November 2015 - ghc-tickets

[GHC] #9383: GHCi parser doesn't properly handle -fobject-code
by GHC 27 Jun '18

27 Jun '18

#9383: GHCi parser doesn't properly handle -fobject-code -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 7.8.2 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Other Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- My complete Haskell source: module Foo where f x = x Shell: $ ghci Foo.hs -fobject-code GHCi, version 7.8.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. target ‘-fobject-code’ is not a module name or a source file Ok, modules loaded: Linker. Prelude Linker> Despite the wanring/error, GHCi *does* generate the object code. This error also prints when I *load* the compiled Foo object file. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9383> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10965: GHC Panic on import with 'OPTIONS_GHC -fobject-code -O'
by GHC 27 Jun '18

27 Jun '18

#10965: GHC Panic on import with 'OPTIONS_GHC -fobject-code -O' --------------------------------------+--------------------------------- Reporter: Orome | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Keywords: | Operating System: MacOS X Architecture: x86_64 (amd64) | Type of failure: None/Unknown Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: --------------------------------------+--------------------------------- I get {{{ > :l Test.hs [1 of 1] Compiling Main ( Test.hs, interpreted ) ghc: panic! (the 'impossible' happened) (GHC version 7.10.2 for x86_64-apple-darwin): floatExpr tick break<11>() Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} with Test.hs: {{{ {-# OPTIONS_GHC -fobject-code -O #-} import Control.Exception (assert) import Data.Char (ord) f :: String -> String f s = assert (all (`elem` letters) s) $ (letters!!) <$> (ix <$> s) where ix ch = (ord ch - ord 'A') letters = ['A'..'Z'] }}} OSX 10.11, GHC 7.10.2; uad-core 64-bit haswell; Homebrew GHC -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10965> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10059: :i doesn't work for ~
by GHC 21 Jun '18

21 Jun '18

#10059: :i doesn't work for ~ -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 7.11 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Other Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- {{{#!hs Prelude> :set -XTypeOperators Prelude> :i (~) <interactive>:1:2: parse error on input ‘~’ Prelude> :k (~) (~) :: k -> k -> Constraint }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10059> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9106: GHC Panic related to functional dependencies - kindFunResult
by GHC 18 Jun '18

18 Jun '18

#9106: GHC Panic related to functional dependencies - kindFunResult ----------------------------+--------------------------------------- Reporter: yuriy | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Operating System: Windows Architecture: x86 | Type of failure: Compile-time crash Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ----------------------------+--------------------------------------- Minimal example: {{{ {-# LANGUAGE MultiParamTypeClasses, DataKinds, FunctionalDependencies, TypeOperators, KindSignatures, PolyKinds, FlexibleInstances, FlexibleContexts, UndecidableInstances #-} import GHC.TypeLits data Proxy (a :: k) = Proxy class FunctorN n f a fa | n f a -> fa where fmapn :: Proxy n -> Proxy f -> (a -> a) -> fa -> fa instance FunctorN 0 f a a where fmapn _ _ a = a instance (Functor f, FunctorN (n - 1) f a fa) => FunctorN n f a (f fa) where fmapn _ pf f = fmap (fmapn (Proxy :: Proxy (n-1)) pf f) }}} Crashes with ghc and ghci: {{{ >ghc test [1 of 1] Compiling Main ( test.hs, test.o ) ghc.exe: panic! (the 'impossible' happened) (GHC version 7.8.20140130 for x86_64-unknown-mingw32): kindFunResult k{tv azb} [sk] >ghci test GHCi, version 7.8.20140130: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( test.hs, interpreted ) ghc.exe: panic! (the 'impossible' happened) (GHC version 7.8.20140130 for x86_64-unknown-mingw32): kindFunResult k{tv aPm} [sk] }}} Removing the functional dependency makes the code compile. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9106> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8619: Support anonymous string literals in C-- (OR) give better ASSERT failure messages in C--
by GHC 17 Jun '18

