- Haskell - Haskell.org

How to get the binary respentation of the Int/Double.
by haihualin 28 Oct '08

28 Oct '08

Hi, I am wondering how to get the binary respentation of the Int/Double. So I can save the integer as the binary data so that the C program can read it from the file. Thanks for your help. Haihua

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Problem with installing wxHaskell
by nlv 28 Oct '08

28 Oct '08

Hello. I cannot install wxHaskell. I've installed wxX11-2.8.8 and tried to install wxcore-0.10.5. But on "make" I've got: wxc/include/wrapper.h:151: error: invalid use of undefined type ‘struct wxDropTarget’ /usr/local/include/wx-2.8/wx/window.h:60: error: forward declaration of ‘struct wxDropTarget’ wxc/include/wrapper.h:170: error: ‘wxDragResult’ does not name a type wxc/include/wrapper.h:172: error: ‘wxDragResult’ does not name a type wxc/include/wrapper.h:173: error: ‘wxDragResult’ does not name a type wxc/include/wrapper.h: In constructor ‘ELJDropTarget::ELJDropTarget(void*)’: wxc/include/wrapper.h:161: error: type ‘wxDropTarget’ is not a direct base of ‘ELJDropTarget’ wxc/include/wrapper.h: At global scope: wxc/include/wrapper.h:199: error: expected class-name before ‘{’ token wxc/include/wrapper.h:222: error: ‘wxDragResult’ does not name a type wxc/include/wrapper.h:224: error: ‘wxDragResult’ does not name a type wxc/include/wrapper.h:225: error: ‘wxDragResult’ does not name a type wxc/include/wrapper.h: In constructor ‘ELJTextDropTarget::ELJTextDropTarget(void*, int (*)(void*, long int, long int, void*))’: wxc/include/wrapper.h:209: error: class ‘ELJTextDropTarget’ does not have any field named ‘wxTextDropTarget’ wxc/include/wrapper.h: At global scope: wxc/include/wrapper.h:236: error: expected class-name before ‘{’ token wxc/include/wrapper.h:259: error: ‘wxDragResult’ does not name a type wxc/include/wrapper.h:261: error: ‘wxDragResult’ does not name a type wxc/include/wrapper.h:262: error: ‘wxDragResult’ does not name a type wxc/include/wrapper.h: In constructor ‘ELJFileDropTarget::ELJFileDropTarget(void*, int (*)(void*, long int, long int, void*, int))’: wxc/include/wrapper.h:246: error: class ‘ELJFileDropTarget’ does not have any field named ‘wxFileDropTarget’ Can anyone help me?

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IORef sharing
by Rodney D Price 28 Oct '08

28 Oct '08

I'm trying to understand how an IORef (MVar, TVar) might be shared between separate instances of function closures. I've defined a function `count` that returns a function with an IORef "inside", > count :: IORef Int -> Int -> IO (Char -> IO Int) > count io i = do > writeIORef io i > return (\c -> if c == 'a' > then modifyIORef io (+1) >> readIORef io > else readIORef io) Now I define an IORef and a couple of counters that share the IORef, > iio :: IO (IORef Int) > iio = newIORef 0 > ic1 = do { io <- iio ; count io 0 } > ic2 = do { io <- iio ; count io 0 } I expected to see the counters sharing the IORef, so that executing `counter1` below would print "1,1,2,3". Instead, it prints "1,0,1,2". > counter1 = do > c1 <- ic1 > c2 <- ic2 > c1 'a' >>= print > c2 'b' >>= print > c2 'a' >>= print > c1 'a' >>= print However, if I create the two counters inside the same do block, I get the result I expected, "1,1,2,3". > counter2 = do > io <- iio > c1 <- count io 0 > c2 <- count io 0 > c1 'a' >>= print > c2 'b' >>= print > c2 'a' >>= print > c1 'a' >>= print So apparently my mental picture of an IORef as a pointer to a value is wrong. I need a new mental picture. What's going on here? Thanks, -Rod

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No fun with phantom types
by Michael Marte 27 Oct '08

