November 2009 - Haskell-Cafe

FFI binding -- different behaviour under compilation and interpretation.
by Jason Dusek 14 Nov '09

14 Nov '09

I'm binding to `wcwidth` to determine the column widths of various Unicode characters. I noticed a lot of -- in fact all -- Chinese characters were being given widths of `-1` when of course they should have width `2`. This only showed up when I compiled my program though -- within GHCi, it never happened. Below my signature is a parred down example that demoes the bug. It tries to get the width of only one Chinese character. You can see it like this: :; ghc --make DemoFailure.hs -o demo && demo [1 of 1] Compiling Main ( DemoFailure.hs, DemoFailure.o ) Linking demo ... 0x00005cff -1 峿 :; chmod ug+x DemoFailure.hs && DemoFailure.hs 0x00005cff 2 峿 Switching between safe/unsafe does not make any difference. This was run on a Macintosh. -- Jason Dusek #!/usr/bin/env runhaskell {- DemoFailure.hs -} {-# LANGUAGE ForeignFunctionInterface #-} import Foreign.C import Data.Char import Text.Printf import qualified System.IO.UTF8 as UTF8 main = do (sequence_ . fmap (UTF8.putStrLn . uncurry fmt)) widths where widths = [ (c, wcwidth c) | c <- ['\x5cff'] ] --widths = [ (c, wcwidth c) | c <- [minBound..maxBound] ] fmt c cols = printf "0x%08x %2d %s" (fromEnum c) cols rep where rep | ' ' == c = "\\SP" | isSpace c = '\\' : show (fromEnum c) | isPrint c = [c] | otherwise = (reverse . drop 1 . reverse . drop 1 . show) c wcwidth :: Char -> Int wcwidth = fromEnum . native . toEnum . fromEnum foreign import ccall unsafe "wchar.h wcwidth" native :: CWchar -> CInt

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looking for a good algorithm
by Hong Yang 13 Nov '09

13 Nov '09

The question is more about algorithm than Haskell. But I am going to code in Haskell which I am still learning. Suppose I have a large table, with hundreds of columns and thousands of rows. But not every cell has a value (of String, or Int, or Double type). I want to shuffle the rows to maximize the number of columns whose first 100 rows have at least one number, given a list of preferred column names since there is no guarantee that every number column will have at least one number in its first 100 rows after shuffling. Can someone provide a good algorithm for this problem? (I do not have any background in algorithms.) You can assume I already know which columns are of Int or Double type. This is not a homework. Thanks, Hong

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Re: [Haskell-cafe] Could someone teach me why we use Data.Monoid?
by David Place 13 Nov '09

13 Nov '09

Magicloud Magiclouds <magicloud.magiclouds(a)gmail.com> wrote: > Message: 26 > Date: Sat, 14 Nov 2009 01:11:27 +0800 > From: Magicloud Magiclouds <magicloud.magiclouds(a)gmail.com> > Subject: Re: [Haskell-cafe] Could someone teach me why we use > Data.Monoid? > To: Stephen Tetley <stephen.tetley(a)gmail.com> > Cc: haskell-cafe <haskell-cafe(a)haskell.org> > Message-ID: > <3bd412d40911130911v4f3ac0b9laebca79f5921491d(a)mail.gmail.com> > Content-Type: text/plain; charset=UTF-8 > > That is OK. Since understand the basic concept of monoid (I mean the > thing in actual math), the idea here is totally not hard for me. But > the sample here does not show why (or how) we use it in programming, > right? For an example of it's use, you might enjoy reading my article in the Monad.Reader "How to Refold a Map." > http://www.haskell.org/sitewiki/images/6/6a/TMR-Issue11.pdf Cheers, David

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A small (?) problem with type families
by Andy Gimblett 13 Nov '09

