phase4 make_phase1 $wpoly_go10 make_phase2 make_phase3 make_phase5 $fNFDataEventRefA4 $fNFDataEventRefA2 $stypeRep#78 $swithCommand8 $stypeRep#54 $swithCommand5 $stypeRep#11 $swithCommand2 $stypeRep#74 $stypeRep#81 $stypeRep#15 $stypeRep#20 $stypeRep#79 $stypeRep#80 $stypeRep#70 $stypeRep#75 $stypeRep#58 $stypeRep#66 $stypeRep#71 $stypeRep#62 $stypeRep#8 $stypeRep#67 $stypeRep#63 $stypeRep#59 $stypeRep#50 $stypeRep#55 $stypeRep#38 $stypeRep#46 $stypeRep#51 $stypeRep#42 $stypeRep#47 $stypeRep#43 $stypeRep#34 $stypeRep#39 $stypeRep#31 $stypeRep#35 $stypeRep#28 $stypeRep#24 $stypeRep#25 $stypeRep#21 $stypeRep#3 $stypeRep#16 $stypeRep#7 $stypeRep#12 $stypeRep#2 $smakeCell8 $smakeCell7 $smakeCell40 $smakeCell39 $smakeCell36 $smakeCell35 $smakeCell32 $smakeCell31 $smakeCell4 $smakeCell28 $smakeCell27 $smakeCell24 $smakeCell23 $smakeCell3 $smakeCell20 $smakeCell19 $smakeCell16 $smakeCell15 $smakeCell12 $smakeCell11 $wpoly_go5 $wgo5 $sfromList_go5 $wpoly_go2 $sfromList2 $s$fOrd(,) $sfromList_fromList'1 $wpoly_go1 $s$fEqEither $s$fOrdEither_$s$fOrdEither_$cp1Ord $s$fEq(,) $s$fOrd(,)_$s$fOrd(,)_$cp1Ord $s$fMonadRWST $s$fMonadReaderrReaderT1 $s$fMonadReaderT $s$fApplicativeReaderT $s$fMonadReaderT_$s$fMonadReaderT_$cfail $s$fMonadReaderT_$s$fMonadReaderT_$c>> $s$fMonadReaderT_$s$fMonadReaderT_$c>>= $s$fMonadReaderT_$s$fMonadReaderT_$cp1Monad $s$fMonadRWST_$s$fMonadRWST_$cfail $s$fMonadRWST_$s$fMonadRWST_$c>> $s$fMonadRWST_$s$fMonadRWST_$c>>= $s$fMonadRWST_$s$fMonadRWST_$cp1Monad $s$fIsTupleconstr(,,,,)_$s$fLatexArg[] $s$fIsTupleconstr(,,,,)1 $s$fIsTupleconstr(,,,)_$s$fLatexArg[] $s$fIsTupleconstr(,,,)_irred2 $s$fIsTupleconstr(,,)_$s$fLatexArg[] $s$fIsTupleconstr(,,)_$dLatexArgFromString $s$fIsTupleconstr(,,)_$s$fLatexArgFromStringConc $s$fIsTupleconstr(,,)_irred1 $s$fIsTupleconstr(,,)_$s$fLatexArgNonEmpty $s$fIsTupleconstr(,,)1 $s$fIsTupleconstr(,)3 $s$fHasMachineP1p $s$fHasMachineP2p1 $s$fHasMachineP2p2 $s$fHasMachineP2p3 $s$fHasMachineP2p4 $s$fHasMachineP2p5 $s$fHasMachineP2p6 $s$fHasMachineP1p_$s$fHasMachineP1p_$cp5HasMachineP1 $s$fHasMachineP1p_$s$fHasMachineP1p_$cp4HasMachineP1 $s$fHasMachineP1p_$s$fHasMachineP1p_$cp3HasMachineP1 $s$fHasMachineP1p_$s$fHasMachineP1p_$cp2HasMachineP1 $s$fHasMachineP1p_$s$fHasMachineP1p_$cp1HasMachineP1 $s$fEq(,)_$dEq1 $s$fEq(,)_$dEq $s$fApplicativeReaderT_$s$fFunctorReaderT_$c<$ $s$fApplicativeReaderT_$s$fFunctorReaderT_$cfmap $s$fApplicativeReaderT_$s$fFunctorReaderT $s$fApplicativeRWST $s$fApplicativeReaderT_$dApplicative $s$fApplicativeReaderT_$s$fApplicativeReaderT_$c<*> $s$fApplicativeReaderT_$s$fMonadReaderT_$creturn $s$fApplicativeReaderT_$s$fApplicativeReaderT_$cp1Applicative $s$fApplicativeRWST_$dFunctor $s$fApplicativeRWST_$s$fApplicativeRWST_$c<*> $s$fApplicativeRWST_$s$fApplicativeRWST_$cpure $s$fApplicativeRWST_$s$fApplicativeRWST_$cp1Applicative $fNFDataEventRefA1 $fNFDataEventRefA3 $w$dNFData2 $w$dNFData1 $w$dNFData $fNFDataEventRefA_$crnf $wgo $fMonoidEventRefA_$cmconcat $fMonoidEventRefA_$cmappend $fMonoidEventRefA_$cmempty $fGenericEventRefA_$cto $fGenericEventRefA_$cfrom make_phase4 ruleProxies_rSKY refinement_parser_rSL2 $w$smiddle $w$sgreater $sfilterGt1 $sfilterLt1 $sinsert_$sgo10 $sinsert_$sgo5 $sleadsTo1 $wpoly_go3 $wpoly_go4 $slookup5 $slookup7 $smakeCell2 $smakeCell6 $smakeCell10 $smakeCell14 $smakeCell18 $smakeCell22 $smakeCell26 $smakeCell30 $smakeCell34 $smakeCell38 $wpoly_go6 $wpoly_go7 $wpoly_go8 $sshowStringP1 $strim1 $strim3 $sunions1 $sunless_eta $swithCommand1 $swithCommand4 $swithCommand7 lvl_r2714 lvl1_r2715 go_r2716 $wgo1_r2717 lvl2_r2718 lvl3_r2719 lvl4_r271a lvl5_r271b lvl6_r271c lvl7_r271d lvl8_r271e lvl9_r271f lvl10_r271g lvl11_r271h lvl12_r271i lvl13_r271j lvl14_r271k lvl15_r271l lvl16_r271m lvl17_r271n lvl18_r271o lvl19_r271p lvl20_r271q lvl21_r271r lvl22_r271s lvl23_r271t lvl24_r271u lvl25_r271v lvl26_r271w lvl27_r271x lvl28_r271y lvl29_r271z lvl30_r271A lvl31_r271B lvl32_r271C lvl33_r271D lvl34_r271E lvl35_r271F lvl36_r271G lvl37_r271H lvl38_r271I lvl39_r271J lvl40_r271K lvl49_r2723 lvl50_r2724 lvl51_r2725 lvl52_r2726 lvl53_r2727 lvl54_r2728 lvl55_r2729 lvl56_r272a lvl57_r272b lvl58_r272c lvl59_r272d lvl60_r272e lvl61_r272f lvl62_r272g lvl63_r272h lvl64_r272i lvl65_r272j lvl66_r272k lvl67_r272l lvl68_r272m lvl69_r272n lvl70_r272o lvl71_r272p lvl72_r272q $s$fApplicativeRWST_$c<*>_r272u $s$fApplicativeRWST_$cpure_r272v lvl74_r272w lvl75_r272x lvl76_r272y $s$fMonadRWST_$c>>_r272z $s$fMonadRWST_$cfail_r272A $s$fMonadRWST_$c>>=_r272B lvl77_r272C lvl78_r272D lvl79_r272E lvl80_r272F lvl81_r272G lvl82_r272H $slesser1_r272S lvl88_r272T lvl89_r272U $wcreate_r272V lvl90_r272W m2_r272X $s$fMonadReaderT_$c>>_r272Y $s$fMonadReaderT_$c>>=_r272Z go10_r2730 $wpoly_create_r2731 lvl91_r2732 lvl92_r2733 lvl93_r2734 lvl94_r2735 lvl95_r2736 lvl96_r2737 lvl97_r2738 lvl98_r2739 lvl99_r273a lvl100_r273b lvl101_r273c lvl102_r273d lvl103_r273e lvl104_r273f lvl105_r273g lvl106_r273h lvl107_r273i lvl108_r273j lvl109_r273k lvl110_r273l lvl111_r273m lvl112_r273n lvl113_r273o lvl114_r273p lvl115_r273q lvl116_r273r lvl117_r273s lvl118_r273t lvl119_r273u lvl120_r273v lvl121_r273w lvl122_r273x lvl123_r273y lvl124_r273z $wpoly_create1_r273A lvl125_r273B lvl126_r273C lvl127_r273D lvl128_r273E lvl129_r273F lvl130_r273G lvl131_r273H lvl132_r273I lvl133_r273J lvl134_r273K lvl135_r273L $wlvl_r273M lvl136_r273N $wlvl1_r273O lvl137_r273P $wlvl2_r273Q lvl138_r273R $wlvl3_r273S lvl139_r273T $wlvl4_r273U lvl140_r273V $wlvl5_r273W lvl141_r273X $wlvl6_r273Y lvl142_r273Z lvl143_r2740 lvl144_r2741 lvl145_r2742 lvl146_r2743 lvl147_r2744 lvl148_r2745 lvl149_r2746 lvl150_r2747 lvl151_r2748 lvl152_r2749 lvl153_r274a lvl154_r274b $s$fFunctorReaderT_$cfmap_r274c $s$fFunctorReaderT_$c<$_r274d lvl155_r274e lvl156_r274f $s$fApplicativeReaderT_$c<*>_r274g $s$fMonadReaderT_$creturn_r274h $s$fMonadReaderT_$cfail_r274i lvl157_r274j lvl158_r274k lvl159_r274l lvl160_r274m $d~_r274n lvl161_r274p lvl162_r274q lvl163_r274s lvl164_r274t lvl165_r274v lvl166_r274w lvl167_r274y lvl168_r274z lvl169_r274B lvl170_r274C lvl171_r274E lvl172_r274F lvl173_r274H lvl174_r274I lvl175_r274K lvl176_r274L lvl177_r274N lvl178_r274O lvl179_r274Q lvl180_r274R lvl181_r274T lvl182_r274U lvl183_r274V lvl184_r274W lvl185_r274X lvl186_r274Y lvl187_r274Z lvl188_r2750 lvl189_r2751 lvl190_r2752 lvl191_r2753 lvl192_r2754 lvl193_r2755 lvl194_r2756 lvl195_r2757 lvl196_r2758 lvl197_r2759 lvl198_r275a lvl199_r275b lvl200_r275c lvl201_r275d lvl202_r275e lvl203_r275f lvl204_r275g lvl205_r275h lvl206_r275i lvl207_r275j lvl208_r275k lvl209_r275l lvl210_r275m lvl211_r275n lvl212_r275o lvl213_r275p lvl214_r275q lvl215_r275r lvl216_r275s lvl217_r275t lvl218_r275u lvl219_r275v lvl220_r275w ww1_r275x ww2_r275y lvl221_r275z ww3_r275A lvl222_r275B pre_r275C x_s27MP eta_s27MQ eta1_s27MR ds_s27MS ds1_s27MT ds2_s27MU ds3_s27MV goal_s27MW prxy'_s27MX sat_s27MY Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13547> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14722: Error message points to wrong location
