- Haskell-Cafe - Haskell.org

How to do this in Haskell
by Chris Smith 13 Nov '07

13 Nov '07

If you wanted to write a Haskell application that included a WYSIWYG HTML editor, how would you do it? More details: - I'll probably be using Gtk2Hs for the app, though that could change with a (very) good reason. - The top priorities for the editor are that it resemble common word processors in editing functions, and that the display looks as near as possible to what can be expected from some major browser such as FireFox or IE. If it's not possible to do extremely well in both of these criteria, then I may as well abandon the project now. - I'd like for the result to be cross-platform, but Windows-only would be just as good from my boss's perspective. So, with all that in mind, I'm looking for options. What looks most realistic that I've found to date is building an XP-COM implementation for Haskell via the FFI and either a code generation tool or Template Haskell, and then using that to try to embed the Mozilla editor component. Given my serious lack of knowledge in XP-COM or the Mozilla project or GTK, that looks sort of scary. Any other ideas? -- Chris Smith

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Data.Set.member vs Data.List.elem
by Neil Mitchell 13 Nov '07

13 Nov '07

Hi, Is there a good reason that Data.Set uses the name "member" while Data.List (or the Prelude) uses the name "elem", for what to me seem identical concepts. I realise that in Set's the traditional test is for "membership", but it seems awfully arbitrary that one jumped one way and one jumped the other. I've just written an entire module's worth of Haskell with Set.elem, as that "felt" right, now I'm going back and fixing it. Thanks Neil

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Re: [Haskell-cafe] Interesting effect of upgrading GHC
by Andrew Coppin 13 Nov '07

13 Nov '07

Gwern Branwen wrote: > FWIW, I was actually discussing compilation with -O2 on 6.8 with Sjannssen and he told me that even with -O2 turned on, GHC now defaults to -fasm instead of -fvia-c. Ah. Yeah, that could well make a big difference... (Especially on a machine with insufficient RAM.) > Also, the release notes are talking about the runtime speed of binaries, not compilation. > But since GHC is compiled with GHC, you would expect a (small) speedup of GHC. (And indeed that is what the release notes specifically suggest.) Either way, I (and probably a number of other people) will go test what speed difference it actually makes to compiled code. ;-) (I've been wondering for a while now whether there's much to choose between -fasm and -fvia-c.)

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Re: [Haskell] GHC6.8.1 can not compile the simple example
by Neil Mitchell 12 Nov '07

12 Nov '07

Hi David, In future, please post emails to haskell-cafe@, the haskell@ list is for annoucements. The correct command line is: ghc --make c.hs Thanks Neil On Nov 12, 2007 1:11 PM, david yu <davidyuhxl(a)gmail.com> wrote: > Link error is: > [davidyu@linuxpower test]$ ghc c.hs > c.o: In function `rM7_info': > (.text+0xaa): undefined reference to > `containerszm0zi1zi0zi0_DataziMap_lookup_closure' > c.o: In function `rMd_info': > (.text+0x1fa): undefined reference to > `containerszm0zi1zi0zi0_DataziMap_fromList_closure' > c.o: In function `sVJ_info': > (.text+0xa3b): undefined reference to > `__stginit_containerszm0zi1zi0zi0_DataziMap_' > c.o: In function `rM7_srt': > (.data+0x18): undefined reference to > `containerszm0zi1zi0zi0_DataziMap_lookup_closure' > c.o: In function `rMd_srt': > (.data+0x60): undefined reference to > `containerszm0zi1zi0zi0_DataziMap_fromList_closure' > collect2: ld returned 1 exit status > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell > >

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Upgrading X11
by Jim Burton 12 Nov '07

12 Nov '07

I have X11 1.2.2 installed and wanted to upgrade to 1.3 (to satisfy the dependencies of another package), but "Setup configure" tells me I don't have the headers installed. I do, and when I configure 1.2.2 they're detected. Is this due to the newer version of Cabal? I have ghc 6.6, Cabal 1.1.6. Thanks, -- View this message in context: http://www.nabble.com/Upgrading-X11-tf4785984.html#a13691779 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Memory Leak Help
by J. Garrett Morris 12 Nov '07

12 Nov '07

Hello, I have code which seems to contain a memory leak, but I'm not sure where it is or what's causing it. Any help would be greatly appreciated: The code is: data Ratings = Ratings { movieCount :: Int , movieLookup :: IOUArray Int Word32 , movieRatings :: IOUArray Word32 Word32 } readRatingsFromText :: Int -> Int -> C.ByteString -> IO Ratings readRatingsFromText movieCount ratingCount text = do movieLookup <- newArray_ (0, movieCount - 1) movieRatings <- newArray_ (0, fromIntegral ratingCount - 1) iter movieLookup movieRatings 0 (C.lines text) return (Ratings movieCount movieLookup movieRatings) where iter :: IOUArray Int Word32 -> IOUArray Word32 Word32 -> Word32 -> [C.ByteString] -> IO () iter !movieLookup !movieRatings !i [] = return () iter !movieLookup !movieRatings !i (s:ss) | c == ':' = do writeArray movieLookup movie i iter movieLookup movieRatings i ss | otherwise = do writeArray movieRatings i rating -- *** iter movieLookup movieRatings (i + 1) ss where Just (x, rest) = C.readInt s c = C.head rest Just (y, _) = C.readInt (C.tail rest) movie = x - 1 rating = fromIntegral x `shiftL` 3 .|. fromIntegral y :: Word32 main = do args <- getArgs ratings <- readRatingsFromText (read $ args !! 0) (read $ args !! 1) =<< B.readFile (args !! 2) return () This attempts to read a file that looks like: 1: 401,2 503,1 ... 2: 506,5 230,4 possibly with additional junk at the end of each line into an array. When I run this, memory usage grows without bound. If I comment out the line with -- ***, the memory profile is flat. Without storing the ratings, it's not fantastically useful; however, it does demonstrate that it should be possible to iterate through the file with a flat memory profile (which is my goal). I'm compiling with ghc --make -O2 Any help would be greatly appreciated. Thanks! /g -- The man who'd introduced them didn't much like either of them, though he acted as if he did, anxious as he was to preserve good relations at all times. One never knew, after all, now did one now did one now did one.

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Implementing the State Monad
by oleg＠pobox.com 12 Nov '07

12 Nov '07

apfelmus showed the implementation of the state monad as free term algebra, using GADT. Here's an implementation that does not use GADT http://okmij.org/ftp/Haskell/types.html#state-algebra All the smarts are in the observation function. This style is _very_ well explained by Ralf Hinze in his Functional Pearl `Deriving Backtracking Monad Transformers' (ICFP00) http://citeseer.ist.psu.edu/hinze00deriving.html He also showed how to optimize this term algebra (aka, `initial') implementation using continuations (sec 3.3). Where this style begins to break down is with complex monads, such as backtracking *with cut* or some other control. Note that Hinze was not able to implement his version of backtracking with cut using _free_ term algebra. The situation can be improved however if we use a special operation to reflect the search (msplit of MonadMinus). We obtain an efficient implementation then and no longer have to match on the contexts.

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Haskell Weekly News: November 11, 2007
by Don Stewart 12 Nov '07

