- Haskell-Cafe - Haskell.org

ICFP07 Call for Participation
by Matthew Fluet (ICFP Publicity Chair) 06 Aug '07

06 Aug '07

===================================================================== Call for Participation The 12th ACM SIGPLAN International Conference on Functional Programming (ICFP 2007) http://www.informatik.uni-bonn.de/~ralf/icfp07.html Freiburg, Germany, 1-3 October 2007 ===================================================================== ICFP 2007 provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. The conference covers the entire spectrum of work, from practice to theory, including its peripheries. Preliminary program: * http://www.informatik.uni-bonn.de/~ralf/schedule.html * Invited speakers: + John Hughes (Chalmers University of Technology) + Frank Pfenning (Carnegie Mellon University) + John Lloyd (Australian National University) * The Program committee has *deliberately* created extra breaks so as to give participants more time to talk with colleagues. Schedule including related workshops: * 30 Sep: ACM SIGPLAN Haskell Workshop * 30 Sep: ACM SIGPLAN Workshop on Scheme and Functional Programming * 1-3 Oct: ICFP07 * 4 Oct: ACM SIGPLAN Commercial Users of Functional Programming * 4 Oct: ACM SIGPLAN Workshop on Mechanizing Metatheory * 5 Oct: ACM SIGPLAN Erlang Workshop * 5 Oct: ACM SIGPLAN Workshop on ML * 5 Oct: ACM SIGPLAN Programming Languages meets Program Verification Registration information: * http://proglang.informatik.uni-freiburg.de/ICFP2007/registration.shtml * Early registration deadline: September 7, 2007 Accommodations information: * http://proglang.informatik.uni-freiburg.de/ICFP2007/accommodation.shtml * Conference reservation/rate deadline: September 1, 2007 * September/October is Freiburg's main tourist season; participants are advised to book rooms as early as possible. Conference organizers: * General Chair: Ralf Hinze (Universität Bonn) * Program Chair: Norman Ramsey (Harvard University) * Local Arrangements Chair: Peter Thiemann (Universität Freiburg) * Workshop Co-Chairs: Graham Hutton (University of Nottingham) and Matthias Blume (Toyota Technological Institute at Chicago) * Programming Contest Chair: Johan Jeuring (Universiteit Utrecht) * Publicity Chair: Matthew Fluet (Toyota Technological Institute at Chicago)

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Zippers, Random Numbers & Terrain
by Thomas Conway 06 Aug '07

06 Aug '07

Hi All, To amuse myself while waiting for test-runs to complete, I was thinking about random terrain generation. I came across a bunch of nice posts by Torben Mogensen, where he describes a neat way of constructing random terrains by recursively subdividing right angled isosceles triangles. It got me thinking - it's all well and good subdividing to give more detail as you zoom in, but what about when you zoom out? This got me thinking that it would be cool to make an infinite terrain generator using a zipper, so you can zoom in/out infinitely, and by implication, infinitely in any direction. One of the key components that seems to be necessary is a random number generator zipper. In Mogensen's scheme, you have a number associated with each point, and when you subdivide, you create a new RNG seed from the numbers at each end of the hypotenuse which you are bisecting. These numbers are used to generate height variation. The trick, is to make the combination order independent (e.g. xor). This is easy for zooming in, but it's not clear how to do this for zooming out. It's probably sufficient to assume a (parameterizable) hashing/mixing scheme, and to simply number the nodes in some deterministic fashion. The subdivision is binary, so we could number the children deterministically. If we use "decimals", from some arbitrary starting point, we can extend in the "fractional" direction when we zoom in, and extend in the "whole number" direction. I'm only just discovering zippers, so my question to the learned members of this forum is: Am I on the right track? Is a scheme like this going to work? cheers, Tom -- Dr Thomas Conway drtomc(a)gmail.com Silence is the perfectest herald of joy: I were but little happy, if I could say how much.

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how to make haskell faster than python at finding primes?
by Alex Jacobson 06 Aug '07

06 Aug '07

This implementation of calculating 10000 primes (compiled with GHC -O2) is 25% slower than the naive python implementation. Shouldn't it be faster? Am I doing something obviously stupid? primes = map (\(x,_,_)->x) $ filter (\(_,isP,_)->isP) candidates candidates = (2,True,[]):(3,True,[]): map next (tail candidates) next (candidate,isP,modCounts) = let newCounts = map incrMod2 modCounts in -- accumulate mods (candidate+2 -- we only need to bother with odd numbers ,(isPrime newCounts) -- track whether this one is prime ,if isP then (candidate,2):newCounts else newCounts) -- add if prime isPrime = and . map ((/=0).snd) incrMod2 (p,mc) = let mc' = mc+2 in if mc'<p then (p,mc') else if mc'>p then (p,1) else (p,0) Note: It is shorter than the python, but I would have assumed that GHC could deliver faster as well. -Alex-

