July 2016 - ghc-tickets

[GHC] #10963: Beginner-targeted language extension
by GHC 30 Aug '16

30 Aug '16

#10963: Beginner-targeted language extension -------------------------------------+------------------------------------- Reporter: kanetw | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 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: -------------------------------------+------------------------------------- FMP significantly complicated type signatures such as `length :: Foldable t => t a -> Int`, which makes teaching Haskell more difficult. I propose a language extension `-XBeginner` which instructs GHC(i) to specialize `Foldable` and `Traversable` to `[]` when possible and in case of an ambiguous instance, default those to `[]`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10963> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 25

[GHC] #12029: Notify user to import * from Data.Kind with TypeInType on
by GHC 29 Aug '16

29 Aug '16

#12029: Notify user to import * from Data.Kind with TypeInType on -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: feature | Status: new request | Priority: lowest | Milestone: Component: GHCi | Version: 8.1 Keywords: TypeInType | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- With `TypeInType` asking for the kind of `*` gives the user a warning to import it {{{ ghci> :set -XTypeInType ghci> :k * <interactive>:1:1: error: Not in scope: type constructor or class ‘*’ NB: With TypeInType, you must import * from Data.Kind <interactive>:1:1: error: Illegal operator ‘*’ in type ‘*’ Use TypeOperators to allow operators in types <interactive>:1:1: error: Operator applied to too few arguments: * }}} Should a similar warning be issued when she asks for information on it {{{ ghci> :i * class Num a where ... (*) :: a -> a -> a ... -- Defined in ‘GHC.Num’ infixl 7 * }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12029> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 7

[GHC] #12368: Demand Analyzer: Cunnig plan not adhered to with aborting fixpoint interation
by GHC 29 Aug '16

29 Aug '16

#12368: Demand Analyzer: Cunnig plan not adhered to with aborting fixpoint interation -------------------------------------+------------------------------------- Reporter: nomeata | Owner: Type: bug | Status: new Priority: low | 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: -------------------------------------+------------------------------------- Not sure how relevant this is, but when reading both paper and code long enough, on inevitably finds some code smell. This is about nested recursion, as in this example {{{ f [] = [] f (x:xs) = let g [] = f xs g (y:ys) = y+1 : g ys in g (h x) }}} The “cunning plan” of fixpoint iteration (see Note [Initialising strictness]) says that in the first time an inner recursive definition is looked at, its strictness is assumed to be `b` (`botSig`), and from then on, its `idInformation` (presumably from the previous iteration or the outer recursive definition) is used. A flag (`virgin`) in the analysis environment is used to detect that. The problem is that the fixpoint iteration code in `dmdFix` aborts after more than 10 iterations: {{{ loop' n env pairs | found_fixpoint = (env', lazy_fv, pairs') | n >= 10 = (env, lazy_fv, orig_pairs) -- Safe output }}} This means that if the iteration does not terminate, we will “not” attach a strictness signature to the inner binders (`g` in the example above), but rather leave the binders untouched. Then, in the second iteration of finding a fixpoint for `f`, the `virgin` flag is `False`, and `idStrictness` is used, which in this case will simply be the default, namely `nopSig`. I could not produce an example where it matters. And it might be that it simply does not matter, so I’m not sure what to do with this information. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12368> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 18

[GHC] #12435: Turn "No alternatives for a case scrutinee not known to diverge for sure" into a warning
by GHC 28 Aug '16

28 Aug '16

#12435: Turn "No alternatives for a case scrutinee not known to diverge for sure" into a warning -------------------------------------+------------------------------------- Reporter: nomeata | Owner: Type: task | Status: new 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: -------------------------------------+------------------------------------- This was discussed in the context of #10180, #11290 and Phab:D851; let’s focus on this here. In changeset:a0678f1f0e62496c108491e1c80d5eef3936474a/ghc we introduced a lint ''error'' that would abort compilation if there is an empty case with a scrutinee not known to diverge for sure. It is then nature of such a test that it is never complete (we’d solve the halting problem). Occasionally, when working on other parts of the code, together with other transformations (such as CSE), we get Core code that does not pass the test, even though the scrutinee ''is'' diverging. Whenever it occurs, we have to find out why needs to be done to make the test pass again, e.g. by ensuring that CSE cse’s things in the right order, or whatnot. This slows down development (e.g. I cannot test my work on #12368 on the auto-builders because of this). It feels wrong to me to give a necessarily non-complete test such a high priority. I therefore propose to turn this check into a lint ''warning''. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12435> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 2