27 Oct '08

Hello *, I am trying to extend the finite-domain (FD) constraint solver proposed by David Overton (http://overtond.blogspot.com/2008/07/pre.html) with arithmetic constraints by means of an embedded DSL. In principle, this is a very natural thing to do in a functional language; it is basically a matter of defining some suitable operators: data FDVar = FDVar Int deriving Show -- the type of FD variables data AExp = --the type of arithmetic expression over FD variables and integers IntegerConstant Int | Variable FDVar | Addition AExp AExp | Subtraction AExp AExp | Multiplication AExp AExp | IntegerDivision AExp AExp deriving Show infixl 7 #* -- multiplication infixl 7 #/ -- integer division infixl 6 #+ -- addition infixl 6 #- -- subtraction (#+), (#-), (#*), (#/) :: (MakeAExp a, MakeAExp b) => a -> b -> AExp (#+) = parseArgs Addition (#-) = parseArgs Subtraction (#*) = parseArgs Multiplication (#/) = parseArgs IntegerDivision with class MakeAExp a where makeAExp :: a -> AExp instance MakeAExp Int where makeAExp = IntegerConstant instance MakeAExp FDVar where makeAExp = Variable instance MakeAExp AExp where makeAExp = id parseArgs :: (MakeAExp a, MakeAExp b) => (AExp -> AExp -> c) -> a -> b -> c parseArgs f x y = f (makeAExp x) (makeAExp y) So far, so good. To avoid that FD variables escape their constraint stores, David employed a phantom type variable s leading to newtype FDVar s = FDVar { unFDVar :: Int } deriving (Ord, Eq) Trying to thread the phantom variable through my DSL implementation, I ended up with the following code: data AExp s = IntegerConstant Int | Variable (FDVar s) | Addition (AExp s) (AExp s) | Subtraction (AExp s) (AExp s) | Multiplication (AExp s) (AExp s) | IntegerDivision (AExp s) (AExp s) class MakeAExp a s where makeAExp :: a -> AExp s instance MakeAExp Int s where makeAExp = IntegerConstant instance MakeAExp (FDVar s) s where makeAExp x = Variable x instance MakeAExp (AExp s) s where makeAExp = id parseArgs :: (MakeAExp a s, MakeAExp b s) => (AExp s -> AExp s -> c s) -> a -> b -> c s parseArgs f x y = f (makeAExp x) (makeAExp y) infixl 7 #* -- multiplication infixl 7 #/ -- integer division infixl 6 #+ -- addition infixl 6 #- -- subtraction (#+), (#-), (#*), (#/) :: (MakeAExp a s, MakeAExp b s) => a -> b -> AExp s (#+) = parseArgs Addition (#-) = parseArgs Subtraction (#*) = parseArgs Multiplication (#/) = parseArgs IntegerDivision This code works if only one operator is applied: *FD> :type let x = (1::Int) in x #+ x let x = (1::Int) in x #+ x :: AExp s but *FD> :type let x = (1::Int) in x #+ x #+ x let x = (1::Int) in x #+ x #+ x :: (MakeAExp (AExp s) s1) => AExp s1 It appears to me that ghci generates two phantom types s and s1 and fails to unify them. Only the extensive use of type constraints seems to help like in the following example, where I used Int as phantom type: *FD> :type ((((1::Int) #+ (1::Int)) :: AExp Int) #+ (2::Int))::AExp Int ((((1::Int) #+ (1::Int)) :: AExp Int) #+ (2::Int))::AExp Int :: AExp Int But this approach only works on the command line and is out of question anyway. Any idea how to make my code work? I am using ghc 6.8.2 with {-# LANGUAGE GeneralizedNewtypeDeriving #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE FlexibleInstances #-} Thanks, Michael

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Publication of InputYourData.com + Project Announcement
by Enzo Haussecker 26 Oct '08

26 Oct '08

I would like to announce the publication of InputYourData.com (beta) − the online resource tool for financial, mathematical and scientific calculations. All web applications found at http://inputyourdata.com/ are written solely in Haskell and based on the Network.CGI framework. This website began as an experiment to familiarize myself with the monadic features of Haskell and their use in web programming. Namely, the mapping and manipulation of user inputs as typed objects. Through these experiments I found that Haskell allows for an efficient system where a variety of operations can be preformed while minimizing as many resources (such as time and memory space) as possible. I am now interested in developing a similar type of website, except wiki style − where all web applications are created by the user. Essentially, I am designing a web application where users can symbolically declare the arguments of a function and that function's call based on arbitrary variables. For example, say a user would like to create a web application to compute the roots of a second degree polynomial. He/she would simply declare the arguments to her function − three complex numbers a b and c, and the call of that function − (-b ± sqrt (b^2 - 4*a*c))/(2*a). The product of that users inputs will render a web application that looks similar to http:// inputyourdata.com/cgi-bin/quadratic.cgi (all text is to be updated by the user as well). As one could imagine, other, more complex functions involving vectors, matrices, stock prices, and other arguments can also be defined in terms of arbitrary variables and declared as an inputs to my wiki-style web application. If you are intrigued by this project and you have substantial experience in designing Haskell-based web applications, please send me your resume and a brief summery of why you are interested. Regards, Enzo Haussecker ehaussecker(a)gmail.com

