June 2018 - ghc-tickets

Re: [GHC] #4806: Make error message more user friendly when module is not found because package is unusable
by GHC 05 Jun '18

05 Jun '18

#4806: Make error message more user friendly when module is not found because package is unusable -------------------------------------+------------------------------------- Reporter: mitar | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.0.1 Resolution: | Keywords: newcomer Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4783 Wiki Page: | -------------------------------------+------------------------------------- Comment (by sgillespie): I added my phabricator revision, but it doesn't appear to be building (we're on Step 1 for over 24 hours now): https://phabricator.haskell.org/harbormaster/build/47009/ -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4806#comment:30> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14879: QuantifiedConstraints: Big error message + can't substitute (=>) with a class alias
by GHC 05 Jun '18

05 Jun '18

#14879: QuantifiedConstraints: Big error message + can't substitute (=>) with a class alias -------------------------------------+------------------------------------- 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: -------------------------------------+------------------------------------- This compiles fine {{{#!hs {-# Language QuantifiedConstraints, FlexibleInstances, MultiParamTypeClasses, ConstraintKinds, UndecidableInstances, GADTs, TypeOperators #-} class (a => b) => Implies a b instance (a => b) => Implies a b data Dict c where Dict :: c => Dict c type a :- b = Dict (Implies a b) class (forall xx. Implies b xx => Implies a xx) => Yo a b instance (forall xx. Implies b xx => Implies a xx) => Yo a b yo :: Yo a b :- Implies a b yo = Dict }}} until you replace `(=>)` with `Implies` (which should be fine?) {{{#!hs class (forall xx. Implies b xx `Implies` Implies a xx) => Yo a b instance (forall xx. Implies b xx `Implies` Implies a xx) => Yo a b }}} and the error message blows up {{{ $ ghci -ignore-dot-ghci SD.hs GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( SD.hs, interpreted ) SD.hs:15:6: error: • Reduction stack overflow; size = 201 When simplifying the following type: Implies b (Implies b (Implies -->8---->8----several-hundred-lines---->8---->8---- b))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))))) Use -freduction-depth=0 to disable this check (any upper bound you could choose might fail unpredictably with minor updates to GHC, so disabling the check is recommended if you're sure that type checking should terminate) • In the expression: Dict In an equation for ‘yo’: yo = Dict | 15 | yo = Dict | ^^^^ Failed, no modules loaded. Prelude> }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14879> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14831: QuantifiedConstraints: Odd superclass constraint
by GHC 05 Jun '18

05 Jun '18

#14831: QuantifiedConstraints: Odd superclass 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: -------------------------------------+------------------------------------- This code works {{{#!hs {-# Language QuantifiedConstraints, RankNTypes, PolyKinds, ConstraintKinds, UndecidableInstances #-} import Data.Semigroup newtype Free cls a = Free () instance (forall xx. cls xx => Semigroup xx) => Semigroup (Free cls a) where (<>) = undefined stimes = undefined sconcat = undefined -- instance (forall xx. cls xx => Semigroup xx) => Monoid (Free cls a) where -- mempty = undefined }}} but uncomment the `Monoid` instance and we get {{{ $ ghci -ignore-dot-ghci /tmp/I.hs GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( /tmp/I.hs, interpreted ) /tmp/I.hs:12:10: error: • Could not deduce: cls (Free cls a) arising from the superclasses of an instance declaration from the context: forall xx. cls xx => Semigroup xx bound by the instance declaration at /tmp/I.hs:12:10-67 • In the instance declaration for ‘Monoid (Free cls a)’ | 12 | instance (forall xx. cls xx => Semigroup xx) => Monoid (Free cls a) where | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Failed, no modules loaded. Prelude> }}} Is this correct behavior? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14831> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #4806: Make error message more user friendly when module is not found because package is unusable
by GHC 05 Jun '18

05 Jun '18

#4806: Make error message more user friendly when module is not found because package is unusable -------------------------------------+------------------------------------- Reporter: mitar | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.0.1 Resolution: | Keywords: newcomer Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4783 Wiki Page: | -------------------------------------+------------------------------------- Changes (by sgillespie): * differential: => Phab:D4783 -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4806#comment:29> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14877: QuantifiedConstraints: Can't deduce `xx' from `(xx => a, xx)'
by GHC 05 Jun '18

