October 2017 - ghc-tickets

[GHC] #9793: Some as-patterns could be accepted in pattern synonyms
by GHC 29 May '18

29 May '18

#9793: Some as-patterns could be accepted in pattern synonyms -------------------------------------+------------------------------------- Reporter: cactus | Owner: cactus Type: feature request | Status: new Priority: normal | Milestone: 7.10.1 Component: Compiler (Type | Version: 7.8.3 checker) | Operating System: Keywords: pattern synonyms | Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: Difficulty: Easy (less than 1 | None/Unknown hour) | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Currently all as-patterns are rejected in pattern synonym definitions, to avoid situations like {{{ pattern P x y <- x@(y:_) }}} since there's no valid reason to be able to then write something like {{{ f [True] False = ... }}} this would just lead to confusion. However, I think we could accept as-patterns where the body of the as- pattern doesn't contain any variable bindings that are accessible via the pattern synonym. In other words, this should be OK: {{{ pattern P x <- x@(y:_) }}} since it's exactly equivalent to {{{ pattern P x <- x@(_:_) }}} which I don't think is as confusing as the other example. I haven't really made up my mind yet if these should be bidirectional; but they certainly could be, by just ignoring the body of the as-pattern in the wrapper; so the following two would be equivalent: {{{ pattern P1 x = [x@(y:_)] pattern P2 x <- [x@(y:_)] where P2 x = [x] }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9793> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12540: RFC: Allow not quantifying every top-level quantifiee
by GHC 28 May '18

28 May '18

#12540: RFC: Allow not quantifying every top-level quantifiee -------------------------------------+------------------------------------- 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: -------------------------------------+------------------------------------- Minor nuisance, does this happen to anyone else? Currently Haskell has an all-nothing policy on quantified type variables, either you quantify none or all (fine, let's ignore non-prenex quantification `foo :: a -> forall b. b -> a`). Can we have a top-level quantification of only a subset of the free variables? Say I'm working on a function {{{#!hs reflected :: forall s m a. (Applicative m, Reifies s a) => TaggedT s m a }}} and I need to make ''s'' a scoped type variable, I always accidentally write {{{#!hs reflected :: forall s. (Applicative m, Reifies s a) => TaggedT s m a reflected = TagT . pure . reflect $ (Proxy :: Proxy s) }}} This causes GHC to complain that the other types — ''m'', ''a'' — are not in scope so and I have to add the remaining quantifiees I don't really care about (may have long names as well). It could be worse (''dramatization'') {{{#!hs ipartsOf :: forall i p f s t a. (Indexable [i] p, Functor f) => Traversing (Indexed i) f s t a a -> Over p f s t [a] [a] ipartsOf l = conjoined (\f s -> let b = inline l sell s in outs b <$> f (wins b)) (\f s -> let b = inline l sell s; (is, as) = unzip (pins b) in outs b <$> indexed f (is :: [i]) as) appendAssocAxiom :: forall p q r as bs cs. p as -> q bs -> r cs -> Dict ((as ++ (bs ++ cs)) ~ ((as ++ bs) ++ cs)) appendAssocAxiom _ _ _ = unsafeCoerce (Dict :: Dict (as ~ as)) }}} It would be nice to be nice to only have to specify the type one is interested in: {{{#!hs ipartsOf :: forall i. (Indexable [i] p, Functor f) => Traversing (Indexed i) f s t a a -> Over p f s t [a] [a] appendAssocAxiom :: forall as. p as -> q bs -> r cs -> Dict ((as ++ (bs ++ cs)) ~ ((as ++ bs) ++ cs)) }}} and have the others chosen in some way. This is not just useful for writing, but it makes it easier to read: If I see a type `forall i p f s t a. ...` any of them may appear in the function body, if I see a type `forall i. ...` I know only one is. Thoughts? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12540> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9427: Do finer-grained dependency analysis to infer more general kinds on type/class declarations
by GHC 27 May '18

27 May '18

#9427: Do finer-grained dependency analysis to infer more general kinds on type/class declarations -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- In the resolution to #9200, as detailed [wiki:GhcKinds/KindInference#Apossiblevariation here], we identified an area for improvement. This improvement was not implemented in the fix for #9200. This ticket will track just this improvement. (In brief: we can do better dependency analysis on mutually recursive type/class declarations to allow more kinds to generalize earlier. This would lessen the burden on programmers to put in kind annotations. See the wiki page.) A solid but unfinished attempt is [https://github.com/goldfirere/ghc/tree /re-sort-non-cusk-decls here]. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9427> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10141: Kind inference regression in closed type families
by GHC 27 May '18

27 May '18

#10141: Kind inference regression in closed type families -------------------------------------+------------------------------------- Reporter: goldfire | Owner: goldfire Type: bug | Status: new Priority: highest | Milestone: 7.10.1 Component: Compiler | Version: 7.10.1-rc2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Take the following definition: {{{ type family G (a :: k) where G Int = Bool G Bool = Int G a = a }}} It compiles in 7.8.3, but not in 7.10.1 RC2. This makes me sad. I will fix. (Found by Jan Stolarek.) -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10141> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11767: Add @since annotations for base instances
by GHC 26 May '18

26 May '18

#11767: Add @since annotations for base instances -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: task | Status: new Priority: normal | Milestone: 8.2.1 Component: Core | Version: 7.10.3 Libraries | 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: -------------------------------------+------------------------------------- Up until now we haven't been adding `@since` annotations for instances that we add to `base` (see Phab:D2051 for the discussion that spurred this ticket). We should try to get better coverage here. Unfortunately, we currently have no way to place Haddock comments on derived instances, which limits our ability consistently add these annotations. We'll need to work out some mechanism for this. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11767> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12447: Pretty-printing of equality `~` without parentheses
by GHC 26 May '18

