March 2018 - ghc-tickets

[GHC] #10859: Generated Eq instance associates && wrongly
by GHC 18 Sep '18

18 Sep '18

#10859: Generated Eq instance associates && wrongly -------------------------------------+------------------------------------- Reporter: nomeata | Owner: simonpj Type: bug | Status: new Priority: low | 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: #10858 Differential Revisions: | -------------------------------------+------------------------------------- Currently, we derive this code for `(==)`: {{{ (==) (ImportDecl a1_acXi a2_acXj a3_acXk a4_acXl a5_acXm a6_acXn a7_acXo a8_acXp) (ImportDecl b1_acXq b2_acXr b3_acXs b4_acXt b5_acXu b6_acXv b7_acXw b8_acXx) = (((((((((a1_acXi == b1_acXq)) && ((a2_acXj == b2_acXr))) && ((a3_acXk == b3_acXs))) && ((a4_acXl == b4_acXt))) && ((a5_acXm == b5_acXu))) && ((a6_acXn == b6_acXv))) && ((a7_acXo == b7_acXw))) && ((a8_acXp == b8_acXx))) }}} To me this looks wrongly associated: Since `&&` is strict in the left but lazy in the right argument, shouldn’t this be associated the other way around? The compiler might clean it up later, but why not produce it correctly right away? The culprit is this line: {{{ = foldl1 and_Expr (zipWith3Equal "nested_eq" nested_eq tys as bs) }}} and I was just about to change that ot `foldr1`, but then I did some git- archeology and found that Simon changed that deliberately from `foldr1` to `foldl1` in changeset:8de16184643ea3c2f9f30b5eaed18db6ef247760. Simon, do you still recall what you were thinking when you applied this commit ... 18½years ago? :-) -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10859> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13617: Segfault in Windows GHCi involving C code compiled with -O4
by GHC 17 Sep '18

17 Sep '18

#13617: Segfault in Windows GHCi involving C code compiled with -O4 ----------------------------------------+------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 8.0.2 Keywords: | Operating System: Windows Architecture: Unknown/Multiple | Type of failure: GHCi crash Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: ----------------------------------------+------------------------------- This is a very elusive bug that I noticed when running an `hmatrix` example under GHCi on Windows. Luckily, this example can be reduced down to a couple of `.hs` files and a single `.c` file (with no other Haskell or C dependencies). Unfortunately, I can't quite figure out a way to reproduce this bug without `cabal`, so I've put the source code at https://github.com/RyanGlScott/hmatrix-segfault. You can reproduce this bug by doing the following: {{{ $ git clone https://github.com/RyanGlScott/hmatrix-segfault $ git reset 2bfe38f964fca64dd776993c89ec59d35fb368a5 $ cd hmatrix-segfault/ $ cabal install $ runghc exe/Main.hs Access violation in generated code when reading ffffffffffffffff }}} Running `Main.hs` in GHCi crashes, whereas compiling it works: {{{ $ ghc exe/Main.hs $ ./exe/Main.exe [1,1,1,1,1,1,1,1,1,1,1,1] }}} I've reproduced this with GHC 7.10.3, 8.0.2, and 8.2.1-rc1. There are a couple of things that appear to be necessary to trigger the segfault: 1. You need to have `-O4` under `cc-options` in `hmatrix-segfault.cabal`. 2. You need to define the [https://github.com/RyanGlScott/hmatrix- segfault/blob/2bfe38f964fca64dd776993c89ec59d35fb368a5/src/Internal/Vectorized.hs#L38 FunCodeS] datatype. Removing either of these things causes the program to work again under GHCi. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13617> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12457: Deriving should be (more closely) integrated with other metaprogramming methods
by GHC 17 Sep '18

