June 2009 - Haskell-Cafe

[ANN] Nemesis : easy task management
by Jinjing Wang 12 Jun '09

12 Jun '09

You can have a description file named Nemesis, with content nemesis = do clean [ "**/*.hi" , "**/*.o" , "manifest" ] task "dist" $ do sh "cabal clean" sh "cabal configure" sh "cabal sdist" task "i" (sh "ghci -isrc src/System/Nemesis.hs") task "manifest" $ do sh "find . | grep 'hs$' > manifest" then after run `nemesis`, there will be a compiled bin `nem` in your local path, which allows you to do `nem clean` Install with `cabal install nemesis` Cheers, -- jinjing

1 0

ghci and applicative
by Paul Keir 12 Jun '09

12 Jun '09

Hi, I'm finding that some data types which use Applicative to instantiate the Num class, give responses I wasn't expecting at the ghci prompt. A simple example is list: import Control.Applicative instance (Num a) => Num [a] where as + bs = (+) <$> as <*> bs (*) = undefined; abs = undefined signum = undefined; fromInteger = undefined f1 = let op = (+) in [1,2,3] `op` [1,1,1] f2 = let op = (+) in op [1,2,3] [1,1,1] Functions f1 and f2 give no problems at the ghci prompt. However if I instead type the body of either interactively, I get an error: *Main> let op = (+) *Main> [1,2,3] `op` [1,1,1] <interactive>:1:0: Couldn't match expected type `Integer' against inferred type `[a]' In the first argument of `op', namely `[1, 2, 3]' In the expression: [1, 2, 3] `op` [1, 1, 1] In the definition of `it': it = [1, 2, 3] `op` [1, 1, 1] I get the same error message with op [1,2,3] [1,1,1]. Any thoughts? Thanks, Paul

4 4

Debugging misbehaving multi-threaded programs
by Evan Klitzke 12 Jun '09

12 Jun '09

I've written a multi-threaded Haskell program that I'm trying to debug. Basically what's happening is the program runs for a while, and then at some point one of the threads goes crazy and spins the CPU while allocating memory; this proceeds until the system runs out of available memory. I can't fix this without figuring out what code is being executed in the loop (or at least which thread is misbehaving, which would narrow things down a lot). I was hopeful that I could compile the program with profiling support and then use +RTS -M100M along with some of the RTS profiling options and get profiling information on CPU and memory usage at the time that my program runs out of memory. The thinking here is that nearly all of the CPU time and heap space will be from the misbehaving thread, so at that point I could do more investigation into exactly what is happening. Unfortunately, this doesn't seem to work; whenever the program terminates due to running out of heap space, the generated .prof file is empty. Another strategy I tried was running the program in ghci and use -fbreak-on-exception and :trace; by hitting Ctrl-C I was hopeful I'd stop the program in whatever is looping (this is all described and suggested in the ghc docs). Unfortunately, this also didn't seem to work, because the Ctrl-C only stops the main thread. Does anyone have any tips for dealing this? Have other people run into similar problems? I'm out of ideas, so I'm open to any suggestions. -- Evan Klitzke <evan(a)eklitzke.org> :wq

4 4

Why do ghc-built binaries use timer_create?
by Maurício 12 Jun '09

12 Jun '09

This comes from an issue in haskell-beginner, although it have already been touched here. If you use recent versions of ghc to build a program and try the resulting binary on an old linux distro, you may get a message about timer_create receiving the wrong parameters. Curiously, as sugested in the other thread, it seems building with ghc -threaded solves the problem. So, I'm just curious: how is timer_create used, and why maybe it's not used with -threaded? Thanks, Maurício

3 3

Software Transactional Memory and LWN
by Ketil Malde 12 Jun '09

12 Jun '09

Hi, Browsing LWN, I ran across this comment: http://lwn.net/Articles/336039/ The author makes a bunch of unsubstantiated claims about STM, namely that all implementations use locking under the hood, and that STM can live- and deadlock. I've seen a lot of similar sentiments in other places as well (comp.arch springs to mind). Now, I'm no expert on STM, but I was pretty sure these are incorrect, and I certainly had the impression that Haskell's STM guarantees some progress - which it couldn't in a deadlock situation. Am I wrong? -k -- If I haven't seen further, it is by standing in the footprints of giants

