February 2018 - ghc-tickets

[GHC] #11534: Allow class associated types to reference functional dependencies
by GHC 22 Dec '18

22 Dec '18

#11534: Allow class associated types to reference functional dependencies -------------------------------------+------------------------------------- Reporter: ekmett | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 (Type checker) | Keywords: TypeFamilies, | Operating System: Unknown/Multiple FunctionalDependencies | Architecture: | Type of failure: Other Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider: {{{#!hs class Foo i j | i -> j where type Bar i :: * type Bar i = j }}} This is rejected as `j` is not an argument to `Bar`, but when we realize any instance `j` will be a known type, determined by `i`, as told to us by the | i -> j functional dependency on the class. This would likely require some sort of extra pass over the instances with class associated types of this form to elaborate them, or a slight modification of the default signature implementation for class associated types. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11534> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8949: switch -msse2 to be on by default
by GHC 21 Dec '18

21 Dec '18

#8949: switch -msse2 to be on by default ----------------------------------------------+---------------------------- Reporter: errge | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (CodeGen) | Version: 7.9 Keywords: | Operating System: Architecture: x86 | Unknown/Multiple Difficulty: Moderate (less than a day) | Type of failure: Other Blocked By: | Test Case: Related Tickets: | Blocking: ----------------------------------------------+---------------------------- I propose msse2 to be on by default. I guess the default was chosen way back, when Pentium III support was still relevant. Nowadays we don't really win on the CPU support, because e.g. https://github.com/tibbe/hashable/blob/master/hashable.cabal is built by default with sse2 on the injected C code level. And hashable has a lot of reverse depends, therefore on an end user system (RedHat or Debian) the user is most probably unlucky with a Pentium III CPU anyways. Flipping this default would also fix the excess precision problem for most users of GHC on the i686 platform. GHC should provide a -mno-sse2 flag for the cases when this needs to be disabled. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8949> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14794: -Weverything should not enable -Wmissing-exported-signatures
by GHC 20 Dec '18

20 Dec '18

#14794: -Weverything should not enable -Wmissing-exported-signatures -------------------------------------+------------------------------------- Reporter: MaxGabriel | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect Unknown/Multiple | error/warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The `-Weverything` compiler flag enables every compiler warning. That includes `-Wmissing-exported-signatures`, which disables `-Wmissing- signatures` (With `-Wmissing-signatures`, every top-level binding without a signature generates a warning, with `-Wmissing-exported-signatures`, only exported bindings generate a warning). While technically "every warning" includes `-Wmissing-exported- signatures`, I don't think this matches how people use `-Weverything`. At least personally I think of it as "`-Wall` isn't enough, give me the strictest possible warning settings", but actually `-Weverything` can have ''fewer'' warnings than `-Wall` because of `-Wmissing-exported- signatures`. Example reproduction case: {{{ -- Main.hs {-# LANGUAGE Safe #-} module Main (main) where import Prelude (IO, putStrLn, show, ($)) import Foo (foo) main :: IO () main = putStrLn $ show foo }}} {{{ -- Foo.hs {-# LANGUAGE Safe #-} module Foo (foo) where import Prelude (Integer) foo :: Integer foo = bar bar = (1 :: Integer) }}} Expected: when compiling with `-Weverything`, I get a superset of `-Wall`'s warnings. Actual: Actual `-Wall`: {{{ maximiliantagher@Maximilians-MacBook-Pro ~/D/C/H/ghc-warning> ghc -Wall Main.hs Foo.hs [1 of 2] Compiling Foo ( Foo.hs, Foo.o ) Foo.hs:10:1: warning: [-Wmissing-signatures] Top-level binding with no type signature: bar :: Integer | 10 | bar = (1 :: Integer) | ^^^ [2 of 2] Compiling Main ( Main.hs, Main.o ) Linking Main ... }}} Actual `-Weverything` (note: you should remove run `rm main.o main.hi main Foo.o Foo.hi` in between invocations of the ghc to avoid it using something pre-built): {{{ maximiliantagher@Maximilians-MacBook-Pro ~/D/C/H/ghc-warning> ghc -Weverything Main.hs Foo.hs [1 of 2] Compiling Foo ( Foo.hs, Foo.o ) <no location info>: warning: [-Wsafe] ‘Foo’ has been inferred as safe! [2 of 2] Compiling Main ( Main.hs, Main.o ) <no location info>: warning: [-Wsafe] ‘Main’ has been inferred as safe! Linking Main ... }}} (I'm not familiar enough with Safe Haskell to get those warnings to go away—I tried the SafeHaskell pragma but that didn't seem to work. In any case, unrelated to this issue) I tested with these versions: * GHC: The Glorious Glasgow Haskell Compilation System, version 8.2.1 * OS Version: Mac OS 10.13.3 (17D47) (High Sierra) -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14794> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8655: Evaluate know-to-terminate-soon thunks
by GHC 19 Dec '18