17 Sep '18

#12457: Deriving should be (more closely) integrated with other metaprogramming methods -------------------------------------+------------------------------------- Reporter: mpickering | 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: -------------------------------------+------------------------------------- It is unfortunate that if we want to (efficiently) derive a new type class then we have to bake the derivation logic into GHC itself. It seems desirable to be able to implement new deriving methods in libraries in some manner. Ryan Scott says: > I think this would be a wonderful thing to have. Matthew Pickering > (cc'd) has expressed a desire to have all the logic for the `bespoke` > deriving strategies compartmentalized into a library that could easily > be expanded on in the future to support more typeclasses in base. > (Bifunctor, anyone?) > > Unfortunately, each of the major players in today's Haskell > metaprogramming scene that I'm aware of have some downfalls that make > them unsuitable as `deriving` replacements: > > * Template Haskell: Not portable. Staging issues make it hard to use as a drop-in replacement for the `deriving` keyword > * GHC generics: Can't express all the optimizations that the bespoke `deriving` algorithms perform. Unperformant. > * Haskell preprocessors: Difficult to integrate in a typical GHC workflow. Probably wouldn't have all the metadata you'd need to be feature-complete with what `deriving` does today. > > The way I see it, the whole `deriving` business as it currently stands > today is a somewhat-grotesque-but-darn-useful hack that gets around > the lack of a truly nice metaprogramming facility in Haskell. I'm > holding out hope that the work in > https://github.com/shayan-najd/NativeMetaprogramming makes things > nicer soon, and then we can revisit this idea. Until then, > -XDerivingStrategies provides a way to contain some of the madness of > `deriving` after having many features tacked onto it in recent GHC > releases. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12457> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #13862: Optional "-v" not allowed with :load in GHCi
by GHC 16 Sep '18

16 Sep '18

#13862: Optional "-v" not allowed with :load in GHCi -------------------------------------+------------------------------------- Reporter: vanto | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 8.0.2 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: -------------------------------------+------------------------------------- When loading a file in GHCi with the command {{{:load}}} and the file must import another file, if that other file is unavailable then GHCi sends the following error\\ {{{ Failed to load interface for `xxx` Use -v to see a list of the files searched for. }}} ' xxx ' is the name of the imported file that GHCi cannot find.\\ But we can not use a flag ( ie -v) with the command {{{:load}}} in GHCi.\\ This error is not appropriate in GHCi when using {{{:load}}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13862> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14921: Type inference breaks on constraint-kinded parameter used by a Rank-2 callback
by GHC 16 Sep '18

16 Sep '18

#14921: Type inference breaks on constraint-kinded parameter used by a Rank-2 callback -------------------------------------+------------------------------------- Reporter: glittershark | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 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: -------------------------------------+------------------------------------- Apologies in advance if the terminology in the summary is incorrect, I'm not really sure of the proper terms to use here The following code: {{{#!hs {-# LANGUAGE AllowAmbiguousTypes #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE ConstraintKinds #-} {-# LANGUAGE ScopedTypeVariables #-} module Scratch where import Prelude data Foo = FooA Int | FooB String useFoo :: forall cls a . (cls Int, cls String) => (forall k . cls k => k -> a) -> Foo -> a useFoo f (FooA a) = f a useFoo f (FooB b) = f b }}} seems like it should be able to infer the `cls` type parameter to `useFoo` based on the first argument, but instead calling with `kshow` gives the following error message: {{{#!hs >>> useFoo show (FooA 1) <interactive>:52:1: error: • Could not deduce: (cls0 Int, cls0 String) arising from a use of ‘useFoo’ • In the expression: useFoo show (FooA 1) In an equation for ‘it’: it = useFoo show (FooA 1) <interactive>:52:8: error: • Couldn't match type ‘a’ with ‘String’ ‘a’ is untouchable inside the constraints: cls0 k bound by a type expected by the context: forall k. cls0 k => k -> a at <interactive>:52:1-20 ‘a’ is a rigid type variable bound by the inferred type of it :: a at <interactive>:52:1-20 Possible fix: add a type signature for ‘it’ Expected type: k -> a Actual type: k -> String • In the first argument of ‘useFoo’, namely ‘show’ In the expression: useFoo show (FooA 1) In an equation for ‘it’: it = useFoo show (FooA 1) • Relevant bindings include it :: a (bound at <interactive>:52:1) }}} Type-applying `useFoo` works as expected: {{{#!hs >>> :set -XTypeApplications >>> useFoo @Show show (FooA 1) "1" }}} This broke both with and without ScopedTypeVariables -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14921> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10651: Type checking issue with existential quantification, rank-n types and constraint kinds
by GHC 16 Sep '18

