February 2017 - ghc-tickets

[GHC] #12978: User guide section on AllowAmbiguousTypes should mention TypeApplications
by GHC 23 Jun '17

23 Jun '17

#12978: User guide section on AllowAmbiguousTypes should mention TypeApplications -------------------------------------+------------------------------------- Reporter: mpickering | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 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: -------------------------------------+------------------------------------- https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_ext… #ghc-flag--XAllowAmbiguousTypes With type applications it is tempting to write {{{ class C x a where meth :: a }}} which will fail the ambiguity check as `x` is not used in any methods. One way to get around this is to use `TypeApplications` in order to specify what `x` should be at each usage of `meth`. A small section in the user guide explaining this and linking to the section on `TypeApplications` would be useful. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12978> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11224: Program doesn't preserve semantics after pattern synonym inlining.
by GHC 23 Jun '17

23 Jun '17

#11224: Program doesn't preserve semantics after pattern synonym inlining. -------------------------------------+------------------------------------- Reporter: anton.dubovik | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Keywords: | Operating System: Windows Architecture: x86_64 | Type of failure: Incorrect result (amd64) | at runtime Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{#!hs {-# language PatternSynonyms , ViewPatterns #-} import Text.Read -- pattern PRead :: () => Read a => a -> String pattern PRead a <- (readMaybe -> Just a) foo :: String -> Int foo (PRead x) = (x::Int) foo (PRead xs) = sum (xs::[Int]) foo _ = 666 bar :: String -> Int bar (readMaybe -> Just x) = (x::Int) bar (readMaybe -> Just xs) = sum (xs::[Int]) bar _ = 666 main :: IO () main = do print $ foo "1" -- 1 print $ foo "[1,2,3]" -- 666 -- ??? print $ foo "xxx" -- 666 print $ bar "1" -- 1 print $ bar "[1,2,3]" -- 6 print $ bar "xxx" -- 666 }}} I'm expecting that semantics of the function `foo` would not change if I inline pattern synonym. However it does. `bar` successfully matches string against list of ints, whereas `foo` apperently skips this pattern and fall back to the catch-all case. Reproducible both in GHCi and GHC. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11224> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13009: Hierarchical Module Structure for GHC
by GHC 23 Jun '17

23 Jun '17

#13009: Hierarchical Module Structure for GHC -------------------------------------+------------------------------------- Reporter: hsyl20 | Owner: Type: task | Status: new Priority: low | 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: | ModuleDependencies/Hierarchical -------------------------------------+------------------------------------- Currently GHC's modules are not hierarchical and have very short cryptic names. I would like to revive this 8-year old proposal: https://ghc.haskell.org/trac/ghc/wiki/ModuleDependencies/Hierarchical Pros: * cleaner code base and overall code structure easier to grasp * better integration with other packages when using GHC API * better documentation (cf https://www.stackage.org/lts-7.14/package/ghc-8.0.1) Cons: * break the GHC (plugin) API * may make "rebase" harder during the transition period -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13009> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12379: WARN pragma gives warning `warning: [-Wdeprecations]'
by GHC 22 Jun '17

22 Jun '17

#12379: WARN pragma gives warning `warning: [-Wdeprecations]' -------------------------------------+------------------------------------- Reporter: zilinc | Owner: Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Linux Architecture: x86_64 | Type of failure: Incorrect (amd64) | warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Example: {{{#!hs -- Warn.hs module Warn where __todo :: String -> a {-# WARNING __todo "TODO" #-} __todo msg = error $ "TODO: " ++ msg }}} {{{#!hs -- Main.hs {- OPTIONS_GHC -Wall #-} import Warn inc :: Int -> Int inc n | n >= 0 = n + 1 inc _ = __todo "what about negatives?" }}} When compile the files (or ghci), I get {{{ UseWarn.hs:9:9: warning: [-Wdeprecations] In the use of ‘__todo’ (imported from Warn): "TODO" }}} Should the flag be `-Wwarnings-deprecations`? And `-Wdeprecations` is not in the user guide, if it is a genuine flag. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12379> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13295: Failure to resolve type parameter determined by type family
by GHC 22 Jun '17

