August 2015 - ghc-tickets

[GHC] #8634: Code valid in GHC 7.6 is impossible to move over GHC 7.7 (because of liberal coverage condition)
by GHC 22 Nov '18

22 Nov '18

#8634: Code valid in GHC 7.6 is impossible to move over GHC 7.7 (because of liberal coverage condition) ------------------------------------+------------------------------------- Reporter: danilo2 | Owner: Type: bug | Status: new Priority: high | Milestone: Component: Compiler | Version: 7.7 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- == Abstract == Hi! I'm writing a compiler, which produces Haskell code. I've discovered it is impossible to keep currently used features / logic using GHC 7.7 instead of 7.6 Below is more detailed description of the problem: == The idea == I'm writing a [DSL][1], which compiles to Haskell. Users of this language can define own immutable data structures and associated functions. By associated function I mean a function, which belongs to a data structure. For example, user can write (in "pythonic" pseudocode): {{{#!python data Vector a: x,y,z :: a def method1(self, x): return x }}} (which is equivalent to the following code, but shows also, that associated functions beheva like type classes with open world assumption): {{{#!python data Vector a: x,y,z :: a def Vector.method1(self, x): return x }}} In this example, `method1` is a function associated with `Vector` data type, and can be used like `v.testid(5)` (where `v` is instance of `Vector` data type). I'm translating such code to Haskell code, but I'm facing a problem, which I'm trying to solve for a long time. == The problem == I'm trying to move the code from GHC 7.6 over GHC 7.7. The code works perfectly under GHC 7.6, but does not under GHC 7.7. I want to ask you how can I fix it to make it working in the new version of the compiler? == Example code == Lets see a simplified version of generated (by my compiler) Haskell code: {{{#!haskell {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE FunctionalDependencies #-} import Data.Tuple.OneTuple ------------------------------ -- data types ------------------------------ data Vector a = Vector {x :: a, y :: a, z :: a} deriving (Show) -- the Vector_testid is used as wrapper over a function "testid". newtype Vector_testid a = Vector_testid a ------------------------------ -- sample function, which is associated to data type Vector ------------------------------ testid (v :: Vector a) x = x ------------------------------ -- problematic function (described later) ------------------------------ testx x = call (method1 x) $ OneTuple "test" ------------------------------ -- type classes ------------------------------ -- type class used to access "method1" associated function class Method1 cls m func | cls -> m, cls -> func where method1 :: cls -> m func -- simplified version of type class used to "evaluate" functions based on -- their input. For example: passing empty tuple as first argument of `call` -- indicates evaluating function with default arguments (in this example -- the mechanism of getting default arguments is not available) class Call a b where call :: a -> b ------------------------------ -- type classes instances ------------------------------ instance (out ~ (t1->t1)) => Method1 (Vector a) Vector_testid out where method1 = (Vector_testid . testid) instance (base ~ (OneTuple t1 -> t2)) => Call (Vector_testid base) (OneTuple t1 -> t2) where call (Vector_testid val) = val ------------------------------ -- example usage ------------------------------ main = do let v = Vector (1::Int) (2::Int) (3::Int) -- following lines equals to a pseudocode of ` v.method1 "test" ` -- OneTuple is used to indicate, that we are passing single element. -- In case of more or less elements, ordinary tuples would be used. print $ call (method1 v) $ OneTuple "test" print $ testx v }}} The code compiles and works fine with GHC 7.6. When I'm trying to compile it with GHC 7.7, I'm getting following error: {{{ debug.hs:61:10: Illegal instance declaration for ‛Method1 (Vector a) Vector_testid out’ The liberal coverage condition fails in class ‛Method1’ for functional dependency: ‛cls -> func’ Reason: lhs type ‛Vector a’ does not determine rhs type ‛out’ In the instance declaration for ‛Method1 (Vector a) Vector_testid out’ }}} The error is caused by new rules of checking what functional dependencies can do, namely `liberal coverage condition` (as far as I know, this is `coverage condition` relaxed by using `-XUndecidableInstances`) == Some attemps to fix the problem == I was trying to overcome this problem by changing the definition of `Method1` to: {{{#!haskell class Method1 cls m func | cls -> m where method1 :: cls -> m func }}} Which resolves the problem with functional dependencies, but then the line: {{{#!haskell testx x = call (method1 x) $ OneTuple "test" }}} is not allowed anymore, causing a compile error (in both 7.6 and 7.7 versions): {{{ Could not deduce (Method1 cls m func0) arising from the ambiguity check for ‛testx’ from the context (Method1 cls m func, Call (m func) (OneTuple [Char] -> s)) bound by the inferred type for ‛testx’: (Method1 cls m func, Call (m func) (OneTuple [Char] -> s)) => cls -> s at debug.hs:50:1-44 The type variable ‛func0’ is ambiguous When checking that ‛testx’ has the inferred type ‛forall cls (m :: * -> *) func s. (Method1 cls m func, Call (m func) (OneTuple [Char] -> s)) => cls -> s’ Probable cause: the inferred type is ambiguous }}} It is also impossible to solve this issue using type families (as far as I know). If we replace `Method1` type class and instances with following code (or simmilar): {{{#!haskell class Method1 cls m | cls -> m where type Func cls method1 :: cls -> m (Func cls) instance Method1 (Vector a) Vector_testid where type Func (Vector a) = (t1->t1) method1 = (Vector_testid . testid) }}} We would get obvious error `Not in scope: type variable ‛t1’`, because type families does not allow to use types, which does not appear on LHS of type expression. == The final question == How can I make this idea work under GHC 7.7? I know the new `liberal coverage condition` allows GHC devs make some progress with type checking, but it should somehow be doable to port idea working in GHC 7.6 over never compiler version. (without forcing user of my DSL to introduce any further types - everything so far, like type class instances, I'm genarating using Template Haskell) Maybe is there a way to indroduce an extension, which will disable liberal coverage condition in such situations? There is also a StackOverflow discussion available, here: http://stackoverflow.com/questions/20778588/liberal-coverage-condition- introduced-in-ghc-7-7-breaks-code-valid-in-ghc-7-6 [1]: http://en.wikipedia.org/wiki/Domain-specific_language -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8634> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10445: Wrong stack space size when using -Ksize
by GHC 19 Nov '18

