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Performance of Knight's Tour
by Artyom Kazak 01 Mar '10

01 Mar '10

Hi! I'm learning Haskell, and now I'm trying to make framework for solving searching problems, such as Knight's Tour. For small boards it answers instantly. For 7x8 board - 23 seconds. For 8x8 board - more than 30 minutes (it hasn't finished yet). Where is the root of the evil? --program module Main where import Data.List import Data.Array.Unboxed import qualified Data.Array.IArray as IArr import Data.Ix data SResult = Good | GoodProcess | Process | Bad data SDPSearch a p r = SDPSearch (a -> p -> [a]) --expand (p -> p) --update (a -> p -> SResult) --sort ([a] -> r) --result runSDPSearch :: SDPSearch a c b -> [a] -> c -> b runSDPSearch (SDPSearch e u s r) list p = r (rec list params) where params = iterate u p rec [] _ = [] rec (l:lp) pr@(n:np) = case s l n of Good -> l : rec lp pr GoodProcess -> l : (rec (e l n) np) ++ (rec lp pr) Process -> (rec (e l n) np) ++ (rec lp pr) Bad -> rec lp pr main = do (a, b) <- (break (== ' ')) `fmap` getLine print (knightTour (read a) (read b)) knightTour :: Int -> Int -> UArray (Int, Int) Int knightTour a b = runSDPSearch (SDPSearch e u s r) [((1, 1), sArray)] 2 where size = a * b range = ((1, 1), (a, b)) sArray = listArray range (1 : (replicate (size - 1) 0)) allTurns :: Array (Int, Int) [(Int, Int)] allTurns = IArr.listArray range [turns x y | x <- [1..a], y <- [1..b]] where shifts = [(1, 2),(1, -2),(2, 1),(2, -1),(-1, 2),(-1, -2),(-2, 1),(-2, -1)] turns x y = [(x+i, y+j) | (i, j) <- shifts, inRange range (x+i, y+j)] e ((x, y), arr) p = [(t, arr // [(t, p)]) | t <- changes] where changes = [t | t <- allTurns ! (x, y), arr ! t == 0] s el p | p == size = Good | otherwise = Process u = (+ 1) r l | not (null l) = snd (head l) | otherwise = error "No solutions!"

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Cabal pre-compiled packages
by Diego Souza 01 Mar '10

01 Mar '10

Hi, currently when one install a cabal package it compiles it and then install generated binaries. I wonder whether or not it would be useful to have pre-compiled binaries as many package managers usually do (e.g. apt). I often think that would save some time on the expense of a busier hackage server capable of generating packages for many different platforms. I'm particularly thinking on the following scenario: suppose that you have code that is ready for production. If cabal supported pre-compiled binaries, there is no need to install ghc or eventually any other compiler, just runtime environment and eventually cabal. I must say that I have no experience in doing this in Haskell (just personal/small projects), so I suppose one have to generate binaries and use other sort of package manager to deploy code to production (which sounds reasonable as well). Thus, if the assumption is correct, cabal is a development tool, not something one could to only deploy runtime-only packages. I also would appreciate if others could share how usually this is managed. Best Regards, ~dsouza

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Ghent Functional Programming Group: First Meeting on April 1, 2010
by Jeroen Janssen 01 Mar '10

01 Mar '10

Dear All, The results of the doodle are in, and the result is that the first meeting of the Ghent Functional Programming Group will be held on April 1, 2010 at 7pm in meeting room "Shannon" in the Technicum building (Sint-Pietersnieuwstraat 41, 9000 Gent) of Ghent University. The automatic doors will be closed at this time, but we will attach a notice with the telephone number of the meeting room at the front door in the outer left of the building, so you can call us to let you in. We would like to ask anyone even remotely considering to participate to fill in the following Google form: http://spreadsheets.google.com/viewform?formkey=dEtsR2ZIdVhqeVdRNkx6bmxCdF9… Filling in this form does not commit you to anything, but merely serves as an indication of the number of participants for our sake. We will also only be sending follow-up mails with the final program etc. to the persons that have registered. If you do not intend to come to this first meeting, but wish to be updated on our activities, you can follow us on Twitter: http://twitter.com/ghentfpg or you can sign-up for our Google group: http://groups.google.com/group/ghent-fpg Our current program already contains one talk by Tom Schrijvers entitled "Functional Pearl: The Monad Zipper", with the following abstract: "This pearl aims to demonstrate the application of Huet's zipper to the monad stack for the development of highly modular programs." Anyone willing to give a talk on a subject that fits the scope can e-mail us with a proposed topic, no matter what experience level you have, or no matter how short or long the talk will be. Kind Regards, Bart Coppens Jeroen Janssen

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Computer Graphics and Haskell - Radiosity Methods
by Hector Guilarte 01 Mar '10