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Navigating Haddock
by David Pollak 06 Aug '07

06 Aug '07

Howdy, As I'm starting to learn the Haskell libraries, I'm having a less than fun time trying to figure out what functions operate on what types. For example, in the documentation for HaXml, there's a description of Document: http://www.cs.york.ac.uk/fp/HaXml/HaXml/Text-XML-HaXml-Types.html#4 However, I can't find any links to what functions accept document as a parameter. Am I missing some magic? A couple of other questions... Can ByteStrings be substituted anywhere that takes a String (e.g., HaXml xmlParse)? Is there a way to deal with UTF-8 encoded byte arrays converting them to Strings other than http://www.gatago.org/fa/haskell/52409638.html ? Thanks, David -- lift, the fast, powerful, easy web framework http://liftweb.net

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c2hs and structs?
by Magnus Therning 06 Aug '07

06 Aug '07

I can't seem to find any information on how to deal with C functions that return a (pointer to a) struct. C2hs tells me there's no automatic support for marshalling structs (I'm using version 0.14.5). If I'm to do it by hand, is there a preferred way? (E.g. make the type adhere to the type Storable.) /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus(a)therning.org Jabber: magnus.therning(a)gmail.com http://therning.org/magnus Unreadable code, Why would anyone use it? Learn a better way. -- Geoff Kuenning's contribution to the 2004 Perl Haiku Contest, Haikus about Perl - 'Dishonerable Mention' winner

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Perfect example
by Jon Harrop 06 Aug '07

06 Aug '07

Any suggestions for a perfect example that uniquely demonstrates the benefits of the Haskell language compared to other languages? -- Dr Jon D Harrop, Flying Frog Consultancy Ltd. OCaml for Scientists http://www.ffconsultancy.com/products/ocaml_for_scientists/?e

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Re: creating graphics the functional way
by Shin-Cheng Mu 06 Aug '07

06 Aug '07

On 05/08/07, Frank Buss <fb(a)frank-buss.de> wrote: > Is it possible to write functions with an arbitrary number of > arguments? > Would be nice if the average function would accept any number of pixel > values. You may be interested to see Oleg Kiselyov's discussion on polyvariadic functions in Haskell: http://okmij.org/ftp/Haskell/types.html#polyvar-fn sincerely, Shin

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Re: [Haskell-cafe] how to make haskell faster than python at finding primes?
by dons＠cse.unsw.edu.au 06 Aug '07

06 Aug '07

ptanimoto: > I haven't tried any of those codes, but Don did you notice you're > calculating 1,000 primes while Alex was talking about 10,000? Yeah, I tested both with 1k and 10k :)

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Re: [Haskell-cafe] How odd...
by Andrew Coppin 06 Aug '07

06 Aug '07

> >> Um... why would infinity * 0 be NaN? That doesn't make sense... > Infinity times anything is Infinity. Zero times anything is zero. So > what should Infinity * zero be? There isn't one right answer. In > this case the "morally correct" answer is zero, but in other contexts > it might be Infinity or even some finite number other than zero. I don't follow. Infinity times any positive quantity gives positive infinity. Infinity times any negative quantity gives negative infinity. Infinity times zero gives zero. What's the problem? > Consider 0.0 / 0.0, which also evaluates to NaN. Division by zero is *definitely* undefined. (The equation 0 * k = v has no solutions.)

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Re: Developing Programs and Proofs Spontaneously using GADT
by Shin-Cheng Mu 06 Aug '07

06 Aug '07

Thanks for the comments. I will take a look at the type family extension. The definition of plusFn proposed by Jim Apple worked nicely. The solution from apfelmus works after fixing a small typo: > newtype Equal a b = Proof (forall f . f a -> f b ) > > newtype Succ f a = InSucc { outSucc :: f (S a) } > > equalZ :: Equal Z Z > equalZ = Proof id > > equalS :: Equal m n -> Equal (S m) (S n) -- was (S n) (S m) > equalS (Proof eq) = Proof (outSucc . eq . InSucc) > > plusFn :: Plus m n i -> Plus m n k -> Equal i k > plusFn PlusZ PlusZ = Proof id > plusFn (PlusS x) (PlusS y) = equalS (plusFn x y) Also thanks to Derek Elkins and Dan Licata for interesting discussions about dependent sum/product. :) sincerely, Shin

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