12 Nov '07

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20071111 Issue 67 - November 11, 2007 --------------------------------------------------------------------------- Welcome to issue 67 of HWN, a newsletter covering developments in the [1]Haskell community. This week sees the release of GHC 6.8.1, to rave reviews. There have been many reports of large performance improvements for Haskell programs, from small to large production systems. Congratulations to the GHC team for such a great release! 1. http://haskell.org/ Announcements GHC 6.8.1. Ian Lynagh [2]announced the release of GHC 6.8.1, a new major release of GHC. There have been a number of significant changes since the last major release, including: Haskell Program Coverage support, the GHCi debugger, pointer tagging in the runtime (with up to 10-15% speedups), constructor specialisation, improved optimisations and much more! The [3]full release notes are available. 2. http://article.gmane.org/gmane.comp.lang.haskell.general/15733 3. http://haskell.org/ghc/docs/6.8.1/html/users_guide/release-6-8-1.html Gtk2Hs 0.9.12.1. Duncan Coutts [4]announced version 0.9.12.1 of gtk2hs is [5]now available. gtk2hs is the standard graphics library for Haskell. 4. http://article.gmane.org/gmane.comp.lang.haskell.general/15738 5. http://haskell.org/gtk2hs/download/ Lazy SmallCheck 0.1. Matthew Naylor [6]announced Lazy SmallCheck 0.1, a library for exhaustive, demand-driven testing of Haskell programs. 6. http://article.gmane.org/gmane.comp.lang.haskell.general/15702 HDBC 1.1.3. John Goerzen [7]announced new releases of HDBC, the Haskell database connectivity kit, and its associated backends (for sqlite3, postgresql, odbc). 7. http://article.gmane.org/gmane.comp.lang.haskell.general/15729 xmobar. Andrea Rossato [8]announced the release of Xmobar-0.8, a minimalistic, text based, status bar. It was specifically designed to work with the XMonad Window Manager. 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/31465 Flymake Haskell. Daisuke Ikegami [9]announced flymake haskell, emacs bindings for interactive Haskell editing. 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/31453 network bytestring. Johan Tibbel [10]announced, strict ByteString versions of the recv/send family of functions for efficient network IO. 10. http://article.gmane.org/gmane.comp.lang.haskell.general/15645 ByteString search. Bryan O'Sullivan [11]announced a cabalised version of the fast Boyer-Moore and Knuth-Morris-Pratt string search code for ByteStrings 11. http://article.gmane.org/gmane.comp.lang.haskell.cafe/31229 Generating free theorems. Janis Voigtlaender [12]announced an improved version of the online and offline free theorems generator for Haskell 12. http://article.gmane.org/gmane.comp.lang.haskell.general/15693 hslogger4j 0.1.1. Bjorn Buckwalter [13]announced Hslogger4j, which provides handlers for hslogger (John Goerzen's Haskell logging framework) that are compatible with log4j's XMLLayout. 13. http://article.gmane.org/gmane.comp.lang.haskell.general/15707 Infinity 0.3. Austin Seipp [14]announced `infinity', an IRC bot in Haskell 14. http://article.gmane.org/gmane.comp.lang.haskell.cafe/31228 hswm. Remi Turk [15]announced the first and last release of hswm, a Haskell window manager. 15. http://article.gmane.org/gmane.comp.lang.haskell.cafe/30804 Hackage This week's new libraries in [16]the Hackage library database. 16. http://hackage.haskell.org/ * xmobar-0.8. Uploaded by Andrea Rossato. [17]xmobar, a minimalistic text based status bar * hsSqlite3-0.0.4. Uploaded by Evgeny Jukov. [18]hsSqlite3, bindings for Sqlite3 * cabal-rpm-0.3.2. Uploaded by Bryan OSullivan. [19]cabal-rpm turns Haskell Cabal source packages into source and binary RPM packages. * selenium-0.2.2. Uploaded by Aaron Tomb. [20]selenium, Haskell bindings to communicate with a Selenium Remote Control server. This package makes it possible to use Haskell to write test scripts that exercise web applications through a web browser. * HPDF-1.3. Uploaded by alpheccar. [21]HPDF. A PDF library with support for several pages, page transitions, outlines, annotations, compression, colors, shapes, patterns, jpegs, fonts, typesetting ... 17. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmobar-0.8 18. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsSqlite3-0.0.4 19. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/cabal-rpm-0.3.2 20. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/selenium-0.2.2 21. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HPDF-1.3 * FileManip-0.3.1. Uploaded by Bryan OSullivan. [22]FileManip, a Haskell library for working with files and directories. Includes code for pattern matching, finding files, modifying file contents, and more. * stringsearch-0.2. Uploaded by Bryan OSullivan. [23]stringsearch, fast search of ByteStrings. * nano-md5-0.1. Uploaded by Don Stewart. [24]nano-md5, ByteString bindings to OpenSSL. * denominate-0.4.1. Uploaded by Calvin Smith. [25]denominate provides a main program for performing bulk file and directory renaming, using a built-in filename converter or user-defined converters. * ContArrow 0.0.2. Uploaded by Evgeny Jukov. [26]ContArrow, Control.Arrow.Transformer.Cont 22. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/FileManip-0.3.1 23. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/stringsearch-0.2 24. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/nano-md5-0.1 25. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/denominate-0.4.1 26. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ * state 0.0.2. Uploaded by Evgeny Jukov. [27]state. State. * infinity 0.3. Uploaded by Austin Seipp. [28]infinity, a tiny, pluggable irc bot. * unix-compat 0.1.2.1. Uploaded by Duncan Coutts. [29]unix-compat, portable implementations of parts of the unix package. * Ranged-sets 0.1.1. Uploaded by Paul Johnson. [30]Ranged-sets. A ranged set is an ordered list of ranges. * IFS 0.1.1. Uploaded by alpheccar. [31]IFS, a library to describe IFS and generate PPM pictures from the descriptions 27. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ 28. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ 29. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ 30. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ 31. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ * bktrees 0.1.3. Uploaded by Josef Svenningsson. [32]bktrees. Burhard-Keller trees provide an implementation of sets which apart from the ordinary operations also has an approximate member search, allowing you to search for elements that are of a certain distance from the element you are searching for. * pqc 0.2. Uploaded by Don Stewart. [33]pqc, a parallel batch driver for QuickCheck * strict 0.2. Uploaded by Don Stewart. [34]strict, strict data types. * HsSVN 0.2. Uploaded by PHO. [35]HsSVN, (Part of) Subversion binding for Haskell * HsHyperEstraier 0.2. Uploaded by PHO. [36]HsHyperEstraier, a HyperEstraier binding for Haskell. HyperEstraier is an embeddable full text search engine which is supposed to be independent to any particular natural languages. 32. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ 33. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ 34. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ 35. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ 36. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ * HsOpenSSL 0.3.1. Uploaded by PHO. [37]HsOpenSSL, a (part of) OpenSSL binding for Haskell. It can generate RSA and DSA keys, read and write PEM files, generate message digests, sign and verify messages, encrypt and decrypt messages. * Finance-Quote-Yahoo 0.4. Uploaded by Brad Clawsie. [38]Finance-Quote-Yahoo, obtain quote data from finance.yahoo.com * LRU 0.1.1. Uploaded by Adam Langley. [39]LRU, an LRU data structure * base 3.0. Uploaded by Ross Paterson. [40]base, the Prelude and its support libraries, and a large collection of useful libraries ranging from data structures to parsing combinators and debugging utilities. * regex-posix 0.72.0.2. Uploaded by Duncan Coutts. [41]regex-posix, posix regex support. 37. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ 38. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Finance-Quote-Ya… 39. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/LRU-0.1.1 40. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/base-3.0.0.0 41. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regex-posix-0.72… * xhtml 3000.0.2.1. Uploaded by Bjorn Bringert. [42]xhtml, combinators for producing XHTML 1.0, including the Strict, Transitional and Frameset variants. * Win32 2.1.0.0. Uploaded by Esa Ilari Vuokko. [43]Win32, a binding to part of the Win32 library * unix 2.2.0.0. Uploaded by Ross Paterson. [44]unix, POSIX functionality * time 1.1.2.0. Uploaded by Ross Paterson. [45]time, time handling. * template-haskell 2.2.0.0. Uploaded by Ross Paterson. [46]template-haskell, support for manipulating Haskell syntax trees 42. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xhtml-3000.0.2.1 43. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Win32-2.1.0.0 44. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/unix-2.2.0.0 45. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/time-1.1.2.0 46. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/template-haskell… * stm 2.1.1.0. Uploaded by Ross Paterson. [47]stm, software transational memory * regex-compat 0.71.0.1. Uploaded by Ross Paterson. [48]regex-compat, backwards compatible regex support * regex-base 0.72.0.1. Uploaded by Ross Paterson. [49]regex-base, regex support. * readline 1.0.1.0. Uploaded by Ross Paterson. [50]readline, an interface to the GNU readline library * random 1.0.0.0. Uploaded by Ross Paterson. [51]random, random number generation 47. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/stm-2.1.1.0 48. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regex-compat-0.7… 49. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/regex-base-0.72.… 50. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/readline-1.0.1.0 51. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/random-1.0.0.0 * QuickCheck 1.1.0.0. Uploaded by Ross Paterson. [52]QuickCheck, automatic testing of Haskell programs * process 1.0.0.0. Uploaded by Ross Paterson. [53]process, jobs and processes * pretty 1.0.0.0. Uploaded by Ross Paterson. [54]pretty, pretty printing library. * parsec 2.1.0.0. Uploaded by Ross Paterson. [55]parsec, monadic parser combinators. * parallel 1.0.0.0. Uploaded by Ross Paterson. [56]parallel, support for parallel programming 52. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/QuickCheck-1.1.0… 53. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/process-1.0.0.0 54. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/pretty-1.0.0.0 55. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/parsec-2.1.0.0 56. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/parallel-1.0.0.0 * packedstring 0.1.0.0. Uploaded by Ross Paterson. [57]packedstring, packed strings * OpenGL 2.2.1.1. Uploaded by Ross Paterson. [58]OpenGL, binding for the OpenGL graphics system * OpenAL 1.3.1.1. Uploaded by Ross Paterson. [59]OpenAL, binding to the OpenAL cross-platform 3D audio API. * old-time 1.0.0.0. Uploaded by Ross Paterson. [60]old-time, time library. * alex 2.2. Uploaded by Simon Marlow. [61]alex is a tool for generating lexical analysers in Haskell 57. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/packedstring-0.1… 58. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/OpenGL-2.2.1.1 59. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/OpenAL-1.3.1.1 60. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/old-time-1.0.0.0 61. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/alex-2.2 * Cabal 1.2.2.0. Uploaded by Duncan Coutts. [62]Cabal is the framework for packaging Haskell software * HaXml 1.19.1. Uploaded by Malcolm Wallace. [63]HaXml, Utilities for manipulating XML documents * HDBC-odbc 1.1.3.0. Uploaded by John Goerzen. [64]HDBC-odbc, ODBC driver for HDBC * HDBC-postgresql 1.1.3.0. Uploaded by John Goerzen. [65]HDBC-postgresql is a PostgreSQL driver for HDBC * HDBC-sqlite3 1.1.3.0. Uploaded by John Goerzen. [66]HDBC-sqlite3 is a Sqlite v3 driver for HDBC 62. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Cabal-1.2.2.0 63. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HaXml-1.19.1 64. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HDBC-odbc-1.1.3.0 65. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HDBC-postgresql-… 66. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HDBC-sqlite3-1.1… * HDBC 1.1.3. Uploaded by John Goerzen. [67]HDBC is a Haskell Database library * X11 1.3.0. Uploaded by Don Stewart. [68]X11 is a library of bindings to the X11 libraries and server * HsOpenSSL 0.3. Uploaded by Masatake Daimon. [69]HsOpenSSL, (Part of) OpenSSL binding for Haskell * Imlib 0.1. Uploaded by Cale Gibbard. [70]Imlib, Haskell binding for Imlib 2 * Stream 0.2.2. Uploaded by Wouter Swierstra. [71]Stream, a library for manipulating infinite lists. 67. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HDBC-1.1.3 68. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/X11-1.3.0 69. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HsOpenSSL-0.3 70. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Imlib-0.1 71. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Stream-0.2.2 * sat 1.0. Uploaded by AndriiZvorygin. [72]sat, CNF SATisfier * torrent 2007.10.27. Uploaded by David Himmelstrup. [73]torrent, BitTorrent file parser * bencode 0.2. Uploaded by David Himmelstrup. [74]bencode, Parser and printer for bencoded data. * SDL 0.5.1. Uploaded by David Himmelstrup. [75]SDL, binding to libSDL * NGrams 1.0. Uploaded by Justin Bailey. [76]ngrams, Simple application for calculating n-grams using Google 72. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/sat-1.0 73. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/torrent-2007.10.… 74. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bencode-0.2 75. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/SDL-0.5.1 76. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/NGrams-1.0 Discussion Bootstrapping Haskell. Andrew Copping [77]wondered how the first Haskell compilers were bootstrapped, leading to some interesting historical details. GHC 6.8 performance. Dan Piponi [78]asked about Haskell performance for low level array manipulation, with some excellent speedups produced by GHC 6.8 77. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/31459 78. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/31314 Jobs Software Development Engineer at MSR. Don Syme announced that [79]the F Sharp team is hiring! We have two positions open right now. The first is a software development engineer specializing in Visual Studio and libraries. The second Post Calendar is a software development engineer 79. http://cs.hubfs.net/blogs/f_team/archive/2007/10/30/3900.aspx PhD position at Chalmers. John Hughes [80]announced that the Functional Programming group at Chalmers is seeking to recruit a PhD student to work on domain-specific languages embedded in Haskell for hardware design, and for programming graphics processors. PhD positions in Sweden are 'real jobs', paying a respectable salary for up to five years. 80. http://article.gmane.org/gmane.comp.lang.haskell.general/15714 Blog noise [81]Haskell news from the [82]blogosphere. * [83]Scala for bioinformatics * [84]I like Haskell a lot * [85]Haskell history * [86]Continuation Passing Style for Monads * [87]Abuse: Is it ruby? Is it Haskell? It's both! * [88]Learning Haskell? * [89]For loops in Haskell * [90]A Simple Programming Puzzle Seen Through Three Differenent Lenses 81. http://planet.haskell.org/ 82. http://haskell.org/haskellwiki/Blog_articles 83. http://tiago.org/ps/2007/10/26/scala-for-bioinformatics/ 84. http://www.noulakaz.net/weblog/2007/04/20/i-like-haskell-a-lot/ 85. http://caos.di.uminho.pt/~ulisses/blog/2007/10/27/haskell-history/ 86. http://r6.ca/blog/20071028T162529Z.html 87. http://meta-meta.blogspot.com/2007/05/abuse-is-it-ruby-is-it-haskell-its-bo… 88. http://www.dynamicdrive.com/forums/showthread.php?t=26094 89. http://osfameron.vox.com/library/post/for-loops-in-haskell.html?_c=feed-atom 90. http://thraxil.org/users/anders/posts/2007/10/30/A-Simple-Programming-Puzzl… * [91]Scala Makes Me Think * [92]Haskell Substring Function * [93]Multicore Programming and Automatic Parallelisation * [94]Spinoza SOS in Haskell * [95]Why not just use Haskell? * [96]Chain delegates (in Haskell) * [97]Parser Combinators in C * [98]Will hybrid languages like D render functional languages like Haskell, OCaml and Common Lisp irrelevant? * [99]Category Theory for the Java Programmer * [100]Benchmarking ray tracing, Haskell vs. OCaml 91. http://blog.snowtide.com/2007/10/31/scala-makes-me-think 92. http://ironmonkey.blogspot.com/2007/10/haskell-substring-function_31.html 93. http://hnr.dnsalias.net/wp/?p=31 94. http://seta07.blogspot.com/2007/10/spinoza-sos-in-haskell.html 95. http://www.hackinghat.com/index.php/c/66 96. http://thoughtspam.spaces.live.com/Blog/cns!253515AE06513617!454.entry 97. http://www.math.chalmers.se/~koen/ParserComboC/parser-combo-c.html 98. http://pinderkent.blogsavy.com/archives/141 99. http://reperiendi.wordpress.com/2007/11/03/category-theory-for-the-java-pro… 100. http://augustss.blogspot.com/2007/11/benchmarking-ray-tracing-haskell-vs.ht… * [101]Some lambda calculus examples * [102]Beautiful timetables * [103]References, Arrows and Categories * [104]Anamorphisms in Ruby * [105]Type metaprogramming in Haskell and C++ * [106]A Small Combinatorial Library 101. http://augustss.blogspot.com/2007/11/some-lambda-calculus-examples-syntax-i… 102. http://onthebalcony.wordpress.com/2007/11/11/beautiful-timetables/ 103. http://twan.home.fmf.nl/blog/haskell/References-Arrows-and-Categories.detai… 104. http://weblog.raganwald.com/2007/11/really-useful-anamorphisms-in-ruby.html 105. http://www.rubinsteyn.com/template_insanity.html 106. http://sigfpe.blogspot.com/2007/11/small-combinatorial-library.html Quotes of the Week * faxathisia: Omg! I spent 2 days writing this code and it's worked the first time I run it. Only possible with Haskell :D * anonymous: The thing is Haskell isn't suited for young people, whereas the OBJECT model of C++ is * sethg: I feel like I still dont understand comonads * fnord123: Haskell mainly helps with my C++ template coding when I'm doing money oriented programming * Tac-Tics: I get the feeling if all I ever use is the IO monad, someone here will shower me in holy monad fire and cleanse the evil from me.... leaving burn marks all over * Anton van Straaten: there's a new movement towards 'functional eating' which involves using a knife and fork (think ML) or chopsticks (Haskell ;) instead of a chainsaw. Its proponents claim that this approach is far superior, but chainsaw fans are skeptical. * SamB: what happens in the monad... stays in the monad... * Brent Yorgey: Friends don't let friends write in COBOL. * Bulat Ziganshin: It's a whole new era in low-level GHC programming About the Haskell Weekly News New editions are posted to [107]the Haskell mailing list as well as to [108]the Haskell Sequence and [109]Planet Haskell. [110]RSS is also available, and headlines appear on [111]haskell.org. Headlines are available as [112]PDF. To help create new editions of this newsletter, please see the [113]contributing information. Send stories to dons at galois.com. The darcs repository is available at darcs get [114]http://code.haskell.org/~dons/code/hwn/ 107. http://www.haskell.org/mailman/listinfo/haskell 108. http://sequence.complete.org/ 109. http://planet.haskell.org/ 110. http://sequence.complete.org/node/feed 111. http://haskell.org/ 112. http://code.haskell.org/~dons/code/hwn/archives/20071111.pdf 113. http://haskell.org/haskellwiki/HWN 114. http://code.haskell.org/~dons/code/hwn/

