November 2016 - 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 27 May '18

27 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] #12854: ghc-8 prints mangled names in error message: ‘GHC.Base.$dm<$’
by GHC 26 May '18

26 May '18

#12854: ghc-8 prints mangled names in error message: ‘GHC.Base.$dm<$’ -------------------------------------+------------------------------------- Reporter: slyfox | 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: -------------------------------------+------------------------------------- Noticed on old omega codebase when built with ghc-8.0.2-rc1. Here is the minimal example: {{{#!hs {-# LANGUAGE FlexibleInstances, KindSignatures #-} {-# OPTIONS_GHC -Wall #-} data F a b instance Functor (F a) where fmap = undefined instance Functor (F String) where fmap = undefined }}} {{{ $ ghci a.hs GHCi, version 8.0.1.20161117: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/slyfox/.ghci [1 of 1] Compiling Main ( a.hs, interpreted ) a.hs:6:10: error: • Overlapping instances for Functor (F String) arising from a use of ‘GHC.Base.$dm<$’ Matching instances: instance Functor (F a) -- Defined at a.hs:5:10 instance Functor (F String) -- Defined at a.hs:6:10 • In the expression: GHC.Base.$dm<$ @F String In an equation for ‘<$’: (<$) = GHC.Base.$dm<$ @F String In the instance declaration for ‘Functor (F String)’ Failed, modules loaded: none. }}} ‘GHC.Base.$dm<$’ is an internal name. Looks like it should be a 'GHC.Base.<$'. Confusingly this does not cause overlapping error at all: {{{#!hs {-# LANGUAGE FlexibleInstances, KindSignatures #-} {-# OPTIONS_GHC -Wall #-} data F a b class C (f :: * -> *) where instance C (F a) where instance C (F String) where }}} {{{ $ ghci a.hs GHCi, version 8.0.1.20161117: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/slyfox/.ghci [1 of 1] Compiling Main ( a.hs, interpreted ) Ok, modules loaded: Main. }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12854> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12794: Out of scope error not reported
by GHC 26 May '18

26 May '18

#12794: Out of scope error not reported -------------------------------------+------------------------------------- Reporter: simonpj | 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: -------------------------------------+------------------------------------- Consider {{{ {-# LANGUAGE TypeApplications #-} {-# OPTIONS -fdefer-type-errors #-} f = bar @Int }}} where `bar` is not in scope. We get {{{ T12768.hs:8:7: error: • Cannot apply expression of type ‘t1’ to a visible type argument ‘Int’ • In the expression: bar @Int }}} Confusingly, the "`bar` is not in scope`" error (the true error) has been suppressed. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12794> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12655: Bizarre parser problem: "Illegal bang-pattern" (something to do with CPP?)
by GHC 26 May '18

26 May '18

#12655: Bizarre parser problem: "Illegal bang-pattern" (something to do with CPP?) -------------------------------------+------------------------------------- Reporter: edsko | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.4.2 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: -------------------------------------+------------------------------------- This is a bug in an old ghc (7.4.2), documenting it here mostly in case anybody else comes across it. If you try to compile http://hackage.haskell.org/package/bytestring-builder-0.10.8.1.0 /bytestring-builder-0.10.8.1.0.tar.gz , ghc fails with {{{ src/Data/ByteString/Builder/Prim.hs:553:13: Illegal bang-pattern (use -XBangPatterns): ! br' }}} However, that file contains a directive {{{#!hs {-# LANGUAGE CPP, BangPatterns, ScopedTypeVariables #-} }}} at the very top of the file, so we should not be getting this error message. Now here's the bizarre thing: if we compile with `cabal --builddir=foo`, compilation happens as normal! I ran ghc with `--keep- tmp-files`, and compared the output of CPP for both variations, and literally the ''only'' difference between the two that lines {{{ # 156 "./dist/build/autogen/cabal_macros.h" }}} in one are {{{ # 156 "./foo/build/autogen/cabal_macros.h" }}} in the other. It gets weirder. I replaced the output of `src/Data/ByteString/Builder/Prim.hs` with the output of `cpp`, and removed the `CPP` language directive. If I start with the version that has `dist` (i.e., the one that results in a compilation error), ''even adding or removing a single blank line in the header of the file made the parser error go away'' (where the "header" is the bit with lines such as {{{ # 134 "./dist/build/autogen/cabal_macros.h" }}} No idea what's going on here; probably not worth investigating further. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12655> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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