- Haskell-Cafe - Haskell.org

parsec, hGetContents
by brian 24 Aug '08

24 Aug '08

I'm writing an NNTP client API. When I connect with connectTo, I get a Handle 'h'. I immediately hGetContents h to get a string 'c' that I hope contains all the data the server will ever send. I put h and c in a data Connection and pass it to the functions that send or receive. I send a command and receive a response. The response might look like "200 OK\r\n". Parsec parses c. No problem. But if I send another command and receive another response, parsec parses the same string because (I think) I haven't trimmed the part I parsed on the previous command. Maybe c looks like "200 OK\r\n200 second thing\r\n". So I was thinking I need to parse and return the unparsed part so that the next parse will work. Parsec has a getInput that returns the unparsed part, but it blocks if there is no more to parse. That is, after I parse "200 OK\r\n", if I ask parsec for the "rest", it blocks because there isn't any "rest" yet. How should I be doing this? Before people suggest I hGetLine and parse that, I don't like hGetLine for this because 1) it assumes '\n' signifies the end of a line, whereas in my case the protocol specifies "\r\n" and 2) if the "line" doesn't have an ending, I can't tell it from what hGetLine returns. That is, hGetLine on a handle having "abc\n" returns the same thing as on a handle having "abc". Also, if I can parse c directly, I can write parsers (especially those for multiline respones) much more naturally than I can if I have to get lines separately. Thanks very much for any help.

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"Real World Haskell" hits a milestone
by Bryan O'Sullivan 23 Aug '08

23 Aug '08

Hi, all - I'm proud to report that John, Don and I have finished the draft manuscript of our book. It is now available online in its entirety. For more details, see here: http://www.realworldhaskell.org/blog/2008/08/22/our-writing-is-now-complete/ We expect the final book to be published around the beginning of November, and to weigh in at about 700 pages. Thank you to everyone in the Haskell community who has encouraged and assisted us so far!

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multi-core programming in Haskell
by Galchin, Vasili 23 Aug '08

23 Aug '08

Hello, With pure side of the Haskell house, there is hope that the generated code could automagically scale as more cores are added yes? It seems that it is on the stateful monadic side of the house in an appplication that it is the programmer responsibility to design the software so that it scales across increasing cores? (I am assuming that things like "par" construct are monadic). On Monday, I am starting a several month project with a company. Alledgely some of the code will be written in Python. I would like engage the manager in a discussion about multi-core "enabling" the code now when we design and implement not later as an afterthought. Seems like a gnarly subject given current "state-of-the-art" software tools. Ideas?! Regards, Vasili

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question about uploads of code contribution
by Galchin, Vasili 22 Aug '08

22 Aug '08

Hello, 1) I want to upload a version with minor changes. Should I send out an "announcement"? 2) does the hackage database have a "reader/writer" lock to protect readers, i.e. people downloading when I am uploading? Regards, Vasili

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Libraries of standard QuickCheck properties
by Sean Leather 22 Aug '08

22 Aug '08

This in response to a comment on a GHC ticket [1]. I thought it was interesting enough to warrant general discussion. Comment (by JeremyShaw): > > > Also, where does the H98 report say all instances of Eq must be > transitive, reflexive, symmetric, and antisymmetric? It just says "The Eq > class provides equality (==)..", whatever that might mean :-) > > Well, it does not say it explicitly, but I suspect H98's usage of Eq > implicitly demands those laws be followed. Indeed. The same goes for the implicit law that (x /= y) /= (x == y), since both (/=) and (==) can be overridden. [2] Hopefully in Haskell' the laws > will not only be stated, but there will be some QuickCheck-style > properties you can use to test your own instances ;) This is an interesting thought. Has there been any work towards collecting properties written with QuickCheck (or similar) into a reusable chunk of some form? It would be very convenient to have parametrized properties that tested implicit laws such as equality, monads, and other things that come with the Haskell Platform. Not only would such a library be useful to use, but it would provide a number of readily available examples for would-be testers, thus encouraging testing. Sean [1] http://hackage.haskell.org/trac/ghc/ticket/2528#comment:6 [2] http://www.haskell.org/tutorial/monads.html#sect9.1

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HTTP package: connection closing bug?
by Arjun Guha 22 Aug '08

22 Aug '08

Hi, I'm using HTTP-3001.0.4 with GHC 6.8.3 under Mac OS X 10.5 and Debian Lenny. If I open and close many connections, I eventually get the error. Anyone else seen this before? *** Exception: socket: resource exhausted (Too many open files) Apparently, the socket is not closing. in TCP.hs:168, the call to shutdown is raising a "socket already closed" exception. So, the program never reaches the sClose on line 171. A simple fix is: wanderlust:Network arjun$ diff TCP.hs.original TCP.hs 172c172 < } --- > } `Exception.catch` (\_ -> sClose sk) The code that demonstrates this problem on Mac OS X is: > module Main where > > import Network.HTTP > import Network.URI (parseURI) > import Data.Maybe (fromJust) > import Control.Monad > > googleM n = do > s <- simpleHTTP $ Request (fromJust $ parseURI "http://www.google.com ") > GET [] "" > when (n `mod` 100 == 0) $ putStrLn (show n) > return () > > main = mapM_ googleM [1..5000] On Debian Lenny, the code works fine with google.com. However, when I try to connect to my local CouchDB server, it throws an exception after roughly 1000 connections. Thanks. Arjun

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the "process" package ...
by Galchin, Vasili 22 Aug '08

22 Aug '08

Hello, The "process" package Internal.hs references HsProcessConfig.h but this include file is not in the package "include" directory. Is this C include supposed to be generated? ?? KInd regards, Vasili

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parser combinators need how many values to report errors?
by Jason Dusek 22 Aug '08

22 Aug '08

I'm looking for a copy of: Predictive Parser Combinators Need four Values to Report Errors. I've poked around on the JFP's web site at Cambridge and they don't seem to have Volume 6 anymore. -- _jsn

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CouchDB module in Yhc source tree: clarification, and small problems with other packages
by Dimitry Golubovsky 22 Aug '08

22 Aug '08

Hi, Don asked: > Are we likely to see the couchdb bindings released as a standalone > library for the wider community? Let me explain a little bit why the CouchDB module appears in the Yhc repo. As the next stage of the Yhc/Javascript project, I am trying to set up a web service where people might upload some Haskell code (or reference to such code, these details haven't been clarified yet), and get it compiled by background Yhc/ycr2js batch job, returning a HTML page with Javascript in response. CouchDB was picked* as intermediate storage for this project. Therefore an interface module was needed, so I wrote one. This module, `Database.CouchDB' is not 100% finished; it was coded up to satisfy the needs of the first utility I wrote, `cdbup' which is currently capable of uploading a text (utf-8 expected) or a binary file as a document attachment to CouchDB. So some functionality hasn't been coded yet, although it will definitely be, as the project develops. Regarding packaging, I hope I will eventually put it on Hackage, but there are some problems with another packages which I'll try to explain here, so maybe those packages' developers suggest something. 1. Text.JSON. This module was picked from Jun Mukai's haskellnet package (http://darcs.haskell.org/SoC/haskellnet/) One problem with this package, it needs too many dependencies (while JSON module relies only on Packrat parser). I had to extract the JSON module into Yhc tree; can the haskellnet package be split somehow into orthogonal sub-packages, and maybe one toplevel package which would pull all parts together if used? 2. Text.JSON again. I had to change the pNumber function (parser for numeric nodes) replacing the '1'..'9' digits range with '0' .. '9'. Otherwise it did not handle JSON-encoded numbers equal to zero. I am not sure I was 100% right, so perhaps some JSON experts here might correct me. 3. Packrat parser does not seem to be packaged anywhere else, does it? 4. The dataenc package (http://code.haskell.org/dataenc/devo/) from which I pulled the Codec.Binary.Base64 module. The package may need some adjustment to GHC 6.8.x packages structure as only "base" listed in build-depends does not seem to be enough: when building it, Cabal complains for hidden packages such as collections (which I had to add manually to the cabal file), etc. Is it some problem with my GHC setup, or does just the dataenc.cabal need to be updated? So, in order to get things done quicker, I pulled the said pieces from their packages into Yhc tree. For packaging the CouchDB interface, definitely these duplicates need to be removed. Other packages that CouchDB relies upon, are**: base, network, HTTP, filepath, base64-string, utf8-string Thanks for the interest in CouchDB interface, and feel free to ask any questions. --- * before support form IBM/Apache was announced, but this recent development definitely increases interest to CouchDB ** Just from the -package options I used to compile it; I may have something omitted here. -- Dimitry Golubovsky Anywhere on the Web

