January 2017 - ghc-tickets

[GHC] #8346: Rank 1 type signature still requires RankNTypes
by GHC 11 Jul '17

11 Jul '17

#8346: Rank 1 type signature still requires RankNTypes ----------------------------+---------------------------------------------- Reporter: | Owner: tinctorius | Status: new Type: bug | Milestone: Priority: normal | Version: 7.6.3 Component: Compiler | Operating System: Unknown/Multiple Keywords: | Type of failure: GHC rejects valid program Architecture: | Test Case: Unknown/Multiple | Blocking: Difficulty: Unknown | Blocked By: | Related Tickets: #2605 | ----------------------------+---------------------------------------------- When trying to figure out which type variable names are *actually* bound in `ScopedTypeVariables`, I tried floating `forall`s into the covariant argument of the function type. Essentially, I ran into the problem that programs like the following are rejected: {{{ #!hs {-# LANGUAGE ExplicitForAll #-} tuple :: forall a. a -> (forall b. b -> (a, b)) tuple = (,) }}} The message is as follows: {{{ Main.hs:2:10: Illegal polymorphic or qualified type: forall b. b -> (a, b) Perhaps you intended to use -XRankNTypes or -XRank2Types In the type signature for `tuple': tuple :: forall a. a -> (forall b. b -> (a, b)) }}} As far as I know, the rank of a type is defined by how deep quantifiers are nested in contravariant parts of that type. Or something like that. Also, as far as I know, `forall a. a -> (forall b. b -> (a, b))` is equivalent to `forall a b. a -> b -> (a, b)`, and more importantly, both are rank-1 polymorphic. There should be no need to use extensions that allow higher-ranked polymorphism. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8346> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 4

[GHC] #12390: List rules for `Coercible` instances
by GHC 11 Jul '17

11 Jul '17

#12390: List rules for `Coercible` instances -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: 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: -------------------------------------+------------------------------------- `:info` on [https://downloads.haskell.org/~ghc/7.8.2/docs/html/libraries/base-4.7.0.0 /Data-Coerce.html#t:Coercible Data.Coerce.Coercible] doesn't display any instances, it's a special class so it should be special-cased {{{ ghci> :i Coercible type role Coercible representational representational class a ~R# b => Coercible (a :: k) (b :: k) -- Defined in ‘GHC.Types’ }}} The base case would be easy to display: {{{#!hs instance Coercible a a -- Generated on the fly }}} but it's trickier to describe the … instance succinctly: {{{#!hs instance Coercible b b' => Coercible (D a b c) (D a b' c') }}} same for newtypes. ---- Maybe it's better to list concrete examples {{{#!hs coerce @(Reader _ _) :: Reader a b -> (a -> Identity b) }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12390> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 13

[GHC] #10150: Suppress orphan instance warning per instance
by GHC 11 Jul '17

11 Jul '17

#10150: Suppress orphan instance warning per instance -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: feature | Status: new request | Milestone: Priority: low | Version: 7.11 Component: Compiler | Operating System: Unknown/Multiple (Type checker) | Type of failure: None/Unknown Keywords: | Blocked By: Architecture: | Related Tickets: Unknown/Multiple | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- In the same way we have overlapping/incoherent instance pragmas which can be applied on a per instance basis, it would be good to have a `-fno-warn- orphan-instance` pragma which can be applied on a per-instance basis. Annoyingly, there is not an obvious syntax to use for this case, since SOP for a module with orphan instances is `{-# GHC_OPTIONS -fno-warn-orphans #-}` -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10150> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 4