16 Sep '18

#10651: Type checking issue with existential quantification, rank-n types and constraint kinds -------------------------------------+------------------------------------- Reporter: Roboguy | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 (Type checker) | Operating System: Unknown/Multiple Keywords: | Type of failure: GHC rejects Architecture: | valid program Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- I'm trying to create a library for working with lists with existentially quantified types and I ran into an issue implementing `mapM_` for my type (note that the `map` and `mapM` implementations both work fine): {{{#!hs {-# LANGUAGE RankNTypes, ExistentialQuantification, ConstraintKinds #-} data ConstrList c = forall a. c a => a :> ConstrList c | CNil infixr :> constrMap :: (forall a. c a => a -> b) -> ConstrList c -> [b] constrMap f (x :> xs) = f x : constrMap f xs constrMap f CNil = [] constrMapM :: Monad m => (forall a. c a => a -> m b) -> ConstrList c -> m [b] constrMapM f = sequence . constrMap f -- Doesn't work, even with the definition as undefined (?) constrMapM_ :: Monad m => (forall a. c a => a -> m b) -> ConstrList c -> m () constrMapM_ = undefined }}} It fails with this error message: {{{ /users/dyoung/Test.hs:16:16: Couldn't match type ‘b0’ with ‘b’ ‘b0’ is untouchable inside the constraints (c a) bound by the type signature for constrMapM_ :: c a => a -> m b0 at /users/dyoung/Test.hs:16:16-77 ‘b’ is a rigid type variable bound by the type signature for constrMapM_ :: Monad m => (forall a. c a => a -> m b) -> ConstrList c -> m () at /users/dyoung/Test.hs:16:16 Expected type: a -> m b0 Actual type: a -> m b In the ambiguity check for the type signature for ‘constrMapM_’: constrMapM_ :: forall (c :: * -> Constraint) (m :: * -> *) b. Monad m => (forall a. c a => a -> m b) -> ConstrList c -> m () To defer the ambiguity check to use sites, enable AllowAmbiguousTypes In the type signature for ‘constrMapM_’: constrMapM_ :: Monad m => (forall a. c a => a -> m b) -> ConstrList c -> m () Failed, modules loaded: none. }}} If I add `AllowAmbiguousTypes`, it works when the definition is `undefined` but it doesn't work if I put the real definition in. I believe that this is accepted and works as I had intended on 7.8.3 (with the correct definition as well) after talking to some people on IRC, but I haven't personally tested it. bitemyapp on IRC noticed that it does compile on 7.10.1 if we add an extra argument of type `b` to both `constrMap_` and `constrMapM` and pass that value from `constrMapM_` to `constrMap` (I think all of those things must happen in order for this to work): {{{#!hs constrMapM :: Monad m => b -> (forall a. c a => a -> m b) -> ConstrList c -> m [b] constrMapM ignoredB f = sequence . constrMap f constrMapM_ :: Monad m => b -> (forall a. c a => a -> m b) -> ConstrList c -> m () constrMapM_ b f xs = fmap (const ()) (constrMapM b f xs) }}} Is this expected behavior? I don't fully understand the new ambiguity check. This kind of reminds me of linear types (or a relevant type system) in a way: we must make explicit use of the `b` to "discharge" it in order to have a valid type. Also, I notice that this works fine as well, which makes me think it the type error above is related to `ConstraintKinds`: {{{#!hs data AnyList f = forall a. (f a) :| (AnyList f) | Nil infixr :| anyMap :: (forall a. f a -> b) -> AnyList f -> [b] anyMap f (x :| xs) = f x : anyMap f xs anyMap _ Nil = [] anyMapM :: Monad m => (forall a. f a -> m b) -> AnyList f -> m [b] anyMapM f xs = sequence (anyMap f xs) anyMapM_ :: Monad m => (forall a. f a -> m b) -> AnyList f -> m () anyMapM_ f xs = fmap (const ()) (anyMapM f xs) }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10651> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #12565: unhelpful error message about enabling TypeApplications
by GHC 14 Sep '18

