April 2009 - Haskell-Cafe

Printing list of enum type
by michael rice 14 Apr '09

14 Apr '09

What do I need to add to this Color enum type to print a list of them? Michael =============== data Color = Red | Blue | Green | Yellow | Orange | Brown | White | Black instance Show Color where show Red = "Red" show Blue = "Blue" show Green = "Green" show Yellow = "Yellow" show Orange = "Orange" show Brown = "Brown" show White = "White" show Black = "Black" ====================== Ok, modules loaded: Main. *Main> Red Red *Main> Black Black *Main> [Red Black White] <interactive>:1:1: Couldn't match expected type `Color -> Color -> t' against inferred type `Color' In the expression: Red Black White In the expression: [Red Black White] In the definition of `it': it = [Red Black White]

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Re: [Haskell-cafe] Printing list of enum type
by michael rice 14 Apr '09

14 Apr '09

Dumb, dumb, dumb, dumb, DUMB!, he Lisped. Thanks. Michael --- On Tue, 4/14/09, Andrew Wagner <wagner.andrew(a)gmail.com> wrote: From: Andrew Wagner <wagner.andrew(a)gmail.com> Subject: Re: [Haskell-cafe] Printing list of enum type To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Tuesday, April 14, 2009, 9:19 AM You don't need to add anything, you just need to use list notation correctly. Try typing [Red, Black, White] in at the prompt instead. The commas are part of the list syntax. On Tue, Apr 14, 2009 at 9:15 AM, michael rice <nowgate(a)yahoo.com> wrote: What do I need to add to this Color enum type to print a list of them? Michael =============== data Color = Red | Blue | Green | Yellow | Orange | Brown | White | Black instance Show Color where show Red = "Red" show Blue = "Blue" show Green = "Green" show Yellow = "Yellow" show Orange = "Orange" show Brown = "Brown" show White = "White" show Black = "Black" ====================== Ok, modules loaded: Main. *Main> Red Red *Main> Black Black *Main> [Red Black White] <interactive>:1:1: Couldn't match expected type `Color -> Color -> t' against inferred type `Color' In the expression: Red Black White In the expression: [Red Black White] In the definition of `it': it = [Red Black White] _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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ANN: hmatrix-static: statically-sized linear algebra
by Reiner Pope 14 Apr '09

14 Apr '09

Hi everyone, I am pleased to announce hmatrix-static[1], a thin wrapper over Alberto Ruiz's excellent hmatrix library[2] for linear algebra. The main additions of hmatrix-static over hmatrix are: - vectors and matrices have their length encoded in their types - vectors and matrices may be constructed and destructed using view patterns, affording a clean, safe syntax. hmatrix-static includes statically-sized versions of hmatrix's linear algebra routines, including: - simple arithmetic (addition, multiplication, of matrices and vectors) - matrix inversion and least squares solutions - determinants / rank / condition number - computing eigensystems - factorisations: svd, qr, cholesky, hessenberg, schur, lu - exponents and sqrts of matrices - norms See http://code.haskell.org/hmatrix-static/examples/ for example code. Installation and requirements: Because of the use of type families and view patterns, hmatrix-static requires GHC 6.10 to build. The linear algebra routines are ultimately implemented by BLAS, GSL and LAPACK; see the hmatrix manual [3] for instructions on installing these, prior to installing hmatrix. Once this has been done, hmatrix-static can be installed by $ cabal install hmatrix-static I would be very interested in your feedback, so please try it out. Kind regards, Reiner Pope [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hmatrix-static [2] http://www.hmatrix.googlepages.com/ [3] http://www.hmatrix.googlepages.com/installation

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The Monad.Reader (14) - Call for copy
by Wouter Swierstra 14 Apr '09

14 Apr '09

Call for Copy The Monad.Reader - Issue 14 Please consider writing something for the next issue of The Monad.Reader. The deadline for Issue 14 is: ** May 15, 2009 ** The Monad.Reader is a electronic magazine about all things Haskell. Check out the website and browse the previous editions to learn more: http://www.haskell.org/haskellwiki/The_Monad.Reader * Submission Details * Get in touch if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue to me by e-mail (wouter at chalmers.se). Articles should be written according to the guidelines available from: http://www.haskell.org/haskellwiki/The_Monad.Reader Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. Looking forward to your submission, Wouter

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Types and hashes of hashes, trouble for a Java-programmer...
by John Smith 14 Apr '09

14 Apr '09

Hi, a java-programmer running into trouble while trying to learn Haskell. I try to make a hash containing hashes but can not getting a value out of the innermost hash - I keep running into type-trouble where IO and Maybe monad is getting mixed? My attempt: removeMaybeHash x = case x of Just ref -> ref Nothing -> HashTable.new (==) (\key -> key) test = do h <- HashTable.new (==) (\key -> key) h1 <- HashTable.new (==) (\key -> key) HashTable.insert h 3 h1 HashTable.insert h1 1 1000 maybeOuterHash <- HashTable.lookup h 3 res <- removeMaybe (removeMaybeHash maybeOuterHash) 1000 return res Any clues? In java this would be as simple as (pseudocode): h.lookup(3).lookup(1000) Cheers, Pedro

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Monads from Functors
by Sebastian Fischer 14 Apr '09

