- Haskell-Cafe - Haskell.org

Foreign.C.Types question ...
by Galchin Vasili 13 Feb '08

13 Feb '08

Hello, http://cvs.haskell.org/Hugs/pages/libraries/base/Foreign-C-Types.html .. I want to look at the actual code for these C types so that I can see how the instances of Storable for each type are coded so e.g. I can see how sizeOf/allignment are coded. Thanks, Vasili

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Announce : Sydney (Australia) Functional Programming Group
by Erik de Castro Lopo 13 Feb '08

13 Feb '08

Hi all, I'd like to announce the FP-SYD, the Sydney (Australia) Functional Programming group. The intro page is here: http://groups.google.com/group/fp-syd To subscribe to the mailing list (mainly for meeting organisation, meeting announcements, and Sydney job offers) either go to the URL above of send mail to: fp-syd-subscribe at googlegroups dot com Cheers, Erik -- ----------------------------------------------------------------- Erik de Castro Lopo ----------------------------------------------------------------- "Hey, I've re-dorkulated." -- Prof. Frink (The Simpsons)

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Windows, or GHC?
by Magnus Therning 13 Feb '08

13 Feb '08

The following code will on Linux print three strings each followed by a NULL byte: module Main where putStr0 = putStr $ s ++ "\0" main = do putStr0 "Hello" putStr0 "Hello" putStr0 "Hello" On Windows however it will print nothing! In order to trigger printing I have to change the definition of putStr0 to putStr0 = putStr (s ++ "\0") >> hFlush stdout Is this difference in behaviour due to a bug in GHC on Windows or just a quirkiness of the platform? /M

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Export Haskell Libraries
by SevenThunders 13 Feb '08

13 Feb '08

I've finished a good sized Haskell project which I now must expose as a library (along with a lot of C code) to my fellow engineers. I had originally devoped my code on Windows XP and managed to learn how to wrap my Haskell code in a DLL and create an MSVC linkable library stub for the DLL. Although involved, the output in windows is relatively transparent in that as long as the paths are available to the Haskell runtime libraries and my DLL, linking is straightforward. Now however I find I must export the same libraries to linux. I decided to get 'smart' and repackage my build tools using CMAKE and cabal. Building the C object files is no problem and even configuring Cabal to generate my executables is not too painful. However the pain begins when one tries to link any Haskell object files or libraries to an external C program. Using ghc -v during the build reveals that gcc has about 50 -u flag arguments and links to 9 or 10 different libraries. Given that my little component is just a tiny part of a large software project involving many more engineers and many man years of work, it's not going to be fun to link my little library using all these crazy build flags to a large external project, especially when order is important. Wouldn't it be nice if there were a single run time library I could link to, (or even generate on the fly) that didn't have this intricate build step. How is ghc/Haskell supposed to integrate with the rest of the software world? A related issue in the building of my export libraries is cabal itself. I want to package my own library with some external object files etc and some Haskell code. It's easily done with ar and the correct object files. Unfortunately it is not clear from the documentation how I can force cabal to place the object files and generated stub files in a place where I can use them. (CMake is kind of forcing me to have them reside with the source for reasons I don't want to go into here.) I instead find myself copying the object files (*.o) back into the source directory from an obscure tmp directory several layers down from the source directory. This is not particularly portable and is probably quite fragile once someone decides to change the directory hiearcharchy in cabal. I guess I am surprised that on linux, with all of it's amazing software development tools, that Haskell export libraries would be this tricky to develop. -- View this message in context: http://www.nabble.com/Export-Haskell-Libraries-tf3569747.html#a9972866 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Datatypes - Haskell
by Mattes Simeon 12 Feb '08

12 Feb '08

Hello to everybody I am an new user of Haskel and generally in functional programming and I could say that I am very impressed from this Language. Though I can't understand the use of datatypes. Let's take a firly simple situtation e.g. data Pair a b = Pair a b i.e. an new type with name Pair parameterized over the types a,b with one Constructor named Paid which take two values of type a,b a more complex one would be data Either a b = Left a | Right b i.e a new type named Either parameterized over the types a, b and two Constructors 1. Left which take one value of type a and 2. Right which takes one value of type b I consider that the definitions above are well formulated. Nevertheless I can't understand them quite well. I have read that datatypes are used to define new structures. So, is there any corresponding example in C, sinch I am quite familiar with structures in it? I hope the word C here is allowed here :o) Thanks

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Haskell Weekly News - February 10, 2008
by Don Stewart 12 Feb '08