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Implementing the State Monad (Was: Can every monad can be implemented with Cont?)
by apfelmus 12 Nov '07

12 Nov '07

jeff p wrote: >>>> Didn't someone already prove all monads can be implemented in terms >>>> of Cont? >>> >>> Cont and StateT, wasn't it? >>> And the schemers have no choice about running in StateT :) >> >> You sure? I want to see the proof :) > > I think this is referring to Andrzej Filinski's paper "Representing > Layered Monads" in which it shown that stacks of monads can be > implemented directly (no layering) by using call/cc and mutable state. Thanks for the reference! I still don't understand Filinski's papers enough to say whether there's more to the embedding than just type Cont m a = forall b . (a -> m b) -> m b reify :: Monad m => Cont m a -> m a reify f = f return reflect :: Monad m => m a -> Cont m a reflect x = (x >>=) Does this already give a performance benefit without further inlining? I doubt it. Anyway, all this leads to a fun and easy way to implement monads, for example the state monad. We have the primitive operations type State s a get :: State s s put :: s -> State s () together with the usual monad laws and the operational semantics evalState :: State s a -> (s -> a) evalState (return x) = \_ -> x evalState (get >>= k) = \s -> evalState (k s ) s -- make the state s available to k evalState (put s >>= k) = \_ -> evalState (k ()) s -- use a new state s Why "operational semantics"? Well, we're just specifying what happens when we "execute" a get or put "instruction" by saying how the execution proceeds with the next instruction k pointed out by >>=. We're not using the "usual" and "elaborate" type like s -> (a,s) to thread the state around, we're using a humble function s -> a to specify that a value a depends on some state s . The operational semantics will do the state plumbing for us. In other words, the we don't have to come with a special implementation like s -> (a,s) that works, we will *mechanically* get one from our intended operational semantics. Now, how to implement? Well, the best way to start is to represent each primitive operation with a new constructor: (GADT notation, needs the -XGADTs flag) data State :: * -> * -> * where Return :: a -> State s a (:>>=) :: State s a -> (a -> State s b) -> State s b Get :: State s s Put :: s -> State s a With this _term representation_, we can implement the operational semantics by evalState ((m :>>= n) :>>= k) = evalState (m :>>= (n :>>= k)) -- monad law associativity evalState (Return x :>>= k) = evalState (k x) -- monad law left unit evalState (Get :>>= k) = \s -> evalState (k s ) s -- semantics of get evalState (Put s :>>= k) = \_ -> evalState (k ()) s -- semantics of put evalState (Return x) = \_ -> x -- semantics of return evalState m = evalState (m :>>= Return) -- monad law right unit Neat, isn't it? Every law and every specification for the primitive instructions has been used exactly once. Simple and painless, but not fully optimized yet. With the first equation, using :>>= left-associatively has similar problems like using ++ left-associatively. Both concat' = foldl (++) [] sequence_' = foldl (>>) (return ()) would show quadratic time behavior. So, just like with difference lists, the idea is to represent the operations in the monad together with the _context_ they are commonly used in. For concatenating lists, the context is (xs ++ _) so that every list xs is represented by \ys -> (xs ++ ys) For our state monad, the context is evalState (m :>>= _) i.e., we make evaluation and sequencing "built-in". More specifically, we also make the evaluation of the next instructions built-in, so that every monadic action m will be represented by \k -> evalState (m :>>= k') where k = evalState . k' In other words, we will represent the type State s a by State' s a = forall b . (a -> (s -> b)) -> (s -> b) But this is just the continuation monad! data Cont m a = Cont (forall b . (a -> m b) -> m b) type State' s a = Cont (s ->) a with m a = s -> a the result type of our semantics evalState . So, we already get >>= and return for free, knowing that they fulfill the monad laws return x = \k -> k x m >>= f = \k -> m (flip f k) Our custom primitive operations get and put are straightforward to implement get = \k -> evalState (Get :>>= k') = \k -> \s -> evalState (k' s ) s = \k -> \s -> k s s put s = \k -> \_ -> k () s These definitions are crystal clear from their operational semantics, given some practice reading them. Last but not least, there is evalState which implements the behavior of the Return instruction evalState m = evalState (m' :>>= Return) = (\k -> evalState (m' :>>= k')) (evalState . Return) = m (\x -> evalState (Return x)) = m (\x -> \_ -> x) That's it for the state monad. For reference, here's the full implementation type State s a = Cont ((->) s) a get = \k s -> k s s put s = \k _ -> k () s evalState m = m const We get >>= and return for free from the predefined Cont (assuming that the "done right"-version with universal quantification would be in the libraries, that is). Of course, this approach isn't limited to the state monad. Here are some parser combinators type Result a = String -> [a] type Parser a = Cont (Result) a run p = p (\x i -> if null i then [x] else []) symbol = \k i -> case i of { c:cs -> k c; [] -> []; } fail = \k i -> [] p +++ q = \k i -> p k i ++ q k i Can you see the operational semantics? (Think of p k as run (p >>= k)). If not, stick to the term implementation and check out Unimo below for a free >>= :) This simple way of implementing monads by their operational semantics is known for quite some time John Hughes. The Design of a Pretty-printing Library. http://citeseer.ist.psu.edu/hughes95design.html Chuan-kai Lin. Programming Monads Operationally with Unimo. http://web.cecs.pdx.edu/~cklin/papers/unimo-143.pdf and is in fact related to the good old continuation passing style of IO and parser combinators. But I think it's powerful and I'd like it to be well-known. Regards, apfelmus PS: Put differently, the question of the original thread "Can every monad can be implemented with Cont?" is whether Unimo can implement strictly more monads than Cont.