19 Dec '18

#8655: Evaluate know-to-terminate-soon thunks ------------------------------------+------------------------------------- Reporter: nomeata | Owner: nomeata Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- I guess I’ll better put my interior monologue in a ticket than on ghc- dev... In order to implement nested CPR (#1600), I had to enrich the CPR type to keep track of whether something, when entered, will converge for sure. My code does not solve the halting problem, but does only simple stuff, so if something is known to converge, is is very likely to be cheap. Simon suggested to measure the effect of evaluating a let-bound thunk with the known-to-terminate property, as if the body was using it strictly. This tickets tracks this suggestion, and reports progress. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8655> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8598: IO hack in demand analyzer gets in the way of CPR
by GHC 19 Dec '18

19 Dec '18

#8598: IO hack in demand analyzer gets in the way of CPR ------------------------------------+------------------------------------- Reporter: nomeata | Owner: Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- After a lot of staring at code and comparing unexpected nofib results I found the following: The IO hack in the demand analyzer (see `dmdAnalAlt` in `StrAnal.lhs` and #1592 for a good discussion) prevents CPR in any function that uses a C call. This is a small example, reduced from the `scaleFloat` method for doubles: {{{ module Float(fun) where import GHC.Float (Double(..)) import GHC.Integer (decodeDoubleInteger, encodeDoubleInteger) fun :: Double -> Double fun x | isFix = x | otherwise = case x of (D# x#) -> case decodeDoubleInteger x# of (# i, j #) -> D# (encodeDoubleInteger i j) where isFix = isDoubleFinite x == 0 foreign import ccall unsafe "isDoubleFinite" isDoubleFinite :: Double -> Int }}} Here, `fun` does current not get the CPR property, and the work gets type `GHC.Prim.Double# -> GHC.Types.Double`. Why? Because in core, there will be a {{{ case {__pkg_ccall main isDoubleFinite GHC.Prim.Double# -> GHC.Prim.State# GHC.Prim.RealWorld -> (# GHC.Prim.State# GHC.Prim.RealWorld, GHC.Prim.Int# #)}_dQD ds_dQA GHC.Prim.realWorld# of _ [Occ=Dead, Dmd=<L,A>] { (# ds_dQC [Dmd=<L,A>, OS=OneShot], ds_dQB [Dmd=<S,1*U>] #) -> ... }}} where the case body has `DmdType m {dQz-><L,1*U(U)> dQA-><L,U>}`, but `dmdAnalAlt` detects that this is evaluating a possibly exiting function and throws aways the information using `alt_ty `lubDmdType` topDmdType`. Would it be ok to `lub` only the demand on the free variables, but keep the CPR information? In nofib (if I managed to compare the right results) this does nothing for almost all benchmarks, `-9.2%` of allocations for `mandel` and `+4.9%` for reverse-complement (but these numbers are not obtained very cleanly, and all is happening on top of the better-ho-cardinality branch. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8598> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10905: Incorrect number of parameters in "role" errors
by GHC 19 Dec '18

19 Dec '18

#10905: Incorrect number of parameters in "role" errors -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Keywords: roles | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect Unknown/Multiple | warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- The following code produces the error: {{{ import Data.Coerce import Data.Functor.Trans.Tagged toTT :: Tagged t [a] -> TaggedT t [] a toTT = coerce }}} {{{ Couldn't match representation of type ‘[a]’ with that of ‘Data.Functor.Identity.Identity [a]’ arising from trying to show that the representations of ‘Tagged t [a]’ and ‘TaggedT t [] a’ are the same Relevant role signatures: type role [] representational type role Data.Functor.Identity.Identity representational type role TaggedT nominal nominal phantom representational nominal The data constructor ‘Data.Functor.Identity.Identity’ of newtype ‘Data.Functor.Identity.Identity’ is not in scope Relevant bindings include toTT :: Tagged t [a] -> TaggedT t [] a (bound at Main.hs:9:1) In the expression: coerce In an equation for ‘toTT’: toTT = coerce }}} First, kudos for correctly identifying the problem: I failed to import the `Identity` constructor. However, it seems that the report for the roles of the `TaggedT` type has too many parameters. It is defined in tagged-transformer [https://hackage.haskell.org/package/tagged-transformer-0.8/docs/Data- Functor-Trans-Tagged.html] as `newtype TaggedT s m b = TagT { untagT :: m b }`, but is listed with 5 roles. I did try to move the definition of `TaggedT/Tagged` into my module so I didn't have to import the library, but when I did the error didn't show up: TaggedT was listed with the expected roles `phantom representational nominal`. As a user, it wasn't clear to me why there are 5 roles listed for a type with 3 parameters, and I didn't know which roles I should be looking at to help me debug the `coerce` error. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10905> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8959: GHCi should honour UnicodeSyntax
by GHC 19 Dec '18

