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debugging memory corruption
by Evan Laforge 02 Dec '12

02 Dec '12

Ever since upgrading to 7.6.1 I regularly get panics like this: seq: internal error: evacuate: strange closure type -1958168540 (GHC version 7.6.1 for x86_64_apple_darwin) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug I've seen some variations, but basically I think it just means someone is corrupting memory and it goes unnoticed until the GC trips over it. This happens infrequently (maybe once in 15m, very roughly, it's extremely inconsistent) in non-optimized code, and frequently (maybe once in 5m) in optimized code. This only happens during interactive use, not during testing or profiling. I had a problem like this once before, and it took a very long time to track down. And in fact, I never really tracked it down, I just got a test that could semi-reliably reproduce it, and then by trial and error discovered that if I changed the alignment of a particular Storable instance from 1 to 4, the problem stopped happening (and 1 should have been correct, it was a struct of chars). Not exactly a satisfying solution, and now I'm thinking all I did was make ghc 6 stop manifesting it, and with 7 it's back again. I'm most suspicious of the FFI usage since it's easy to corrupt memory in C++ and even easier to write a bogus Storable instance that does the same, but I really have no idea what or where. My first thought was to start cutting out portions (especially FFI-using portions) to try to isolate it, but it's slow going because it can sometimes take quite a while to the bug to come out again. My second thought was that I need a more automated way to reproduce it, but it's nontrivial because it only comes up when I'm using the interactive GUI parts, which are also incidentally a big chunk of FFI. And even if I do get a repro, as I did before, it just means I can more quickly poke randomly at things hoping they change it, but even if I can get it to stop happening it doesn't mean I understood it, or even really fixed it. This is also the kind of bug (well, it was last time), which is highly dependent on the code, so add one print and it stops happening. I have a sort of complicated scheme where I pass a wrapped haskell function callback along with a wrapped freeHaskellFunPtr to free the last one, along with itself, maybe it's something to do with that. Anyone out there with ideas or advice on how to track down this kind of bug? My next thought is to try to automate the GUI parts, or maybe just the FFI part, so I can write a program to randomly fuss with it until it crashes. I've also tried valgrind, but it doesn't report anything suspicious. But it also doesn't seem to work on FFI Storable corruption, I've tried intentionally inserting a bad poke and valgrind still won't report it. Thanks in advance for any insight! Actually, there's a whole other discussion which has been nagging at me for a while, though another thread would be more appropriate. But in short it's that it feels like hsc2hs is just too low level, and too error-prone. It's tempting to use because it comes with ghc, but it seems bad to tell people "haskell is a safe language, but as soon as you want to talk to C you're writing totally unchecked pokes and peeks". Maybe I should go evaluate the alternatives like c2hs, or maybe safety features can added to hsc2hs. Wouldn't it be nice to have ghc come with a high level and safe FFI language?

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Why is boxed mutable array so slow?
by Branimir Maksimovic 02 Dec '12

02 Dec '12

I have made benchmark test inspired by http://lemire.me/blog/archives/2012/07/23/is-cc-worth-it/ What surprised me is that unboxed array is much faster than boxed array.Actually boxed array performance is on par with standard Haskell listwhich is very slow.All in all even unboxed array is about 10 times slower than Java version.I don't understand why is even unboxed array so slow.But! unboxed array consumes least amount of RAM.(warning, program consumes more than 3gb of ram) bmaxa@maxa:~/examples$ time ./Cumulboxed arraylast 262486571 seconds 4.972unboxed arraylast 262486571 seconds 0.776listlast 262486571 seconds 6.812 real 0m13.086suser 0m11.996ssys 0m1.080s -------------------------------------------------------------------------{-# LANGUAGE CPP, BangPatterns #-}import System.CPUTimeimport Text.Printfimport Data.Array.IOimport Data.Array.Baseimport Data.Intimport Control.DeepSeqimport System.Mem main :: IO()main = do (newArray_ (0,n'-1) :: IO(A)) >>= test "boxed array" performGC (newArray_ (0,n'-1) :: IO(B)) >>= test "unboxed array" performGC begin <- getCPUTime printf "list\nlast %d" $ last $ force $ take n' $ sum' data' end <- getCPUTime let diff = (fromIntegral (end - begin)) / (10^12) printf " seconds %.3f\n" (diff::Double) test s a = do putStrLn s begin <- getCPUTime init' a partial_sum a end <- getCPUTime let diff = (fromIntegral (end - begin)) / (10^12) last <- readArray a (n'-1) printf "last %d seconds %.3f\n" last (diff::Double) n' :: Intn' = 50 * 1000 * 1000 type A = IOArray Int Int32type B = IOUArray Int Int32 init' a = do (_,n) <- getBounds a init a 0 n where init a k n | k > n = return () | otherwise = do let !x = fromIntegral $ k + k `div` 3 unsafeWrite a k x init a (k+1) n partial_sum a = do (_,n) <- getBounds a k <- unsafeRead a 0 ps a 1 n k where ps a i n s | i > n = return () | otherwise = do k <- unsafeRead a i let !l = fromIntegral $ s + k unsafeWrite a i l ps a (i+1) n l data' :: [Int32]data' = [k + k `div` 3 | k <- [0..] ] sum' = scanl1 (+)

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what is the purpose of GHC.Exts.lazy?
by jared simpson 01 Dec '12

01 Dec '12

docs say: The call '(lazy e)' means the same as e, but lazy<http://hackage.haskell.org/packages/archive/base/latest/doc/html/GHC-Exts.h…> has a magical strictness property: it is lazy in its first argument, even though its semantics is strict. why do i want to use magic during strictness analysis?

