- Haskell - Haskell.org

re: ordinary/accumulator recursion
by Randles 10 Sep '01

10 Sep '01

Hi, Im a computer science undergraduate and we are studying haskell as part of our course. I have a question on recursion that i'd like to receive some sort of answer to. The question is to write a function to sum the first n terms in the series: 1/1 + 1/3 + 1/5+ .. + 1/(2n-1) using ordinary recursion and then using accumulator recursion. Please help! Regards Stephanie

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Deriving ..?
by Amit Garg 10 Sep '01

10 Sep '01

Im curious to know how the `deriving' keyword has been implemented, and more so as to how it may be extended for any user defined type class w/o resorting to an external program like DriFt. Thanks! -Amit. ------------------------------------------------------------------- Amit Garg | Office: ACES 6SEo4E Graduate Student | Phone : (512) 232-7875 Computer Sciences | Res : 2000 Pearl St. #207 University of Texas at Austin | Phone : (512) xxx-xxxx Homepage: http://www.cs.utexas.edu/~amitji -------------------------------------------------------------------

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XML
by Steinitz, Dominic J 09 Sep '01

09 Sep '01

dominic.steinitz(a)blueyonder.co.uk 08/09/2001 12:09 To: Dominic Steinitz/HEATHROW/BRITISH AIRWAYS/GB@BRITISH AIRWAYS cc: bcc: Subject: XML If, from Windows, I click on the file: C:\Dom\xml\HaXml-1.02\examples\album.xml then I get the following: The XML page cannot be displayed Cannot view XML input using style sheet. Please correct the error and then click the Refresh button, or try again later. ------------------------------------------------------------------------------ Cannot have multiple DOCTYPE declarations. Line 2, Position 11 <!DOCTYPE album SYSTEM "album.dtd" [ ----------^ Is this what should happen? Dominic Steinitz

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closing file handles
by Hal Daume 09 Sep '01

09 Sep '01

I'm working with data spanning a lot of separate files and am having difficulty getting ghc to close the files. I've reduced my code to the following bit of code which should show my problem: >go filelist = do return (mapM go' filelist) > where go' f = do h <- openFile f ReadMode > text <- hGetContents h > let c = head $! text > return (c `seq` (hClose h `seq` c)) basically, you give "go" a list of files and it'll give you the first character from each of these. if i run this with 30-40 files, it's okay. but when i run it with a list of 400 files, i get: *** Exception: resource exhausted Action: openFile Reason: process file table full File: /nfs/isd/marcu/RST-CORPUS-LDC/RSTtrees-WSJ-main-1.0/DATA/wsj_1355.out.dis which doesn't make sense to me, since it should be (as far as i can tell) closing each of the handles (the yucky "c `seq` (hClose h `seq` c)" is my way of making sure that it reads the first character before it closes the file and then makes sure to close the file before returning the character -- if there is a better way, please let me know). if anyone can tell me what my problem is, that would be great! also, along the same lines, are there any good papers on using haskell with large amounts of data? i frequently have problems like this and if there were a unified theory behind dealing with them, it would be great. any pointers would be helpful. thanks! -- 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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controlling resource usage
by Richard Uhtenwoldt 07 Sep '01

07 Sep '01

First I must retract what I wrote, ie, >> GHC code can interface with C code, but not as efficiently or as >> flexibly as C or C++ code can interface with C code. --to which Alastair replied: > the foreign function interfaces have been getting >pretty damn good and, if you know your tools > can generate pretty good code. Whilst I don't speak for the >GHC team and can only speak for one of the FFI tools, I think you >should report is as a bug when you find inefficiency of the FFI to be >a bottleneck or that the effort in interfacing to a piece of C code is >way out of proportion to the size of the interface. I was just kind of guessing and surmising, so Alastair's evaluation of FFI is probably the correct one. Alastair: >Haskell's a great programming language for most programming because it >is so good at hiding resource usage from the programmer but since one >of the primary goals of an OS is to control resource usage, this is >exactly what you don't want when you're programming an OS. To kind of summarize, I would like people to voice agreement or disagreement with these next few points. (I'm anticipating wide agreement with all of them.) One of Haskell's strongest points is that more than almost any other PL it allows the programmer to avoid paying attention to the details of memory and cpu usage when memory and cpu usage are unimportant. (But sometimes it is important, so...) For a programmer to learn to control memory and cpu use *well* in Haskell is a very big job --bigger than, eg, learning monads (a *big* stumbling block for the industrial programmers than blow through here and the FP newsgroup) or existential types. So let me talk concretely about that a little, just so people have some words and images to hang their thoughts off of. The way I program in Haskell, I do not concern myself with resource control till I run my program and get empirical evidence that it's too strict (stack overflow or infinite loop) or too lazy, then I ask myself how I can add a ~ to make it lazier or add a seq or rewrite, eg, foo::[(Foo,Bar)]->Baz foo xs = bar xs with foo ((x1,x2):xs) = bar ((x1,x2):xs) (which the mathematician in me wants to consider equivalent in all ways, but sometimes they lead to different resource usage patterns) to make it stricter. Resource control in Haskell is a murky business to me. monads and existential types are the kind of thing I enjoy learning and working with --while not exactly easily and quickly learned, they're "tight" and "mathematical"-- but controlling cpu cycles and memory in Haskell does not have that tight, mathematical flavor I like. Finally, are we all in agreement that it would be a great thing if someone invented a Haskell-like language as good as Haskell at hiding resource usage when resource usage is unimportant, but that made it easier to control resource usage than Haskell does when resource usage is important?

