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[GHC] #14861: QuantifiedConstraints: Can't use forall'd variable in context
by GHC 27 Feb '18

27 Feb '18

#14861: QuantifiedConstraints: Can't use forall'd variable in context -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 (Type checker) | Keywords: | Operating System: Unknown/Multiple QuantifiedConstraints wipT2893 | Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This fails to typecheck, to my surprise: {{{#!hs {-# LANGUAGE AllowAmbiguousTypes #-} {-# LANGUAGE ConstraintKinds #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE QuantifiedConstraints #-} {-# LANGUAGE UndecidableInstances #-} module Bug where import Data.Kind data ECC1 :: (Type -> Constraint) -> (Type -> Type) -> Type -> Type where ECC1 :: c p => f p -> ECC1 c f p class Foldable' f where foldMap' :: Monoid m => (a -> m) -> f a -> m instance (forall p. c p => Foldable' f) => Foldable' (ECC1 c f) where foldMap' f (ECC1 x) = foldMap' f x instance Foldable' [] where foldMap' = foldMap test :: ECC1 Show [] Ordering -> Ordering test = foldMap' id }}} {{{ $ ghc-cq/inplace/bin/ghc-stage2 --interactive Bug.hs GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Bug.hs:18:25: error: • Could not deduce: c p0 arising from a use of ‘foldMap'’ from the context: forall p. c p => Foldable' f bound by the instance declaration at Bug.hs:17:10-63 or from: Monoid m bound by the type signature for: foldMap' :: forall m a. Monoid m => (a -> m) -> ECC1 c f a -> m at Bug.hs:18:3-10 or from: c a bound by a pattern with constructor: ECC1 :: forall (f :: * -> *) p (c :: * -> Constraint). c p => f p -> ECC1 c f p, in an equation for ‘foldMap'’ at Bug.hs:18:15-20 • In the expression: foldMap' f x In an equation for ‘foldMap'’: foldMap' f (ECC1 x) = foldMap' f x In the instance declaration for ‘Foldable' (ECC1 c f)’ • Relevant bindings include foldMap' :: (a -> m) -> ECC1 c f a -> m (bound at Bug.hs:18:3) | 18 | foldMap' f (ECC1 x) = foldMap' f x | ^^^^^^^^^^^^ }}} I would have expected the `(forall p. c p => Foldable' f)` quantified constraint to kick in there. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14861> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #5927: A type-level "implies" constraint on Constraints
by GHC 27 Feb '18

27 Feb '18

#5927: A type-level "implies" constraint on Constraints -------------------------------------+------------------------------------- Reporter: illissius | Owner: (none) Type: feature request | Status: closed Priority: normal | Milestone: Component: Compiler (Type | Version: 7.4.1 checker) | Keywords: Resolution: duplicate | QuantifiedConstraints Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #2893 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Hmm. This is interesting. We have {{{ $dmshowsPrec :: Show a => Int -> a -> ShowS }}} and the implied instance decl is {{{ instance (forall a. c a => Show a) => Show (Exists c) where show (Exists a) = show a showsPrec = $dmshowsPrec }}} From the RHS of `showsPred` we get `[W] Show (Exists c)`. And ''alas'' we try to solve that from the quantified constraint (which takes precedence over top-level instances), by instantiating `a` to `(Exists c)`. So we get {{{ [W} c (Exists c) }}} which we can't solve. This would happen on any recursive invocation of a method at the same type. It's not clear what we want here. The quantified constraint claims to provide an instance ''for every type a'' which is jolly suspicious. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5927#comment:33> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14864: Suggestion: Turn on +RTS -xt by default
by GHC 27 Feb '18

27 Feb '18

#14864: Suggestion: Turn on +RTS -xt by default -------------------------------------+------------------------------------- Reporter: ryantrinkle | Owner: (none) Type: feature | Status: new request | Priority: low | Milestone: Component: Runtime | Version: 8.2.2 System | 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 found it quite surprising and difficult to debug that my heap profile showed no leak while the GC stats showed a clear leak. As a user who is somewhat familiar with this stuff, I was quite surprised that those two data sources didn't agree. I wasn't sure how to determine why they disagreed, since none of the output referenced anything interesting. I'm not familiar with the reasoning behind disabling -xt by default, but I think it may be easier for users to figure out that they need to *disable* it (since they'll be able to see its results in the heap profile) than to figure out that they need to *enable* it. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14864> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14835: QuantifiedConstraints: Can't deduce "(a, b)" from "a" and "b"
by GHC 27 Feb '18

27 Feb '18

#14835: QuantifiedConstraints: Can't deduce "(a, b)" from "a" and "b" -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Keywords: | Operating System: Unknown/Multiple QuantifiedConstraints wipT2893 | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{#!hs {-# Language QuantifiedConstraints, RankNTypes, PolyKinds, ConstraintKinds, UndecidableInstances, GADTs, LiberalTypeSynonyms, TypeOperators, MultiParamTypeClasses, FlexibleInstances #-} data Dict c where Dict :: c => Dict c infixr |- class (a => b) => (a |- b) instance (a => b) => (a |- b) type a :- b = Dict (a |- b) class ((a, b) => c) => Curried a b c instance ((a, b) => c) => Curried a b c curryC :: forall a b c. ((a, b) |- c) :- (a |- b |- c) curryC = Dict }}} This fails with {{{ GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( /tmp/Test.hs, interpreted ) /tmp/Test.hs:16:10: error: • Could not deduce (a, b) arising from a use of ‘Dict’ from the context: (a, b) |- c bound by a quantified context at /tmp/Test.hs:1:1 or from: a bound by a quantified context at /tmp/Test.hs:1:1 or from: b bound by a quantified context at /tmp/Test.hs:1:1 Possible fix: add (a, b) to the context of the type signature for: curryC :: forall (a :: Constraint) (b :: Constraint) (c :: Constraint). ((a, b) |- c) :- (a |- (b |- c)) • In the expression: Dict In an equation for ‘curryC’: curryC = Dict | 16 | curryC = Dict | ^^^^ Failed, no modules loaded. }}} but it works by changing the type of `curryC`: {{{#!hs curryC :: forall a b c. Curried a b c :- (a |- b |- c) }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14835> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14833: QuantifiedConstraints: GHC can't deduce (() :: Constraint)?
by GHC 27 Feb '18

