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Haskell Weekly News - August 14, 2006
by dons＠cse.unsw.edu.au 14 Aug '06

14 Aug '06

--------------------------------------------------------------------------- Haskell Weekly News http://haskell.org/haskellwiki/HWN Issue 40 - August 14, 2006 --------------------------------------------------------------------------- Welcome to issue 40 of HWN, a weekly newsletter covering developments in the Haskell community. Each week, new editions are posted to [1]the Haskell mailing list as well as to [2]the Haskell Sequence and [3]Planet Haskell. [4]RSS is also available, and headlines appear on [5]haskell.org. A mega super bumper issue, for the 1st birthday of the Haskell Weekly News 1. http://www.haskell.org/mailman/listinfo/haskell 2. http://sequence.complete.org/ 3. http://planet.haskell.org/ 4. http://sequence.complete.org/node/feed 5. http://haskell.org/ Announcements * The Haskell Workshop . Andres Loeh [6]announced the preliminary schedule of the Haskell Workshop 2006, part of the 2006 International Conference on Functional Programming (ICFP) 6. http://article.gmane.org/gmane.comp.lang.haskell.general/14104 * dbus haskell bindings . Evan Martin [7]announced preliminary D-Bus Haskell bindings. D-Bus is a message bus system, a simple way for applications to talk to one another. [8]More 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/13771 8. http://neugierig.org/software/hdbus/ * The GHC typechecker is Turing-complete . Robert Dockins was able to [9]show how that the GHC typechecker with multi-parameter typeclasses, functional dependencies, and undecidable instances is Turing-complete. 9. http://article.gmane.org/gmane.comp.lang.haskell.general/14088 * Haskell Program Coverage . Colin Runciman [10]announced the first release of hpc, a new tool for Haskell developers. Hpc records and displays Haskell program coverage. It provides coverage information of two kinds: source coverage and boolean-control coverage. [11]More here 10. http://article.gmane.org/gmane.comp.lang.haskell.general/14087 11. http://www.galois.com/~andy/hpc-intro.html * Smash your boiler-plate without class and Typeable . Oleg Kiselyov [12]described a new generic programming technique, expressive enough to traverse a term and return another term of a different type, determined by the original term's type/structure. [13]More details 12. http://article.gmane.org/gmane.comp.lang.haskell.general/14086 13. http://okmij.org/ftp/Haskell/syb4.hs * Paper: Software Extension and Integration with Type Classes . Ralf Laemmel and Klaus Ostermann [14]invite comments towards the final version of their paper [15]Software Extension and Integration with Type Classes 14. http://article.gmane.org/gmane.comp.lang.haskell.general/14040 15. http://homepages.cwi.nl/~ralf/gpce06/ * HSP.Clientside 0.01 . Joel Bj�rnson [16]announced a release of his Summer of Code project HSP.Clientside 0.01. Present features include an embedding of (typed) JavaScript language in Haskell, a small combinator library for generating JavaScript code, and high-level interface to Ajax functionality. 16. http://article.gmane.org/gmane.comp.lang.haskell.general/14023 * Monadic probabilistic functional programing . Stefan Karrmann [17]announced that he had extended Martin Erwig's PFP library to support abstract monads, cabal and darcs 17. http://article.gmane.org/gmane.comp.lang.haskell.general/14012 * hdbc-odbc 1.0.0.1 . John Goerzen [18]released DBC-odbc, the ODBC backend driver for HDBC, version 1.0.0.1. 18. http://article.gmane.org/gmane.comp.lang.haskell.general/13998 * Few Digits 0.5.0 . Russell O'Connor This year, Few Digits competed in the [19]More Digits contest. To celebrate, version 0.5.0 of Few Digits is available. Few Digits 0.5.0 is now ten times faster and three times more complicated. Few Digits has been Cabalized for your convenience. [20]More info 19. http://rnc7.loria.fr/competition.html 20. http://r6.ca/FewDigits/ * System.FilePath 0.9 . Neil Mitchell [21]announced System.FilePath 0.9 21. http://article.gmane.org/gmane.comp.lang.haskell.general/13985 * The History of Haskell . Phil Wadler, John Hughes, Paul Hudak and Simon Peyton Jones [22]have been writing a paper, The History of Haskell, for the History Of Programming Languages conference (HOPL'07), and they invite feedback. Wiki page [23]here. 22. http://article.gmane.org/gmane.comp.lang.haskell.general/13983 23. http://haskell.org/haskellwiki/History_of_Haskell * AngloHaskell . Lemmih [24]mentioned that AngloHaskell will be held at Cambridge in August. The agenda includes beer, unicycles, hacking and other fun. [25]More info 24. http://article.gmane.org/gmane.comp.lang.haskell.general/13979 25. http://haskell.org/haskellwiki/AngloHaskell * Haskell XML Toolbox Version 6.0, 6.1 . Uwe Schmidt [26]announced [27]two new versions of the Haskell XML Toolbox. New features include ghc 6.4.2 support, better XPath integration, separate documentation for filter API and an arrow API 26. http://article.gmane.org/gmane.comp.lang.haskell.general/13924 27. http://article.gmane.org/gmane.comp.lang.haskell.general/13949 * Down the rabbit hole . Bulat Ziganshin [28]announced the availability of a new tutorial directed toward comprehensive explanation of the IO monad, and it's use in complex programs 28. http://article.gmane.org/gmane.comp.lang.haskell.general/13914 * ldap-haskell, arch2darcs and darcs-buildpackage . John Goerzen [29]posted new versions of these packages 29. http://article.gmane.org/gmane.comp.lang.haskell.general/13912 * Internships on GHC and Haskell at MSR Cambridge . Simon Peyton-Jones [30]announced that MSR Cambridge is taking interns year-round, not just in the summer months. GHC HQ are keen to attract motivated and well-qualified folk to work on improving or developing GHC. [31]More details 30. http://article.gmane.org/gmane.comp.lang.haskell.glasgow.user/10219 31. http://hackage.haskell.org/trac/ghc/wiki/Internships * FGL . Martin Erwig [32]announced a new release of his well known Functional Graph Library (FGL). 