December 2001 - Haskell

random
by GACOGNE Louis equipe Bernadette Bouchon-Meunier 19 Dec '01

19 Dec '01

Is there a random function in Hugs 98 on macintosh. I found something about random in the library but I was not able to find a random function running. Louis Gacogne IIE-CNAM allee Jean Rostand 91025 Evry (tel 01 69 36 73 41) Laboratoire d'Informatique de l'Universite Paris VI Place Jussieu 75252 Paris 5 (tel 01 44 27 88 07) http://www.iie.cnam.fr/~gacogne/

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x^y
by Toralf Wittner 19 Dec '01

19 Dec '01

Hello, you can count me as a newbie in functional programming. I'm attempting to define a function that computes the value of x^y for whole numbers. Intuively (all efficiency considerations aside) I would start with something like this: power x y | x == 0 = 0 | y == 0 = 1 | 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. If I would use Python I could write something like this: def power(m, n): if m == 0: return 0 if n == 0: return 1 if n > 0: return m * power(m, (n-1)) if n < 0: return (1 / float(m)) * power(m, (n+1)) How then would I write this function in Haskell (concerning types)? Thanks in advance! Kind regards, Toralf

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ANN (bis): Release 0.1 of Haskell/Java VM Bridge
by Ashley Yakeley 19 Dec '01

19 Dec '01

OK, the SourceForge file release system bug has been fixed, so I put the release files up there. Please download from there rather than from semantic.org. <http://sourceforge.net/project/showfiles.php?group_id=32318&release_id=654 34> Revised, expanded announcement... ------- The first release, 0.1, of Haskell/Java VM Bridge is now available. Haskell/Java VM Bridge allows Haskell programs access to the Java Virtual Machine. Features include: * Calling of Java methods, access to fields, array-handling; * Integration of garbage collectors; * Reconciliation of exception models; * Reconciliation of thread models, including 'synchronized' monitor support and the ability to fork Haskell actions as Java threads; * Strong typing for Java classes and method argument lists (as tuples), and use of Haskell lists as arrays; * Creation of Java classes 'on the fly' (using DefineClass and the Java Class File Format) that can have Haskell callback methods; * Lifted monads which do all the necessary JNI class and method/field ID preloading and 'JNIEnv'-pointer variable handling for you -- these can be automatically generated via a tool (MakeJVMModule); * Layered design includes lower-level IO-based interface; * Automatic generation of modules for Java API classes; * All relevant imports, flags and libraries etc. handled by a single GHC package 'javavm'; * No 'unsafe' Haskell calls or pure function FFI anywhere. This release should be 'beta' quality, but has undergone little testing. You will need: * An x86 machine running some form of Unix; * GHC 5.02 or later; * The appropriate JVM, installed in the appropriate place: - IBM JDK/JRE 1.3 for x86, installed in /usr/lib/ibm-java/IBMJava2-13/ - Blackdown Port of Sun's JRE 1.3/Sun JDK 1.3 for x86, installed in /usr/lib/j2sdk1.3/ and /usr/lib/j2re1.3/ No documentation is currently available, sorry. Two examples have been included: a trivial "hello world" program, and a program that shows a Java Frame containing an instance of a Haskell-defined subclass of Canvas that has a Haskell 'paint' method that draws an oval. You should be able to figure out most of it from that... and of course the source is available from SourceForge CVS. <http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/jvm-bridge/source/> Be sure to download the correct .tar.gz file for your Java VM. Simple installation instructions are included in the file. <http://sourceforge.net/project/showfiles.php?group_id=32318&release_id=654 34> Haskell/JVM Bridge and source code is licensed under the GNU Lesser GPL. <http://sourceforge.net/projects/jvm-bridge/> Future plans: * documentation * port to Cygwin * port to Darwin/OS X (waiting on GHC) * use of Java 1.4 API (waiting on Sun, though use of beta versions is possible) -- Ashley Yakeley, Seattle WA

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gcd oops
by Michael Ackerman 19 Dec '01