1 0

GHC 6.8.1 Documentation
by Cale Gibbard 11 Nov '07

11 Nov '07

Hello all, Recently I noticed that all of my bookmarks to the hierarchical libraries documentation broke, and I'm not entirely happy with the solution of just correcting all the links to point at the new URLs since the URLs all have package version numbers in their names, which means that I'll have to update them all over again next time. Is there any chance we could get stable URLs for the latest version of each module somewhere? (Perhaps via symlinks on the webserver?) I also noticed that the links to the source code seem to be missing from all the 6.8.1 Haddock docs now, which is something that I used quite a lot in the past. It'd be nice to have those back. - Cale

2 1

missed clause???
by Ryan Bloor 11 Nov '07

11 Nov '07

hi I was testing my code when I came across a strange predicament. The input is a list of ints and a Results type which is of type [(int,.......),(Int......)......]. I am comparing each int from the list to the first element in each member of results. But it works for 1-9 but not for 10 onwards.... why? Have I missed a clause anywhere...... it only does the first one after that. --Print the points of eight games poolsPoints :: [Int] -> Results -> Int poolsPoints [] [] = 0 poolsPoints [] _ = 0 poolsPoints _ [] = 0 poolsPoints (x:xs) ((a,b,c,d,e):t) | (x == a && c>d) = 1 + poolsPoints xs t | (x == a && c<d) = 1 + poolsPoints xs t | (x == a && c==0 && d==0) = 2 + poolsPoints xs t | (x == a && c==d && c>0) = 3 + poolsPoints xs t | otherwise = 0 + poolsPoints [x] t Thanks Ryan _________________________________________________________________ The next generation of MSN Hotmail has arrived - Windows Live Hotmail http://www.newhotmail.co.uk

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Announce: xmobar-0.8 released!
by Andrea Rossato 11 Nov '07

11 Nov '07

Hello! I'm very glad to announce the release of Xmobar-0.8. You can grab the source code from Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmobar-0.8 Xmobar is a minimalistic, text based, status bar. It was specifically designed to work with the XMonad Window Manager. Inspired by the Ion3 status bar, it supports similar features, like dynamic color management, output templates, and extensibility through plugins. A precompiled binary is available here: http://code.haskell.org/~arossato/xmobar/xmobar-0.8.bin A recent screen shot can be found here: http://code.haskell.org/~arossato/xmobar/xmobar-0.8.png A screen shot of xmonad and xmobar (my desktop): http://haskell.org/sitewiki/images/a/ae/Arossato-config.png Xmobar depends on X11-1.3.0 and Cabal-1.2 but works both with ghc-6.6.x and ghc-6.8.1. Here you can find a version of the 0.8 source tree that works with Cabal-1.1.x, but will *not* compile with ghc-6.8.1: http://code.haskell.org/~arossato/xmobar/xmobar-0.8-Cabal-1.1-ghc-6.6.tar.gz Some basic documentation is available at the following address: http://code.haskell.org/~arossato/xmobar/ I would like to thank Robert Manea and Spencer Janssen for their help in understanding X and Xlibs. Thanks to Claus Reinke for making me understand existential types (or at least for letting me think I grasp them ;). Xmobar incorporates patches from: Krzysztof Kosciuszkiewicz, Spencer Janssen, Jens Petersen, Dmitry Kurochkin, and Lennart Kolmodin. Hope you will enjoy! Andrea Rossato What's new ========== There are a lot of changes since the last release: a. Configuration: - removed xPos, yPos, width, height, align, and refresh configuration options; - added position and alignSep configuration options: the first is used to set the position (Top, Bottom or Static; the second to set the characters to be used to separate, in the output template, text that must be aligned to the left, centered, or aligned to the right; (please have look at xmobar.config-sample and the README for more details) b. Plugins: - added a plugin to read Unix named pipe (useful to display XMonad logs); - added a plugin to read from standard input; - a new plugin API makes it possible to write plugins that perform asynchronous actions. The new API is totally backward compatible; c. Output template: - it is now possible to align text in the output template. By default text before a '}' will be align to left, the text in between '}' and '{' will be centered, and the text after a '{' will be align to the right, like: "left } centered { right" - strings can have also a background color set: "<fc=red,white>%date%<fc>" will display the output of the "date" command in red with a white background. d. Concurrency: now Xmobar uses Software Transactional Memory and the main window is updated only when needed e. Documentation: I now use Pandoc for the README, this way I have also created an HTML version of. I hope that makes the documentation a bit more accessible: http://code.haskell.org/~arossato/xmobar/ f. The darcs repository has been moved to: http://code.haskell.org/xmobar g. XRandR: since positioning is now done automatically, XRandR should be supported too, even thought support could be buggy due to the fact that I cannot actually test it, since I don't have capable hardware. h. Cabal-1.2 and ghc-6.8.1 support. i. Many other small changes and code cleanup: darcs changes will tell you everything. Configuration Example ===================== This is my ~/.xmobarrc: Config { font = "-bitstream-charter-*-r-normal-*-10-*-*-*-*-56-*-*" , bgColor = "black" , fgColor = "grey" , position = Top , commands = [ Run Weather "LIPB" ["-L","18","-H","27","--normal","green","--high","red","--low","lightblue"] 36000 , Run Network "eth1" ["-L","0","-H","32","--normal","green","--high","red"] 10 , Run Cpu ["-L","3","-H","50","--normal","green","--high","red"] 10 , Run Memory ["-t","Mem: <usedratio>%"] 10 , Run Battery ["-L","50","-H","75","--high","green","--normal","yellow", "--low", "red"] 10 , Run Swap [] 10 , Run Com "uname" ["-s","-r"] "" 36000 , Run Date "%a %b %_d %Y * %H:%M:%S" "mydate" 10 , Run PipeReader "/home/andrea/.xmonad-status" "xlog" ] , sepChar = "%" , alignSep = "}{" , template = "%xlog% }{ %cpu% | %memory% * %swap% | %eth1% | %LIPB% | <fc=orange>%mydate%</fc> | %battery%" }

1 0

Sinus in Haskell
by Hans van Thiel 11 Nov '07

11 Nov '07

Hello All, Can anybody explain the results for 1.0, 2.0 and 3.0 times pi below? GHCi yields the same results. I did search the Haskell report and my text books, but to no avail. Thanks in advance, Hans van Thiel Hugs> sin (0.0 * pi) 0.0 Hugs> sin (0.5 * pi) 1.0 Hugs> sin (1.0 * pi) 1.22460635382238e-16 Hugs> sin (1.5 * pi) -1.0 Hugs> sin (2.0 * pi) -2.44921270764475e-16 Hugs> sin (2.5 * pi) 1.0 Hugs> sin (3.0 * pi) 3.67381906146713e-16 Hugs>

12 15

WideFinder
by manu 11 Nov '07

11 Nov '07

Haskell is conspicuously absent from the languages used to tackle Tim Bray's Wide Finder problem (http://www.tbray.org/ongoing/When/200x/ 2007/10/30/WF-Results?updated). So far we have Ocaml, Erlang, Python, Ruby, etc... Bryan quickly wrote a program on his blog (http://www.serpentine.com/ blog/2007/09/25/what-the-heck-is-a-wide-finder-anyway/) that would place Haskell right in the second position. JoCaml is the fastest so far (http://eigenclass.org/hiki.rb?fast- widefinder)... Can Haskell do better ? Care to take a shot ? Manu

6 8

Re: Flymake Haskell
by Daisuke IKEGAMI 11 Nov '07

11 Nov '07

<cbf55b100711081320k10988f5fke345b9d01bcf64e9(a)mail.gmail.com> <20071109.090048.74745692.ikegami(a)madscientist.jp> <jwvwssq4r4f.fsf-monnier%2Bemacs(a)gnu.org> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Hi, Stefan wrote: > I looked at http://www.emacswiki.org/cgi-bin/emacs/FlymakeHaskell > but it's not clear what's going on. I've been written the instruction of Flymake Haskell for both Emacs21 and Emacs22 at; http://www.emacswiki.org/cgi-bin/emacs/FlymakeHaskell Note that it will not work in the future, since the flymake-mode has been developed so actively. I'm not a native English speaker. Please fix typo or inappropriate expressions. I wish it helps to write Haskell code on Emacs. Best wishes, ike

1 0

O'Reilly "Real World Haskell" book
by Galchin Vasili 11 Nov '07

11 Nov '07

Hello, What is the proposed table of contents for "Real World Haskell"? Regards, Vasili

2 1

Whither memo and hash-consing?
by Chung-chieh Shan 11 Nov '07

11 Nov '07

Hello, What is the current status of the Memo module, for memoizing functions? It seems to have disappeared from the standard Hugs/GHC distributions; are we supposed to just find the old code on Google which uses System.Mem.Weak? A related question is whether anyone has built a library for hash-consing. Ideally it's just a function "hashCons" with a type like "(Ord a) => a -> a". Thank you. -- Edit this signature at http://www.digitas.harvard.edu/cgi-bin/ken/sig The old name for the Giraffe was Cameleopard, literally a Camel-Leopard.