5 6

[ANN] Data.Reify.CSE
by Sebastiaan Visser 12 Jun '09

12 Jun '09

Hi all, This module[1] implements common sub-expression elimination for graphs generated by the Data.Reify package. (Which is a really neat tool!) The algorithm performs a simple fixed point iteration and is not (consciously) optimized for speed. This package might especially be useful for optimizing simple compilers for referential transparent domain specific languages. Which is exactly the reason I've written this code. An example of the usage is included in the Haddock documentation. Gr, -- Sebastiaan Visser [1] http://hackage.haskell.org/package/data-reify-cse

1 0

Documentation bug -- building ghc from darcs sources
by Michael Vanier 12 Jun '09

12 Jun '09

I've been trying to build ghc head from the darcs repo using these instructions: http://hackage.haskell.org/trac/ghc/wiki/Building/GettingTheSources Unfortunately, when I do ./darcs-all --extra get as described under "Getting more packages" it fails because the darcs-all script doesn't recognize the --extra option. Was this removed recently, and if so, how do I achieve the same effect? TIA, Mike

2 1

exporting Data.ByteString functions via FFI
by kenny lu 12 Jun '09

12 Jun '09

Hi, I was trying to write a FFI wrapper for my Haskell program which manipulates ByteString. But I am unable to compile/link it. Here is the toy program. {-# LANGUAGE ForeignFunctionInterface #-} module B where import Foreign.C.Types import Foreign.C.String import qualified Data.ByteString as BS rev :: BS.ByteString -> BS.ByteString rev bstr = BS.reverse bstr rev_hs :: CString -> IO CString rev_hs cstr = do { bstr <- BS.packCString cstr ; let bstr' = rev bstr ; cstr' <- newCString (show bstr') ; return cstr' } foreign export ccall rev_hs :: CString -> IO CString And here is the C counter-part. #include "B_stub.h" #include <stdio.h> int main(int argc, char *argv[]) { char *str; hs_init(&argc, &argv); str = rev_hs("it works."); printf("Rev: %s\n", str); hs_exit(); return 0; } Compiling B.hs alone seems fine, but errors popped up when I was trying to compile/link it with C. $ ghc -c -O B.hs $ ghc -optc-O test_b.c B.o B_stub.o -o test_b Undefined symbols: "___stginit_bytestringzm0zi9zi1zi4_DataziByteString_", referenced from: ___stginit_Lib_ in B.o "_bytestringzm0zi9zi1zi4_DataziByteString_zdwreverse_info", referenced from: _s19w_info in B.o "_bytestringzm0zi9zi1zi4_DataziByteStringziInternal_zdwshowsPrec_info", referenced from: _s19v_info in B.o "_bytestringzm0zi9zi1zi4_DataziByteStringziInternal_zdwshowsPrec_closure", referenced from: _Lib_zdwa_srt in B.o "_bytestringzm0zi9zi1zi4_DataziByteString_zdwa4_info", referenced from: _Lib_zdwa_info in B.o "_bytestringzm0zi9zi1zi4_DataziByteString_reverse_info", referenced from: _Lib_rev_info in B.o ld: symbol(s) not found collect2: ld returned 1 exit status If I replace ByteString with the ordinary String, the above programs can be compiled and linked. Can someone tell me what I did wrong here? -Kenny

2 2

Re: [Haskell] ANNOUNCE: OpenGLRaw 1.0.0.0
by Felipe Lessa 12 Jun '09

12 Jun '09

On Thu, Jun 11, 2009 at 06:56:01PM +0200, Sven Panne wrote: > Version 1.0.0.0 covers the OpenGL 3.1 core, all ARB extensions > and all EXT extensions. What about OpenGL 2.1? As I understand, Linux won't have OpenGL 3.0 or 3.1 for at least some months, $ glxinfo | grep 'OpenGL version' OpenGL version string: 2.1 Mesa 7.5-rc3 Thanks! -- Felipe.

5 7

Semantic web RDF store
by Vasili I. Galchin 12 Jun '09

12 Jun '09

Hello, Currently I am working on re-packaging Swish written by Graham Klyne so that is cabalized... I am chugging along. I am also thinking about writing a FFI for some RDF store API like http://www.rdflib.net/store/ so that Swish could be hooked into a "reliable"persistent store where the store type can be switched out like MySQL dor SQLite, etc. Has any one already done something like this? I don't like reinventing the wheel. Regards, Vasili

1 1