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[GHC] #10873: Bad error message for incorrect pattern synonym signature
by GHC 29 Feb '16

29 Feb '16

#10873: Bad error message for incorrect pattern synonym signature -------------------------------------+------------------------------------- Reporter: mpickering | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.10.2 (Type checker) | Keywords: newcomer | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- Consider the following program {{{ {-# LANGUAGE PatternSynonyms #-} pattern Pat :: Show a => () => a -> Maybe a pattern Pat a = Just a }}} GHC complains that {{{ test.hs:4:9: No instance for (Show a) arising from a pattern }}} I think this is quite difficult to understand. The problem is that matching on `Just a` doesn't provide the show constraint (it provides no constraints). A better error message here would explain this fact and maybe a short explanation of the difference between prov/req in a pattern synonym signature. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10873> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #6166: Performance regression in mwc-random since 7.0.x
by GHC 29 Feb '16

29 Feb '16

#6166: Performance regression in mwc-random since 7.0.x -------------------------------------+------------------------------------- Reporter: bos | Owner: Type: bug | Status: new Priority: high | Milestone: 8.2.1 Component: Compiler | Version: 7.4.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: x86_64 Type of failure: Runtime | (amd64) performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by bgamari): * milestone: 8.0.1 => 8.2.1 Comment: Nothing will be happening on this front for 8.0.1. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/6166#comment:19> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11370: Redundant superclass warnings being included in -Wall destroys the "3 Release Policy"
by GHC 29 Feb '16

29 Feb '16

#11370: Redundant superclass warnings being included in -Wall destroys the "3 Release Policy" -------------------------------------+------------------------------------- Reporter: ekmett | Owner: Type: bug | Status: new Priority: highest | Milestone: 8.0.1 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: #11369 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- These redundant constraint warnings being in -Wall make it seemingly impossible for us to live up to the '3 Release Policy' in many situations. https://prime.haskell.org/wiki/Libraries/3-Release-Policy Consider the AMP: `(Monad m, Functor m)` gets used as a replacement for `Monad m` in many situations for compatibility with code before GHC 7.8, where that wants to call both `fmap` and something from `Monad`, but from 7.10+ that `Functor` is a redundant constraint. We have no backwards- facing 3-release-compatible constraint that someone could use in a type signature without warning to get both `fmap` and `return` that won't trigger this warning. Similarly if we split a class we can't advocate that folks use the subclass until the superclass is widely available or they'll get spammed with warnings. This affects splitting out `Semigroup` from `Monoid`. A similar issue arises with the `MonadFail` proposal as there is a stage around 8.4 where the 3-release-compatible advice would be to incur a `MonadFail` constraint, even when the active desguaring will be through `Monad`. I don't actually have a fix from the libraries side for the fact that with these showing up in -Wall, the 3 release policy doesn't work. Every single proposal the core libraries committee currently has in the works (except for adding `log1p`, `expm1`, etc. to Floating) is affected by this concern. One possible fix is to demote the redundant superclass warnings out of `-Wall`. This would resolve #11369 as well. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11370> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11625: Data.Type.Equality is not Trustworthy
by GHC 29 Feb '16

29 Feb '16

#11625: Data.Type.Equality is not Trustworthy -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: bug | Status: new Priority: high | Milestone: 8.0.1 Component: Core | Version: 7.10.3 Libraries | 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: -------------------------------------+------------------------------------- `Data.Type.Equality` is not inferred safe because it has unsafe imports. I believe it is safe, and should therefore claim `Trustworthy`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11625> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #7325: threadDelay mistreats minBound and maxBound in some configurations
by GHC 29 Feb '16

29 Feb '16

#7325: threadDelay mistreats minBound and maxBound in some configurations -------------------------------------+------------------------------------- Reporter: joeyadams | Owner: Type: bug | Status: new Priority: high | Milestone: 8.2.1 Component: Runtime System | Version: 7.6.1 Resolution: | Keywords: Operating System: Windows | Architecture: | Unknown/Multiple Type of failure: Incorrect result | Test Case: at runtime | base/tests/T8089 Blocked By: | Blocking: Related Tickets: #9722 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by bgamari): * milestone: 8.0.1 => 8.2.1 Comment: This won't be fixed in 8.0.1. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7325#comment:12> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11589: Reexports of wired-in packages don't work.
by GHC 29 Feb '16

