May 2017 - ghc-tickets

[GHC] #13320: Unfortunate compiler loop when creating type loop (with UndecidableInstances)
by GHC 04 May '17

04 May '17

#13320: Unfortunate compiler loop when creating type loop (with UndecidableInstances) -------------------------------------+------------------------------------- Reporter: Ptival | Owner: (none) Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple UndecidableInstances loop | Architecture: | Type of failure: Compile-time Unknown/Multiple | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I'm afraid this will simply be seen as "that's what happens when you use UndecidableInstances", but I might as well document this issue I had. Trying to play with a "Trees that Grow" syntax, I encountered an issue when making a mistake, that can be boiled down to the following: {{{#!hs {-# language ConstraintKinds, FlexibleContexts, TypeFamilies, UndecidableInstances #-} module Loop where import GHC.Exts (Constraint) import Test.QuickCheck type family X_Var ξ data TermX ξ = Var (X_Var ξ) type ForallX (φ :: * -> Constraint) ξ = ( φ (X_Var ξ) ) --genTerm :: ForallX Arbitrary ξ => Int -> Gen (TermX ξ) genTerm 0 = Var <$> arbitrary genTerm n = Var <$> genTerm (n - 1) --instance ForallX Arbitrary ξ => Arbitrary (TermX ξ) where --arbitrary = sized genTerm }}} This code will compile correctly, and generate: {{{#!hs genTerm :: (X_Var ξ ~ TermX ξ, Arbitrary (TermX ξ), Eq t, Num t) => t -> Gen (TermX ξ) }}} Which is correct (though, not the type I had intended, since my code had a mistake). Now, if you uncomment the "instance" line only, the compiler will loop. Adding the commented out type, of course, gives a type error where it's due. I was just wondering whether this type of error failed with the loops that should be caught by the compiler. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13320> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13642: GHCi 8.2 simply ignores TH splice using datatype with a forall'd kind signature
by GHC 04 May '17

04 May '17

#13642: GHCi 8.2 simply ignores TH splice using datatype with a forall'd kind signature -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Template | Version: 8.2.1-rc1 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: -------------------------------------+------------------------------------- This is a pretty bizarre behavior I've noticed recently that only happens in GHC 8.2 or later. If you try to use a Template Haskell splice with a very particular feature (a datatype whose kind uses `forall`) in GHCi, then GHCi will flat-out ignore it! {{{ $ /opt/ghc/8.2.1/bin/ghci GHCi, version 8.2.0.20170427: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci λ> :set -XGADTs -XTypeInType -XTemplateHaskell -XRankNTypes λ> import Language.Haskell.TH (stringE, pprint) λ> import Data.Kind (Type) λ> $([d| data Foo :: forall a. a -> Type where MkFoo :: Foo Int |] >>= stringE . pprint) λ> print (5 + length $([d| data Foo :: forall a. a -> Type where MkFoo :: Foo Int |] >>= stringE . pprint)) λ> 5 5 λ> it 5 λ> $([d| data Foo :: forall a. a -> Type where MkFoo :: Foo Int |] >>= stringE . pprint) λ> it 5 }}} Notice how none of my attempts to use the splice seemed to register with GHCi. This isn't really a regression //per se//, since GHC 8.0.1 nor 8.0.2 even allowed you to get that far: {{{ $ /opt/ghc/8.0.2/bin/ghci GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci λ> :set -XGADTs -XTypeInType -XTemplateHaskell -XRankNTypes λ> import Language.Haskell.TH (stringE, pprint) λ> import Data.Kind (Type) λ> $([d| data Foo :: forall a. a -> Type where MkFoo :: Foo Int |] >>= stringE . pprint) <interactive>:4:3: error: Exotic form of kind not (yet) handled by Template Haskell forall a. a -> Type }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13642> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13645: whoCreated produces an uninformative stack trace when an exception is raised in a CAF
by GHC 04 May '17