13 Nov '09

Hi all, This email is literate Haskell. I'm trying to use type families to express some dependencies between type classes, and I'm running into trouble, I think because I'm producing chains of dependencies which the checker can't resolve... Here's a minimised version of the state I've got myself into. :-) > {-# LANGUAGE FlexibleContexts #-} > {-# LANGUAGE TypeFamilies #-} > module Families where First a type family where the type Y is functionally dependent on the type X, and we have a function from Y to (). > class X a where > type Y a > enact :: Y a -> () Now another type family, where the type Q is functionally dependent on the type P, _and_ it must also be an instance of the X class. > class (X (Q s)) => P s where > type Q s (Perhaps there's a better way to express that dependency?) Now a function which takes a value whose type is an instance of the Y depending on the Q depending on the P. (Phew!) The function just tries to call enact on that value. > bar :: P s => Y (Q s) -> () > bar w = enact w The error we get is: src/Families.lhs:35:16: Couldn't match expected type `Y a' against inferred type `Y (Q s)' In the first argument of `enact', namely `w' In the expression: enact w In the definition of `bar': bar w = enact w Presumably this way I'm chaining type dependencies is flawed. Any suggestions on how to improve it, and/or what to read to understand what I'm dealing with better? (So far I've read "Fun with type functions V2", but that's about it, and I admit I didn't grok it all.) Thanks! -Andy

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ANN: wxHaskell 0.12.1.2
by Jeremy O'Donoghue 13 Nov '09

13 Nov '09

Hi all, On behalf of the wxHaskell maintainers, I am very pleased to announce the release of wxHaskell 0.12.1.2. The key feature of this release is that it is now possible to install wxHaskell entirely using cabal on all targets (with a minor proviso on Windows). The credit for this achievement is almost all down to new contributor Brian Lewis, who has worked very hard to make this work 'just right', and given me (at least) an object lesson in how to use Cabal. The small proviso for Windows machines is that wxWidgets should be built using MSys - none of the other compilers supported by wxWidgets is currently working, but we consider this a small price to pay for the convenience of getting a GUI straight from cabal. For Unix machines which have wxWidgets 2.8.x installed, you should now be able to install wxHaskell with a simple: cabal install wx For Windows machines, there are a few prerequisites: 1) You will need the Windows port of wx-config in your path. This can be downloaded from http://wxconfig.googlepages.com/ 2) You will need to compile wxWidgets using MSys. We have tested against MinGW 5.1.6 with g++ compiler, MinGW Make, MSYS-1.0.11, wxMSW-2.8.10, which are the latest versions, and can be downloaded from their respective websites. I used a completely clean install of the latest Haskell Platform to validate the procedure. Please note: all commands to be entered in an MSys shell. cd /c/path/to/wxWidgets-2.8.10/build/msw mingw32-make -f makefile.gcc BUILD=release MONOLITHIC=1 SHARED=1 UNICODE=1 set PATH=$PATH:/c/path/to/wx-config set WXWIN=/c/path/to/wxWidgets-2.8.10 set WXCFG=gcc_dll/mswu cabal install wx I will be updating the wxHaskell wiki to reflect these changes over the next day or so. Best Regards Jeremy O'Donoghue on behalf of the wxHaskell maintainers.

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Pattern Matching
by Casey Hawthorne 13 Nov '09

13 Nov '09

Why in a pattern match like score (1 3) = 7 can I not have sizeMax = 3 score (1 sizeMax) = 7 -- Regards, Casey

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Weird dependency failure log
by Maurício CA 13 Nov '09

13 Nov '09

Hi, all, Hackage shows a log failure for 'bindings-gsl': Configuring bindings-gsl-0.1.1... cabal-setup: At least the following dependencies are missing: bindings-DSL ==1.0.* But here is, at version 1.0.1, no building problems: http://hackage.haskell.org/package/bindings-DSL There's one thing special about bindings-DSL. It's a package with a set of macros for hsc2hs, and contains no Haskell code. Maybe this revealed some hidden error in package dependency checking. Thanks, Maurício

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Cabal and autogenerated files
by Jeremy O'Donoghue 13 Nov '09

