July 2013 - Glasgow-haskell-users

match_co: needs more cases
by Evan Laforge 07 Nov '13

07 Nov '13

I made some strictifying changes and started getting this msg from ghc: match_co: needs more cases vector-0.10.0.1:Data.Vector.Generic.Mutable.MVector{tc r46} (Sym <(vector-0.10.0.1:Data.Vector.TFCo:R:MutableVector{tc r45})>) <main:Util.TimeVectorStorable.Sample{tc r4e} y{tv a4lK} [tv]> It's not just TimeVectorStorable.Sample (which is indeed stored in Data.Vectors), we also have some mysterious compiler-generated symbols: match_co: needs more cases vector-0.10.0.1:Data.Vector.Generic.Mutable.MVector{tc rQA} (Sym <(vector-0.10.0.1:Data.Vector.TFCo:R:MutableVector{tc rQz})>) <a{tv a6IH} [tv]> I assume this is harmless, but I didn't see any other references to this error on the web or on the ghc trac so maybe it's new?

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Liberalising IncoherentInstances
by Simon Peyton-Jones 19 Aug '13

19 Aug '13

Friends I've realised that GHC's -XIncoherentInstances flag is, I think, over-conservative. I propose to liberalise it a bit. This email describes the issue. Please yell if you think this is a bad idea. Simon Suppose we have class C a where { op :: a -> String } instance C [a] where ... instance C [Char] where ... f :: [b] -> String f xs = "Result:" ++ op xs With -XOverlappingInstances, but without -XIncoherentInstances, f won't compile. Reason: if we call 'f' at Char (e.g. f "foo") then you might think we should use instance C [Char]. For example, if we inlined 'f' at the call site, to get ("Result:" ++ op "foo"), we certainly then would use the C [Char] instance, giving perhaps different results. If we accept the program as-is, we'll permanently commit 'f' to using the C [a] instance. The -XIncoherentInstances flag says "Go ahead and use an instance, even if another instance might become relevant if you were to specialise or inline the enclosing function." The GHC user manual gives a more precise spec [1]. Now consider this class D a b where { opD :: a -> b -> String } instance D Int b where ... instance D a Int where ... g (x::Int) = opD x x Here 'g' gives rise to a constraint (D Int Int), and that matches two instance declarations. So this is rejected regardless of flags. We can fix it up by adding instance D Int Int where ... but this is pretty tiresome in cases where it really doesn't matter which instance you choose. (And I have a use-case where it's more than tiresome [2].) The underlying issue is similar to the previous example. Before, there was *potentially* more than one way to generate evidence for (C [b]); here there is *actually* more than one instance. In both cases the dynamic semantics of the language are potentially affected by the choice -- but -XIncoherentInstnaces says "I don't care". So the change I propose to make IncoherentInstances to pick arbitrarily among instances that match. More precisely, when trying to find an instance matching a target constraint (C tys), a) Find all instances matching (C tys); these are the candidates b) Eliminate any candidate X for which another candidate Y is strictly more specific (ie Y is a substitution instance of X), if either X or Y was complied with -XOverlappingInstances c) Check that any non-candidate instances that *unify* with (C tys) were compiled with -XIncoherentInstances d) If only one candidate remains, pick it. Otherwise if all remaining candidates were compiled with -XInccoherentInstances, pick an arbitrary candidate All of this is precisely as now, except for the "Otherwise" part of (d). One could imagine different flags for the test in (c) and (d) but I really don't think it's worth it. Incidentally, I think it'd be an improvement to localise the Overlapping/Incoherent flags to particular instance declarations, via pragmas, something like instance C [a] where {-# ALLOW_OVERLAP #-} op x = .... Similarly {-# ALLOW_INCOHERENT #-}. Having -XOverlappingInstances for the whole module is a bit crude., and might be missed when looking at an instance. How valuable would this be? [1] http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensio… [2] http://ghc.haskell.org/trac/ghc/wiki/NewtypeWrappers

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throwTo semantics
by Roman Cheplyaka 13 Aug '13