32. http://article.gmane.org/gmane.comp.lang.haskell.libraries/4948 * Takusen . Alistair Bayley and Oleg Kiselyov [33]released a new version of Takusen, a library for accessing DBMSs. The most significant code change is a new internal design, giving better separation of concerns like statement preparation, binding, and result-set processing. Takusen is now held in darcs, and [34]hosted at haskell.org 33. http://article.gmane.org/gmane.comp.lang.haskell.libraries/4887 34. http://darcs.haskell.org/takusen * Text.Regex.Lazy 0.44, 0.56, 0.66 and 0.70 . Chris Kuklewicz [35]announced [36]Text.Regex.Lazy 0.44-0.70, with many enhancements. Multiple backends are supported, in addition to the "full lazy" and the DFA backends. Text.Regex.Lazy is a replacement and enhancement for Text.Regex. More details [37]here 35. http://article.gmane.org/gmane.comp.lang.haskell.libraries/4977 36. http://sourceforge.net/projects/lazy-regex 37. http://article.gmane.org/gmane.comp.lang.haskell.libraries/5028 * Streams 0.2.1 beta . Bulat Ziganshin [38]released Streams 0.2.1 beta, featuring various bug fixes and improvements to the streams library 38. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/4865/focus=4865 Haskell' This section covers the [39]Haskell' standardisation process. * [40]Numeric Class reworking 39. http://hackage.haskell.org/trac/haskell-prime 40. http://thread.gmane.org/gmane.comp.lang.haskell.prime/1568/focus=1568 Discussion * Package "mounting" . Sven Hallberg [41]proposed drop the assumption that Haskell modules are closed entities, and rather always consider them to be seen in the context of a particular package. 41. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/4900/focus=4900 * Cabal and DLLs . Jason Dagit [42]described how he was able to use Cabal with Visual Haskell to build Dlls in Windows. 42. http://thread.gmane.org/gmane.comp.lang.haskell.libraries/4952/focus=4952 * Replacing GMP . Peter Tanski [43]sparked a long discussion on the merits and difficulties of replacing the GMP numerical library used by GHC and nhc98 with a library using a less restrictive license. More [44]here, [45]here and [46]here 43. http://thread.gmane.org/gmane.comp.lang.haskell.glasgow.user/10287/focus=10… 44. http://thread.gmane.org/gmane.comp.lang.haskell.glasgow.user/10324/focus=10… 45. http://thread.gmane.org/gmane.comp.lang.haskell.glasgow.user/10393/focus=10… 46. http://thread.gmane.org/gmane.comp.lang.haskell.glasgow.user/10408/focus=10… * The cost of Integer . Serge Mechveliani [47]asked about the runtime cost of Integer, triggering an interesting discussion 47. http://thread.gmane.org/gmane.comp.lang.haskell.glasgow.user/10313/focus=10… * Thread-local variables . Frederik Eaton [48]started a large discussion on the merits or otherwise of thread-local variables. 48. http://thread.gmane.org/gmane.comp.lang.haskell.general/14033/focus=14033 * Type level programming made simple . Oleg Kiselyov [49]composed a useful comparison between Prolog and type level programming in Haskell 49. http://article.gmane.org/gmane.comp.lang.haskell.cafe/13785 * IRC channel statistics . Don Stewart [50]created some graphs and analysis of activity and growth of the [51]#haskell IRC channel 50. http://article.gmane.org/gmane.comp.lang.haskell.cafe/13789 51. http://www.haskell.org/haskellwiki/IRC_channel Quotes of the Week * Dan Piponi : "Writing introductions to monads seems to have developed into an industry" * John Meacham : "I liken learning Haskell to tipping over a vending machine. You can't just push it, you gotta rock it back and forth a few times building up momentum until bam! suddenly the flash of insight hits and it all makes sense." * Andrew Bromage : "What was considered 100 milli-Olegs of type hackery five years ago is standard operating procedure th ese days" * Tim Toady : "Learning Haskell itself is easy -- I've done it several times already" * Adam : "Lisp is like Ruby with an ugly syntax, impossible stdlib, and macros that don't compensate for above shortcomings" * Lennart A : "Hbc still has some unique features, like views. That I've never used.." * Edward K : "I've been playing with type level 2s complement arithmetic" * Edwin B : "I think I'd panic if I had to write something that wasn't a compiler" * Ozone : "Our C++ guru at work is getting a bit sick of me saying: Ah, so that's like in Haskell, but not quite as elegant..." * roconnor : "Life lesson from today: 1 closure is nice, 500 000 closures sucks" * scsibug : "I could have sworn it was incorrect until it type-checked" * stepcut : "I wrote 1 + 1 in php once, and got 11 -- wasn't quite what I was hoping for" Contributing to HWN To help create new editions of this newsletter, please see the [52]contributing information. Send stories to dons at cse.unsw.edu.au . The darcs repository is available at darcs get http://www.cse.unsw.edu.au/~dons/code/hwn 52. http://haskell.org/haskellwiki/HWN

