November 2017 - ghc-tickets

[GHC] #13555: Typechecker regression when combining PolyKinds and MonoLocalBinds
by GHC 03 Aug '18

03 Aug '18

#13555: Typechecker regression when combining PolyKinds and MonoLocalBinds -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: highest | 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: -------------------------------------+------------------------------------- `lol-0.6.0.0` from Hackage currently fails to build with GHC 8.2.1 because of this regression. Here is a minimized example: {{{#!hs {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE InstanceSigs #-} {-# LANGUAGE MonoLocalBinds #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE RankNTypes #-} module Crypto.Lol.Types.FiniteField (GF(..)) where import Data.Functor.Identity (Identity(..)) data T a type Polynomial a = T a newtype GF fp d = GF (Polynomial fp) type CRTInfo r = (Int -> r, r) type Tagged s b = TaggedT s Identity b newtype TaggedT s m b = TagT { untagT :: m b } class Reflects a i where value :: Tagged a i class CRTrans mon r where crtInfo :: Reflects m Int => TaggedT m mon (CRTInfo r) instance CRTrans Maybe (GF fp d) where crtInfo :: forall m . (Reflects m Int) => TaggedT m Maybe (CRTInfo (GF fp d)) crtInfo = undefined }}} This typechecks OK with GHC 8.0.2, but with 8.2.1, it complains: {{{ $ /opt/ghc/8.2.1/bin/ghci Bug.hs GHCi, version 8.2.0.20170403: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Crypto.Lol.Types.FiniteField ( Bug.hs, interpreted ) Bug.hs:25:14: error: • Couldn't match type ‘k0’ with ‘k2’ because type variable ‘k2’ would escape its scope This (rigid, skolem) type variable is bound by the type signature for: crtInfo :: forall k2 (m :: k2). Reflects m Int => TaggedT m Maybe (CRTInfo (GF fp d)) at Bug.hs:25:14-79 Expected type: TaggedT m Maybe (CRTInfo (GF fp d)) Actual type: TaggedT m Maybe (CRTInfo (GF fp d)) • When checking that instance signature for ‘crtInfo’ is more general than its signature in the class Instance sig: forall (m :: k0). Reflects m Int => TaggedT m Maybe (CRTInfo (GF fp d)) Class sig: forall k2 (m :: k2). Reflects m Int => TaggedT m Maybe (CRTInfo (GF fp d)) In the instance declaration for ‘CRTrans Maybe (GF fp d)’ | 25 | crtInfo :: forall m . (Reflects m Int) => TaggedT m Maybe (CRTInfo (GF fp d)) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Bug.hs:25:14: error: • Could not deduce (Reflects m Int) from the context: Reflects m Int bound by the type signature for: crtInfo :: forall k2 (m :: k2). Reflects m Int => TaggedT m Maybe (CRTInfo (GF fp d)) at Bug.hs:25:14-79 The type variable ‘k0’ is ambiguous • When checking that instance signature for ‘crtInfo’ is more general than its signature in the class Instance sig: forall (m :: k0). Reflects m Int => TaggedT m Maybe (CRTInfo (GF fp d)) Class sig: forall k2 (m :: k2). Reflects m Int => TaggedT m Maybe (CRTInfo (GF fp d)) In the instance declaration for ‘CRTrans Maybe (GF fp d)’ | 25 | crtInfo :: forall m . (Reflects m Int) => TaggedT m Maybe (CRTInfo (GF fp d)) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ }}} Notably, both `PolyKinds` and `MonoLocalBinds` are required to trigger this bug. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13555> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10087: DefaultSignatures: error message mentions internal name
by GHC 02 Aug '18

02 Aug '18

#10087: DefaultSignatures: error message mentions internal name -------------------------------------+------------------------------------- Reporter: | Owner: andreas.abel | Status: new Type: feature | Milestone: request | Version: 7.8.4 Priority: normal | Operating System: Unknown/Multiple Component: Compiler | Type of failure: Other (Type checker) | Blocked By: Keywords: | Related Tickets: Architecture: | Unknown/Multiple | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- {{{#!hs {-# LANGUAGE DefaultSignatures #-} class C a where reflexive :: a -> Bool default reflexive :: Eq a => a -> Bool reflexive x = x == x data D instance C D where -- /home/abel/play/haskell/bugs/DefaultSig.hs:10:10: -- No instance for (Eq D) arising from a use of ‘Main.$gdmreflexive’ -- In the expression: Main.$gdmreflexive -- In an equation for ‘reflexive’: reflexive = Main.$gdmreflexive -- In the instance declaration for ‘C D’ }}} Error looks odd: The user has not written $gdmreflexive in his code. TODO: Better error message. Maybe this should just trigger a warning that method {{{reflexive}}} is undefined for instance {{{D}}} of {{{C}}}. Like when I remove the default method. {{{ /home/abel/play/haskell/bugs/DefaultSig.hs:10:10: Warning: No explicit implementation for ‘reflexive’ In the instance declaration for ‘C D’ }}} It seems the semantics of a default signature is that each instance *must* implement this method. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10087> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12096: Attach stacktrace information to SomeException
by GHC 02 Aug '18

