January 2012 - Haskell-Cafe

Patch combinators
by Emil Axelsson 16 Jan '12

16 Jan '12

Hi all! Based on ideas by Koen Claessen, I have made a small module for what might be called patch combinators: http://hpaste.org/56501 Examples are found as comments. Before I push this to Hackage, I just wanted to check if there is any package that already provides this sort of functionality. (We're planning to use this module for expressing compile-time constraints on Feldspar functions.) Thanks for any feedback! / Emil

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named pipe interface
by Serge D. Mechveliani 16 Jan '12

16 Jan '12

People, (I wonder: is this for beginners(a)haskell.org ?) I need to organize a string interface for a Haskell function Main.axiom and a C program fifoFromA.c via a pair of named pipes (in Linux, UNIX). The pipes are created before running, by the commands > mkfifo toA > mkfifo fromA Main.axiom outputs a string to toA and inputs the respond string from fromA as the result. fifoFromA inputs a string from toA, converts it to the string resStr, outputs resStr to fromA. Main.axiom must be able to be applied in a loop, and there must be avoided repeated opening of a file/channel in a loop. As an example, the string conversion in fifoFromA.c is put the conversion of each character to the lower case: -------------------------- fifoFromA.c ------------------------------ #include <string.h> #include <stdio.h> #include <fcntl.h> #define BOUND 64 int main() { int toA, fromA, i, numread; char buf[BOUND]; toA = open("toA", O_RDONLY); fromA = open("fromA", O_WRONLY); for (;;) { numread = read(toA, buf, BOUND); buf[numread] = '\0'; // printf("A: Read from toA: %s\n", buf); i = 0; // convert the string to the lower case while (i < numread) {buf[i] = tolower(buf[i]); i++;} write(fromA, buf, strlen(buf)); } } ----------------------------------------------------------------------- For the "to-A" part writen in C (instead of Haskell), this interface loop works all right. With Haskell, I manage to process only a single string in the loop, and then it ends with an error. Main.hs is given below. I never dealt with such an IO in Haskell. Can you, please, fix the code or give comments? Please, copy the response to mechvel(a)botik.ru (I am not in the list). Thank you in advance for your notes, ------ Sergei mechvel(a)botik.ru ------------------------------------------------------------------- import System.IO (IOMode(..), IO(..), Handle, openFile, hPutStr, hGetLine, hFlush) import System.IO.Unsafe (unsafePerformIO) dir = showString "/home/mechvel/ghc/axiomInterface/byLowerLevel/" toA_IO = openFile (dir "toA") WriteMode :: IO Handle fromA_IO = openFile (dir "fromA") ReadMode -- used as global values toA = unsafePerformIO toA_IO -- fromA = unsafePerformIO fromA_IO -- axiomIO :: String -> IO String axiomIO str = do hPutStr toA str hFlush toA hGetLine fromA axiom :: String -> String -> String axiom str = showString (unsafePerformIO $ axiomIO str) -- Examples of usage -------------------------------------------- -- main = putStr (axiom "ABC1" "\n") -- I -- putStr (axiom "ABC1" $ showChar '\n' $ axiom "ABC2" "\n") -- II {- III: putStr (shows resPairs "\n") where n = 9000 str0 = "ABC" strings = [str0 ++ (show i) | i <- [1 .. n]] resPairs = [(str, axiom str "") | str <- strings] -} ----------------------------------------------------------------- I use Glasgow Haskell ghc-7.01. Build: > gcc -o fifoFromA fifoFromA.c > ghc --make Main Running: first, command > ./fifoFromA on terminal-2, then command > ./Main on terminal-1. Now, the example (I) works -- in the sense that terminal-1 shows the result "abc1" after the program on terminal-2 is interrupted. II and III do not work. And the aim is the examples like III.