1 0

Trouble using unboxed arrays
by Justin Bailey 11 Nov '07

11 Nov '07

I would like to create a data structure that uses an unboxed array as one of its components. I would like the data structure to be parameterized over the type of the elements of the array. Further, I'd like to build the array using runSTUArray. I can't make the code work though. My naive approach: data Ring a = Ring (UArray Int a) and the code to make the array: makeArray :: [a] -> Ring a makeArray ls = Ring (ST.runSTUArray (ST.newListArray (0, length ls - 1) ls)) But that doesn't work. I get from GHC (6.8.1): Could not deduce (MArray (STUArray s) a (ST s)) from the context () arising from a use of `newListArray' I am pretty sure I need to constrain 'a' to primitive types only, but how? Can I do it in the data definition? Thanks in advance for any help! Justin

3 2

recursion issues...
by Ryan Bloor 11 Nov '07

11 Nov '07

hiya I was wondering how I would get the second function do recursively do the function for poolNews xs.... tried that and it fails. Ryan --Give wins, draws a rating. poolNews :: Result -> PoolNews -> PoolNews poolNews (a,b,c,d,e) (home,away,goaless,scoredraw) | c > d = (home+1,away,goaless,scoredraw) | c < d = (home,away+1,goaless,scoredraw) |(c == 0) && (d == 0) = (home+1,away,goaless+1,scoredraw) | otherwise = (home,away,goaless,scoredraw+1) --Do for all Results poolNewsB :: Results -> PoolNews poolNewsB (x:xs) = poolNews x (0,0,0,0) _________________________________________________________________ Celeb spotting – Play CelebMashup and win cool prizes https://www.celebmashup.com

3 2

C++ vs. Haskell: Type-Level Death Match
by Don Stewart 11 Nov '07

11 Nov '07

http://www.rubinsteyn.com/template_insanity.html The C++ response to Conrad's 'instant insanity' type program from the last Monad.Reader ! Can we do better still with the planned type-level programming of type families? -- Don

1 0

Some More Sinus Results
by Hans van Thiel 11 Nov '07

11 Nov '07

Thanks to all who've replied; Carl's explanation in particular was very interesting. So the precision, suggested by the many decimals in the 'show', is not the actual precision the user should 'count on'. If you take 1/60 of a degree to be approximately 0.0003 radians, you should not use sin for smaller values. In all cases the actual precision of sin appears to be 4 to 5 decimals, and results should be rounded to that before using them. Now I'm wondering about cos, tan and also the inverses, asin etc. :-) Regards, Hans van Thiel Hugs> sin (1.000 * pi) 1.22460635382238e-16 Hugs> sin (0.999 * pi) 0.00314158748587949 Hugs> sin (1.00001 * pi) -3.14159265309255e-05 Hugs> sin (0.99999 * pi) 3.14159265307264e-05 Hugs> sin (1.001 * pi) -0.00314158748587925 Hugs> sin (0.999 * pi) 0.00314158748587949 Hugs> sin (1.0001 * pi) -0.000314159260191213 Hugs> sin (0.9999 * pi) 0.000314159260191458 Hugs> sin (1.00001 * pi) -3.14159265309255e-05 Hugs> sin (1.0001 * pi) -0.000314159260191213 Hugs> sin (0.9999 * pi) 0.000314159260191458 Hugs> sin (1.00001 * pi) -3.14159265309255e-05 Hugs> sin (0.99999 * pi) 3.14159265307264e-05 Hugs> sin (6.0001 * pi) 0.000314159260189269 Hugs> sin (5.9999 * pi) -0.000314159260190738 Hugs> sin (6.00001 * pi) 3.14159265298692e-05 Hugs> sin (5.99999 * pi) -3.14159265313387e-05 Hugs> sin pi 1.22460635382238e-16

3 3

FW: please help... small problem
by Ryan Bloor 11 Nov '07

11 Nov '07

hi I've attempted to cut down this module... but I cannot see where... can someone help... Ryan thanks From: ryanbloor(a)hotmail.comTo: Subject: FW: please help... small problemDate: Fri, 9 Nov 2007 21:57:30 +0000 sorry heres the code.... I always do that. From: ryanbloor(a)hotmail.comTo: Subject: please help... small problemDate: Fri, 9 Nov 2007 21:44:35 +0000 hi Is there anyway to cut down this code and to not use auxillary functons, but instead use pattern matching? The code basically splits up a list 'rslis' into a list of lists - but so each word is split up and the integers have been parsed. so.... ["hi ryan 1","hi jeff 2"] becomes [["hi","ryan" 1], ["hi","jeff", 2]].The code is far too long. I don't wanna use premade functions too much... pattern matching is required. Ryan The next generation of MSN Hotmail has arrived - Windows Live Hotmail Are you the Quizmaster? Play BrainBattle with a friend now! _________________________________________________________________ 100’s of Music vouchers to be won with MSN Music https://www.musicmashup.co.uk

2 1

Doubly-linked zipper list w/ insert implementation
by Justin Bailey 11 Nov '07

11 Nov '07

The other day I decided to implement a ring buffer with a current element (i.e. a doubly-linked zipper list). In order to allow inserts (and, in the future, deletes and updates), I have a special sentinel element called "Join" in the structure. When inserting, I find the join first, insert and then rebuild the buffer using circular programming techniques. This also allows the buffer to be converted back to a list. The current element can be changed by rotating right or left, which never fails. Rotating n positions takes n steps. I'm posting it here for comments and feedback. How could the structure be smarter? Would storing a unique ID with each element make more sense? Any comments on the space behavior under insert and rotates? I wanted to "maximize" sharing. Thanks in advance. Justin p.s. The original motivation for writing this was to model cellular automata. The CA world is "circular", so that got me thinking about a structure that made connecting the ends easy to do. -- cut here --- module Ring (Ring, create, insert, current, rotR, rotL, toListL, toListR) -- Thanks to Keith Wansbrough for his posting describing a doubly-linked list -- in Haskell for inspiration here. -- -- http://groups.google.com/group/comp.lang.functional/msg/8c65fdd16f7e91e1 where import Data.List (foldl') -- For testing import Test.QuickCheck import Control.Monad (replicateM_) import System.Random (randomRIO) import System.Environment (getArgs) data Ring a = Ring (Ring a) a (Ring a) | Join (Ring a) (Ring a) instance (Show a) => Show (Ring a) where show r = "{" ++ show' (findLeftOfJoin r) where show' (Join _ _) = "}" show' (Ring l v _) | isJoin l = show v ++ show' l | otherwise = show v ++ "," ++ show' l create v = let me = Ring join v join join = Join me me in me insert r a = let (left, right) = insert' left right start a start = findLeftOfJoin r in left where insert' left right (Join _ _) val = let last = Ring join val right join = Join left last in (last, join) insert' left right (Ring l v _) val = let this = Ring newL val right (newL, newR) = insert' left this l v in (this, newR) fromList [] = error "Can't create empty ring" fromList ls = let (left, right) = fromList' left right ls in left where -- compute this ring, given left and right pointers. Return -- left and right pointers for this segment fromList' left right [] = let join = Join left right in (join, right) fromList' left right (x:xs) = let this = Ring l x right (l, r) = fromList' left this xs in (this, r) toList = toListL -- View of list from left hand side toListL = toList' . findLeftOfJoin where toList' (Join _ _) = [] toList' (Ring l v _) = v : toList' l toListR = toList' . findRightOfJoin where toList' (Join _ _) = [] toList' (Ring _ v r) = v : toList' r current (Ring r v l) = v current _ = error "Join is never current (curr)" rotR r amt | amt > 0 = goRight r amt | amt < 0 = goLeft r (negate amt) | otherwise = r rotL r amt | amt > 0 = goLeft r amt | amt < 0 = goRight r (negate amt) | otherwise = r goRight r 0 = r goRight (Ring _ _ r@(Ring _ _ _)) amt = goRight r (amt - 1) goRight (Ring _ _ (Join _ r)) amt = goRight r (amt - 1) goRight (Join _ _) _ = error "Join is never current (goRight)" goLeft r 0 = r goLeft (Ring l@(Ring _ _ _) _ _) amt = goLeft l (amt - 1) goLeft (Ring (Join l _) _ _) amt = goLeft l (amt - 1) goLeft (Join _ _) _ = error "Join is never current (goLeft)" isRing (Ring _ _ _) = True isRing _ = False isJoin (Join _ _) = True isJoin _ = False findLeftOfJoin (Join l _) = l findLeftOfJoin (Ring l _ _) = findLeftOfJoin l findRightOfJoin (Join _ r) = r findRightOfJoin (Ring _ _ r) = findRightOfJoin r