12 Feb '08

--------------------------------------------------------------------------- Haskell Weekly News Issue 69 - February 10, 2008 --------------------------------------------------------------------------- Welcome to issue 69 of HWN, a newsletter covering developments in the [1]Haskell community. A quick update HWN this week, with a gazillion new libraries on Hackage. (> 150 new and updated libs!) 1. http://haskell.org/ Hackage New and updated libraries in [2]the Hackage library database. 2. http://hackage.haskell.org/ * WordNet 0.1.1. Uploaded by Max Rabkin. [3]WordNet: Haskell interface to the WordNet database. * lazysmallcheck 0.1. Uploaded by Gwern Branwen. [4]lazysmallcheck: A library for demand-driven testing of Haskell programs. * DrIFT 2.2.3. Uploaded by Gwern Branwen. [5]DrIFT: Program to derive type class instances. * highlighting-kate 0.2. Uploaded by John MacFarlane. [6]highlighting-kate: Syntax highlighting. * leksah 0.1. Uploaded by Juergen NicklischFranken. [7]leksah: Genuine Haskell Face. * frag 1.1. Uploaded by Gwern Branwen. [8]frag: 3-D First Person Shooter (FPS). * GoogleChart 0.2. Uploaded by Evan Martin. [9]GoogleChart: Generate web-based charts using the Google Chart API. * HFuse 0.1. Uploaded by Gwern Branwen. [10]HFuse: HFuse is a binding for the Linux FUSE library. * GoogleChart 0.1. Uploaded by Evan Martin. [11]GoogleChart: Generate web-based charts using the Google Chart API. * Finance-Quote-Yahoo 0.5.0. Uploaded by Brad Clawsie. [12]Finance-Quote-Yahoo: Obtain quote data from finance.yahoo.com. 3. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/WordNet-0.1.1 4. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/lazysmallcheck-0… 5. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/DrIFT-2.2.3 6. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/highlighting-kat… 7. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/leksah-0.1 8. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/frag-1.1 9. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/GoogleChart-0.2 10. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HFuse-0.1 11. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/GoogleChart-0.1 12. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Finance-Quote-Ya… * binary-strict 0.3.0. Uploaded by Adam Langley. [13]binary-strict: Binary deserialisation using strict ByteStrings. * Stream 0.2.3. Uploaded by Wouter Swierstra. [14]Stream: A library for manipulating infinite lists.. * Finance-Treasury 0.1.1. Uploaded by Stephen Lihn. [15]Finance-Treasury: Obtain Treasury yield curve data. * Hedi 0.1. Uploaded by Paolo Veronelli. [16]Hedi: Line oriented editor. * newports 1.1. Uploaded by Brad Clawsie. [17]newports: List ports newer than N days on a FreeBSD system. * Finance-Treasury 0.1. Uploaded by Stephen Lihn. [18]Finance-Treasury: Obtain Treasury yield curve data. * GPLib 0.0. Uploaded by Gwern Branwen. [19]GPLib: Generic library for genetic programming. * nano-hmac 0.1.1. Uploaded by Hitesh Jasani. [20]nano-hmac: Bindings to OpenSSL HMAC.. * multiset 0.1. Uploaded by Twan VanLaarhoven. [21]multiset: The Data.MultiSet container type. * hpodder 1.1.0. Uploaded by John Goerzen. [22]hpodder: Podcast Aggregator (downloader). 13. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/binary-strict-0.… 14. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Stream-0.2.3 15. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Finance-Treasury… 16. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Hedi-0.1 17. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/newports-1.1 18. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Finance-Treasury… 19. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/GPLib-0.0 20. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/nano-hmac-0.1.1 21. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/multiset-0.1 22. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hpodder-1.1.0 * bimap 0.2.1. Uploaded by Stuart Cook. [23]bimap: Bidirectional mapping between two key types. * monadLib 3.4.4. Uploaded by Iavor Diatchki. [24]monadLib: A collection of monad transformers.. * IOSpec 0.2. Uploaded by Wouter Swierstra. [25]IOSpec: A pure specification of the IO monad.. * bimap 0.2. Uploaded by Stuart Cook. [26]bimap: Bidirectional mapping between two key types. * strictify 0.1. Uploaded by Sterling Clover. [27]strictify: Find a local optimum of strictness annotations.. * heap 0.2.2. Uploaded by Stephan Friedrichs. [28]heap: Heaps in Haskell. * LDAP 0.6.4. Uploaded by John Goerzen. [29]LDAP: Haskell binding for C LDAP API. * HStringTemplate 0.2. Uploaded by Sterling Clover. [30]HStringTemplate: StringTemplate implementation in Haskell.. * RJson 0.3.2. Uploaded by Alex Drummond. [31]RJson: A reflective JSON serializer/parser.. * network-dns 0.1.1. Uploaded by Adam Langley. [32]network-dns: A pure Haskell, asyncronous DNS client library. 23. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bimap-0.2.1 24. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/monadLib-3.4.4 25. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/IOSpec-0.2 26. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bimap-0.2 27. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/strictify-0.1 28. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/heap-0.2.2 29. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/LDAP-0.6.4 30. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HStringTemplate-… 31. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/RJson-0.3.2 32. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/network-dns-0.1.1 * lcs 0.2. Uploaded by Ian Lynagh. [33]lcs: Find longest common sublist of two lists. * tracker 0.1. Uploaded by Will Thompson. [34]tracker: Client library for Tracker metadata database, indexer and search tool. * CC-delcont 0.2. Uploaded by Dan Doel. [35]CC-delcont: Delimited continuations and dynamically scoped variables. * control-timeout 0.1.2. Uploaded by Adam Langley. [36]control-timeout: Timeout handling. * network-dns 0.1. Uploaded by Adam Langley. [37]network-dns: A pure Haskell, asyncronous DNS client library. * binary-strict 0.2.4. Uploaded by Adam Langley. [38]binary-strict: Binary deserialisation using strict ByteStrings. * heap 0.1.1. Uploaded by Stephan Friedrichs. [39]heap: Heaps in Haskell. * HCL 1.3. Uploaded by Justin Bailey. [40]HCL: High-level library for building command line interfaces.. * yi 0.3. Uploaded by Jean PhilippeBernardy. [41]yi: The Haskell-Scriptable Editor. * binary-strict 0.2.3. Uploaded by Adam Langley. [42]binary-strict: Binary deserialisation using strict ByteStrings. 33. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/lcs-0.2 34. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/tracker-0.1 35. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/CC-delcont-0.2 36. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/control-timeout-… 37. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/network-dns-0.1 38. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/binary-strict-0.… 39. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/heap-0.1.1 40. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HCL-1.3 41. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/yi-0.3 42. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/binary-strict-0.… * heap 0.1. Uploaded by Stephan Friedrichs. [43]heap: Heaps in Haskell. * pureMD5 0.1.2. Uploaded by Thomas DuBuisson. [44]pureMD5: MD5 implementations that should become part of a ByteString Crypto package.. * RJson 0.3.1. Uploaded by Alex Drummond. [45]RJson: A reflective JSON serializer/parser.. * template 0.1.1.1. Uploaded by Johan Tibell. [46]template: Simple string substitution. * network-bytestring 0.1.1.1. Uploaded by Johan Tibell. [47]network-bytestring: Fast and memory efficient low-level networking. * ftphs 1.0.4. Uploaded by John Goerzen. [48]ftphs: FTP Client and Server Library. * mersenne-random-pure64 0.1.1. Uploaded by Donald Stewart. [49]mersenne-random-pure64: Generate high quality pseudorandom numbers purely using a Mersenne Twister. * Diff 0.1. Uploaded by Sterling Clover. [50]Diff: O(ND) diff algorithm in haskell.. * crack 0.1. Uploaded by Trevor Elliott. [51]crack: A haskell binding to cracklib. * miniplex 0.3.3. Uploaded by Lukas Mai. [52]miniplex: simple 1-to-N interprocess communication. 43. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/heap-0.1 44. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/pureMD5-0.1.2 45. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/RJson-0.3.1 46. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/template-0.1.1.1 47. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/network-bytestri… 48. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ftphs-1.0.4 49. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/mersenne-random-… 50. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Diff-0.1 51. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/crack-0.1 52. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/miniplex-0.3.3 * colock 0.2.2. Uploaded by Lukas Mai. [53]colock: thread-friendly file locks that don't block the entire program. * mersenne-random-pure64 0.1. Uploaded by Donald Stewart. [54]mersenne-random-pure64: Generate high quality pseudorandom numbers purely using a Mersenne Twister. * network-rpca 0.0.1. Uploaded by Adam Langley. [55]network-rpca: A cross-platform RPC library. * xmonad-contrib 0.6. Uploaded by Spencer Janssen. [56]xmonad-contrib: Third party extensions for xmonad. * xmonad 0.6. Uploaded by Spencer Janssen. [57]xmonad: A tiling window manager. * codec-libevent 0.1.2. Uploaded by Adam Langley. [58]codec-libevent: Cross-platform structure serialisation. * bytestringparser 0.3. Uploaded by Bryan OSullivan. [59]bytestringparser: Combinator parsing with Data.ByteString.Lazy. * HStringTemplate 0.2. Uploaded by Sterling Clover. [60]HStringTemplate: StringTemplate implementation in Haskell.. * value-supply 0.1. Uploaded by Iavor Diatchki. [61]value-supply: A library for generating values without having to thread state.. * derive 0.1.1. Uploaded by Neil Mitchell. [62]derive: A program and library to derive instances for data types. 53. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/colock-0.2.2 54. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/mersenne-random-… 55. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/network-rpca-0.0… 56. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmonad-contrib-0… 57. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmonad-0.6 58. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/codec-libevent-0… 59. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bytestringparser… 60. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HStringTemplate-… 61. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/value-supply-0.1 62. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/derive-0.1.1 * control-timeout 0.1.1. Uploaded by Adam Langley. [63]control-timeout: Timeout handling. * mkcabal 0.4.1. Uploaded by Donald Stewart. [64]mkcabal: Generate cabal files for a Haskell project. * regexpr 0.2.9. Uploaded by Yoshikuni Jujo. [65]regexpr: regular expression like Perl/Ruby in Haskell. * mtlparse 0.0.0.5. Uploaded by Yoshikuni Jujo. [66]mtlparse: parse library use mtl package. * cgi 3001.1.5.2. Uploaded by Bjorn Bringert. [67]cgi: A library for writing CGI programs. * xhtml 3000.0.2.2. Uploaded by Bjorn Bringert. [68]xhtml: An XHTML combinator library. * harpy 0.4. Uploaded by Martin Grabmueller. [69]harpy: Runtime code generation for x86 machine code. * editline 0.2. Uploaded by Judah Jacobson. [70]editline: Bindings to the editline library (libedit).. * hmatrix 0.2.0.0. Uploaded by Alberto Ruiz. [71]hmatrix: Linear algebra and numerical computations. * regexpr 0.2.8. Uploaded by Yoshikuni Jujo. [72]regexpr: regular expression like Perl/Ruby in Haskell. 63. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/control-timeout-… 64. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/mkcabal-0.4.1 65. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regexpr-0.2.9 66. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/mtlparse-0.0.0.5 67. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/cgi-3001.1.5.2 68. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xhtml-3000.0.2.2 69. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/harpy-0.4 70. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/editline-0.2 71. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hmatrix-0.2.0.0 72. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regexpr-0.2.8 * pcre-light 0.3. Uploaded by Donald Stewart. [73]pcre-light: A small, efficient and portable regex library for Perl 5 compatible regular expressions. * mersenne-random 0.1. Uploaded by Donald Stewart. [74]mersenne-random: Generate high quality pseudorandom numbers using a SIMD Fast Mersenne Twister. * AvlTree 2.4. Uploaded by Adrian Hey. [75]AvlTree: Balanced binary trees using AVL algorithm.. * fec 0.1.1. Uploaded by Adam Langley. [76]fec: Forward error correction of ByteStrings. * COrdering 2.1. Uploaded by Adrian Hey. [77]COrdering: An algebraic data type similar to Prelude Ordering.. * i18n 0.3. Uploaded by Eugene Grigoriev. [78]i18n: Internationalization for Haskell. * binary-strict 0.2.2. Uploaded by Adam Langley. [79]binary-strict: Binary deserialisation using strict ByteStrings. * regexpr 0.2.6. Uploaded by Yoshikuni Jujo. [80]regexpr: regular expression like Perl/Ruby in Haskell. * RJson 0.2. Uploaded by Alex Drummond. [81]RJson: A reflective JSON serializer/parser.. * regexpr 0.2.5. Uploaded by Yoshikuni Jujo. [82]regexpr: regular expression like Perl/Ruby in Haskell. 73. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/pcre-light-0.3 74. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/mersenne-random-… 75. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/AvlTree-2.4 76. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/fec-0.1.1 77. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/COrdering-2.1 78. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/i18n-0.3 79. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/binary-strict-0.… 80. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regexpr-0.2.6 81. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/RJson-0.2 82. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regexpr-0.2.5 * fec 0.1. Uploaded by Adam Langley. [83]fec: Forward error correction of ByteStrings. * dataenc 0.10.2. Uploaded by Magnus Therning. [84]dataenc: Data encoding library. * regexpr 0.2.3. Uploaded by Yoshikuni Jujo. [85]regexpr: regular expression like Perl/Ruby in Haskell. * regexpr 0.2.2. Uploaded by Yoshikuni Jujo. [86]regexpr: regular expression like Perl/Ruby in Haskell. * reactive 0.3. Uploaded by Conal Elliott. [87]reactive: Simple foundation for functional reactive programming. * regexpr 0.2.1. Uploaded by Yoshikuni Jujo. [88]regexpr: regular expression like Perl/Ruby in Haskell. * djinn 2008.1.18. Uploaded by Lennart Augustsson. [89]djinn: Generate Haskell code from a type. * Etherbunny 0.3. Uploaded by Nicholas Burlett. [90]Etherbunny: A network analysis toolkit for Haskell. * regexpr 0.2.0. Uploaded by Yoshikuni Jujo. [91]regexpr: regular expression like Perl/Ruby in Haskell. * RJson 0.1. Uploaded by Alex Drummond. [92]RJson: A reflective JSON serializer/parser.. 83. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/fec-0.1 84. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dataenc-0.10.2 85. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regexpr-0.2.3 86. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regexpr-0.2.2 87. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/reactive-0.3 88. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regexpr-0.2.1 89. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/djinn-2008.1.18 90. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Etherbunny-0.3 91. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regexpr-0.2.0 92. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/RJson-0.1 * regexpr 0.1.7. Uploaded by Yoshikuni Jujo. [94]regexpr: regular expression like Perl/Ruby in Haskell. * reactive 0.2. Uploaded by Conal Elliott. [95]reactive: Simple foundation for functional reactive programming. * srcinst 0.8.10. Uploaded by John Goerzen. [96]srcinst: Build and install Debian packages completely from source. * dfsbuild 1.0.2. Uploaded by John Goerzen. [97]dfsbuild: Build Debian From Scratch CD/DVD images. * darcs-buildpackage 0.5.12. Uploaded by John Goerzen. [98]darcs-buildpackage: Tools to help manage Debian packages with Darcs. * anydbm 1.0.5. Uploaded by John Goerzen. [99]anydbm: Interface for DBM-like database systems. * HDBC-sqlite3 1.1.4.0. Uploaded by John Goerzen. [100]HDBC-sqlite3: Sqlite v3 driver for HDBC. * HDBC-postgresql 1.1.4.0. Uploaded by John Goerzen. [101]HDBC-postgresql: PostgreSQL driver for HDBC. * HDBC-odbc 1.1.4.0. Uploaded by John Goerzen. [102]HDBC-odbc: ODBC driver for HDBC. 94. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regexpr-0.1.7 95. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/reactive-0.2 96. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/srcinst-0.8.10 97. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dfsbuild-1.0.2 98. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/darcs-buildpacka… 99. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/anydbm-1.0.5 100. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HDBC-sqlite3-1.1… 101. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HDBC-postgresql-… 102. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HDBC-odbc-1.1.4.0 * magic 1.0.7. Uploaded by John Goerzen. [103]magic: Interface to C file/magic library. * ListLike 1.0.1. Uploaded by John Goerzen. [104]ListLike: Generic support for list-like structures. * LDAP 0.6.3. Uploaded by John Goerzen. [105]LDAP: Haskell binding for C LDAP API. * hg-buildpackage 1.0.4. Uploaded by John Goerzen. [106]hg-buildpackage: Tools to help manage Debian packages with Mercurial. * HDBC 1.1.4. Uploaded by John Goerzen. [107]HDBC: Haskell Database Connectivity. * HSH 1.2.5. Uploaded by John Goerzen. [108]HSH: Library to mix shell scripting with Haskell programs. * editline 0.1. Uploaded by Judah Jacobson. [109]editline: Bindings to the editline library (libedit).. * pureMD5 0.1.1. Uploaded by Thomas DuBuisson. [110]pureMD5: MD5 implementations that should become part of a ByteString Crypto package.. * hmp3 1.4. Uploaded by Donald Stewart. [111]hmp3: An ncurses mp3 player written in Haskell. * binary-strict 0.2.1. Uploaded by Adam Langley. [112]binary-strict: Binary deserialisation using strict ByteStrings. 103. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/magic-1.0.7 104. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ListLike-1.0.1 105. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/LDAP-0.6.3 106. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hg-buildpackage-… 107. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HDBC-1.1.4 108. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HSH-1.2.5 109. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/editline-0.1 110. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/pureMD5-0.1.1 111. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hmp3-1.4 112. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/binary-strict-0.