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VIRUS IN EINER MAIL VON <olafmennerich@dev.mcd-panasonic.de>
by Wolfgang Boenschen 07 Sep '01

07 Sep '01

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VIRUS IN EINER MAIL VON <olafmennerich@dev.mcd-panasonic.de>
by Wolfgang Boenschen 07 Sep '01

07 Sep '01

V I R U S A L A R M Unser Viruswall fand folgende Viren: in einer E-Mail an Sie von: <olafmennerich(a)dev.mcd-panasonic.de> Die E-Mail wird mit einer Verz�gerung von einigen Minuten zugestellt. Sie erkennen Sie an dem Hinweis [VIRUS IN DIESER MAIL!!!] im Betreff. Falls Sie nicht im Besitz eines aktuellen Virenscanners sein sollten empfehlen wir Ihnen, die Mail zu loeschen und den Absender zu informieren. Seien Sie auf jeden Fall sehr vorsichtig mit dieser Mail!!! ########################################################################## V I R U S A L E R T Our viruschecker found the virus(es) in an email to you from: <olafmennerich(a)dev.mcd-panasonic.de> The email will be delivered in a few minutes. You may identify it by the info [VIRUS IN DIESER MAIL!!!] in the subject line. If you don't have a virus scanner tool with actual patterns we advise you to delete the email without opening it. Please be very careful with this email.

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VIRUS IN EINER MAIL VON <olafmennerich@dev.mcd-panasonic.de>
by Wolfgang Boenschen 07 Sep '01

07 Sep '01

V I R U S A L A R M Unser Viruswall fand folgende Viren: in einer E-Mail an Sie von: <olafmennerich(a)dev.mcd-panasonic.de> Die E-Mail wird mit einer Verz�gerung von einigen Minuten zugestellt. Sie erkennen Sie an dem Hinweis [VIRUS IN DIESER MAIL!!!] im Betreff. Falls Sie nicht im Besitz eines aktuellen Virenscanners sein sollten empfehlen wir Ihnen, die Mail zu loeschen und den Absender zu informieren. Seien Sie auf jeden Fall sehr vorsichtig mit dieser Mail!!! ########################################################################## V I R U S A L E R T Our viruschecker found the virus(es) in an email to you from: <olafmennerich(a)dev.mcd-panasonic.de> The email will be delivered in a few minutes. You may identify it by the info [VIRUS IN DIESER MAIL!!!] in the subject line. If you don't have a virus scanner tool with actual patterns we advise you to delete the email without opening it. Please be very careful with this email.

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VIRUS IN EINER MAIL VON <olafmennerich@dev.mcd-panasonic.de>
by Wolfgang Boenschen 07 Sep '01

07 Sep '01

V I R U S A L A R M Unser Viruswall fand folgende Viren: in einer E-Mail an Sie von: <olafmennerich(a)dev.mcd-panasonic.de> Die E-Mail wird mit einer Verz�gerung von einigen Minuten zugestellt. Sie erkennen Sie an dem Hinweis [VIRUS IN DIESER MAIL!!!] im Betreff. Falls Sie nicht im Besitz eines aktuellen Virenscanners sein sollten empfehlen wir Ihnen, die Mail zu loeschen und den Absender zu informieren. Seien Sie auf jeden Fall sehr vorsichtig mit dieser Mail!!! ########################################################################## V I R U S A L E R T Our viruschecker found the virus(es) in an email to you from: <olafmennerich(a)dev.mcd-panasonic.de> The email will be delivered in a few minutes. You may identify it by the info [VIRUS IN DIESER MAIL!!!] in the subject line. If you don't have a virus scanner tool with actual patterns we advise you to delete the email without opening it. Please be very careful with this email.

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Fwd: Re: Anyone working on Scientific Squeak?
by Galchin Vasili 06 Sep '01

06 Sep '01

More FYI from Squeak land. Regards, Bill Halchin Note: forwarded message attached. __________________________________________________ Do You Yahoo!? Get email alerts & NEW webcam video instant messaging with Yahoo! Messenger http://im.yahoo.com

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