November 2001 - Haskell

Re: Functional Metapost maintained?
by Olof Oberg 07 Nov '01

07 Nov '01

Ferenc Wagner wrote: > Hi, > > I got to know Haskell through Functional Metapost. Those > who use LaTeX and feel like programming some graphics in > Haskell, might have a look at this: > > http://tex.loria.fr/prod-graph/functional-metapost-latex.pdf > > Trying to use the package I had some problems with the > current Hugs and GHC, too. It seems like the version I have > is quite dated: it uses fromDouble, fromInt, fromRealFrac > etc. which GHC barks at. > > I made some modifications to have it compile, so that I can > use it know, but I still have a couple of problems that > should be washed out. But before I go for it, I'd like to > know if somebody have got a newer version, or somebody else > maintains it, or have some relevant information about that > package, or anything... On the Web I found nothing, really. I am interested in using FuncMP, so if you get it to work with a current GHC or Hugs.. Plain metapost is just too awkward to use when one wants to make more complex graphics, like 3D. I think Haskell might help there. /mill -- ############################################################# # S-mail: Olof Oberg # mill(a)pedgr571.sn.umu.se # # Pedagoggr. 7A # mill(a)ludd.luth.se # # S-907 30 Umea # tdv94oog(a)cs.umu.se # # Phone: 090-197395 # http://pedgr571.sn.umu.se/~mill # #############################################################

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Non-monolithic version of 'sequence' for IO monad?
by Joe English 07 Nov '01

07 Nov '01

Is there any way, short of using unsafePerformIO, to implement a combinator f :: [IO a] -> IO [a] in a way that the the result is produced lazily? The problem with sequence :: (Monad m) => [m a] -> m [a] sequence = foldr (liftM2 (:)) (return []) where liftM2 f a b = do { a' <- a; b' <- b; return (f a' b') } is that it consumes the entire input before producing a result. Thanks for any advice, --Joe English jenglish(a)flightlab.com

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subsequencing!
by Stephanie Randles 07 Nov '01

07 Nov '01

Hi, Im studying for an exam in Haskell program and for practice we have been given exercises to complete on our own. The answers have not been put up on the website either. So i was hoping for some help with this little program here. The idea is to be able to substitute the first occurrence of a given subsequence in a list with another subsequence. The problem im not sure how to compare the lists for equality. Here the is what i have so far..... substitute :: Eq a => [a] -> [a] -> [a] -> [a] substitute _ [] _ = [] substitute ys (x:xs) zs | ys == x = zs:xs | otherwise = x:substitute ys xs zs Any help would be greatly appreciated. Thanks Stephanie

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penalty for proper tail recursion
by Richard Uhtenwoldt 06 Nov '01

06 Nov '01

What is the penalty that compilers pay these days for allowing "proper" tail-recursion (that does not grow the stack)? It used to be that implementations targetting C would use the while (1) {cont=(*cont)();} trick, which would lose a factor of 2 or so in speed IIRC. If the compiler generates native code, then the penalty I believe consists mainly of the "opportunity cost" of not targetting C and thus not being able to piggyback on GCC development efforts, but there may be costs which I'm failing to visualize from the fact that the hardware is specialized to handle the conventional way of iterating (ie, structured looping constructs). Please share answers for other languages like Mercury, even though some might think it slightly off-topic...

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ANN: tagsh 0.1
by Jonas Svensson 06 Nov '01

06 Nov '01

Hello, I've written a version of the tags program for Haskell, tagsh. It does things differently compared to the hasktags program but the main reason I wrote this is that I didn't knew hasktags existed. It uses the hssource and posix library from hslibs, works with GHC 5.02.1. tags file has been checked to work with vim and nedit. Features: Run files trough cpp (optional) Only write tags for things which are exported (optional) Download: http://www.dtek.chalmers.se/~d99josve/tagsh.tar.gz Run with "-h" for list of options. /jonas

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PLI 2002-Second call for workshop proposals
by Carsten Schuermann 06 Nov '01