… * GLFW 0.3. Uploaded by Paul Liu. [113]GLFW: A binding for GLFW, An OpenGL Framework. * hbeat 0.1.1. Uploaded by Tim Docker. [114]hbeat: A simple step sequencer GUI.. * hackage2hwn 0.2.1. Uploaded by Donald Stewart. [115]hackage2hwn: Convert hackage = Hackage RSS feeds to Haskell Weekly News format. * tagsoup 0.4. Uploaded by Neil Mitchell. [116]tagsoup: Parsing and extracting information from (possibly malformed) HTML documents. * HaXml 1.19.2. Uploaded by Malcolm Wallace. [117]HaXml: Utilities for manipulating XML documents. * Emping 0.3.1. Uploaded by Hans VanThiel. [118]Emping: derives heuristic rules from nominal data. * hbeat 0.1. Uploaded by Tim Docker. [119]hbeat: A simple step sequencer GUI.. * Imlib 0.1.1. Uploaded by Cale Gibbard. [120]Imlib: Added by CaleGibbard, Sun Jan 13 22:26:59 PST 2008.. * pcre-light 0.2. Uploaded by Donald Stewart. [121]pcre-light: A small, efficient and portable regex library for Perl 5 compatible regular expressions. * YamlReference 0.8. Uploaded by Oren BenKiki. [122]YamlReference: YAML reference implementation. 113. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/GLFW-0.3 114. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hbeat-0.1.1 115. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hackage2hwn-0.2.1 116. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/tagsoup-0.4 117. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HaXml-1.19.2 118. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Emping-0.3.1 119. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hbeat-0.1 120. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Imlib-0.1.1 121. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/pcre-light-0.2 122. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/YamlReference-0.8 * ContArrow 0.0.4. Uploaded by Evgeny Jukov. [123]ContArrow: Control.Arrow.Transformer.Cont. * ContArrow 0.0.3. Uploaded by Evgeny Jukov. [124]ContArrow: Control.Arrow.Transformer.Cont. * mkcabal 0.4. Uploaded by Donald Stewart. [125]mkcabal: Generate cabal files for a Haskell project. * pcre-light 0.1. Uploaded by Donald Stewart. [126]pcre-light: A lightweight binding to PCRE. * YamlReference 0.7. Uploaded by Oren BenKiki. [127]YamlReference: YAML reference implementation. * Crypto 4.1.0. Uploaded by Dominic Steinitz. [128]Crypto: DES, Blowfish, AES, TEA, SHA1, MD5, RSA, BubbleBabble, Hexdump, Support for Word128, Word192 and Word256 and Beyond, PKCS5 Padding, Various Encryption Modes e.g. Cipher Block Chaining all in one package.. * containers 0.1.0.1. Uploaded by Ross Paterson. [129]containers: Assorted concrete container types. * ConfigFile 1.0.4. Uploaded by John Goerzen. [130]ConfigFile: Configuration file reading & writing. * MissingH 1.0.0. Uploaded by John Goerzen. [131]MissingH: Large utility library. * hslogger 1.0.4. Uploaded by John Goerzen. [132]hslogger: Versatile logging framework. 123. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ContArrow-0.0.4 124. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ContArrow-0.0.3 125. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/mkcabal-0.4 126. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/pcre-light-0.1 127. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/YamlReference-0.7 128. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Crypto-4.1.0 129. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/containers-0.1.0… 130. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ConfigFile-1.0.4 131. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/MissingH-1.0.0 132. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hslogger-1.0.4 * hslogger 1.0.2. Uploaded by John Goerzen. [133]hslogger: Versatile logging framework. * BerkeleyDB 0.3. Uploaded by John McCall. [134]BerkeleyDB: Bindings for Berkeley DB v1.x. * BitSyntax 0.3.2. Uploaded by Adam Langley. [135]BitSyntax: A module to aid in the (de)serialisation of binary data. * Hashell 0.15. Uploaded by Gwern Branwen. [136]Hashell: Simple shell written in Haskell. * binary-strict 0.2. Uploaded by Adam Langley. [137]binary-strict: Binary deserialisation using strict ByteStrings. * Shu-thing 1.1. Uploaded by Gwern Branwen. [138]Shu-thing: A vector shooter game. * zlib 0.4.0.2. Uploaded by Duncan Coutts. [139]zlib: Compression and decompression in the gzip and zlib formats. * i18n 0.2. Uploaded by Eugene Grigoriev. [140]i18n: Internationalization for Haskell. * pandoc 0.46. Uploaded by John MacFarlane. [141]pandoc: Conversion between markup formats. * hscolour 1.9. Uploaded by Malcolm Wallace. [142]hscolour: Colourise Haskell code.. 133. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hslogger-1.0.2 134. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/BerkeleyDB-0.3 135. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/BitSyntax-0.3.2 136. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Hashell-0.15 137. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/binary-strict-0.2 138. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Shu-thing-1.1 139. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/zlib-0.4.0.2 140. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/i18n-0.2 141. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/pandoc-0.46 142. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hscolour-1.9 * regex-pcre 0.94.1. Uploaded by ChrisKuklewicz. [143]regex-pcre: Replaces/Enhances Text.Regex. * regex-posix 0.93.1. Uploaded by Chris Kuklewicz. [144]regex-posix: Replaces/Enhances Text.Regex. * regex-base 0.93.1. Uploaded by Chris Kuklewicz. [145]regex-base: Replaces/Enhances Text.Regex. * regex-compat 0.91. Uploaded by Chris Kuklewicz. [146]regex-compat: Replaces/Enhances Text.Regex. * haddock 2.0.0.0. Uploaded by David Waern. [147]haddock: Added by DavidWaern 143. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regex-pcre-0.94.1 144. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regex-posix-0.93… 145. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regex-base-0.93.1 146. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regex-compat-0.91 147. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/haddock-2.0.0.0 Blog noise [148]Haskell news from the [149]blogosphere. 148. http://planet.haskell.org/ * [150]Scala Buzzzzings * [151]Taxicab Numbers * [152]Tuppence Tour of Haskell Concurrency Constructs * [153]Power of Functional Programming, its Features and its Future * [154]Monads in Python (with nice syntax!) * [155]Monads in Ruby (with nice syntax!) * [156]The Marvels of Monads (in C#) * [157]Pointfree programming in OCaml * [158]HSOE Chapter 3 * [159]Extra type safety using polymorphic types as first-level refinements * [160]Why you should use Haskell for your next domain specific language 149. http://haskell.org/haskellwiki/Blog_articles 150. http://patricklogan.blogspot.com/2008/01/scala-buzzzzings.html 151. http://python-tips-gw.blogspot.com/2008/01/taxicab-numbers.html 152. http://mult.ifario.us/p/tuppence-tour-of-haskell-concurrency-constructs 153. http://www.canerten.com/power-of-functional-programming-its-features-and-it… 154. http://www.valuedlessons.com/2008/01/monads-in-python-with-nice-syntax.html 155. http://www.valuedlessons.com/2008/01/monads-in-ruby-with-nice-syntax.html 156. http://blogs.msdn.com/wesdyer/archive/2008/01/11/the-marvels-of-monads.aspx? 157. http://alaska-kamtchatka.blogspot.com/2008/01/pointless-polymorphism.html 158. http://apocalisp.wordpress.com/2008/01/08/hsoe-chapter-3/ 159. http://blog.jbapple.com/2008/01/extra-type-safety-using-polymorphic.html 160. http://paulspontifications.blogspot.com/2008/01/why-haskell-is-good-for-emb… * [161]Record system headaches * [162]Programming Erlang * [163]Haskell syntax is not Java syntax; good or bad? * [164]Judging programming languages by the results * [165]One line binary reader in Haskell * [166]On the utility of functional programming * [167]The Rise Of Functional Programming: F#/Scala/Haskell and the failing of Lisp * [168]Seeqsuq: A Seeqpod URL Ripper (in Haskell) * [169]2008 predictions * [170]The Haskell Program Coverage Toolkit (part 1) 161. http://jpmoresmau.blogspot.com/2008/01/nested-records-headaches-in-haskell.… 162. http://www.oxtremists.co.uk/?p=68 163. http://jpmoresmau.blogspot.com/2008/01/haskell-syntax-is-not-java-syntax-go… 164. http://www.pvv.org/~alexanro/2008/01/11/judging-programming-languages-by-th… 165. http://tehgeekmeister.wordpress.com/2008/01/11/one-line-binary-reader-in-ha… 166. http://geekrant.wordpress.com/2008/01/12/on-the-utility-of-functional-progr… 167. http://www.brandonwerner.com/2008/01/13/the-rise-of-functional-programming-… 168. http://cmars232.blogspot.com/2008/01/seeqsuq-seeqpod-url-ripper-in-haskell.… 169. http://lambda-the-ultimate.org/node/2600#comment-39191 170. http://blog.unsafeperformio.com/?p=18 * [171]Haskell Code Coverage tool available * [172]A Little Lesson on Laziness and Unsafety * [173]Haskell's do syntax for python and ruby * [174]Error handling in Python: monads are too much for me * [175]Simple POSIX regular expression example * [176]Read the content of a file into a list of string * [177]How not to explain Haskell monads * [178]HStringTemplate: An Elegant, Functional, Nifty Templating Engine for Haskell. * [179]A (hopefully) painless introduction to monads * [180]Simple Type Inference in Haskell 171. http://leibnizdream.wordpress.com/2008/01/15/haskell-code-coverage-tool-ava… 172. http://mult.ifario.us/p/a-little-lesson-on-laziness-and-unsafety 173. http://www.valuedlessons.com/2008/01/recently-i-wrote-about-ways-to-add.html 174. http://ndanger.org/blog/2008/01/16/error-handling-in-python-monads-are-too-… 175. http://athashdollarpercent.blogspot.com/2008/01/simple-posix-regular-expres… 176. http://athashdollarpercent.blogspot.com/2008/01/read-content-of-file-into-l… 177. http://mvanier.livejournal.com/1205.html 178. http://fmapfixreturn.wordpress.com/2008/01/14/hstringtemplate-an-elegant-fu… 179. http://www.prairienet.org/~dsb/monads.htm 180. http://notvincenz.blogspot.com/2008/01/simple-type-inference-in-haskell.html * [181]Quality assurance for Haskell code via code coverage * [182]Getting started with Yi, the haskell editor * [183]Haskell Impressions, Part I * [184]Software Transactional Memory for F# * [185]One or so word summary of the programming languages I work with * [186]HStringTemplate: Turtles all the way * [187]Scraping my boilerplate: Generics instead of Records * [188]Functional Programming is Hard * [189]Erik Meijer: Evaluating Functional Programming Language Purity :: Video * [190]Solving XKCD's Ghost problem in Haskell: First player wins 181. http://blog.unsafeperformio.com/?p=18 182. http://nobugs.org/developer/yi/index.html 183. http://blog.safira.com/2008/01/haskell-impressions-part-i.html 184. http://cs.hubfs.net/blogs/hell_is_other_languages/archive/2008/01/16/4565.a… 185. http://berlinbrowndev.blogspot.com/2008/01/one-or-so-word-summary-of-progra… 186. http://fmapfixreturn.wordpress.com/2008/01/18/turtles-all-the-way/ 187. http://jpmoresmau.blogspot.com/2008/01/scraping-my-boilerplate-generics.html 188. http://blog.magenic.com/blogs/aarone/archive/2008/01/18/Functional-Programm… 189. http://channel9.msdn.com/ShowPost.aspx?PostID=374141 190. http://squing.blogspot.com/2008/01/first-player-wins-superghost.html * [191]Haskell Function calls and Parentheses * [192]an elementary proof of the undecidability of the halting problem * [193]Invasion of the multi-core machines * [194]Monads for Imperative Peeps * [195]Haskell Shuffling * [196]Notes on Haskell * [197]Functional Programming Languages * [198]Case Study: Using Haskell and HAppS for Openomy API v2.0 * [199]A Haskell Hackathon (in Japanese) * [200]Control.Parallel.Strategies introduction 191. http://berlinbrowndev.blogspot.com/2008/01/haskell-function-calls-and-paren… 192. http://www.bemuzed.com/lucasd/halting-poem.html 193. http://blog.susheelspace.com/?p=126 194. http://climbing-the-hill.blogspot.com/2008/01/haskell-monads-for-imperative… 195. http://brett-hall.blogspot.com/2008/01/haskell-shuffling.html 196. http://penberg.blogspot.com/2008/01/its-not-dead-yet.html 197. http://blog.inquirylabs.com/2008/01/20/functional-programming-languages/ 198. http://blog.openomy.com/2008/01/case-study-using-haskell-and-happs-for.html 199. http://d.hatena.ne.jp/yukoba/20080121/p1 200. http://thoughtpolice.stringsandints.com/index.cgi/code/haskell#parallel_str… * [201]'I asked the manager if they were considering using such languages. He replied that they had actually considered using Haskell for some smaller projects' * [202]Solving IO Sequencing in Haskell with a Monad * [203]Haskell Music * [204]Matching checklists using Haskell * [205]State monads * [206]Haskell is a very expressive language * [207]Counting Infinity * [208]Haskell, del.icio.us, and JSON * [209]It's time for Haskell in the web browser * [210]OpenGL programming with Haskell 201. http://pinderkent.blogsavy.com/archives/150 202. http://wry-observer.blogspot.com/2008/01/solving-io-sequencing-in-haskell-w… 203. http://samdanielson.com/2008/1/23/haskell-music 204. http://porg.es/blog/matching-checklists-using-haskell 205. http://mvanier.livejournal.com/1901.html 206. http://berlinbrowndev.blogspot.com/2008/01/haskell-is-very-expressive-langu… 207. http://muaddibspace.blogspot.com/2008/01/counting-infinity.html 208. http://mult.ifario.us/p/haskell-del-icio-us-and-json 209. http://pinderkent.blogsavy.com/archives/151 210. http://metavar.blogspot.com/2008/01/opengl-programming-with-haskell.html * [211]unfolds in scala * [212]Haskell Snippet: looking at foldl (also a java version) * [213]MatLab: Why OCaml and Haskell are not ready for general scientists -- missing libraries * [214]Problems in Lisp: Historical relics, poor FFI, libraries, forks * [215]On blub * [216]Refining Pipelines * [217]Purely functional recursive types in Haskell and Python * [218]Counting matches in a list * [219]SPJ on .NET Rocks * [220]Hackage, cabal and the google charts api * [221]Haskell Rocks! * [222]How many functions are there from () to ()? 211. http://scala-blogs.org/2008/01/roman-numerals-in-scala.html 212. http://berlinbrowndev.blogspot.com/2008/01/haskell-snippet-looking-at-foldl… 213. http://alexandermikhalev.blogspot.com/2008/01/why-ocaml-and-haskell-is-not-… 214. http://nightschool.near-time.net/news/2008/1/31/some-improvements-are-distr… 215. http://etalog.blogspot.com/2008/01/on-blub.html 216. http://alaska-kamtchatka.blogspot.com/2008/02/refining-pipelines.html 217. http://sigfpe.blogspot.com/2008/02/purely-functional-recursive-types-in.html 218. http://haskell101.wordpress.com/2008/02/03/counting-matches-in-a-list/ 219. http://weblogs.asp.net/gbarnett/archive/2008/02/06/something-that-when-unno… 220. http://community.livejournal.com/evan_tech/241080.html 221. http://l33tminion.livejournal.com/189554.html 222. http://sigfpe.blogspot.com/2008/02/how-many-functions-are-there-from-to.html Quotes of the Week * Peter: I hardly know Haskell, but I can already write some code much faster and easier than I could do in C/C++ (and I've been programming 2 decades in that language) * Miguel Mitrofanov: Since NaN /= NaN, I think, we should decipher 'NaN' as 'Not a NaN' * Achim Schneider: The essence of non-strictness, though, is another kind of story. Like a golem plowing half of the country until you remember that you placed him a bit absent-mindedly into your backyard and said 'plow', that still won't plow mountains. The essence of strictness is easy, though: get stuck on a stone, fall over and continue moving until you break. * monochrom: OCaml tells you 'map f xs is bad because it takes O(n) space'. Haskell tells you 'map f xs' is good because it takes O(1) space * ddarius: It says *right on the box* that it it's intended for *both* research and applications. * Clifford Beshers: Why Haskell? Medical researchers announced today that MRI studies reveal that the structure of the human brain is significantly different in adolescence than adulthood. * Larry O'Brien: I really had a hard time refactoring my Ruby into a packrat parser. This was due, in no small part, because of the difficulty in understanding the types being built-up in the data structure. The paper, in explicitly-typed Haskell: clear as a bell. My code's behavior on unit tests: virtually indistinguishable from random. * SuperGrade: Haskell kind of herds you into functional programming. There are advantages to this manner of coding, and I'm getting to thinking you should attempt code this way almost all of the time, regardless of language. * Berlin Brown: Haskell is readable, fast, and expressive. I like it. * Yaakov Nemoy: Changing the type of a function in Python will lead to strange runtime errors that take some work to debug, whereas, when I tinker with a program in Haskell, I already know it will work once it compiles. * Michael Reid: Learning Haskell has completely reversed my feeling that static typing is an old outdated idea. The power of Haskell's type system makes it feel like you are programming in a dynamic language to some degree, yet all of it is type-checked, and that is just *really* cool. * weavejester: I've come across many parser generators that are a dream to work with in comparison to lex and yacc. The best one I've found so far is Haskell's Parsec. * David Roundy: What's the good of having a haskell-programming computational linguist on board if we can't get static compile-time guarantees of grammatical correctness? * Bulat Ziganshin: When FP just pass all the functions and data required for specialization of generic algorithm, OOP provides interfaces, virtual functions, anonymous classes, delegates and lots of other interesting ways to hide the fact of lack of first-class functions :) * [Douglas] Adams: was interested in computing --- I think his reaction to being told about functional programming was to wonder what non-functional programming might be. * ezekiel: I find Haskell to be like Lisp and APL and Python all put together in a way that leads me quickly to a solid result. Other languages work, but the road to the result is bumpier. * AlanYx: I've found that Haskell's uber-strong, static type system does help me be productive, because it increases the class of bugs that can be found at compile time, forces thinking at a higher level, and can help avoid edits to existing code breaking things elsewhere in the code * anonymous: Closures in today's world are a 'language geek' feature. Unless done extremely carefully and in a way that supports the various skill levels of developers, they end up being unusable and unsupportable by anything less than computer language savants. In their inherent obscurity and complexity, 'language geek' style closures are about as anti-Java as you can get. * anonymous: I work in a Smalltalk shop, where we extend and maintain an application that has been around over a decade. During the maintenance part of our job, it would be really nice to have type declarations on variables. For maintenance, the more dependable information we have, the better! The fewer things that can escape as runtime exceptions, the better! * consultant barbie: Languages are hard. Let's write web frameworks and go shopping! About the Haskell Weekly News New editions are posted to [223]the Haskell mailing list as well as to [224]the Haskell Sequence and [225]Planet Haskell. [226]RSS is also available, and headlines appear on [227]haskell.org. Headlines are available as [228]PDF. To help create new editions of this newsletter, please see the [229]contributing information. Send stories to dons at galois.com. The darcs repository is available at darcs get [230]http://code.haskell.org/~dons/code/hwn/ 223. http://www.haskell.org/mailman/listinfo/haskell 224. http://sequence.complete.org/ 225. http://planet.haskell.org/ 226. http://sequence.complete.org/node/feed 227. http://haskell.org/ 228. http://code.haskell.org/~dons/code/hwn/archives/20080210.pdf 229. http://haskell.org/haskellwiki/HWN 230. http://code.haskell.org/~dons/code/hwn/