05 Jun '18

#14877: QuantifiedConstraints: Can't deduce `xx' from `(xx => a, xx)' -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 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: -------------------------------------+------------------------------------- This works with the [https://ghc.haskell.org/trac/ghc/ticket/2893#comment:34 latest change to the -XQuantifiedConstraints branch]. {{{#!hs {-# Language QuantifiedConstraints, FlexibleInstances, MultiParamTypeClasses, AllowAmbiguousTypes, RankNTypes, PolyKinds, ConstraintKinds, UndecidableInstances #-} class (forall xx. (xx => a) => Implies xx b) => F a b instance (forall xx. (xx => a) => Implies xx b) => F a b class (a => b) => Implies a b instance (a => b) => Implies a b }}} but replacing `Implies xx b` with `(xx => b)` causes it to fail. I don't know if the cause of this overlaps with an existing ticket. {{{#!hs class (forall xx. (xx => a) => (xx => b)) => F a b instance (forall xx. (xx => a) => (xx => b)) => F a b -- $ ghci -ignore-dot-ghci /tmp/ASD.hs -- GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help -- [1 of 1] Compiling Main ( /tmp/ASD.hs, interpreted ) -- -- /tmp/ASD.hs:4:10: error: -- • Could not deduce: xx0 -- arising from the superclasses of an instance declaration -- from the context: forall (xx :: Constraint). (xx => a, xx) => b -- bound by the instance declaration at /tmp/ASD.hs:4:10-53 -- or from: (xx => a, xx) -- bound by a quantified context at /tmp/ASD.hs:1:1 -- • In the instance declaration for ‘F a b’ -- | -- 4 | instance (forall xx. (xx => a) => (xx => b)) => F a b -- | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ -- Failed, no modules loaded. -- Prelude> }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14877> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14832: QuantifiedConstraints: Strengthening context causes failure
by GHC 05 Jun '18

05 Jun '18

#14832: QuantifiedConstraints: Strengthening context causes failure -------------------------------------+------------------------------------- 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: -------------------------------------+------------------------------------- With Simon's latest changes (ticket:14733#comment:9) this works: {{{#!hs {-# Language QuantifiedConstraints, RankNTypes, PolyKinds, ConstraintKinds, UndecidableInstances, GADTs #-} import Control.Category import Data.Kind import Data.Coerce data With cls a b where With :: cls a b => With cls a b type Coercion = With Coercible type Refl rel = (forall xx. rel xx xx :: Constraint) type Trans rel = (forall xx yy zz. (rel xx yy, rel yy zz) => rel xx zz :: Constraint) instance Refl rel => Category (With rel) where id = With (.) = undefined }}} But strengthening the context with `Trans rel`: {{{#!hs instance (Refl rel, Trans rel) => Category (With rel) where }}} causes it to fail {{{ GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( J.hs, interpreted ) J.hs:15:10: error: • Could not deduce: rel xx zz from the context: (Refl rel, Trans rel) bound by an instance declaration: forall k (rel :: k -> k -> Constraint). (Refl rel, Trans rel) => Category (With rel) at J.hs:15:10-53 or from: (rel xx yy, rel yy zz) bound by a quantified context at J.hs:15:10-53 • In the ambiguity check for an instance declaration To defer the ambiguity check to use sites, enable AllowAmbiguousTypes In the instance declaration for ‘Category (With rel)’ | 15 | instance (Refl rel, Trans rel) => Category (With rel) where | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ J.hs:15:10: error: • Could not deduce: rel xx xx from the context: (Refl rel, Trans rel) bound by an instance declaration: forall k (rel :: k -> k -> Constraint). (Refl rel, Trans rel) => Category (With rel) at J.hs:15:10-53 • In the ambiguity check for an instance declaration To defer the ambiguity check to use sites, enable AllowAmbiguousTypes In the instance declaration for ‘Category (With rel)’ | 15 | instance (Refl rel, Trans rel) => Category (With rel) where | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Failed, no modules loaded. Prelude> }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14832> 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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Re: [GHC] #436: Declare large amounts of constant data
by GHC 04 Jun '18

04 Jun '18

#436: Declare large amounts of constant data -------------------------------------+------------------------------------- Reporter: ajk | Owner: (none) Type: feature request | Status: closed Priority: normal | Milestone: ⊥ Component: Compiler | Version: None Resolution: duplicate | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: #9719 | -------------------------------------+------------------------------------- Changes (by bgamari): * status: new => closed * failure: => None/Unknown * resolution: None => duplicate * wikipage: => #9719 Comment: I think this will be addressed with the solution to #9719 (https://github.com/ghc-proposals/ghc- proposals/pull/135/files?short_path=d675984#diff- d6759846a84df1af033ef6e51346e80f) -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/436#comment:5> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14939: StaticPointers + -dcore-lint: cause Core Lint error??
by GHC 04 Jun '18