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Publication of InputYourData.com + Project Announceme
by Enzo Haussecker 26 Oct '08

26 Oct '08

I would like to announce the publication of InputYourData.com (beta) − the online resource tool for financial, mathematical and scientific calculations. All web applications found at http://inputyourdata.com/ are written solely in Haskell and based on the Network.CGI framework. This website began as an experiment to familiarize myself with the monadic features of Haskell and their use in web programming. Namely, the mapping and manipulation of user inputs as typed objects. Through these experiments I found that Haskell allows for an efficient system where a variety of operations can be preformed while minimizing as many resources (such as time and memory space) as possible. I am now interested in developing a similar type of website, except wiki style − where all web applications are created by the user. Essentially, I am designing a web application where users can symbolically declare the arguments of a function and that function's call based on arbitrary variables. For example, say a user would like to create a web application to compute the roots of a second degree polynomial. He/she would simply declare the arguments to her function − three complex numbers a b and c, and the call of that function − (-b ± sqrt (b^2 - 4*a*c))/(2*a). The product of that users inputs will render a web application that looks similar to http:// inputyourdata.com/cgi-bin/quadratic.cgi (all text is to be updated by the user as well). As one could imagine, other, more complex functions involving vectors, matrices, stock prices, and other arguments can also be defined in terms of arbitrary variables and declared as an inputs to my wiki-style web application. If you are intrigued by this project and you have substantial experience in designing Haskell-based web applications, please send me your resume and a brief summery of why you are interested. Regards, Enzo Haussecker ehaussecker(a)gmail.com

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Haskell Weekly News: Issue 90 - October 25, 2008
by Brent Yorgey 25 Oct '08