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ANN: nano-hmac 0.2.0
by Hitesh Jasani 12 Feb '08

12 Feb '08

nano-hmac provides bindings to OpenSSL's HMAC interface. With this release the set of hashing functions supported is: MD5, SHA, SHA1, SHA224, SHA256, SHA384, SHA512. If you're unfamiliar with HMAC's then you may want to check out the second link below where I explain a little bit about them in a blog entry. The hackage pages mentioned that they're not running haddock 2.0, so I don't know if the docs will generate. If not, you can see the docs online at the third link below. * http://hackage.haskell.org/cgi-bin/hackage-scripts/package/nano-hmac-0.2.0 * http://www.jasani.org/2008/02/nano-hmac-020-released.html * http://docs.jasani.org/nano-hmac/0.2.0/ Any and all comments/suggestions/criticisms/fortune-cookie-proverbs are welcome. Thanks, Hitesh

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Announce: Melbourne Functional Programming Union
by Bernie Pope 12 Feb '08

12 Feb '08

Hello Haskellers, After a few years hiatus, I'm pleased to announce that the Melbourne Functional Programming Union (FPU) is back. What is the FPU? It is a group of people who are interested in all things functional programming. We hold regular informal talks, and have friendly discussions afterwards. We are situated at the University of Melbourne (Australia), but the Union is open to all comers. Obviously it helps if you live near this part of the World. To give you an idea of the kinds of things we talk about, here is a list of our most recent topics: - the GHCi debugger - Arrows for parser combinators - a curious stack overflow in Haskell - a demonstration of Coq - deriving monads from vague ideas in several easy steps (this coming Friday) Here is our old outdated web page for historical reference: http://www.cs.mu.oz.au/fpu/ A new page is under construction. At the moment, talks are held on Fridays, 1-2pm, in the ICT Building at Melbourne Uni. Contact Bernie Pope if you are interested in participating in the FPU.

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example packages that use Storable in Hackage ??
by Galchin Vasili 12 Feb '08

12 Feb '08

Hello, http://hackage.haskell.org/packages/archive/pkg-list.html .. what are some packages that use Storable? Regards, Vasili

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Parsec: Transforming between parsers with different token streams
by Maciej Podgurski 12 Feb '08

12 Feb '08

Hi, I'm writing a simple parser for a line-oriented language using Parsec. A group of lines is parsed by a parser that tokens are of type String. Each token is again parsed by a parser that tokens are of type Char (i.e. this parser has the Parsec type Parser a). Now I wrote a transforming function that converts a char parser to a line parser by running the char parser on the current token of the line parser and accepting the token, if the char parser did not fail: type LineParser = GenParser String () a toLineParser :: Parser a -> LineParser a toLineParser p = tokenPrim showLine nextPos testLine <?> "how to get the inner error message???" where showLine l = "\": " ++ l ++ "\"" nextPos pos t rest = incSourceLine pos 1 testLine l = case parse p "" l of Left msg -> Nothing Right res -> Just res My problem is to extract the possible error message of the inner char parser (msg) and pass it to the outer line parser. Maybe using tokenPrim is no the best approach here. Someone how can help me with that problem? Besh wishes, Maciej

1 0

Create a list without duplicates from a list with duplicates
by news＠lyra.net 12 Feb '08

12 Feb '08

Hallo! Let's suppose I have a list [a,b,c,d,c,d]. I'd like to write a function that returns a new list without duplicates (in the example [a,b,c,d]). How can I do that? What is the most general way? I'd like to use the same function for a list of Int or String or some other user defined data type. Thank for your attention!

8 9

Compulsory relation 1 to many from entity A to entity A
by news＠lyra.net 12 Feb '08

12 Feb '08

Hallo! I'd like to build a database model with winHugs that allows a "recursive relation". For example a single instance of entity "components" is related with at least another row of the entity "components" (1 to many relationship). How can I do that? Thank you for your attention!

2 1

f(g(x), h(y)) - Correct syntax
by news＠lyra.net 11 Feb '08

11 Feb '08

Hallo! I have this code: ---------------------------------------- q1 :: EName -> [ApprenticeInfo] q1 c = [apprenticeInfo n | n <- allApprentices, member ((sq4 c) (firstOf5(n))) == True] sq4 :: ESurname -> [IDB] sq4 c = (sq3 (sq1 (c))) firstOf5 :: (a,b,c,d,e) -> a firstOf5 (n,_,_,_,_) = n member :: [IDB] -> IDB -> Bool member [] y = False member(x:xs) y = (x==y) || member xs y ---------------------------------------- sq4 works correctly and returns [IDB] Unfortunately member ((sq4 c) (firstOf5(n))) gives this error: Type error in application *** Expression : sq4 c (firstOf5 n) *** Term : sq4 *** Type : ESurname -> [IDB] *** Does not match : a -> b -> c As far as I see it I'm not writing the correct syntax for a function of 2 functions. I have g(x): Esurname->[IDB] h(y): (IDB,_,_,_,_) -> IDB f(x,y): ([IDB], IDB) -> Bool I want to obtain f(g(x), h(y)) and I obtain something else. Where am I wrong? Thanks

3 2

Re: [Haskell-cafe] using the writer monad to better understand foldl and foldr, and haskell debugging techniques in general
by Thomas Hartman 11 Feb '08

11 Feb '08

same behavior with myfoldrD (:) [] [1..] -- uses Debug.Trace.trace So, I would say this proves my main point, which was that you could accomplish the same thing using the writer monad that you could do using the more "ad hoc" trace function from Debug.Trace. It's good that you point this out though, because understanding that foldr can take an infinite list and foldl not is a very key point. > 2008/2/10, Felipe Lessa <felipe.lessa(a)gmail.com>: > > On Feb 10, 2008 9:33 PM, Thomas Hartman <tphyahoo(a)gmail.com> wrote: > > > -- using writer monad > > > -- Nothing unsafe here, pure referrentially transparent goodness > > > myfoldrW f z [] = return z > > > myfoldrW f z (x:xs) = do > > > r <- (myfoldrW f z xs) > > > tell ("x,r: " ++ (show (x,r)) ++ "\n" ) > > > return $ x `f` r > > > > *Main> foldr const 0 [1..] > > 1 > > *Main> putStrLn $ snd $ runWriter $ myfoldrW const 0 [1..] > > Interrupted. > > > > One of the good things from foldr is the possibility of > > "short-circuiting", so to speak. However I don't know if it is > > possible to show this using the writer monad, as is would involve > > observing if the function is strict or not in its second argument. > > > > Cheers, > > > > -- > > Felipe. > > >