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poll: how can we help you contribute to darcs?
by Eric Kow 22 Aug '08

22 Aug '08

Dear Haskellers, I would like to take an informal poll for the purposes of darcs recruitment. Could you please complete this sentence for me? "I would contribute to darcs if only..." The answers I am most interested in hearing go beyond "... I had more time". For instance, if you are contributing to other Haskell/volunteer projects, why are you contributing more to them, rather than darcs? The context: Lately, darcs has suffered a setback: the GHC team has decided that it is now time to switch to a different system, like git or Mercurial. This is probably a good thing for GHC and for us. By the way, good luck to them, and thanks for everything! (better GHC == better darcs) But where is darcs going? For now, we are going to have to focus on what we do best, providing precision merging and a consistent user interface for small-to-medium sized projects. I want more, though! I want to see darcs 2.1 come out next year, performance enhanced out the wazoo, and running great on Windows. And I want to see Future Darcs, the universal revision control system, seamlessly integrating with everybody else. We need to learn to do better so that darcs can achieve this kind of wild success. For example, whereas darcs suffers from the "day job" problem, xmonad has had to turn developers away! As Don mentions, this is partly thanks to their extreme accessibility (better self-documentation). But does anyone have more specific ideas about things we need to change so that you can contribute to darcs? How do we hit critical hacker mass? I have jotted down some other thoughts here regarding recruitment here: http://wiki.darcs.net/index.html/Recruitment In the meantime, if you have been discouraged from hacking on darcs, we want to know why, and how we can change things! Thanks,

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cabal build command and package versions
by Nicolas Frisby 21 Aug '08

21 Aug '08

I have a question about cabal's behavior for the build command. When using the build command on a cabalized project, any version changes for installed packages go unnoticed - the necessary modules in the project are not re-compiled. If however, you run the configure command (though the .cabal file for the project has not changed) and then the build command, the appropriate modules (and only the appropriate modules) are re-compiled. Not knowing that the configure command is necessary to detect changes in package that the current project depends on and proceeding only with the build command has led to BusErrors and GHC incurring the impossible in my exploration. Is there a reason that the build command does not check the packages for version changes? It seems fair to expect package-sensitivity of the process that determines if modules need to be re-compiled. This process, I think, is part of the build command and not the configure command. Here's an example scenario: - Imagine package Q depends on package P. - We "runhaskell Setup.hs clean/configure/build/install" them both, from scratch, at version 0. - Then we change package P (by, say, introducing new fields to a constructor that Q cases over), bump its version to 1, and "runhaskell Setup.hs clean/configure/build/install" on it. - If we now "runhaskell build" on package Q, nothing is re-compiled, though a package dependency has changed. "runhaskell Setup.hs build" does not notice this. - However, if we "runhaskell configure/build" on package Q, then the (necessary) modules are re-compiled. Thanks for your time.

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Haskell Xcode Plugin
by Dennis Buchmann 21 Aug '08

21 Aug '08

Hello, during my search for an acceptable development environment under Mac OS X I found this Plugin for Xcode: http://hoovy.org/HaskellXcodePlugin/ Unfortunately, I'm not able to get it to run in Xcode v3.0 and the developer seems to be not contactable at the moment. So, has anyone installed this plugin successfully (with problems in the beginning)? With best regards Dennis Buchmann

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unknown flags scramble
by Evan Laforge 21 Aug '08

21 Aug '08

I've been getting a mysterious error for a while now. It's come and gone, but seems to be showing up slightly more consistently now. Before I was on ghc 6.8.2 and now I'm on ghc 6.8.3, OS X 10.4 and now 10.5, i386 arch. The problem is that randomly while compiling I get an error like this: unknown flags in {-# OPTIONS #-} pragma: -XDeriveDataTypeabl<Data/Array/IArray.hs" #<e Ui.Font ( TextStyle(..), Font(..), FontFace(..) ) where import qualified Data.Generics as <Data/Array/IArray.lhs.L<array-0.1.0.0ali<Color as Color import Foreign import qualified Ui.Util as Util {-# LINE 14 "Ui/Font.hsc" It looks like somehow pragma parsing gets a bunch of random junk scrambled in. In the past this would happen consistently until I did a make clean and rebuild, but more recently all I have to do is try again and it doesn't happen the second time. I haven't gotten it while compiling for a while, but since I'm using hint which uses the ghc api to interpret haskell, I'm getting this quite frequently inside a GhcException when hint wants to interpret some code. I've also gotten this from ghci occasionally, and the usual fix is just to do a reload, and the second time it works. Does this sound familiar to anyone? It's not fatal since a retry clears things up, but it's a little worrying and would not look very good in released software. I cast about a bit on the ghc trac and didn't find any open bugs like this.

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a recent Haskell contribution?
by Galchin, Vasili 21 Aug '08

21 Aug '08

Hello, Recently someone made a post about a message-passing IPC that they contributed (within one month?). I have been searching the Haskell cafe archive to no avail. Can the contributor (or any one else) tell where the code is and any papers? Thank you, Vasili

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lines of code metrics
by Greg Fitzgerald 21 Aug '08

21 Aug '08

Does anyone know of a good case study comparing a project written in C versus one written in Haskell? I'm mostly looking for a comparison of lines of code, but any other metric, such as time to market and code quality metrics could also be

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"build depends:" in a .cabal file
by Galchin, Vasili 21 Aug '08

21 Aug '08

Hello, It seems to me that the "build depends" attribute/field is only informational, i.e. it doesn't cause "faulting in" dependencies if not present? If true, this seems to be a deficiency in cabal. ?? Kind regards, Vasili PS It seems to me that for HaskellDB the "build depends" is incomplete in that it should contain "old time" and "TextPrettyPrint.HughesPJ"??

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FRP question
by Tim Newsham 21 Aug '08

21 Aug '08

I'm interested in FRP, have read several of the papers and am wondering if it could be applied to writing multi-client server programs. What would be the input type events for such a system? What would be the output type events? How would the system handle the fact that it has to multiplex several IO streams (sockets)? I'm trying to answer these questions myself and having a hard time. Should input events include new connection requests and shutdowns? Individual bytes? Streams of bytes? or higher level PDUs? What kind of output should be generated? Full PDUs? streams of bytes? Pairs of (connection identifier,data)? How would such a system effectively hide the multiplexed IO going on? Is this sort of problem poorly suited for FRP? Tim Newsham http://www.thenewsh.com/~newsham/

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Re: [Haskell] Compiler Construction course using Haskell?
by Johannes Waldmann 21 Aug '08

21 Aug '08

Martin Erwig wrote (on a similar course he taught): > * Many students did not know Haskell before, so I had to teach some > Haskell and was forced to avoid some more advanced features, such as > type classes and monads. Yeah, in my case I know they don't know Haskell... Still, I trust them to pick it up quickly, and understand type classes (by analogy to interfaces in Java) and - they have to learn monads. These are master students, they should be open to a challenge ... NB: The course is declared "optional". As a prerequisite, there is a (mandatory) course in Principles of Programming Languages. J.W.