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Simple GUI Form
by Rune Harder Bak 01 Dec '12

01 Dec '12

Hi I have some input parameters data Input = ... that I need the user to enter in a gui pop-up. (windows people...) The rest of the app is not gui (or perhaps progress could be displayed in a log-window) What is the easiest way to make such a GUI form? It need to compile for both Linux and Windows, so I though WxWidgets was ideal, and I got wx[1] < 0.90 to install (using wxWidgets2.8) on both windows and linux. Now I just need to create the form, but how do you do that? Any clues or links to examples? I have never used wxwidgets on any platform or done any other form of GUI before for that matters. (apart from some Visual Basic ten years ago, and html). I tried looking at wx examples, but I couldn't find this simple use case explained anywhere. I installed wx in the first place because WxGeneric[2] seemed exactly what I needed, but I can't get it to compile using ghc7.4.2 from haskell-platform. Anybody got that working or have some other simple method? Help much appreciated! -Rune [1] http://hackage.haskell.org/package/wx-0.13.2.3 [2] http://hackage.haskell.org/package/WxGeneric

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computation over containers, greatly simplified notation.
by Takayuki Muranushi 01 Dec '12

01 Dec '12

Dear all, I came up with an idea to greatly simplify some kinds of array computations. It should work well with many kinds of arrays. Is this new? https://gist.github.com/4162375 These few days, I've been trying to rewrite a hydrodynamic simulation code that used Data.Vector (~250 lines), to Repa [1] . It seemed promising, but soon I realized that I needed to use Repa.map and Repa.zipWith almost everywhere. I need careful thinking to transform every lines (that used vector's indexing) to Repa's point-free stile. Is there any better ways? Then I realized that I was the author of Paraiso [2], a DSL for stencil computation. One of its feature is to write down array computation just as if it were scalar computation. Basically what I need is ZipList-like Applicative instances for vectors and Repa arrays. Why not they support ZipVector? Because 'pure' of zipList was an infinite list and you can't do infinite vectors. Then I came up with this idea. https://gist.github.com/4162375 the wrapper W does several things: it represents the 'pure,' homogeneous array in a space-efficient manner, and also serves as a newtype-wrapper of Num (and possibly Fractional, Floating...) instances. Questions are: is this technology new? or promising? doomed? It seems to me like a free-Applicative, like the free-Monad theory. Are they related? The function 'backend' helps to mix in the non-zip-like computations. How can we remove the 'undefined' in the 'backend?' Some of Repa computations are Monads. W needs to be a monad transformer to incooperate this. Also I'm grateful to past cafe discussion on existing Zippable implementations [3][4] . [1] hackage.haskell.org/package/repa [2] http://hackage.haskell.org/package/Paraiso [3] http://www.haskell.org/pipermail/haskell-cafe/2009-July/064403.html [4] http://hackage.haskell.org/packages/archive/category-extras/latest/doc/html… -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

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Are storable-mutable-vectors in two dimensions possible in Haskell?
by Fixie Fixie 01 Dec '12

01 Dec '12

Hello haskellers I am struggling with this package: Data.Vector.Storable.Mutable I am creating vectors like this: MV.new 1000 :: IO (V.MVector (PrimState IO) Int) Now I would like access to this vectors in linear time, like I could have done in C using pointers. The problem is that I do not know which datastructure to keep my vectors in. If I put them in a list, the access is very slow. Even a binary tree is very slow compared to a C-version. I have tried to make an array og vector of storable-mutable vectors, but have not manged to accomplish this - even though there must be a haskell way :-) Any clues? Cheers, Felix

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Why Kleisli composition is not in the Monad signature?
by damodar kulkarni 01 Dec '12

01 Dec '12

Hi, The Monad class makes us define bind (>>=) and unit (return) for our monads. Why the Kleisli composition (>=>) or (<=<) is not made a part of Monad class instead of bind (>>=)? Is there any historical reason behind this? The bind (>>=) is not as elegant as (>=>), at least as I find it. Am I missing something? - Damodar

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Can a GC delay TCP connection formation?
by Jeff Shaw 01 Dec '12

01 Dec '12

Hello, I've run into an issue that makes me think that when the GHC GC runs while a Snap or Warp HTTP server is serving connections, the GC prevents or delays TCP connections from forming. My application requires that TCP connections form within a few tens of milliseconds. I'm wondering if anyone else has run into this issue, and if there are some GC flags that could help. I've tried a few, such as -H and -c, and haven't found anything to help. I'm using GHC 7.4.1. Thanks, Jeff

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ANN: zeromq3-conduit, Conduit bindings for zeromq3-haskell
by Nicolas Trangez 30 Nov '12

30 Nov '12

Hi, I published zeromq3-conduit, a small library which facilitates using zeromq3-haskell, a binding for ZeroMQ 3.x, in a Conduit-based application. The 'examples' folder in the source repository contains ports of the zeromq3-haskell examples. The library also contains a module which might ease using plain zeromq3-haskell: it lifts some of the IO actions into MonadIO and provides a ReaderT which carries a Context around, so sockets can be created without passing this around explicitly. Package is on Hackage [1] and Github [2]. Comments/feedback/pull requests/... welcome! Nicolas [1] http://hackage.haskell.org/package/zeromq3-conduit [2] https://github.com/NicolasT/zeromq3-conduit/

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ANN: base-compat
by Simon Hengel 30 Nov '12

30 Nov '12

Hi, I just released the first version of a package that "provide new additions to base for older versions of base" [2]. So far the following is covered: readMaybe readEither lookupEnv getExecutablePath Source is on GitHub [2]; patches welcome ;) Cheers, Simon [1] http://hackage.haskell.org/package/base-compat [2] https://github.com/sol/base-compat

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