December 2018 - ghc-tickets

[GHC] #14865: GHC Defeats Manual Worker Wrapper with Unboxed Sum
by GHC 06 Dec '18

06 Dec '18

#14865: GHC Defeats Manual Worker Wrapper with Unboxed Sum -------------------------------------+------------------------------------- Reporter: andrewthad | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 Keywords: UnboxedSums | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Here's the code in question: {{{ {-# LANGUAGE BangPatterns #-} {-# LANGUAGE LambdaCase #-} {-# LANGUAGE MagicHash #-} {-# LANGUAGE TypeInType #-} {-# LANGUAGE UnboxedSums #-} {-# LANGUAGE UnboxedTuples #-} {-# OPTIONS_GHC -O2 #-} module Byte.Array.Window ( findByte , boxMaybeInt ) where import Data.Primitive (ByteArray) import Data.Word (Word8) import GHC.Types (RuntimeRep,TYPE) import GHC.Int (Int(I#)) import GHC.Exts (Int#) import qualified Data.Primitive as PM type Maybe# (a :: TYPE (r :: RuntimeRep)) = (# (# #) | a #) boxMaybeInt :: Maybe# Int# -> Maybe Int boxMaybeInt = \case (# | a #) -> Just (I# a) (# (# #) | #) -> Nothing unboxInt :: Int -> Int# unboxInt (I# i) = i -- | Finds the first occurrence of the given byte. -- TODO: optimize this to search through a whole -- Word64 at a time if the bytearray is pinned. findByte :: Int -> Int -> Word8 -> ByteArray -> Maybe Int findByte !off !len !w !arr = boxMaybeInt (go off) where go :: Int -> Maybe# Int# go !ix = if ix < len then if PM.indexByteArray arr ix == w then (# | unboxInt ix #) else go (ix + 1) else (# (# #) | #) }}} When compiled with GHC 8.5 with `-ddump-simpl -dsuppress-all`, here is the relevant part of the resulting Core: {{{ -- RHS size: {terms: 33, types: 13, coercions: 0, joins: 1/1} $wfindByte $wfindByte = \ ww_s38C ww1_s38G ww2_s38K ww3_s38O -> joinrec { $wgo_s38v $wgo_s38v ww4_s38t = case <# ww4_s38t ww1_s38G of { __DEFAULT -> Nothing; 1# -> case indexWord8Array# ww3_s38O ww4_s38t of wild_a36w { __DEFAULT -> case eqWord# wild_a36w ww2_s38K of { __DEFAULT -> jump $wgo_s38v (+# ww4_s38t 1#); 1# -> Just (I# ww4_s38t) } } }; } in jump $wgo_s38v ww_s38C -- RHS size: {terms: 21, types: 12, coercions: 0, joins: 0/0} findByte findByte = \ w_s38w w1_s38x w2_s38y w3_s38z -> case w_s38w of { I# ww1_s38C -> case w1_s38x of { I# ww3_s38G -> case w2_s38y of { W8# ww5_s38K -> case w3_s38z of { ByteArray ww7_s38O -> $wfindByte ww1_s38C ww3_s38G ww5_s38K ww7_s38O } } } } }}} I expected that the tail recursive `go` helpful function from my original code would still be a function that returns an unboxed sum when optimized and turned into Core. However, it isn't. The call to `boxMaybeInt` gets pushed into `go`. This means that when `findByte` is called and the result cased on, an allocation is going to happen. I think it would be preferable for `boxMaybeInt` to not get pushed into the worker, since `boxMaybeInt (go off)` could be inlined and the allocation of `Maybe` could be prevented (assuming that it was cased on right afterward). -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14865> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15677: Valid hole fits and GADT type variable names
by GHC 06 Dec '18