27 Feb '18

#14833: QuantifiedConstraints: GHC can't deduce (() :: Constraint)? -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Keywords: | Operating System: Unknown/Multiple QuantifiedConstraints wipT2893 | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{#!hs {-# Language QuantifiedConstraints, RankNTypes, PolyKinds, ConstraintKinds, UndecidableInstances, GADTs #-} import Data.Kind data Dict c where Dict :: c => Dict c class (a => b) => Implies a b instance (a => b) => Implies a b type a :- b = Dict (Implies a b) iota :: (Implies () a) :- a iota = Dict }}} GHC claims that it can't deduce `(() :: Constraint)` :) {{{ GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( /tmp/H.hs, interpreted ) /tmp/H.hs:14:8: error: • Could not deduce () :: Constraint arising from a use of ‘Dict’ from the context: Implies () :: Constraint a bound by a quantified context at /tmp/H.hs:1:1 Possible fix: add () :: Constraint to the context of the type signature for: iota :: forall (a :: Constraint). Implies () :: Constraint a :- a • In the expression: Dict In an equation for ‘iota’: iota = Dict | 14 | iota = Dict | ^^^^ Failed, no modules loaded. Prelude> }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14833> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14824: automatically select instance
by GHC 27 Feb '18

27 Feb '18

#14824: automatically select instance -------------------------------------+------------------------------------- Reporter: zaoqi | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 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: -------------------------------------+------------------------------------- There is {{{#!hs class C a b where m :: [a] -> b instance C x0 x0 instance C x1 x2 instance C x0 x3 }}} x0 x1 x2 x3 is Types. FunctionalDependencies can't work. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14824> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13075: Top-level bang pattern accepted
by GHC 27 Feb '18

27 Feb '18

#13075: Top-level bang pattern accepted -------------------------------------+------------------------------------- Reporter: goldfire | Owner: goldfire 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: -------------------------------------+------------------------------------- When I compile/link {{{#!hs {-# LANGUAGE BangPatterns #-} module Main where !(Just x) = Nothing main = putStrLn "hi there!" }}} I get {{{ rae:09:50:49 ~/temp> ghc Bug.hs [1 of 1] Compiling Main ( Bug.hs, Bug.o ) Linking Bug ... ld: can't open output file for writing: Bug, errno=21 for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation) `gcc' failed in phase `Linker'. (Exit code: 1) }}} This bogus code should be rejected more gracefully (and earlier). It actually also loads into GHCi, but shouldn't. Will fix while I'm in the area. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13075> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14855: Implementation of liftA2 for Const has high arity
by GHC 26 Feb '18

26 Feb '18

#14855: Implementation of liftA2 for Const has high arity -------------------------------------+------------------------------------- Reporter: lyxia | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: | Version: 8.2.2 libraries/base | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Runtime Unknown/Multiple | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{ instance Monoid m => Applicative (Const m) where pure _ = Const mempty liftA2 _ (Const x) (Const y) = Const (x `mappend` y) (<*>) = coerce (mappend :: m -> m -> m) }}} https://hackage.haskell.org/package/base-4.10.1.0/docs/src/Data.Functor.Con… `(<*>)` is implemented with a `coerce` but `liftA2` isn't. Would the following not have better inlining behavior? {{{ liftA2 _ = coerce (mappend :: m -> m -> m) }}} Going further, should the unused argument also be moved to the RHS? What about `pure`? What are the pros and cons compared to this other alternative: {{{ pure = \_ -> mempty liftA2 = \_ -> coerce (mappend :: m -> m -> m) }}} This came up while implementing `Applicative` for `K1` in phab:D4447. `K1` is essentially the same type as `Const` and thus their instances should be identical for the sake of consistency. Is `Const`'s `Applicative` instance already optimal? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14855> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13596: Try disabling inlining of DynFlags combinators
by GHC 26 Feb '18

26 Feb '18

#13596: Try disabling inlining of DynFlags combinators -------------------------------------+------------------------------------- Reporter: bgamari | Owner: (none) Type: task | 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: -------------------------------------+------------------------------------- `DynFlags` requires a surprisingly amount of effort to compile. One thing I've been meaning to try is disabling inlining of `make_ord_flag` and such to characterize how much of an impact these inlinings have on simplification effort. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13596> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #855: Improvements to SpecConstr
by GHC 26 Feb '18

26 Feb '18

#855: Improvements to SpecConstr -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: task | Status: new Priority: normal | Milestone: ⊥ Component: Compiler | Version: 6.4.2 Resolution: | Keywords: SpecConstr Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: N/A Blocked By: | Blocking: 915 Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Calling the very simple optimiser (in `CoreOpt`) on the LHS might help. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/855#comment:18> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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