13 Aug '13

The documentation for throwTo says: throwTo does not return until the exception has been raised in the target thread. The calling thread can thus be certain that the target thread has received the exception. This is a useful property to know when dealing with race conditions: eg. if there are two threads that can kill each other, it is guaranteed that only one of the threads will get to kill the other. I don't see how the last sentense follows. I understood it so that the implication throwTo has returned => exception has been delivered is true, but not the reverse. If my understanding is correct, then both exceptions could be delivered without any of throwTos returning. Or is it true that throwTo has returned <=> exception has been delivered ? Roman

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cascading type errors in ghc
by Evan Laforge 09 Aug '13

09 Aug '13

I frequently see one logical mistake turn into many type errors in ghc. Of course in general "one logical mistake" to a human and a type checker can be completely different things, so that's not surprising. The thing is, I feel like I started seeing more with the upgrade to ghc 7.6 (or was it 7.4?) a while back, which I guess coincides with a big typechecker overhaul. So I tracked down a specific example, though I haven't checked if this gives different results in older ghcs: complicated :: Bool -> Int -> (Double -> Double) -> Int -> Char -> Int -> Char -> () complicated _ _ _ _ _ _ _ = () t0 = complicated 0 id x1 y1 x1 y1 where x1 :: Int x1 = 0 y1 :: Char y1 = 'a' In this case, the call to complicated is missing the initial Bool. In ghci the above gives 5 separate type errors, for the 2nd, 3rd, 4th, 5th, and 6th arguments. The curious thing is, if I replace x1 and y1 with their values: t0 = complicated 0 id 0 'a' 0 'a' I'm now down to only 3 type errors, for 2nd, 4th, and 6th args. So I'm just curious: what's the difference between the where-bound stuff and literals? Also, why don't I get an error that 0 isn't a Bool? Or I suppose, that there's no instance Integral Bool?

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Threadscope reporting that nothing is happening (MUT or GC) for ~2 seconds
by Patrick Palka 30 Jul '13

30 Jul '13

Hi, I'm analyzing the performance of my parallel --make branch of GHC ( https://github.com/parcs/ghc/tree/cherry-picking if anyone is interested) by compiling the Cabal repository in parallel, using two capabilities. I'm seeing very strange results from Threadscope: at some point during the compile, absolutely no work is being performed on any HEC for about 2 seconds. How do I interpret this kind of behavior? What does it mean? Relevant screenshot: http://i.imgur.com/uDQk9zG.png Patrick

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Call for talks: Haskell Implementors Workshop 2013, Sept 22, Boston
by Ryan Newton 30 Jul '13