04 Jun '18

#14939: StaticPointers + -dcore-lint: cause Core Lint error?? -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Keywords: | Operating System: Unknown/Multiple StaticPointers | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This is an odd bug {{{#!hs {-# Language StaticPointers #-} import Data.Kind type Cat ob = ob -> ob -> Type type Alg cls ob = ob newtype Frí (cls::Type -> Constraint) :: (Type -> Alg cls Type) where Frí :: { with :: forall x. cls x => (a -> x) -> x } -> Frí cls a data AlgCat (cls::Type -> Constraint) :: Cat (Alg cls Type) where AlgCat :: (cls a, cls b) => (a -> b) -> AlgCat cls a b leftAdj :: AlgCat cls (Frí cls a) b -> (a -> b) leftAdj (AlgCat f) a = undefined }}} causes a {{{ $ ./ghc-stage2 --interactive -ignore-dot-ghci -dcore-lint 222-bug.hs GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( 222-bug.hs, interpreted ) *** Core Lint errors : in result of Float out(FOS {Lam = Just 0, Consts = True, OverSatApps = False}) *** <no location info>: warning: In the type ‘forall (cls :: * -> Constraint) (b :: Alg cls *). b’ Variable escape in forall: forall (cls :: * -> Constraint) (b :: Alg cls *). b *** Offending Program *** with :: forall (cls :: * -> Constraint) a. Frí cls a -> forall x. cls x => (a -> x) -> x [LclIdX[[RecSel]], Arity=2] with .. . --->8------->8------->8--- .. }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14939> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #2893: Implement "Quantified constraints" proposal
by GHC 04 Jun '18

04 Jun '18

#2893: Implement "Quantified constraints" proposal -------------------------------------+------------------------------------- Reporter: porges | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: ⊥ Component: Compiler | Version: 6.10.1 Resolution: | Keywords: | QuantifiedConstraints Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #5927 | Differential Rev(s): Phab:D4724 Wiki Page: | -------------------------------------+------------------------------------- Comment (by Ben Gamari <ben@…>): In [changeset:"7df589608abb178efd6499ee705ba4eebd0cf0d1/ghc" 7df5896/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="7df589608abb178efd6499ee705ba4eebd0cf0d1" Implement QuantifiedConstraints We have wanted quantified constraints for ages and, as I hoped, they proved remarkably simple to implement. All the machinery was already in place. The main ticket is Trac #2893, but also relevant are #5927 #8516 #9123 (especially! higher kinded roles) #14070 #14317 The wiki page is https://ghc.haskell.org/trac/ghc/wiki/QuantifiedConstraints which in turn contains a link to the GHC Proposal where the change is specified. Here is the relevant Note: Note [Quantified constraints] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The -XQuantifiedConstraints extension allows type-class contexts like this: data Rose f x = Rose x (f (Rose f x)) instance (Eq a, forall b. Eq b => Eq (f b)) => Eq (Rose f a) where (Rose x1 rs1) == (Rose x2 rs2) = x1==x2 && rs1 >= rs2 Note the (forall b. Eq b => Eq (f b)) in the instance contexts. This quantified constraint is needed to solve the [W] (Eq (f (Rose f x))) constraint which arises form the (==) definition. Here are the moving parts * Language extension {-# LANGUAGE QuantifiedConstraints #-} and add it to ghc-boot-th:GHC.LanguageExtensions.Type.Extension * A new form of evidence, EvDFun, that is used to discharge such wanted constraints * checkValidType gets some changes to accept forall-constraints only in the right places. * Type.PredTree gets a new constructor ForAllPred, and and classifyPredType analyses a PredType to decompose the new forall-constraints * Define a type TcRnTypes.QCInst, which holds a given quantified constraint in the inert set * TcSMonad.InertCans gets an extra field, inert_insts :: [QCInst], which holds all the Given forall-constraints. In effect, such Given constraints are like local instance decls. * When trying to solve a class constraint, via TcInteract.matchInstEnv, use the InstEnv from inert_insts so that we include the local Given forall-constraints in the lookup. (See TcSMonad.getInstEnvs.) * topReactionsStage calls doTopReactOther for CIrredCan and CTyEqCan, so they can try to react with any given quantified constraints (TcInteract.matchLocalInst) * TcCanonical.canForAll deals with solving a forall-constraint. See Note [Solving a Wanted forall-constraint] Note [Solving a Wanted forall-constraint] * We augment the kick-out code to kick out an inert forall constraint if it can be rewritten by a new type equality; see TcSMonad.kick_out_rewritable Some other related refactoring ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * Move SCC on evidence bindings to post-desugaring, which fixed #14735, and is generally nicer anyway because we can use existing CoreSyn free-var functions. (Quantified constraints made the free-vars of an ev-term a bit more complicated.) * In LookupInstResult, replace GenInst with OneInst and NotSure, using the latter for multiple matches and/or one or more unifiers }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/2893#comment:39> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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