[GHC] #12921: initTc: unsolved constraints
by GHC 10 Jul '17

10 Jul '17

#12921: initTc: unsolved constraints -------------------------------------+------------------------------------- Reporter: jlp | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 (Type checker) | Keywords: | Operating System: MacOS X Architecture: x86_64 | Type of failure: Compile-time (amd64) | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- While working on a small project of mine, GHC panicked with the following error: {{{ lua-types-0.1.0.0: build Preprocessing executable 'lua-types' for lua-types-0.1.0.0... [2 of 3] Compiling Lua.Parser ( src/Lua/Parser.hs, .stack- work/dist/x86_64-osx/Cabal-1.24.1.0/build/lua-types/lua-types- tmp/Lua/Parser.o ) ghc: panic! (the 'impossible' happened) (GHC version 8.0.1.20161117 for x86_64-apple-darwin): initTc: unsolved constraints WC {wc_insol = [W] explist_a3nf :: t_a3ne[tau:1] (CHoleCan: explist) [W] funccall_a4dv :: t_a4du[tau:1] (CHoleCan: funccall)} Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} From a quick google search, it seems like this is a bug in the type checker. I'm not willing nor able to dive any further into the GHC code myself, though. I'll attach the code as it is, before looking for a workaround. I'm using stack, GHC version 8.0.1.20161117 for x86_64-apple-darwin. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12921> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 11

[GHC] #11954: Associated pattern synonyms not included in haddock
by GHC 08 Jul '17

08 Jul '17

#11954: Associated pattern synonyms not included in haddock -------------------------------------+------------------------------------- Reporter: darchon | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 8.0.1-rc3 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: -------------------------------------+------------------------------------- Running haddock on: {{{#!hs {-# LANGUAGE PatternSynonyms #-} module Foo ( A(P) ) where data A a = A pattern P = A }}} Just gives a data type A with no constructors, and no mention of the associated pattern synonym P. Attached is a patch for haddock that prints associated patterns as GADT constructors -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11954> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 11

[GHC] #10648: Some 64-vector SIMD primitives are absolutely useless
by GHC 03 Jul '17

03 Jul '17

#10648: Some 64-vector SIMD primitives are absolutely useless -------------------------------------+------------------------------------- Reporter: mniip | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- The primitives `packInt8X64#`, `packWord8X64#`, `unpackInt8X64#`, `unpackWord8X64#` cannot be used because their types include unboxed 64-tuples, but any haskell code using them does not compile due to the 62-tuple limitation. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10648> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 6

[GHC] #12610: Emit tab warning promptly
by GHC 03 Jul '17

03 Jul '17

#12610: Emit tab warning promptly -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: bug | Status: new Priority: high | Milestone: 8.0.2 Component: Compiler | Version: 8.0.1 (Parser) | 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: -------------------------------------+------------------------------------- {{{#!hs p = let x = 4 [tab]y = 5 in 12 }}} produces `error: parse error on input ‘y’` and nothing else. The problem is caused by a tab appearing to the user to line up `x` with `y` but actually placing `y` in the wrong column. The user typically will scratch their head and then post a question on StackOverflow. There they'll be told to use `-fwarn-tabs`, but ''that option won't help'' (and it's probably already on) because GHC will exit with a parse error before it gets around to giving the warning. I suspect the solution is probably to emit the tab warning in the lexer, but one of the parsing gurus will probably know better. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12610> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 12

[GHC] #12203: Allow constructors on LHS of (implicit) bidirectional pattern synonym
by GHC 01 Jul '17