06 Dec '18

#15677: Valid hole fits and GADT type variable names -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 Component: Compiler | Version: 8.6.1 Keywords: TypedHoles, | Operating System: Unknown/Multiple GADTs | Architecture: | Type of failure: Poor/confusing Unknown/Multiple | error message Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider the following code: {{{#!hs {-# LANGUAGE DataKinds #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeOperators #-} module Foo where import Data.Kind data HList :: [Type] -> Type where HNil :: HList '[] HCons :: x -> HList xs -> HList (x:xs) foo :: HList a -> HList a foo HNil = HNil foo (HCons (b :: bType) bs) = HCons _ bs }}} Here is the suggestion that the typed hole in `foo` provides: {{{ $ /opt/ghc/8.6.1/bin/ghc Bug.hs [1 of 1] Compiling Foo ( Bug.hs, Bug.o ) Bug.hs:16:37: error: • Found hole: _ :: x Where: ‘x’ is a rigid type variable bound by a pattern with constructor: HCons :: forall x (xs :: [*]). x -> HList xs -> HList (x : xs), in an equation for ‘foo’ at Bug.hs:16:6-26 • In the first argument of ‘HCons’, namely ‘_’ In the expression: HCons _ bs In an equation for ‘foo’: foo (HCons (b :: bType) bs) = HCons _ bs • Relevant bindings include bs :: HList xs (bound at Bug.hs:16:25) b :: x (bound at Bug.hs:16:13) foo :: HList a -> HList a (bound at Bug.hs:15:1) Constraints include a ~ (x : xs) (from Bug.hs:16:6-26) Valid hole fits include b :: x (bound at Bug.hs:16:13) | 16 | foo (HCons (b :: bType) bs) = HCons _ bs | ^ }}} One thing immediately stands out here: the hole has type `x`, but `x` appears no where in the definition of `foo`! I had expected this suggestion to mention `bType`, since I went through the effort of declaring `b` to have that type through a pattern signature, but GHC instead uses types from the definition of the `HCons` constructor itself. This seems less than ideal, since one would expect GHC to only ever mention types that are lexically in scope at a particular definition site. One thing which complicates this idea is that there can be multiple in- scope type variables that all refer to the same type. For instance, if I define this function: {{{#!hs bar :: HList a -> HList a -> HList a bar HNil HNil = HNil bar (HCons (b :: bType) bs) (HCons (c :: cType) cs) = HCons _ bs }}} What should the suggested type of the hole be: `bType`, or `cType`? Either choice is equally valid. After talking with Tritlo and simonpj about this, we came to the consensus that we should just pick one of the type variables to report at the top of the error message: {{{ • Found hole: _ :: bType }}} And then later in the message, include any type variable synonyms that have been brought into scope (via pattern signatures or otherwise). I imagine this might look something like: {{{ • Type variable synonyms include `cType` equals `bType` }}} This is quite similar to an existing feature of valid hole fits where we report `Constraints include`. (Indeed, we briefly considered just reporting these type variable synonyms as explicit equality constraints, but doing so would be somewhat misleading, since that's not how pattern signatures actually work in practice.) One implementation challenge is to figure out how to construct a mapping from `x` to `bType`. One place where inspiration can be drawn from is the `ATyVar` constructor of `TcTyThing`: {{{#!hs data TcTyThing = ... | ATyVar Name TcTyVar -- The type variable to which the lexically scoped type -- variable is bound. We only need the Name -- for error-message purposes; it is the corresponding -- Name in the domain of the envt }}} `ATyVar` already stores a "reverse mapping" of sorts to give better a more accurate `Name` in the event that it is pretty-printed, which is quite similar to what we need to do with `x` and `bType`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15677> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #5793: make nofib awesome
by GHC 06 Dec '18

06 Dec '18

#5793: make nofib awesome -------------------------------------+------------------------------------- Reporter: dterei | Owner: michalt Type: task | Status: new Priority: normal | Milestone: ⊥ Component: NoFib benchmark | Version: suite | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: 9571 | Blocking: Related Tickets: #15357 #15333 | Differential Rev(s): #15999 | Wiki Page: | -------------------------------------+------------------------------------- Changes (by sgraf): * related: #15357 #15333 => #15357 #15333 #15999 Comment: I opened #15999 to track the implicit task in comment:38. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5793#comment:40> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #7495: generalizing overloaded list syntax to Sized Lists, HLists, HRecords, etc
by GHC 06 Dec '18

06 Dec '18

#7495: generalizing overloaded list syntax to Sized Lists, HLists, HRecords, etc -------------------------------------+------------------------------------- Reporter: nwf | Owner: carter Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: 9883 Related Tickets: #9883 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by int-index): * cc: int-index (added) Comment: Lovely design in comment:11. I don't think anything stops us from extending it to support pattern matching and ranges by adding more classes for these features. Another use case that I have in mind is `NonEmpty`. As it stands, using it with `OverloadedLists` is unsafe, as `[]` will fail at run-time instead of compile-time. With the design of comment:11 we could do this: {{{#!hs instance Cons (NonEmpty a) where type Head (NonEmpty a) = a type Tail (NonEmpty a) = [a] cons = (:|) instance TypeError ... => Nil (NonEmpty a) }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7495#comment:20> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15203: Wrong location reported for kind error
by GHC 06 Dec '18