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Functor0?
by Evan Laforge 16 Jan '12

16 Jan '12

Often when I define some type that wraps something else, I find myself writing a function like the following: newtype Thing = Thing X liftThing f (Thing x) = Thing (f x) It's like a Functor, but I can't make it an instance because Functor requires that the type be parametric. So I've been using type families to make a kind of "0 argument functor": class Functor0 a where type Elem a :: * fmap0 :: (Elem a -> Elem a) -> a -> a instance Functor0 Thing where type Elem Thing = X fmap0 = liftThing Is there a name for this? A better way to do it?

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porting feed to xml-enumerator
by Simon Michael 16 Jan '12

16 Jan '12

Hi Sigbjorn (and Don), I'm back for another reason. feed leaks and uses a lot of memory due to the xml package. Rather than fix xml I ported feed to xml-enumerator, which is used by yesod and more actively maintained than xml. This seems to have fixed the problem so I'm thinking of uploading this version to hackage as feed-1.0 (which I'll use for rss2irc and hackagebot.) Please let me know whether you agree. Also you might be interested in moving your repo (http://code.galois.com/cgi-bin/gitweb?p=feed.git;a=summary) to github ? This would make it easier to publish my changes, either to the main repo or a branch or fork. Otherwise I'll need to get them to your repo somehow. Thanks again, -Simon On Feb 21, 2011, at 5:14 PM, Simon Michael wrote: > thanks for feed. I'm just investigating a bug with rss2irc, and I think I'm seeing problems in the current feed on hackage. It lookas as if there's no way to get item updated date as opposed item published date, and getItemPublishDate actually gets the updated date in the case of an atom feed (http://hackage.haskell.org/packages/archive/feed/0.3.8/doc/html/src/Text-Fe…) > > I'd like to contribute a fix. Would you be able to make feed's repo public, eg on darcsden.com or github ?

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HaskellWiki upload failed
by Henk-Jan van Tuyl 15 Jan '12

15 Jan '12

L.S., I am trying to upload a file of about 12 Mbyte to the HaskellWiki; the upload fails without error message. Is the file too big? Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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hackage trac broken
by Joachim Breitner 15 Jan '12

15 Jan '12

Hi, a Debian user tried to report a bug against cabal-install (http://bugs.debian.org/655752) but the trac instance linked from the Cabal homepage seems to be down: http://hackage.haskell.org/trac/hackage/ Internal Server Error TracError: IOError: [Errno 13] Permission denied: '/srv/trac/hackage/VERSION' Is this known and will it be fixed? Thanks, Joachim -- Joachim "nomeata" Breitner Debian Developer nomeata(a)debian.org | ICQ# 74513189 | GPG-Keyid: 4743206C JID: nomeata(a)joachim-breitner.de | http://people.debian.org/~nomeata

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SBLP - call for papers
by Simon Thompson 15 Jan '12