04 May '17

#13645: whoCreated produces an uninformative stack trace when an exception is raised in a CAF -------------------------------------+------------------------------------- Reporter: refold | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider the following program: {{{#!hs {-# LANGUAGE ScopedTypeVariables #-} module Main where import Control.Exception import GHC.Stack {-# NOINLINE caf #-} caf :: [Int] caf = [1..500] {-# NOINLINE caf_exc #-} caf_exc :: Int caf_exc = caf !! 1000 {-# NOINLINE foo #-} foo :: Int -> Int foo 0 = caf_exc foo n = foo $ n - 1 {-# NOINLINE bar #-} bar :: Int -> Int bar n = bar' n where bar' 0 = foo n bar' m = bar' $ m - 1 main :: IO () main = print (bar 10) `catch` \(e :: SomeException) -> do stacktrace <- whoCreated e print stacktrace }}} By default, when built with profiling, `whoCreated` in the example above produces a quite uninformative stack trace: {{{#!shell $ ./caf-nostack ["GHC.List.CAF (<entire-module>)"] }}} However, if you run the program with `+RTS -xc`, you'll see that it prints a stack trace with much more context: {{{#!shell $ ./caf-nostack +RTS -xc *** Exception (reporting due to +RTS -xc): (THUNK_2_0), stack trace: GHC.List.CAF --> evaluated by: Main.caf_exc, called from Main.CAF --> evaluated by: Main.foo, called from Main.bar.bar', called from Main.bar, called from Main.main, called from Main.CAF --> evaluated by: Main.main ["GHC.List.CAF (<entire-module>)"] }}} It'd be nice if `whoCreated` produced something closer to the `+RTS -xc` output in this case. Cabalised test project: https://github.com/23Skidoo/caf-nostack -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13645> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13638: configure error: GHC is required.
by GHC 04 May '17

04 May '17

#13638: configure error: GHC is required. -------------------------------------+------------------------------------- Reporter: QuRyu | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Build System | Version: 8.0.1 Keywords: | Operating System: MacOS X Architecture: | Type of failure: Building GHC Unknown/Multiple | failed Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- While I was trying to download GHC source code from Github and to build it myself, I encountered the error "configure: error: GHC is required." when I was doing "./configure". The "perl boot" raised no warning or error, and the source code was directly downloaded from Github using "git clone --recursive git://github.com/ghc/ghc.git". -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13638> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13631: In GHCi a result is wrong when -fdefer-typed-holes is used with underscore alone
by GHC 03 May '17

03 May '17

#13631: In GHCi a result is wrong when -fdefer-typed-holes is used with underscore alone -------------------------------------+------------------------------------- Reporter: vanto | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect result Unknown/Multiple | at runtime Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I open this ticket instead of {{{#13579}}} to restate it and I closed the other.\\ In fact a result of the compiler which I think is wrong misled me.\\ Therefore I close tickets {{{#13602}}} and {{{#13557}}}.\\ In Haskell2010 Language Report it is said that :\\ 1. underscore "_ " all by itself is a reserved identifier.\\ 2. underscore "_" is treated as a lowercase letter, and can occur wherever a lowercase letter can.\\ So {{{_e}}} is an identifier like {{{__}}}\\ GHCi gives a bad result when he computed the code below.\\ {{{ Prelude> :set -fdefer-typed-holes Prelude> let f = map (\x -> True) [_, _] <interactive>:2:27: warning: [-Wtyped-holes] * Found hole: _ :: a0 Where: `a0' is an ambiguous type variable * In the expression: _ In the second argument of `map', namely `[_, _]' In the expression: map (\ x -> True) [_, _] * Relevant bindings include f :: [Bool] (bound at <interactive>:2:5) <interactive>:2:30: warning: [-Wtyped-holes] * Found hole: _ :: a0 Where: `a0' is an ambiguous type variable * In the expression: _ In the second argument of `map', namely `[_, _]' In the expression: map (\ x -> True) [_, _] * Relevant bindings include f :: [Bool] (bound at <interactive>:2:5) Prelude> f [True,True] }}} The underscore "_" is alone , all by itself and yet is recognized as an identifier, and GHCi gives a result.\\ This is the bug. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13631> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11333: GHCi does not discharge satisfied constraints
by GHC 03 May '17

