February 2018 - ghc-tickets

[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] #14662: Partial type signatures + mutual recursion = confusion
by GHC 27 May '18

27 May '18

#14662: Partial type signatures + mutual recursion = confusion -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: bug | Status: new 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: -------------------------------------+------------------------------------- I'm trying to understand better how partial type signatures interact with mutual recursion. This is all in 8.4.1-alpha1. Example 1: {{{#!hs f :: forall a. _ -> a -> a f _ x = g True x g :: forall b. _ -> b -> b g _ x = f 'x' x }}} This works -- no problem. Example 2: {{{#!hs f :: forall a. _ -> a -> a f _ x = snd (g True 'a', x) g :: forall b. _ -> b -> b g _ x = f 'x' x }}} This fails, explaining that GHC inferred `g :: Bool -> a -> a` and that `a` doesn't match `Char` (in the second argument of the call site in the body of `f`). This is unsatisfactory because clearly `g` should be ''instantiated'' at `Char`. The manual does say that polymorphic recursion isn't available with partial type signatures, and I suppose this is an example of polymorphic (mutual) recursion. Example 3: {{{#!hs f :: forall a. _ -> a -> a f _ x = snd (g True 'a', x) where g :: forall b. _ -> b -> b g _ y = f 'x' y }}} This is accepted. This is the same example as the last one, but now `g` is local. It does not capture any variables (including type variables) from `f`, so it seems to me that it should be equivalent to Example 2. But somehow GHC is clever enough to accept. Example 4: {{{#!hs thdOf3 (_, _, c) = c f :: forall a. _ -> a -> a f _ x = thdOf3 (g True 'a', g False 5, x) where g :: forall b. _ -> b -> b g _ y = f 'x' y }}} This works, too. Note that `g` is applied at several different types, so it really is being generalized. Example 5: {{{#!hs f :: Int -> forall a. _ -> a -> a f n _ x = snd (g n True 'a', x) g :: Int -> forall b. _ -> b -> b g n _ x = f n 'x' x }}} This is accepted. This is the same as Example 2, but each function now takes an `Int`, which is simply passed back and forth. Evidently, when you write the type non-prenex, polymorphic recursion is OK. Example 6: {{{#!hs f :: Int -> forall a. _ -> a -> a f n _ x = snd (f n True 'x', x) }}} This is accepted, even though it's blatantly using polymorphic recursion. Example 7: {{{#!hs f :: forall a. _ -> a -> a f _ x = snd (f True 'x', x) }}} This is rejected as polymorphically recursive. -------------------------- Something is fishy here. It's not the explicit prenex `forall`s -- leaving those out doesn't change the behavior. I guess my big question is this: * If the user quantifies a partial type signature (either by using `forall`, or just using an out-of-scope type variable and using Haskell's implicit quantification), why can't we use polymorphic recursion with that variable? I understand why we can't use polymorphic recursion with wildcards. ----------------------------------- A little background for context: I'm struggling (in my work on #14066) with GHC's use of `SigTv`s for partial type signatures. I don't have a better suggestion, but `SigTv`s never make me feel good. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14662> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14777: panic
by GHC 27 May '18

27 May '18

#14777: panic -------------------------------------+------------------------------------- Reporter: zilinc | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Keywords: | Operating System: Linux Architecture: | Type of failure: Compile-time Unknown/Multiple | crash or panic Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The error message I got from GHC was: {{{ <no location info>: error: ghc: panic! (the 'impossible' happened) (GHC version 8.2.2 for x86_64-unknown-linux): idInfo t_XG0J Call stack: CallStack (from HasCallStack): prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1133:58 in ghc:Outputable callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in ghc:Outputable pprPanic, called at compiler/basicTypes/Var.hs:526:34 in ghc:Var Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} How was it triggerred (in short): There's function definition, `f x = undefined`. If I change it to `f x = __impossible "Oops!"`, ghc crashes. The definition of `__impossible` is simply `__impossible s = error $ s ++ "some text"`. The code can be found here: https://github.com/NICTA/cogent/blob/92c40530cbcbac77469865e096dab3228a2fb9… and https://github.com/NICTA/cogent/blob/92c40530cbcbac77469865e096dab3228a2fb9… It seems that this bug is not limited to 8.2.2; it also happened in 8.2.1. I'm happy to provide more information, if someone can advice me what is needed. I'll also try to shrink it to a small test case. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14777> 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] #14442: InstanceSigs fails
by GHC 26 May '18

26 May '18

#14442: InstanceSigs fails -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.3 Keywords: InstanceSigs | 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 may be expected behavior, resulting from instance signatures being allowed to be "more polymorphic" than the method `@Bool`. But better safe than sorry (can't think of a better title). This works fine: {{{#!hs {-# Language KindSignatures, GADTs, DataKinds, TypeOperators, PolyKinds, TypeFamilies, TypeFamilyDependencies, InstanceSigs #-} import Data.Kind import Data.Type.Equality type family Sing = (res :: k -> Type) | res -> k type instance Sing = SBool data SBool :: Bool -> Type where SFalse :: SBool False STrue :: SBool True class EQ a where eq :: Sing (x::a) -> Sing (y::a) -> Maybe (x :~~: y) instance EQ Bool where eq :: Sing (x :: Bool) -> Sing (y :: Bool) -> Maybe (x :~~: y) eq SFalse SFalse = Just HRefl }}} Removing the kind annotation from `x :: Bool` and/or `y :: Bool` causes it to fail: Here I have removed it from `x`: `eq :: Sing x -> Sing (y :: Bool) -> Maybe (x :~~: y)` {{{ $ ghci -ignore-dot-ghci B.hs GHCi, version 8.3.20170920: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( B.hs, interpreted ) B.hs:19:6: error: • Couldn't match type ‘SBool’ with ‘Sing’ Expected type: Sing x Actual type: SBool a0 • In the pattern: SFalse In an equation for ‘eq’: eq SFalse SFalse = Just HRefl In the instance declaration for ‘EQ Bool’ • Relevant bindings include eq :: Sing x -> Sing y -> Maybe (x :~~: y) (bound at B.hs:19:3) | 19 | eq SFalse SFalse = Just HRefl | ^^^^^^ B.hs:19:22: error: • Could not deduce: (x :: k1) ~~ ('False :: Bool) from the context: a0 ~ 'False bound by a pattern with constructor: SFalse :: SBool 'False, in an equation for ‘eq’ at B.hs:19:6-11 or from: y ~ 'False bound by a pattern with constructor: SFalse :: SBool 'False, in an equation for ‘eq’ at B.hs:19:13-18 ‘x’ is a rigid type variable bound by the type signature for: eq :: forall k1 (x :: k1) (y :: Bool). Sing x -> Sing y -> Maybe (x :~~: y) at B.hs:18:9-54 Expected type: Maybe (x :~~: y) Actual type: Maybe (x :~~: x) • In the expression: Just HRefl In an equation for ‘eq’: eq SFalse SFalse = Just HRefl In the instance declaration for ‘EQ Bool’ • Relevant bindings include eq :: Sing x -> Sing y -> Maybe (x :~~: y) (bound at B.hs:19:3) | 19 | eq SFalse SFalse = Just HRefl | ^^^^^^^^^^ Failed, 0 modules loaded. Prelude> }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14442> 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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