02 Aug '18

#12096: Attach stacktrace information to SomeException -------------------------------------+------------------------------------- Reporter: ndtimofeev | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: | Version: 8.0.1 libraries/base | 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: -------------------------------------+------------------------------------- Now (base 4.9.0.0) ErrorCall contain field for stacktrace information and error fills it. If you add field for stacktrace information to SomeException {{{#!hs data SomeException where SomeExceptionWithLocation :: Exception e => String -> e -> SomeException pattern SomeException :: () => Exception e => e -> SomeException pattern SomeException err <- SomeExceptionWithLocation _ err where SomeException err = SomeExceptionWithLocation "" err }}} and will fills it in throw {{{#!hs throw :: (HasCallStack, Exception e) => e -> a throw e = unsafeDupablePerformIO $ do stack <- currentCallStack raise# (CallStackException e $ if stack /= [] then prettyCallStack ?callStack ++ "\n" ++ renderStack stack else prettyCallStack ?callStack) }}} it will be more useful for ghci users. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12096> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11759: can't decompose ghc.exe path
by GHC 02 Aug '18

02 Aug '18

#11759: can't decompose ghc.exe path --------------------------------------+---------------------------- Reporter: erisco | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Keywords: | Operating System: Windows Architecture: x86_64 (amd64) | Type of failure: Other Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: --------------------------------------+---------------------------- {{{ erisco@ERICWIN C:\Users\erisco\haskell-platform-bin $ ghc.exe ghc.exe: panic! (the 'impossible' happened) (GHC version 7.8.3 for x86_64-unknown-mingw32): can't decompose ghc.exe path: "C:\\Users\\erisco\\haskell- platform-bin\\ghc.exe" }}} haskell-platform-bin is a symbolic directory link to "C:\Program Files\Haskell Platform\2014.2.0.0\bin". I did this so there would not be spaces in the path because this is causing other issues for me. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11759> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14346: 8.2.1 regression: heap corruption after safe foreign calls
by GHC 31 Jul '18

31 Jul '18

#14346: 8.2.1 regression: heap corruption after safe foreign calls -------------------------------------+------------------------------------- Reporter: andrewchen | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: Component: Runtime | Version: 8.2.1 System | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Runtime crash Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Test case: (compile with ghc 8.2.1 and -threaded option) {{{#!haskell module Main where import Control.Concurrent import Control.Monad import Data.Word import Foreign.Marshal.Alloc import Foreign.Ptr import Foreign.Storable foreign import ccall safe "test" c_test :: Ptr Word32 -> IO () main :: IO () main = do replicateM_ 1000 $ threadDelay 1000 _ <- forkIO $ forever $ threadDelay 100 allocaBytes 4 $ \p -> forever $ do c_test p x <- peek p unless (x == 0xDEADBEEF) $ putStrLn "value mismatch" }}} {{{#!c void test(unsigned int *buf) { *buf = 0xDEADBEEF; } }}} On my machine, it detects a few value mismatches before crashing with sigsegv. {{{ $ time ./.stack-work/install/x86_64-linux- nopie/nightly-2017-10-10/8.2.1/bin/bug value mismatch value mismatch value mismatch value mismatch zsh: segmentation fault (core dumped) ./.stack-work/install/x86_64-linux- nopie/nightly-2017-10-10/8.2.1/bin/bug ./.stack-work/install/x86_64-linux-nopie/nightly-2017-10-10/8.2.1/bin/bug 2.11s user 0.25s system 66% cpu 3.543 total }}} I believe this is what is causing crashes in xmobar. See discussion: https://github.com/jaor/xmobar/issues/310. Note that the crash in xmobar still happens without -threaded option, while this example only breaks when compiled with -threaded. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14346> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12030: GHCi Proposal: Display (Data.Kind.)Type instead of *
by GHC 31 Jul '18