15 Jan '12

============================================================ CALL FOR PAPERS 16th BRAZILIAN SYMPOSIUM ON PROGRAMMING LANGUAGES Natal, Rio Grande do Norte, Brazil September 24-28, 2012 http://www.cbsoft.dimap.ufrn.br ============================================================ IMPORTANT DATES Paper abstract submission (15 lines): April 20nd, 2012 Full paper submission: April 27th, 2012 Notification of acceptance: May 28th, 2012 Final papers due: June 29th, 2012 INTRODUCTION The 16th Brazilian Symposium on Programming Languages, SBLP 2012, will be held in Natal, Brazil, on September 24-28, 2012. SBLP provides a venue for researchers and practitioners interested in the fundamental principles and innovations in the design and implementation of programming languages and systems. The symposium will be part of the 3nd Brazilian Conference on Software: Theory and Practice, CBSoft 2012, http://www.cbsoft.dimap.ufrn.br, which will host four traditional, well-established symposia: * XXVI Brazilian Symposium on Software Engineering (SBES) * XVI Brazilian Symposium on Programming Languages (SBLP) * XV Brazilian Symposium on Formal Methods (SBMF) * VI Brazilian Symposium on Components, Software Architecture and Software Reuse (SBCARS) SBLP 2012 invites authors to contribute with technical papers related (but not limited) to: * Program generation and transformation, including domain- specific languages and model-driven development in the context of programming languages. * Programming paradigms and styles, including functional, object-oriented, aspect-oriented, scripting languages, real-time, service-oriented, multithreaded, parallel, and distributed programming. * Formal semantics and theoretical foundations, including denotational, operational, algebraic and categorical. * Program analysis and verification, including type systems, static analysis and abstract interpretation. * Programming language design and implementation, including new programming models, programming language environments, compilation and interpretation techniques. SUBMISSIONS Contributions should be written in Portuguese or English. We solicit papers that should fall into one of two different categories: full papers, with at most 15 pages, or short papers, with at most 5 pages. All papers should be prepared using the SBC template. In particular, we encourage the submission of short papers reporting on partial results of on-going master dissertations or doctoral theses. All accepted papers will be published in the conference proceedings. Submissions should be done using SBLP 2012 installation of the EasyChair conference mangement system at http://www.easychair.org/conferences/?conf=sblp2012. As in previous editions, a journal special issue, with selected papers from accepted contributions, is anticipated. GENERAL CO-CHAIRS Nélio Cacho, UFRN Gibeon Aquino, UFRN Martin Musicante, UFRN PROGRAMME CHAIRS Francisco Heron de Carvalho Junior, UFC Luis Soares Barbosa, Univ. do Minho PROGRAMME COMMITTEE * Alberto Pardo, Univ. de La Republica * Alex Garcia, IME * Alvaro Freitas Moreira, UFRGS * André Rauber Du Bois, UFPel * Andre Santos, UFPE * Carlos Camarao, UFMG * Christiano Braga, UFF * Fernando Castor Filho, UFPE * Fernando Quintão Pereira, UFMG * Francisco Heron de Carvalho Junior, UFC (co-chair) * Joao Saraiva, Universidade do Minho * João F. Ferreira, Teeside University * Jonathan Aldrich, Carnegie Mellon Univ. * Jose Luiz Fiadeiro, Univ. of Leicester * Lucilia Figueiredo, UFOP * Luis Soares Barbosa, Univ. do Minho * Manuel António Martins, Univ. de Aveiro * Marcelo A. Maia, UFU * Marcello Bonsangue, Leiden Univ/CWI * Marcelo d'Amorim, UFPE * Marco Tulio Valente, UFMG * Mariza A. S. Bigonha, UFMG * Martin A. Musicante, UFRN * Noemi Rodriguez, PUC-Rio * Paulo Borba, UFPE * Peter Mosses, Swansea University * Qiu Zongyang, Beijing University * Rafael Dueire Lins, UFPE * Renato Cerqueira, PUC-Rio * Ricardo Massa, UFPE * Roberto S. Bigonha, UFMG * Roberto Ierusalimschy, PUC-Rio * Sandro Rigo, UNICAMP * Sergio Soares, UFPE * Simon Thompson, Univ. of Kent * Varmo Vene, Univ. de Tartu Simon Thompson | Professor of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt

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function composition
by TP 15 Jan '12

15 Jan '12

Hi, I have a basic question concerning function composition. I have used http://www.haskell.org/tutorial/functions.html to write a composition function: Prelude> let f°g = f g Prelude> let p = (*2) Prelude> let q = (+3) Prelude> p°q 4 14 Prelude> :t (°) (°) :: (t1 -> t) -> t1 -> t If I understand well, this means that the infix operator "°" takes a function of type t1, i.e. g in f°g, and applies f on it, which takes a type t1 as input and returns f(g) which is of type t. The final result is of type t. So the first argument is represented above by "(t1->t)", and the second by "t1", the final result being of type t. However, I am not able to get the type of p°q Prelude> :t p°q <interactive>:1:3: Couldn't match expected type `Integer' with actual type `Integer -> Integer' In the second argument of `(°)', namely `q' In the expression: p ° q Prelude> What's the problem here? How can I obtain the type of p°q? Thanks in advance, TP