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"class []" proposal Re: [Haskell-cafe] One thought: Num to 0 as ? to list?
by Bulat Ziganshin 14 Aug '06

14 Aug '06

Hello Marc, Sunday, August 13, 2006, 10:36:39 PM, you wrote: > In other words: why not overload (:) ? i have such proposal, more or less complete: 1) define [] as type class and [] and ':' as operations of this class: class [] c where [] :: c a -- creates empty container (:) :: a -> c a -> c a -- prepends new element to the head of container head :: c a -> a -- returns first container's element tail :: c a -> c a -- returns remaining container's elements 2) allow to use type classes in type declarations like the types itself. for example, allow the following: f :: Num a => a -> Int write as f :: Num -> Int and following: sequence :: Monad m => [m a] -> m [a] write as sequence :: [Monad a] -> Monad [a] these two changes together will change the meaning of existing list functions declarations, making them polymorphic functions that are ready to work with any container which is an instance of [] class. For example, the following declaration: foldr :: (a -> b -> b) -> b -> [a] -> b now will be treated as: foldr :: ([] c) => (a -> b -> b) -> b -> c a -> b 3) translate pattern matching on the left side of function equations to the equivalent pattern guards: foldr k z [] = z foldr k z (x:xs) = x `k` foldr k z xs turns into: foldr k z xx | xx==[] = z | x<-head xx, xs<-tail xx = x `k` foldrL k z xs 4) improve Haskell defaulting mechanism to allow declaring default container type used when there is not enough context: default [] => List main = print (head [1,2,3]) Here, [1,2,3] desugared into 1:2:3:[]. Because there is no information which allow to decide which container should be used, we defaults to use List instance: type List a = Cons a (List a) | Nil instance [] List where ..... Some Haskeller once said that lists are widely used in Haskell primarily because they are supported much better than any other data structure. All these changes together will allow to use any other container with the same easy as lists. i have attached a file which implements the idea without language changes. at the last end, it computes length and sum of two containers - list and array slice - using the same polymorphic functions: lengthL xs = foldrL (\_->(+1)) 0 xs sumL xs = foldrL (+) 0 xs Of course, this idea is not yet completely developed. Two problems that i see just now is dealing with pattern matching failures (f.e. trying to call 'head' for empty list) and building an hierarchy of classes representing various list features. at least, array slices don't support efficient ':' implementation. -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Haskell Workshop 2006 Call for participation
by Andres Loeh 14 Aug '06