01 Jul '17

#12203: Allow constructors on LHS of (implicit) bidirectional pattern synonym -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: feature | Status: new request | Priority: low | 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: -------------------------------------+------------------------------------- Inspired by the recent Ghostbuster paper http://www.cs.ox.ac.uk/people/timothy.zakian/ghostbuster.pdf , I was experimenting with using pattern synonyms to simulate GADTs. In the process, I noticed that we should be able to generalize the implicit bidirectional synonyms to allow constructors on the LHS. Consider this program: {{{ {-# LANGUAGE KindSignatures #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE PatternSynonyms #-} {-# LANGUAGE ViewPatterns #-} {-# LANGUAGE TypeOperators #-} {-# OPTIONS_GHC -fwarn-incomplete-patterns #-} module GhostBuster where import GHC.TypeLits newtype Vec a (n :: Nat) = Vec { unVec :: [a] } pattern VNil :: Vec a 0 pattern VNil = Vec [] pattern VCons :: a -> Vec a n -> Vec a (n + 1) pattern VCons x xs <- Vec (x : (Vec -> xs)) where VCons x (Vec xs) = Vec (x : xs) headVec :: Vec a (n + 1) -> a headVec (VCons x _) = x }}} In effect, we simulate a vector GADT using pattern synonyms to handle the newtype `Vec`. I would like it if I could specify the `VCons` pattern more simply, as just: {{{ pattern VCons :: a -> Vec a n -> Vec a (n + 1) pattern VCons x (Vec xs) = Vec (x : xs) }}} And GHC would infer the correct pattern (the constructor can be read off immediately), automatically replacing occurrences of `xs` with a view pattern that reconstructs the constructors that were stripped off on the LHS. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12203> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 12

[GHC] #9588: Add `MonadPlus {IO, Either e}` and `Alternative (Either e)` instances
by GHC 29 Jun '17

29 Jun '17

#9588: Add `MonadPlus {IO,Either e}` and `Alternative (Either e)` instances -------------------------------------+------------------------------------- Reporter: hvr | Owner: hvr Type: task | Status: new Priority: normal | Milestone: 7.10.1 Component: libraries/base | Version: Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- The following 3 instances are currently Orphans in `transformers` but shall be defined in `base` instead: {{{#!hs instance MonadPlus IO instance MonadPlus (Either e) instance Alterantive (Either e) }}} This proposal by SPJ already passed the CLC back in April and only needs implementing. I'll submit a patch shortly to Phab -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9588> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 11

[GHC] #11990: Custom Type Error not getting triggered in the nested Type function call
by GHC 27 Jun '17

27 Jun '17

#11990: Custom Type Error not getting triggered in the nested Type function call -------------------------------------+------------------------------------- Reporter: magesh.b | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 8.0.1-rc3 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: -------------------------------------+------------------------------------- I have partial type function which is invoked by another type function. When the inner type function fails with TypeError, outer type function is not been able to propagate that type error to its caller. As a result of it, I'm getting following error • No instance for (KnownSymbol (NestedPartialTF (TypeError ...))) instead of • Unexpected type @ NestedPartialTF: Char {{{#!hs {-# LANGUAGE DataKinds, TypeOperators, TypeFamilies, UndecidableInstances, ScopedTypeVariables, FlexibleContexts #-} -- | module CErrs where import GHC.TypeLits import Data.Proxy type family PartialTF t :: Symbol where PartialTF Int = "Int" PartialTF Bool = "Bool" PartialTF a = TypeError (Text "Unexpected type @ PartialTF: " :<>: ShowType a) type family NestedPartialTF (tsym :: Symbol) :: Symbol where NestedPartialTF "Int" = "int" NestedPartialTF "Bool" = "bool" NestedPartialTF a = TypeError (Text "Unexpected type @ NestedPartialTF: " :<>: ShowType a) testPartialTF :: forall a.(KnownSymbol (PartialTF a)) => a -> String testPartialTF t = symbolVal (Proxy :: Proxy (PartialTF a)) --t1 = testPartialTF 'a' {- Above code rightly fails with the following error: • Unexpected type: Char • In the expression: testPartialTF 'a' In an equation for ‘t1’: t1 = testPartialTF 'a' -} -- Bug? testNesPartialTF :: forall a.(KnownSymbol (NestedPartialTF (PartialTF a))) => a -> String testNesPartialTF t = symbolVal (Proxy :: Proxy (NestedPartialTF (PartialTF a))) t2 = testNesPartialTF 'a' {- Above code fails with the following error: • No instance for (KnownSymbol (NestedPartialTF (TypeError ...))) arising from a use of ‘testNesPartialTF’ • In the expression: testNesPartialTF 'a' In an equation for ‘t2’: t2 = testNesPartialTF 'a' -} }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11990> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 12