August 2018 - ghc-tickets

[GHC] #13191: Make liftA2 a method of Applicative
by GHC 16 Jan '19

16 Jan '19

#13191: Make liftA2 a method of Applicative -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: dfeuer Type: feature | Status: new request | Priority: normal | Milestone: 8.2.1 Component: Core | Version: 8.0.1 Libraries | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): Phab:D3031 | Wiki Page: -------------------------------------+------------------------------------- Some functors support implementations of `liftA2` that are considerably faster than the usual definition. Furthermore, some people see `liftA2` as more intuitive and fundamental to the idea of `Applicative` than `<*>` is. This has been discussed on the libraries list in https://mail.haskell.org/pipermail/libraries/2017-January/027579.html and (as an official proposal) in https://mail.haskell.org/pipermail/libraries/2017-January/027612.html. All who expressed an opinion favored the proposal. One (David Menendez) specified that his support was weak. The others (Conal Elliot, Kris Nuttycombe, Wren Romano, Bardur Arantsson, and Mario Blažević) either supported the proposal strongly or merely indicated they were "+1". Conal Elliot wrote: [[BR]] [[BR]] > +1. > > I also sometimes define a specialized `liftA2` and then use it to define > `(<*>)`, which then gets used to define the real `liftA2`. > > I think of `liftA2` as playing a role similar to `foldMap` and `traverse`, while > `(<*>)` corresponds to `fold` and `sequenceA`. The first three self- compose > nicely: `liftA2.liftA2.liftA2`, `foldMap.foldMap.foldMap`, and > `traverse.traverse.traverse`. With functor composition, it's so much nicer to > write `liftA2.liftA2` (in the style of `Functor`, `Foldable`, and `Traversable`) > rather than `liftA2 (<*>)`. [[BR]] Wren Romano wrote [[BR]] [[BR]] > I'm also all for adding liftA2 to the class and have noticed this > inefficiency/asymmetry when working on the class hierarchies for other > languages [[BR]] Kris NuttyCombe wrote [[BR]] [[BR]] > I'm in favor of this change. From my perspecive, liftA2 is actually the > fundamental Applicative operation, an <*> is merely a convenient > isomorphism. When I'm teaching, showing the symmetry between the following > always seems to help students: > > fmap :: (a -> b) -> f a -> f b > liftA2 :: (a -> b -> c) -> f a -> f b -> f c > flip (>>=) :: (a -> f b) -> f a -> f b > > <*> is obviously exceptionally useful in practice. But liftA2 seems like the > more essential shape of that operation. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13191> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12045: Visible kind application
by GHC 16 Jan '19

16 Jan '19

#12045: Visible kind application -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.1 Keywords: | Operating System: Unknown/Multiple TypeApplications TypeInType | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I've wanted this for a while {{{ ghci> :kind (:~:) (:~:) :: k -> k -> Type }}} {{{ ghci> :kind (:~:) @(Type -> Type) (:~:) @(Type -> Type) :: (Type -> Type) -> (Type -> Type) -> Type ghci> :kind (:~:) @(Type -> Type) [] (:~:) @(Type -> Type) [] :: (Type -> Type) -> Type ghci> :kind (:~:) @(Type -> Type) [] Maybe (:~:) @(Type -> Type) [] Maybe :: Type }}} Working like {{{ ghci> type Same k (a::k) (b::k) = a :~: b ghci> :kind Same Same :: forall k -> k -> k -> * }}} {{{ ghci> :kind Same (Type -> Type) Same (Type -> Type) :: (Type -> Type) -> (Type -> Type) -> * ghci> :kind Same (Type -> Type) [] Same (Type -> Type) [] :: (Type -> Type) -> * ghci> :kind Same (Type -> Type) [] Maybe Same (Type -> Type) [] Maybe :: * }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12045> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15362: Template Haskell ignores bad type family definitions
by GHC 16 Jan '19

16 Jan '19

#15362: Template Haskell ignores bad type family definitions -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Template | Version: 8.4.3 Haskell | 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 garbage is accepted: {{{#!hs data Nat = Zero | Succ Nat $( [d| type family a + b where Maybe Zero b = b Succ a + b = Succ (a + b) |] ) }}} Note the `Maybe` photo-bombing in the first equation. The problem is that TH uses the symbol declared in the family head for all the equations, ignoring what's actually there. This is sad. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15362> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15360: Template Haskell splicing drops lots of type arguments
by GHC 16 Jan '19

16 Jan '19

#15360: Template Haskell splicing drops lots of type arguments -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Template | Version: 8.4.3 Haskell | 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: -------------------------------------+------------------------------------- The conversion functions (in the `Convert` module) drop type arguments in various places. Here are two examples: {{{ data T a b c = Mk a b c bar :: $( return $ AppT (InfixT (ConT ''Int) ''T (ConT ''Bool)) (ConT ''Double) ) bar = Mk 5 True 3.14 }}} This correct code leads to an error {{{ Expecting one more argument to ‘T Int Bool’ }}} because the `Double` is forgotten. Similarly, {{{ baz :: $( return $ AppT (ParensT (ConT ''Maybe)) (ConT ''Int) ) baz = Just 5 }}} fails because `baz :: (Maybe)` isn't valid. On the other hand, {{{ frob :: forall (a :: $( [t| * Int |] )). a -> a frob x = x }}} is spuriously accepted, because GHC has forgotten about the `Int` argument. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15360> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13648: ApplicativeDo selects "GHC.Base.Monad.return" when actions are used without patterns.
by GHC 15 Jan '19

