- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Type level sets with GADTs, fundeps etc
by J.Burton＠brighton.ac.uk 17 Jul '08

17 Jul '08

At Tue, 15 Jul 2008 10:27:59 -0400, Jeff Polakow wrote: > > [1 <text/plain; US-ASCII (7bit)>] > > [2 <text/html; US-ASCII (7bit)>] > Hello, > > > data LSet t where > > Nil :: LSet Nil > > Ins :: (Member a t b > > , If b t (a ::: t) r) > > => L a -> LSet t -> LSet r > > > Try replacing both original occurrences of r, i.e. (untested) > > Ins :: (Member a t b, If b t (a ::: t) (LSet r)) => L a -> LSet t -> LSet r > Thanks. This sort of works, but shifts the problem to another context. Now it seems that I can't hide the extra type information in the existential types, which is what I want to do. In the function `insertChar' below I want the type LSetBox to be opaque (i.e. it will be called by users who don't need to know about the fancy types): data LSet t where Nil :: LSet Nil Ins :: (Member a t b , If b t (a ::: t) (LSet r)) => L a -> LSet t -> LSet r -- I have to supply a type for `insert' now and it must include the constraints insert :: (Member a t b , If b t (a ::: t) (LSet r)) => L a -> LSet t -> LSet r insert = Ins --insertChar (and the boxing) doesn't work insertChar :: Char -> LSetBox -> LSetBox insertChar c (LSetBox s) = case fromChar c of LBox t -> LSetBox (insert t s) The error: Could not deduce (If b t1 (a ::: t1) (LSet t), Member a t1 b) from the context () arising from a use of `insert' at /home/jim/sdf-bzr/dsel/TF/Set-July08.hs:54:25-34 Possible fix: add (If b t1 (a ::: t1) (LSet t), Member a t1 b) to the context of the constructor `LBox' or add an instance declaration for (If b t1 (a ::: t1) (LSet t)) In the first argument of `LSetBox', namely `(insert t s)' In the expression: LSetBox (insert t s) In a case alternative: LBox t -> LSetBox (insert t s) Failed, modules loaded: none. Jim > -Jeff > > --- > > This e-mail may contain confidential and/or privileged information. If you > are not the intended recipient (or have received this e-mail in error) > please notify the sender immediately and destroy this e-mail. Any > unauthorized copying, disclosure or distribution of the material in this > e-mail is strictly forbidden. > >

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Searchig for modules
by fero 17 Jul '08

17 Jul '08

Hi I am learnig Haskell form SOE book, but I can't find out how to import SOE module. I use eclipsefp IDE and I get no error when I have SOE.hs in my project and in SimpleGraphics.hs I make "import SOE". But when trying to run it I get error: "Could not find module `SOE': Use -v to see a list of the files searched for." Where ghc/ghci search for modules? Is it possible put some modules somewhere that every Haskell project can find it (something as global classpath in java). And is it posible in eclipsefp to link some modules to project that this project will use them. As I wrote even when I have 2 modules in src in project I when trying to run one of then the others are not found. I can't find any site about searching for modules (in java there is a lot options, global/local classpath, directory in java runtime used only with this rt or directory in web server used in all apps run on this server...). Can you put a link? Thanks Fero -- View this message in context: http://www.nabble.com/Searchig-for-modules-tp18505770p18505770.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Fixed-Point Combinators
by Adrian Neumann 17 Jul '08

17 Jul '08

Hello, while studying for a exam I came across this little pearl: Y = (L L L L L L L L L L L L L L L L L L L L L L L L L L L L) where L = λabcdefghijklmnopqstuvwxyzr. (r (t h i s i s a f i x e d p o i n t c o m b i n a t o r)) posted by Cale Gibbard to this list. Now I'm wondering how exactly does one finde such awesome λ expressions? Is there some mathemagical background that lets one conjure such beasts? Adrian

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Linker problems linking FFI import call in Windows
by PJ Durai 17 Jul '08

17 Jul '08

Greetings I am trying to import some functions from a windows DLL. I am getting strange errors. (This used to work with previous versions of GHC. 6.6 I think.). This is my import statement: foreign import stdcall "ace.h AdsConnect" adsConnect' :: CString -> Ptr CInt -> IO CInt This is the build output. C:\moonar>\bin\make.exe ghc --make -ffi main.hs -ffi adsapi.hs -optc -DWIN32 native.cpp -lace32 -L/hom e/pjdurai/dev/moonar -L/tmp/Advantage -o moonar.exe [1 of 2] Compiling AdsApi ( adsapi.hs, adsapi.o ) [2 of 2] Compiling Main ( main.hs, main.o ) Linking moonar.exe ... adsapi.o(.text+0x124):fake: undefined reference to `AdsConnect@8' adsapi.o(.text+0x260):fake: undefined reference to `AdsDisconnect@4' collect2: ld returned 1 exit status make: *** [main.exe] Error 1 It actually links without errors if I use 'ccall' instead of 'stdcall'. But the executable segfaults when the function is called. I had the same code working with 'stdcall' declaration a while ago. (With GCH 6.6 I think) I am using ghc-6.8.3 on Windows XP SP2. appreciate your time. thanks pj

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ANN: Topkata
by c-bauer-olsbruecken＠t-online.de 17 Jul '08

17 Jul '08

Hi All, Topkata is a simple OpenGL Game written in Haskell. It's not very advanced. Goal so far is to guide a ball trough an labyrinth to the opposite corner. The web page shows an screenshot. It's only tested under Linux. http://home.arcor.de/chr_bauer/topkata.html Feedback&Patches are welcome. BTW, I had a lot of support in #haskell! Christoph Bauer

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curl installation
by Thomas Fuhrmann 17 Jul '08

17 Jul '08

Hello, I'm trying to install the curl package 1.3.2.1 for haskell, I'm using Ubuntu and I've already installed curl-7.18.2. I can't get rid of the configuration error. According to the log entry the curl.h doesn't exist. Shell output: runghc Setup.hs configure Configuring curl-1.3.2.1... checking for gcc... gcc checking for C compiler default output file name... a.out checking whether the C compiler works... yes checking whether we are cross compiling... no checking for suffix of executables... checking for suffix of object files... o checking whether we are using the GNU C compiler... yes checking whether gcc accepts -g... yes checking for gcc option to accept ANSI C... none needed checking how to run the C preprocessor... gcc -E configure: error: curl libraries not found, so curl package cannot be built See `config.log' for more details. Part of Log entry: configure:2439: gcc -E conftest.c conftest.c:9:23: error: curl/curl.h: No such file or directory configure:2445: $? = 1 configure: failed program was: | /* confdefs.h. */ | | #define PACKAGE_NAME "Haskell curl package" | #define PACKAGE_TARNAME "curl" | #define PACKAGE_VERSION "1.1" | #define PACKAGE_STRING "Haskell curl package 1.1" | #define PACKAGE_BUGREPORT "dons(a)galois.com" | /* end confdefs.h. */ | #include <curl/curl.h> configure:2469: error: curl libraries not found, s o curl package cannot be built See `config.log' for more details. Any ideas?

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Haskell Weekly News: Issue 77 - July 16, 2008
by Brent Yorgey 17 Jul '08