by GHC 21 May '18

21 May '18

#14722: Error message points to wrong location -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.5 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect Unknown/Multiple | error/warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This rightly fails, but I have an issue with the error message. I think points to the visible type application eagerly.. rather than the type annotation `(f_xx :: ())`. {{{#!hs {-# Language RankNTypes, PolyKinds, GADTs, TypeApplications, ScopedTypeVariables #-} import Data.Kind newtype Limit :: (k -> Type) -> Type where Limit :: (forall xx. f xx) -> Limit f foo :: forall f a. Limit f -> f a foo (Limit (f_xx :: ())) = f_xx @a }}} gives {{{ $ ghci -ignore-dot-ghci /tmp/Test.hs GHCi, version 8.5.20180105: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( /tmp/Test.hs, interpreted ) /tmp/Test.hs:9:28: error: • Cannot apply expression of type ‘()’ to a visible type argument ‘a’ • In the expression: f_xx @a In an equation for ‘foo’: foo (Limit (f_xx :: ())) = f_xx @a | 9 | foo (Limit (f_xx :: ())) = f_xx @a | ^^^^^^^ Failed, no modules loaded. Prelude> }}} I would have expected: {{{ /tmp/Test.hs:9:13: error: • Couldn't match expected type ‘()’ with actual type ‘f xx0’ • When checking that the pattern signature: () fits the type of its context: forall (xx :: k1). f xx In the pattern: f_xx :: () In the pattern: Limit (f_xx :: ()) • Relevant bindings include foo :: Limit f -> f a (bound at /tmp/Test.hs:9:1) }}} Thoughts -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14722> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14370: non-deferred typed hole despite -fdefer-typed-holes
by GHC 21 May '18

21 May '18

#14370: non-deferred typed hole despite -fdefer-typed-holes -------------------------------------+------------------------------------- Reporter: int-e | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect Unknown/Multiple | error/warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider the following ghci session. {{{ GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help Prelude> :set -fdefer-typed-holes -Wno-typed-holes Prelude> :set -fdefer-out-of-scope-variables -Wno-deferred-out-of-scope- variables Prelude> let x = [_]; y = [r] Prelude> (length x, length y) (1,1) Prelude> length [_] <interactive>:4:9: error: • Found hole: _ :: a0 Where: ‘a0’ is an ambiguous type variable • In the expression: _ In the first argument of ‘length’, namely ‘[_]’ In the expression: length [_] • Relevant bindings include it :: Int (bound at <interactive>:4:1) Prelude> length [r] 1 }}} Why does the `length [_]` expression produce a type error immediately instead of being deferred? (I've asked the same question in #14367 but this looks like a real bug.) -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14370> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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