03 May '17

#11333: GHCi does not discharge satisfied constraints ----------------------------------------+-------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Keywords: | Operating System: Linux Architecture: Unknown/Multiple | Type of failure: Other Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: ----------------------------------------+-------------------------- Don't know if this is the intended behaviour (GHCi, version 8.1.20151231), with the new extension TypeApplications. For the function [https://hackage.haskell.org/package/base-4.8.1.0/docs /Text-Show.html#v:show show]: {{{#!hs Prelude> :set -XTypeApplications Prelude> :t show show :: Show a => a -> String Prelude> :t show @Int show @Int :: Show Int => Int -> String }}} And the function [https://hackage.haskell.org/package/base/docs/Data- Typeable.html#v:eqT eqT] (`:: forall a b. (Typeable a, Typeable b) => Maybe (a :~: b)`): {{{#!hs Prelude> :set -XTypeApplications Prelude> import Data.Typeable Prelude Data.Typeable> :t eqT eqT :: forall k1 (a :: k1) (b :: k1). (Typeable a, Typeable b) => Maybe (a :~: b) Prelude Data.Typeable> :t eqT eqT :: forall k1 (a :: k1) (b :: k1). (Typeable a, Typeable b) => Maybe (a :~: b) Prelude Data.Typeable> :t eqT @Int eqT @Int :: (Typeable Int, Typeable b) => Maybe (Int :~: b) }}} Should the types of `show @Int` and `eqT @Int` not be `Int -> String` and `(Typeable b) => Maybe (Int :~: b)` respectively? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11333> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13585: ala from Control.Lens.Wrapped panics
by GHC 03 May '17

03 May '17

#13585: ala from Control.Lens.Wrapped panics -------------------------------------+------------------------------------- Reporter: fumieval | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: (Type checker) | Keywords: | Operating System: Linux Architecture: | Type of failure: Compile-time Unknown/Multiple | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Panic.hs: {{{ module Panic where import Control.Lens.Wrapped import Data.Monoid foo :: Maybe String foo = ala Last foldMap [Just "foo"] }}} main.hs: {{{ module Main where import Panic (foo) main :: IO () main = print foo }}} {{{ $ ghc -c -O2 Panic.hs $ ghc -c -O2 main.hs ghc: panic! (the 'impossible' happened) (GHC version 8.2.0.20170404 for x86_64-unknown-linux): splitTyConApp (Exchange (Unwrapped (Last String)) (Unwrapped (Last String)) |> <* -> * -> *>_N) (Maybe [Char]) ((Identity |> <* -> *>_N) (Maybe [Char])) Call stack: CallStack (from HasCallStack): prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1134:58 in ghc:Outputable callStackDoc, called at compiler/utils/Outputable.hs:1138:37 in ghc:Outputable pprPanic, called at compiler/types/Type.hs:1105:34 in ghc:Type Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} The GHC version is 8134f7d4ba2c14b2f24d2f4c1f5260fcaff3304a. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13585> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13333: Typeable regression in GHC HEAD
by GHC 03 May '17