06 Dec '18

#15203: Wrong location reported for kind error -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.5 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: -------------------------------------+------------------------------------- When I try to compile {{{#!hs {-# LANGUAGE PolyKinds, ConstraintKinds, TypeFamilies, FlexibleContexts #-} module Bug where import Data.Proxy type T (a :: k1) (b :: k2) = (a ~ b, Show (Proxy (a :: k1)), Show (Proxy (b :: k2))) }}} I get {{{ Bug.hs:7:80: error: • Couldn't match ‘k1’ with ‘k2’ • In the type declaration for ‘T’ | 7 | type T (a :: k1) (b :: k2) = (a ~ b, Show (Proxy (a :: k1)), Show (Proxy (b :: k2))) | ^^ }}} The problem is that the `k2` that's highlighted has nothing at all to do with the error. Indeed, some experimentation shows that GHC will highlight the first occurrence of `k2` to the right of the `=`. The error is actually from unification caused by `a ~ b`. I noticed this because I've been tightening up the way GHC does left-to- right ordering during implicit quantification (while working on something more substantial). In so doing, I made sure to prefer kind variable occurrences to the ''left'' of the = over those to the right. But then I got a testsuite failure due to a changed location of an error. This is caused by the call to `report_sig_tv_err` in `tcTyClTyVars`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15203> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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Re: [GHC] #5793: make nofib awesome
by GHC 06 Dec '18

06 Dec '18

#5793: make nofib awesome -------------------------------------+------------------------------------- Reporter: dterei | Owner: michalt Type: task | Status: new Priority: normal | Milestone: ⊥ Component: NoFib benchmark | Version: suite | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: 9571 | Blocking: Related Tickets: #15357 #15333 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by AndreasK): > I blame non-determinism in code layout or some other caching effect until I have a better hypothesis. These are the candidats we want to flag away for runtime measurements, especially real/eff, or make them actually do something meaningful. They generally compile down to a loop of a handfull of instructions. So flipping an `if` (invert the condition, swap then and else expressions) can already lead to noteable differences as the cpu might need proccess one more instruction. For the really small loops we can also get counterintuitive behaviour. Eg I reduced on of them by one instruction and performance got slower because of instruction sheduling effects. Having an "effective runtime" modus that excludes these seems worthwhile. But I'm torn on excluding them by default. Ideally people would see these unexpected results. Then look at the generated code and be able to tell if the code is actually worse or if it's just one of these edge cases. But that requires a lot of low level knowledge, which not everyone has. So people might throw out perfectly good ideas/patches because of missleading nofib results. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5793#comment:39> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9091: print and/or apply constraints when showing info for typed holes
by GHC 06 Dec '18

06 Dec '18

#9091: print and/or apply constraints when showing info for typed holes -------------------------------------------+------------------------------- Reporter: kosmikus | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type checker) | 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: -------------------------------------------+------------------------------- Typed holes are fantastic already, but could be even better. Consider the following three examples: == Example 1 == {{{ f :: Eq a => a -> a -> Bool f x y = _ }}} GHC-7.8.2 shows: {{{ Found hole ‘_’ with type: Bool Relevant bindings include y :: a (bound at THC.hs:4:5) x :: a (bound at THC.hs:4:3) f :: a -> a -> Bool (bound at THC.hs:4:1) In the expression: _ In an equation for ‘f’: f x y = _ }}} GHC additionally knows that `Eq a` holds. It would be great if it would print that, too. == Example 2 == {{{ g :: (forall a. Eq a => a -> Bool) -> Bool g f = f _ }}} GHC-7.8.2 shows: {{{ Found hole ‘_’ with type: a0 Where: ‘a0’ is an ambiguous type variable Relevant bindings include f :: forall a. Eq a => a -> Bool (bound at THC.hs:7:3) g :: (forall a. Eq a => a -> Bool) -> Bool (bound at THC.hs:7:1) In the first argument of ‘f’, namely ‘_’ In the expression: f _ In an equation for ‘g’: g f = f _ }}} GHC additionally knows the `Eq a0` must hold. It would be nice if it could indicate that. == Example 3 == {{{ data X a where Y :: X Int h :: X a -> a -> a h Y x = _ }}} GHC-7.8.2 shows: {{{ Found hole ‘_’ with type: Int Relevant bindings include x :: a (bound at THC.hs:13:5) h :: X a -> a -> a (bound at THC.hs:13:1) In the expression: _ In an equation for ‘h’: h Y x = _ }}} Interestingly, GHC shows that we have to produce an `Int`. Unfortunately, it shows that `x` is of type `a`. GHC additionally knows that `a ~ Int`. It would be nice if GHC could either apply the constraint to show that `x :: Int` or at least show the constraint. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9091> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15942: Type family used invisibly (with Visible Kind Applications)
by GHC 06 Dec '18