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Re: [Haskell] Compiler Construction course using Haskell?
by Johannes Waldmann 21 Aug '08

21 Aug '08

> This has me worried slightly: do you intend to showcase > monads, or do you intend to teach compiler construction? The subject is Compiler Construction ... Doing tree traversals via monads would, in OO-Speak, just correspond to using some Visitor pattern for the AST. The students know about these patterns. J.W.

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Haskell Weekly News: Issue 82 - August 20, 2008
by Brent Yorgey 21 Aug '08

21 Aug '08

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20080820 Issue 82 - August 20, 2008 --------------------------------------------------------------------------- Welcome to issue 82 of HWN, a newsletter covering developments in the [1]Haskell community. Announcements Lava2000 on Hackage. Emil Axelsson [2]announced that [3]Lava2000 has been uploaded to Hackage. Lava is a structural hardware description library embedded in Haskell. This version of Lava focuses on verification, and connects to a number of different verification engines (although only Smv and Satzoo are maintained in this version). QuickCheck and HPC Tutorial. Andy Gill [4]announced a half day tutorial on QuickCheck and HPC to be held at [5]DEFUN, taught by Andy Gill and Koen Claessen. The latest version of QuickCheck will also be released in conjunction with the tutorial. Compiler Construction course using Haskell?. Johannes Waldmann [6]announced that he plans to give a course in compiler construction, using Haskell as the implementation language. Any comments are appreciated, especially from anyone who has given or taken such a course. witness 0.1, open-witness 0.1. Ashley Yakeley [7]announced the release of these new packages and the draft paper on which they are based. posix-realtime 0.0.1. Vasili Galchin [8]announced the release of the posix-realtime package. benchpress 0.2.1. Johan Tibell [9]announced the first public release of [10]benchpress, a micro-benchmark library that produces statistics such as min, mean, standard deviation, median, and max execution time. wavconvert 0.1.1. Tim Chevalier [11]announced the release of [12]wavconvert, a tool which takes a directory tree as an argument, and converts any .wav files in it to .ogg (using an external OGG encoder) while filling in the ID3 tags based on the directory names. darcs hacking sprint (25-26 October). Eric Y. Kow [13]announced a darcs hacking sprint, to be held on the 25-26 of October, in Cambridge and Portland (and over IRC). The perfect opportunity to get involved, if you are interested in doing some hacking on darcs. Let Eric know if you are interested. X Haskell Bindings. Antoine Latter [14]announced that he is slowly [15]porting XCB (the X C Bindings) to Haskell, and would like input from any interested parties. logfloat. wren ng thornton [16]announced the release of the logfloat package for manipulating log-domain floating numbers. The main reason for casting numbers into the log-domain is to prevent underflow when multiplying many small probabilities as is done in Hidden Markov Models and other statistical models often used for natural language processing. The log-domain also helps prevent overflow when multiplying many large numbers. Roguestar 0.2.2. Christopher Lane Hinson [17]announced the release of version 0.2.2 of Roguestar. Google Summer of Code Progress updates from participants in the 2008 [18]Google Summer of Code. Hoogle 4. Neil Mitchell (ndm) is working on [19]Hoogle 4. [20]This week, he worked on getting out the last kinks to make [21]Hoogle 4 usable as a replacement for [22]Hoogle 3. Although this is the last official week of the GSoC, he plans to continue working on it when he has time. DPH physics engine. Roman Cheplyaka (Feuerbach) is working on a [23]physics engine using [24]Data Parallel Haskell. GHC plugins. Max Bolingbroke is working on dynamically loaded plugins for GHC. Cabal dependency framework. Andrea Vezzosi (Saizan) is working on a [25]make-like dependency analysis framework for Cabal. Language.C. Benedikt Huber (visq) is [26]working on Language.C, a standalone parser/pretty printer library for C99. Generic tries. Jamie Brandon is working on a library for efficient maps using generalized tries. GHC API. Thomas Schilling (nominolo) is working on [27]improvements to the GHC API. Jobs Platform Architect at Peerium, Inc.. Don Stewart [28]forwarded a job opportunity for a Haskell programmer at [29]Peerium, Inc. Peerium is building a new software platform for direct, real-time communication and collaboration within graphically rich environments, and is looking for a platform architect proficient in Haskell to work with the founders to design and implement components of the runtime and supporting libraries for our software platform. Resumes should be forwarded to hr [30]peerium.com. Blog noise [31]Haskell news from the [32]blogosphere. * Don Stewart: [33]The 500 Packages: Haskell, Distros, and Maintainership. * Neil Mitchell: [34]Hoogle New Features. Neil has finished his Google Summer of Code work on the [35]new version of Hoogle, but still intends to continue working on it when he gets the chance. A number of new features have been added. * Well-Typed.Com: [36]Solving the diamond dependency problem. Duncan explains how the newest version of Cabal (mostly) solves the DDDP (dreaded diamond dependency problem). * Real-World Haskell: [37]Two new chapters, preorder links, and a page count estimate. Two new beta chapters are up! * Dan Piponi (sigfpe): [38]Untangling with Continued Fractions: Part 1. Dan continues his fascinating series on rational tangles, showing how to describe a tangle using do-notation. * Christopher Lane Hinson: [39]ANN: Roguestar 0.2.2. * Luis Araujo: [40]Himerge (and Haskell) out to the street!. * Neil Mitchell: [41]GSoC Hoogle: Week 12. * Twan van Laarhoven: [42]A generic merge function. * Luke Palmer: [43]Slicing open the belly of the IO monad in an alternate universe. Luke explores methods of safely performing some I/O without the IO monad in the context of FRP. * Ketil Malde: [44]The wee beginnings of a biohaskell tutorial? -- and some thoughts on programming productivity. * Douglas M. Auclair (geophf): [45]Using Difference Lists. * Real-World Haskell: [46]Mid-August status update. Quotes of the Week * SimonM: People don't seem to believe me when I say this. In retrospect we shouldn't have called it forkOS, we should have called it forkReallyExpensiveOnlyNecessaryForCallingOpenGL_IO. * quicksilver: unsafeInterleaveIO gives you a way to cheat if you're happy to descend into a mire of programs with no semantics. * dmhouse: Eurgh, metastereo quotes. * quicksilver: [on @yhjulwwiefzojcbxybbruweejw] it's the noise a haskell developer makes if you kick him in the de bruijn index. About the Haskell Weekly News New editions are posted to [47]the Haskell mailing list as well as to [48]the Haskell Sequence and [49]Planet Haskell. [50]RSS is also available, and headlines appear on [51]haskell.org. To help create new editions of this newsletter, please see the information on [52]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [53]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.general/16381 3. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/chalmers-lava2000 4. http://article.gmane.org/gmane.comp.lang.haskell.general/16380 5. http://www.deinprogramm.de/defun-2008/ 6. http://article.gmane.org/gmane.comp.lang.haskell.general/16379 7. http://article.gmane.org/gmane.comp.lang.haskell.general/16376 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43295 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43292 10. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/benchpress 11. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43282 12. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/wavconvert 13. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43240 14. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43230 15. http://community.haskell.org/~aslatter/code/xhb 16. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43211 17. http://blog.downstairspeople.org/2008/08/16/ann-roguestar-022/ 18. http://hackage.haskell.org/trac/summer-of-code/wiki/SoC2008 19. http://code.haskell.org/hoogle/ 20. http://neilmitchell.blogspot.com/2008/08/gsoc-hoogle-week-12.html 21. http://haskell.org/hoogle/beta 22. http://haskell.org/hoogle 23. http://haskell.org/haskellwiki/Hpysics 24. http://www.haskell.org/haskellwiki/GHC/Data_Parallel_Haskell 25. http://code.haskell.org/~Saizan/cabal 26. http://www.sivity.net/projects/language.c/wiki/ 27. http://hackage.haskell.org/trac/ghc/wiki/GhcApiStatus 28. http://article.gmane.org/gmane.comp.lang.haskell.cafe/43175 29. http://www.peerium.com/ 30. file://localhost/home/brent/haskell/hwn/at] 31. http://planet.haskell.org/ 32. http://haskell.org/haskellwiki/Blog_articles 33. http://cgi.cse.unsw.edu.au/~dons/blog/2008/08/21#the_500 34. http://neilmitchell.blogspot.com/2008/08/hoogle-new-features.html 35. http://haskell.org/hoogle/ 36. http://blog.well-typed.com/2008/08/solving-the-diamond-dependency-problem/ 37. http://www.realworldhaskell.org/blog/2008/08/18/two-new-chapters-preorder-l… 38. http://sigfpe.blogspot.com/2008/08/untangling-with-continued-fractions_16.h… 39. http://blog.downstairspeople.org/2008/08/16/ann-roguestar-022/ 40. http://araujoluis.blogspot.com/2008/08/himerge-and-haskell-out-to-street.ht… 41. http://neilmitchell.blogspot.com/2008/08/gsoc-hoogle-week-12.html 42. http://twan.home.fmf.nl/blog/haskell/generic-merge.details 43. http://luqui.org/blog/archives/2008/08/14/slicing-open-the-belly-of-the-io-… 44. http://blog.malde.org/index.php/2008/08/14/the-wee-beginnings-of-a-biohaske… 45. http://logicaltypes.blogspot.com/2008/08/using-difference-lists.html 46. http://www.realworldhaskell.org/blog/2008/08/13/mid-august-status-update/ 47. http://www.haskell.org/mailman/listinfo/haskell 48. http://sequence.complete.org/ 49. http://planet.haskell.org/ 50. http://sequence.complete.org/node/feed 51. http://haskell.org/ 52. http://haskell.org/haskellwiki/HWN 53. http://code.haskell.org/~byorgey/code/hwn/