17 Jul '08

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20080716 Issue 77 - July 16, 2008 --------------------------------------------------------------------------- Welcome to issue 77 of HWN, a newsletter covering developments in the [1]Haskell community. Announcements Takusen 0.8.3. Alistair Bayley [2]announced the release of [3]Takusen 0.8.3, with ODBC support, more Cabal improvements, bug fixes, and some basic result-set validation. Launching Haskell Group in Vancouver, Canada. Jon Strait [4]announced that a [5]Haskell Programmers Group has been created in Vancouver; the first meeting is scheduled for next Monday, July 21st. Feel free to join the Google Groups list to be notified of future events, or just show up to a meeting to bounce ideas and questions off of other Haskell programmers. Sphinx full-text searching client on Hackage. Chris Eidhof [6]announced work on a [7]client for the [8]sphinx full-text search engine. Help hacking on it, testing it or improving documentation is welcome. haskell-src-exts 0.3.5. Niklas Broberg [9]announced that the [10]haskell-src-exts package is now updated to understand the current version of Template Haskell syntax. Bug reports welcome. Prime time for Haskell. Janis Voigtlaender [11]announced that Haskell STM is featured in an [12]article in this month's Communications of the ACM. vector 0.1 (efficient arrays with lots of fusion). Roman Leshchinskiy [13]announced an initial release of the [14]vector library, which will eventually provide fast, Int-indexed arrays with a powerful fusion framework. Galois Tech Talks: Stream Fusion for Haskell Arrays. Don Stewart [15]announced that he was giving this week's Galois Tech Talk, on stream fusion for Haskell arrays. The talk was yesterday, July 15, but hopefully some sort of recording or slides will be made available. protocol-buffers. Chris Kuklewicz [16]announced a very early version of the [17]protocol-buffers package, a Haskell interface to Google's newly released [18]data interchange format. GHC IRC meeting. Simon Marlow [19]announced the (first weekly?) IRC meeting to discuss GHC, a scheduled time when the developers turn up on #ghc, discuss current topics around GHC, and users can chime in with questions, points for discussion, complaints and so on. The first meeting took place on July 16 at 1600 BST (UTC +1)/0800 PDT (UTC-7)/1100 EDT (UTC-4), in the #ghc channel on chat.freenode.net; hopefully this will become a weekly event. Google Summer of Code Progress updates from participants in the 2008 [20]Google Summer of Code. GHC API. Thomas Schilling (nominolo) is working on [21]improvements to the GHC API. He recently [22]asked for comments on a proposed refactoring to the GHC API, creating a new Ghc monad to capture error handling and single-threaded use of Sessions. GHC plugins. Max Bolingbroke is working on dynamically loaded plugins for GHC. [23]This week, he wrote a ton of Haddock documentation for GHC internals. He also added the ability for compiler plugins to generate their own source annotations, to allow plugins to use intermediate results from previous plugins. He's currently working on an exciting, secret feature: tune in next week to find out what it is! Generic tries. Jamie Brandon is working on a library for efficient maps using generalized tries. Jamie is [24]currently working on, tweaking the [25]api, writing tests and writing reference implementations on sorted and unsorted association lists. Hoogle 4. Neil Mitchell (ndm) is working on [26]Hoogle 4. [27]This week, he worked on type search and database generation. Next week he plans to finish up type search and release and command-line version. Language.C. Benedikt Huber (visq) is [28]working on Language.C, a standalone parser/pretty printer library for C99. DPH physics engine. Roman Cheplyaka (Feuerbach) is working on a [29]physics engine using [30]Data Parallel Haskell. Cabal dependency framework. Andrea Vezzosi (Saizan) is working on a [31]make-like dependency analysis framework for Cabal. Libraries Proposals and extensions to the [32]standard libraries. adding split to Data.List. Gwern Branwen [33]proposed adding some split-like functions to Data.List. Will they actually get added this time? Will people be able to agree on one of the seventeen possible sets of semantics? Tune in next time... Discussion GHC API: monad and error handling. Thomas Schilling [34]asked for comments on a proposed refactoring to the GHC API, creating a new Ghc monad to capture error handling and single-threaded use of Sessions. Jobs Research positions on Modeling and Analyzing Software Adaptation, University of Koblenz. Ralf Lammel [35]announced two research positions for a postdoc and a PhD student, available initially for 2 years. The successful applicants will work on the research theme of "ADAPT: Modeling and Analyzing Software Adaptation". The objective of [36]ADAPT is to relate, advance, combine, and challenge adaptation methods and associated methods of modeling and analyzing that are used by the communities of software engineering, programming languages, logic-based modeling, multi-agent systems, formal methods, SOA, web systems, and mobile, autonomous systems. Blog noise [37]Haskell news from the [38]blogosphere. * Real-World Haskell: [39]Real World Haskellers at OSCON next week. John Goerzen and Bryan O'Sullivan will be in Portland, Oregon next week for OSCON (along with Don Stewart, who lives in Portland). * Russell O'Connor: [40]ICFP 2008 Post-Mortem. * Brent Yorgey: [41]Call for an ICFP Mars Server. Will someone make a Mars Server where we can all submit our rovers and new maps, watch them compete, improve them, and generally have fun getting some feedback without bugging the organizing committee? Pretty please? * Jamie Brandon: [42]Week 5 progress. An update on Jamie's Google Summer of Code project. * Brent Yorgey: [43]ICFP programming contest reflections. * >>> Chris Bogart: [44]Help with study of functional programmers. Chris is doing a study as part of a research internship at Microsoft, and is seeking people currently working on a real project in a functional language whom he can observe as part of his study. * >>> eigenclass: [45]Quicksort erratum. A time and space analysis of a classic quicksort implementation in Haskell. * Ralf Lammel: [46]Research positions on Modeling and Analyzing Software Adaptation, University of Koblenz. * Max Bolingbroke: [47]Compiler Plugins For GHC: Week Five. An update on Max's Google Summer of Code project. * Luke Plant: [48]Haskell Blog Rewrite - Session 7. Luke documents setting up CentOS in VirtualBox VM in order to compile Haskell code for a server to which he does not have ssh access. Not for the faint of heart. * Chris Eidhof: [49]Stemming with Haskell. A [50]stemming library and [51]Sphinx client for Haskell. * >>> Duane Johnson: [52]Haskell is Popular on IRC. The large amount of community participation and academic brainshare gives Duane a lot of confidence in Haskell. * >>> Mike Harris: [53]ICFP Programming Contest '08. Mike participated in the ICFP programming contest, and might try porting his solution to Haskell. * Luke Palmer: [54]The Curry-Howard isomorphism and the duality of x and ->. * Magnus Therning: [55]Playing with prefixes. Magnus explores different ways to encode units of storage (bytes, kilobytes, kibibytes...) in Haskell. * Dan Piponi (sigfpe): [56]MSFP 2008. * Shin-Cheng Mu: [57]Tail-Recursive, Linear-Time Fibonacci. * Neil Mitchell: [58]GSoC Hoogle: Week 7. * FP Lunch: [59]CCC-ness of the category of containers. * JP Moresmau: [60]instance Data Map where -- half done!. JP writes an Data.Generics.Data instance for Data.Map. * >>> Rick Carback: [61]AutoKey in Haskell. Rick is learning Haskell and has implemented a simple AutoKey cipher. * >>> Jeremy Frens: [62]PE Problem #2 in All Languages (Part II). * Luke Palmer: [63]Required Optimization. Annotations for specifying expected compiler optimizations? * London Haskell Users Group: [64]AngloHaskell 2008. * >>> Justus: [65]random programming. Solving a simple programming challenge in Haskell. Quotes of the Week * Japsu: segfault cat is watching you unsafeCoerce * z0MB13: who can say hello to me as a md5 or decipher what kind of technique can be used to remove the password * denq: [upon experiencing a moment of enlightenment] oh! something bing in my brain :) * Pseudonym: Pseudonym needs codependent types - useful when you're doing dysfunctional programming. About the Haskell Weekly News New editions are posted to [66]the Haskell mailing list as well as to [67]the Haskell Sequence and [68]Planet Haskell. [69]RSS is also available, and headlines appear on [70]haskell.org. Headlines are available as [71]PDF. To help create new editions of this newsletter, please see the information on [72]how to contribute. Send stories to byorgey at seas dot upenn dot edu. The darcs repository is available at darcs get [73]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.libraries/9570 3. http://hackage.haskell.org/packages/archive/Takusen/0.8.3/Takusen-0.8.3.tar… 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/42287 5. http://groups.google.com/group/hugvan 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/42263 7. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/sphinx 8. http://www.sphinxsearch.com/ 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/42233 10. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/haskell-src-exts 11. http://article.gmane.org/gmane.comp.lang.haskell.cafe/42227 12. http://research.microsoft.com/~larus/Papers/p80-larus.pdf 13. http://article.gmane.org/gmane.comp.lang.haskell.cafe/42207 14. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/vector 15. http://article.gmane.org/gmane.comp.lang.haskell.cafe/42196 16. http://article.gmane.org/gmane.comp.lang.haskell.libraries/9575 17. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/protocol-buffers 18. http://code.google.com/apis/protocolbuffers/docs/overview.html 19. http://article.gmane.org/gmane.comp.lang.haskell.glasgow.user/14741 20. http://hackage.haskell.org/trac/summer-of-code/wiki/SoC2008 21. http://hackage.haskell.org/trac/ghc/wiki/GhcApiStatus 22. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/9589 23. http://blog.omega-prime.co.uk/?p=69 24. http://jamiiecb.blogspot.com/2008/07/week-5-progress.html 25. http://code.haskell.org/gmap/api 26. http://code.haskell.org/hoogle/ 27. http://neilmitchell.blogspot.com/2008/07/gsoc-hoogle-week-7.html 28. http://www.sivity.net/projects/language.c/wiki/ 29. http://haskell.org/haskellwiki/Hpysics 30. http://www.haskell.org/haskellwiki/GHC/Data_Parallel_Haskell 31. http://code.haskell.org/~Saizan/cabal 32. http://haskell.org/haskellwiki/Library_submissions 33. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/9558 34. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/9589 35. http://blogs.msdn.com/ralflammel/archive/2008/07/15/research-positions-on-m… 36. http://adapt.uni-koblenz.de/ 37. http://planet.haskell.org/ 38. http://haskell.org/haskellwiki/Blog_articles 39. http://www.realworldhaskell.org/blog/2008/07/16/real-world-haskellers-at-os… 40. http://r6.ca/blog/20080716T131235Z.html 41. http://byorgey.wordpress.com/2008/07/16/call-for-an-icfp-mars-server/ 42. http://jamiiecb.blogspot.com/2008/07/week-5-progress.html 43. http://byorgey.wordpress.com/2008/07/15/icfp-programming-contest-reflection… 44. http://sambangu.blogspot.com/2008/07/help-with-study-of-functional 45. http://eigenclass.org/hiki/quicksort-erratum 46. http://blogs.msdn.com/ralflammel/archive/2008/07/15/research-positions-on-m… 47. http://blog.omega-prime.co.uk/2008/07/14/compiler-plugins-for-ghc-week-five/ 48. http://lukeplant.me.uk/blog.php?id=1107301689 49. http://blog.tupil.com/stemming-with-haskell/ 50. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/stemmer 51. http://www.sphinxsearch.com/ 52. http://blog.inquirylabs.com/2008/07/14/haskell-is-popular-on-irc/ 53. http://thingsthatmikethinks.blogspot.com/2008/07/icfp-programming-contest-0… 54. http://luqui.org/blog/archives/2008/07/13/ch-isomorphism-duality/ 55. http://therning.org/magnus/archives/354 56. http://sigfpe.blogspot.com/2008/07/msfp-2008.html 57. http://www.iis.sinica.edu.tw/~scm/2008/tail-recursive-linear-time-fibonacci/ 58. http://neilmitchell.blogspot.com/2008/07/gsoc-hoogle-week-7.html 59. http://sneezy.cs.nott.ac.uk/fplunch/weblog/?p=108 60. http://jpmoresmau.blogspot.com/2008/07/instance-data-map-where-half-done.ht… 61. http://carback.us/rick/blog/?p=24 62. http://jdfrens.blogspot.com/2008/07/pe-problem-2-in-all-languages-part-ii.h… 63. http://luqui.org/blog/archives/2008/07/10/required-optimization/ 64. http://www.londonhug.net/2008/07/10/anglohaskell-2008/ 65. http://hoostus.blogspot.com/2008/07/random-programming.html 66. http://www.haskell.org/mailman/listinfo/haskell 67. http://sequence.complete.org/ 68. http://planet.haskell.org/ 69. http://sequence.complete.org/node/feed 70. http://haskell.org/ 71. http://code.haskell.org/~byorgey/code/hwn/archives/20080716.pdf 72. http://haskell.org/haskellwiki/HWN 73. http://code.haskell.org/~byorgey/code/hwn/