2 5

Disable echo in POSIX terminal
by Taylor Venable 10 Nov '07

10 Nov '07

Hello all, I've written a little Haskell program to get information from a MySQL database (great thanks to anybody reading this who works on HSQL, by the way) but I want to keep the user's password concealed, obviously. Currently I prompt for it from the terminal, but the problem is that it's echoed just like all other input. I would like to disable input echo when asking for the password, but I'm not sure how. I notice there's a System.POSIX.Terminal module, but I have no idea how to use it, or whether it will do what I want. Any ideas would be greatly appreciated. Best regards. -- Taylor Venable taylor(a)metasyntax.net http://real.metasyntax.net:2357/

6 8

checking regular expressions
by Uwe Schmidt 10 Nov '07

10 Nov '07

Hi all, what's the simplest way to check, whether a given string is a wellformed regular expression? In the API there's just a "mkRegex" which does not make any checks, and the "matchRegex" which throws an exception when the regex isn't wellformed. So do I need the IO monad for checking a regex? Uwe

3 2

Odd error report against ghc-6.8.1
by ChrisK 10 Nov '07

10 Nov '07

thomas.hartman(a)db.com has sent me a new bug report. Apparently he can crash ghc-6.8.1 when compiling regex-tdfa-0.93 (darcs under http://darcs.haskell.org/packages/regex-unstable/regex-tdfa/ ). Should I open a ticket for this? > > I darcs got the newest regex-tdfa (0.93 iirc) and made all the obvious changes involving LANGUAGE/OPTION_GHC pragmas to get this to work on ghc-6.8.1 > > prereq regex_base 0.93 installs ok > regex-tdfa panics with > > hartthoma@linuxpt:~/installs/regexstuff/regex-newest/regex-tdfa>runghc Setup.hs build > Preprocessing library regex-tdfa-0.93... > Building regex-tdfa-0.93... > [ 1 of 21] Compiling Data.IntMap.CharMap ( Data/IntMap/CharMap.hs, dist/build/Data/IntMap/CharMap.o ) > > Data/IntMap/CharMap.hs:9:27: > Warning: Imported from `Data.Char' but not used: `chr' > [ 2 of 21] Compiling Data.IntSet.EnumSet ( Data/IntSet/EnumSet.hs, dist/build/Data/IntSet/EnumSet.o ) > [ 3 of 21] Compiling Data.IntMap.EnumMap ( Data/IntMap/EnumMap.hs, dist/build/Data/IntMap/EnumMap.o ) > [ 4 of 21] Compiling Text.Regex.TDFA.IntArrTrieSet ( Text/Regex/TDFA/IntArrTrieSet.hs, dist/build/Text/Regex/TDFA/IntArrTrieSet.o ) > [ 5 of 21] Compiling Text.Regex.TDFA.Common ( Text/Regex/TDFA/Common.hs, dist/build/Text/Regex/TDFA/Common.o ) > [ 6 of 21] Compiling Text.Regex.TDFA.Pattern ( Text/Regex/TDFA/Pattern.hs, dist/build/Text/Regex/TDFA/Pattern.o ) > [ 7 of 21] Compiling Text.Regex.TDFA.ReadRegex ( Text/Regex/TDFA/ReadRegex.hs, dist/build/Text/Regex/TDFA/ReadRegex.o ) > [ 8 of 21] Compiling Text.Regex.TDFA.CorePattern ( Text/Regex/TDFA/CorePattern.hs, dist/build/Text/Regex/TDFA/CorePattern.o ) > ghc-6.8.1: panic! (the 'impossible' happened) > (GHC version 6.8.1 for i386-unknown-linux): > splitFunTy > <nt>mtl-1.1.0.0:Control.Monad.RWS.Lazy.RWS{tc rsRo} > (base:Data.Maybe.Maybe{tc r6} > regex-tdfa-0.93:Text.Regex.TDFA.Common.GroupIndex{tc raaa}) > [base:Data.Either.Either{tc 344} > regex-tdfa-0.93:Text.Regex.TDFA.Common.Tag{tc raai} > regex-tdfa-0.93:Text.Regex.TDFA.Common.GroupInfo{tc raa4}] > ([regex-tdfa-0.93:Text.Regex.TDFA.Common.OP{tc raae}] > -> [regex-tdfa-0.93:Text.Regex.TDFA.Common.OP{tc raae}], > regex-tdfa-0.93:Text.Regex.TDFA.Common.Tag{tc raai}) > (base:GHC.Prim.Any{(w) tc 31N}, > [regex-tdfa-0.93:Text.Regex.TDFA.Common.Tag{tc raai}]) > > Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug Note that "Text.Regex.TDFA.Common.Tag" is "type Tag = Int" and compiling the CorePattern.hs file works fine under ghc-6.6.1. The only unusual thing in CorePattern is the use of "mdo" recursive-do syntax. His message continues with the steps he used, and the minimal patch that got it from ghc-6.6.1 to ghc-6.8.1 : > > hartthoma@linuxpt:~/installs/regexstuff/regex-newest/regex-tdfa>runghc Setup.hs configure > Configuring regex-tdfa-0.93... > hartthoma@linuxpt:~/installs/regexstuff/regex-newest/regex-tdfa>runghc Setup.hs build > Preprocessing library regex-tdfa-0.93... > Building regex-tdfa-0.93... > > Text/Regex/TDFA/MutRunBS.hs:12:17: > Could not find module `Data.ByteString.Base': > Use -v to see a list of the files searched for. > hartthoma@linuxpt:~/installs/regexstuff/regex-newest/regex-tdfa>darcs whatsnew > { > hunk ./Text/Regex/TDFA/CorePattern.hs 1 > +{-# OPTIONS_GHC -XRecursiveDo #-} > hunk ./Text/Regex/TDFA/RunMutState.hs 1 > -{-# LANGUAGE CPP #-} > +{-# OPTIONS_GHC -fglasgow-exts -XCPP #-} > + > hunk ./regex-tdfa.cabal 16 > -Build-Depends: regex-base >= 0.80, base >= 2.0, parsec, mtl > +Build-Depends: regex-base >= 0.80, base >= 2.0, parsec, mtl, containers, array, bytestring > hunk ./regex-tdfa.cabal 45 > -GHC-Options: -Wall -Werror -O2 -funbox-strict-fields > +GHC-Options: -Wall -Wwarn -O2 -funbox-strict-fields > } > hartthoma@linuxpt:~/installs/regexstuff/regex-newest/regex-tdfa> > >

1 0

An exceptional exception
by Andrew Coppin 10 Nov '07

10 Nov '07

I've been doing some testing to try to figure out the tangled maze that is exception handling. (Is there a *reason* why half of all exceptions are dynamic, and the other half aren't? Couldn't we have just 1 system?) Anyway, I wrote a cute little "test" function for figuring out how each kind of exception works: > test (fail "go away") isAlreadyExistsError=False isDoesNotExistError=False isAlreadyInUseError=False isFullError=False isEOFError=False isIllegalOperation=False isPermissionError=False isUserError=True Next I tried this: > test (getDirectoryContents "banana") isAlreadyExistsError=False isDoesNotExistError=True isAlreadyInUseError=False isFullError=False isEOFError=False isIllegalOperation=False isPermissionError=False isUserError=False And now, this: > test (writeFile "C:\\C:\\x" "test") isAlreadyExistsError=False isDoesNotExistError=False isAlreadyInUseError=False isFullError=False isEOFError=False isIllegalOperation=False isPermissionError=False isUserError=False Erm... so this exception registers as "False" on every single predicate? How exactly am I meant to detect this error?

3 4

FW: please help... small problem
by Ryan Bloor 10 Nov '07

10 Nov '07

sorry heres the code.... I always do that. From: ryanbloor(a)hotmail.comTo: Subject: please help... small problemDate: Fri, 9 Nov 2007 21:44:35 +0000 hi Is there anyway to cut down this code and to not use auxillary functons, but instead use pattern matching? The code basically splits up a list 'rslis' into a list of lists - but so each word is split up and the integers have been parsed. so.... ["hi ryan 1","hi jeff 2"] becomes [["hi","ryan" 1], ["hi","jeff", 2]].The code is far too long. I don't wanna use premade functions too much... pattern matching is required. Ryan The next generation of MSN Hotmail has arrived - Windows Live Hotmail _________________________________________________________________ Get free emoticon packs and customisation from Windows Live. http://www.pimpmylive.co.uk

2 1

please help... small problem
by Ryan Bloor 10 Nov '07

10 Nov '07

hi Is there anyway to cut down this code and to not use auxillary functons, but instead use pattern matching? The code basically splits up a list 'rslis' into a list of lists - but so each word is split up and the integers have been parsed. so.... ["hi ryan 1","hi jeff 2"] becomes [["hi","ryan" 1], ["hi","jeff", 2]].The code is far too long. I don't wanna use premade functions too much... pattern matching is required. Ryan _________________________________________________________________ 100’s of Music vouchers to be won with MSN Music https://www.musicmashup.co.uk

3 2

XMonad architecture paper?
by Galchin Vasili 10 Nov '07

10 Nov '07

Hello, I was watching Simon Peyton-Jones' video on "A Taste of Haskell" Part 1. .. Is there any paper discussing the architecture? I am not afraid to read code but sometimes a paper overview is good ... Regards, Vasya

2 3

ByteString search code available in easy-to-digest form
by Bryan O'Sullivan 10 Nov '07

10 Nov '07

I've packaged up the fast Boyer-Moore and Knuth-Morris-Pratt code that Chris Kuklewicz posted a few months ago: http://article.gmane.org/gmane.comp.lang.haskell.libraries/7363 The consensus at the time was that the code was not ready for rolling into the bytestring package, but now it's easy to install and start working with. API docs: http://darcs.serpentine.com/stringsearch/dist/doc/html/stringsearch/ Patches against the darcs repo welcome: darcs get http://darcs.serpentine.com/stringsearch Credit to Justin Bailey, Daniel Fischer, and Chris Kuklewicz for their hard work. (Currently only tested against GHC 6.6.1, FYI.) <b

5 7

[OT] GHC uninstall on Linux
by Dusan Kolar 09 Nov '07

09 Nov '07

Hello all, I use tar.bz2 binary distribution of GHC compiler as my distro does not use any supported packaging system. Everything is fine, but... I want to install the new version of the GHC compiler. Is there any (easy) way, how to get information about what was copied and where during installation? (./configure; make install) There seems to be no uninstall target in the Makefile. :-( And I want to uninstall the previous version of the compiler. Is it safe to delete files/folders just from /usr/local/lib/ghc-6.6.1 and /usr/local/bin/gh* ? Thanks for any hint Dusan P.S. Tried google with no usable results. D.