[GHC] #12116: No easy way to add cost-centre to top-levels
by GHC 28 Aug '16

28 Aug '16

#12116: No easy way to add cost-centre to top-levels -------------------------------------+------------------------------------- Reporter: osa1 | Owner: 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: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- As far as I can see from the user manual (I also tried placing some `SCC`s), there's no way to easily add cost-centres to top-level definitions. We can do things like: {{{ f x y z = ... => f = {-# SCC f #-} \x y z -> ... }}} but it becomes too much work once we have "equations" style definitions like {{{ head [] = {-# SCC head #-} Nothing head (x : _) = {-# SCC head #-} Just x }}} Also, `where` clauses work differently after the transformations. Example: {{{ f x y z = ... where <can use x and y here> => f = {-# SCC f #-} \x y z -> ... where <can't use x and y here> }}} So it gets annoying real quick. It'd be really great if we could just add `{-# SCC ... #-}` just like `{-# INLINE ... #-}` pragmas. Any ideas/opinions? Looking at the code, it seems like `-fprof-auto` and similar flags are adding ticks to `FunBind.fun_tick`. We could do the same with this new `SCC` pragma. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12116> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 3

[GHC] #10547: feature request: expanding type synonyms in error messages
by GHC 26 Aug '16

26 Aug '16

#10547: feature request: expanding type synonyms in error messages -------------------------------------+------------------------------------- Reporter: osa1 | Owner: osa1 Type: feature | Status: new request | Milestone: 7.12.1 Priority: low | Version: Component: Compiler | Operating System: Unknown/Multiple (Type checker) | Type of failure: None/Unknown Keywords: | Blocked By: Architecture: | Related Tickets: Unknown/Multiple | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Type errors sometimes becoming confusing because of synonyms, for example, expected type and found type sometimes use different synonyms or sometimes only one of them use a synonym etc. We discussed this in the mailing list: https://mail.haskell.org/pipermail /ghc-devs/2015-June/009247.html And also moved it to the Wiki to collect examples: https://wiki.haskell.org/Expanding_type_synonyms_in_error_messages_proposal I'm currently working on a patch, I made some progress and I'll hopefully submit something for reviews in a couple of days. This ticket is for further discussions and keep track of the progress. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10547> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 23

[GHC] #12407: Template Haskell thinks an unboxed tuple name is illegal to splice in
by GHC 26 Aug '16

26 Aug '16

#12407: Template Haskell thinks an unboxed tuple name is illegal to splice in -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.0.2 Component: Template | Version: 8.0.1 Haskell | 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: -------------------------------------+------------------------------------- {{{#!hs {-# LANGUAGE TemplateHaskell #-} {-# LANGUAGE UnboxedTuples #-} module Bug where import Language.Haskell.TH.Lib import Language.Haskell.TH.Syntax $(do let ubxTup = conT (unboxedTupleTypeName 2) `appT` conT ''Int `appT` conT ''Int x <- newName "x" y <- newName "y" [d| f :: $(ubxTup) -> $(ubxTup) f $(conP (unboxedTupleDataName 2) [varP x, varP y]) = $(conE (unboxedTupleDataName 2) `appE` varE x `appE` varE y) |]) }}} {{{ $ /opt/ghc/8.0.1/bin/ghc Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) Bug.hs:8:3: error: Illegal type constructor or class name: ‘(#,#)’ When splicing a TH declaration: f_0 :: GHC.Tuple.(#,#) GHC.Types.Int GHC.Types.Int -> GHC.Tuple.(#,#) GHC.Types.Int GHC.Types.Int }}} Patch coming soon. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12407> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 5

[GHC] #12401: GHC panic! Template variable unbound in rewrite rule
by GHC 26 Aug '16