15 Jan '19

#13648: ApplicativeDo selects "GHC.Base.Monad.return" when actions are used without patterns. -------------------------------------+------------------------------------- Reporter: AaronFriel | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- GHC 8.0.2 and 8.2.1-rc1 (rc2 not checked) have a bug where -XApplicativeDo causes "GHC.Base.Monad.return" to be used instead of the locally available "return", and a spurious "return ()" shows up. This desugaring is not adhering to the -XRebindableSyntax spec (see: #12490). Example: {{{#!hs {-# LANGUAGE NoImplicitPrelude #-} {-# LANGUAGE RebindableSyntax #-} -- Bug vanishes if this next line is removed: {-# LANGUAGE ApplicativeDo #-} module Main where import Prelude (String, print) class MyFunctor f where fmap :: (a -> b) -> f a -> f b class MyApplicative f where pure :: a -> f a (<*>) :: f (a -> b) -> f a -> f b class MyMonad m where return :: a -> m a (>>) :: m a -> m b -> m b (>>=) :: m a -> (a -> m b) -> m b fail :: String -> m a join :: m (m a) -> m a testCase1 m1 m2 = do m1 m2 return () testCase2 m1 m2 = do _ <- m1 _ <- m2 return () main = print "42" }}} {{{ :t testCase1 testCase1 :: (MyFunctor f, MyApplicative f, MyMonad f, Monad f) => f a2 -> f a1 -> f () :t testCase2 :: testCase2 :: (MyFunctor f, MyApplicative f) => f t -> f a -> f () }}} The desugaring for testCase1 shows the issue: {{{#!hs testCase1' m1 m2 = (<*>) (fmap (\() () -> ()) (m1 >> (GHC.Base.Monad.return ()))) (m2 >> (GHC.Base.Monad.return ())) -- or: testCase1' m1 m2 = (fmap (\() () -> () ) (m1 >> (return ()))) <*> (m2 >> (return ())) }}} I would be able to work on this if someone pointed me in the right direction. It looks like it would be in `compiler/rename/RnEnv` and `compiler/rename/RnExpr`, as with #12490? As a proposed fix, I would want to implement a limited-scope fix before the 8.2.1 release which would not address the thornier issue of #10892. The patch would: 1. Replace `GHC.Base.Monad.return` with local `pure`, removing the `Monad` constraint. 2. Replace `>>` with `*>`, removing the `MyMonad` constraint. This isn't a _complete_ fix, as this would still induce an unnecessary use of the local `fmap`, but it would reduce the desugaring bug in `testCase1` to only that local `fmap`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13648> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15010: Application (warp) server crashing periodically with "TSO object entered"
by GHC 14 Jan '19

14 Jan '19

#15010: Application (warp) server crashing periodically with "TSO object entered" -------------------------------------+------------------------------------- Reporter: ramirez7 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.4.3 Component: Runtime | Version: 8.0.2 System | 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: -------------------------------------+------------------------------------- We are seeing one of our warp servers periodically crash with the following error: {{{ Apr 06 10:44:28 stdouterr.log: binary-name-elided: internal error: TSO object entered! Apr 06 10:44:28 stdouterr.log: (GHC version 8.0.2 for x86_64_unknown_linux) }}} We are sometimes seeing CPU/memory spikes concurrently with this error, but not always. We are running very similar servers for other applications and they are '''not''' experiencing this error. But this server is one of our highest- load ones. Other tickets I found with this error: * https://ghc.haskell.org/trac/ghc/ticket/12383 * https://ghc.haskell.org/trac/ghc/ticket/8316 But these seem to be happening in ghci and ghc itself, not in a binary compiled by ghc. Questions I have: * What exactly is this error? * Is there anything else we can do on our side to help debug? Any artifacts that should be produced upon crash? * Should we consider upgrading our GHC version from 8.0.2 to get a bugfix? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15010> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14645: Allow type family in data family return kind
by GHC 14 Jan '19

14 Jan '19

#14645: Allow type family in data family return kind -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.4.1-alpha1 Keywords: TypeInType, | Operating System: Unknown/Multiple TypeFamilies | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- GHC currently allows {{{#!hs data family DF1 :: k1 -> k2 }}} where it's expected (and checked) that all data ''instances'' have a return kind of `Type`. (Perhaps `k2` expands to `Type -> Type`, for example.) However, it rejects {{{#!hs type family TF (x :: Type) :: Type data family DF2 :: x -> TF x }}} when that's clearly just as sensible as the first definition. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14645> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10271: Typed Template Haskell splice difficulty when resolving overloading
by GHC 11 Jan '19