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A type signature inferred by GHCi that is rejected when written explicitly
by Pablo Nogueira 17 Jul '08

17 Jul '08

I find this interesting, GHCi accepts a function |dmap| which I show below and infers its type, but if I annotate the function with the inferred type, GHCi's type-checker rejects it. I'm trying to generalise the datatype-generic dmap: < dmap :: Bifunctor s => (a -> b) -> Fix s a -> Fix s b < dmap f = In . bimap (dmap f) f . out where > data Fix s a = In { out :: s (Fix s a) a } > > class Bifunctor s where > bimap :: (a -> c) -> (b -> d) -> s a b -> s c d The idea is that recursive types are represented by their (lamba-lifted) functors, eg: > data ListF b a = NilF | ConsF a b > instance Bifunctor ListF where > bimap f g NilF = NilF > bimap f g (ConsF a b) = ConsF (g a) (f b) I now define two classes: > class From a c x where > from :: a x -> c x > > class To a c y where > to :: c y -> a y And a generic |gmap| which given the map for |c| and a mapping for |x| delivers the map for |a|: > type GMap t x y = (x -> y) -> t x -> t y > > gmap :: (From a c x, To a c y) => GMap c x y -> GMap a x y > gmap gmc gmx = to . gmc gmx . from I want to write |dmap| as a special case of |gmap|, but I can't even get there. If I write > dmap f = to . bimap (dmap f) f . from GHCi infers it has type (up to renaming): (From a1 (s (a1 x)) x, Bifunctor s, To a2 (s (a2 y)) y) => (x -> y) -> a1 x -> a2 y But if I cut and paste the type into the code I get type errors: Could not deduce (From a1 (s1 (a11 x)) x) ... Could not deduce (From a11 (s1 (a11 x)) x, To a21 (s1 (a21 y)) y) ... Could not deduce (From a1 (s1 (a11 x)) x) ... Interesting,

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Euler 201 performance mystery
by nadine.and.henry＠pobox.com 16 Jul '08

16 Jul '08

Dear Group, I've spend the last few days figuring out the solution to Euler Problem 201 in haskell. I first tried a relatively elegant approach based on Data.Map but the performance was horrible. I never actually arrived at the answer. I then rewrote the same algorithm using STUArrays and it was lightning. I have posted both versions of the code at: http://www.maztravel.com/haskell/euler_problem_201.html and would appreciate any insights that you master haskellers can provide on why the speed difference is so huge. Thanks in advance. Henry Laxen

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Strange space leak
by Grzegorz Chrupala 16 Jul '08

16 Jul '08

Hi all, I just noticed that a tiny change to the program which I posted recently in the "More idiomatic use of strictness" thread causes a space leak. The code is: {-# LANGUAGE BangPatterns, PatternGuards #-} import Data.List (foldl') import Data.Char split delim s | [] <- rest = [token] | otherwise = token : split delim (tail rest) where (token,rest) = span (/=delim) s main = do putStrLn =<< fmap (show . stats ["the","a","and"] . split "<DOC>" . words) getContents stats ws docs = foldl' f ((map (const 0) ws),0) docs where f (dfs,n) d = let dfs' = zipWith (\w df -> (df + fromEnum (w `elem` d))) ws dfs in sum dfs' `seq` (dfs',n+1) If I change this line: putStrLn =<< fmap (show . stats ["the","a","and"] . split "<DOC>" . words) getContents to this: putStrLn =<< fmap (show . stats ["the","a","and"] . split "<DOC>" . words . map toLower) getContents suddenly the programs starts using tons of memory instead of running in small constant space. What's going on? Best, -- Grzegorz -- View this message in context: http://www.nabble.com/Strange-space-leak-tp18439685p18439685.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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win32: haddock: internal Haddock or GHC error?
by Dmitri O.Kondratiev 16 Jul '08

16 Jul '08

Using GHC 6.8.2 and haddock 2.0.0.0. in WinXP (SP3) I am trying to follow instructions from "How to write a Haskell program<http://www.haskell.org/haskellwiki/How_to_write_a_Haskell_program#Build_som…> ". Everything goes well, until I try to generate html documentation for the project. Then I get the following errors: C:\wks\haq>runhaskell Setup.lhs haddock --executables Preprocessing executables for haq-0.0... Running Haddock for haq-0.0... Preprocessing executables for haq-0.0... Warning: Cannot read C:/usr/ghc-6.8.2/doc/libraries/base\base.haddock: "Magic number mismatch: couldn't load interface file: C:/usr/ghc-6.8.2/doc/li braries/base\\base.haddock" Skipping this interface. Warning: main:Main: could not find link destinations for: GHC.IOBase.IO haddock: internal Haddock or GHC error: c:/builds/haddock-2/install\haddock-2.0. 0.0/html/haddock.css: openFile: does not exist (No such file or directory) Any ideas how to solve this? Thanks!