06 Dec '18

#15942: Type family used invisibly (with Visible Kind Applications) -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.3 Component: Compiler | Version: 8.6.2 Keywords: | Operating System: Unknown/Multiple TypeApplications | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I want to run the following past you (using [https://phabricator.haskell.org/D5229 Visible Kind Applications] but may be unrelated). The following compiles {{{#!hs {-# Language DataKinds #-} {-# Language KindSignatures #-} {-# Language TypeFamilies #-} {-# Language AllowAmbiguousTypes #-} import Data.Kind type Cat ob = Bool -> Type data Fun :: Cat Type class F (bool :: Bool) where type Not bool :: Bool foo :: Fun (Not bool) }}} but quantifying `Bool` invisibly all of a sudden I can't use `Not` {{{#!hs {-# Language DataKinds #-} {-# Language RankNTypes #-} {-# Language TypeApplications #-} {-# Language PolyKinds #-} {-# Language KindSignatures #-} {-# Language TypeFamilies #-} {-# Language AllowAmbiguousTypes #-} import Data.Kind type Cat ob = forall (b :: Bool). Type data Fun :: Cat Type class F (bool :: Bool) where type Not bool :: Bool foo :: Fun @(Not bool) }}} {{{ $ ghc-stage2 --interactive -ignore-dot-ghci 739_bug.hs GHCi, version 8.7.20181017: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( 739_bug.hs, interpreted ) 739_bug.hs:17:16: error: • Type constructor ‘Not’ cannot be used here (it is defined and used in the same recursive group) • In the first argument of ‘Fun’, namely ‘(Not bool)’ In the type signature: foo :: Fun @(Not bool) In the class declaration for ‘F’ | 17 | foo :: Fun @(Not bool) | ^^^ Failed, no modules loaded. }}} Is this restriction warranted -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15942> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #15997: EventManager could benefit from Data.Primitive.UnliftedArray
by GHC 06 Dec '18

06 Dec '18

#15997: EventManager could benefit from Data.Primitive.UnliftedArray -------------------------------------+------------------------------------- Reporter: chessai | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: 8.6.3 Component: | Version: 8.6.2 libraries/base | 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 -- | The event manager state. data EventManager = EventManager { emBackend :: !Backend , emFds :: {-# UNPACK #-} !(Array Int (MVar (IntTable [FdData]))) , emState :: {-# UNPACK #-} !(IORef State) , emUniqueSource :: {-# UNPACK #-} !UniqueSource , emControl :: {-# UNPACK #-} !Control , emLock :: {-# UNPACK #-} !(MVar ()) } }}} the field in question is an `Array Int (MVar (IntTable [FdData]))`. `EventManager` could instead be: {{{#!hs type MVarIO = MVar# RealWorld -- type synonym for brevity -- | The event manager state. data EventManager = EventManager { emBackend :: !Backend , emFds :: {-# UNPACK #-} !(UnliftedArray (MVarIO (IntTable [FdData]))) , emState :: {-# UNPACK #-} !(IORef State) , emUniqueSource :: {-# UNPACK #-} !UniqueSource , emControl :: {-# UNPACK #-} !Control , emLock :: {-# UNPACK #-} !(MVar ()) } }}} now the `UnliftedArray` contains non-thunks. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15997> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13480: GHCi display visible type application
by GHC 05 Dec '18

05 Dec '18

#13480: GHCi display visible type application -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: feature | Status: new request | 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: #8751 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Similar to #8751, have a way to get GHCi to dump which types an expression is applied to: {{{ ghci> :tyapp length [1,2,3] length @[] @Integer [1,2,3] }}} It could also function as the [https://downloads.haskell.org/~ghc/master /users-guide/ghci.html#ghci-cmd-:all-types :all-types], [https://downloads.haskell.org/~ghc/master/users-guide/ghci.html#ghci-cmd -:type-at :type-at] commands where we can specify a region of code (from [https://www.researchgate.net/publication /282487445_Combining_Deep_and_Shallow_Embedding_of_Domain- Specific_Languages?ev=pubitem-pub_cit_inc Combining Deep and Shallow Embedding of Domain-Specific Languages]) {{{#!hs fromFunC :: FunC (m (Internal a)) -> Mon m a fromFunC m = M $ \k -> fromFunC Bind m k }}} displays {{{#!hs fromFunC :: FunC (m (Internal a)) -> Mon m a fromFunC m = M @m @a $ \(k :: a -> FunC (m b)) -> fromFunC @(FunC (m (Internal a)) -> (a -> FunC (m b)) -> FunC (m b)) (Bind @m @(Internal a) @b) m k }}} Would be great to get this feature as an IDE support. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13480> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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