19 Dec '18

#8959: GHCi should honour UnicodeSyntax -------------------------------------------+------------------------------- Reporter: Fuuzetsu | Owner: Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 7.6.3 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Easy (less than 1 hour) | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: -------------------------------------------+------------------------------- When we load a module into GHCi with -XUnicodeSyntax, GHCi should default to printing information with the relevant unicode symbols. {{{ *Communication> let a ∷ Int; a = 7 *Communication> :t a a :: Int }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8959> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14146: Can GHC propose kind restrictions?
by GHC 17 Dec '18

17 Dec '18

#14146: Can GHC propose kind restrictions? -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 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 is UX. As my code gets more polykinded I find myself in situations such as these all the more often {{{#!hs instance (Comonad f, Representable f, Comonad g, Representable g) => Comonad (Compose f g) where extract = undefined duplicate = undefined instance ComonadApply (Compose f g) where (<@>) = undefined }}} {{{ /tmp/a.hs:20:10-35: error: • No instance for (Comonad (Compose f g)) arising from the superclasses of an instance declaration • In the instance declaration for ‘ComonadApply (Compose f g)’ | 20 | instance ComonadApply (Compose f g) where | ^^^^^^^^^^^^^^^^^^^^^^^^^^ Failed, modules loaded: none. }}} It would help me if GHC looked up a `Comonad (Compose _ _)` instance, compared the kinds {{{#!hs Comonad (Compose (f :: Type -> Type) (g :: Type -> Type) :: Type -> Type) ComonadApply (Compose (f :: k -> Type) (g :: Type -> k) :: Type -> Type) }}} a simple suggestion like this would be helpful {{{ • No instance for (Comonad (Compose f g)) arising from the superclasses of an instance declaration Try adding a kind signature (ComonadApply (Compose (f :: Type -> Type) g)). }}} Even more amazing would be {{{ • No instance for (Comonad (Compose f g)) arising from the superclasses of an instance declaration Try adding a context (Comonad f, Representable f, Comonad g, Representable g) => ComonadApply (Compose f g) }}} Which the compiler could in theory guess, since following GHC's suggestion iteratively you end up with {{{#!hs ComonadApply (Compose (f :: Type -> Type) g) -- ==> add (Comonad f) to the context of the instance declaration Comonad f => ComonadApply (Compose f g) -- ==> add (Comonad g) to the context of the instance declaration (Comonad f, Comonad g) => ComonadApply (Compose f g) -- ==> add (Representable f) to the context of the instance declaration (Comonad f, Comonad g, Representable g) => ComonadApply (Compose f g) -- ==> add (Representable f) to the context of the instance declaration (Comonad f, Comonad g, Representable g, Representable f) => ComonadApply (Compose f g) }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14146> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13992: Error message, room for improvement (polykinds)
by GHC 17 Dec '18

17 Dec '18

#13992: Error message, room for improvement (polykinds) -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: PolyKinds | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Something like {{{#!hs -- • No instance for (Show (Compose Proxy Proxy a)) -- arising from the 'deriving' clause of a data type declaration -- Possible fix: -- use a standalone 'deriving instance' declaration, -- so you can specify the instance context yourself -- • When deriving the instance for (Show (FlipProxy a)) {-# Language DerivingStrategies, GeneralizedNewtypeDeriving, PolyKinds, KindSignatures #-} import Data.Functor.Compose import Data.Proxy import Data.Kind newtype FlipProxy a = FlipProxy_ (Compose Proxy Proxy a) deriving newtype Show }}} where the solution is to constraint the kind of `a :: Type`, it would be nice if GHC could reference kind variables. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13992> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14473: Implement Underscores in Numeric Literals Proposal (NumericUnderscores extension)
by GHC 14 Dec '18

14 Dec '18

#14473: Implement Underscores in Numeric Literals Proposal (NumericUnderscores extension) -------------------------------------+------------------------------------- Reporter: takenobu | Owner: (none) Type: task | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.2.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: #13126 #9224 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Implement Underscores in Numeric Literals Proposal. GHC supports various numeric literals such as decimal, octal, hexadecimal, binary, and floating point numbers. However, large numeric literals are hard to read. This proposal improves the readability, quality, expressiveness of numeric literals. This proposal allows underscores to numeric literals when the `NumericUnderscores` language extension is enabled. Underscores (`_`) in numeric literals are simply ignored. The specification of the feature is available here:\\ https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0009 -numeric-underscores.rst For a discussion:\\ https://github.com/ghc-proposals/ghc-proposals/pull/76 -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14473> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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