13 Nov '09

Hi all, Another, probably simple, question regarding cabalization. Part of wxcore, the low level abstraction in wxHaskell, consists of haskell modules which are generated automatically by parsing C headers using another tool, wxdirect. When trying to create an sdist package, we run into the problem that because we export modules which are automatically generated, after a 'clean', they do not exist, so the sdist package generation fails. We have tried using 'extra-tmp-files' to list the modules which are autogenerated, but this isn't working. Is this because we are generating the autogen modules in an autogen directory, or is this approach likely to fail wherever we put the autogenerated files? Our use case seems a reasonable one, as it could reasonably exist for any project which relies on automatically generated code. Thanks for any suggestions. Jeremy

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Strange parallel behaviour with Ubuntu Karmic / GHC 6.10.4
by Michael Lesniak 13 Nov '09

13 Nov '09

Hello, I'm currently developing some applications with explicit threading using forkIO and have strange behaviour on my freshly installed Ubuntu Karmic 9.10 (Kernel 2.6.31-14 SMP). Setup: Machine A: Quadcore, Ubuntu 9.04, Kernel 2.6.28-13 SMP Machine B: AMD Opteron 875, 8 cores, 2.6.18-164 SMP- (some redhat) Machine C: Dual-Core, Ubuntu 9.10, Kernel 2.6.31-14 SMP Compiler on all machines: ghc 6.10.4 (downloaded from GHCs official website) Program, Compilation, Execution A simple taskqueue with independent tasks and explicit parallelization (hence should deliver more or less perfect speedup). For one core wall-times around 16 are ok, for 2 a bit more than 8 seconds. Since I used the same sources and Makefiles on all machines all files were compiled with -threaded and started with +RTS -N2 -RTS. Testing: Machine A: Ok (meaning works and delivers the expected speedup) Machine B: Ok Machine C: Not ok (with -N2 wall times around 14-15 seconds) Looking at the core usage, for example with htop, I see that the second core is not really used on C. Executing OpenMP programs shows the expected speedup and usage of both cores, hence I do not think its a kind of general linux configuration problem. So, after all the testing I think its either the Linux Kernel or some other component of Ubuntu 9.10. But: Ubuntu is often used and I did not found any information regarding this problem. The simple solution of installing the old version of Ubuntu would probably help but should not be the way to go, should it? I'd be glad for any hints or comments, Michael -- Dipl.-Inf. Michael C. Lesniak University of Kassel Programming Languages / Methodologies Research Group Department of Computer Science and Electrical Engineering Wilhelmshöher Allee 73 34121 Kassel Phone: +49-(0)561-804-6269

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Final CFP: WFLP 2010. Deadlines extended: Abstract due Nov 18; Full paper due Nov 25 (LNCS)
by Pablo Nogueira 13 Nov '09

