October 2001 - Haskell

numbers game
by Graham Hutton 18 Oct '01

18 Oct '01

Dear all, The following new paper may be of interest to readers of this group: http://www.cs.nott.ac.uk/~gmh/countdown.pdf It shows how to develop a Haskell program to solve the numbers game from countdown, a popular quiz show on British television. The aim wasn't to produce solutions as fast as possible (although in absolute terms the program actually performs rather well), but rather to show how the program itself could be developed in systematic way in conjunction with a proof of its correctness. It's the kind of example that could be covered in one or two lectures as part of a Haskell course. Have fun! Graham p.s. The paper will appear as a functional pearl in JFP. +---------------------------------------------------------------------+ | 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 | | United Kingdom | +---------------------------------------------------------------------+

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proofs
by Richard 18 Oct '01

18 Oct '01

I want to learn how to go about proving things like (p >>= j) >= k == p >>= (\x->(j x >>= k)) where p >>= k = \s0 -> let (s1, a) = p s0 in k a s1 eg, verifying that the 3 monad laws hold for a specific monad. also, verifying the more numerous laws every arrow should obey. I could teach myself to do it clumsily, but I want to learn from others. would learning category theory help me do this? pointers to documents? proof-assistant software?

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RE: Haskell 98 - Standard Prelude - Floating Class
by Simon Peyton-Jones 18 Oct '01

18 Oct '01

| > 1. Actually, I wouldn't even call that "default | definitions". These ARE | > definitions of sinh and cosh. | | Mathematically, yes. Numerically, no. Even if 'exp' is | implemented with high accuracy, the suggested defaults may | return a very inaccurate (in ulps) result. Take sinh near | zero. sinh(x) with x very close to 0 should return x. With | the above 'default' sinh(x) will return exactly 0 for a | relatively wide interval around 0, which is the wrong result | except for 0 itself. Indeed. The proposal is only to give "default declarations" in the class defn for sinh, cosh, and perhaps as Lennart suggests asinh, acosh, atanh. They give a reasonable first cut if you don't write definitions yourself. But you can overrride them at will. The only reason not to do this (which amounts to giving a default decl of "error") is if the default decl is so awful that it's badly misleading to provide it. Which doesn't look true in this case. I don't propose to change any of the actual instance declarations, because (as Lennart says) it's entirely possible that they have better numerical properties than the default declarations. Simon

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Re: proofs
by Janis Voigtlaender 18 Oct '01

18 Oct '01

I wrote: > Rijk-Jan van Haaften writes: > > > I used 3 main properties for the process: > > ... > > 2. Lifting local let definitions to a higher level > > let > > f = > > let > > g = gExpr > > in > > fExpr > > in > > expr > > > > can be translated into > > ... > > Care has to be taken if g (which might be bound outside this let-block) occurs > in expr, because with the translated version it is now bound to gExpr. Renaming > helps (for some unused g'): > > let > > g' = gExpr > > f = fExpr[g <- g'] > > in > > expr Oops: let g' = gExpr[g <- g'] f = fExpr[g <- g'] in expr Hope, I got it right now. Janis. -- Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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Re: proofs
by Janis Voigtlaender 18 Oct '01

18 Oct '01

Rijk-Jan van Haaften writes: > I used 3 main properties for the process: > ... > 2. Lifting local let definitions to a higher level > let > f = > let > g = gExpr > in > fExpr > in > expr > > can be translated into > let > g = gExpr > f = fExpr > in > expr > > The definition of g which is local for f in the first > expression is made more global in the second > one. Care has to be taken if g (which might be bound outside this let-block) occurs in expr, because with the translated version it is now bound to gExpr. Renaming helps (for some unused g'): > let > g' = gExpr > f = fExpr[g <- g'] > in > expr -- Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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ANNOUNCE: linkchk-0.01 released
by Jens-Ulrik Petersen 18 Oct '01

18 Oct '01

Two months after the first release, linkchk version 0.01 is released! New in this version: * displays ping time instead of ok status * IPv6 support * command option to set delay between pings * uses netstat instead of /proc/net/route * uses popenhs * uses automake and autoconf The source and rpm files are available from: http://www.01.246.ne.jp/~juhp/haskell/linkchk/ Feedback, comments and code contributions appreciated, Jens

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please add me
by lo janus 18 Oct '01

18 Oct '01

me(a)lojanus.com

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Examples of some nice Haskell programs
by Andre W B Furtado 17 Oct '01

17 Oct '01

There are some nice examples (binaries) of [Haskell + HOpenGL] programs at: www.cin.ufpe.br/~haskell/hopengl/examples.html I think they may be of interest to the community because they confirm that it's really possible to use Haskell in the development of complex, fast and robust graphic applications and games that involve animation and other interesting issues (texturing, lighting, checking and analyzing multiple user inputs, etc.) -- Andre

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ANNOUNCE: A Haskell-CORBA interface
by Zoltan.2.Varga＠nokia.com 17 Oct '01

17 Oct '01

Dear All, I'm happy to announce the first release of my Haskell-CORBA interface package, which is available from: http://sourceforge.net/projects/haskell-corba/ This package allows Haskell programmers to write CORBA clients and servers using the MICO open-source CORBA implementation (www.mico.org) It defines a Haskell language mapping for CORBA, and includes an IDL compiler which generates Haskell stub and skeleton modules from IDL files. Comments and suggestions are welcome Zoltan

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ANNOUNCE: popenhs-0.00 released
by Jens-Ulrik Petersen 17 Oct '01

17 Oct '01

I am happy to announce the first release of popenhs, version 0.00. popenhs is a very small Haskell library, based on runProcess in hslibs' posix library. It provides lazy output from subprocesses through two functions popen2 and popen3. The source and rpm files are available from: http://www.01.246.ne.jp/~juhp/haskell/popenhs/ I welcome comments and feedback. Jens

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