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Re: [Haskell] Compiler Construction course using Haskell?
by Derek Elkins 21 Aug '08

21 Aug '08

On Wed, 2008-08-20 at 16:03 +0200, Johannes Waldmann wrote: > Hello. > > I plan to give a course in compiler construction, > using Haskell as the implementation language > (not as source or target language). > > Something along these lines: > 1. combinator parsers (Parsec), > 2. simple interpreter (arithmetical expressions) > 3. add algebraic data types, functions > 4. type checker > 5. code generator. > Ideally, 2..5 would be using the very same tree traversal code > and just change the monad for evaluation. > > Any comments appreciated. Have you given such a course? Taken? [I've replied to Haskell-Cafe which is a better list for this discussion.] 2 & 3 are going to have different trees so using the same tree traversal code would require some finesse, though the code for 2 should only need extension not change. One thing you may want to look at is attribute grammars which can be cast into a monadic framework and gives a natural example of using the backward state monad and allows you to connect to other formalisms used for compiler construction. Another possibility is abstract interpretation. Both code generation and type checking can be viewed as examples of abstract interpretation (as well as, of course, actual interpretation.) Also many analyses fit into the model of abstract interpretation. I'd also recommend transforming the initial AST to some intermediate form before doing 2-5. Not only is this a virtually universal practice, but it will the later stages, particularly the interpreter and the code generator easier to write and, in the code generator's case, able to produce better output. Ultimately, trying to have the same code produce all of these is going to lead to some compromises. You are either going to have to forsake some quality in the output, or you are going to have unnatural or, at least, non-trivial implementations of some the functions. The recommendation to for the use of a intermediate language mitigates this somewhat.

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500 Haskell packages in Arch Linux
by Don Stewart 21 Aug '08

21 Aug '08

Monday was something of a landmark, as the 500th Haskell package was added to the Arch Linux distribution. I've written about the approach we took to fully automating the construction of native packages from Hackage, http://cgi.cse.unsw.edu.au/~dons/blog/2008/08/21#the_500 As well as a survey of 10 cool packages that are a little unknown. And some fun stats. On Hackage we have: * 723 unique packages * Over a million lines of Haskell code * 2600 pages of API documentation And in Arch, there are: 6 Erlang packages 21 OCaml packages 28 Lisp packages 101 Ruby packages 495 Python packages and now, 504 Haskell packages :) -- Don

1 0

Cyclic Inclusions
by C.M.Brown 20 Aug '08

20 Aug '08

Hi, I have a question about cyclic inclusions. It appears in the Haskell 98 report that mutually recursive modules are allowed, however GHC complains at any Haskell project that has cyclic inclusions (implicit or explicit). Am I right in thinking that this is a GHC limitation? http://www.haskell.org/onlinereport/modules.html Kind regards, Chris.

8 24

Re: [Haskell-cafe] Unboxing VT_VARIANT in hscom
by Krasimir Angelov 20 Aug '08

20 Aug '08

Aha. I got it. You should compile with -fglasgow-exts option. The extra type signatures doesn't matter. On Wed, Aug 20, 2008 at 8:20 AM, Praki Prakash <praki.prakash(a)gmail.com> wrote: > Krasimir - thanks for your reply. I had tried explicit typing but I still > get the same error. I have some code below and the error message. > > import Control.Exception > import Foreign > import Foreign.COM.Client > import Foreign.COM.Automation > import System.IO > > data WmiConnection = > WmiConnection { > execQuery :: String -> IO () > } > > main = withCOM $ do > conn <- wmiconnect "." "root\\cimv2" > execQuery conn "Select * From ..." > print "done" > > wmiconnect :: String -> String -> IO WmiConnection > wmiconnect comp cimroot = > do > clsid <- progid2clsid "WbemScripting.SWbemLocator" > (bracket (createInstance clsid iidIUnknown) release $ \iunkn -> > bracket (queryInterface iidIDispatch iunkn) release $ \idisp -> do > dispId <- getMethodID "ConnectServer" idisp > withBSTR comp $ \pComputer -> > withBSTR cimroot $ \pCimRoot -> do > (res,args) <- invoke dispId InvokeMethod [Variant VT_BSTR > pComputer, > Variant VT_BSTR > pCimRoot > ] idisp > print res > let conn = WmiConnection{ execQuery = queryFunc res } > return conn) > where > queryFunc :: Variant -> String -> IO () > queryFunc (Variant VT_DISPATCH idisp) query = do > dispId <- getMethodID "ExecQuery" idisp > return () > {- > queryFunc (Variant vt idisp) query = > if vt == VT_DISPATCH > then do > dispId <- getMethodID "ExecQuery" idisp > return "" > else > fail "error" > > return () > -} > > C:\>ghc --make test.hs > [1 of 1] Compiling Main ( test.hs, test.o ) > > test.hs:35:47: > Couldn't match expected type `IDispatch a' > against inferred type `a1' > `a1' is a rigid type variable bound by > the constructor `Variant' at test.hs:34:22 > In the second argument of `getMethodID', namely `idisp' > In a 'do' expression: dispId <- getMethodID "ExecQuery" idisp > In the expression: > do dispId <- getMethodID "ExecQuery" idisp > return () > > Any further suggestions? > > Thanks, > Praki > > On Tue, Aug 19, 2008 at 1:49 AM, Krasimir Angelov <kr.angelov(a)gmail.com> > wrote: >> >> This looks like a GHC bug to me. I am pretty sure that this worked >> before. Variant is defined like this: >> >> data Variant = forall a . Variant (VarType a) a >> >> data VarType a where >> .... >> VT_DISPATCH :: VarType (IDispatch ()) >> >> From this it clear that val is of type (IDispatch ()) because the >> VarType has value VT_DISPATCH. A workaround is to add explicit type >> singnature for val: >> >> someFunc (Variant VT_DISPATCH val) query = do >> dispId <- getMethodID "MethodName" (val :: IDispatch ()) >> >> I don't know why this doesn't work without the signature. >> >> Regards, >> Krasimir >> >> >> On Tue, Aug 19, 2008 at 7:09 AM, Praki Prakash <praki.prakash(a)gmail.com> >> wrote: >> > I am a Haskell newbie trying to do COM automation using Haskell. I am >> > using >> > hscom (Krasimir's implementation of COM automation). I have run into a >> > problem >> > and need some help. >> > >> > I have a Variant returned from a COM method invocation. When I print it, >> > it >> > shows up as below. >> > >> > Variant VT_DISPATCH <interface 0x00039f00> >> > >> > I need to invoke methods on the wrapped interface. My attempt to unbox >> > it as >> > below runs into 'rigid type' error. >> > >> > someFunc (Variant VT_DISPATCH val) query = do >> > dispId <- getMethodID "MethodName" val >> > >> > The code above generates this error. >> > >> > Couldn't match expected type `IDispatch a' >> > against inferred type `a1' >> > `a1' is a rigid type variable bound by... >> > >> > I am probably missing something pretty basic. Any help on this is >> > greatly >> > appreciated! >> > >> > Thanks >> > >> > >> > >> > >> > >> > _______________________________________________ >> > Haskell-Cafe mailing list >> > Haskell-Cafe(a)haskell.org >> > http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > >

1 0

ANN: Tutorial on information visualization and visual analytics in Haskell
by Jefferson Heard 20 Aug '08

20 Aug '08

This is the tutorial I'll be presenting at DEFUN 2008. I'll be building a site around it until then, complete with compilable code examples, but I thought I would let everyone get a sneak peek at the long version of the tutorial before I'm done with it. The code is as yet untested, and keep in mind, advanced Haskellers, that I'm purposefully simplifying some things to be understood by the beginner to the intermediate Haskeller. Comments and questions are welcome and encouraged. Please do ignore typos in the inline code for now, though, as I'll be spending this week testing it out and making sure everything works. The link is: http://bluheron.europa.renci.org/docs/BeautifulCode.pdf -- Jeff

10 12

darcs hacking sprint (25-26 October)
by Eric Y. Kow 19 Aug '08

19 Aug '08

Dear Haskellers, As Jason mentions, there has been recently been a lot of interest in darcs and an influx of new contributors. Thanks, everybody! Things are moving faster now with a few extra brains on board. We have some very ambitious plans, though and we still need all the help we can get. Do you think you might be interested in participating, but don't really know where to start? We may have just the solution for you: Darcs hacking sprint -------------------- You are cordially invited to hack on darcs with us for a weekend, specifically, that of 25-26 October. We will primarily be working on performance hacking of all sorts http://wiki.darcs.net/DarcsWiki/Roadmap If there are sufficient new hackers, I plan to prepare an overview and a set of starter tasks to help you get to grips with the darcs code. Venues ------ The venues are not yet fixed, although we have some tentative offers for space in London (thanks to Ganesh) and Portland (thanks to Galois). We will also be available via IRC, with some possibility for video-conferencing of some sort. For more information, please see http://wiki.darcs.net/index.html/Sprints If you have any suggestions for possible venues in London, please let me know. Our current London host can accommodate a handful of people, but we would love to have more people joining in on the fun! Also, if there are people who cannot make it to either London and Portland, perhaps you could gather into satellite teams? I hear Boulder, Colorado is a possibility. RSVP ---- If you would like to participate in any way, please let me know * when you are available (25/26 October) * roughly where you are based * if you are willing to travel Again, if you have suggestions for a good London venue, please send them my way. Many thanks! -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

2 2

Unboxing VT_VARIANT in hscom
by Praki Prakash 19 Aug '08

19 Aug '08

I am a Haskell newbie trying to do COM automation using Haskell. I am using hscom (Krasimir's implementation of COM automation). I have run into a problem and need some help. I have a Variant returned from a COM method invocation. When I print it, it shows up as below. Variant VT_DISPATCH <interface 0x00039f00> I need to invoke methods on the wrapped interface. My attempt to unbox it as below runs into 'rigid type' error. someFunc (Variant VT_DISPATCH val) query = do dispId <- getMethodID "MethodName" val The code above generates this error. Couldn't match expected type `IDispatch a' against inferred type `a1' `a1' is a rigid type variable bound by... I am probably missing something pretty basic. Any help on this is greatly appreciated! Thanks

2 1

Reader monad, implicit parameters, or something else altogether?
by Bjorn Buckwalter 19 Aug '08

19 Aug '08

All, I have a growing amount of astrodynamics code that relies on various physical "constants". The problem with these so called "constants" are that they either aren't really constants or aren't well known. An example is the leap second table (see Data.Time.Clock.TAI). I'd like to be able to fetch current values of these constants at runtime and make them accessible to my astrodynamics functions by some means. To clarify, once initialized the constants will be considered constant for the remainder of the program. I'd store the constants in a data structure along the lines of: > data AstroData a = AstroData > { mu_Earth :: GravitationalParameter a > , leapSeconds :: LeapSecondTable > } I would like to know if there is any consensus on what is the best way to make such a data structure accessible in pure functions. Passing it explicitly would be a mess. It seems that two options are to use either a Reader monad or implicit parameters. Using a Reader monad is straight forward enough though it requires writing/converting code in/to monadic style and adds some clutter to the formulae. It seems implicit parameters could be cleaner but I've seen them referred to as everything from evil to "just what you need" and rendering the Reader monad obsolete... What do you people recommend? Thanks, Bjorn Buckwalter

4 7

ANN: posix-realtime 0.0.1 (system category)
by Galchin, Vasili 19 Aug '08

19 Aug '08

Hello, 1) I have added more to "the posix realtime" support. - I put in parens above because I have currently uploaded outside the Haskell Unix library .. i.e. I am still open to submitting to the Unix library. - I have also put in parens because I have not personally implemented all of the posix realtime support .... others have also implemented Posix realtime support. 2) I am still trying to learn the Haskell FFI .. my bad .. thank you Brandon, Derek, Don(2?), Jason, Bulat, Duncan, Felipe, Bryan, Ketli, Luke, Jonthan, Jeremy, Jules. . .. Sorry for list .. alittle corney .. but nonetheless very grateful! Who did I forget who answered my questions? Is this a pre-order? ;^) 3) There are still issues with RTDataTypes.hsc that I am working with .. in particular Sigevent. 4) This has been a learning exercise for me as I said in terms of the Haskell API and Haskell monad class. Someone one in the 2) list suggested aHaskell Posix AIO (async IO) API implemented in terms of the State monad or Continuation monad ... I didn't ignore this suggestion ... I am still thinking about .. i.e. the AIO "API" is a somewhat moving target. Kind regards, Vasili

1 1

(fwd) OT: Cartoon Guide to Lob's Theorem
by Benjamin L.Russell 19 Aug '08

19 Aug '08

While this is somewhat off-topic, I found this interesting reference to a cartoon guide to Lob's Theorem, so I am forwarding it to this mailing list for some enjoyment in mathematical logic: On Mon, 18 Aug 2008 12:31:11 -0400, in gmane.lisp.scheme.plt Psy-Kosh <psykosh(a)earthlink.net> wrote: >This is a bit off topic, but I suspect some here would appreciate this: >http://www.overcomingbias.com/2008/08/lobs-theorem.html > >Remenicent of the graphical lambda game thing. > >Psy-Kosh >_________________________________________________ > For list-related administrative tasks: > http://list.cs.brown.edu/mailman/listinfo/plt-scheme -- Benjamin L. Russell

