December 2001 - Haskell

installation problems with graphics-2.0.4
by Bernhard Reus 20 Dec '01

20 Dec '01

The graphics lib of the new December release does not compile with the shipped makefile. The makefile does not do anything at all??? Maybe there is a problem with wildcard extensions? To be honest, I have no clue. I am using a Sparc5/Solaris machine. Any help appreciated. Thanks in advance, Bernhard

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global counters
by Hal Daume III 20 Dec '01

20 Dec '01

Please, no tirade about banishing unsafePerformIO. I've seen this done before I just don't remember how. I want to use a state monad to count things, but don't want to monadify the thing I'm using the counter in. basically, i want a function getVar :: () -> String which returns a new string every time. i tried this: > curVarId = newSTRef (0 :: Integer) > > {-# NO-INLINE newVar -} > > newVar = \_ -> ('\0' : show x) > where y = unsafePerformIO (stToIO curVarId) > x = unsafePerformIO $ stToIO $ > do x <- readSTRef y > writeSTRef y (x + 1) > return x and in ghci it works wonderfully, but when i actually compile, all i get is "\NUL0". what's the proper way to write this? thanks - hal

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Working Conference on Generic Programming, 2nd call for papers
by Johan Jeuring 20 Dec '01

20 Dec '01

WCGP '02 IFIP WG2.1 Working Conference on GENERIC PROGRAMMING http://www.generic-programming.nl/wcgp/cfp.html Organised in conjunction with MPC'02 July 8 - July 13, 2002 Dagstuhl, Germany CALL FOR PAPERS Generic programming is about making programs more adaptable by making them more general. Generic programs often embody non-traditional kinds of polymorphism; ordinary programs are obtained from them by suitably instantiating their parameters. In contrast with normal programs, the parameters of a generic programs are often quite rich in structure. For example they may be other programs, types or type constructors, class hierarchies, or even programming paradigms. Generic programming techniques have always been of interest, both to practitioners and to theoreticians, but only recently have generic programming techniques become a specific focus of research in the functional and object-oriented programming language communities. This working conference will bring together leading researchers in generic programming from around the world, and feature papers capturing the state of the art in this important emerging area. We welcome contributions on all aspects, theoretical as well as practical, of generic programming, aspect-oriented programming, polytypic programming, adaptive object-oriented programming, generic components, and so on. SUBMISSION Full papers should be submitted in Postscript or pdf format by e-mail to reach Jeremy.Gibbons(a)comlab.ox.ac.uk by February 16, 2002. The details of the submission procedure can be found at http://www.generic-programming.nl/wcgp/submit.html Although there is no page limit, submissions should strive for brevity and clarity. IMPORTANT DATES Submission February 16, 2002 Notification April 12, 2002 Final version due May 24, 2002 PROGRAMME COMMITTEE Matt Austern Eerke Boiten Ulrich Eisenecker Jeremy Gibbons (co-chair) Ralf Hinze Johan Jeuring (co-chair) Gary Leavens Karl Lieberherr Lambert Meertens Eugenio Moggi Bernhard Moeller Oege de Moor David Musser Martin Odersky Ross Paterson Simon Peyton Jones Colin Runciman Doaitse Swierstra Stephanie Weirich LOCAL ORGANISATION Jeremy Gibbons Johan Jeuring Bernhard Moeller CORRESPONDENCE Jeremy Gibbons (Jeremy.Gibbons(a)comlab.ox.ac.uk) Johan Jeuring (johan(a)jeuring.net)

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x^y. Reply
by S.D.Mechveliani 20 Dec '01