4 4

Memory-mapped arrays?
by Joel Reymont 09 Nov '07

09 Nov '07

Is there such a thing as memory-mapped arrays in GHC? I'm looking for something that would let me memory-map a file of floats and access it as an array. Thanks, Joel -- http://wagerlabs.com

6 6

ANN: FileManip 0.1, an expressive file manipulation library
by Bryan O'Sullivan 09 Nov '07

09 Nov '07

The FileManip package provides expressive functions and combinators for searching, matching, and manipulating files. It provides three modules. System.FilePath.Find lets you search a filesystem hierarchy efficiently: find always (extension ==? ".rb") >>= mapM_ remove System.FilePath.GlobPattern lets you perform glob-style pattern matching, without going through a regexp engine: "foo.c" ~~ "*.c" ==> True System.FilePath.Manip lets you rename files procedurally, edit files in place, or save old copies as backups: modifyWithBackup (<.> "bak") (unlines . map (takeWhile (/= ',')) . lines) "myPoorFile.csv"

5 11

Fibbonachi numbers algorithm work TOO slow.
by gitulyar 09 Nov '07

09 Nov '07

Please help me. I'm new in Haskell programming, but wrote some things in Scheme. I make so function: fib 1 = 1 fib 2 = 2 fib n = fib (n-1) + fib (n-2) And when I call "fib 30" it works about 5 seconds. As for me it's really TOO SLOW. Tell me please if I have something missed, maybe some compiler (interpretaitor) options (I use ghc 6.6.1). P.S. As I understand function "fib n" should be calculated one time. For example if I call "fib 30" compiler builds tree in which call function "fib 28" 2 times and so on. But as for lazy calculation principle it should be calculated just ones and then it's value is used for all other calls of this function with the same argument. But it seems that this principle doesn't work in this algorithm. -- View this message in context: http://www.nabble.com/Fibbonachi-numbers-algorithm-work-TOO-slow.-tf4752338… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

14 20

Re: [Haskell-cafe] Flymake Haskell
by Daisuke IKEGAMI 09 Nov '07

09 Nov '07

Hi, Thanks Thomas for your advice. The speed is improved very much. The latest code about Flymake Haskell will be developed and discussed on EmacsWiki:FlymakeHaskell. http://www.emacswiki.org/cgi-bin/emacs/FlymakeHaskell However, we can continue the discussion via email, too. Thomas wrote: > It seems you need multi-line pattern matching. Note that shim already > can do this, but it doesn't do it automatically. Unfortunately, shim > chokes on files that need pre-processing. There's probably a way to > integrate these two. You are right; the multi-line pattern matching is needed. However, I failed some tries, and finally I invoked ghci from Emacs as I did usually. Anyone helps? I don't know shim's work. Gwern wrote: > Is there further setup needed? I think your setting maybe wrong. > (defun flymake-Haskell-init () > (interactive) ; this line should be removed > brabra I didn't test Gwern's setting on XEmacs, however I wish it helps. Best wishes, Ike

1 0

Composing trading strategies (was: Should I step into a minefield? / Writing a trading studio in Haskell)
by Greg Fitzgerald 09 Nov '07

09 Nov '07

> My idea is to write something like TradeStation [1] or NinjaTrader, only for the Mac. > It would be quite nifty to use SPJ's financial combinator approach I was experimenting with a Haskell EDSL for financial trading not too long ago. My favorite strategy so far is the parallel parser combinator approach. It allows users to run many parallel trading strategies in a nice single-threaded way that's easy to reason about and straightforward to compose new trading strategies. The idea is that the user composes an 'openPosition' and 'closePosition' trading strategies from a combinator library and gives them to the trading platform. The user's trading strategies are nothing more than a numeric parser combinators. With each incoming quote, the trading platform kicks off the user's 'openPosition' trading strategy. When a strategy succeeds (a successful parse), the trading platform opens a position for the user. Once a position is opened, the platform starts running the user's 'closePosition' strategy. The closePosition parser is run the same way and when it succeeds, the user's position is closed. To implement, I leveraged Twan van Laarhoven's ParseP library ( http://twan.home.fmf.nl/parsep/) which at least for my simple experiments, was incredibly complete and stable given its 0.1 version number. Why ParseP and not Parsec or ReadP? Parsec doesn't have an unbiased choice operator and ReadP only works on strings. Implementing context-free trading strategies like limit orders is very easy (limitOrder price = satisfy (>= price)), but context-sensitive strategies, such as "match the longest string of increasing numbers" is quite a bit more painful. Below are some of my experiments (also on hpaste: http://hpaste.org/3756) If anybody could offer more elegant solutions, I'd love to hear because this still feels overly complicated to me. 'testUpThenDown' matches an input stream that increases then decreases. 'testNumericFib' will try to match as much of the Fibonacci sequence as possible. import Text.ParserCombinators.ParseP import qualified Text.ParserCombinators.ParseP.Greedy as Greedy import Control.Applicative import Data.Monoid testUpThenDown = print $ runParser coaster [1, 2, 1] testNumericFib = print $ runParser (fib (Sum 1) (Sum 1)) (map Sum [2, 3, 5, 7, 1]) testAlphaFib = print $ runParser (fib "a" "b") ["ab", "bab", "cad"] fib :: (Eq a, Monoid a) => a -> a -> Parser a p [a] fib n1 n2 = Greedy.option [] (do n3 <- satisfy (== (n1 `mappend` n2)) ns <- fib n2 n3 return (n3:ns)) coaster :: Ord a => Parser a p ([a], [a]) coaster = do seed <- get up <- increasing seed down <- decreasing (last up) return (seed:up, down) increasing seed = trend (<) seed decreasing seed = trend (>) seed trend :: (Monad (Parser a p)) => (a -> a -> Bool) -> a -> Parser a p [a] trend f = many1WithContext (satisfy . f) manyWithContext p = Greedy.option [] . many1WithContext p many1WithContext p seed = do n <- p seed ns <- manyWithContext p n return (n:ns) Thanks, Greg On Nov 7, 2007 5:02 PM, Joel Reymont <joelr1(a)gmail.com> wrote: > I need to pick among the usual list of suspects for a commercial > product that I'm writing. The suspects are OCaml, Haskell and Lisp and > the product is a trading studio. My idea is to write something like > TradeStation [1] or NinjaTrader, only for the Mac. > > It would be quite nifty to use SPJ's financial combinator approach > and, for example, embed Yi (Haskell editor). > > One of the key features of the product would be the ability to model > your trading logic using a trading DSL. I'm thinking that this DSL > could well be Haskell but I'm concerned about stepping into a minefield. > > I will need to embed GHC into the app, for example, and I understand > that the GHC API does not offer unloading of code at the moment. I > would prefer not to bundle GHC separately so I don't think the hs- > plugins approach would work for me. Maybe I'm mistaken. > > Most of all, I'm concerned that my users will need to face the error > reports from GHC and could get tripped by laziness, i.e. write > something that would make the app run out of memory. Off the top of my > head I can't figure out a way to limit what my users can do without > analyzing the Haskell AST within the GHC API and complaining if > necessary. > > Can someone with experience in offering a Haskell DSL to their users > please comment? > > Notice that I'm not even mentioning being concerned with the > unpredictable effects of laziness. There's probably a reason why Jane > St Capital is using OCaml instead of Haskell. I'm not going to play in > that league but my knee-jerk reaction is to use OCaml or Lisp and > avoid laziness altogether. I just can't see how laziness can help in > processing real-time price data. > > Thanks, Joel > > [1] http://www.tradestation.com/default_2.shtm > [2] http://www.ninjatrader.com/webnew/index.htm > > -- > http://wagerlabs.com > > > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

2 2

Re: [Haskell-cafe] Haskell performance question
by Dan Piponi 09 Nov '07

09 Nov '07

On Nov 8, 2007 11:36 AM, Paul Brown <paulrbrown(a)gmail.com> wrote: > All that said, I'm not sure where I got the GHC that I used to build > the 6.6.1 via MacPorts; I think it shipped with MacOS once upon a time. > sudo port install ghc . . . configure: error: GHC is required unless bootstrapping from .hc files. But I do have a plan... -- Dan

1 0

Standalone PNG module?
by Peter Verswyvelen 09 Nov '07

09 Nov '07

I would like to load 32-bit images (RGB+alpha) for use with GLUT/OpenGL. I know GTK2HS has support for loading images, but does a standalone Haskell (wrapper) module exists for loading images? PNG or TGA would be enough for me. Yes, I could of course just use the GTK2HS OpenGL widget support, but I already have written most of my code on top of GLUT. Or I could use the SDL wrapper, but that one does not work with GHCI under Windows. Thanks, Peter