1 0

how can I receive the mail?
by shoulders 19 Aug '08

19 Aug '08

-----邮件原件----- 发件人: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] 代表 haskell-cafe-request@haskell. org 发送时间: 2008年8月19日 4:23 收件人: haskell-cafe(a)haskell.org 主题: Haskell-Cafe Digest, Vol 60, Issue 20 Send Haskell-Cafe mailing list submissions to haskell-cafe(a)haskell.org To subscribe or unsubscribe via the World Wide Web, visit http://www.haskell.org/mailman/listinfo/haskell-cafe or, via email, send a message with subject or body 'help' to haskell-cafe-request(a)haskell.org You can reach the person managing the list at haskell-cafe-owner(a)haskell.org When replying, please edit your Subject line so it is more specific than "Re: Contents of Haskell-Cafe digest..." Today's Topics: 1. Re: permutations and performance (John D. Ramsdell) 2. Houston area FPers.... (Galchin, Vasili) 3. Re: Fwd: Haskell job opportunity: Platform Architect at Peerium, Inc. (Benjamin L.Russell) 4. the cabal category field and "uploads" (Galchin, Vasili) 5. Re: Houston area FPers.... (Galchin, Vasili) 6. Re: the cabal category field and "uploads" (Don Stewart) 7. Re: the cabal category field and "uploads" (Galchin, Vasili) 8. Re: Re: Phantoms (Jason Dagit) 9. "build depends:" in a .cabal file (Galchin, Vasili) 10. Re: "build depends:" in a .cabal file (Galchin, Vasili) 11. Serial COM port access on Windows ( Kol?? Du?an ) 12. Re: FRP question (Ryan Ingram) 13. Re: "build depends:" in a .cabal file (Henning Thielemann) 14. missingH 0.18.6 - problems with cabal configure (Andrew U. Frank) 15. Re: "build depends:" in a .cabal file (Duncan Coutts) 16. Reader monad, implicit parameters, or something else altogether? (Bjorn Buckwalter) 17. Reader monad, implicit parameters, or something else altogether? (Bjorn Buckwalter) 18. Re: Reader monad, implicit parameters, or something else altogether? (Henning Thielemann) 19. Re: Reader monad, implicit parameters, or something else altogether? (Bjorn Buckwalter) 20. Re: Reader monad, implicit parameters, or something else altogether? (Brandon S. Allbery KF8NH) 21. Re: Reader monad, implicit parameters, or something else altogether? (Brandon S. Allbery KF8NH) 22. Re: Reader monad, implicit parameters, or something else altogether? (Henning Thielemann) 23. Re: Reader monad, implicit parameters, or something else altogether? (Henning Thielemann) 24. Fwd: [Haskell-cafe] Reader monad, implicit parameters, or something else altogether? (Bjorn Buckwalter) 25. Re: Fwd: [Haskell-cafe] Reader monad, implicit parameters, or something else altogether? (Henning Thielemann) 26. Re: X Haskell Bindings (David Roundy) 27. Re: Fwd: [Haskell-cafe] Reader monad, implicit parameters, or something else altogether? (Evan Laforge) 28. Re: Reader monad, implicit parameters, or something else altogether? (Brandon S. Allbery KF8NH) ---------------------------------------------------------------------- Message: 1 Date: Sun, 17 Aug 2008 22:40:30 -0400 From: "John D. Ramsdell" <ramsdell0(a)gmail.com> Subject: Re: [Haskell-cafe] permutations and performance To: "Henning Thielemann" <schlepptop(a)henning-thielemann.de> Cc: Haskell Cafe <haskell-cafe(a)haskell.org> Message-ID: <7687290b0808171940y49c67e93n4866a276a5b8a50(a)mail.gmail.com> Content-Type: text/plain; charset=UTF-8 On Sat, Aug 16, 2008 at 5:28 PM, Henning Thielemann <schlepptop(a)henning-thielemann.de> wrote: > There is _one_ permutation with no elements, namely [], thus it must be > | n == 0 = [[]] I see. John ------------------------------ Message: 2 Date: Sun, 17 Aug 2008 21:47:33 -0500 From: "Galchin, Vasili" <vigalchin(a)gmail.com> Subject: [Haskell-cafe] Houston area FPers.... To: mad.one(a)gmail.com Cc: Galchin Vasili <vigalchin(a)gmail.com>, "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> Message-ID: <5ae4f2ba0808171947v52dae7a2gf9582c1b31f50d20(a)mail.gmail.com> Content-Type: text/plain; charset="iso-8859-1" Hi Austin, I live next to the NASA Johnson Space Center. JSC is minutes away from Uof H Clear Lake. Which campus are you going to? (main campus?). Kind regards, Vasili

1 0

Fwd: [Haskell-cafe] Reader monad, implicit parameters, or something else altogether?
by Bjorn Buckwalter 19 Aug '08

19 Aug '08

On Mon, Aug 18, 2008 at 2:02 PM, Henning Thielemann <lemming(a)henning-thielemann.de> wrote: > > On Mon, 18 Aug 2008, Bjorn Buckwalter wrote: > >> On Mon, Aug 18, 2008 at 11:16 AM, Henning Thielemann >> <lemming(a)henning-thielemann.de> wrote: >>> >>> On Mon, 18 Aug 2008, Bjorn Buckwalter wrote: >>> >>>> I would like to know if there is any consensus on what is the best way >>>> to make such a data structure accessible in pure functions. Passing it >>>> explicitly would be a mess. It seems that two options are to use >>>> either a Reader monad or implicit parameters. Using a Reader monad is >>>> straight forward enough though it requires writing/converting code >>>> in/to monadic style and adds some clutter to the formulae. It seems >>>> implicit parameters could be cleaner but I've seen them referred to as >>>> everything from evil to "just what you need" and rendering the Reader >>>> monad obsolete... >>> >>> I expect that you will get the same range of opinions as you got from >>> your >>> search. As far as I know implicit parameters break referential >>> transparency. >>> >>> http://www.haskell.org/haskellwiki/The_Monad.Reader/Issue2/FunWithLinearImp… >>> So I prefer Reader monad. The burden of converting to monadic style pays >>> off when you need to use the same code with different values for the >>> "constants". (E.g. find out for which value of the Planck constant the >>> universe collapses and for which it oscillates etc. :-) >> >> Love the example but could you elaborate a little on how monadic style >> helps with this? (This is probably a matter of it not being obvious to >> me what approach you would take to solving the problem.) > > Instead of > muEarth :: GravitationalParameter a > muEarth = ??? > > escapeVelocity :: a > escapeVelocity = ... muEarth ... > > you would write > > data AstroData a = AstroData > { muEarth :: GravitationalParameter a > , leapSeconds :: LeapSecondTable > } > > escapeVelocity :: Reader (AstroData a) a > escapeVelocity = > do mu <- asks muEarth > return (... mu ...) > > Even better you would introduce a newtype for Reader (AstroData a). This way > you can add any monadic functionality later (Writer et.al.). Right, and I'd evaluate it using e.g.: runReader escapeVelocity myAstroData But with implicit params I suppose I'd define (untested) e.g.: escapeVelocity :: (?astro :: AstroData a) => a escapeVelocity = ... mu ... where mu = muEarth ?astro To evaluate this I'd use: let ?astro = myAstroData in escapeVelocity Which is comparable to the Reader version (with the advantage/disadvantage of the body of 'escapeVelocity' not being monadic). In retrospect I think I misunderstood what you were saying in you first email. I thought you were arguing that the monadic style would have an advantage over implicit params in the Planck problem. But you probably only meant to reemphasize the advantage (of either solution) over hard-coding constants... Thanks again, your Reader example is virtually identical to what I started off with so at least I know I'm not completely off target for a monadic implementation. (Sorry about the reposts Henning, I keep forgetting to cc the café!)

4 5

Kieran Beltran/Watson/IBM is out of the office.
by Kieran Beltran 19 Aug '08

19 Aug '08

I will be out of the office starting 08/18/2008 and will not return until 08/25/2008. I will be on vacation for a few days. If this is urgent please reach my Admin Kristine Smester ((917) 472-3387), otherwise I will respond to your message when I return.

