January 2002 - Haskell

Three new lectureships in Nottingham
by Graham Hutton 10 Jan '02

10 Jan '02

Dear all, We are currently advertising three new lectureships in Nottingham. There are no particular research areas specified for these positions, but applications in the area of the Foundations of Programming research group (http://www.cs.nott.ac.uk/Research/fop/) would be most welcome. Further details about the positions are available from: http://www.nottingham.ac.uk/personnel/vacancies/academic.html#LEG/597 The closing date for applications is 1st February 2002. Graham Hutton +---------------------------------------------------------------------+ | Dr Graham Hutton Email : gmh(a)cs.nott.ac.uk | | Foundations of Programming Group Web : www.cs.nott.ac.uk/~gmh | | School of Computer Science and IT | | University of Nottingham | | Jubilee Campus, Wollaton Road | | Nottingham NG8 1BB Phone : +44 (0)115 951 4220 | | United Kingdom Fax : +44 (0)115 951 4254 | +---------------------------------------------------------------------+

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possible collaboration on tutorial with study group?
by David Alan Black 09 Jan '02

09 Jan '02

Hello -- I'm writing at the suggestion of Shae Erisson, who mentioned to me that there was some interest in writing a collaborative online Haskell tutorial. I happen to be involved in organizing something called the Language of the Year project (LotY), which, in brief, involves following up on the advice offered by the Pragmatic Programmers, that advice being that programmers should try to learn at least one new language every year. (See <http://www.pragmaticprogrammer.com>, where there's a Language of the Year link.) The purpose of LotY is for people who are basically capable of teaching themselves new programming languages (not programming novices) to do it in a structured and somewhat synchronized way -- which, for some of us, may be the difference between not doing it and really doing it. Now, here's the thing: Our language of the year for 2002 is Haskell -- which means that a whole bunch of programmers are just now entering into a worldwide Haskell-learning support network. There are various books and resources available to us, of course, and that's fine. But when Shae mentioned that there's talk of a new Haskell tutorial coming into being, that got me thinking. We've got a rather large group of programmers about to learn Haskell, and we might have some Haskell programmers who want to write a new tutorial. Would it be possible, perhaps, to bring those groups together? What I'm thinking is that we, the LotY people, could serve as tutorial testers, while at the same time the tutorial under construction could help us learn Haskell. We could try out exercises, point out unclarities (or clarities for that matter), and generally troubleshoot the project. And there'd be something in it for us, obviously. So, if anyone's thinking along tutorial-writing lines, and would like some tutees, please contact me and we can try to weave that into to the LotY project. Thanks -- David -- David Alan Black home: dblack(a)candle.superlink.net work: blackdav(a)shu.edu Web: http://pirate.shu.edu/~blackdav

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Re: differentiation. Reply
by Dimitre Novatchev 09 Jan '02

09 Jan '02

I wrote: > > super :: (Floating a, RealFrac a) => [a] -> [a] > > super xs = map second (myRepeat improve xs) ^^^^^^^^ The definition must be: > super xs = map second (iterate improve xs) Cheers, Dimitre Novatchev. __________________________________________________ Do You Yahoo!? Send FREE video emails in Yahoo! Mail! http://promo.yahoo.com/videomail/

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Re: differentiation. Reply
by Dimitre Novatchev 09 Jan '02

09 Jan '02

> how about this: > > diff f x = (f (x + epsilon) - f (x - epsilon)) / (2 * epsilon) > where epsilon = 0.00001 > > every call to say "diff sin" will cost you about as much as every call to > "sin" Following John Hughes' article "Why Functional Programming Matters", here's one implementation: > within :: (Num a, Ord a) => a -> [a] -> a > within eps (a:b:xs) | abs(a - b) <= eps = b > | otherwise = within eps (b:xs) > relative :: (Num a, Ord a) => a -> [a] -> a > relative eps (a:b:xs) | abs(a - b) <= eps * abs(b) = b > | otherwise = relative eps (b:xs) > easydiff :: Fractional a => (a -> a) -> a -> a -> a > easydiff f x h = (f(x + h) - f x) / h > halve :: Fractional a => a -> a > halve x = x /2 > log2 x = log(x) / log(2) > elimerror :: Floating a => a -> [a] -> [a] > elimerror n (a:b:xs) = (((b * (2 ** n) - a) / (2 ** n - 1)):elimerror n (b:xs)) > order :: (Num a, Floating b, RealFrac b) => [b] -> a > order (x:y:z:xs) = fromInteger(round(log2((x-z)/(y-z) - 1))) > improve xs = elimerror (order xs) xs > differentiate :: Fractional a => a -> (a -> a) -> a -> [a] > differentiate h0 f x = map (easydiff f x) (iterate halve h0) > super :: (Floating a, RealFrac a) => [a] -> [a] > super xs = map second (myRepeat improve xs) > second (x:y:ys) = y > e2 = within 0.001 (super (differentiate 0.1 (^3) 2)) > data Dif_args = Diffargs {eps, h0, xx :: Float, func :: (Float -> Float)} > myDiff :: Dif_args -> Float > myDiff df = within (eps df) (super (differentiate (h0 df) (func df) (xx df))) > e3 = myDiff Diffargs {eps=0.001, h0=0.1, func= sin, xx=(3.1415926 / 2) } I am new to Haskell and this was my first haskell application. Therefore, I'd appreciate any comments on improving the style of expression or on any defficiencies of this implementation. Cheers, Dimitre Novatchev. __________________________________________________ Do You Yahoo!? Send your FREE holiday greetings online! http://greetings.yahoo.com