25 Oct '08

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20081025 Issue 90 - October 25, 2008 --------------------------------------------------------------------------- Welcome to issue 90 of HWN, a newsletter covering developments in the [1]Haskell community. One day a Haskell n00b asked the master coder, "Master, does (const 3 undefined) have the terminating nature, or not?" The master replied, "Of course." Cried the n00b, "But that changes everything! Why did you not tell me this before?" "You never asked." Immediately, the n00b was enlightened. Announcements Darcs hacking sprint. Eric Kow [2]announced that the [3]darcs hacking sprint is taking place this weekend! Lambdabot 4.2.2. Gwern Branwen [4]announced the release of [5]version 4.2.2 of lambdabot, the famous Haskell IRC bot. The new release has innumerable new features and bugfixes, trained suckling pigs, mermaids, etc. Autoproc Change of Maintainer (if you use procmail you should read this). Jason Dagit [6]announced that Gwern Branwen will be taking over the [7]autoproc project. Autoproc makes it quick and easy for Haskell programmers to make procmail recipes by using an embedded domain specific language. Once your recipes type check and compile, you simply run autoproc and it generates the corresponding procmail recipe. External Sort: Sort a 10-million integer file with just 256M of ram.. Thomas Hartman [8]announced the external-sort package. It implements an on-disk external sort algorithm in Haskell, which you can use to sort lists that will not fit in memory. IEEE-utils. Sterling Clover [9]announced the [10]IEEE-utils package, providing a number of bindings for anyone interested in doing hardcore floating-point programming. rewriting-0.1. Thomas van Noort [11]announced the release of [12]rewriting, a generic rewriting library for regular datatypes. Features include generic rewriting machinery, generic traversals, and rewrite rules defined as values instead of functions. REMINDER: Haskell Communities and Activities Report. Janis Voigtlaender [13]reminded everyone that the deadline for the November 2008 edition of the Haskell Communities and Activities Report (Friday, October 31) is approaching fast! If you haven't already, please write an entry for your new project, or update your old entry. For more information, see the original [14]call for contributions. lhs2tex-1.14. Andres Loeh [15]announced the release of [16]lhs2TeX version 1.14, a a preprocessor to generate LaTeX code from literate Haskell sources. colour 0.0.0. roconnor [17]announced an initial release of the [18]colour package. It is hoped that this library will become the standard colour library for Haskell. Discussion Hackage Improvement Ideas. Jason Dagit [19]suggested some improvements to Hackage and asked for others to contribute their ideas as well. Spine-lazy "multiqueue". Luke Palmer [20]asked for help implementing an efficient spine-lazy multiqueue. Jobs Functional programming job opening. Simon Peyton-Jones [21]announced a [22]job opening in functional programming at Microsoft. They are looking for "an experienced software development engineer who has mastered C/C++ and/or C# development and is now learning or is already an expert using F# (or Haskell)." Blog noise [23]Haskell news from the [24]blogosphere. * Eric Kow (kowey): [25]darcs hacking sprints - some pictures from Team Brighton. * Jason Dagit: [26]Understanding Darcs Commute. * Ketil Malde: [27]Optimization again: befuddled by bytestrings. * Creighton Hogg: [28]Revisiting old concepts. * Tupil: [29]On unit testing and type checking. * Eric Kow (kowey): [30]darcs weekly news #9. * >>> Alberto Corona: [31]Axioms and properties for haskell classes. Alberto demonstrates some code for adding checkable algebraic properties to type classes. * Creighton Hogg: [32]A serious problem. Creighton with some straight talk about typeclass abuse. * London Haskell Users Group: [33]Next meeting: Duncan Coutts on the Haskell Platform. * Manuel M T Chakravarty: [34]Keep those cores busy!. A comprehensive account of the full vectorisation transformation underlying the implementation of nested data parallelism in GHC. * Conal Elliott: [35]Simpler, more efficient, functional linear maps. * Conal Elliott: [36]Vector space bases via type families. * >>> Sebastian Fischer: [37]Constrained Monadic Computations. * Luke Palmer: [38]I know what I'm going to get for christmas. "I believe I have found a truly marvelous implementation of FRP which this blog post is too short to contain." * >>> codeflow: [39]On Haskell and C++. * >>> Simon Dobson: [40]Why Haskell will take over the world. Simon has some thoughts on functional programming and the future of programming languages. * >>> Falko: [41]Learning Haskell's Basics - Problems from Project Euler. Quotes of the Week * vixey: debugging code is admitting defeat * kaomoji: [on #haskell] man, you guys are really nerdy * sclv: I guess I'd believe the universe was lazy if the hubble looked at that stuff way at the edge of the big bang diaspora and when we magnified the picture we saw "stack overflow" * rwbarton: relational calculus? "DIFFERENTIATE TABLE users WITH RESPECT TO name" * heatsink: ban :: (BanContext no, UserIdentifer u) => u -> no u * rwbarton: I tried typechecking the value of unwords (replicate 125 "fmap") in ghci once, and it consumed all my memory * ghc: Step 3: Zonk the kinds About the Haskell Weekly News New editions are posted to [42]the Haskell mailing list as well as to [43]the Haskell Sequence and [44]Planet Haskell. [45]RSS is also available, and headlines appear on [46]haskell.org. To help create new editions of this newsletter, please see the information on [47]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [48]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://www.haskell.org//pipermail/haskell-cafe/2008-October/049775.html 3. http://wiki.darcs.net/index.html/Sprints 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/46842 5. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/lambdabot 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/46841 7. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/autoproc 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/46776 9. http://www.haskell.org//pipermail/haskell-cafe/2008-October/049760.html 10. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ieee-utils 11. http://article.gmane.org/gmane.comp.lang.haskell.cafe/46750 12. http://www.cs.uu.nl/wiki/GenericProgramming/Rewriting 13. http://article.gmane.org/gmane.comp.lang.haskell.cafe/46696 14. http://www.haskell.org/pipermail/haskell/2008-October/020651.html 15. http://article.gmane.org/gmane.comp.lang.haskell.general/16538 16. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/lhs2tex 17. http://article.gmane.org/gmane.comp.lang.haskell.general/16524 18. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/colour 19. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/46764 20. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/46710 21. http://article.gmane.org/gmane.comp.lang.haskell.general/16511 22. http://members.microsoft.com/careers/search/details.aspx?JobID=E29D3886-A15… 23. http://planet.haskell.org/ 24. http://haskell.org/haskellwiki/Blog_articles 25. http://koweycode.blogspot.com/2008/10/darcs-hacking-sprints-some-pictures.h… 26. http://blog.codersbase.com/2008/10/24/understanding-darcs-commute/ 27. http://blog.malde.org/index.php/2008/10/24/optimization-again-befuddled-by-… 28. http://abstractabsurd.blogspot.com/2008/10/revisiting-old-concepts.html 29. http://blog.tupil.com/on-unit-testing-and-type-checking/ 30. http://koweycode.blogspot.com/2008/10/darcs-weekly-news-9.html 31. http://haskell-web.blogspot.com/2008/10/axioms-properties-for-haskell-class… 32. http://abstractabsurd.blogspot.com/2008/10/serious-problem.html 33. http://www.londonhug.net/2008/10/22/next-meeting-duncan-coutts-on-the-haske… 34. http://justtesting.org/post/55387637 35. http://feeds.feedburner.com/~r/conal/~3/425946113/ 36. http://feeds.feedburner.com/~r/conal/~3/425935010/ 37. http://www-ps.informatik.uni-kiel.de/~sebf/projects/constraint-monad.html 38. http://luqui.org/blog/archives/2008/10/19/i-know-what-im-going-to-get-for-c… 39. http://codeflow.wordpress.com/2008/10/18/on-haskell-and-c/ 40. http://www.simondobson.org/?p=179 41. http://informatikr.wordpress.com/2008/10/13/learning-haskells-basics-proble… 42. http://www.haskell.org/mailman/listinfo/haskell 43. http://sequence.complete.org/ 44. http://planet.haskell.org/ 45. http://sequence.complete.org/node/feed 46. http://haskell.org/ 47. http://haskell.org/haskellwiki/HWN 48. http://code.haskell.org/~byorgey/code/hwn/