03 May '17

#13333: Typeable regression in GHC HEAD -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: high | Milestone: 8.2.1 Component: Compiler | Version: 8.1 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I recently wrote some code while exploring a fix for #13327 that heavily uses poly-kinded `Typeable`. This compiles without issue on GHC 8.0.1 and 8.0.2: {{{#!hs {-# LANGUAGE ConstraintKinds #-} {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeInType #-} {-# LANGUAGE TypeOperators #-} module DataCast where import Data.Data import GHC.Exts (Constraint) data T (phantom :: k) = T data D (p :: k -> Constraint) (x :: j) where D :: forall (p :: k -> Constraint) (x :: k). p x => D p x class Possibly p x where possibly :: proxy1 p -> proxy2 x -> Maybe (D p x) dataCast1T :: forall k c t (phantom :: k). (Typeable k, Typeable t, Typeable phantom, Possibly Data phantom) => (forall d. Data d => c (t d)) -> Maybe (c (T phantom)) dataCast1T f = case possibly (Proxy :: Proxy Data) (Proxy :: Proxy phantom) of Nothing -> Nothing Just D -> gcast1 f }}} But on GHC HEAD, it barfs this error: {{{ Bug.hs:28:29: error: • Could not deduce (Typeable T) arising from a use of ‘gcast1’ GHC can't yet do polykinded Typeable (T :: * -> *) from the context: (Typeable k, Typeable t, Typeable phantom, Possibly Data phantom) bound by the type signature for: dataCast1T :: (Typeable k, Typeable t, Typeable phantom, Possibly Data phantom) => (forall d. Data d => c (t d)) -> Maybe (c (T phantom)) at Bug.hs:(22,1)-(25,35) or from: (k ~ *, (phantom :: k) ~~ (x :: *), Data x) bound by a pattern with constructor: D :: forall k (p :: k -> Constraint) (x :: k). p x => D p x, in a case alternative at Bug.hs:28:23 • In the expression: gcast1 f In a case alternative: Just D -> gcast1 f In the expression: case possibly (Proxy :: Proxy Data) (Proxy :: Proxy phantom) of Nothing -> Nothing Just D -> gcast1 f | 28 | Just D -> gcast1 f | ^^^^^^^^ }}} That error message itself is hilariously strange, since GHC certainly has the power to do polykinded `Typeable` nowadays. Marking as high priority since this is a regression—feel free to lower the priority if you disagree. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13333> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11669: Incorrectly suggests RankNTypes for ill-formed type "forall a. Eq a. Int"
by GHC 03 May '17

03 May '17

#11669: Incorrectly suggests RankNTypes for ill-formed type "forall a. Eq a. Int" -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | 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: -------------------------------------+------------------------------------- {{{ baldur@Loki:~$ ghci -ignore-dot-ghci GHCi, version 8.1.20160117: http://www.haskell.org/ghc/ :? for help Prelude> let foo :: forall a. Eq a. Int; foo = undefined <interactive>:1:20: error: Illegal symbol '.' in type Perhaps you intended to use RankNTypes or a similar language extension to enable explicit-forall syntax: forall <tvs>. <type> Prelude> :set -XRankNTypes Prelude> let foo :: forall a. Eq a. Int; foo = undefined <interactive>:3:26: error: Illegal symbol '.' in type Perhaps you intended to use RankNTypes or a similar language extension to enable explicit-forall syntax: forall <tvs>. <type> Prelude> }}} also {{{ Prelude> :kind forall a. Eq a. Int <interactive>:1:15: error: Illegal symbol '.' in type Perhaps you intended to use RankNTypes or a similar language extension to enable explicit-forall syntax: forall <tvs>. <type> }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11669> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13578: Incorrect Record Pattern Synonym example in users guide
by GHC 03 May '17

03 May '17

#13578: Incorrect Record Pattern Synonym example in users guide -------------------------------------+------------------------------------- Reporter: cjholdbrooks | Owner: (none) Type: task | Status: new Priority: normal | Milestone: Component: Documentation | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Documentation Unknown/Multiple | bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The first example in section 9.7.1 of the users guide provides the example {{{#!hs pattern Point :: (Int, Int) pattern Point{x, y} = (x, y) }}} which results in the error {{{ • Pattern synonym ‘Point’ has two arguments but its type signature has none • In the declaration for pattern synonym ‘Point’ }}} upon load. I believe the correct code is as follows. {{{#!hs pattern Point :: Int -> Int -> (Int, Int) pattern Point{x, y} = (x, y) }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13578> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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