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readfile not so lazy
by Hal Daume III 09 Jan '02

09 Jan '02

Why, in the following program: > createFile = unsafePerformIO $ > writeFile "test.xxx" > ('1' : (take 10000000 (repeat '0')) ++ "1") > > processFile = > unsafePerformIO $ > do f <- readFile "test.xxx" > return (dropWhile (=='0') $ > dropWhile (/='0') f) does the amount of memory used by 'processFile' depend on the size of the file created? that is, if the take 10000000 is replaced by 100000000, the execution of processFile takes (around) ten times as much memory. it appears that it's storing all of f in memory even though all it really needs is the last character. how can i make this more lazy? (this was all done in ghc) - Hal

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differentiation. Reply
by S.D.Mechveliani 09 Jan '02

09 Jan '02

Zhe Fu <fuzhezheng(a)hotmail.com> writes > Is there any built-in functions in Haskell to implement > diffential operation and partial diffential operation? > Or can anyone give me some advices about how to implement them > with Haskell? Thanks. There are no differential operators in Haskell-98 standard library, nor even polynomial arithmetic. Very probably, there exist application programs treating differential operators. Try to search in http://www.haskell.org If you are going to implement differential operators, try first to implement, for example, polynomial multiplication. After this, differentiation would not look hard. But it depends on what one wants to differentiate, one has to formulate first what kind of expressions are supposed to be differentiated. ----------------- Serge Mechveliani mechvel(a)botik.ru

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[qforeign] examples/Glob.hsc patch for segv on nomatch
by Jens Petersen 09 Jan '02

09 Jan '02

This patch fixes a problem with the glob binding example in qforeign, where it would segfault when no files were found for the glob exp. Btw, Qrczak, have you considered adding the glibc glob flag extensions to Glob (eg GLOB_BRACE, GLOB_TILDE, etc)? They would be nice to have too, but I guess there is the issue of portability? In that case perhaps they should then going into a separate submodule? Jens ps There is also still the issue with attr_set in the Curses.hsc binding, which I haven't managed to resolve yet (except by working "around" it by using attrset instead). 2002-01-09 Jens Petersen <juhp(a)01.246.ne.jp> * examples/Glob.hsc (glob): Test pathc before trying to read pathv to avoid segfault on nomatch. Index: examples/Glob.hsc =================================================================== RCS file: /cvsroot/qforeign/qforeign/examples/Glob.hsc,v retrieving revision 1.4 diff -u -r1.4 Glob.hsc --- examples/Glob.hsc 2001/03/18 19:40:00 1.4 +++ examples/Glob.hsc 2002/01/09 14:18:14 @@ -81,8 +81,12 @@ (#const GLOB_NOMATCH) -> GlobNoMatch (#const GLOB_NOSPACE) -> GlobNoSpace _ -> GlobOther - pathv <- (#peek glob_t, gl_pathv) ptr - paths <- peekArray0 nullPtr pathv >>= mapM peekCString + pathc <- (#peek glob_t, gl_pathc) ptr :: IO CUInt + paths <- if pathc > 0 then + do + pathv <- (#peek glob_t, gl_pathv) ptr + peekArray0 nullPtr pathv >>= mapM peekCString + else return [] globfree ptr return (res, paths)

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Extended deadline
by icalp2002＠informatica.uma.es 09 Jan '02

09 Jan '02

We apologize for possible multiple postings. One week extension of paper submission deadline and invited speakers ------------------------------------------------------------------------- Within the last two weeks the conference chairs have received multiple requests to extend the paper submission deadline. In response to it, the submission deadline for papers is extended for a week (until 23:59:59 GMT of 21th January). Note that we will not be able to extend this deadline any further. The complete list of invited speakers is as follows: Manuel Hermenegildo Heikki Mannila Madhav Marathe Andrew Pitts John Reif The 2002 Gödel Prize and The 2002 EATCS Prize: Maurice Nivat For information about the conference visit the web page: http://www.lcc.uma.es/ICALP2002 Best Regards,

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by thefortune＠cc.com 09 Jan '02

09 Jan '02

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how to implement the differential operation with Haskell?
by Zhe Fu 09 Jan '02

09 Jan '02

Is there any built-in functions in Haskell to implement diffential operation and partial diffential operation? Or can anyone give me some advices about how to implement them with Haskell? Thanks.

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