19 Nov '18

#10445: Wrong stack space size when using -Ksize -------------------------------------+------------------------------------- Reporter: asr | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.10.2 Component: Compiler | Version: 7.10.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- After compiling the first example in https://wiki.haskell.org/Performance/Accumulating_parameter: {{{ $ cat Test.hs len :: [a] -> Int len [] = 0 len (x:xs) = len xs + 1 main :: IO () main = print $ len [1..1000000] }}} {{{ $ ghc Test.hs -rtsopts }}} and running {{{ ./Test +RTS -K10K }}} GHC 7.6.3 reports {{{ Stack space overflow: current size 10240 bytes. }}} GHC 7.8.4 reports {{{ Stack space overflow: current size 33632 bytes. }}} and GHC 7.10.1 reports {{{ Stack space overflow: current size 99136 bytes. }}} GHC 7.8.3 and 7.10.1 report an incorrect stack space size. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10445> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10347: Spurious "unused constructor" warning with Coercible
by GHC 19 Nov '18

19 Nov '18

#10347: Spurious "unused constructor" warning with Coercible -------------------------------------+------------------------------------- Reporter: goldfire | Owner: goldfire Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.11 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect Unknown/Multiple | warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- When I say {{{ {-# LANGUAGE RoleAnnotations #-} module Bug (N, twiddle) where import Data.Coerce newtype N a = MkN Int type role N representational twiddle :: N a -> N b twiddle = coerce }}} I get {{{ Bug.hs:7:15: warning: Defined but not used: data constructor ‘MkN’ }}} But the constructor ''is'' used, in order to do the coercion. The role annotation is to force GHC to use the constructor to do the coercion -- otherwise, it might not have. This bug exists in 7.10 and in HEAD. I will fix. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10347> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9207: Detect obvious cases of infinite recursion.
by GHC 18 Nov '18

18 Nov '18

#9207: Detect obvious cases of infinite recursion. --------------------------------------+------------------------------------ Reporter: mrugiero | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Keywords: infinite recursion | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Unknown | Type of failure: None/Unknown Blocked By: | Test Case: Related Tickets: | Blocking: --------------------------------------+------------------------------------ Pure functions are guaranteed to return the same value if the same arguments are used. Because of that, a compiler could be able to detect infinite recursion in the form of a function calling itself with the same arguments (case, some distracted programmer forgot to apply the function tail to a list). The way I see it, one of Haskell's main selling points is that it helps the programmer to avoid sources of bugs as much as it can, and infinite recursion is one of the few that stand unchecked. I specifically point to this example because both, I'm a newbie and made that newbie mistake, and think it shouldn't be hard to implement to the parsing stage of the compiler/interpreter. It probably should throw an error when this happens in pure functions, and maybe an opt-in warning if it happens in monads. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9207> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #10675: GHC does not check the functional dependency consistency condition correctly
by GHC 16 Nov '18