11 Jan '19

#10271: Typed Template Haskell splice difficulty when resolving overloading -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- J Garrett Morris describes the following surprising behaviour for typed Template Haskell {{{ {-# LANGUAGE TemplateHaskell, FlexibleInstances #-} module PrintfLib where import Language.Haskell.TH class Printf t where printf :: String -> Q (TExp String) -> Q (TExp t) instance Printf [Char] where tf s t | '%' `notElem` s = [|| $$t ++ s ||] | otherwise = fail ("Unexpected format %" ++ [c]) where (_, _:c:_) = break ('%' ==) s instance Printf t => Printf (Char -> t) where printf s t | c /= 'c' = fail ("Unexpected format %" ++ [c] ++ " for character") | otherwise = [|| \c -> $$(printf s'' [|| $$t ++ s' ++ [c] ||]) ||] where (s', '%':c:s'') = break ('%' ==) s ------------------------- {-# LANGUAGE TemplateHaskell #-} module Printf where import PrintfLib f :: Char -> String f = $$(printf "foo %c" [||""||]) h :: Char -> String h y = $$(printf "foo %c" [||""||]) y }}} Oddly, `f` typechecks but `h` does not, even though `h` is just an eta- expanded version of `f`: {{{ Printf.hs:9:10: No instance for (Printf t0) arising from a use of ‘printf’ The type variable ‘t0’ is ambiguous Note: there are several potential instances: instance Printf t => Printf (Char -> t) -- Defined in ‘PrintfLib’ instance Printf [Char] -- Defined in ‘PrintfLib’ In the expression: printf "foo %c" [|| "" ||] }}} What is going on? Here's the deal * To run the splice, GHC must solve any constraints that arise form the expression `(printf "foo %c" ...)`. * Since `printf` is overloaded, and overloaded on its result type, the type needed by the context of the splice is what determines which instance of `Printf` is needed. * In `f` the context needs `Char -> String`, and so the call to `printf` must have type `TExpr (Char -> String)`, so we get the constraint `Printf (Char -> String)` which we can solve. * But in `h` the rule for application tries to ''infer'' a type for the splice. So the context for the call just says `TExp t0` for some unification variable `t0`; and that leads to the insoluble constraint. You may say that GHC should be cleverer, and push that type signature information into the application. And perhaps it should. But you can never win. For example: {{{ hard x = [ $$(printf "gluk" [|| "" ||]), undefined :: Char -> String ] }}} Here the RHS of `hard` is a 2-element list. Since all elements of a list have the same type, the splice must have the same type as the second element of the list, namely `Char->String`. But seeing that would mean that we'd have to typecheck right-to-left. In general GHC tries very very hard NOT to depend on traveral order. There is no way in general to ensure that we have all the information now that constraint solving may ultimately produce. I'm not sure what to do about this. * It seldom matters, because resolving the overloading on the splice seldom depends on the result type. * When it does matter, you can fix it by giving a type signature to the splice itself. But it seems unsatisfactory. Ideas welcome. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10271> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12488: Explicit namespaces doesn't enforce namespaces
by GHC 11 Jan '19

11 Jan '19

#12488: Explicit namespaces doesn't enforce namespaces -------------------------------------+------------------------------------- Reporter: mpickering | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: newcomer | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- It seems to me that the type keyword should permanently enforce that a RdrName lives in a particular namespace. Currently it only acts as a syntactic marker so that type constructors which looks like variables can be included in export lists. The following example is accepted by GHC. As `A` is parsed as a type constructor but later put into the data constructor namespace. {{{#!hs {-# LANGUAGE ExplicitNamespaces #-} {-# LANGUAGE TypeFamilies #-} module M ( T (type A) ) where data T = A class Foo a where type A a foo :: a -> Int }}} The fix is to track uses of ExplicitNamespaces in the AST and then use this information in the renamer. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12488> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11610: Remove IEThingAll constructor from IE datatype
by GHC 11 Jan '19

11 Jan '19

#11610: Remove IEThingAll constructor from IE datatype -------------------------------------+------------------------------------- Reporter: mpickering | Owner: Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 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: -------------------------------------+------------------------------------- It seems that now `IEThingWith` can include a wildcard that the `IEThingAll` constructor is a special case. I'm not sure how involved this would be but it seems worth doing. {{{ | IEThingAll (Located name) -- ^ Class/Type plus all methods/constructors -- -- - 'ApiAnnotation.AnnKeywordId's : 'ApiAnnotation.AnnOpen', -- 'ApiAnnotation.AnnDotdot','ApiAnnotation.AnnClose', -- 'ApiAnnotation.AnnType' -- For details on above see note [Api annotations] in ApiAnnotation -- See Note [Located RdrNames] in HsExpr | IEThingWith (Located name) IEWildcard [Located name] [Located (FieldLbl name)] -- ^ Class/Type plus some methods/constructors -- and record fields; see Note [IEThingWith] -- - 'ApiAnnotation.AnnKeywordId's : 'ApiAnnotation.AnnOpen', -- 'ApiAnnotation.AnnClose', -- 'ApiAnnotation.AnnComma', -- 'ApiAnnotation.AnnType' -- For details on above see note [Api annotations] in ApiAnnotation }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11610> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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