October 2015 - ghc-tickets

[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] #9718: Avoid TidyPgm predicting what CorePrep will do
by GHC 10 Dec '18

10 Dec '18

#9718: Avoid TidyPgm predicting what CorePrep will do -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 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 `TidyPgm` pass is forced to predict, accurately but unpleasantly, some aspects of what `CorePrep` and `Core2Stg` will do. Reason: * Each `Id` in the interface file records (among other things) the '''arity''' of the `Id`, and whether it has '''CAF references''' * We really only know these two things for sure after `CorePrep`. The conversion from Core to STG makes no structural changes. * However the result of `TidyPgm` (which preceded `CorePrep`) is used to generate the interface file. So it has to predict the arity and CAF-ref status of each `Id`. * This is not good. It restricts what `CorePrep` can do (notably, it must not change the arity of a top-level `Id`), and it leads to unsavoury code (e.g. look at the call to `CorePrep.cvtLitInteger` in `TidyPgm.cafRefsL`. It's also dangerous: an inconsistency could lead to a crash. This is a long-standing problem. My current thought for how to unravel it is this: * `TidyPgm` does not attach arity or CAF-ref info. * Instead, run `CorePrep` after `TidyPgm`, and generate accurate arity and CAF-ref info * Then use that auxiliary mapping during the conversion from tidied program to `ModIface`. I don't think this would be hard. It would mean that the tidied program and the core-prep'd program would have to exist in memory at the same time. An alternative would be to generate the `ModIface` from the tidied program ''sans'' arity and CAF-ref info, and then, after `CorePrep` run over it to add arity and CAF-ref info. (You'd have to do this before generating the fingerprints.) The advantage of this is that the `ModIface` can be a lot smaller than the code for the entire module. A long-standing wart which needs some careful attention. Simon -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9718> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8109: Type family patterns should support as-patterns.
by GHC 09 Dec '18

09 Dec '18

#8109: Type family patterns should support as-patterns. ------------------------------------+------------------------------------- Reporter: carlhowells | Owner: Type: feature request | 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 recently wrote code similar to the following: {{{ {-# LANGUAGE DataKinds, ScopedTypeVariables, TypeOperators, TypeFamilies #-} {-# LANGUAGE PolyKinds, FlexibleInstances, FlexibleContexts #-} import GHC.TypeLits data P n = P fromNat :: forall (p :: Nat -> *) (n :: Nat). SingI n => p n -> Integer fromNat _ = fromSing (sing :: Sing n) class C (a :: [Nat]) where type T a :: * val :: p a -> T a instance SingI n => C '[n] where type T '[n] = Integer val _ = fromNat (P :: P n) instance (SingI n, C (n2 ': ns)) => C (n ': n2 ': ns) where type T (n ': n2 ': ns) = (Integer, T (n2 ': ns)) val _ = (fromNat (P :: P n), val (P :: P (n2 ': ns))) }}} There were semantic constraints in my case that made an empty list nonsensical, so it wasn't an appropriate base case. But the resulting effort in ensuring type family patterns didn't overlap got unwieldy. I would have much preferred to write that second instance more like this: {{{ instance (SingI n, C ns) => C (n ': ns@(_ ': _)) where type T (n ': ns) = (Integer, T ns) val _ = (fromNat (P :: P n), val (P :: P ns)) }}} The reasoning here is identical to as-patterns in value-level pattern matching. If you only want to match an expression when it has a particular sub-structure, it's way more convenient to do it with an as-pattern. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8109> 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 05 Dec '18

05 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] #9221: (super!) linear slowdown of parallel builds on 40 core machine
by GHC 04 Dec '18

04 Dec '18

#9221: (super!) linear slowdown of parallel builds on 40 core machine -------------------------+------------------------------------------------- Reporter: | Owner: carter | Status: new Type: bug | Milestone: Priority: high | Version: 7.8.2 Component: | Operating System: Unknown/Multiple Compiler | Type of failure: Compile-time performance bug Keywords: | Test Case: Architecture: | Blocking: Unknown/Multiple | Difficulty: | Unknown | Blocked By: | Related Tickets: | -------------------------+------------------------------------------------- im seeing slowdowns in parallel builds of a (simple!!) 6 module project when I build it on a 40 core server i'm using for work. for any given ghc invocation with -jn, once n>10, i start to see a super linear slow down as a function of n heres some basic numbers at -j1 0m2.693s at -j4 0m2.507s at -j10 0m2.763s at -j25 0m12.634s at -j30 : 0m39.154s at -j40 : 0m57.511s at -j60 : 2m21.821s these timings are another 2-4x worse if ghc is invoked indirectly via cabal-install / setup.hs according to the linux utility latencytop, 100% of ghc's cpu time was spent on user-space lock contention when I did the -j40 invocation. the timing in the -j40 case stayed the same even when ghc was also passed -O0 (and -fforce-recomp to ensure it did the same ) a bit of experimentation makes me believe that in *ANY* cabalized project on a 40 core machine will exhibit this perf issue. cabal clean ; cabal configure --ghc-options="-j" ; cabal build -j1 should be enough to trigger the lock contention. That said, I'll try to cook up a minimal repro that i can share the source for post haste. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9221> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10739: Resuscitate the humble ticky-ticky profiler
by GHC 04 Dec '18