2 2

Newbie question: mutually exclusive strict / lazy
by Peter Verswyvelen 11 Feb '08

11 Feb '08

Consider the function cond x y z = if x then y else z I guess we can certainly say cond is strict in x. But what about y and z? If x is true, then cond is strict in y If x is false, then cond is strict in z So we can't really say cond is lazy nor strict in its second or third argument. Of course, this is the case for many more functions, but in the case of the if-then-else primitive, does the strictness analyzer make use of this "mutually exclusive strictness" fact? Cheers, Peter

4 4

using the writer monad to better understand foldl and foldr, and haskell debugging techniques in general
by Thomas Hartman 11 Feb '08

11 Feb '08

The following is a simple introduction to debugging techniques in haskell, illustrated with a canonical use of foldr and foldl. Comments welcome. import Control.Monad.Writer import Debug.Trace -- We use the writer monad to better understand foldl and foldr -- and show a debugging technique in haskell -- you could get similar output using Debug.Trace, but this relies on unsafePerformIO -- ... http://www.haskell.org/ghc/docs/latest/html/libraries/base/src/Debug-Trace.… -- which is Ugly. -- Though ugly, trace isn't really unsafe as far as my understanding goes. But still, I find it nice -- that there's a way to accomplish the exact same thing by using the writer or "debugger" monad -- which is pure as pure can get -- In this particular example, it doesn't matter if you use Debug.Trace or the writer monad. -- However, I am fidding around in another scenario (debugging a series of graphs using Data.Graph.Inductive) -- which seems not to lend itself well to trace. If I can find a nice way to explain what I am doing and why -- I may do a follow-up to this post describing that. -- same as prelude, I think myfoldr f z [] = z myfoldr f z (x:xs) = x `f` r where r = (myfoldr f z xs) myfoldl f z [] = z myfoldl f z (x:xs) = myfoldl f l xs where l = z `f` x -- canonical uses of fold, no debug output tfr = myfoldr (:) [] [1..10] -- copy a list tfl = myfoldl (flip (:)) [] [1..10] -- reverse a list -- debugging output using Debug.Trace (unsafePerfomIO) -- (Not really unsafe, but unsafePerformIO (which is used in the trace function) sounds kind of scary) myfoldrD f z [] = z myfoldrD f z (x:xs) | trace ("x,r: " ++ (show (x,r))) True = x `f` r where r = (myfoldrD f z xs) myfoldlD f z [] = z myfoldlD f z (x:xs) | trace (("z,x") ++ (show (z,x))) True= myfoldlD f l xs where l = z `f` x -- run these to see the functions with debug output from trace tfrD = myfoldrD (:) [] [1..10] -- copy a list tflD = myfoldlD (flip (:)) [] [1..10] -- reverse a list -- using writer monad -- Nothing unsafe here, pure referrentially transparent goodness myfoldrW f z [] = return z myfoldrW f z (x:xs) = do r <- (myfoldrW f z xs) tell ("x,r: " ++ (show (x,r)) ++ "\n" ) return $ x `f` r myfoldlW f z [] = return z myfoldlW f z (x:xs) = do tell ("z,x): " ++ (show (z,x)) ++ "\n") l <- return $ (z `f` x) myfoldlW f l xs -- display the debug output from the writer monad tfrW = putStrLn $ snd $ runWriter $ myfoldrW (:) [] [1..10] tflW = putStrLn $ snd $ runWriter $ myfoldlW (flip (:)) [] [1..10]

2 1

Re: RE: Extensible records: Static duck typing
by Ben Franksen 11 Feb '08

11 Feb '08

[redirecting discussion from @haskell to @cafe] Barney Hilken wrote: >> What about just implementing the cheapest solution that still gets >> us most >> of the way? > >> (3) If it is as cheap (to implement) as advertised then there is no >> great >> risk involved. If it turns out the missing features are a great >> show-stopper for some people (which I don't believe) then let them >> present >> their case afterwards, with good examples at hand. We can still >> decide to >> aim for a higher goal in the long term. > >> If in doubt, chose the solution that is easier to implement. > > Since this paper, there have been several proposals which can be 90% > implemented as libraries, using either functional dependencies or > associated types. These all have much more expressive type systems > than the SPJ paper, yet need very little compiler support. The > question is, which one (if any) should get this small but necessary > support? Which proposals exactly do you refer to? Where can I read about the amount of compiler support these proposals need to make them practically usable? I know about HList/OOHaskell papers+library. This is very interesting stuff but it is not something that comes even close to being usable in day-to-day programming. It is too hard to understand, the (type) error messages are completely uncomprehensible, it is completely unclear how to to make it efficient and how much compiler support would be needed to overcome all these problems. Besides, the question of "functional dependencies or associated types" is still open, right? I think it would be a very bad idea to wait until all those issues have been resolved. So, unless there are other library based proposals on the table that I am not aware of and which don't share these problems, I still think the proposal I was referring to (http://research.microsoft.com/~simonpj/Haskell/records.html) comes closest to what people want and need, while still requiring minimal implementation effort. Cheers Ben

1 0

ANN: highlighting-kate-0.2
by John MacFarlane 11 Feb '08

11 Feb '08

I've just uploaded highlighting-kate-0.2 to HackageDB. highlighting-kate is a source code syntax highlighting library, based on syntax description files from the Kate editor (http://kate-editor.org/) Currently, the following languages are supported: Ada Alert Asp Awk Bash Bibtex C Cmake Coldfusion Commonlisp Cpp Css D Djangotemplate Doxygen Dtd Eiffel Erlang Fortran Haskell Html Java Javadoc Javascript Json Latex Lex LiterateHaskell Lua Makefile Matlab Mediawiki Modula3 Nasm Objectivec Ocaml Pascal Perl Php Postscript Prolog Python Rhtml Ruby Scala Scheme Sgml Sql SqlMysql SqlPostgresql Tcl Texinfo Xml Xslt Yacc The tarball includes a utility for generating new syntax highlighting modules from XML syntax descriptions. It also includes the source code for a command-line syntax highlighting program, Highlight. Please send bug reports and suggestions my way. John

1 0

ANN: Finance-Treasury 0.1.1
by Steve Lihn 10 Feb '08

10 Feb '08

A new experimental package, Finance-Treasury, has been uploaded to hackage. It automates the fetching of Treasury's daily yield curve data (XML) and translates the data into Data.Map representation. The collection of historical data can go back to 1992. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Finance-Treasury The APIs are largely working. But the internal code still needs improvement. Welcome suggestions and fixes. Recommendation on proper error handling design will be particularly appreciated. steve

1 0

Inverting a Monad
by Bas van Dijk 09 Feb '08

09 Feb '08

Hello, Is there a way to 'invert' an arbitrary Monad? By 'inverting' I mean to turn success into failure and failure into success. Here are some specific inversions of the Maybe and List Monad: invM :: Maybe a -> Maybe () invM Nothing = Just () invM (Just _) = Nothing invL :: [] a -> [] () invL [] = [()] invL (_:_) = [] How can I define this for an arbitrary Monad m? More specifically, I would like to define: inv :: (Monad m, M.MonadPlus m, ???) => m a -> m () inv m = if m fails then return () if m succeeds then fail The following obviously doesn't work: inv m = (m >> mzero) `mplus` return () because it will always return (). There's also a 'inversion' for natural numbers: invN :: Int -> Int invN 0 = 1 invN n = 0 but how can I define that without pattern matching, so only using arithmetic operations, +, -, *, ^, ...? The reason I ask this is that I'm writing a parser combinator library to understand parser a bit better. And I would like to define the combinator: notFollowedBy :: P t m a -> P t m () 'notFollowedBy p' fails when p succeeds and returns () when p fails. I will share the code when it's a bit more polished. Thanks, Bas

8 16

Internships at GHC HQ
by Simon Peyton-Jones 09 Feb '08

09 Feb '08

Would you be interested in working at Microsoft Research for three months? If so, you might want to think about applying for an internship. Simon and I are looking for interns, starting in summer 2008. Lots of background info here: http://hackage.haskell.org/trac/ghc/wiki/Internships including a bunch of possible projects, although you may also have ideas of your own. But the bottom line is - apply by end Feb 2008 for this round - tell one of us that you have done so (None of this is restricted to Haskell stuff. You can apply to work at any Microsoft Research lab, on any topic. But there are a lot of applicants, so you are more likely to be successful if you are fairly specific about who at MSR you'd like to work with and why, and contact that person to say that you've applied.) Simon

5 4

parsec3 pre-release [attempt 2]
by Derek Elkins 09 Feb '08

09 Feb '08

[Now with 100% more correct darcs get URLs.] I'm currently getting Paolo Martini's Google Summer of Code project, an updated version of Parsec, into a releasable state, and I will be maintaining it for at least a while. Paolo's major additions are: * The Parser monad has been generalized into a Parser monad transformer * The parsers have been generalized to work over a stream of any type, in particular, with bytestrings. I have made a few minor additions as well: * There is Haddock documentation for almost all functions * The Parser monad now has Applicative/Alternative instances Currently, I am looking for people to give it a go reporting any bugs in the library or documentation, troubles building it, or changes/features they would like. I'm also interested in performance information. Most old Parsec code should be relatively easy but not trivial to port. There is a darcs repository on code.haskell.org. If nothing comes up, I'll put a package on Hackage in about a week or so. To get the code: darcs get http://code.haskell.org/parsec3 To build it, the standard cabal commands should work: http://haskell.org/haskellwiki/Cabal/How_to_install_a_Cabal_package Alternatively, you can use the cabal-install application: http://hackage.haskell.org/trac/hackage/wiki/CabalInstall The documentation can be generated also via the normal cabal routine, or via cabal-install. The Text.Parsec modules should be preferred to the Text.ParserCombinators.Parsec modules.

5 11

ANN: Finance-Quote-Yahoo 0.5.0
by brad clawsie 09 Feb '08

09 Feb '08

a new version of Finance-Quote-Yahoo has been uploaded to hackage that breaks an api of the previous version for getting bulk historical quote information. specifically, notice this updated type signature: getHistoricalQuote :: QuoteSymbol -> Day -> Day -> QuoteFrequency -> IO (Maybe [HistoricalQuote]) the QuoteFrequency can be one of Daily, Weekly, Monthly or Dividend old code using this function will break, which is why i am notifying the list explicitly of this upload. since the versioning is still 0.* and the library is listed as experimental, users should expect changes until a 1.* version is released. brad

1 0

Slightly Offtopic in Part
by PR Stanley 09 Feb '08

09 Feb '08

Hi folks The disjunction elimination rule: I've been trying to make sense of it and I think I have had some success; however, it's far from adequate. I wonder, is there a way of demonstrating it in Haskell? A code frag with a jargon-free explanation would be mucho appreciated. Cheers, Paul

5 5

question concerning ANSI "void *"
by Galchin Vasili 09 Feb '08

09 Feb '08

Hello, Ok .. I am writing a Haskell function that will call down into the ANSI C library .. blah :: .... -> Ptr Word8 ....-> .... The underlying C function that "blah" is calling has a "void *" so I am using "Ptr Word 8" to model the "void *". I propose to have the callers of function "blah" to populate a data structure something like "Ptr Buz" where "data Buz = { ........} and then do a "recast :: Ptr Word 8 -> Ptr Buz" when invoking function "blah". Does this seem reasonable? Is there a better way? Thanks, Vasili

3 7

Fwd: [pdxfunc] pdxfunc meeting: Monday, February 11, 7pm, CubeSpace (Portland, OR)
by Justin Bailey 09 Feb '08

09 Feb '08

---------- Forwarded message ---------- From: Igal Koshevoy <igal(a)pragmaticraft.com> Date: Feb 8, 2008 12:01 PM Subject: [pdxfunc] pdxfunc meeting: Monday, February 11, 7pm, CubeSpace To: Igal Koshevoy <igal(a)pragmaticraft.com> Join us at the next meeting of pdxfunc, the Portland Functional Programming Study Group. We'll have presentations, demos and discussions. We welcome programmers interested in all functional languages and our meetings have content for coders of all skill levels. If interested, please also subscribe to our mailing list at http://groups.google.com/group/pdxfunc PRESENTATIONS (1) Kevin Scaldeferri: Effortless Concurrent Programming with Erlang Abstract: Erlang is a dynamic functional programming language designed for concurrent programming. After a quick introduction to the syntax and primitives, we'll dive into some code to see example of how you can easily write programs that take advantage of multiple CPUs, and that even scale effortlessly to clusters of machines. (You are highly encouraged to download and install Erlang from http://erlang.org/download.html before the meeting.) Speaker: Kevin Scaldeferri has spent the last 6 years building high-volume, high-reliability systems at Yahoo. His interests include programming languages, the interaction between online and real-life communities, and techniques for making the development process more reliable, more successful, and more fun. (2) Tim Chevalier: Towards a GraphicsMagick Binding for Haskell Abstract: I will discuss my work in progress on implementing a Haskell Foreign Function Interface (FFI) binding for the C-based GraphicsMagick image manipulation library. I will introduce portions of the FFI through examples, as well as tools for simplifying FFI use. I will also explain how I have begun to use Haskell's type system to develop a simplified yet equally powerful interface (as compared to the C version) to the GraphicsMagick library. Given the ongoing nature of my work, I will conclude by soliciting comments on what *you* would like to see in a Haskell image manipulation library. Speaker: Tim Chevalier is a PhD student at Portland State University, where he thinks about alternative back-ends for Haskell and languages like it. He has been programming in Haskell for seven years, the first six of which he spent trying to understand monads.