16 Nov '18

#10675: GHC does not check the functional dependency consistency condition correctly -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- Consider {{{ {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances, ScopedTypeVariables #-} class C a b | a -> b where op :: a -> b instance C [x] [x] instance {-# OVERLAPS #-} C x y => C [x] [Maybe y] f x = op [x] }}} Should these two instance declarations be accepted? They are simply ''inconsistent'' in the sense of Definition 6 of the [http://research.microsoft.com/en-us/um/people/simonpj/papers/fd- chr/jfp06.pdf FDs through CHRs paper]. Sadly GHC does not currently reject these as inconsistent. As a result it'll use ''both'' instance for improvement. In the definition of `f` for example we get {{{ C [alpha] beta }}} where `x:alpha` and the result type of `f` is `beta`. By using both instances for improvement we get {{{ C [Maybe gamma] [Maybe gamma] }}} Is that what we want? The two instances don't ''contradict'' each other, but neither do they agree as all published work on FDs says they should! Examples in the testsuite that exploit this loophole are {{{ ghci/scripts ghci047 polykinds T9106 typecheck/should_compile FD4 typecheck/should_compile T7875 }}} I'm not sure what the right thing here is. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10675> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9692: Template Haskell reification for Data Families is buggy?
by GHC 08 Nov '18

08 Nov '18

#9692: Template Haskell reification for Data Families is buggy? -------------------------------------+------------------------------------- Reporter: hpacheco | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 Keywords: TemplateHaskell | Operating System: MacOS X Architecture: Unknown/Multiple | Type of failure: Difficulty: Unknown | None/Unknown Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- I have found an example where I think that the TH reification for data/newtype families is incorrect, in that if forgets non-instantiated type variables in instance declarations. For the attached example, it will infer the Info: {{{#!hs FamilyI (FamilyD DataFam TyFamReifyDefs.F [PlainTV a_1627399879,PlainTV b_1627399880] (Just StarT)) [DataInstD [] TyFamReifyDefs.F [ConT GHC.Types.Int] [NormalC TyFamReifyDefs.FInt [(NotStrict,VarT x_1627399881)]] []] }}} Note how the variable x is not declared in the body of the DataInstD. Since it does not need to have the same name as the family-declared variable, substituing is non-trivial this way... -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9692> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[GHC] #9545: Evaluate Takano Akio's foldrW/buildW fusion framework as a possible replacement for foldr/build
by GHC 29 Oct '18

29 Oct '18

#9545: Evaluate Takano Akio's foldrW/buildW fusion framework as a possible replacement for foldr/build -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: task | Status: new Priority: normal | Milestone: Component: libraries/base | Version: 7.9 Keywords: | Operating System: Architecture: Unknown/Multiple | Unknown/Multiple Difficulty: Project (more | Type of failure: than a week) | None/Unknown Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Takano Akio's [https://github.com/takano-akio/ww-fusion worker-wrapper fusion] modifies `foldr/build` fairly conservatively to allow safe fusion of `foldl` and friends without relying on arity analysis. This advantage is important for two reasons that I know of: 1. As discussed in Joachim Breitner's original paper, the current arity analysis is unable to infer arity correctly when facing certain forms of recursion. 2. The current arity analysis, and probably ''any'' practical arity analysis, depends on inlining to a degree that can sometimes be unhealthy. I would love to make functions like `hPutStr` fuse, but I suspect doing so safely at present would likely cause a code explosion—the function being folded is too large to want to inline to make it available for arity analysis following fusion. I don't understand it completely yet, but it looks like `foldrW/buildW` can eliminate a lot of this uncertainty. Furthermore, it appears that functions currently written using `foldr` in a "right-handed" way can remain unchanged, using an implementation of `foldr` in terms of `foldrW`. Only "left-handed" folds will need to be rewritten to take advantage of this framework, along with all the `build`s. That said, `foldrW/buildW` probably has its weaknesses too. The basic fusion rule looks like this. {{{#!hs {-# RULES "foldrW/buildW" forall f z (i :: Wrap f b) (g :: forall c g . (Wrap g c) -> (a -> c -> c) -> c -> c) . foldrW i f z (buildW g) = g i f z #-} }}} If `g` and `i` are not inlined sufficiently to merge with each other, I imagine this could produce worse results than `foldr/build` when the latter works properly. The only way to really get a feel for performance would be to carefully modify everything to work as well as it can with this framework and see how the results compare. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9545> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

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[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] #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] #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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