1 0

X Haskell Bindings
by Antoine Latter 19 Aug '08

19 Aug '08

Haskellers, I'm slowly porting XCB (the X C Bindings) to Haskell, and I would like input from any interested parties. The following is a summary of my plan so far. I'm interested in hearing any suggestions or concerns about what a Haskell library for writing X clients should look like. This is not a release announcement, and I can't make any promises about when this will be delivered. Code is available in darcs: -- darcs get http://community.haskell.org/~aslatter/code/xhb Some of the advantages XCB claims over xlib are: + smaller API + latency hiding when communicating with the X server + direct access to the X protocol + improved threading support + easier to extend What I plan for the X Haskell Bindings (XHB) are as follows: + All requests to the server are non-blocking (under most circumstances) + Requests which produce a reply from the server return a "token" or "receipt" + The caller may then, at a time of their choosing, query the receipt for the response (or error) from the server. This query is blocking. The API will look something like: > -- | Create a window as specified > createWindow :: Connection -> CreateWindow -> IO () > -- | Instruct the server that it should begin displaying the named window > mapWindow :: Connection -> WINDOW -> IO () > -- | List all of the extensions supported by the server > listExtensions :: Connection -> IO (Receipt (ListExtensionsReply)) > -- | Query a receipt for a response > getReply :: Receipt a -> IO (Either XError a) Note that the first two requests do not have replies, whereas the third request expects a reply from the server. Since the request to create a window has so many parameters, these parameters are all wrapped up into a "CreateWindow" data type, which is only ever used by the "createWindow" function. The "mapWindow" request only has one parameter, so it does not need it's own "MapWindow" data type. What I don't have planned out is what to do with the stream of events and errors that come back from the server. If an error is related to an outstanding receipt, it gets dumped there for the caller to examine directly. Other errors go into an error queue. Events go into a similar event queue. How should this queue be exposed in the API? Should the user of the library register an error/event callback? > registerErrorCallback :: Connection -> (XError -> IO ()) -> IO () Or is something like this enough: > pollForError :: Connection -> IO (Maybe (XError)) > waitForError :: Connection -> IO XError Each X extension defines its own set of errors and events (potentially). Should all of these be lumped together into one giant sum-type for errors and one for events? Take care, Antoine

4 3

Serial COM port access on Windows
by Kolář Dušan 18 Aug '08

18 Aug '08

Dear all, Is there any more or less specific library for access to serial COM port from Haskell on Windows? Or am I expected to use a file mapping? Could you point me to some texts/examples? I tried some google (GHC specific) with no expected result. The issue is the following - I have a device communicating via Bluetooth with my PC, PC can create a serial COM port over the Bluetooth connection... I used C and a specific library for C so far, but I'd rather not encapsulate it by some Foreign calls in Haskell. Thanks for hints, Dusan

1 0

Phantoms
by Andrew Coppin 18 Aug '08

18 Aug '08

I just (re)discovered that I can do things like data Foo x = Foo Int Int Now "Foo Int" and "Foo Double" are, as far as the type checker cares, two completely different types, even though in fact they are the same. This is actually Quite Useful, in the particular case I'm working on. Here's the thing though... How do I get it so that "Foo Int" and "Foo Double" produce slightly different strings when printed?

7 11

the cabal category field and "uploads"
by Galchin, Vasili 18 Aug '08

18 Aug '08

Hi Ross, When I upload a code contribution, how do I specify which category in the Hackage database, e.g. System? Kind regards, Vasili

2 2

Houston area FPers....
by Galchin, Vasili 18 Aug '08

18 Aug '08

Hi Austin, I live next to the NASA Johnson Space Center. JSC is minutes away from Uof H Clear Lake. Which campus are you going to? (main campus?). Kind regards, Vasili

1 1

Fwd: Haskell job opportunity: Platform Architect at Peerium, Inc.
by Don Stewart 18 Aug '08

18 Aug '08

Hey all, Below is a job opportunity for a Haskell programmer at Peerium, Inc. I was asked for forward to the community. Enjoy! -- Don ------------------------------------------------------------------------ Subject: Haskell Job Opportunity Platform Architect at Peerium, Inc. Please mail resumes to: hr(a)peerium.com Location: Cambridge, MA (Greater Boston Area) http://www.peerium.com Type: Full-time Skills: Proficiency and a strong interest in Haskell programming :) Bachelor's degree in computer science or equivalent from a four-year institution. Job Description Platform architects work with the founders to design and implement components of the runtime and supporting libraries for our software platform. Company Description At Peerium, we're striving to bring a new level of quality and efficiency to online communication and collaboration within virtual communities, social networks, and business environments. We believe that a new environment that supports the effortless sharing of both information and software will enable a level of online cooperation far beyond current Web-based technologies -- modern programming techniques will enable the creation of more robust and more powerful programs within these environments. ----- To this end, we're building a new software platform for direct, real- time communication and collaboration within graphically rich environments. We're currently building our first publicly available software product using many in-house technologies, including a powerful new programming language. Our first software product brings together a broad range of technologies, including: * Concurrent, distributed, functional programming * Graphically-rich, OpenGL interfaces * Peer-to-peer network protocols * Cryptographically secure communication * Distributed computation * Ubiquitous online agents ----- Peerium is located in the heart of Harvard Square in Cambridge, Massachusetts.

2 3

Re: [Haskell-cafe] permutations and performance
by Henning Thielemann 18 Aug '08

18 Aug '08

John D. Ramsdell schrieb: > Try deleting it and see what happens. Erm, yes, this case is wrong: >>>> | n <= 0 = [] There is _one_ permutation with no elements, namely [], thus it must be | n == 0 = [[]]

2 1

permutations and performance
by John D. Ramsdell 18 Aug '08

18 Aug '08

I tried to replace a permutation generator with one that generates each permutation from the previous one, in a stream-like fashion. I had hoped the stream-based algorithm would be more efficient because I use only one permutation at a time, so only the permutation and the previous one need be in memory at one time. I thought I'd share the results of testing the two algorithms. I first forced the algorithms to produce answers by printing the length of their results. Bad idea. The stream-based algorithm produces a stack overflow on an input that it can handle when the contents of every permutation is forced. In this run, touch = length. $ ghc -O perms.lhs $ echo '(True, 9)' | ./a.out Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize' to increase it. $ echo '(False, 9)' | ./a.out 362880 $ I forced all parts of the computation by summing all of the numbers in the output. The result show the more obvious algorithm is faster. $ ghc -O perms.lhs $ echo '(True, 12)' | time ./a.out 31614105600 299.56user 0.97system 5:00.75elapsed 99%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (0major+479minor)pagefaults 0swaps $ echo '(False, 12)' | time ./a.out 31614105600 213.86user 0.55system 3:34.90elapsed 99%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (0major+841minor)pagefaults 0swaps $ > module Main(main) where > main = > do (new, n) <- readLn :: IO (Bool, Int) > case new of > True -> print $ touch $ npermutations n > False -> print $ touch $ permutations n Touch all the numbers in the output. Originally, touch = length. > touch :: [[Int]] -> Int > touch xs = > sum (map sum xs) The permutation algorithm used by Serge Mechveliani in The Algebraic Domain Constructor DoCon. The idea of the algorithm was suggested to him by S.M.Abramov. > npermutations :: Int -> [[Int]] > npermutations n = > first : next (spanMonotoneous first) > where > first = take n [0..] > next (_ , []) = [] > next (decr, j:js) = > p : next (spanMonotoneous p) > where > p = concat [reverse smallers, [j], reverse greaters, [i], js] > (greaters, i:smallers) = span (> j) decr > spanMonotoneous (x:y:xs) > | x <= y = ([x], y:xs) > | otherwise = (x:ys, zs) > where > (ys,zs) = spanMonotoneous (y:xs) > spanMonotoneous xs = (xs, []) > p : next (spanMonotoneous p) > where > p = concat [reverse smallers, [j], reverse greaters, [i], js] > (greaters, i:smallers) = span (> j) decr > spanMonotoneous (x:y:xs) > | x <= y = ([x], y:xs) > | otherwise = (x:ys, zs) > where > (ys,zs) = spanMonotoneous (y:xs) > spanMonotoneous xs = (xs, []) Straight forward permation algorithm. > permutations :: Int -> [[Int]] > permutations n > | n <= 0 = [] > | n == 1 = [[0]] > | otherwise = > concatMap (insertAtAllPos (n - 1)) (permutations (n - 1)) > where > insertAtAllPos x [] = [[x]] > insertAtAllPos x (y : l) = > (x : y : l) : map (y :) (insertAtAllPos x l)