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Mutually recursive modules and google protocol-buffers
by Chris Kuklewicz 16 Jul '08

16 Jul '08

I have reached an impasse in designing a Haskell API for the google's protocol-buffers data language / format. ( http://code.google.com/apis/protocolbuffers/docs/overview.html ) The messages in protobuf are defined in a namespace that nests in the usual hierarchical OO style that Java encourages. To avoid namespace conflicts, I made a hierarchy of modules. But...this is a legal pair protobuf message definitions: > // Test that mutual recursion works. > message TestMutualRecursionA { > optional TestMutualRecursionB b = 1; > optional int32 content = 2; > } > > message TestMutualRecursionB { > optional TestMutualRecursionA a = 1; > optional int32 content = 2; > } And there is no way ghc can compile these in separate modules. But the overlap of record accessors names "content" makes defining these messages in a single module with a common namespace quite verbose. Any opinions on the least worst way to design this? -- Chris

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CAL (OpenQuark) and enterprise
by fero 16 Jul '08

16 Jul '08

Hi Haskellers and CALers, I have the feeling that a lot of code in my jee application can be done better by using functional programming. There is a lot of searching in object trees, transforming objects to another objects, aggregation functions... All written in java. Sequential logic can by done declarative with statemachines and workflows (not handcoded, dut drawed) all other with functions. Has somebody experience with this? Or is that a bad idea? Should I use pure functions, or imperative-functional language such as Scala? Is somebody here using CAL (OpenQuark) in jee application? What is your experience -- View this message in context: http://www.nabble.com/CAL-%28OpenQuark%29-and-enterprise-tp18366397p1836639… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Getting latest GHC-head/libraries takes forever
by Austin Seipp 16 Jul '08

16 Jul '08

For the purpose of experimenting with NDP I went through the process of getting the GHC head from darcs.haskell.org. As specified in the developer wiki[1], using darcs get is basically not possible because there're so many patches. So I downloaded http://darcs.haskell.org/ghc-HEAD-2008-06-06-ghc-corelibs-testsuite.tar.bz2 This alone took over *7 hours* using wget because it could never get a connection faster than 5kb/s. After downloading I changed to the directory and ran 'darcs pull -a'. Nothing bad happened here, darcs pulled most of the changes pretty quick so I had an up to date GHC. However, when trying to use the ./darcs-all script to pull in the latest library/testsuite changes, things go horribly, horribly wrong. I let darcs sit for over another hour and a half trying to pull patches from darcs.haskell.org for the libraries and the testsuite. It never got past packages/base. In fact, it never even got to *getting* the patches from the base repository, it sat there, stuck at the 'identifying repository' stage (darcs 2.0.2) for over an hour, making my processor go round and round at 99% CPU usage the entire way, never accomplishing anything. What makes it even stranger is that it got the latest patches for the testsuite and the array package pretty quick. It just got permanently stuck on base for some reason and never went further. Also, I tried pulling package-ndp a few days ago while using an older development snapshot of GHC (not the darcs repository, a nightly HEAD tarball of ghc.) Just pulling 500 patches from http://darcs.haskell.org/packages/ndp took over an hour, but at least it got done. It didn't build properly so I thought I'd get the latest darcs version of GHC instead, and here I am. Has anybody experienced anything like this recently? Off the top of my head the only things that possibly come to mind are: 1) darcs.haskell.org is just really slow or under -constant- heavy load. 2) the darcs repositories are in darcs-1 format, so darcs-2 is having issues when trying to pull the patches. 3) Diabolical connection-killing ninjas. None of these 3 possibilities are really 'great,' and none of them help me get the latest version of GHC, the libs, and NDP any faster then a narcoleptic snail. If instead this is a techincal issue with, say, darcs that someone is aware of, I'm using OS X Leopard, 10.5.4 with a statically linked darcs-2.0.2 binary from [2]. [1] http://hackage.haskell.org/trac/ghc/wiki/Building/GettingTheSources [2] http://wiki.darcs.net/DarcsWiki/CategoryBinaries#head-25c8108e9d719be30a8cc…

3 2

Can't get Haddock 2.0 anchor reference to work
by Mike Gunter 16 Jul '08

16 Jul '08

How are anchor references in Haddock supposed to work? When I use "Dir.Mod#foo", the resulting HTML contains: <A HREF="Dir-Mod#foo.html"> instead of the more desirable: <A HREF="Dir-Mod.html#foo"> User error or bug? thanks, -m

2 1

Remote control of firefox through Haskell?
by Jefferson Heard 16 Jul '08

16 Jul '08

Is there a library out there that will allow me to remote-control the firefox or mozilla browsers, e.g. change the current page, open a new tab? -- I try to take things like a crow; war and chaos don't always ruin a picnic, they just mean you have to be careful what you swallow. -- Jessica Edwards

3 3

ANN: haskell-src-exts 0.3.5
by Niklas Broberg 15 Jul '08

15 Jul '08

Hi all, I'm pleased to report that haskell-src-exts is now updated to understand Template Haskell syntax (it used to understand pre-6.4 TH, but now it works with the current version). At least I hope so, I didn't have much TH code to try it on so if you find some bugs just let me know. It wasn't all that hard to fix though, so I'm pretty confident it actually works. Get the code: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/haskell-src-exts… darcs get http://code.haskell.org/HSP/haskell-src-exts Cheers, /Niklas

2 1

Re: [Haskell-cafe] Type level sets with GADTs, fundeps etc
by J.Burton＠brighton.ac.uk 15 Jul '08

15 Jul '08

At Tue, 15 Jul 2008 10:02:23 -0400, Jeff Polakow wrote: [...] > > > Type classes are open so there is nothing to prevent you from adding another > instance for If, perhaps in a different module, which returns some arbitrary > type. I see what you mean...so I tried to make Ins return an LSet of "something", like this: data LSet t where Nil :: LSet Nil Ins :: (Member a t b , If b t (a ::: t) r) => L a -> LSet t -> LSet r /home/jim/sdf-bzr/dsel/TF/Set-July08.hs:44:9: No instance for (If b t (a ::: t) r) arising from a use of `Ins' at /home/jim/sdf-bzr/dsel/TF/Set-July08.hs:44:9-11 Possible fix: add an instance declaration for (If b t (a ::: t) r) In the expression: Ins In the definition of `insert': insert = Ins /home/jim/sdf-bzr/dsel/TF/Set-July08.hs:68:6: Inferred type is less polymorphic than expected Quantified type variable `a' is mentioned in the environment: insert :: L a -> LSet t -> LSet r (bound at /home/jim/sdf-bzr/dsel/TF/Set-July08.hs:44:0) When checking an existential match that binds t :: L a The pattern(s) have type(s): LBox The body has type: LSetBox In a case alternative: LBox t -> LSetBox (insert t s) In the expression: case fromChar c of LBox t -> LSetBox (insert t s) Failed, modules loaded: none. > > -Jeff > > > --- > > This e-mail may contain confidential and/or privileged information. If you > are not the intended recipient (or have received this e-mail in error) > please notify the sender immediately and destroy this e-mail. Any > unauthorized copying, disclosure or distribution of the material in this > e-mail is strictly forbidden. > [2 <text/html; US-ASCII (7bit)>] >

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Re: [Haskell-cafe] Type level sets with GADTs, fundeps etc
by J.Burton＠brighton.ac.uk 15 Jul '08

15 Jul '08

At Tue, 15 Jul 2008 09:43:40 -0400, Jeff Polakow wrote: > Hello, Hi Jeff, > > > data LSet t where > > Nil :: LSet Nil > > --either add the new element or do nothing > > Ins :: (Member a t b > > , If b (LSet t) (LSet (a ::: t)) r) > > => L a -> LSet t -> r > > > The constructor Ins needs to return an LSet. Maybe try replacing occurrences of r with (LSet r). > I expected that r would be an LSet, as its the output of If which returns its 2nd or 3rd argument: class If p x y z | p x y -> z where if' :: p -> x -> y -> z instance If T x y x instance If F x y y Jim > -Jeff > > --- > > This e-mail may contain confidential and/or privileged information. If you > are not the intended recipient (or have received this e-mail in error) > please notify the sender immediately and destroy this e-mail. Any > unauthorized copying, disclosure or distribution of the material in this > e-mail is strictly forbidden. > >