14 Aug '06

Please note that the early registration deadline is August 18, 2006. Cheers, Andres

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2 programs VERY URGENT
by Mohammad Mehdi Bezyan 14 Aug '06

14 Aug '06

Hello All There are 2 questions for my project and I need to aswer them as soon as possible. The answers need to be written in Haskell using recursive methods. The questions are the following: Number 1: The set of n-combinations of a set S is the set of subsets of S of size n. Write a function combs :: Int -> [a] -> [[a]] which, representing sets as lists, calculates n-combinations of a given list. For example, combs 3 [1,3,5,6,9] is [[1,3,5],[1,3,6],[1,3,9],[1,5,6],[1,5,9], [1,6,9],[3,5,6],[3,5,9],[3,6,9],[5,6,9]] Within each combination, the n values should appear in the same order as they appear in the original list (so, for example, [1,5,3] should not appear instead of [1,3,5] in the result for combs 3 [1,3,5,6,9]). But the list of combinations may appear in any order (ie, [1,3,5] need not appear first in the result of combs 3 [1,3,5,6,9]). You may assume that the list argument to combs does not contain duplicate members. Number 2: Given a list of lists, a list is in the Cartesian product of them if it contains one member from each of them. Write a function cart :: [[a]] -> [[a]] which calculates this. For example, cart [[1,2], [4], [7,8,9]] is [[1,4,7],[1,4,8],[1,4,9],[2,4,7],[2,4,8],[2,4,9]] Each member of the Cartesian product must appear in the appropriate order: eg, in [1,4,7] above, 1 appears first, then 4, then 7, because 1 is from the first input list [1,2], 4 is from the second input list [4], and 7 is from the third input list [7]. The list of members of the Cartesian product may appear in any order (ie, [1,4,7] need not appear first in the list). It would be very much appreciated if you could help me or provide me with some code, please. I need them very soon. Thanks! Send instant messages to your online friends http://au.messenger.yahoo.com

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haskell.org account cleanup / administration
by John Peterson 14 Aug '06

14 Aug '06

I'm going to try and be more organized about administering haskell.org. I've started a wiki page, http://haskell.org/haskellwiki/Haskell.org, that will allow me to track the accounts I've given out and tell everyone what haskell.org is for and who to contact for various matters. Would everyone with an account on haskell.org please add themselves to the list on this page? We'll be shutting down accounts that have become inactive so this is the best way to let us know you're still around. Anyone that administers something on haskell.org should also place themselves in the contact list so that people will know who to turn to if something isn't working. It would be nice if the other haskell machines (like cvs.haskell.org) added something to this page (or created a new page). Thanks!!! John

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AJAX applications in Haskell
by Pasqualino 'Titto' Assini 14 Aug '06