06 Nov '01

SECOND CALL FOR WORKSHOP PROPOSALS Principles, Logics, and Implementations of high-level programming languages (PLI 2002) Pittsburgh, USA October 3 - 8, 2002 Carsten Schuermann http://www.cs.yale.edu/~carsten/pli02 PLI 2002, a federation of colloquia which includes ICFP 2002 (ACM-SIGPLAN International Conference on Functional Programming), PPDP 2002 (ACM-SIGPLAN International Conference on Principles and Practice of Declarative Programming), and GC 2002 (ACM-SIGPLAN Conference on Generative Component-based Software Engineering (GCSE), and the Semantics, Applications, and Implementation of Program Generators (SAIG) Generators and Components). PLI 2002 will be held in Pittsburgh, USA, October 3 - 8 2002. Affiliated workshops will be scheduled from October 3 through October 8. Researchers and practitioners are invited to submit workshop proposals, that may be sent to the PLI 2002 Workshop Chair Carsten Schuermann, carsten(a)cs.yale.edu by e-mail (Postscript, Pdf, ASCII) with "PLI02 Workshop Submission" in the subject header. Following SIGPLAN's sponsorship procedures, proposals should include: 1. Name of the workshop and the sponsoring conference (ICFP/PPDP/GC). Workshops associated with ICFP may be scheduled during PPDP/GC, workshops associated with PPDP may be scheduled during ICFP, workshops associated with GC may be scheduled during ICFP. 2. The goals of the workshop. 3. Names and addresses of organizers. 4. Names of potential participants, such as program committee members. 5. Plans for call for participation (e.g., call for papers). 6. Expected number of attendees. 7. Plans for publicity. 8. Plans for a proceedings. 9. Description of past versions of the workshop including date, organizers, paper submission/acceptance counts, attendance, site, registration fees and summary budget information. 10. Url address of the workshop description. Proposals will be evaluated by the PLI 2002 Workshop Chair, the ICFP, PPDP, and GC Program and Conference Chairs, and by the SIGPLAN Executive Committee. Be advised that workshops must be sponsored by SIGPLAN and require therefore SIGPLAN approval (see http://www.acm.org/sigplan/sigplan_workshop_proposal.htm for more information.) THE DEADLINE FOR RECEIPT OF PROPOSALS IS DECEMBER 7, 2001. Notification of acceptance will be made by February 1, 2002. Workshop selection committee: Don Batory : Program Co-Chair of GC 2002 Charles Consel : Program Co-Chair of GC 2002 Claude Kirchner : Program Chair of PPDP 2002 Simon Peyton Jones : Program Chair of ICFP 2002 Frank Pfenning : General Chair of PPDP 2002 Carsten Schuermann : Workshop Chair of PLI 2002 Walid Taha : General Chair of GC 2002 Mitch Wand : General Chair of ICFP 2002

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Haskell Wiki Upgraded
by John Heron 05 Nov '01

05 Nov '01

Hi All, As you may know, I've had to fix the Haskell wiki quite a number of times due to a bug, probably a bombing CGI script. I did a trial upgrade to a new version of pywiki a couple of weeks ago and didn't find problems or get any complaints so tonight (Nov 5, 2001) I went ahead and updated the wiki to a new version. There hasn't been anything like what I usually like in the way of testing, but you can't wait forever, and I'm comforted by a backup, and the fact that RCS lies underneath all. In any case, please let me know right away if you've any problems with it. Cheers, John Heron

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using haskell as a shell language
by Hal Daume 05 Nov '01

05 Nov '01

Hi, As we all know well, we can write scripts like: #!/usr/bin/perl -w print "foo" and whatnot. i'd like to be able to write something like: #!/usr/local/ghc/bin/ghc main = do putStrLn "foo" and whatnot, even if I have to explicitely say something like: #!/usr/local/ghc/bin/ghc module Main(main) where main = do putStrLn "foo" it doesn't really matter. but i'd like to be able to write something like this, chmod +x it and be able to execute it, interpreted. i have *no* idea what the semantics of #!... is in shell scripts, so i don't know if it's easy or hard to hack something like this together, but if anyone has any pointer (or if anyone's done something like this before), please please please let me know. thanks! - hal -- Hal Daume III "Computer science is no more about computers | hdaume(a)isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume

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URGENT: File exists
by bracaman＠box.sk 05 Nov '01

05 Nov '01

I need to create a function that checks if a file exists. My intention is to do something like: teste :: String -> Bool (Beeing the String the FilePath) I've tried to use openFile and catch functions, but i couldn't put it working. The purpose of this function is to test a file before trying to read it. Example: if (test "filepath") then readfile else fail Thank you very much for your help, Joao ------------------------------------ geek code inside :: powered by linux

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Re: Non-monolithic version of 'sequence' for IO monad?
by Marcin 'Qrczak' Kowalczyk 05 Nov '01

05 Nov '01

Mon, 05 Nov 2001 14:23:18 -0800, Joe English <jenglish(a)flightlab.com> pisze: > Is there any way, short of using unsafePerformIO, > to implement a combinator > > f :: [IO a] -> IO [a] > > in a way that the the result is produced lazily? No. If evaluation of list elements causes IO, then it's unsafe. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ QRCZAK

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