2 1

Type level sets with GADTs, fundeps etc
by J.Burton＠brighton.ac.uk 15 Jul '08

15 Jul '08

I have a problem with applying a type constraint in the constructor of a GADT...I wrote some type level code for sets using empty types and fundeps, along the lines of Conrad Parker's Instant Insanity and Oleg's Lightweight Static Resources. At this level things works OK so I have empty types A, B ... Nil with an infix cons (:::) and fundeps determining constraints like Member, Disjoint etc. (Links to code at the end of message.) Then I can evaluate: > :t insert (undefined::A) (undefined:: A ::: Nil) insert (undefined::A) (undefined:: A ::: Nil) :: A ::: Nil But what I really want to do is wrap this up so that it can be used at runtime, not just in the type-checker, so that (just a sketch) I could have insert 'A' empty :: Set (A ::: Nil) where the runtime value of the set is fully determined by its type. I got some help with this and existential boxing seems to be the answer. I got a version that uses this strategy working OK until it comes to applying constraints (e.g. Member, needed when inserting an element). My problem is below. Here's one with no constraints that will type-check: ---------------------- {-box for labels -} data LBox = forall a. LBox (L a) {-box for sets of labels-} data LSetBox = forall t. LSetBox (LSet t) {-sets of labels -} data LSet t where Nil :: LSet Nil -- 'dumb' insertion Ins :: L a -> LSet t -> LSet (a ::: t) insert = Ins empty :: LSetBox empty = LSetBox Nil {-take a Char into a boxed LSet-} insertChar :: Char -> LSetBox -> LSetBox insertChar c (LSetBox s) = case fromChar c of LBox t -> LSetBox (insert t s) {-populate a box-} fromChar :: Char -> LBox fromChar 'A' = LBox AL {- ... B, etc -- use Template Haskell to generate alphabet and the various instances needed? -} fromChar _ = error "fromChar: bad Char" ------------------------ With some Show instances then I can evaluate > insertChar 'A' empty LSet {A,} But what about constraints on the Ins constructor, e.g. on insertion, ignore elements that are already there: --------------------- data LSet t where Nil :: LSet Nil --either add the new element or do nothing Ins :: (Member a t b , If b (LSet t) (LSet (a ::: t)) r) => L a -> LSet t -> r insert = Ins [...] class Member x ys b | x ys -> b where member :: x -> ys -> b instance Member x Nil F instance (Eq x y b', Member x ys m, If b' T m b) => Member x (y:::ys) b class If p x y z | p x y -> z where if' :: p -> x -> y -> z instance If T x y x instance If F x y y --------------------- But this evidently isn't the way to use fundeps -- here's the error: Malformed constructor result type: r In the result type of a data constructor: r In the data type declaration for `LSet' Failed, modules loaded: none. (The other place I tried putting the constraints is in the type of `insert' rather than the Ins constructor...) How can I express this constraint? BTW, I would rather be using type families for this, for forward compatibility and their greater generality, so if I can also get round this with them I'd love to know how... Thanks, and sorry for the long post! Jim [1] Purely type level version: http://jim.sdf-eu.org/TypeLevelSets2.hs [2] Interactive version using GADTs/Existential boxing: http://jim.sdf-eu.org/Set-July08.hs

2 1

ANN: Launching Haskell Group in Vancouver, Canada
by Jon Strait 15 Jul '08

15 Jul '08

For those in Vancouver, a short announcement about the recently created Haskell Programmers Group and a meeting scheduled for next Monday, July 21st. http://groups.google.com/group/hugvan Future group announcements will be posted on this Google Groups site, so feel free to join the group list to be notified of future events or just show up to a meeting to bounce ideas and questions off of other Haskell programmers. Thanks, Jon

1 0

A question about algebra and dependent typing
by Nathan Bloomfield 15 Jul '08

15 Jul '08

There's something I want to do with Haskell, and after tinkering for a while I think it's not possible. Before giving up entirely, I thought I'd try this mailing list. I'm working on an abstract algebra library, using the "types are sets" strategy. For the algebraists out there, I'm trying to implement as much as I can of "Abstract Algebra" by Dummit & Foote in Haskell. I've got a Ring class definition that looks approximately like > class Ring t where > (<+>) :: t -> t -> t > (<*>) :: t -> t -> t > neg :: t -> t > zero :: t So, for example, we can say >instance Ring Integer where > (<+>) = (+) > (<*>) = (*) > neg = negate > zero = 0 >From here I can subclass to domains, PIDs, EDs, UFDs, fields, et cetera, and write some useful algorithms in great generality. The ring R[x] can be modeled by the type [r], direct products are tuples, and fractions in the domain r have type Fraction r. I can even model the set (i.e. type) of nxn matrices over a ring using type-level integers as demonstrated in the paper "Number-Parameterized Types" by Oleg Kiselyov [1]. All good stuff. However, I'm running into problems modeling some other useful and computationally interesting things, in particular adjoining an algebraic element and taking a quotient by an ideal. For example, I've tried the following (using GHC extensions): >class (Ring r) => Ideal i r | i -> r > >data (Ring r) => FinGenIdeal r = Ideal [r] > >instance (Ring r) => Ideal (FinGenIdeal r) r > >data (Ring r, Ideal i r) => Coset r i = r :+: i But of course I can't say >instance (Ring r, Ideal i r) => Ring (Coset r i) where > (r :+: i) <+> (s :+: j) = (r <+> s) :+: i > zero = zero :+: ? > ... It shouldn't make sense to add 2 :+: Ideal [3] and 3 :+: Ideal [5], for example, and zero is ambiguous. This problem happems because Ideal [3] and Ideal [5] have the same type, namely FinGenIdeal Integer. I am pretty new to Haskell and type theory, but I think what I'm wanting is dependent types. So, for instance, the type of Coset r i is parameterized by an particular value in FinGenIdeal r. Is there another way around this? I'm sure there is some sophisticated type-fu one could perform (similar to the examples in [1]) to construct ideals of integers, tuples of integers, et cetera, but this would quickly get unwieldy and sacrifices some generality. I have heard lots of praise of GADT as an approximation to dependent types, but I don't yet see how they could apply in this situation. Has anyone else encountered a similar problem, and if so, how did you get around it? Would I be better off working in a dependently-typed language like Agda? [1] - http://okmij.org/ftp/papers/number-parameterized-types.pdf

5 4

Sphinx full-text searching client on Hackage
by Chris Eidhof 15 Jul '08

15 Jul '08

Hey everyone, We started working on a client [1] for the sphinx full-text search engine [2], which is a very fast full-text search engine that has either SQL or XML as a backend. While our version is far from done (it only supports the query command, and a limited number of parameters), we still think that it might be useful for other people as well. If you want to help us hacking on it, testing it or improving documentation, you're more than welcome. The communication with Sphinx is done using a quite low-level binary protocol, but Data.Binary saved the day: it made it very easy for us to parse all the binary things. Especially the use of the Get and Put monads are a big improvement over the manual reading and keeping track of positions, as is done in the PHP/Python clients. Thanks, -chris [1]: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/sphinx [2]: http://www.sphinxsearch.com/

2 1

vector vs uvector
by stefan kersten 15 Jul '08

15 Jul '08

hi, what's the difference between the vector [1] and uvector [2] packages? should one of those preferred over the other? thanks, <sk> [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ vector-0.1 [2] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ uvector-0.1.0.1

3 3

Human-friendly compiler errors for GHC
by Max Bolingbroke 14 Jul '08

14 Jul '08

I had some free time this afternoon so I put together an (experimental) patch for GHC that implements helpful errors messages. Have a look at this GHCi session to see what I mean: "" $ stage2/ghc-inplace --interactive -fhelpful-errors GHCi, version 6.9.20080711: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> let foo = 10 Prelude> foa <interactive>:1:0: Not in scope: `foa' Maybe you meant one of: `foo' `fst' `not' -- Maybe the matching threshold could stand to be tweaked a bit, e.g. scaled with identifier string length.. Prelude> let myIdentifier = 10 Prelude> myIdentfiier <interactive>:1:0: Not in scope: `myIdentfiier' Maybe you meant `myIdentifier' Prelude> "" The feature was inspired by the equivalent feature in the Boo programming language (http://boo.codehaus.org/) I use the restricted Damerau–Levenshtein distance to do the fuzzy match (http://en.wikipedia.org/wiki/Damerau-Levenshtein_distance) What do you think about this feature? Would it be genuinely helpful or annoying? Max

10 15

Re: [Haskell-cafe] ANNOUNCE: vector 0.1 (efficient arrays with lots of fusion)
by Levi Stephen 14 Jul '08

14 Jul '08

On Sun, Jul 13, 2008 at 12:31 AM, Roman Leshchinskiy <rl(a)cse.unsw.edu.au> wrote: > Hi all, > > the vector library will eventually provide fast, Int-indexed arrays with a > powerful fusion framework. It's very immature at the moment (I haven't > tested most of the code) and implements just a few combinators but I > thought releasing early wouldn't hurt. Use at your own risk and expect > things to break horribly! > Sounds interesting. How does this compare to the uvector library? Thanks, Levi