1 0

:i and :t give different types
by Chad Scherrer 09 Feb '08

09 Feb '08

Hello haskell-cafe, In ghci, I tried to get info for Data.Stream.Stream: $ ghci GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> :m Data.Stream Prelude Data.Stream> :i Stream data Stream a where Stream :: forall a s. (Data.Stream.Unlifted s) => !s -> Step a s -> !s -> Stream a -- Defined in Data.Stream instance Functor Stream -- Defined in Data.Stream This didn't seem right to me, so I asked tried this: Prelude Data.Stream> :t Stream Stream :: (Data.Stream.Unlifted s) => (s -> Step a s) -> s -> Stream a What's going on here? forall a s. (Data.Stream.Unlifted s) => !s -> Step a s -> !s -> Stream a and (Data.Stream.Unlifted s) => (s -> Step a s) -> s -> Stream a are completely different, right? And really, neither one makes much sense to me. I would have expected forall s. (Data.Stream.Unlifted s) => (s -> Step a s) -> s -> Stream a -- Chad

5 7

User groups meeting all over the world
by Don Stewart 08 Feb '08

08 Feb '08

The community might be interested to know that there are (at least) 7 Haskell/FP user groups meeting across the world in the next two weeks! FringeDC Washington DC/USA February 9 PDXFunc Portland/USA February 11 Fun in the afternoon London/UK February 12 BayFP San Francisco/USA February 13 Saint-Petersburg Haskell User Group Saint-Petersburg/Russia. February 16 NYFP Network New York/USA February 19 NWFPIG: Seattle FP Group Seattle/USA February 20 The Austin (Texas) group met a couple of weeks ago. This is a great development -- a year ago I don't think we had *any* groups meeting regularly. Consider dropping in to one, if there's a meetting near you, or starting your own group! All groups we know about are listed here: http://www.haskell.org/haskellwiki/User_groups -- Don

5 4

Re: [Haskell-cafe] Mutable arrays
by John Lato 08 Feb '08

08 Feb '08

Ok, I've done some testing on different array accessing strategies, and at this point I believe that, as Stefan previously suggested, using an Oleg-inspired fold for access offers the best performance. My first non-unsafe idea was to try: > import Data.Array.MArray > import Data.Array.IO > import Data.Int (Int32) > import Data.List (foldl') > > -- |Create an array. Change the type signature to experiment with different array types. > makeAry :: IO (IOUArray Integer Float) > makeAry = newListArray (1,1000000) [1..] > getElemsLazy :: (MArray a e m, Ix i) => a i e -> m [e] > getElemsLazy myAry = getBounds myAry >>= > mapM (readArray myAry) . range this works on a 1 million element Int32 array (IOArray or IOUArray): > myElems <-getElemsLazy myAry > let myMax = foldl' max 0 myElems But with a 10-million element array, I get a stack overflow. I next tried the foldA functions Stefan posted, but had some problems with them. I think that the array elements need to be instances of Bounded and Fractional, and I don't know of a suitable type. Anyway, his code is a little too compact for me to understand without further study, so I moved on. Next I created my own fold operators. These are fairly directly copied from an email Oleg posted to the haskell mailing list: > leftFoldA' myArray self iteratee seed = do > elem <- readArray myArray $ fst seed > case iteratee seed elem of > Left seed -> return seed > Right seed -> self iteratee seed > > leftFoldA :: (MArray a e m, Ix a1) => a a1 e -> ((a1, b) -> e -> Either t (a1, b)) -> a1, b) -> m t > leftFoldA array iteratee seed = > let g = (leftFoldA' array) g in > g iteratee seed > > leftFoldModA' myArray self iteratee seed = do > elem <- readArray myArray $ fst seed > result <- iteratee seed elem > case result of > Left seed -> return seed > Right seed -> self iteratee seed > > leftFoldMA :: (MArray a e m, Ix a1) => a a1 e -> ((a1, b) -> e -> m (Either t (a1, b))) -> a1, b) -> m t > leftFoldMA array iteratee seed = > let g = (leftFoldModA' array) g in > g iteratee seed the two functions to use are leftFoldA and leftFoldMA. The difference, show in the type signature, is that the iteratee in leftFoldA is pure while the iteratee in leftFoldMA operates in the monad. So if you're going to modify the array, for example, you need to use the leftFoldMA version. These folds let me define: > findMax myAry = do > (lowB, highB) <- getBounds myAry > let maxReader (ix, acc) c = > let > ix' = ix + 1 > acc' = max c acc > in acc' `seq` (if ix' > highB then Left else Right) (ix', acc') > (_, myMax) <- leftFoldA myAry maxReader (lowB, 0) > return myMax > > normalizeArray myAry = do > myMax <- findMax myAry > (lowB, highB) <- getBounds myAry > let normMod = 1/myMax > let normalizer (ix, _) c = do > let ix' = ix + 1 > curVal <- readArray myAry ix > writeArray myAry ix (curVal * normMod) > return $ (if ix' > highB then Left else Right) (ix', ()) > leftFoldMA myAry normalizer (lowB, ()) > > -- |show all elements from lowB to highB > -- this only works with IO arrays, since the readArray and print happen in the same monad. > -- you could possibly make it work with ST arrays if you use a monad transformer, but you'd have > -- to change leftFoldMA as well. > showElems myAry lowB highB = do > let showFunc (ix, _) c = do > let ix' = ix + 1 > readArray myAry ix >>= print > return $ (if ix' > highB then Left else Right) (ix', ()) > leftFoldMA myAry showFunc (lowB, ()) There are probably better ways to define the folds, but I hope these are pretty easy to follow. If I was doing a lot of different operations, I'd probably define functions like > makeIteratee highB func = (\(ix, acc) c -> > let > ix' = ix + 1 > acc' = func c acc > in acc' `seq` (if ix' > highB then Left else Right) (ix', acc')) so that findMax becomes: > findMax2 myAry = do > (lowB, highB) <- getBounds myAry > (_, myMax) <- leftFoldA myAry (makeIteratee highB max) (lowB, 0) > return myMax instead of having to define a bunch of iteratees in let clauses. Now, running the program > main :: IO () > main = do > myAry <- makeAry > (lowB, highB) <- getBounds myAry > normalizeArray myAry > showElems myAry 1 10 > showElems myAry 999990 1000000 > return () takes about 5 seconds, with no optimizations. It takes less than a second with -O2. Much better. It does take longer with boxed arrays, but that's to be expected. No unsafe functions, either. The only sketchy bit is using seq in the iteratees (e.g. maxReader in findMax) to force the accumulator. I say sketchy not because it's unsafe or unreliable, but it would be nice if it was unnecessary. Notes: I used GHC 6.8.1 on WinXP, 3.2GHZ P4, 1.5GB RAM. All of my original tests were done without any optimizations, which is why I didn't include timing information for the earlier stuff. The getElemsLazy function is fast, but the stack overflow is annoying. Using the leftFold functions is both faster and avoids the space leak. I was also using the Data.List.Stream and Control.Monad.Stream versions of foldl' and mapM, although I don't think those made much difference. Anyway, this was a good exercise for me. I learned a lot, especially about the power of left-fold enumerators. Cheers, John

2 2

Re: [Haskell-cafe] Best practice for embedding files in a GHC-compiled tool?
by Dave Bayer 08 Feb '08

08 Feb '08

On Feb 7, 2008, at 12:27 AM, gwern0(a)gmail.com wrote: > Are you assuming that the various users have GHC/Hugs installed? You > know about scripting through the 'runhaskell' binary, right? I do, and I've used this. I don't want to do that here. Let me say this again: I am making no assumptions whatsoever about various users, other than platform. Haskell is not a niche language, with the right compile options, it CAN be used in this way. Here's the extreme case: When one is installing Mac OS X, one has access to a command line via a terminal application, but the operating system is otherwise very stripped down. Nevertheless, if one customizes an install DVD by adding a single command-line tool, one can execute that tool in this environment. I'd rather use Haskell than C for such applications. With C, we can introduce one file to an alien environment, and it will run. I've linked GHC Haskell programs that can be used in this way. Such programs can be used by anyone on a given platform. Assuming that GHC/Hugs is installed divides the potential audience by a large factor. Under this extreme hypothesis, how do I embed a compressed tar file into a single file command line tool written in Haskell and compiled by GHC? Thanks, Dave

2 1

Draft chapters of "Real World Haskell" now publicly available
by Bryan O'Sullivan 08 Feb '08

08 Feb '08

John, Don and I are pleased to announce the beginning of the public beta programme for our upcoming book, "Real World Haskell". For further details, please see the following blog entry: http://www.realworldhaskell.org/blog/2008/01/21/finally-the-public-beta-pro… Thanks to all of the Haskell community members who have so far performed sterling service in commenting on our closed drafts. We look forward to your feedback! <b

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Re: [Haskell-cafe] FP and Quality
by PR Stanley 08 Feb '08

08 Feb '08

Thanks, keep the tips coming. I like the ones about the type safety and line counts. Cheers, Paul At 23:33 04/02/2008, you wrote: >Good luck with this - I'd love to see the outcome. > >My experience is that FP tends to result in a lot less code, so if >there are x >bugs per line of code, FP has less bugs per complete application. > >Talking about haskell, the typesystem dissalows whole classes of bugs. Things >simply do not compile if you stitch the innards together in the wrong order >(particuarly if you are agressive about using the most general types >possible). Since this accounts for perhaps 90% of where I do things wrong in >Java, I get a corresponding decrease in run-time bugs in haskell. However, >this is somewhat compensated for by the effort needed to get haskell programs >through the compiler in the first place - debug at compile or debug at >runtime is the tradeoff here. > >FP is easier to verify mechanically than imperative programming - more of the >logic is exposed directly. It's easier to do by-case proofs, even if they are >by-hand rather than mechanical. > >However, all of this is annecdotal. Good luck collecting real stats, or >peer-reviewed annecdotes. You may have luck looking at bug-fix times vs >number of developers for equivalent FP and Java apps/libs. Worth a shot, >given that the bug-trackers tend to be public. You could probably tie it back >to the size of the 'fix' patches. Get some nice graphs? > >Matthew > >On Monday 04 February 2008, you wrote: > > Hi folks > > I'm thinking of writing a little essay arguing the case for the > > advantages of FP for producing quality software. Can the list > > recommend any papers/articles which I can use as sources of my > > argument? I have access to the IEEE database too although earlier I > > couldn't find anything on the subject. > > Thanks, Paul > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe

3 2

background question about IO monad
by Uwe Hollerbach 08 Feb '08

08 Feb '08

Hello, haskellers, I have a question for you about the IO monad. On one level, I seem to be "getting" it, at least I seem to be writing code that does what I want, but on another level I am almost certainly not at all clear on some concepts. In various tutorials around the web, I keep finding this notion that the IO monad is "one-way", that you can put stuff into it, but you can't take it out. (And, damn, some of the contortions I went through while trying to figure out how to do exceptions in my little scheme interpreter certainly bear that out! I was sure beating my head against liftIO et al for a fair while...) But let me post some code snippets here: > lispUTCTime [] = doIOAction (getClockTime) toS allErrs > where toS val = String (calendarTimeToString (toUTCTime val)) > lispUTCTime [IntNumber n] = > return (String (calendarTimeToString (toUTCTime (TOD n 0)))) This is a little function I added to the interpreter a couple of days ago: if you enter (UTCtime) with no arguments, it gets the current time and formats it as UTC: like so; this came from the first alternative above: lisp> (UTCtime) "Wed Feb 6 03:57:45 UTC 2008" and if you give an argument, you get that interpreted as a number of seconds since epoch, and that gets formatted as UTC; this is from the second alternative above: lisp> (UTCtime 1.203e9) "Thu Feb 14 14:40:00 UTC 2008" And here's the doIOAction routine: I wrote this, it's not some system-level routine. > doIOAction action ctor epred = > do ret <- liftIO (try action) > case ret of > Left err -> if epred err > then throwError (Default (show err)) > else return (Bool False) > Right val -> return (ctor val) OK, with all that as background, on one level I understand why I need the doIOAction routine in the first version of lispUTCTime: I'm calling getClockTime, that's an IO action, so I enter the IO monad, get the time, and return: all is cool. In the second version, all I'm doing is taking a number and interpreting it as a time, and writing that in a particular format; again, no problem. But after that, it sure seems to me as if I've taken data out of the IO monad... haven't I? Given that the second alternative never entered doIOAction and that after both are done I have a string of characters, prettily formatted to indicate a time, that's what it feels like to this unwashed C programmer. So, what's going on there? What's the difference between the two alternatives? I would appreciate any enlightenment you can send my way! regards, Uwe

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modeling ANSI C structs in Haskell?
by Galchin Vasili 08 Feb '08

08 Feb '08

Hello, I am reading through the FFI doc. Any suggestions on enabling Haskell programmers to model ANSI C structs that will be passed down to C run-time? Vasili

4 3

Signature for non-empty filter
by Henning Thielemann 08 Feb '08

08 Feb '08

Is Haskell's type system including extensions strong enough for describing a function, that does not always return a trivial value? E.g. (filter (\x -> x==1 && x==2)) will always compute an empty list. Using an appropriate signature for the function it shall be rejected at compile time, because it is probably a mistake - probably (filter (\x -> x==1 || x==2) xs) was meant. I assume that the type must contain somehow an example input for which the function value is non-trivial. If Haskell's type system is not strong enough, what about dependently typed languages like Cayenne or Epigram? (I know, theorem provers have no problem with such types.)

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Two Lectureships in Nottingham
by Graham Hutton 08 Feb '08

08 Feb '08

Dear all, We are advertising two new Lectureships (Assistant Professorships) in Computer Science at the University of Nottingham in England. Applications in the area of functional programming are particularly welcome. Further details are available from: http://jobs.nottingham.ac.uk/vacancies.aspx?cat=160#j2797 The closing date for applications is 22nd Feburary 2008 Best wishes, Graham Hutton +-----------------------------------------------------------------+ | Dr Graham Hutton Email : gmh(a)cs.nott.ac.uk | | School of Computer Science | | University of Nottingham Web : www.cs.nott.ac.uk/~gmh | | Jubilee Campus, Wollaton Road | | Nottingham NG8 1BB, UK Phone : +44 (0)115 951 4220 | +-----------------------------------------------------------------+ This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

1 0

FP and Quality
by PR Stanley 08 Feb '08

08 Feb '08

Hi folks I'm thinking of writing a little essay arguing the case for the advantages of FP for producing quality software. Can the list recommend any papers/articles which I can use as sources of my argument? I have access to the IEEE database too although earlier I couldn't find anything on the subject. Thanks, Paul

3 2

Mutable arrays
by Jeff φ 08 Feb '08

08 Feb '08

Hello, I'm trying to write code that will take a mutable 2D array and normalize it by dividing all elements by the largest element. I managed to write code to do this, but it seems overly complex. I could write something much simpler in Clean or C++. Most likely, my code is complex because I don't have any experience with mutable arrays in Haskell. I couldn't find any tutorials on the Internet. I'd be grateful for suggestions on simplifying the following code. Thanks. {-# OPTIONS_GHC -fglasgow-exts -fbreak-on-exception #-} -- normalize_ary This takes a mutable array. Determines the largest -- element in the array (max_elem) and then divides every element by -- max_elem. normalize_ary :: (Num t1, Num t, Ix t, Ix t1, MArray a e t2, Ord e, Fractional e, Enum t, Enum t1) => a (t, t1) e -> t2 () normalize_ary ary = do -- The following two commented out lines of code show my first -- attempt at determining a value for max_elem. However, this -- produces a stack overflow. -- elem_ary <- getElems ary -- let max_elem = foldl1 max elem_ary max_elem <- calc_max_2d_elem ary max_elem `seq` map_in_place_2d_arr (\x -> x / max_elem) ary map_in_place_2d_arr :: (MArray a e t, Enum t2, Enum t1, Ix t1, Ix t2) => (e -> e) -> a (t1, t2) e -> t () map_in_place_2d_arr fn arr = ret where ret = do ((i1,j1),(i2,j2)) <- getBounds arr ( mapM_ (\i -> do v <- readArray arr i writeArray arr i (fn v) ) [(i,j) | i <- [i1..i2], j <- [j1..j2]]) calc_max_2d_elem :: (Ord t, MArray a t t1, Ix t2, Ix t3, Num t3, Num t2) => a (t3, t2) t -> t1 t calc_max_2d_elem arr = do m <- readArray arr (0,0) (_,(i_max, j_max)) <- getBounds arr let calc_max_loop arr m (i,j) | j == j_max = return m | otherwise = do e <- readArray arr (i,j) let m2 = max e m m2 `seq` calc_max_loop arr m2 nxt_idx where nxt_idx | i == i_max - 1 = (0,j+1) | otherwise = (i+1,j) calc_max_loop arr m (0,0)

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Why is this so inefficient?
by Jefferson Heard 07 Feb '08

07 Feb '08

I thought this was fairly straightforward, but where the marked line finishes in 0.31 seconds on my machine, the actual transpose takes more than 5 minutes. I know it must be possible to read data in haskell faster than this. I'm trying to read a 100MB comma delimited file. I've tried both CSV modules, and these take even longer to read the file. Is there some general best-practice for reading and parsing large amounts of data that I'm not aware of? I have tried, by the way, a couple of things, including putting a bang (!) before row in transposeRow and using foldr instead of foldl', but that didn't change anything other than force me to increase the stack size to 100M on the command line. I'm running in the profiler now, and I'll update this, but I thought I would check and see if my head was on remotely straight to begin with. -- Jeff --- module ColumnMajorCSV where import qualified Data.ByteString.Char8 as S import qualified Data.Map as M import qualified Data.List as L transposeRow cols row = zipWith (:) row cols transposeCSV :: [[S.ByteString]] -> M.Map String [S.ByteString] transposeCSV (header:rows) = M.fromList (zip (map S.unpack header) spreadsheet) where spreadsheet = L.foldl' transposeRow emptySheet rows emptySheet = take (length header) $ repeat [] dataFromFile :: String -> IO (M.Map String [S.ByteString]) dataFromFile filename = do f <- S.readFile filename print . length . map (S.split ',' $!) . S.lines $ f -- finishes in 0.31 seconds return . transposeCSV . map (S.split ',' $!) . S.lines $ f -- this takes 5 minutes and 6 seconds

4 4

classes for data structures
by Matthew Pocock 07 Feb '08

07 Feb '08

Hi, I've been working a lot with maps, sets and lists. While the process of getting things out of them is abstracted by foldable, traversable and friends, the process of building one up is not. Would it be possible to have something like: class Buildable b where empty :: b a --makes an empty b with elements of type a insert :: a -> b a -> b a --inserts the new element into the buildable I'm not particularly wedded to the names. It's just that it would be very convenient sometimes to collect data items into e.g. a list or a set, without pushing in some klunky foralled insert function. I see that this is realted to MonadPlus, but the space of possible container classes is wider than that of monads e.g. set. Matthew

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Re: [Haskell] Extensible records: Static duck typing
by Tim Chevalier 07 Feb '08

07 Feb '08

[redirecting to haskell-cafe] On 2/6/08, Barney Hilken <b.hilken(a)ntlworld.com> wrote: > This is what I was trying to do with the wiki page. I stopped because > the only other contributor decided he could no longer contribute, and > I felt I was talking to myself. If we want to be rational about the > design, we need real examples to demonstrate what is genuinely useful, > and I don't have that many of them. It's obvious that records are a language feature that people besides just you care about. And so everybody would benefit from your effort if you chose to continue adding more examples to the wiki page. Records clearly seem to be an important issue if so many people have replied to your thread, and your comment expressing frustration at "arbitrary decisions" getting made about design seems to suggest you have some passion about the issue. On the other hand, if you can't think of real examples offhand, and no one else can either, maybe it's not that important... (Examples don't have to be very complicated to be useful, by the way. Simpler is better.) Cheers, Tim -- Tim Chevalier * http://cs.pdx.edu/~tjc * Often in error, never in doubt "If pots couldn't call kettles black, there'd be a lot less talking going on." -- Larissa Ranbom

1 0

weird ghci behavior with gadts and existentials
by Chris Casinghino 07 Feb '08

07 Feb '08

Hi all, I've been playing a bit with gadts and existentials lately, and I have an example where I don't quite understand the behavior of ghc. The expression in question typechecks sometimes in some versions of ghc, depending on where you write it, and not in other versions. Some other people I've asked report not getting any errors, even when using apparently one of the same versions of ghc I checked. If I create a file which contains: > data TypeGADT t where > TyInt :: TypeGADT Int > > data ExpGADT t where > ExpInt :: Int -> ExpGADT Int > > data HiddenTypeExp = forall t . Hidden (TypeGADT t, ExpGADT t) > > weird = case Hidden (TyInt, ExpInt 3) of Hidden (TyInt, e) -> e I am able to :load it into ghci just fine (with -fglasgow-exts) with version 6.8.2. However, if I then copy the line: let weird2 = case Hidden (TyInt, ExpInt 3) of Hidden (TyInt, e) -> e into ghci, I get a type error. In the HEAD version 6.9, I get a type error on the definition of "weird" right away when I :load the file. The type error goes away if I add the line > weird :: ExpGADT Int before the definition of weird. The type error in question is this: <interactive>:1:46: Inferred type is less polymorphic than expected Quantified type variable `t' escapes When checking an existential match that binds e :: ExpGADT t The pattern(s) have type(s): HiddenTypeExp The body has type: ExpGADT t In a case alternative: Hidden (TyInt, e) -> e In the expression: case Hidden (TyInt, ExpInt 3) of Hidden (TyInt, e) -> e So, several questions. 1) Why the discrepancy in behavior between ":load"ing the file and copying in the definition in 6.8.2. It seems like, one way or the other, this should be consistent. 2) Several other people report not getting any errors at all, even people with ghc 6.8.2, one of the versions I tested. What's the "right" behavior? My guess would be that this should cause no type error, even without the type annotation. The GADT pattern match against "TyInt" in the case statement should refine the existential type variable t to Int, so no existential type variables are escaping. Is that right? 3) Is there a bug here? Are there two bugs (one, the typing error, two, the difference between ghc and ghci)? Or, do I just not understand what is going on? Sorry for the length of this email! --Chris Casinghino

2 2

Data.Ord and Heaps (Was: Why functional programming matters)
by apfelmus＠quantentunnel.de 06 Feb '08

