April 2016 - ghc-tickets

[GHC] #9374: Investigate Static Argument Transformation
by GHC 29 Oct '18

29 Oct '18

#9374: Investigate Static Argument Transformation -------------------------------------+------------------------------------- Reporter: jstolarek | Owner: Type: task | Status: new Priority: lowest | Milestone: Component: Compiler | Version: 7.9 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: Blocked By: | None/Unknown Related Tickets: | Test Case: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- At the moment the Static Argument Transformation (SAT) optimisation is not run unless explicitly enabled with `-fstatic-argument-transformation`. The comment in DynFlags says: {{{ Max writes: I think it's probably best not to enable SAT with -O2 for the 6.10 release. The version of SAT in HEAD at the moment doesn't incorporate several improvements to the heuristics, and I'm concerned that without those changes SAT will interfere with some attempts to write "high performance Haskell", as we saw in some posts on Haskell-Cafe earlier this year. In particular, the version in HEAD lacks the tail call criterion, so many things that look like reasonable loops will be turned into functions with extra (unneccesary) thunk creation. }}} 6.10 was a long time ago. Has anything changed since then? Does it make sense to enable that optimisation now? What are the mentioned heuristics and were they finally implemented? Does anyone know what Haskell-cafe posts does Max refer to? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9374> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11719: Cannot use higher-rank kinds with type families
by GHC 29 Oct '18

29 Oct '18

#11719: Cannot use higher-rank kinds with type families -------------------------------------+------------------------------------- Reporter: ocharles | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1-rc2 (Type checker) | 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 ticket came out of a discussion with Richard in this mailing list post: https://mail.haskell.org/pipermail/haskell- cafe/2016-March/123462.html Here's the code that should work, but doesn't: {{{ import Data.Kind data TyFun :: * -> * -> * type a ~> b = TyFun a b -> * type family (f :: a ~> b) @@ (x :: a) :: b data Null a = Nullable a | NotNullable a type family ((f :: b ~> c) ∘ (g :: a ~> b)) (x :: a) :: c where (f ∘ g) x = f @@ (g @@ x) type family BaseType (k :: forall a. a ~> Type) (x :: b) :: Type where -- this fails :( -- BaseType k x = (@@) k x }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11719> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9163: Ptr should have a phantom role
by GHC 28 Oct '18

28 Oct '18

#9163: Ptr should have a phantom role ------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- In `GHC.Ptr` we see {{{ type role Ptr representational data Ptr a = Ptr Addr# deriving (Eq, Ord) }}} with no comments. Why is `Ptr` representational? In the same module we have `castPtr`: {{{ castPtr :: Ptr a -> Ptr b castPtr (Ptr addr) = Ptr addr }}} which unpacks and repacks a `Ptr`. If `Ptr` was phantom, we could use `coerce`. And that in turn would actually make a lot of code more efficient – there are lots of calls to `castPtr`. Specifically, in `nofib`, I tried implementing `castPtr` with `unsafeCoerce`. Then I found: * 12% less allocation in `reverse-complem` * 7.3% less allocation in `fasta`. * Binary sizes fell 0.1%. Both these benchmarks are ones that do a ''lot'' of I/O, and it turns out that `GHC.IO.Handle.Text.$wa1` has a `castPtr` that (all by itself) accounts for 12% of `reverse-complem`'s total allocation! So making `castPtr` free is good. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9163> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11238: Redesign the dynamic linking on ELF systems
by GHC 27 Oct '18

27 Oct '18

#11238: Redesign the dynamic linking on ELF systems -------------------------------------+------------------------------------- Reporter: trommler | Owner: trommler Type: task | Status: new Priority: normal | Milestone: 8.2.1 Component: Runtime | Version: 7.10.3 System (Linker) | Keywords: | Operating System: Linux Architecture: | Type of failure: Other Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This ticket is to follow up on my comment on #11042. Let's give dynamic linking on ELF systems a second chance. I propose the following steps: 1. Collect required features 1. Write up and discuss and agree on a specification 1. Create regression tests for all features 1. Design and implement a new dynamic linker for GHCi 1. (optional) Make it work on Mac OS X too -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11238> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10353: Haddock for Data.List should list instances
by GHC 26 Oct '18

26 Oct '18

#10353: Haddock for Data.List should list instances -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: | Version: 7.10.1 Documentation | Operating System: Unknown/Multiple Keywords: | Type of failure: None/Unknown Architecture: | Blocked By: Unknown/Multiple | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- I'd like to know the set of classes for which there are instances for `[]`. So, I naturally looked in the documentation for `Data.List`. However, because `Data.List` doesn't export `[]`, this list isn't there. Can this be fixed? (It's an honest question -- because of `[]`'s special syntax, I could see this fix being non-trivial.) -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10353> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10453: \case should trigger auto-multiline mode in ghci
by GHC 24 Oct '18