3 2

Architecturally flawed
by Andrew Coppin 14 Jul '08

14 Jul '08

OK, so I just spent an entire day trying to write some code, and after hours of struggling I have something that's semi-working but very ugly and rather unreliable. My usual guideline for Haskell programming is "if it seems hard, you're doing it wrong"... After many hours of effort, I came up with these: data Writer x instance Monad Writer run_writer :: Writer () -> ByteString write1 :: Bool -> Writer () write8 :: Word8 -> Writer () write16 :: Word16 -> Writer () write32 :: Word32 -> Writer () write64 :: Word64 -> Writer () writeN :: Word8 -> Word32 -> Writer () data Reader x instance Monad Reader run_reader :: Reader x -> ByteString -> x is_EOF :: Reader Bool read1 :: Reader Bool read8 :: Reader Word8 read16 :: Reader Word16 read32 :: Reader Word32 read64 :: Reader Word64 readN :: Word8 -> Reader Word32 Notice that, unlike the Binary package, all of these are bit-aligned rather than byte-aligned. This is what took so much effort. I strived to make each operation as efficient as possible - by performing as few steps as possible. It would have been *far* easier to, say, implement read8 by calling read1 eight times and then packing the bits back into a byte. But that wouldn't be very fast. So actually read8 reads a pair of bytes, bit-shifts them as required, and then ANDs them together. And so on. (Note that the Writer monad uses Data.Binary.Builder (only) from the Binary package. I assume this is more efficient...) I even sat down and wrote a battery of QuickCheck properties because - as you can imagine - there's a hell of a lot to go wrong here! After many hours, I managed to get it to pass all tests... Next I decided to write an LZW compressor. Here's what I came up with: data EncodeLZW symbol eLZW_start :: (Ord symbol) => EncodeLZW symbol eLZW_encode :: (Ord symbol) => EncodeLZW symbol -> symbol -> Writer (EncodeLZW symbol) eLZW_end :: (Ord symbol) => EncodeLZW symbol -> Writer () data DecodeLZW symbol dLZW_start :: DecodeLZW symbol dLZW_decode :: DecodeLZW symbol -> Reader (DecodeLZW symbol, symbol) So each step of the encoder (possibly) writes some bits, and also returns a new encoder state. The decoder is similar. (At least one program bug was caused by the wrong version of the state being used somewhere!) Then I tried to run the code, and oh look, there's a bug somewhere. Emphasis "somewhere". At this point I have several KB of code and no idea how in hell to test it. What I *want* to do is single-step through the encoder algorithm, watch it write bits out and update state, and then single-step through the decoder watching it read bits in and update its state, etc. But I can't. All the encoder does is return a giant Writer object which you can't look inside, only run. And likewise for the decoder. I could try GHC's new debugger. But my experiences with it so far have shown that for all but the most trivial programs possible, it becomes intractably difficult to figure out what the debugger is actually showing you. I actually tried to debug a "normal" LZW implementation once - one that didn't involve two highly convoluted custom monads and a stateful execution model with manually threaded state. This is *not* my idea of a fun time... In a "normal" programming language, at this point you would sprinkle a few print statements around so you can see the intermediate steps the program is taking. But I can't. I'm in the wrong monad. Curses! In the end, the only solution I could come up with was to design a second, "hacked" version of the two monads. Instead of importing one module, you import another that provides the same interface. However, now Reader and Writer are aliases to IO. All write requests cause pretty-printed binary to be sent to stdout, and all read requests pop up a prompt for input from stdin. It worked reasonably well in that I could now add the vitally necessary print statements, and see intermediate values and so forth... It wasn't very pretty though. At this point I decided that lugging 5 individual functions around was silly, and I should write a class: class Encoder e where encoder_start :: e x encode :: e x -> x -> Writer (e x) encoder_end :: e x -> Writer () class Decoder d where decoder_start :: d x decode :: d x -> Reader (d x, x) 10 points to the first person to spot the fatal flaw in this plan... Yep, that's right. The type system won't accept this. The reason? x needs an Ord context. If you turn on multi-parameter type classes *and* flexible instances, apparently this works: class Encoder e x where... instance (Ord x) => Encoder EncodeLZW x where... (I haven't tried but I *presume* it means what I think it does...) Alternatively, you can avoid using dangerous type system extensions and just write class Encoder e where encode :: (Symbol x) => e x -> x -> Writer (e x) ... and make Symbol encompass everything any possible encoder could want. (In reality, x is almost always Word8 or something. But I dislike loss of generality just for the hell of it...) So at least I got that part fixed. Heh. But now I find myself worrying about yet *another* problem: is Writer lazy enough? I mean, the idea is that you can write a program that lazily reads from a file, pushes it through a Writer, and lazily writes the result back to another file. The thing should chug along reasonably fast and use a constant amount of RAM. But all this is only true of Writer is actually lazy enough. I have a horrible feeling that all the complicated Origami inside makes it too strict. And I have no way to check! Actually, thinking about it, is Reader lazy enough? You call run_reader and it hands over your data, but if that data is, say, a list, will run_reader build the entire damn list before it'll hand it over? Or will the monadic code by called as-needed to generate the list elements? Obviously I desire the latter, but I'm really not sure what the actual behaviour is... In summary, I've spent several days coding, and I still don't have a single algorithm working. I've spent all my time wrestling with the mundane details of infrastructure rather than the interesting algorithms I actually set out to implement. This makes me very sad. :-( If anybody can think of a better set of abstractions or another way to do debugging, I'd be interested to hear about it. (This would all be so trivial in an OO language. Just make an Encoder object that updates its own state internally and talks to a Source object and a Destination object for its data...)

8 11

Prime time for Haskell
by Janis Voigtlaender 14 Jul '08

14 Jul '08

It is really just a short discussion, but Haskell STM is featured in an article of this month's Communications of the ACM: http://doi.acm.org/10.1145/1364782.1364800 -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

3 3

Existential quantification problem
by Marco Túlio Gontijo e Silva 14 Jul '08

14 Jul '08

Hello, how do I unbox a existential quantificated data type? > {-# LANGUAGE ExistentialQuantification #-} > data L a = forall l. L (l a) > unboxL (L l) = l is giving me, in GHC: Inferred type is less polymorphic than expected Quantified type variable `l' escapes When checking an existential match that binds l :: l t The pattern(s) have type(s): L t The body has type: l t In the definition of `unboxL': unboxL (L l) = l Thanks. -- Marco Túlio Gontijo e Silva Página: http://marcotmarcot.googlepages.com/ Blog: http://marcotmarcot.blogspot.com/ Correio: marcot(a)riseup.net XMPP: marcot(a)jabber.org IRC: marcot(a)irc.freenode.net Telefone: 25151920 Celular: 98116720 Endereço: Rua Turfa, 639/701 Prado 30410-370 Belo Horizonte/MG Brasil

6 7

memoization
by Logesh Pillay 14 Jul '08

14 Jul '08

I know we can perform memoization in Haskell. The well known recursive Fibonacci example works v. well. f(10000) returns a virtually instant answer which would not be possible otherwise. My (probably naive) function to give the number of partitions of a number :- p = ((map p' [0 .. ]) !!) where p' 0 = 1 p' 1 = 1 p' n = sum [(((-1) ^ (k+1)) * ( p' (n-((k*(3*k-1)) `div` 2)) + p' (n-((k*(3*k+1)) `div` 2)))) | k <- [1 .. n]] It is an attempt to apply the Euler recurrence formula (no 11 in http://mathworld.wolfram.com/PartitionFunctionP.html ) It works but it is shockingly slow. It is orders of magnitude slower than the Python memoized version which runs very fast. parts = {0:1, 1:1} def P(n): if not n in parts: parts[n] = sum ([( ((-1) ** (k+1)) * ( P(n-((k*(3*k-1))//2)) + P(n-((k*(3*k+1))//2)) ) ) for k in xrange (1, n+1)]) return parts[n] Why? Its as if memoization is being ignored in the haskell version. How to fix?