06 Feb '08

Stephan Friedrichs wrote: > apfelmus wrote: >> [...] >> Feedback: I think the HeapPolicy thing is too non-standard. The >> canonical way would be to use a MinHeap and let the Ord instance >> handle everything. A MaxHeap can then be obtained via a different >> Ord instance >> newtype Ord a => Reverse a = Reverse { unReverse :: a } >> instance Ord a => Ord (Reverse a) where >> compare = comparing unReverse >> This newtype should be in Data.Ord, of course. Being > > This solution should be used for all collections depending on Ord > instances, including Data.Map, Data.Set and others. As long as I > only include it in my tiny heap package, it is as 'non-standard' as > my approach, isn't it? Yes. I mean "non-standard" in the software-reuse sense, i.e. Ord is for user-defined orderings and should be the only such mechanism in order to enable reuse. In fact, Data.Heap clearly shows that Data.Ord is currently missing functionality. >> Simply setting >> type MaxHeap a = MinHeap (Reverse a) >> is inferior to a "native" MaxHeap since we'd have to pack/unpack >> the Reverse all the time. But a type class for heaps - which >> should be present anyway - can solve that problem: >> class Heap h where >> [...] >> instance Heap MinHeap where ... >> newtype MaxHeap a = M (MinHeap (Reverse a)) >> instance Heap MaxHeap where ... > > I've actually thought about this. Realising MinHeap and MaxHeap is > no problem, but I decided against it, because implementing a custom > order becomes quite complicated: You have to declare an > > newtype MyHeap a = ... > > instance Heap MyHeap where > -- about 10 functions > > instead of just > > data PriorityPolicy > > instance HeapPolicy PP MyPriorityType where > heapCompare = const (comparing priority) Note that the Heap class contains only three primitive operations (empty, insert, viewHead), all the others have default implementations in terms of those three. There is even an underappreciated unfold among them :) toAscList = unfoldr viewHead The structure becomes especially clear by noting that any Heap is defined by just two primitives inject :: Ord a => Maybe (a, Heap a) -> Heap a view :: Ord a => Heap a -> Maybe (a, Heap a) We have inject = maybe empty (uncurry insert) . This is just like lists, except that view . inject ≠ id since view returns the smallest element. However, just that we managed to reduce the number of primitive operations doesn't mean that the policy approach isn't preferable. It needs 0 primitive operations, after all. But as foreshadowed in my reply, it's possible to do policies within Ord. Don't stop thinking about your good idea just because you can start coding :) Here's one way to do it: module Data.Ord where ... class (Ord p a) => OrdPolicy p a where -- the policy p is a type constructor to :: a -> p a from :: p a -> a instance OrdPolicy Identity a where ... newtype Reverse a = Reverse a instance Ord a => Reverse a where compare = flip $ comparing from instance OrdPolicy Reverse a where to = Reverse; from (Reverse x) = x module Data.Heap where ... newtype Heap p a = Heap (MinHeap (p a)) type MaxHeap a = Heap Reverse a class Ord a => Heap h a | h -> a where empty :: h insert :: a -> h -> h viewHead :: h -> Maybe (a, h) instance OrdPolicy p a => Heap (Heap p a) a where ... What I don't like about this is that the policy is not polymorphic in the element types, forcing the Heap class to be multi-parameter. I'd really like to write class (forall a . Ord p a) => OrdPolicy p where but I guess that's (currently) not possible. The original "phantom policy" approach can't quite do this either: module Data.Ord where ... newtype OrdBy p a = OrdBy { unOrdBy :: a } data Reverse instance Ord a => Ord (OrdBy Reverse a) where compare = flip $ comparing unOrdBy module Data.Heap where ... newtype Heap p a = Heap (MinHeap (OrdBy p a)) type MaxHeap a = Heap Reverse a class Heap h where empty :: Ord a => h a insert :: Ord a => a -> h a -> h a viewHead :: Ord a => h a -> Maybe (a, h a) instance (Ord (OrdBy p a)) => Heap (Heap p) where -- forall a? ... However, a distinct advantage of using OrdBy for all ordering policies is that the from and to functions are no longer necessary. All ordering policies use the same type OrdBy which automatically guarantees that from and to are inverse to each other. This would be an informal requirement otherwise, so I think that phantom policies are clearly superior to type constructor policies. Fortunately, this is orthogonal to making Heap a multi- parameter type class and ensuring that OrdBy p a instances are polymorphic in a . >> In conclusion: the ordering policy stuff should not be part of >> Data.Heap, this is a job for Data.Ord. > As mentioned above: This sounds really useful. How about you > propose this to the base-package maintainers? :) What, me? :D Regards, apfelmus

4 4

lazy evaluation
by Peter Padawitz 06 Feb '08

06 Feb '08

Can anybody give me a simple explanation why the second definition of a palindrome checker does not terminate, although the first one does? pal :: Eq a => [a] -> Bool pal s = b where (b,r) = eqrev s r [] eqrev :: Eq a => [a] -> [a] -> [a] -> (Bool,[a]) eqrev (x:s1) ~(y:s2) acc = (x==y&&b,r) where (b,r) = eqrev s1 s2 (x:acc) eqrev _ _ acc = (True,acc) pal :: Eq a => [a] -> Bool pal s = b where (b,r) = eqrev' s r eqrev' :: Eq a => [a] -> [a] -> (Bool,[a]) eqrev' (x:s1) ~(y:s2) = (x==y&&b,r++[y]) where (b,r) = eqrev' s1 s2 eqrev' _ _ = (True,[]) Peter

4 4

fmap vs. liftM
by Felipe Lessa 06 Feb '08

06 Feb '08

Hi there, Reading http://www.haskell.org/haskellwiki/Things_to_avoid I found an interesting saying: "By the way, in the case of IO monad the Functor class method fmap and the Monad based function liftM are the same." I always tought that prop :: (Functor m, Monad m, Eq (m b)) => (a -> b) -> m a -> Bool prop f x = fmap f x == liftM f x was True regardless of 'm'. Is there any exception? If so, why? I've even done s/fmap/liftM/g and s/liftM/fmap/g in the past for consistency =). Thanks! -- Felipe.

7 11

Linear algebra
by Jon Harrop 06 Feb '08

06 Feb '08

Has anyone written anything on the use of FP (e.g. point free style) in linear algebra problems? -- Dr Jon D Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/products/?e

3 2

bimap 0.2
by Stuart Cook 06 Feb '08

06 Feb '08

I've updated the bimap package to version 0.2. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bimap-0.2 The main difference is a pretty comprehensive interface shakeup: the Either variants have been dropped, all L variants have had the L removed from their name, and a few functions have been curried. The end result is an interface much closer to that of Data.Map. (This also means that 0.2 isn't backwards-compatible.) In addition, the package now supports GHC 6.8, and has a few more tests in the test suite. Stuart

5 9

Parsec3 stream properties
by Philippa Cowderoy 06 Feb '08

06 Feb '08

I'm having a little difficulty finding full properties for Parsec3's Stream class, largely because I don't want to overspecify it with regard to side-effects. Here's the class: > class Stream s m t | s -> t where > uncons :: s -> m (Maybe (t,s)) The idea is that: * unfoldM uncons gives the [t] corresponding to the stream * Assuming no relevant side-effects, unconsing the same value twice will yield the same result What's a relevant side-effect? Well, that's up to the stream and the monad it's built in - but unconsing once shouldn't affect the result of unconsing a second time unless you've found something fiendishly clever that I haven't thought of. Using stream mutability to implement a programming language from within the parsing monad is not a good idea, though no doubt someone'll do it anyway! -- flippa(a)flippac.org Society does not owe people jobs. Society owes it to itself to find people jobs.

1 0

Re: [Haskell-cafe] Mutable arrays
by John Lato 06 Feb '08

06 Feb '08

At least for file processing, I don't think the lazy solution is as bad as some people on this list indicate. My solution was to define a function processAudioFile :: (Handle, Handle) -> (ASig -> ASig) -> IO (), similar to interact. The function reads from the first handle and writes to the second (the problem domain requires two separate files). Contents of the first are read into a lazy bytestring with hGetContents (from Data.ByteString.Lazy), decoded into an ASig (which in the current version is actually a tuple of a list and an Int of the total length, but I'm reworking this into a monad), and processed. The processed list is then encoded back into a bytestring and written with hPut. I then stick the whole thing in a bracket to open and close the filehandles, and call the bracketed function when I'm ready to do processing. I'm pretty happy with this solution, for several reasons: 1. The actual processing code remains purely functional. 2. I didn't have to write imperative-style looping constructs. 3. Handles get closed after use (even with exceptions, thanks to bracket). 4. Because all IO, processing, and writing is encapsulated in one function, everything happens sequentially as it's supposed to, so I don't get exceptions about lazy filehandles being closed. 5. Performance has been good. Memory usage is lower than expected, and it's fairly fast (at least when I remember to use a non-profiled version). I've tested this approach with wave files into the 100's of MB so far. Perhaps not quite as fast as optimized C, but good enough for me. I'm not quite sure how to get around the problem of getElems being strict, though. I do have one idea, but I don't know how it would work in practice: -- let ary_max = foldl1' max $ elems $ unsafeFreeze myArray If you use a boxed array type (IOArray or STArray) for myArray, and compiled with GHC, no copying is necessary (you may need to use type annotations to guarantee this). Then use the foldl' function to get array_max, and map it onto the original mutable array. I think it would be safe provided that you calculate ary_max before you start to modify the array, which is true for normalization. It's worth a try, anyway. John Lato > > Changing the subject slightly, I once wrote code in Concurrent Clean that > > filtered a file that was larger than the available memory on my PC. I did > > this by creating a function that returned the contents of the original file > > as a lazy list. Then, I created functions to process the list and write the > > processed list to a results file. The code was not imperative at all. The > > function that wrote the results file forced the evaluation of the lazy list. > > As the lazy list was consumed, the contents of the original file were read. > > Is this possible with Monads in Haskell? > > Yes, using hGetContents, which is considered bad practice by many > people here. The problem is that hGetContents breaks referential > transparency, and I suspect that whatever Clean does to lazily read > files also does (though I can't be sure, I haven't looked in any > detail at uniqueness types). That is, the contents of the returned > list depend on when you read it, which is not allowed in a > referentially transparent language. > > The same applies to your problem. getElems cannot return a lazy list > of elements*, because what if the array were changed between the point > that you did the getElems and the point you required the element. So > it seems that actually specifying the order of evaluation using an > imperative-style loop is the only pure way to do this. > > * Well, it could, but it would require some cleverness like > copy-on-write logic under the hood.

2 2

Parameterisations of Monads
by Matthew Sackman 06 Feb '08

06 Feb '08

So I was thinking how dull and uninspiring the current definiton of Monad really is and came up with some more interesting parameterisations. The only problem with this one is I'm a) not sure if it still is a Monad and b) very unsure if it's of any use. There's the possibility that chucking Cont in there or using newtype to simultate multiple arrows / type lambdas may lead to more interesting instances, but can anyone think of exciting use cases for this stuff? Feel free to fill in the instances! It's also not a parameterisation I've seen before. Matthew > class SuperMonad (m1 :: * -> * -> *) (m2 :: * -> *) where > (>>~) :: m1 (m2 a) (m1 (m2 b) (m2 b)) > (>>=~) :: m1 (m2 a) (m1 (m1 a (m2 b)) (m2 b)) > returns :: m1 a (m2 a) > > instance (Monad m) => SuperMonad ((->)) m where > (>>~) :: m a -> m b -> m b > (>>~) = (>>) > (>>=~) :: m a -> (a -> m b) -> m b > (>>=~) = (>>=) > returns :: a -> m a > returns = return > > instance (Monad m) => SuperMonad ((,)) m where > (>>~) :: (m a, (m b, m b)) > (>>=~) :: (m a, ((a, m b), m b)) > returns :: (a, m a) > > instance (Monad m) => SuperMonad Either m where > (>>~) :: Either (m a) (Either (m a) (m b)) > (>>=~) :: Either (m a) (Either (Either a (m b)) (m b)) > returns :: Either a (m a) > > instance (Monad m) => SuperMonad State m where > (>>~) :: State (m a) (State (m a) (m b)) > (>>=~) :: State (m a) (State (State a (m b)) (m b)) > returns :: State a (m a)

4 3