20 Dec '01

Toralf Wittner <wittner(a)uni-duesseldorf.de> writes > [..] > data PowerNum = INT Integer | DBL Double deriving (Eq, Show, Read) > [..] > Now it basically works. However wouldn't it have been easier to write > something like this: > > powPos :: Integer -> Integer -> Integer > [..] > powNeg :: Integer -> Integer -> Double > [..] > | y < 0 = 1 / fromInteger x * powNeg x (y + 1) > > Initially I wanted something as terse as the Python version, now I either > have to write two functions or I need to define a type. Is there not an > easier way to come as close as possible to the Python version? > > Thanks anyway - learnt a lot! For this particular task, the most natural solution is, probably, pow :: Fractional a => a -> Integer -> a pow a n = if n > 0 then a^n else (1/a)^(- n) (^) is of standard, only pow adds the facility of negative n. Then, you need each time to convert the argument to appropriate type of Fractional: pow (fromInteger 2 :: Ratio.Rational) 2 --> 4 % 1 pow (fromInteger 2 :: Ratio.Rational) (-2) --> 1 % 4 pow (2 :: Double) (-2) --> 0.25 pow (2 :: Integer) (-2) --> ... No instance for (Fractional Integer) If you replace standard (and not lucky) Fractional with some class Foo (with multiplication mul and division div), make Integer an instance of Foo (where div may fail for some data), and program pow :: Foo a => a -> Integer -> a via mul, div, then it would work like this: pow (2 :: Integer) 2 --> 4 pow (2 :: Integer) (-2) --> "Error: cannot invert 2 :: Integer" pow (2 :: Rational) (-2) --> 1%4 Another way is to try to straggle with overlapping instances by defining something like this: class Pow a b where pow :: a -> Integer -> b instance Num a => Pow a a where pow = (^) Fractional ? instance Num a => Pow Integer a where pow = ? If this succeeds, there will be also no need in new type constructors. ----------------- Serge Mechveliani mechvel(a)botik.ru

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Re: x^y. Reply
by Toralf Wittner 19 Dec '01

19 Dec '01

Ahh - sorry, too much cut and paste ;-) data PowerNum = INT Integer | DBL Double deriving (Eq, Show, Read) pow :: PowerNum -> Integer -> PowerNum pow x y = z x y where z (INT x) y = if y > 0 then INT $ powInteger x y else if y == 0 then INT $ 1 else DBL $ powDouble (fromInteger x) y z (DBL x) y = DBL $ powDouble x y powInteger x y | x == 0 = 0 | y == 0 = 1 | y > 0 = x * powInteger x (y - 1) powDouble x y | x == 0 = 0 | y == 0 = 1 | y < 0 = 1 / x * powDouble x (y + 1) Now it basically works. However wouldn't it have been easier to write something like this: module Main where import System main = do [a1, a2] <- getArgs let x = read a1 let y = read a2 in if y >= 0 then putStrLn (show x ++ " ^ " ++ show y ++ " = " ++ show (powPos x y)) else putStrLn (show x ++ " ^ " ++ show y ++ " = " ++ show (powNeg x y)) powPos :: Integer -> Integer -> Integer powPos x y | x == 0 = 0 | y == 0 = 1 | y > 0 = x * powPos x (y - 1) powNeg :: Integer -> Integer -> Double powNeg x y | x == 0 = 0 | y == 0 = 1 | y < 0 = 1 / fromInteger x * powNeg x (y + 1) Initially I wanted something as terse as the Python version, now I either have to write two functions or I need to define a type. Is there not an easier way to come as close as possible to the Python version? Thanks anyway - learnt a lot! Toralf

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x^y. Reply
by S.D.Mechveliani 19 Dec '01