4 3

confusion with fgl and ghc6
by David Bremner 14 Jul '08

14 Jul '08

I don't know whether this is a bug in fgl, in ghc, or I'm just confused. Any ideas? This is on debian/sid. GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> import Data.Graph.Inductive.Example Prelude Data.Graph.Inductive.Example> star 6 <interactive>:1:0: Ambiguous type variable `gr' in the constraint: `Data.Graph.Inductive.Graph.Graph gr' arising from a use of `star' at <interactive>:1:0-5 Probable fix: add a type signature that fixes these type variable(s) Prelude Data.Graph.Inductive.Example>

2 1

ANNOUNCE: vector 0.1 (efficient arrays with lots of fusion)
by Roman Leshchinskiy 13 Jul '08

13 Jul '08

Hi all, the vector library will eventually provide fast, Int-indexed arrays with a powerful fusion framework. It's very immature at the moment (I haven't tested most of the code) and implements just a few combinators but I thought releasing early wouldn't hurt. Use at your own risk and expect things to break horribly! What it provides: * Boxed and unboxed arrays with a generic interface and a very basic set of combinators. * A powerful loop fusion framework. It is based on stream fusion but already goes beyond that (in particular, it can do some things in-place) and will be significantly extended in the future. * Extensibility. * Rudimentary documentation. * Bugs. The code is based on the DPH libraries but is much more generic and a *lot* simpler. This has only been made possible by the tremendous progress in GHC's simplifier in the recent months/years (thanks Simon!). Consequently, you'll need a recent development version of GHC to build this, 6.8 won't work. It might be able to compile the library but you'll get terrible code, much worse than with lists. If you want to try it out, compile with -O2 -fno-spec-constr-count and please don't forget to let me know about things that didn't work (or, miraculously, did). Grab it from http://hackage.haskell.org/cgi-bin/hackage-scripts/package/vector or darcs get http://darcs.haskell.org/vector Again, special thanks to Simon for doing such a wonderful job with the optimiser. Enjoy, Roman

4 4

Template Haskell and haskell-src-exts
by ChrisK 13 Jul '08

13 Jul '08

Hi, Can one represent the ''Type template Haskell syntax of $( makeMergeable ''FileDescriptorProto ) in haskell-src.exts Language.Haskell.Exts.Syntax ? And what are the HsReify data (e.g. HsReifyType and HsReifyDecl and HsReifyFixity )? I don't see any pretty print capability to produce the ''Type so I am wondering what else I might use...

3 2

Re: [Haskell-cafe] Human-friendly compiler errors for GHC
by Max Bolingbroke 13 Jul '08

13 Jul '08

> Personally, I would suggest make it take up less space. A newline for each match may be alright if there are only 3 suggestions, but past that it begins to take up too much of the screen. Columns are nice, or perhaps a limit on how many matches will be displayed (with more available if the user asks?). I've actually limited it to 3 matches displayed in the current implementation, but indeed it would probably be good to display the matches in columns instead. Max

1 0

FPGA / Lava and haskell
by Marc Weber 12 Jul '08

12 Jul '08

Is Haskell still used (in industry as well ?) to write (V)HDL code to program FPGAs and create circuits on chips? The Chalmers Lava homepage tells abouta Xilinx version which should be merged in soon. But on the xilinx homepage there was no reference to neither Lava nor haskell.. I'm thinking about designing a similar tool to www.combimouse.com. Sincerly Marc

6 7

Getting module functions
by rodrigo.bonifacio 12 Jul '08

12 Jul '08

Hi all, Is there any function that can be used for retrieving the exposed functions of a given module? Thanks, Rodrigo.

3 2

Typeable and fancy types
by Ron Alford 12 Jul '08

12 Jul '08

Hey all, I've reduced my previous problem to a small example. Anyone know why typeOf (...) would work, but typeOf [...] would not? Is the derivation for lists funky? data Expr f = In (f (Expr f)) instance Typeable1 f => Typeable (Expr f) where typeOf (In x) = mkTyConApp (mkTyCon "TypeTest.Expr") [typeOf1 x] data Foo e = Foo deriving instance Typeable1 Foo *TypeTest> typeOf (In Foo) TypeTest.Expr TypeTest.Foo *TypeTest> typeOf [In Foo] *** Exception: Prelude.undefined

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Newbie: Appending arrays?
by Dmitri O.Kondratiev 12 Jul '08

12 Jul '08

What is the best way to extend array? I would use a list instead of array as it is easy to append, but need to have random access to its elements later. So in fact I need to start with an integer array of size 1. Next I may need to add new elements to this array or modify values of the existing ones. Function: array :: (Ix a) => (a,a) -> [(a,b)] -> Array a b allows construct an array of a fixed size. How to add more elements to the array later? Thanks! -- Dmitri O. Kondratiev dokondr(a)gmail.com http://www.geocities.com/dkondr

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Inductive graphs memory usage
by Andre Nathan 12 Jul '08

12 Jul '08

Hello I'm trying to create a directed graph using the Data.Graph.Inductive. The graph is a random graph using the G(n, p) model, that is, each of the n nodes is linked to every other node with probability p. I'm seeing a large increase of memory usage when n grows (this is using p = 0.1): n = 1000 -> 96MB n = 2000 -> 283MB n = 3000 -> 760MB So, I'm probably doing something very stupid :) The code is below. Is there anything I could do to optimize memory usage here? module Main where import Control.Monad import Data.Graph.Inductive import System.Random createEdges :: Int -> Double -> IO [LEdge Int] createEdges n prob = foldM create [] [1..n] where create es i = foldM (flip $ link i) es [i, i-1 .. 1] link i j es | i == j = return es -- no self-loops | otherwise = do es' <- maybeCreateEdge i j prob es es'' <- maybeCreateEdge j i prob es' return es'' maybeCreateEdge :: Node -> Node -> Double -> [LEdge Int] -> IO [LEdge Int] maybeCreateEdge i j prob es = do r <- randomDouble return $ if r < prob then (i, j, 0):es else es randomDouble :: IO Double randomDouble = getStdRandom $ random main :: IO () main = do let (n, p) = (3000, 0.1) es <- createEdges n p let g = mkGraph [(i, 0) | i <- [1..n]] es :: Gr Int Int return () Thanks, Andre

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ANN: Takusen 0.8.3
by Alistair Bayley 12 Jul '08

12 Jul '08

Changes since 0.8.1 (I put 0.8.2 up on hackage with an error in Setup.hs, so it's been skipped): - ODBC support: datetime marshalling is improved. For bind parameters this uses the timestamp struct for most back-ends, but String for MS SQL Server because populating the timestamp struct always failed. - more Cabal improvements: now uses configurations, so the Setup.hs script should be both simpler and more robust. Requires Cabal >= 1.4. Oracle backend on Linux should build nicely. - bug fix for a resource leak if an exception was thrown when initiating a query (the Statement handle was not closed). - some basic result-set validation against the iteratee: if you try to fetch a column that is not in the result-set, an error is thrown (rather than garbage returned). The release bundle: http://hackage.haskell.org/packages/archive/Takusen/0.8.3/Takusen-0.8.3.tar… The latest code: darcs get http://darcs.haskell.org/takusen Docs: http://darcs.haskell.org/takusen/doc/html/index.html A comprehensive description of API usage can be found in the documentation for module Database.Enumerator (look for the Usage section): http://darcs.haskell.org/takusen/doc/html/Database-Enumerator.html Future plans: - Output bind-parameters and multiple-result sets for ODBC - FreeTDS backend (Sybase and MS Sql Server) - support for Blobs and Clobs For those of you unfamiliar with Takusen, here is our HCAR blurb: Takusen is a DBMS access library. Like HSQL and HDBC, we support arbitrary SQL statements (currently strings, extensible to anything that can be converted to a string). Takusen's 'unique-selling-point' is safety and efficiency. We statically ensure all acquired database resources - such as cursors, connection and statement handles - are released, exactly once, at predictable times. Takusen can avoid loading the whole result set in memory, and so can handle queries returning millions of rows in constant space. Takusen also supports automatic marshalling and unmarshalling of results and query parameters. These benefits come from the design of query result processing around a left-fold enumerator. Currently we fully support ODBC, Oracle, Sqlite, and PostgreSQL.