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Current XML libraries status
by Krasimir Angelov 25 Oct '08

25 Oct '08

Hi, Does some one have made performance tests on the different XML libraries for Haskell? I have a 20MB xml file that I want to read. I remember from my earlier experiments (years ago) that all libraries were too slow and were consuming too much memory. I hoped that this situation had changed but maybe not. I looked at HaXML, libxml, HXML and HXT. HaXML eats a lot of memory and is still very slow. libxml is unfinished binding to the C library. Currently it only allows to create documents. HXML seems to be very promising. It works fast and it doesn't eat memory. Unfortunately it is that it seems to be rather old. It uses its own Arrow and Tree libraries instead of the standard libraries. I have not jumped into HXT yet because it seems to be very large library. Could someone recomend which one is the state of the art? Best Regards, Krasimir

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ANNOUNCE: colour 0.0.0
by roconnor＠theorem.ca 24 Oct '08

24 Oct '08

http://hackage.haskell.org/cgi-bin/hackage-scripts/package/colour-0.0.0 I hope for this library to become the standard colour library for Haskell. Most software does not properly blend colours because they fail to gamma-correct the colours before blending. Hopefully by using this library, Haskell programs dealing with colour blending will avoid this problem. I am making an early release of my colour library to get some feedback. I am especially interested in getting feedback on the interfaces: should functions be renamed, should functions be moved, etc. Should I put black and white colours into Data.Colour? Which is better form making a colour: (sRGB r g b) or (sRGB (r,g,b))? Bug reports and any patches are also welcome. Be warned, I haven't extensively tested this library yet. -- Russell O'Connor <http://r6.ca/> ``All talk about `theft,''' the general counsel of the American Graphophone Company wrote, ``is the merest claptrap, for there exists no property in ideas musical, literary or artistic, except as defined by statute.''

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Abusing quickcheck to check existential properties
by nr＠cs.tufts.edu 23 Oct '08

23 Oct '08

I recently used QuickCheck to check on some calculations for image compression. (I love exact rational arithmetic!) But I thought only to check for inverse properties, and I realized afterward I had failed to check for ranges. For example I should have checked that boundedB block = -1 <= b && b <= 1 where Cos a b c d = cos_of_block block which turns out to be correct. But actually my calculations were wrong, and the real bounds on b are not +/- 1 but rather +/- 0.5, so I might equally well have passed a more stringent test. It's quite embarrassing as I have only 5 bits of precision to represent b, and throwing away a bit on values that don't exist is not the best idea I have had recently. What I would really like to do is write some combinators for interval checking that say * It is a *universal* property that for *every* input, the output lies within the stated interval. * It is an *existential* property of *some* input that the output lies close to the ends of the stated interval. But how do I use QuickCheck to check an existential? I realize I could run boundsNotAchieved block = -0.9 <= b && b <= 0.9 where Cos a b c d = cos_of_block block and then if the test of boundsNotAcheived fails, my test passes (by exists x : P(x) iff not forall(x) : not P(x)). But if I set up a test suite in which some tests are supposed to fail and others are supposed to succeed, I will never keep track of which is which. I would like to build an existential test. Is it sufficient for me to write boundsAchieved block = expectFailure (-0.9 <= b && b <= 0.9) where Cos a b c d = cos_of_block block and then run 'quickCheck boundsAchieved'? In the long run I'd love something like fillsIntervalWithin :: (Num a, Ord a, Arbitrary a) => (a, a) -> a -> a -> Property with the idea that I want to test the partial application fillsIntervalWithin (lo, hi) epsilon and have the test succeed if every input x satisfies lo <= x <= hi and if some input x satisfies not (lo + epsilon <= x <= hi - epsilon). I'm betting someone on this list has already thought about similar problems and can advise me. (Because I have yet to write the specialized instance declaration for Arbitrary that guarantees the numerical invariants of the inputs, I have yet to test any of the examples in this email.) Norman

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