17 Jun '18

#8619: Support anonymous string literals in C-- (OR) give better ASSERT failure messages in C-- ------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 7.7 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: Other Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- So if I say "foobar" in C--, it turns into: {{{ section "rodata" { fresh_name : bits8[] "foobar\0"; } }}} This would let us fix the ASSERT macro in Cmm.h to also get the filename. Requires parser changes. Alternatively, we can redefine the ASSERT macro per C-- file to have a hardcoded filename with some preprocessor trickery. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8619> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9539: TQueue can lead to thread starvation
by GHC 12 Jun '18

12 Jun '18

#9539: TQueue can lead to thread starvation -------------------------------------+------------------------------------- Reporter: jwlato | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: libraries (other) | Version: 7.8.2 Keywords: stm | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- For background, please see [http://stackoverflow.com/questions/25536604 /huge-memory-consumption-for-simple-multithreaded- haskell/25560047#25560047 this SO question] When using TQueue, a sufficiently fast/persistent writer can result in the reader never getting scheduled, causing the queue to continually grow and lost concurrency. I believe the issue is caused by the definition of readTQueue: {{{ readTQueue :: TQueue a -> STM a readTQueue (TQueue read write) = do xs <- readTVar read case xs of (x:xs') -> do writeTVar read xs' return x [] -> do ys <- readTVar write case ys of [] -> retry _ -> case reverse ys of [] -> error "readTQueue" (z:zs) -> do writeTVar write [] writeTVar read zs return z }}} Note that the last `case` needs to traverse the write-end of the queue within the STM transaction. If the list is sufficiently large, a writer can commit a new transaction much more quickly, invalidating the read transaction. If threads continue to write to the queue, the reader will never get an opportunity to commit (and the list will grow, exacerbating the problem). This alternative definition seems to fix the problem, but I don't know if there are other drawbacks to using it. {{{ readTQueue' :: TQueue a -> STM a readTQueue' (TQueue read write) = do xs <- readTVar read case xs of (x:xs') -> do writeTVar read xs' return x [] -> do ys <- readTVar write case ys of [] -> retry _ -> do writeTVar write [] let (z:zs) = reverse ys writeTVar read zs return z }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9539> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8516: Add (->) representation and the Invariant class to GHC.Generics
by GHC 04 Jun '18

04 Jun '18

#8516: Add (->) representation and the Invariant class to GHC.Generics ----------------------------------------------+---------------------------- Reporter: nfrisby | Owner: Type: feature request | Status: new Priority: low | Milestone: Component: Compiler (Type checker) | Version: 7.7 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Moderate (less than a day) | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: ----------------------------------------------+---------------------------- We currently disallow any use of the parameter in the domain of (->). {{{ newtype F a = F ((a -> Int) -> Int) deriving Generic1 <interactive>:4:38: Can't make a derived instance of `Generic1 (F g)': Constructor `F' must use the last type parameter only as the last argument of a data type, newtype, or (->) In the data declaration for `F' }}} !DeriveFunctor succeeds for this F. I'd like to add this representation type to GHC.Generics and !DeriveGeneric. {{{ newtype (f :->: g) a = FArrow1 (f a -> g a) }}} We could then represent the first example above. We could also derive the more interesting Generic1 (F g). {{{ newtype F g a = F (g a -> Int) deriving Generic1 type instance Rep1 (F g) = Rec1 g :->: Rec0 Int instance Generic1 (F g) where to x = F $ unRec0 . unArrow1 x . Rec1 from (F x) = FArrow1 $ Rec0 . x . unRec1 }}} Admittedly, there's not many generic definitions impeded by not having (:->:). Contra- and in-variant types are uncommon. I'm suggesting this feature without strong motivating examples because I think this would streamline the implementation of -XDeriveGenerics in some ways while also making it more general — assuming that we added the Invariant class to base or ghc-prim. {{{ class Invariant t where invmap :: (a -> b) -> (b -> a) -> t a -> t b invmap_covariant :: Functor t => (a -> b) -> (b -> a) -> t a -> t b invmap_covariant f _ = fmap f instance (Invariant f,Invariant g) => Invariant (FArrow f g) where invmap co contra (FArrow h) = FArrow $ invmap co contra . h . invmap contra co }}} (Of course, Invariant should be a super class of Functor. :/ ) Now we can handle quite involved examples: {{{ newtype F g h a = F (g (h a)) deriving Generic1 instance Invariant g => Generic1 (F g h) where to x = invmap unRec1 Rec1 $ unComp1 x from (F x) = Comp1 $ invmap Rec1 unRec1 }}} All of that said, I'm mostly opening this ticket so I can get feedback on difficulties I might not be anticipating and have a place to reference from the compiler source code comments. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8516> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9247: Document -XDatatypeContexts in flag reference
by GHC 03 Jun '18