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Combining Wouter's expressions with extensible records
by Ron Alford 12 Jul '08

12 Jul '08

Well, my extension of Wouter's datatypes proved to be unweildy.... So, I'm trying to use http://fmapfixreturn.wordpress.com/2008/05/03/simple-extensible-records-now… for extensible records. I ran across my first problem rather quickly! data Expr f = In (f (Expr f)) Ok, but to make it part of a record, it needs to implement Data: data Expr f = In (f (Expr f)) deriving Data but this gives No instances for (Data (f (Expr f)), Typeable (Expr f)) arising from the 'deriving' clause of a data type declaration at Planning/Wouter.hs:77:0-42 Any hints? Thanks, -Ron

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More idiomatic use of strictness
by Grzegorz Chrupala 11 Jul '08

11 Jul '08

Hi all, Is there a less ugly way of avoiding laziness in the code pasted below then the use of seq in the last line? The program is supposed to split a large input file into chunks and check in how many of those chunks each of a list of words appear, as well as the total number of chunks. Without the seq it consumes huge amounts of memory. Thanks! Grzegorz {-# LANGUAGE BangPatterns, PatternGuards #-} import Data.List (foldl') split delim s | [] <- rest = [token] | otherwise = token : split delim (tail rest) where (token,rest) = span (/=delim) s main = do putStrLn =<< fmap (show . stats ["the","a","and"] . split "<DOC>" . words) getContents stats ws docs = foldl' f ((map (const 0) ws),0) docs where f (dfs,n) d = let dfs' = zipWith (\w df -> (df + fromEnum (w `elem` d))) ws dfs in sum dfs' `seq` (dfs',n+1) -- View this message in context: http://www.nabble.com/More-idiomatic-use-of-strictness-tp18379800p18379800.… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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FPGA / Lava and haskell
by Satnam Singh 11 Jul '08

11 Jul '08

Hello Marc. I've been distracted with the Kiwi project (parallel C# programs to hardware) and right now a Bluespec project so I've not made any progress on a merge of the Lava I produced for designing circuits (with layout) for Xilinx's FPGAs and the Chalmers version (which I recommend for situations where you don't care about circuit layout or when you don't want to use FPGA architecture specific features). The version of Lava on my website at http://raintown.org/lava/ still builds under the current version of ghc and it provides support for generating Xilinx EDIF netlists that supports the basic FPGA circuit building blocks. If you need things like ODDRs etc. which appear in the new FPGAs then you have to either add them yourself (not too difficult) or wait for me to do it :) Cheers, Satnam ________________________________ Satnam Singh Microsoft 7 JJ Thomson Avenue Cambridge CB3 0FB United Kingdom Email: satnams(a)microsoft.com<mailto:satnams@microsoft.com> UK tel: +44 1223 479905 Fax: +44 1223 479 999 UK mobile: +44 7979 648412 USA cell: 206 330 1580 USA tel: 206 219 9024 URL: http://research.microsoft.com/~satnams Live Messenger: satnam(a)raintown.org<mailto:satnam@raintown.org>

1 0

Data.Derive problems
by Ron Alford 11 Jul '08

11 Jul '08

I'm using GHC 6.8.3 with $ cabal --version cabal-install version 0.5.1 using version 1.4.0.1 of the Cabal library I installed Data.Derive from hackage, only to be unable to find the 'derive' binary! Trying it directly from darcs, I get: $ ghc --make Setup.hs [1 of 1] Compiling Main ( Setup.hs, Setup.o ) Linking Setup ... silverback-wired:derive ronwalf$ ./Setup configure Warning: defaultUserHooks in Setup script is deprecated. Configuring derive-0.1.1... Warning: No 'build-type' specified. If you do not need a custom Setup.hs or ./configure script then use 'build-type: Simple'. -Ron

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Poor Parsec error message
by Lyle Kopnicky 10 Jul '08

10 Jul '08

Hi folks, I'm using Parsec to parse a stream of tokens. The token primitive takes, among other arguments, a function to print tokens. However, this function is not always applied. Try the code below: --------- import Text.ParserCombinators.Parsec import Text.ParserCombinators.Parsec.Pos(newPos) mytoken :: (Eq t, Show t) => t -> GenParser (SourcePos,t) () t mytoken x = token showTok posFromTok testTok where showTok (pos,t) = "<" ++ show t ++ ">" posFromTok (pos,t) = pos testTok (pos,t) = if (x == t) then Just t else Nothing main = do putStrLn "" case parse the123Parser "" [(newPos "" 1 n, n) | n <- [1,2,3,4]] of (Left err) -> putStrLn (show err) (Right _) -> putStrLn "parsed correctly" putStrLn "" case parse the123Parser "" [(newPos "" 1 n, n) | n <- [1,3,4]] of (Left err) -> putStrLn (show err) (Right _) -> putStrLn "parsed correctly" the123Parser = do mytoken 1 mytoken 2 mytoken 3 eof return 123 ------- The output I get looks like this: (line 1, column 4): unexpected [((line 1, column 4),4)] expecting end of input (line 1, column 3): unexpected <3> In the second parse case, it correctly uses my showTok function to show the token. But in the first case, it just uses the regular show method. I guess that's because the eof parser doesn't know anything about how to show the token it sees. Any ideas on how I can get the error message in the first case to look more like the second case? Thanks, Lyle

1 1

Lazy IO
by Ronald Guida 10 Jul '08

10 Jul '08

Suppose I have a lazy function f :: [Int] -> [Int], and I happen to know that for all n, the n-th element of the output may only depend on the first (n-1) elements of the input. I want to print a number from f's output list, and then ask the user for the next number in f's input list, and then loop until the user stops providing valid numbers. I also need to be able to do IO after my loop exits. Consider the following code: ---------------------------------------------- module Main where import Control.Monad.Fix import System.IO.Unsafe promptInt :: String -> IO (Maybe Int) promptInt p = do putStr p s <- getLine let rs = reads s if not $ null rs then return $ Just $ fst $ head rs else return $ Nothing promptInts :: [String] -> IO [Int] promptInts [] = return [] promptInts (p:ps) = do m <- promptInt p case m of Just n -> do ns <- unsafeInterleaveIO $ promptInts ps return $ n:ns Nothing -> return [] -- assume accumulator is an opaque function accumulator :: [Int] -> [Int] accumulator = scanl (+) 0 makeAccPrompt :: Int -> String makeAccPrompt n = "[Acc = " ++ show n ++ "] ? " main :: IO () main = do xs <- mfix $ promptInts . map makeAccPrompt . accumulator seq (length xs) $ print xs ---------------------------------------------- Question: If I can't change my function f (in this case, accumulator), then is it possible to get the effect I want without having to resort to "unsafeInterleaveIO"?

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Re: [Haskell-cafe] More idiomatic use of strictness
by Reinier Lamers 10 Jul '08

10 Jul '08

Hi all, Op Thursday 10 July 2008 12:16:25 schreef Grzegorz Chrupala: > Is there a less ugly way of avoiding laziness in the code pasted below then > the use of seq in the last line? You could replace the list dfs' with a strict list type, like: data StrictList a = Cons !a !(StrictList a) | Nil Then you wouldn't have to make useless calls to sum and seq to force strictness. It would be more work though because you'd have to define your own higher order functions to work with the strict list. Reinier

1 0

use of the GHC universal quantifier
by Galchin, Vasili 10 Jul '08

10 Jul '08

Hello, It seems to me by its name that "forall" denotes a logical universal quantifier. In any case, hsql-1.7/Database/HSQL/Types.hs uses "forall" at line #134. I got a nasty build so I added {-# LANGUAGE ExistentialQuantification #-} at the top of the module. Now I get the following a coupleof lines up: Database/HSQL/Types.hs:131:5: Illegal polymorphic or qualified type: forall a. Int -> FieldDef -> FieldDef -> CString -> Int -> IO a -> IO a In the definition of data constructor `Statement' In the data type declaration for `Statement' If seems that GHC doesn't like "a". Why? Kind regards, Vasili

2 2

Qualified import syntax badly designed (?)
by Neil Mitchell 10 Jul '08

10 Jul '08

Hi, It seems that the qualified import syntax is a bit awkward. At the moment, its common to see: import qualified Data.Map as M import Data.Map(Map) i.e. import a module, give it an alias (M), and put some things in the current namespace (Map). Another way some people sometimes do it is: import qualified Data.Map as M import Data.Map hiding (lookup) i.e. import a module, give it an alias (M), and exclude some things from the current namespace. Both of these require two imports, yet feel like they should require only one. It seems as though the import syntax more naturally promotes security (preventing access to some functions), rather than namespacing. I think a better design for namespacing might be: import Data.Map as M implicit (Map) import Data.Map as M explicit (lookup) If this was the design, I'm not sure either qualified or hiding would be necessary for namespacing. You'd get module names aligning up in the same column after the import rather than being broken up with qualified. You'd only need one import of a module for most purposes. The hiding keyword might still be nice for lambdabot style applications, but that is probably a secondary concern, and better handled in other ways. Thoughts? Is this design flawed in some way? Does the existing design have some compelling benefit I've overlooked? Thanks Neil

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re: [Haskell-cafe] Swapping Monads
by Greg Meredith 10 Jul '08

10 Jul '08

Dominic, You can also reference Eugenia Cheng's paper on arXiv<http://arxiv.org/abs/0710.1120> . Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 806 55th St NE Seattle, WA 98105 +1 206.650.3740 http://biosimilarity.blogspot.com

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