29 Feb '16

#11589: Reexports of wired-in packages don't work. -------------------------------------+------------------------------------- Reporter: ezyang | Owner: ezyang Type: bug | Status: new Priority: high | Milestone: 8.0.1 Component: Compiler | Version: 8.0.1-rc2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): Phab:D1926 | Wiki Page: -------------------------------------+------------------------------------- My fault. Have a patch. This bug is to make sure it goes into 8.0. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11589> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11652: Cyclical dependencies aren't reported in pure functions
by GHC 29 Feb '16

29 Feb '16

#11652: Cyclical dependencies aren't reported in pure functions -------------------------------------+------------------------------------- Reporter: keynan | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC accepts Unknown/Multiple | invalid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- No compile error. found in 7.10.2 {{{#!hs main :: IO () main = do putStrLn $ "Hello " ++ (show x) where x = y + 1 y = x + 1 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11652> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11134: Limit frequency of idle GCs
by GHC 29 Feb '16

29 Feb '16

#11134: Limit frequency of idle GCs -------------------------------------+------------------------------------- Reporter: dcturner | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Runtime | Version: 7.10.2 System | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Runtime Unknown/Multiple | performance bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- In a server process with a relatively large heap (100s of MB) serving regular but relatively infrequent requests (~1 per sec) the idle GC runs after essentially every request which turns out to be extremely expensive: switching it off takes us from ~30% CPU to ~0.3% CPU. It is certainly useful have the GC run when idle but running it *every* time the process goes idle isn't necessary. It doesn't work to increase the delay (e.g. say -I2) as then the idle GC would never run, because the requests are rather regular. I'd quite like to be able to say something of the form "do a GC if idle for at least 0.3s as long as the idle GC has not run in the last 120s" - where 120s is another option passed to the RTS. Does this sound like a reasonable idea? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11134> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11636: Promoting newtype destructor
by GHC 29 Feb '16

29 Feb '16

#11636: Promoting newtype destructor -------------------------------------+------------------------------------- Reporter: phadej | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 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: -------------------------------------+------------------------------------- given {{{ newtype LOL a = MkLOL { unLOL :: [[a]] } }}} I'd like to write {{{ type family UnLOL (lol :: LOL a) = (result :: [[a]]) | result -> lol type instance UnLOL ('MkLOL xss) = xss }}} This is currently not accepted, but AFAICS it should be injective, as `lol` is kind restricted. Especially the example in injective type families paper do not apply, as `UnLOL Int` is ill-kinded. With this GHC could infer `lol ~ MkLOL xss` from `UnLOL lol ~ xss`, so newtypes would be useful for type-level programming, as type (kind) synonyms cannot be partially applied. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11636> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11658: Type synonym with context in pattern synonym
by GHC 29 Feb '16

29 Feb '16

#11658: Type synonym with context in pattern synonym -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.1 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: -------------------------------------+------------------------------------- {{{#!hs pattern :: (Num a, Eq a) => a pattern Fiskur = 42 }}} unlike {{{#!hs type Diskur a = Num a => a fiskur :: Num a => a fiskur = 42 }}} we cannot write {{{#!hs type Fugl a = (Num a, Eq a) => a pattern Fiskur :: Fugl a pattern Fiskur = 42 -- Exp.hs:54:18-19: error: … -- • No instance for (Eq a) arising from a pattern -- Possible fix: -- add (Eq a) to the context of -- the type signature for pattern synonym ‘Fiskur’: -- Fugl a -- • In the pattern: 42 -- In the declaration for pattern synonym ‘Fiskur’ -- Compilation failed. }}} Given that the story of pattern synonym contexts is more complicated than usual patterns given provided constraints, I know there are other issues (#11524, #10928). ---- For fun why not type synonym with provided and required constraints: {{{#!hs data T a where MkT :: (Show b) => a -> b -> T a f1 :: (Eq a, Num a) => T a -> String f1 (MkT 42 x) = show x type MKNUMPAT a b = (Num a, Eq a) => (Show b) => b -> T a -- pattern ExNumPat :: (Num a, Eq a) => (Show b) => b -> T a pattern ExNumPat :: MKNUMPAT a b pattern ExNumPat x = MkT 42 x }}} What would that mean in a regular type signature? `CE := CP + CR` where `EXNUMPAT a b` means the same as `EXNUMPAT' a b`? (#8581) {{{#!hs type MKNUMPAT' a b = (Num a, Eq a, Show b) => b -> T a -- This works: mkNumPat :: MKNUMPAT a b mkNumPat = ExNumPat }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11658> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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