19 Dec '01

Sorry for an error in my previous message. The definition there of a gcd works only in a prinicpal ideal domain (which covers all the rings mentioned in the examples). According to Bourbaki, chapter on ordered groups, the gcd of two non-zero elements of a UFD A is well-defined as an element of (A-{0})/units, a quotient monoid. So in this context gcd(0, _) is undefined. But Bourbaki adds that this concept is `sometimes' extended to the gcd of a finite family (x_i) of elements some of which may be zero by taking the gcd to be an element d such that for all z, z divides d iff z divides each x_i. Cheers, Michael Ackerman

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RE: gcd 0 0 = 0
by Simon Peyton-Jones 18 Dec '01

18 Dec '01

| Probably, the best specification would be | | gcd n m :: Integer = if n == 0 && m == 0 then 0 | else | greatest integer that divides both n and m Well, thank you all those that have contributed. My original point was simply to say greatest (positive) integer that divides both n and m but debate seems to have swirled round whether (gcd 0 0) should be 0 or an error. Currently in H98 it's an error; but it is the kind of thing I'm willing to change IF there is a consensus, because it will only make more programs work. Is there a consensus that such a change would be desirable? If someone could write a sentence or two to explain why gcd 0 0 = 0, (ideally, brief ones I can put in the report by way of explanation), I think that might help those of us who have not followed the details of the discussion. Simon

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RE: space efficiency question
by Simon Peyton-Jones 18 Dec '01

18 Dec '01

Now: if I have a couple of thousand of these Instances, created with the same Attributes value, as in: a = Attributes ["a","bc"] i0000 = Instance a [1,2] i0001 = Instance a [1,1] ... i7896 = Instance a [2,1] can I be assured that there are not thousands of actual copies of a but that they simply have a pointer to a ? I'm interested in the answer for both ghc (compiled/interpreted) as interpreted Hugs. Yes for GHC: you'll only get one copy of 'a'. Simon

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Haskell 98 IO
by Simon Peyton-Jones 18 Dec '01

18 Dec '01

| Anyway, what should the report say? I think it is reasonable | to expect that stdin & stdout should both be unbuffered in | order for interact to work right. So the defn of interact should be | | interact f = do | hSetBuffering stdin NoBuffering -- new | hSetBuffering stdout NoBuffering -- new | s <- hGetContents | putStr (f s) Simon Marlow raises a good point here. If 'interact' is to be interactive it had better be unbuffered. So I propose to add the two new lines above to the Report. This does not change the intended behaviour at all. Simon

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RE: Confused about default
by Simon Peyton-Jones 18 Dec '01

18 Dec '01

| Let me try to clarify what I was asking: | | What is a monotype? For example, is "[Foo]" a monotype and is | "Foo -> Int" a monotype? The context free grammar implies | that a monotype is identical to a type, in which case why is | a different name used? A monotype is a type with no for-alls in it. But you are right that it's confusing that this is the only occurrence. I think the simplest thing is to change 'monotype' to 'type' here; after all, Num (forall a.a->a) does not hold! thanks for pointing this out. Simon

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ANNOUNCE: hsclock-0.01.0 release
by Jens-Ulrik Petersen 18 Dec '01

18 Dec '01

I am pleased to announce the initial release of hsclock, another gtk+hs "applet" I've written. hsclock is an accurate multi-zone gtk clock, which can also run in a tty. hsclock uses gtk timeouts to synchronise the time updates to occur on the second or minute "tick" (dependent on the clock format string), making display updates accurate to within milliseconds. More info, source and rpm's are available at: http://www.01.246.ne.jp/~juhp/haskell/hsclock/ As usual feedback and comments welcome. Enjoy, Jens

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Re: gcd 0 0 = 0
by George Russell 18 Dec '01

18 Dec '01

I've reconsidered my earlier position and think now that the Prelude is wrong to make gcd 0 0 an error, and should return 0. It probably doesn't make much difference to anyone, but it's like 1 not being a prime; it may be slightly harder to explain, but it makes the maths come out nicer and is in the end a simplification.

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