31 Jul '18

#12030: GHCi Proposal: Display (Data.Kind.)Type instead of * -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: GHCi | Version: 8.1 Keywords: TypeInType | 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 premature but what the darn, {{{ >>> :kind Maybe Maybe :: Type -> Type >>> :kind StateT StateT :: Type -> (Type -> Type) -> Type -> Type >>> :kind Eq Eq :: Type -> Constraint >>> :info Functor class Functor (f :: Type -> Type) where ... }}} `*` throws students off and makes it seem scarier than it is. Symbols are harder to search for in general and to understand without documentation, `Type` on the other hand is descriptive. There are valid arguments against displaying `Type`: 1. It's a recent feature subject to change. 2. Although symbols are more difficult to search for, `*` is established in educational materials, logs, blogs and questions. 3. By default `Type` is not in scope so a user cannot ask for information in GHCi. `*` is established and searching for “Haskell asterisk” yields a lot resources but [https://support.google.com/websearch/answer/2466433?hl=en ‘*’ is also a wildcard] in Google and ignored by GitHub. With time `Type` would be a good search term but currently it's chicken-and-the-egg. Previous versions of GHCi error on `:kind *` and `:info *` only shows multiplication so that wouldn't be a huge difference but we can qualify by default: {{{ >>> :kind Maybe Maybe :: Data.Kind.Type -> Data.Kind.Type >>> :kind StateT StateT :: Data.Kind.Type -> (Data.Kind.Type -> Data.Kind.Type) -> Data.Kind.Type -> Data.Kind.Type >>> :kind Eq Eq :: Data.Kind.Type -> Constraint >>> :info Functor class Functor (f :: Data.Kind.Type -> Data.Kind.Type) where ... }}} or display `*` normally and only when `TypeInType` is set do we display `Type`. I don't love it (and love `GHC.Types.Type` slightly less) but there is a precedent for unqualified names, browsing the Prelude for example: {{{#!hs ($) :: forall (r :: GHC.Types.RuntimeRep) a (b :: TYPE r). (a -> b) -> a -> b undefined :: forall (r :: GHC.Types.RuntimeRep) (a :: TYPE r). GHC.Stack.Types.HasCallStack => a }}} __If__ the consensus is that this will happen sometime down the line consider that each passing release means many more books and blog posts get written using `*`. I wasn't planning on resulting to scare tactics but [https://www.peoria.com/spaw/spawimages/members/member60763/shoot_this_dog.j… here we are]... ---- If needed a migration plan can be drafted like the Semigroup/FTP/AMP/BBP/MonadFail/expanding Float/... proposals, possibly culminating in `Type` fully replacing `*` and being imported by default. I'm sure there are some further reaching consequences to this and better arguments against. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12030> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10056: Inconsistent precedence of ~
by GHC 30 Jul '18

30 Jul '18

#10056: Inconsistent precedence of ~ -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.4 (Parser) | Operating System: Unknown/Multiple Keywords: | Type of failure: GHC rejects Architecture: | valid program Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- The following compiles as expected: {{{#!hs {-# LANGUAGE TypeFamilies #-} type family Foo a b f :: (Foo a b ~ Int) => a -> b -> b f = error "" }}} but this fails: {{{#!hs {-# LANGUAGE TypeFamilies #-} type family a \\ b f :: (a \\ b ~ Int) => a -> b -> b f = error "" }}} with the error > "The second argument of `(\\)` should have kind `*`, but `b ~ Int` has kind `Constraint`." Thus the first example is being parsed as `(Foo a b) ~ Int`, while the second is parsed as `a \\ (b ~ Int)`. I believe the second example should compile, i.e. `(\\)` and `Foo` should have the same precedence, both of which are higher than `(~)`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10056> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13451: Bind implicit parameter in patterns
by GHC 30 Jul '18

30 Jul '18

#13451: Bind implicit parameter in patterns -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple ImplicitParams | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Is there any reason why variables for `ImplicitParams` can't be bound in patterns, so you can write {{{#!hs data Exp = Val Int | Op Op Exp Exp data Op = Add | Mul data Protect where Protect :: (?op :: Op) => Protect protectOp :: Exp -> Maybe Protect protectOp (Op ?op _ _) = Just Protect protectOp _ = Nothing }}} instead of {{{#!hs protectOp :: Exp -> Maybe Protect protectOp (Op op _ _) = Just Protect where ?op = op protectOp _ = Nothing }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13451> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11549: Add -fshow-runtime-rep
by GHC 29 Jul '18

29 Jul '18

#11549: Add -fshow-runtime-rep -------------------------------------+------------------------------------- Reporter: goldfire | Owner: goldfire Type: bug | Status: new Priority: highest | Milestone: 8.0.1 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: -------------------------------------+------------------------------------- As discussed in several interwoven threads ([https://mail.haskell.org/pipermail/ghc-devs/2016-February/011268.html original], [https://mail.haskell.org/pipermail/haskell- cafe/2016-February/122914.html café]), it has been suggested to add a flag `-fshow-runtime-rep`. Without this flag enabled, the pretty printer will instantiate any `RuntimeRep` type parameters to `PtrRep Lifted`. This has the effect of changing {{{ ($) :: forall (r :: RuntimeRep) (a :: *) (b :: TYPE r). (a -> b) -> a -> b }}} to {{{ ($) :: (a -> b) -> a -> b }}} under the default GHCi settings. Note that `Levity` becomes `RuntimeRep` after #11471 is complete. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11549> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10184: Coercible solver incomplete with recursive newtypes
by GHC 29 Jul '18

29 Jul '18

#10184: Coercible solver incomplete with recursive newtypes -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: low | Milestone: ⊥ Component: Compiler | Version: 7.10.1-rc3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- The following module fails to compile due to infinite recursion: {{{ import Data.Coerce newtype Bar a = Bar (Either a (Bar a)) newtype Age = MkAge Int x :: Bar Age x = coerce (Bar (Left (5 :: Int))) }}} It tries to coerce `Bar Int` to `Bar Age`. This is clearly doable, via tyconapp decomposition. But we don't find it. Before worrying too much about it, though, we'll wait for a real example. Do speak up if this problem is ruining your day! -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10184> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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