24 Oct '18

#10453: \case should trigger auto-multiline mode in ghci -------------------------------------+------------------------------------- Reporter: dmwit | Owner: Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.10.1 Component: Compiler | Operating System: Unknown/Multiple Keywords: | Type of failure: None/Unknown Architecture: | Blocked By: Unknown/Multiple | Related Tickets: Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- In ghci, most block heralds trigger the multiline mode when :set +m is on. However, \case does not (and should). Compare: {{{ Prelude> ($True) (\case Prelude| x -> x Prelude| ) True Prelude> ($True) $ \case <interactive>:11:11: Empty list of alternatives in case expression Use EmptyCase to allow this }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10453> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11307: Regresssion: parsing type operators
by GHC 17 Oct '18

17 Oct '18

#11307: Regresssion: parsing type operators -------------------------------------+------------------------------------- Reporter: alanz | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.11 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: -------------------------------------+------------------------------------- GHC 7.10.3 will parse (but not type-check) the following {{{#!hs {-# LANGUAGE TypeOperators #-} type family (r1 :++: r2); infixr 5 :++: type instance r :++: Nil = r type instance r1 :++: r2 :> a = (r1 :++: r2) :> a }}} Current master (c7830bdb14e0a85a78617314156d101155fdf7aa) fails with {{{#!hs /tmp/Foo.hs:5:15: error: Malformed head of type or class declaration: r1 :++: r2 :> a }}} @simonpj comment on ghc-devs mailing list > `| type instance r1 :++: r2 :> a = (r1 :++: r2) :> a` > > What do you expect this to mean? I suppose you could hope that GHC will > unravel the fixity of :++: and :>, to determine whether you are giving an > instance for :++: or for :>? > > That sounds reasonable, but it's not trivial. > > Currently the LHS of a 'type instance' decl is ultimately a TyFamEqn, and > it looks like this: > > {{{#!hs > data TyFamEqn name pats > = TyFamEqn > { tfe_tycon :: Located name > , tfe_pats :: pats > , tfe_rhs :: LHsType name } > }}} > > So we've already decided (in the parser) what the type-function name is. > But we can't do that in this case, because the parser doesn't understand > fixity. > > To deal with this we'd need to change to > > {{{#!hs > data TyFamEqn name pats > = TyFamEqn > { tfe_lhs :: LHSType name > , tfe_rhs :: LHsType name } > }}} > > so that the LHS was just a type. Now the renamer can re-jiggle its > fixities in the usual way, and only in the type checker will we need to > decide exactly which type it's an instance of. Easy! > > This might be a good change to make! It's a bit like in ClsInstDecl, you'll > see that the instance cid_poly_ty is just a LHsSigType, i.e. not decomposed > into which class it is. > -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11307> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8763: forM_ [1..N] does not get fused (10 times slower than go function)
by GHC 13 Oct '18

13 Oct '18

#8763: forM_ [1..N] does not get fused (10 times slower than go function) ------------------------------+-------------------------------------------- Reporter: nh2 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Runtime performance bug Unknown/Multiple | Test Case: Difficulty: Unknown | Blocking: Blocked By: | Related Tickets: | ------------------------------+-------------------------------------------- Apparently idiomatic code like {{{forM_ [1.._N]}}} does not get fused away. This can give serious performance problems when unnoticed. {{{ -- Slow: forM_ [0.._N-1] $ \i -> do ... -- Around 10 times faster: loop _N $ \i -> do ... {-# INLINE loop #-} loop :: (Monad m) => Int -> (Int -> m ()) -> m () loop bex f = go 0 where go !n | n == bex = return () | otherwise = f n >> go (n+1) }}} Full code example: https://gist.github.com/nh2/8905997 - the relevant alternatives are commented. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8763> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #11368: Pattern synonym name is mangled when patterns are non-exhaustive
by GHC 10 Oct '18

10 Oct '18

#11368: Pattern synonym name is mangled when patterns are non-exhaustive ---------------------------------------+--------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: low | Milestone: Component: Compiler | Version: 8.1 Keywords: PatternSynonyms | Operating System: Linux Architecture: x86 | Type of failure: Other Test Case: | Blocked By: Blocking: | Related Tickets: #11367 Differential Rev(s): | Wiki Page: ---------------------------------------+--------------------------- {{{ pattern A n <- ... where A 0 = ... }}} When a pattern synonym is non-exhaustive its name is mangled: {{{ >>> A 3 "*** Exception: /tmp/tmp.t0h0pMgwWb.hs:(4,1)-(5,14): Non-exhaustive patterns in function $bA }}} “Non-exhaustive patterns in pattern synonym A” [https://ghc.haskell.org/trac/ghc/ticket/11367?replyto=5#comment:1 Comment] from ticket #11367. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11368> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10783: Partial type signatures should work with pattern synonym signatures
by GHC 10 Oct '18

10 Oct '18

#10783: Partial type signatures should work with pattern synonym signatures -------------------------------------+------------------------------------- Reporter: mpickering | Owner: Type: bug | Status: new 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 Revisions: | -------------------------------------+------------------------------------- {{{#!hs {-# LANGUAGE PatternSynonyms #-} pattern MyJust :: _ pattern MyJust a = Just a }}} GHC can correctly infer the type for the pattern without the type signature so (to me) it is unexpected that this inferred type is not reported to me when using partial type signatures. It just seems to not be implemented currently. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10783> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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