22 Jun '17

#13295: Failure to resolve type parameter determined by type family -------------------------------------+------------------------------------- Reporter: alexvieth | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 (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: -------------------------------------+------------------------------------- {{{#!hs {-# LANGUAGE GADTs #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE ScopedTypeVariables #-} import Data.Proxy -- Defined to get the type forall x . 'D x data D where D :: a -> D type family F t :: D -- Will try to create values of this type through the type family -- F, i.e. G (F t) data G (d :: D) where G :: G ('D x) -- This is rejected because the type variable x in 'D x is ambiguous. -- But is it really? If there's an instance of F t then x is determined. introG :: Proxy t -> G (F t) introG _ = G -- introG is well-typed if we add an equality constraint. -- Can GHC not figure this out without our help? introG' :: forall t x . ( F t ~ 'D x ) => Proxy t -> G (F t) introG' _ = G }}} Here's the type error: {{{ • Couldn't match type ‘F t’ with ‘'D x0’ Expected type: G (F t) Actual type: G ('D x0) The type variable ‘x0’ is ambiguous • In the expression: G In an equation for ‘introG’: introG _ = G }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13295> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11758: Drop x86_64 binutils <2.17 hack
by GHC 20 Jun '17

20 Jun '17

#11758: Drop x86_64 binutils <2.17 hack -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: task | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.0.1-rc2 (NCG) | 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: -------------------------------------+------------------------------------- `X86.CodeGen.genSwitch` contains this comment, {{{ else -- HACK: On x86_64 binutils<2.17 is only able to generate -- PC32 relocations, hence we only get 32-bit offsets in -- the jump table. As these offsets are always negative -- we need to properly sign extend them to 64-bit. This -- hack should be removed in conjunction with the hack in -- PprMach.hs/pprDataItem once binutils 2.17 is standard. }}} binutils 2.17 is now quite standard; even CentOS 5 shipped with 2.17. I think it is likely safe to remove this hack at this point. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11758> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8697: Type rationals
by GHC 16 Jun '17

16 Jun '17

#8697: Type rationals ------------------------------------+------------------------------------- Reporter: MikeIzbicki | 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've used GHC's Type Nats to implement my own type level rationals (code below). This works fine, but the syntax is a little funky because you have to write out the number as a fraction. For example, you must write 13/10 instead of 1.3; or even worse, 13/1 instead of 13. It would be nice to just write the decimal notation. I only see one potential problem with this feature: the dot in the decimal notation could interfere with the dot in forall statements. I guess this can be parsed unambiguously, but maybe not. {{{ data Frac = Frac Nat Nat data instance Sing (n::Frac) = SFrac Integer Integer instance (SingI a, SingI b) => SingI ('Frac a b) where sing = SFrac (fromSing (sing :: Sing a)) (fromSing (sing :: Sing b)) instance Fractional r => SingE (Kind :: Frac) r where fromSing (SFrac a b) = fromIntegral a/fromIntegral b type (/) a b = 'Frac a b }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8697> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13267: Constraint synonym instances
by GHC 15 Jun '17