14 Sep '18

#12565: unhelpful error message about enabling TypeApplications -------------------------------------+------------------------------------- Reporter: mauke | Owner: Type: bug | Status: new 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: -------------------------------------+------------------------------------- {{{ % cat bug.hs }}} {{{#!hs {-# LANGUAGE TypeApplications #-} main = return (id@() ()) }}} {{{ % ghc bug.hs [1 of 1] Compiling Main ( bug.hs, bug.o ) bug.hs:2:16: error: Pattern syntax in expression context: id@() Did you mean to enable TypeApplications? }}} This error message makes no sense because `TypeApplications` is already enabled. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12565> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14770: Allow static pointer expressions to have static pointer free variables
by GHC 14 Sep '18

14 Sep '18

#14770: Allow static pointer expressions to have static pointer free variables -------------------------------------+------------------------------------- Reporter: TheKing01 | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Keywords: | Operating System: Unknown/Multiple StaticPointers | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Static pointer expressions can not have free variables. For example: {{{#!hs addPointers :: StaticPtr Int -> StaticPtr Int -> StaticPtr Int addPointers x y = static (deRefStaticPtr x + deRefStaticPtr y) }}} would be invalid, since `x` and `y` are free variables. My proposal would be to make this code sample valid. In particular, my proposal is to allow free variables in `static` expressions if the free variables of the type `StaticPtr a` for some type `a`. Let's say that `m` `n` are Static pointers to integers. The serialized form would of `addPointers m n` would be: A pointer to the code `deRefStaticPtr x + deRefStaticPtr y` The serialized form of `m`, associated with `x`. The serialized form of `n`, associated with `y`. When this is sent over the wire and dereferenced, the other machine would Find `m` and `n`'s pointers Find the code `deRefStaticPtr x + deRefStaticPtr y` Evaluate `deRefStaticPtr x + deRefStaticPtr y`, substituting `m`'s value for `x`, and `n`'s value for y. The reason this feature would be useful is would let the user modify and combine static pointers. You might ask "why not let the user create a GADT to do that (like in https://hackage.haskell.org/package/distributed-static-0.3.8/docs/Control- Distributed-Static.html#t:Static)?" The reason is that then it won't fuse (with recreating every fusion rule for that GADT). For example, if `m` is `static 7` and `n` is `static 3`, then `addPointers m n` can fuse to `static 11`. This is more important then it seems. For example, say you have `composePointers :: StaticPtr (a -> b) -> StaticPtr (b -> c) -> StaticPtr (a -> c)`, and you compose a bunch of pointers. With the GADTS, every `composePointers` would be its own node, but with GHC support, it would probably fuse. In general, dealing with a GADT requires a lot of "glue code" that can be eliminated with GHC support. I'm not sure if this proposal makes a performance difference (it probably makes it more efficient), but it would make code more elegant, cutting out all the glue code. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14770> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14313: “Result signatures are no longer supported in pattern matches” lost
by GHC 14 Sep '18

14 Sep '18

#14313: “Result signatures are no longer supported in pattern matches” lost -------------------------------------+------------------------------------- Reporter: nomeata | Owner: (none) Type: task | Status: new Priority: low | 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: #2310 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This commit (b98ff25f4c8cb4bf18b784c848fabaaa6e4b11b8) from two years ago broke the error message (at least according to when `T2310.stderr` was modified): {{{ T2310.hs:5:22: Illegal result type signature ‘a’ Result signatures are no longer supported in pattern matches In a lambda abstraction: \ x :: a -> (x :: a) }}} that was displayed previously. Either we do want that error message (then this is a bug) or we don’t care about this error message any more, then probably we can remove the `m_type` field of `Match` in `HsExpr`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14313> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #14229: Contraditions in users_guide/using-warnings.html
by GHC 14 Sep '18

14 Sep '18

#14229: Contraditions in users_guide/using-warnings.html -------------------------------------+------------------------------------- Reporter: | Owner: (none) MikolajKonarski | Type: bug | Status: new Priority: low | Milestone: Component: Documentation | Version: 8.2.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Documentation Unknown/Multiple | bug Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Lists are not updated in https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/using- warnings.html E.g., it says "The warnings that are not enabled by -Wall are ...-Wredundant-constraints", but then it says "This option is on by default". Also "-Worphans" is on none of the list of enabled or not enabled flags and also there is no mention in its description if its enabled by default or not. I guess there may be more. Also, it may be worth mentioning that obsolete warnings or warnings subsumed by others are not on any of the list, or whatever the actual rule is. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14229> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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