01 Mar '10

Hello cafe, While I was studying for my computer graphics test I have tomorrow I realized that maybe some of the major problems I've read so far about Radiosity Rendering Algorithms may be reduced significantly if it was implemented in Haskell and taking advantage of the lazy evaluation so that only what can be seen from the viewer's perspective point of view is calculated, and the rest of the scene just remains as thunks waiting for them to be calculated in case they are needed. I guess that probably most of the people on this list doesn't really work on the computer graphics area, but if anybody out there actually does I'd really like if he or she could explain me more of why what I'm saying could be really useful and what has already been done (or why I'm so lost that I'm probably gonna fail my exam tomorrow hahaha)... One of the things I'm studying is the Monte Carlo Radiosity Methods which takes advantages of non-determinism to create stochastic algorithms, I guess this is more difficult to implement in Haskell, please if you know any of this Monte Carlo Methods for Radiosity Algorithms share that knowledge with me (and the list). Thank you, Hector Guilarte

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pddl parser
by Vojtěch Knyttl 01 Mar '10

01 Mar '10

Has anyone seen any PDDL(Planning Domain Definition Language) parser in Haskell? Thanks. VK

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View patterns
by Andrew Coppin 01 Mar '10

01 Mar '10

One somewhat neat thing about Haskell is that you can say case list of [[x], [y,_], [z,_,_]] -> x + y + z _ -> 0 In Java, you'd have to write something like if (list.length() == 3) { List t1 = list.at(0); if (t1.length() == 1) { int x = t1.at(0); List t2 = list.at(1); if (t2.length() == 2) ... I can't even be bothered to finish typing all that lot! However, as somebody pointed out, the Java version is polymorphic. Assuming that length() is defined for multiple types of container, the Java version works with lists, arrays, sets, etc. If you try to do this in Haskell, you end up with case size c of 3 -> case (c ! 0, c ! 1, c ! 2) of (xs, ys, zs) | size x == 1 && size y == 2 & size z == 3 -> (xs ! 0) + (ys ! 0) + (zs ! 0) _ -> 0 _ -> 0 or similar. Which is shorter than Java, but nowhere near as nice as the original list-only version. Now I was under the impression that "view patterns" fix this problem, but it seems they don't: case c of (size -> 3) -> case (c ! 0, c ! 1, c ! 2) of (size -> 1, size -> 2, size -> 3) -> (c ! 0 ! 0) + (c ! 1 ! 0) + (c ! 2 ! 0) Any suggestions?

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Is the Summer of Code TRAC relevant for this years SoC?
by Nathan Hunter 01 Mar '10

01 Mar '10

Hello. I was looking into the possibility of doing Summer of Code as a student, when I found Haskell.org's TRAC<http://hackage.haskell.org/trac/summer-of-code/report/1>. I was particularly interested in Efficient Maps using Generalised Trees and the New Purely Functional Data Structures/Containers Library. I'd love to work on either of those, but are they or any of the projects listed on the TRAC still relevant, or are they just the leftovers from SoC 2009? -Nathan Hunter

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Haskell Weekly News: Issue 151 - February 28, 2010
by jfredett＠gmail.com 01 Mar '10

01 Mar '10

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20100228 Issue 151 - February 28, 2010 --------------------------------------------------------------------------- Welcome to issue 151 of HWN, a newsletter covering developments in the [1]Haskell community. A light HWN this week, some interesting discussion about view patterns and free monads. Also announced this week was [2]TryHaskell a work-in-progress online interactive haskell interpreter and tutorial. The author (based on his reddit post) has asked us not to post this to proggit yet, but I don't think he'll mind a plug here. So after you're done reading, give it a try! Until next week, haskellers, your Haskell Weekly News. Announcements wxHaskell for ghc-6.12.1. Daniel Fischer [3]announced a new release of wxHaskell. concurrent-extra-0.2. Roel van Dijk [4]announced a new release of concurrent-extra jhc-0.7.3. John Meacham [5]announced a new version of jhc, including many bug fixes. Try Haskell! An interactive tutorial in your browser. Benjamin L. Russell [6]told us about the TryHaskell Project, available at [7]TryHaskell.org. TryHaskell is an interactive, online Haskell interpreter and tutorial. MSFP: Call for Papers. Venanzio Capretta [8]announced a call for papers for the Third Workshop on Mathematically Structured Functional Programming. HLedger 0.8. Simon Michael [9]announced a new version of hledger last week, but it slipped through my radar. This release includes bug fixes, some refactoring, and High-Res Graphical Chart generation capability. Discussion View patterns. Andrew Coppin [10]asked about making view patterns polymorphic. What are 'free' Monads? Guenther Schmidt [11]asked what exactly it meant to be a 'free monad' Multiple Interpretations for a monad? Guenther Schmidt [12]asked about monads which can have multiple levels of meaning and interpretation. Blog noise [13]Haskell news from the [14]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Joachim Breitner: [15]Exploiting sharing in arbtt. * Martijn van Steenbergen: [16]Colors in GHCi. * Don Stewart (dons): [17]Evaluation strategies and synchronization: things to watch for. * Don Stewart (dons): [18]Fusion makes functional programming fun!. * Michael Snoyman: [19]Simpler is Better. * Kevin Reid (kpreid): [20]Hvm.hs: an exercise in Haskell golf. * Bryn Keller: [21]Why Isn't Cabal Installed with GHC?. * Don Stewart (dons): [22]Modern Benchmarking in Haskell. Quotes of the Week * roconnor: sounds like you need a zygohistomorphic prepromorphism * copumpkin: These are not the monads you are looking for. * noggle: this language is like having programming super powers About the Haskell Weekly News New editions are posted to [23]the Haskell mailing list as well as to [24]the Haskell Sequence and [25]Planet Haskell. [26]RSS is also available, and headlines appear on [27]haskell.org. To help create new editions of this newsletter, please see the information on [28]how to contribute. Send stories to jfredett . at . gmail . dot . com. The darcs repository is available at darcs get [29]http://patch-tag.com/r/jfredett/HWN2/pullrepo HWN2 . References 1. http://haskell.org/ 2. http://tryhaskell.org/ 3. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70857 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70689 5. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70961 6. http://article.gmane.org/gmane.comp.lang.haskell.general/17819 7. http://tryhaskell.org/ 8. http://article.gmane.org/gmane.comp.lang.haskell.general/17818 9. http://www.haskell.org/pipermail/haskell-cafe/2010-February/073185.html 10. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70895 11. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70874 12. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70800 13. http://planet.haskell.org/ 14. http://haskell.org/haskellwiki/Blog_articles 15. https://www.joachim-breitner.de/blog/archives/381-Exploiting-sharing-in-arb… 16. http://martijn.van.steenbergen.nl/journal/2010/02/27/colors-in-ghci/ 17. http://donsbot.wordpress.com/2010/02/26/evaluation-strategies-and-synchroni… 18. http://donsbot.wordpress.com/2010/02/26/fusion-makes-functional-programming… 19. http://snoyberg.wordpress.com/2010/02/26/simpler-is-better/ 20. http://kpreid.livejournal.com/23060.html 21. http://www.xoltar.org/?p=33 22. http://donsbot.wordpress.com/2010/02/23/modern-benchmarking-in-haskell/ 23. http://www.haskell.org/mailman/listinfo/haskell 24. http://sequence.complete.org/ 25. http://planet.haskell.org/ 26. http://sequence.complete.org/node/feed 27. http://haskell.org/ 28. http://haskell.org/haskellwiki/HWN 29. http://patch-tag.com/r/jfredett/HWN2/pullrepo%20HWN2