14 Aug '06

Hi, Those among you who have an interest in AJAX-style web development - that is to say the development of web applications that run entirely into the browser environment, calling back to the server back-end only to get raw data -- will probably have noticed the recent appearance of the Google Web Toolkit (http://code.google.com/webtoolkit) The main idea behind it is very simple: you write your Ajax application in Java and your code is compiled down to JavaScript/HTML (rather then to Java Virtual Machine bytecode) so that it can be executed in any browser. The main advantages of this solution are: - Development using a typed language with good compile-time checks and debugger support - No plugins (e.g. a Java VM) required on the user machine The disadvantages are: - An additional compilation step - Probably, a significant loss of performance with respect to hand-written code Assuming that the balance is positive, this naturally raises the question: why not doing the same with our favourite language? This would require to: - Retarget one of the existing Haskell compilers to generate JavaScript (other possible targets would be Flash or higher level UI languages such as OpenLaszlo that in turn compiles down to either Flash or JavaScript/HTML) - Write a suitable runtime library (including support for native JavaScript or Flash UI components, remote method invocations, multi-threading, etc.) - Port a usable subset of the Haskell Libraries - Select an approach to write user interfaces in Haskell. There have been many attempts in this direction but I am unsure if any of them has proven particularly successful. All considered, quite a significant amount of work -- that would require a concerted group effort -- with a rather uncertain pay-off. Is anyone working on anything similar or that might be interested in such a project? And has anyone any thoughts to offer on what compilers/libraries/UI frameworks would be more suitable for the task? Regards, titto assini

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We komen langs
by Henk-Jan van Tuyl 13 Aug '06

13 Aug '06

Hallo Diny, We komen vanmiddag langs, zoals afgesproken. Voor geval je het vergeten bent, hoop ik, dat je dit op tijd leest. -- Groeten, Henk-Jan -- http://Van.Tuyl.eu/ -- Using Opera's revolutionary e-mail client: https://secure.bmtmicro.com/opera/buy-opera.html?AID=789433

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aosd 2007
by Oege.de.Moor＠comlab.ox.ac.uk 12 Aug '06

12 Aug '06

AOSD 2007: CALL FOR RESEARCH PAPERS 6th Conference on Aspect-Oriented Software Development http://www.aosd.net/2007/cfc/research.php >> on-line submission is now open << Deadline September 22 ------------------------------------------------------ The program committee especially welcomes submissions from researchers in functional programming on any topic relating to aspect-oriented extensions of Haskell (or why you don't need them!)

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The history of Haskell
by Simon Peyton-Jones 11 Aug '06

11 Aug '06

Friends Some while ago I posted an announcement about our draft paper "The History of Haskell". A dozen or so people typed in pages of comments, or sent us email. Thank you so much! I have incorporated almost everything we received, and would like to invite your renewed attention. You'll find Version 2 (dated 11 August) in the same place as before: http://haskell.org/haskellwiki/History_of_Haskell I have started a new Talk page so that old comments do not get mixed up with new ones. If you'd like to check the revisions, or suggest other improvements, we'd love to hear from you. Our deadline for the Final Version is 14 Sept. Feedback before end Aug is more likely, therefore, to be used. I hope I have acknowledged everyone who contributed -- please yell if not. However, I don't know the real names of the following (these are their Wiki account names): Weihu, Joesf, Ruiz, Plareplane, Padator, Hoan, Saccade, davidk01 If you tell me who you are, I'll add you to the acks. Thanks! Simon

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hs-plugins on MacOS X: unknown symbol
by Ulf Norell 11 Aug '06

11 Aug '06

Hi. I'm playing with hs-plugins (recent darcs version) on MacOS X (ghc-6.4.1) and I get some weird unresolved symbols. Here's what I'm trying to do: Test.hs ------- module Test where test = putStrLn "test" Main.hs ------- import System.Plugins main = do r <- load_ "Test.o" [] "test" case r of LoadFailure msg -> print msg LoadSuccess _ m -> m Now I do: $ ghc -c Test.hs $ ghc Main.hs -package plugins $ ./a.out a.out: unknown symbol `_SystemziIO_putStrLn_closure' a.out: user error (resolvedObjs failed.) I get the same error when I try with plugins-0.9.10. I've tried to use loadPackage "base" and resolveObjs to no avail. What does work (not surprisingly) is to use putStrLn in Main.hs to force it to be linked in but that's not really a solution. If someone knows what I'm doing wrong I'd be very grateful. / Ulf

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