26 May '18

#12447: Pretty-printing of equality `~` without parentheses -------------------------------------+------------------------------------- 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: -------------------------------------+------------------------------------- Based off on [https://github.com/ekmett/constraints/blob/master/src/Data/Constraint/Defer… Data.Constraint.Deferrable] {{{#!hs {-# Language RankNTypes, ConstraintKinds #-} import Data.Typeable class Deferrable p where deferEither :: proxy p -> (p => r) -> Either String r instance (Typeable a, Typeable b) => Deferrable (a ~ b) where deferEither = undefined }}} `PolyKinds` aren't enabled so `deferEither @(_ ~ _)` is not enough to select the `Deferrable (a ~ b)` instance, but it is displayed without parentheses {{{ $ ghci -XTypeApplications -ignore-dot-ghci tyiS.hs GHCi, version 8.0.1: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( tyiS.hs, interpreted ) Ok, modules loaded: Main. *Main> :t deferEither @(_ ~ _) deferEither @(_ ~ _) :: Deferrable t ~ t1 => proxy t ~ t1 -> (t ~ t1 => r) -> Either String r }}} Instead of a preferable {{{#!hs deferEither @(_ ~ _) :: Deferrable (t ~ t1) => proxy (t ~ t1) -> (t ~ t1 => r) -> Either String r }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12447> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11540: ghc accepts non-standard type without language extension
by GHC 26 May '18

26 May '18

#11540: ghc accepts non-standard type without language extension -------------------------------------+------------------------------------- Reporter: augustss | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.4 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: -------------------------------------+------------------------------------- Ghc accepts this {{{ f :: Eq a => Eq b => a -> b -> Bool f a b = a==a && b==b }}} The nested contexts should require some language extension. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11540> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14246: Probably AllowAmbiguousTypes + UndecidableInstances + TypeInType
by GHC 26 May '18

26 May '18

#14246: Probably AllowAmbiguousTypes + UndecidableInstances + TypeInType -------------------------------------+------------------------------------- Reporter: Toricon | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 Keywords: | Operating System: Windows Architecture: x86_64 | Type of failure: Compile-time (amd64) | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- ghc.exe: panic! (the 'impossible' happened) (GHC version 8.0.2 for x86_64-unknown-mingw32): isInjectiveTyCon sees a TcTyCon KLN {{{#!hs {-# LANGUAGE RankNTypes, GADTs, TypeOperators, PolyKinds, DataKinds, TypeFamilies, AllowAmbiguousTypes, UndecidableInstances, TypeInType #-} module MCS.Function where import MCS.Core --import MCS.Value import Data.Kind -- necessary for * type family KLN (n :: k) :: Nat where KLN (f :: v -> k) = S (KLN (forall t. f t)) KLN (f :: *) = Z type family Reveal (n :: k) (l :: Vect (KLN n) L) :: * where Reveal (f :: v -> k) (Cons (Label (t :: v)) l) = Reveal (f t) l Reveal (a :: *) Nil = a }}} The problem *has* to be somewhere in the KLN / Reveal interaction, so I removed everything else for this. Every other weird area is commented out or unused, and the bug vanished when KLN / Reveal was commented out. KLN shouldn't even work. If it _could_ be interaction with the rest of the document, I'll put that back in. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14246> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14172: GHC hangs during type-checking
by GHC 26 May '18

26 May '18

#14172: GHC hangs during type-checking -------------------------------------+------------------------------------- Reporter: lightandlight | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 Keywords: | Operating System: Linux Architecture: x86_64 | Type of failure: Compile-time (amd64) | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- GHC 8.0.2 doesn't terminate when type checking this file: {{{#!hs import Control.Lens import Data.Functor.Compose traverseCompose :: (a -> f b) -> g a -> f (h _) traverseCompose = _Wrapping Compose . traverse }}} This is the smallest type signature I could find that still causes the problem. GHCi infers the type without issue, and when I annotate it with the correct type signature, the file is successfully compiled. Using base-4.9.1.0, lens-4.14.1, and transformers-0.5.2.0 -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14172> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13746: Typeable changes in base-4.10 break ChasingBottoms's test suite
by GHC 26 May '18

26 May '18

#13746: Typeable changes in base-4.10 break ChasingBottoms's test suite -------------------------------------+------------------------------------- Reporter: refold | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: | Version: 8.2.1-rc2 libraries/base | 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: -------------------------------------+------------------------------------- When compiled against base-4.10, `ChasingBottoms`'s test suite fails with: {{{ Approx: ChasingBottomsTestSuite: typeRepTyCon: FunTy CallStack (from HasCallStack): error, called at libraries/base/Data/Typeable/Internal.hs:308:33 in base:Data.Typeable.Internal }}} This is due to the use of `error` in the following function in [https://github.com/ghc/ghc/blob/master/libraries/base/Data/Typeable/Interna… `Data.Typeable.Internal`]: {{{#!hs -- | Observe the type constructor of a type representation typeRepTyCon :: TypeRep a -> TyCon typeRepTyCon (TrTyCon _ tc _) = tc typeRepTyCon (TrApp _ a _) = typeRepTyCon a typeRepTyCon (TrFun _ _ _) = error "typeRepTyCon: FunTy" -- TODO }}} `ChasingBottoms`'s test suite works fine when compiled with GHC 8.0.2. It seems to me that the change in behaviour constitutes a bug or a breaking change (for which there is no mention in the changelog). -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13746> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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