15 Jun '17

#13267: Constraint synonym instances -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC accepts Unknown/Multiple | invalid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Something funny happens when you try to declare an instance of a constraint synonym: {{{ {-# LANGUAGE ConstraintKinds #-} module F where type ShowF a = Show (a -> Bool) instance ShowF Int where show _ = "Fun" }}} I get: {{{ F.hs:8:5: error: ‘show’ is not a (visible) method of class ‘ShowF’ | 8 | show _ = "Fun" | ^^^^ }}} OK, but it gets weirder. Look at: {{{ {-# LANGUAGE ConstraintKinds, FlexibleContexts, FlexibleInstances #-} module F where type ShowF a = (Show (a -> Bool)) instance ShowF Int where }}} This is accepted (with a complaint that `show` is not implemented.) It gets even more awful: {{{ {-# LANGUAGE ConstraintKinds, FlexibleContexts, FlexibleInstances, MultiParamTypeClasses #-} module F where type ShowF a = (Show Bool, Show Int) instance ShowF Int where }}} This is awful: GHC treats `Show Bool` and `Show Int` as if they were constraints, and then emits the following DFun: {{{ df9d1b4635f2a752f29ff327ab66d1cb $f(%,%)ShowShow :: (Show Bool, Show Int) DFunId {- Strictness: m, Inline: CONLIKE, Unfolding: DFun: @ a @ b. @ (Show Bool) @ (Show Int) $fShowBool $fShowInt -} }}} I don't even know what this is supposed to mean. OK, so what should we do? I think there are a few possibilities: 1. Completely outlaw instance declarations on constraint synonyms. 2. Allow instance declarations on constraint synonyms, but only if after desugaring the synonym, you end up with a single class head. I would find this useful in a few cases, for example, if you are writing `instance MonadSample (Impl t) MyMonad`, if you had `type MonadSample2 t a = MonadSample (Impl t) a` you might prefer writing `instance MonadSample t MyMonad` instead 3. Figure out what instance declarations with multiple class heads, and proceed accordingly. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13267> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12056: Too aggressive `-w` option
by GHC 12 Jun '17

12 Jun '17

#12056: Too aggressive `-w` option -------------------------------------+------------------------------------- Reporter: asr | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect Unknown/Multiple | warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- By running {{{ ghc -w -Wunrecognised-warning-flags -Wfoo Main.hs }}} I would expect the warning {{{ on the commandline: warning: unrecognised warning flag: -Wfoo }}} however this warning isn't reported. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12056> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12545: Compilation time/space regression in GHC 8.0/8.1 (search in type-level lists and -O)
by GHC 12 Jun '17

12 Jun '17

#12545: Compilation time/space regression in GHC 8.0/8.1 (search in type-level lists and -O) -------------------------------------+------------------------------------- Reporter: | Owner: mikhail.vorozhtsov | Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Compile-time Unknown/Multiple | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- After upgrading to GHC 8.0.1 I've noticed that compilation time of one of my libraries got unacceptably long. Please find the attached test case. Here are some numbers from my machine: {{{ $ time ~/Prefixes/ghc-7.10.3/bin/ghc -O -c -Rghc-timing TypeList.hs Regression.hs <<ghc: 19103924976 bytes, 1441 GCs, 73048470/256579104 avg/max bytes residency (18 samples), 542M in use, 0.000 INIT (0.002 elapsed), 14.163 MUT (14.257 elapsed), 5.707 GC (5.830 elapsed) :ghc>> real 0m20.160s user 0m19.937s sys 0m0.197s $ time ~/Prefixes/ghc-8.0.1/bin/ghc -O -c -Rghc-timing TypeList.hs Regression.hs <<ghc: 147662211104 bytes, 9635 GCs, 190336418/433097448 avg/max bytes residency (38 samples), 1210M in use, 0.000 INIT (0.001 elapsed), 101.707 MUT (101.840 elapsed), 27.907 GC (28.259 elapsed) :ghc>> real 2m10.195s user 2m9.773s sys 0m0.400s }}} Both versions are the official Debian 8 x86_64 binaries from haskell.org. I also compiled the git master and unfortunately it is affected as well. It is worth mentioning that turning off optimization makes the problem disappear: {{{ $ time ~/Prefixes/ghc-8.0.1/bin/ghc -O0 -c -Rghc-timing TypeList.hs Regression.hs <<ghc: 3779057184 bytes, 225 GCs, 17923355/81566792 avg/max bytes residency (8 samples), 164M in use, 0.000 INIT (0.001 elapsed), 2.847 MUT (2.941 elapsed), 0.553 GC (0.572 elapsed) :ghc>> real 0m3.593s user 0m3.507s sys 0m0.073s }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12545> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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