13 Nov '09

[Deadlines extended: Abstract due Nov 18; Full paper due Nov 25] ******************************************************************** Final Call For Papers 19th International Workshop on Functional and (Constraint) Logic Programming Madrid, Spain, January 17, 2010 http://babel.ls.fi.upm.es/events/wflp2010/ ********* colocated with Principles of Programming Languages POPL 2010 http://www.cse.psu.edu/popl/10/ ******************************************************************** IMPORTANT DATES Abstract Submission: November 18, 2009 **extended** Full Paper Submission: November 25, 2009 **extended** Acceptance Notification: December 20, 2009 Preliminary Proceedings: January 7, 2010 Workshop: January 17, 2010 SCOPE The aim of the Workshop on Functional and (Constraint) Logic Programming is to bring together researchers interested in functional programming and (constraint) logic programming with special emphasis on the integration of both paradigms and of other declarative programming extensions. It promotes the cross-fertilizing exchange of ideas and experiences among researchers and students from the different communities interested in the foundations, applications, and combinations of high-level declarative programming languages and related areas. The previous WFLP editions are: WFLP 2009 (Brasilia, Brazil), WFLP 2008 (Siena, Italy), WFLP 2007 (Paris, France), WFLP 2006 (Madrid, Spain), WCFLP 2005 (Tallinn, Estonia), WFLP 2004 (Aachen, Germany), WFLP 2003 (Valencia, Spain), WFLP 2002 (Grado, Italy), WFLP 2001 (Kiel, Germany), WFLP 2000 (Benicassim, Spain), WFLP'99 (Grenoble, France), WFLP'98 (Bad Honnef, Germany), WFLP'97 (Schwarzenberg, Germany), WFLP'96 (Marburg, Germany), WFLP'95 (Schwarzenberg, Germany), WFLP'94 (Schwarzenberg, Germany), WFLP'93 (Rattenberg, Germany), and WFLP'92 (Karlsruhe, Germany). LOCATION WFLP 2010 will be held on January 17, 2010 in Madrid, Spain, colocated with the 37th ACM SIGACT-SIGPLAN Symposium on Principles of Programming Languages (POPL 2010). WFLP 2010 solicits papers in all areas of functional and (constraint) logic programming, including but not limited to: * Foundations: formal semantics, logic variables, binding and abstract syntax, rewriting and narrowing, unification, constraint solving, dynamics, type theory, meta-theory, effects, etc. * Language Design: security, services, modules, type systems, multi-paradigm languages, concurrency and distribution, objects, libraries, generic programming, interoperability, etc. * Implementation: abstract machines, parallelism, compile-time and run-time optimizations, foreign-language interfaces, memory management, multi-threading, exploiting parallel hardware, etc. * Transformation and Analysis: abstract interpretation, specialization, partial evaluation, program transformation, program calculation, program proof, meta-programming, generative programming, etc. * Software Development: algorithms, data structures, design patterns, components and composition, specification, proof assistants, verification and validation, model checking, debugging, testing, profiling, tracing, etc. * Paradigm Integration: integration of declarative programming with other paradigms or features such as imperative, object-oriented, aspect-oriented, concurrent, real-time programming, event-driven architectures, etc. * Applications: education, industry, commercial uses, domain-specific languages, visual/graphical user interfaces, embedded systems, WWW applications, XML processing, artificial intelligence, knowledge representation and machine learning, deductive databases, advanced programming environments and tools, etc. SUBMISSIONS and PROCEEDINGS Authors are invited to submit papers presenting original unpublished work. Papers must be at most 15 pages long. Exceptionally, authors may surpass the page limit by providing well-marked appendices intended as reviewing aids. Appendices will not appear in the final publication. Submission categories include regular research papers, system descriptions, and short papers describing on-going work (at most 8 pages). Submissions must be formatted in Lecture Notes in Computer Science (LNCS) style. This requirement needs not apply to appendices. Papers should be submitted in pdf format electronically via the web-based submission site http://www.easychair.org/conferences/?conf=wflp2010 Preliminary proceedings will be available at the workshop. Selected authors will be invited to submit a full version of their papers after the workshop. Contributions accepted for the post-workshop proceedings will be published in Springer's Lecture Notes in Computer Science series. INVITED SPEAKER Mariangiola Dezani (University of Torino, Italy) PROGRAM CHAIR Julio Marino (Universidad Politecnica de Madrid, Spain) PROGRAM COMMITTEE Maria Alpuente (Universidad Politecnica de Valencia, Spain) Sergio Antoy (Portland State University, USA) Bernd Brassel (CAU Kiel, Germany) Olaf Chitil (Univ. of Kent, UK) Rachid Echahed (CNRS-IMAG, France) Santiago Escobar (Universidad Politecnica de Valencia, Spain) Moreno Falaschi (Universita di Siena, Italy) Murdoch Gabbay (Heriot-Watt University, UK) Maria Garcia de la Banda (Monash University, Australia) Victor Gulias (Lambdastream SL, Spain) Michael Hanus (CAU Kiel, Germany) Herbert Kuchen (Univ. of Muenster, Germany) Francisco Lopez-Fraguas (Universidad Complutense de Madrid, Spain) James Lipton (Wesleyan University, USA) Mircea Marin (Univ. of Tsukuba, Japan) Juan Jose Moreno-Navarro (Ministry of Science & Innovation, Spain) Brigitte Pientka (McGill University, Canada)

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