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ANNOUNCE: jhc-0.7.3
by John Meacham 01 Mar '10

01 Mar '10

It's been a long time coming, but jhc-0.7.3 is here. If you have been following the darcs repository, there are not a whole lot of new changes, but there have been substantial fixes since 0.7.2 for those that use the tarballs or rpms. http://repetae.net/computer/jhc/ One of the most important ones is I have begun the process of standardizing on YAML for all external interfaces that may be useful to automation, this should make interfacing jhc with external tools written in any language much easier, and it will prevent me from spending time trying to invent proprietary formats every time I think of something new to spit out. right now it is used in 3 places: - dumping dependency info with --deps - library dumping with --list-libraries -v - annotating source code, for inputting to documentation generators via --annotate-source The main place it isn't used yet that I desire too is to replace the 'cabal' library description format with a true YAML file. jhc cabal files were never quite compatible with cabal ones anyway and naming them the same thing has been a source of connfusion. some of the other changes have been: features: - a lot more regression tests, many user submitted bugs are now regressions (or fixed) - support for using 'mock' to build rpms - System.Exit, System.Cmd added to base - better compatibility with ghc in some library functions - ui improvements in showing progress and errors - extended --list-libraries info - --deps dependency dumping - more francise compatible thanks to patches from droundy - more instances for bigger tuples added - uses editline if readline not available - select libraries by hash as well as version - add System.IO.Pipe - detect version of gcc needed for cross compiling to windows performance: - storage analysis - a very basic region inference enabled by default - deadcode analysis can see through partial applications now - profiled and sped up some library routines that were producing inellegant core. - figure out when top level grin functions call themselves recursively and turn them into explicit loops. bug fixes: - do expression in infix - fix strict newtype bug - U2U bug fixed - desugaring inside of list comprehensions fix - c generator doesn't mess up on unknown values - the compiler version is stored in the cache files, so multiple versions of jhc don't clobber each others cache entries - Ord instance for lists fixed - recursive type synonyms detected properly - make sure errno.h is included when errno is referenced - don't give a parse error when seeing ghc extensions to INLINE - qualified method names fix John -- John Meacham - ⑆repetae.net⑆john⑈ - http://notanumber.net/

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OpenSUSE 11.2
by Andrew Coppin 28 Feb '10

28 Feb '10

OK, I imagined to install OpenSUSE 11.2. However, I can't find a way to make it install either GHC or the Haskell Platform. A quick Google search turns up a folder on download.opensuse.org which is stuffed full of Haskell stuff, yet no ammount of prodding YaST will convince it that it's possible to install anything remotely Haskell-related. (Now I remember why I don't use Linux much...) Does anybody know how to install this stuff on OpenSUSE? Alternatively, does anybody know of an alternative Linux distribution where this kind of thing is likely to work better? (I think I've got a Debian ISO somewhere - but IME everything on Debian is always 8 years old.)

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