3 3

Houston-area FPers?
by Austin Seipp 17 Aug '08

17 Aug '08

Hi, In less than a week I'll be moving to Houston TX in order to start school at university (University of Houston.) I'm wondering if there are any functional programmers (particularly haskellers) in that part of the state? If so, a group meeting and perhaps eventually a user-group would be lovely - if only to chat and talk about what everybody is working on for the time being until something more formal can be set up. I'm posting this a little early to try and get the most responses possible - unfortunately at the moment I will not have my own mode of transportation around town (see: car) and I'm not a houston native as you can see, so it will take time for me to get up to speed with college *and* getting around town, but if anybody is in that area I'd like to hear back from you. Austin

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GHC 6.8.3 binary for older linux
by Agent Zhang 17 Aug '08

17 Aug '08

Halo~ I ran into the following error while trying to run a binary generated by GHC 6.8.x on our production machines with a not-so-recent linux installed (kernel 2.6.9). $ ./restyscript restyscript: timer_create: Invalid argument Evan building a pure static-linking executable did not solve the problem. Then I downloaded the binary GHC 6.8.3 from haskell.org on that machine, but the ghc crashes as well: $ ghc Floating point exception My teammate chaos++ found that the last time precision argument passed to the timer_create function was just too big. Manually editing the binary executable (the restyscript file in the previous example) solved this issue completely, but, yeah, it's terribly hacky. So I decided to build a GHC 6.8.3 from source on that old system using GHC 6.4.2. Fortunately, the binary GHC 6.4.2 from haskell.org does work there. The newly-built GHC solves all the problem. No floating-point exception nor invalid argument for timer_create. I've put my binary distribution on my site here: http://agentzh.org/misc/ghc-6.8.3-i386-old-linux.tar.bz2 Hopefully it'll be useful for someone else ;) Not sure if it's worth putting to the official download page as well :) Ideally GHC should inspect the kernel version and timer_create support at *runtime*, rather compile-time. It'll make our lives much easier ;) Thanks! -agentzh

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Cabal has trouble with projects that have both src and lib directories?
by Nicholas Andrews 17 Aug '08

17 Aug '08

Hi! I'm trying to set up a project with both src and lib directories with cabal. My blah.cabal file looks like: Library Build-Depends: base Exposed-Modules: Foo hs-source-dirs: lib/foo Executable hai Build-depends: base Main-is: Bar.hs ghc-options: -O hs-source-dirs: src/bar Other-Modules: Foo extra-libraries: Foo extra-lib-dirs: lib/bar But I get: $ runhaskell blah.cabal configure $ runhaskell blah.cabal build src/bar/Bar.hs:6:7: Could not find module `Foo': Use -v to see a list of the files... In Bar.hs, line 6 says "import Svm" What's wrong? Thanks!

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duplicate symbols for ODBC-1.1.4.4 when running with GHCi
by Andrew U. Frank 17 Aug '08

17 Aug '08

when i run a program (which works with GHC) under GHCi (in eclipse with FP support) then i get GHCi linker duplicate definition for symbol _sqlSucceeded whilst processing .... HSHDBC-odbc-1.1.4.4.o what is wrong here? any suggestion what i can change? thank you andrew

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Sun OpenSPARC project?
by Galchin, Vasili 17 Aug '08

17 Aug '08

Hello, Is this project open to the entire Haskell community or only students? Regards, Vasili

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ANN: logfloat
by wren ng thornton 17 Aug '08

17 Aug '08

-------------------------------------------- -- Announcing: logfloat 0.8.2 -------------------------------------------- I just released a new package, logfloat, for manipulating log-domain floating numbers. The main reason for casting numbers into the log-domain is to prevent underflow when multiplying many small probabilities as is done in Hidden Markov Models and other statistical models often used for natural language processing. The log-domain also helps prevent overflow when multiplying many large numbers. In rare cases it can speed up numerical computation (since addition is faster than multiplication, though logarithms are exceptionally slow), but the primary goal is to improve accuracy of results. A secondary goal has been to maximize efficiency since these computations are frequently done within a /O(n^3)/ loop. The Data.Numeric.LogFloat module provides a new data type LogFloat which handles all the conversions and optimizations and can be treated as any other number thanks to type classes. -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/logfloat Darcs: http://code.haskell.org/~wren/logfloat -------------------------------------------- -- Details -------------------------------------------- The package's version numbers start from 0.8 since this began as a port of my Perl module for doing the same thing. The code is very heavily documented, largely for pedagogical reasons. Since Haddock doesn't play very nicely with literate Haskell, there's also a small lhs2hs converter script which gets run when you call: runhaskell Setup.hs haddock. This converter requires the bytestring package. The library proper does not, so I didn't include this dependency for the package itself. The package uses GHC's rewrite rules to do log/exp fusion. In general, since a newtype is used, the module shouldn't add overhead (beyond crossing the log/exp boundary). In the future I'll be experimenting with strictness as well to try to ensure that everything stays in registers for those /O(n^3)/ loops. I'm not sure whether it'd be helpful to keep a lazy version around as well... The LogFloat type is restricted to non-negative numbers. In the future I'll also add a signed variant for those who aren't just doing statistical work. -- Live well, ~wren

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What's in a name?
by Andrew Coppin 17 Aug '08

17 Aug '08

The naming of cats is a difficult matter... Ahem. So as you may have noticed, we seem to have a profusion of packages all called "binary" or something dangeriously similar. There's also several "MD5" packages. I could point out a few others. So what I'm wondering is... Do we have a formal convention for the naming of packages and/or the naming of the modules they contain? How are name collisions supposed to be avoided? (E.g., Java uses domain names for this. If I write a package named Foo, I put all the classes in orphi.me.uk.Foo.*)

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why isn't the thread blocked?
by Changying Li 17 Aug '08

17 Aug '08

I writed a little program to test forkIO. in fact, I want to know how to implement the 'select' system call in haskell: module Main where import Control.Concurrent.Chan import Control.Concurrent import System.IO main = do chan <- newChan handles <- mapM ((flip openFile) ReadMode) ["/tmp/a","/tmp/b","/tmp/c"] let readF h = do myID <- myThreadId chan' <- dupChan chan char <- hGetChar h writeChan chan' $ show myID putStrLn [char] threads <- mapM (\h -> forkIO $ readF h) handles nr <- readChan chan mapM killThread $ filter (\x -> show x == nr ) threads putStrLn nr I first mkfifo /tmp/{a,b,c} , then run 'echo "hello" >/tmp/a', then 'runhaskell thisProgram.hs' but I got an error: test.hs: /tmp/b: hGetChar: end of file test.hs: /tmp/c: hGetChar: end of file I think the thread will be blocked when /tmp/b has nothing. but it get EOF, why ? -- Thanks & Regards Changying Li

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GHC 6.8.3 package for ArchLinux x86_64
by Dusan Kolar 16 Aug '08

16 Aug '08

Hello all, Is there any plan to make a package for archlinux x86_64 with (more or less) complete GHC 6.8.3 binaries? I know I can download and install myself, usually I do so myself, but the reason is to prepare a list of packages for others. So far, the 6.8.2 is available. Is there any possibility I could help with it? Some guidelines, how to prepare such a package, submit to a right place, etc. Thanks, Dusan

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