19 Dec '01

Toralf Wittner <wittner(a)uni-duesseldorf.de> writes > [..] > power x y > [..] > | y > 0 = x * power x (y-1) > | y < 0 = 1 / fromInteger x * power x (y+1) > > One recognizes that the function returns either an integer value > if y > 0 or a float value if y < 0. Therefore I can't write a > signature like > pow :: Integer -> Integer -> Integer nor can I do > pow :: Integer -> Integer -> Double. > > [..] > How then would I write this function in Haskell (concerning types)? The type Rational fits the case n < 0 too, and it includes Integer. But if you still need Integer | Double, you can, for example, introduce a new type of a disjoint union of the above two, and then, to compute like this: pow (Intg 2) 2 --> Intg 4 pow (Intg 2) (-2) --> D 0.25 pow (D 2.0) (-2) --> D 0.25 This is achieved by data PowerDom = Intg Integer | D Double deriving(Eq,Show) pow :: PowerDom -> Integer -> PowerDom pow x n = p x n where p (Intg m) n = if n > 0 then Intg $ powerInteger m n else D $ powerDouble (fromInteger m :: Double) n p(D d) n = D $ powerDouble d n powerInteger m n = m^n :: Integer powerDouble :: Double -> Integer -> Double powerDouble d n = ... usual way for float - something like this. ----------------- Serge Mechveliani mechvel(a)botik.ru

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RE: FiniteMap for standard
by Simon Peyton-Jones 19 Dec '01

19 Dec '01

Much, much too late, however desirable. | -----Original Message----- | From: S.D.Mechveliani [mailto:mechvel@math.botik.ru] | Sent: 19 December 2001 13:25 | To: haskell(a)haskell.org | Subject: FiniteMap for standard | | | Long ago I suggested to include FiniteMap into Haskell-98 | standard library. | Is it still late to do this, or maybe, the situation has changed? | | ----------------- | Serge Mechveliani | mechvel(a)botik.ru | | _______________________________________________ | Haskell mailing list | Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell |

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ICALP2002 call for papers
by icalp2002＠lcc.uma.es 19 Dec '01