26 Aug '16

#12401: GHC panic! Template variable unbound in rewrite rule -------------------------------------+------------------------------------- Reporter: vagarenko | Owner: Type: bug | Status: new Priority: high | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Windows Architecture: x86_64 | Type of failure: Compile-time (amd64) | crash Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- When compiling following code with `-O`: {{{#!hs {-# LANGUAGE FlexibleContexts, TypeFamilies, TypeSynonymInstances #-} module Bug where import Data.Word import Foreign.Storable import qualified Data.Vector.Storable as VS data Image a = Image { imageWidth :: !Int , imageHeight :: !Int , imageData :: VS.Vector (PixelBaseComponent a) } class (Storable (PixelBaseComponent a)) => Pixel a where type PixelBaseComponent a :: * instance Pixel Pixel8 where type PixelBaseComponent Pixel8 = Word8 instance Pixel Pixel16 where type PixelBaseComponent Pixel16 = Word16 data DynamicImage = ImageY8 (Image Pixel8) | ImageY16 (Image Pixel16) type Pixel8 = Word8 type Pixel16 = Word16 -- | imageMirrorY :: DynamicImage -> DynamicImage imageMirrorY dynImg = case dynImg of ImageY8 img -> ImageY8 $ mirror img 1 ImageY16 img -> ImageY16 $ mirror img 1 where mirror img channels = img { imageData = VS.concat $ reverse $ map (\y -> VS.slice (y * rowLen) rowLen (imageData img)) [0 .. imageHeight img - 1] } where rowLen = channels * imageWidth img }}} I get error: {{{ > ghc Bug -O [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) ghc: panic! (the 'impossible' happened) (GHC version 8.0.1 for x86_64-unknown-mingw32): Template variable unbound in rewrite rule Variable: cobox_s5Rv Rule "SPEC mirror @ Pixel8 @ Pixel8" Rule bndrs: [cobox_s5Rv, $dStorable_s5Rw] LHS args: [TYPE: Pixel8, TYPE: Pixel8, CO: <PixelBaseComponent Pixel8>_N, $dStorable_s5Rw] Actual args: [TYPE: Pixel8, TYPE: Pixel8, CO: <PixelBaseComponent Pixel8>_N, $fStorableWord8 `cast` ((Storable (Sym D:R:PixelBaseComponentWord8[0]))_R :: (Storable Word8 :: Constraint) ~R# (Storable (PixelBaseComponent Pixel8) :: Constraint)), img_a4IS, lvl_s7o4] Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12401> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 4

[GHC] #12125: Field accessors unnecessarily kept alive
by GHC 26 Aug '16

26 Aug '16

#12125: Field accessors unnecessarily kept alive -------------------------------------+------------------------------------- Reporter: osa1 | Owner: osa1 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: Differential Rev(s): Phab:D2270 | Wiki Page: -------------------------------------+------------------------------------- Suppose you have a record with some non-exported field accessors, like so: {{{#!haskell module Lib (Rec) where data Rec = Rec { f1 :: Int , f2 :: Int , f3 :: Int , f4 :: Int , f5 :: Int } }}} Code for fields `f1`, `f2` ... are redundantly generated, optimized etc. and they made it to the object file, causing wasted compile times and bloated object files. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12125> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 3

[GHC] #11442: Segfault when showing (undefined :: Type)
by GHC 26 Aug '16

26 Aug '16

#11442: Segfault when showing (undefined :: Type) -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Keywords: | Operating System: Linux Architecture: x86_64 | Type of failure: Compile-time (amd64) | crash Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This [https://ghc.haskell.org/trac/ghc/ticket/11311?replyto=3#comment:3 comment] made me wonder about the relationship between `Void` and `Type`, `id :: Void -> Void` and `id :: Type -> Type`: https://ghc.haskell.org/trac/ghc/ticket/11311?replyto=3#comment:3 {{{#!hs {-# LANGUAGE TypeSynonymInstances, FlexibleInstances #- import Data.Kind (Type) instance Show Type where show _ = "..." main = print (undefined :: Type) }}} running gives: {{{ $ runghc --version runghc 8.1.20160113 $ runghc -ignore-dot-ghci Segfault.hs Segmentation fault (core dumped) }}} Verbose log attached. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11442> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

1 3