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Installing dph-examples in Mac OS X Version 10.7.2
by mukesh tiwari 15 Jan '12

15 Jan '12

Hello all I am trying to install dph-examples on Mac OS X. First I have Xcode 4.2.1 and during the installation i got this <http://hpaste.org/56445> error Installing library in /Users/mukesh/.cabal/lib/dph-seq-0.5.1.1/ghc-7.2.1Registering dph-seq-0.5.1.1...Downloading dph-examples-0.5.1.2...Configuring dph-examples-0.5.1.2...Building dph-examples-0.5.1.2...Preprocessing executable 'dph-sumsq-seq' for dph-examples-0.5.1.2...[1 of 5] Compiling SumSquaresVectorised ( imaginary/SumSquares/dph/SumSquaresVectorised.hs, dist/build/dph-sumsq-seq/dph-sumsq-seq-tmp/SumSquaresVectorised.o )Error (fd:12: hGetLine: end of file)Warning: Couldn't figure out LLVM version!Make sure you have installed LLVMghc: could not execute: optcabal: Error: some packages failed to install:dph-examples-0.5.1.2 failed during the building phase. The exception was:ExitFailure 1 On stackoverflow<http://stackoverflow.com/questions/8864696/installing-dph-examples-in-mac-o…> , i got suggestion to remove Xcode 4.2 and install Xcode 3.2. After installing Xcode 3.2 , I am still getting this error. Macintosh:~ mukesh$ cabal install dph-examples Resolving dependencies... Configuring dph-examples-0.5.1.2... Building dph-examples-0.5.1.2... Preprocessing executable 'dph-sumsq-seq' for dph-examples-0.5.1.2... [1 of 5] Compiling SumSquaresVectorised ( imaginary/SumSquares/dph/SumSquaresVectorised.hs, dist/build/dph-sumsq-seq/dph-sumsq-seq-tmp/SumSquaresVectorised.o ) Error (fd:11: hGetLine: end of file) Warning: Couldn't figure out LLVM version! Make sure you have installed LLVM ghc: could not execute: opt cabal: Error: some packages failed to install: dph-examples-0.5.1.2 failed during the building phase. The exception was: ExitFailure 1 Macintosh:~ mukesh$ uname -a Darwin Macintosh 11.2.0 Darwin Kernel Version 11.2.0: Tue Aug 9 20:56:15 PDT 2011; root:xnu-1699.24.8~1/RELEASE_I386 i386 Macintosh:~ mukesh$ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.2.1 Xcode 3.2 ( 64 bit ) Could some one please tell me how to resolve this issue. Regards Mukesh Tiwari

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Is it possible to create an instance of MonadBaseControl for PropertyM (QuickCheck)/continuations?
by Oliver Charles 15 Jan '12

15 Jan '12

Hi I'm working on a little experiment at the moment: using monadic QuickCheck to test the integration of my code and the database. I see some of my functions having properties like - given a database in this state, selectAll should return all rows, and so on. My initial attempt has worked nicely, and now I'm trying to test some more complicated properties, but I'm hitting a problem with overlapping primary keys, and this is because I'm not correctly cleaning up after each check. The simplest solution to this is to bracket property itself, and for that I turned to Control.Monad.Trans.Control in lifted-base, but I am struggling to actually write an instance for MonadBaseContol IO (PropertyM IO). It seems that PropertyM is a continuation monad transformer: newtype PropertyM m a = MkPropertyM { unPropertyM :: (a -> Gen (m Property)) -> Gen (m Property) } Given this, is it possible to even write an instance of MonadBaseControl? From the lifted-base documentation, it explicitly calls out ConT as *not* having instances, so I wonder if it can't be done. Sadly, I also lack intuition as to what MonadBaseControl really means - I think it means 'capture the state of this monad, so later we can go back to exactly the same state (or sequence of actions?)', but this is very flakey. So... is this possible, and if so how can I move forward? Thanks for any help or advice! - Ollie

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