04 Dec '18

#10739: Resuscitate the humble ticky-ticky profiler -------------------------------------+------------------------------------- Reporter: bgamari | Owner: Type: task | 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: | -------------------------------------+------------------------------------- The [https://ghc.haskell.org/trac/ghc/wiki/Debugging/TickyTicky ticky- ticky profiler] is known to be quite bit-rotted. Indeed there currently isn't even a definitive list of the counters, much less what they counter and whether they are still accurate. That being said, it can be quite useful and is worth putting effort into bringing back up to grade. There are a few things that need to be done here, 1. Collect a list of the existing counters 2. Work out what they are supposed to be measuring 3. Decide which still have a chance of being useful and accurate 4. ??? 5. Profile! -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10739> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10930: missing empty-Enumeration warning for `Natural`
by GHC 03 Dec '18

03 Dec '18

#10930: missing empty-Enumeration warning for `Natural` -------------------------------------+------------------------------------- Reporter: hvr | Owner: Type: bug | Status: new Priority: low | Milestone: 8.0.1 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: #7881, #10929 Differential Rev(s): | -------------------------------------+------------------------------------- -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10930> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9505: Bounded instance for Word (and possibly others) uses unboxed literals
by GHC 03 Dec '18

03 Dec '18

#9505: Bounded instance for Word (and possibly others) uses unboxed literals -------------------------------------+------------------------------------- Reporter: schyler | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | 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: | Differential Revisions: -------------------------------------+------------------------------------- There's a comment above saying GHC won't optimise. I think since 7.8 they are unboxed by default, so that comment and the explicit constructors can be removed. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9505> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8043: Feature Request : Qualified module exports
by GHC 01 Dec '18

01 Dec '18

#8043: Feature Request : Qualified module exports ------------------------------------+------------------------------------- Reporter: erikd | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.7 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: Other Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- I quite like the practice of using `-XNoImplicitPrelude` and then defining my own (often per-project) Prelude doing things like: {{{ module MyProject.Prelude ( module P ) import Data.Text as P (Text) import Prelude as P hiding (head, init, tail, last) }}} As an extension to the above idea, I would like to add qualified module exports defined so that I can do this: {{{ module MyProject.Prelude ( module P , qualified module T ) import Data.Text as P (Text) import Prelude as P hiding (head, init, tail, last) import qualified Data.Text as T }}} so that any file which imports `MyProject.Prelude` will then have all the identifiers that they would normally get if they did: {{{ import qualified Data.Text as T }}} Is this something that would have to go through the Haskell Next procedure? If so, how does one get that kicked off? -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8043> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #8177: Roles for type families
by GHC 29 Nov '18

29 Nov '18

#8177: Roles for type families ------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | 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: | ------------------------------------+------------------------------------- Now that we have [wiki:Roles roles], it might be helpful to be able to give a role signature for a data/type family. At the moment, data/type family constructors have all parameters conservatively assigned to be role `N`. Thus {{{ data family D a -- Parameter conservatively assumed to be N class C a where op :: D a -> a instance C Int where .... newtype N a = MkN a deriving( C ) -- Rejected }}} The generalised-newtype-deriving clause `deriving( C )` is rejected because `D` might use its parameter at role `N` thus: {{{ data instance D [b] = MkD (F b) -- F is a type function }}} It would be strictly more expressive if we could * '''Declare''' the roles of D's arguments (as we can declare their kinds). E.g. {{{ data family D a@R }}} * '''Check''' that each family instance obeys that role signature. E.g. given the preceding role signature, reject this instance: {{{ data instance D [b] = MkD (F b) -- F is a type function }}} I think there is no technical difficulty here. Just a question of doing it. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8177> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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