19 Dec '01

We apologize for possible multiple postings. In http://www.lcc.uma.es/icalp2002 you can find a pdf version of this call for paper. +++++++++++++++++++++++++++++++++++++ Last information: Second version of the WEB pages at http://www.lcc.uma.es/icalp2002 Workshops confirmed: Computability and Complexity in Analysis (CCA 2002) Algorithmic Methods and Models for Optimization of Railways ATMOS 2002 7 th International Workshop on Formal Methods for Industrial Critical Systems Foundations of Wide Area Network Computing Invited speakers confirmed: Heikki Mannila Manuel Hermenegildo +++++++++++++++++++++++++++++++++++++ Call for Papers ICALP 2002 29th International Colloquium on Automata, Languages and Programming July 8-13, 2002, Málaga, Spain Camera Ready: April 16, 2002 The 29th annual meeting of the European Association of Theoretical Computer Science will be held in Málaga, Spain, July 8-13, 2002 (at the E.T.S. Ingeniería Informática). As with the Journal Theoretical Computer Science (TCS), the scientific program of the Colloquium will be split into two parts: Track A of the meeting will correspond to Algorithms, Automata, Complexity and Games, while Track B to Logic, Semantics and Theory of Programming. SUBMISIONS: Authors are invited to submit extended abstract of their papers, presenting original contributions to the theory of computer science. Detailed instructions for paper submissions will be found on the conference webpage (http://www.lcc.uma.es/icalp2002) and in future calls for papers. Submissions should consist of: a cover page, with the author's full name, address, fax number, e-mail address, a 100-word abstract, keywords and to which track (A or B) the paper is being submitted and an extended abstract describing original research. At least one author of an accepted paper should be available to present it at the conference. Simultaneous submission to other conferences with published proceedings is not allowed. Further information (dates and instructions for submissions, workshops, registration, location and travel) will be provided in future announcements. ORGANIZING COMMITEE: Buenaventura Clares (University of Granada), Ricardo Conejo (University of Málaga), Inmaculada Fortes (University of Málaga), Llanos Mora (University of Málaga), Rafael Morales (co-Chair, University of Málaga), Marlon Nuñez (University of Málaga), José Luis Pérez de la Cruz (University of Málaga), Gonzalo Ramos (University of Málaga), Francisco Triguero (co-Chair, University of Málaga), José Luis Triviño (University of Málaga). IMPORTANT DATES: Workshops proposal: November 8, 2001 Submissions: January 14, 2002 Notification: March 20, 2002 CONFERENCE CO-CHAIRS Prof. Francisco Triguero Prof. Rafael Morales Universidad de Málaga E.T.S. Ingeniería Informática Dept. Lenguajes y Ciencias de la Computación Bulevar Louis Pasteur, 35 29071 - Málaga (SPAIN) e-mail: icalp2002(a)informatica.uma.es PROGRAM COMMITEE Track A Ricardo Baeza-Yates (U. Chile) Volker Diekert (U. Stuttgart) Paolo Ferragina (U. Pisa) Catherine Greenhill (U. Melbourne) Torben Hagerup (U. Frankfurt) Johan Håstad (KTH, Stockholm) Gabriel Istrate (Los Alamos) Claire Kenyon (U. Paris XI) Der-Tsai Lee (Acad. Sinica, Taipei) Heikki Mannila (Nokia, Helsinki) Elvira Mayordomo (U. Zaragoza) Helmut Prodinger (U. Witwatersrand, South Africa) Jan van Leeuwen(U. Utrecht) Paul Vitányi (CWI, Amsterdam) Peter Widmayer (ETH Zürich) (Chair) Gerhard Woeginger (T.U. Graz) Christos Zaroliagis (U. Patras) Track B Martín Abadi (Bell Labs Research, Lucent) Roberto Amadio (U. Provence) Gilles Barthe (INRIA-SophiaAntipolis) Manfred Droste (University of Technology Dresden) Cédric Fournet (Microsoft Cambridge) Matthew Hennessy (U. Sussex) (Chair) Furio Honsell (U. Udine) Peter O'Hearn (Queen Mary & W. C. London) Fernando Orejas (U.P.Catalunya) Ernesto Pimentel (U. Málaga) David Sands (Chalmers University of Technology and Göteborg University) Dave Schmidt (U. Kansas) Gheorghe Stefanescu (U. Bucharest) Vasco Vasconcelos (U. Lisbon) Thomas Wilke (U. Kiel) +++++++++++++++++++++++++++++++++++++++++++ Malaga University uses Christmas holidays for backup and maintenance of his network. If you have problem to arrive to ICALP 2002 main page, please retry again later. If you get to the page http://www.lcc.uma.es but not to the page http://www.lcc.uma.es/icalp2002 contact with us: conejo(a)lcc.uma.es morales(a)lcc.uma.es

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FiniteMap for standard
by S.D.Mechveliani 19 Dec '01

19 Dec '01

Long ago I suggested to include FiniteMap into Haskell-98 standard library. Is it still late to do this, or maybe, the situation has changed? ----------------- Serge Mechveliani mechvel(a)botik.ru

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RE: gcd 0 0 = 0
by Simon Peyton-Jones 19 Dec '01

19 Dec '01

If everyone likes this I'll put it in; otherwise I'll simply state that gcd 0 0 is defined to be 0. Christoph does not like this, but the weight of world opinion seems to be fairly clearly in favour of gcd 0 0 = 0. Let's try to wrap this one up. Simon | -----Original Message----- | From: Alan Bawden [mailto:Alan@LCS.MIT.EDU] | Sent: 17 December 2001 18:45 | To: haskell(a)haskell.org; Simon Peyton-Jones | Subject: Re: gcd 0 0 = 0 | | | From: Lars Henrik Mathiesen <thorinn(a)diku.dk> | Date: 17 Dec 2001 14:50:21 -0000 | ... | In case it isn't clear already, these definitions make a lattice on | the positive integers, with divides ~ leq, gcd ~ meet and | lcm ~ join, | using the report's definitions of gcd and lcm. | | Indeed, that's a nice way of putting it. How about if the report just | says: | | In order to make the non-negative integers into a lattice | under `gcd' | and `lcm', we define `gcd 0 0 = 0'. |

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