5 4

FP design
by Donn Cave 08 Nov '07

08 Nov '07

I have been working on a little Java project lately at work, by way of an introduction to the language, and naturally I often have cause to regret that it isn't Haskell instead. But in the specific matter I'm wrestling with, the Java library's OOP model is, to its credit, allowing me to do some things. I'm using their standard LDAP client library, but swapping in my own function to read X509 certificates for the SSL. Actually, swapping in my own SSL socket "implementation", which in my case just calls the standard library SSL socket implementation to do most of the work. Now it's not like I can't imagine it working better - it may be a little fragile, for one thing - but I have wondered what facilities a Haskell design could have drawn on to de-couple implementation components like that. Let's say you download an IMAP mail client library, and look to see if it can operate on a UNIX pipe; on an SSL socket; authenticate with GSSAPI Kerberos 5 -- when none of those things are supported out of the box. (As I have needed, and done, all three of those with the standard Python IMAP library module.) You may also want its I/O operations to integrate with some dispatching core, for a GUI. But of course you also want the basic interface to be simple in this area - the IMAP protocol itself is complicated enough! So I'm the one user in a thousand that will want to provide my own I/O functions, for example. In the old world, I guess I would be looking for some extended API where my I/O functions are parameters to the "open" or "init" function, and the IMAP functions take over from there. In a more pure functional oriented model, could it be an extended API that exposes the IMAP functionality as operations on data, and leaves it to me to deal with the I/O? Donn Cave, donn(a)drizzle.com

18 28

FP design
by Levi Stephen 08 Nov '07

08 Nov '07

Hi, I'm was wondering how most people work during when designing a functional program. Do you create data structures/types first? Do you work from some type signatures? For example, take a blog. Is the first step likely to be something like: data BlogEntry = BlogEntry { title::String,content::String,comments::[Comment] } type Blog = [BlogEntry] or more likely thinking about what functions will be required: addEntry :: BlogEntry -> Blog -> Blog displayBlog :: Blog -> HTML displayEntry :: BlogEntry -> HTML I'm trying to get my brain out of thinking OO thinking more functionally. Thanks, Levi lstephen.wordpress.com

10 15

Type error with simple list function
by Fernando Rodriguez 08 Nov '07

08 Nov '07

Hi, This simple function definition that should rotate a list (put the first item in the last place) fails with a rather cryptic error with ghc: f :: [a] ->[a] ->[a] f (w : ws) = ws : w Couldn't match expected type `[a] -> [a]' against inferred type `[[a]]' In the expression: ws : w In the definition of `f': f (w : ws) = ws : w What's Haskell trying to tell me? I'm a newby so please forgive my ignorance. Thanks! Fernando

5 4

type/class question: toString
by Graham Fawcett 08 Nov '07

08 Nov '07

Hi folks, Is there a way to declare a 'toString' function, such that toString x | <x is a String> = x toString x | <x's type is an instance of Show> = show x Perhaps, in the type system, there's a way to declare a ToString class, and somehow "inherit" all instances of Show as ToString instances? Thanks, Graham

7 12

Should I step into a minefield? / Writing a trading studio in Haskell
by Joel Reymont 08 Nov '07

08 Nov '07

I need to pick among the usual list of suspects for a commercial product that I'm writing. The suspects are OCaml, Haskell and Lisp and the product is a trading studio. My idea is to write something like TradeStation [1] or NinjaTrader, only for the Mac. It would be quite nifty to use SPJ's financial combinator approach and, for example, embed Yi (Haskell editor). One of the key features of the product would be the ability to model your trading logic using a trading DSL. I'm thinking that this DSL could well be Haskell but I'm concerned about stepping into a minefield. I will need to embed GHC into the app, for example, and I understand that the GHC API does not offer unloading of code at the moment. I would prefer not to bundle GHC separately so I don't think the hs- plugins approach would work for me. Maybe I'm mistaken. Most of all, I'm concerned that my users will need to face the error reports from GHC and could get tripped by laziness, i.e. write something that would make the app run out of memory. Off the top of my head I can't figure out a way to limit what my users can do without analyzing the Haskell AST within the GHC API and complaining if necessary. Can someone with experience in offering a Haskell DSL to their users please comment? Notice that I'm not even mentioning being concerned with the unpredictable effects of laziness. There's probably a reason why Jane St Capital is using OCaml instead of Haskell. I'm not going to play in that league but my knee-jerk reaction is to use OCaml or Lisp and avoid laziness altogether. I just can't see how laziness can help in processing real-time price data. Thanks, Joel [1] http://www.tradestation.com/default_2.shtm [2] http://www.ninjatrader.com/webnew/index.htm -- http://wagerlabs.com

3 2

Flymake Haskell
by Daisuke IKEGAMI 08 Nov '07

08 Nov '07

Hello emacsen users, Here is a setting to check your Haskell code /on-the-fly/ with 'flymake-mode'. (require 'flymake) ;; flymake for Haskell (defun flymake-Haskell-init () (flymake-simple-make-init-impl 'flymake-create-temp-with-folder-structure nil nil buffer-file-name 'flymake-get-Haskell-cmdline)) (defun flymake-get-Haskell-cmdline (source base-dir) (list "ghc" (list "--make" (concat "-i" base-dir) source))) (push '(".+\\.hs$" flymake-Haskell-init) flymake-allowed-file-name-masks) (push '(".+\\.lhs$" flymake-Haskell-init) flymake-allowed-file-name-masks) (push '("^\$.*\$:\$[0-9]+\$:\$[0-9]+\$: \$.*\$$" 1 2 3 4) flymake-err-line-patterns) The 'flymake-mode' has been developed at http://flymake.sourceforge.net/ Emacs22 has this mode already. dons recommends to me at IRC to write this code into the wiki, however, I don't know where we should put the snippet. You can watch a short demo at the following blog: http://madscientist.jp/~ikegami/diary/20071108.html#p01 I have some complaints though it works: - it's slow (hmm) - error and warning seems not distinguished (we may have to modify 'flymake-err-line-patterns') Any comment is appreciated. Best regards, Ike P.S. I like vi-clone, such as vim, too. ;-)

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words function
by Ryan Bloor 08 Nov '07

08 Nov '07

hi I am trying to create a function that uses the words function... I am doing the same thing to each element in a list so I am using mapping techniques. Code... --Define the main first function rStrings2Results :: ([String] -> String) -> [[String]] -> [String] rStrings2Results f(head:tail) = (f head : rStrings2Results f tail) I just want take a list and on the first member ("hello my name is ryan") to say [("hello", "my", "name", "is", "ryan"),..............] using the words function. _________________________________________________________________ 100’s of Music vouchers to be won with MSN Music https://www.musicmashup.co.uk

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generating Maybe
by Tim Newsham 08 Nov '07

08 Nov '07

Data.Maybe has functions for processing Maybe's but nothing useful for creating maybe. I think the following would be a very useful addition, a guarded function: guarded :: (a -> Bool) -> (a -> b) -> a -> Maybe b guarded p f x | p x = Just (f x) | otherwise = Nothing such a function in the std libs would make functions like "unfoldr" more attractive -- uses of foldr nearly always encapsulate this notion. Tim Newsham http://www.thenewsh.com/~newsham/

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type annotations elsewhere
by Jason Dusek 08 Nov '07

08 Nov '07

Haskell offers a few different annotations, for example strictness, to optimize programs. It would sure be nice if there were a way to pull the annotations out and put them somewhere else, enabling them, or not, at compile time. There could be several different annotation sets, even -- you could try different ones for different things. However, there are some real practical difficulties here -- how do you write an 'expresssion selector' to pair with the annotation, in order to find its target? It may involve walking the AST -- a fancy and potentially brittle way to go about optimization (a brittle thing to start with!). -- _jsn

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CPP, Leopard and Haskell Objective-C bindings
by Joel Reymont 08 Nov '07

08 Nov '07

It seems that the current approach taken by HOC is to strip preprocessing directives. This may not have been a problem before Leopard but Cocoa header files are now full of macros in most unusual places, e.g. @interface NSObject (NSDeprecatedMethods) + (void)poseAsClass:(Class)aClass DEPRECATED_IN_MAC_OS_X_VERSION_10_5_AND_LATER; @end I'm trying to figure out the easiest way to run CPP over the header files before processing them in HOC. Is there a CPP written in Haskell and packaged as a library? Does someone have a quick Haskell solution for running CPP over a header file (with include paths?), redirecting into a temp file and reading that back into Haskell? Should I preprocess all Cocoa header files using make and then read these in? Thanks, Joel -- http://wagerlabs.com

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ANN: infinity 0.3
by Austin Seipp 08 Nov '07

08 Nov '07

After quite a bit of work and other such stuff, I pretty much totally rewrote my previous IRC bot and instead updated it from v0.1 to v0.3. You may wonder where 0.2 went; so do I. This is basically an entire rewrite (I threw the first one out.) Now all code can be dynamically hot-swapped via the techniques described in Don's Yi paper. This can happen across multiple servers and as servers are taken down and the like. All code swaps near flawlessly, and even after the initial mechanism was working, the bot ran for quite a while with a good amount of success. I didn't plan to release it in this state, however, with it's dependencies on hs-plugins (which is ghc 6.6 only as of right now,) and my recent installation of GHC 6.8.1, I can't work on it because apparently although both versions appear to be installed, it seems as if ghc-6.6.1 is picking up configuration and package info from 6.8.1 (and therefore ghc-6.6.1 tells me it can't find packages like irc and plugins, although 'ghc-pkg-6.6.1 list' obviously shows they exist.) I also can't work on this on my OpenBSD box which only has 6.6.1, due to some arcane error with hs-plugins (can't find the __errno symbol, I believe.) So because of that, development is temporarily stalled and if you want to test it, you'll need 6.6.1 installed and working properly. I don't think this would have been *too* far from the actual, official release though. At this point I only planned minor tweaks and its release was pretty inevitable anyway. You can get a tarball from hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/infinity-0.3 The darcs repository is here: http://code.haskell.org/infinity/src/ Personally, it was real fun to work on this thing. I wrote almost all the base code for plugins, and for the Net and Monitor to use before the bot ever made its first connection, and a lot of it seemed to just piece together really easily. The code is undeniably cleaner and more extensible than the previous 0.1 code, but I still feel by most standards it's somewhat cluttered, so if anybody here would like to give any comments just from reading the source, that'd be pretty awesome. Until I can continue to work on it (whether hs-plugins is available for 6.8.1 or I get another box with 6.6.1 installed,) this will be considered the stable version by my definition, and all patches are of course welcome. darcs send austin at youareinferior dot net. Criticism is advised. -- - Austin

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