30 Jul '13

Please pass on this announcement! The deadline is in two weeks.* * * * * Call for Talks* ACM SIGPLAN Haskell Implementors' Workshop http://haskell.org/haskellwiki/HaskellImplementorsWorkshop/2013 Boston, USA, September 22th, 2013 The workshop will be held in conjunction with ICFP 2013 http://www.icfpconference.org/icfp2013/ *Important dates* Proposal Deadline: *13th August 2013 (by midnight, any timezone)* Notification: 27th August 2013 Workshop: 22th September 2013 The Haskell Implementors' Workshop is to be held alongside ICFP 2013 this year in Boston. There will be no proceedings; it is an informal gathering of people involved in the design and development of Haskell implementations, tools, libraries, and supporting infrastructure. This relatively new workshop reflects the growth of the user community: there is a clear need for a well-supported tool chain for the development, distribution, deployment, and configuration of Haskell software. The aim is for this workshop to give the people involved with building the infrastructure behind this ecosystem an opportunity to bat around ideas, share experiences, and ask for feedback from fellow experts. We intend the workshop to have an informal and interactive feel, with a flexible timetable and plenty of room for ad-hoc discussion, demos, and impromptu short talks. Scope and target audience ------------------------- It is important to distinguish the Haskell Implementors' Workshop from the Haskell Symposium which is also co-located with ICFP 2013. The Haskell Symposium is for the publication of Haskell-related research. In contrast, the Haskell Implementors' Workshop will have no proceedings -- although we will aim to make talk videos, slides and presented data available with the consent of the speakers. In the Haskell Implementors' Workshop, we hope to study the underlying technology. We want to bring together anyone interested in the nitty-gritty details behind turning plain-text source code into a deployed product. Having said that, members of the wider Haskell community are more than welcome to attend the workshop -- we need your feedback to keep the Haskell ecosystem thriving. The scope covers any of the following topics. There may be some topics that people feel we've missed, so by all means submit a proposal even if it doesn't fit exactly into one of these buckets: * Compilation techniques * Language features and extensions * Type system implementation * Concurrency and parallelism: language design and implementation * Performance, optimisation and benchmarking * Virtual machines and run-time systems * Libraries and tools for development or deployment Talks ----- At this stage we would like to invite proposals from potential speakers for a relatively short talk. We are aiming for 20 minute talks with 10 minutes for questions and changeovers. We want to hear from people writing compilers, tools, or libraries, people with cool ideas for directions in which we should take the platform, proposals for new features to be implemented, and half-baked crazy ideas. Please submit a talk title and abstract of no more than 200 words. Submissions should be made via EasyChair. The website is: https://www.easychair.org/conferences/?conf=hiw2013 If you don't have an account you can create one here: https://www.easychair.org/account/signup.cgi Because the submission is an abstract only, please click the "abstract only" button when you make your submission. There is no need to attach a separate file. We will also have a lightning talks session which will be organised on the day. These talks will be 2-10 minutes, depending on available time. Suggested topics for lightning talks are to present a single idea, a work-in-progress project, a problem to intrigue and perplex Haskell implementors, or simply to ask for feedback and collaborators. Organisers ---------- * Ryan Newton (Indiana University) * Neal Glew (Intel Labs) * Edward Yang (Stanford University) * Thomas Schilling (University of Kent) * Geoffrey Mainland (Drexel University)

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GHC and GSL
by Adrian Victor Crisciu 29 Jul '13

29 Jul '13

Hi, I need to import in one of my programs the minimization module form Numeric.GSL.Minimization. When I run even the example code in interactive mode, ghci crashes with a "Bus error" message. Is this a bug, or is something wrong with my code? The offending program follows: module MinMaxTest where import Numeric.GSL.Minimization objFunc :: [Double] -> Double objFunc [x, y] = 10 * (x-1)^2 + 20 * (y-2)^2 + 30 main :: IO () main = do let (s, p) = minimize NMSimplex2 1e-2 30 [1, 1] objFunc [5, 7] print s print p I use ghc-7.6.3 on a Slackware-Linux platform. Can anyone help? Thank you in advance. Adrian-Victor

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change location of user's package directory
by harry 28 Jul '13

28 Jul '13

How can I change the location that ghc and ghc-pkg use for the user's package directory? I'm running GHC in a very restricted environment where I don't have access to $HOME, but I can use specific subdirectories. -- View this message in context: http://haskell.1045720.n5.nabble.com/change-location-of-user-s-package-dire… Sent from the Haskell - Glasgow-haskell-users mailing list archive at Nabble.com.

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Installing GHC and libgmp.so
by harry 24 Jul '13

24 Jul '13

I've had a lot of trouble installing the pre-build binary of GHC on various Linux systems for which packages don't exist - all of this stems from GHC looking for libgmp.so, instead of libgmp.so.3. Various combinations of --with-gmp-libraries haven't helped, particularly as I need a packaged build for copying onto shared machines where I don't have enough disk space to install from the binary package locally, and don't have root access to create a shortcut in /usr/lib64. Is it possible for someone to recompile the pre-build binaries, linked against a specific version of libgmp, instead of a default which happens to be on their machine? Or is there some other solution to my predicament? -- View this message in context: http://haskell.1045720.n5.nabble.com/Installing-GHC-and-libgmp-so-tp5733330… Sent from the Haskell - Glasgow-haskell-users mailing list archive at Nabble.com.

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GHC and GSL
by Adrian Victor Crisciu 22 Jul '13

22 Jul '13

That's exactly what happened. Indeed, I work on a 64-bit platform, and the above program works flawlessly in compiled mode (stand-alone executable). It's a bit annoying, because I have to tinker a bit with each module in order to test it, but it works. Any idea what happens with ghci/64-bit? Thank you so much! Adrian-Victor.

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