03 Jun '18

#9247: Document -XDatatypeContexts in flag reference ------------------------------------+------------------------------------- Reporter: goldfire | Owner: goldfire Type: bug | Status: new Priority: low | Milestone: Component: Documentation | Version: 7.8.2 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- It may be a deprecated feature, but it still exists and should be listed. I can do this. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9247> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11066: Inacessible branch should be warning - otherwise breaks type soundness?
by GHC 03 Jun '18

03 Jun '18

#11066: Inacessible branch should be warning - otherwise breaks type soundness? -------------------------------------+------------------------------------- Reporter: rrnewton | Owner: Type: bug | Status: new Priority: high | Milestone: Component: Compiler | Version: 7.10.2 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: -------------------------------------+------------------------------------- GHC 7.10.2 reports inaccessible branches as a type error, and I think this is really a bug. First, the implicit thinking seems to be "why would a user want to write inaccessible cases, they'd be crazy!". But that only applies to human-written code. Program generators can have a great deal of trouble avoiding this error, unless they replicate the GHC type checker to predict the problem and prune branches. (That's why we are hitting this error and are stuck.) Second, it seems like this is a problem for type soundness. See the attached program where "step1" typechecks but "step2" does not. And yet, the operational semantics should allow step1 to reduce to step2. Indeed, in the "GADTs meet their match" paper it seems like the intent was to warn for these inaccessible cases, not error. For example, the paper contains the language: "If we warn that a right-hand side of a non-redundant clause is inaccessible," -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11066> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9724: reexport IsList class from a trustworthy module
by GHC 01 Jun '18

01 Jun '18

#9724: reexport IsList class from a trustworthy module -------------------------------------+------------------------------------- Reporter: int-e | Owner: ekmett Type: feature request | Status: new Priority: normal | Milestone: Component: Core Libraries | 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: -------------------------------------+------------------------------------- ==== Motivation GHC's Safe Haskell extension relies to a large extent on inferring safety of modules automatically. As a concrete example, [[http://hackage.haskell.org/package/data-default|data-default]] used to be inferred safe. This is no longer the case, because [[http://hackage.haskell.org/package/dlist-0.7.1|dlist-0.7.1]] added an {{{IsList}}} instance for difference lists. The only way to do that currently is to import the relevant parts of {{{GHC.Exts}}}, and that module is not safe. Of course the resulting non-safety of the {{{Data.DList}}} module can be fixed by adding a {{{TrustWorthy}}} pragma to the module and trusting the {{{dlist}}} package. But that is unsatisfactory, because there is nothing unsafe about providing an instance for the {{{IsList}}} class. This issue is bound to pop up in other packages as well. ==== Proposed Solution (this is one of many possibilities) Add a {{{GHC.Exts.Safe}}} module that is marked {{{TrustWorthy}}} and reexports the {{{IsList}}} class, and possibly other things from {{{GHC.Exts}}} that are safe to use. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9724> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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