14 Apr '09

> {-# LANGUAGE Rank2Types #-} Dear Haskellers, I just realized that we get instances of `Monad` from pointed functors and instances of `MonadPlus` from alternative functors. Is this folklore? > import Control.Monad > import Control.Applicative In fact, every unary type constructor gives rise to a monad by the continuation monad transformer. > newtype ContT t a = ContT { unContT :: forall r . (a -> t r) -> t r } > > instance Monad (ContT t) > where > return x = ContT ($x) > m >>= f = ContT (\k -> unContT m (\x -> unContT (f x) k)) Both the `mtl` package and the `transformers` package use the same `Monad` instance for their `ContT` type but require `t` to be an instance of `Monad`. Why? [^1] If `f` is an applicative functor (in fact, a pointed functor is enough), then we can translate monadic actions back to the original type. > runContT :: Applicative f => ContT f a -> f a > runContT m = unContT m pure If `f` is an alternative functor, then `ContT f` is a `MonadPlus`. > instance Alternative f => MonadPlus (ContT f) > where > mzero = ContT (const empty) > a `mplus` b = ContT (\k -> unContT a k <|> unContT b k) That is no surprise because `empty` and `<|>` are just renamings for `mzero` and `mplus` (or the other way round). The missing piece was `>>=` which is provided by `ContT` for free. Are these instances defined somewhere? Cheers, Sebastian [^1] I recognized that Janis Voigtlaender defines the type `ContT` under the name `C` in Section 3 of his paper on "Asymptotic Improvement of Computations over Free Monads" (available at http://wwwtcs.inf.tu-dresden.de/~voigt/mpc08.pdf) and gives a monad instance without constraints on the first parameter.

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Language.Haskell.Parser and literate Haskell source
by Vasyl Pasternak 13 Apr '09

13 Apr '09

Hello, Are there ability to parse literate haskell source with Language.Haskell.Parser, or maybe there is some tool, which converts .lhs files to .hs ? Thanks in advance. -- Best regards, Vasyl Pasternak

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implementing multilingual interface with dynamic language switching
by Vasyl Pasternak 13 Apr '09

13 Apr '09

Hello, Recently, I've tried to use hgettext to implement dynamic language switching and support of several languages simultaneously. All my tries I've described in blog article http://progandprog.blogspot.com/2009/04/multilingual-ui-and-dynamic-languag… . Comments and suggestions are welcome :) -- Best regards, Vasyl Pasternak

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Haskell Weekly News: Issue 113 - April 13, 2009
by Brent Yorgey 13 Apr '09

13 Apr '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090413 Issue 113 - April 13, 2009 --------------------------------------------------------------------------- Welcome to issue 113 of HWN, a newsletter covering developments in the [1]Haskell community. Announcements xmobar-0.9.2. Andrea Rossato [2]announced the release of [3]xmobar-0.9.2, which features a fix for a longstanding resource leakage bug, new nested color definitions, and more. CFP Haskell Symposium 2009. Stephanie Weirich [4]reminded everyone that there are only 4 weeks until the submission deadline for the [5]2009 Haskell Symposium! hmatrix-static: statically-sized linear algebra. Reiner Pope [6]announced the release of [7]hmatrix-static, a thin wrapper over Alberto Ruiz's excellent [8]hmatrix library for linear algebra. The main additions of hmatrix-static over hmatrix are that vectors and matrices have their length encoded in their types, and vectors and matrices may be constructed and destructed using view patterns, affording a clean, safe syntax. haskellmode for Vim now at projects.haskell.org. Claus Reinke [9]announced that the Haskell mode plugins for Vim have just completed their move to their [10]new home, and took the opportunity to reiterate what they can do (quite a lot, it seems), and mention that some screencasts are available. MSem replacement for QSem. ChrisK [11]announced [12]MSem, a proposed replacement module for Control.Concurrent.QSem. Hac5 is almost upon us!. Sean Leather [13]reminded everyone that in six days, tens of crazy/obsessed, type-safe, functional programmers will be converging on Utrecht to commence execution of the [14]5th Haskell Hackathon, from April 17-19, 2009 in Utrecht, The Netherlands! The local organizing team welcomes you all and looks forward to all of the new developments that come out of everyone's undying quest to write more and better code. Yogurt-0.4. Martijn van Steenbergen [15]announced version 0.4 of [16]Yogurt, a functional MUD client. Version 0.4 makes Yogurt available as a standalone executable that is able to dynamically load and reload Yogurt scripts. Elerea, another FRP library. Patai Gergely [17]announced the release of [18]Elerea, aka "Eventless reactivity", a minimalistic FRP implementation that comes with a convenient applicative interface, supports recursive definition of signals and signals fed from outside by IO actions, plays nice with resources, and is the result of some furious hacking. There are working examples to show off the current capabilities of the library, found in the separate [19]elerea-examples package. tree-monad 0.2. Sebastian Fischer [20]announced version 0.2 of the [21]tree-monad package, which provides instances of MonadPlus that represent the search space of non- deterministic computations as a tree. Version 0.2 implements an optimized CPS version of the tree. HCard -- A library for implementing card-like structures.. Joe Fredette [22]announced the release of [23]HCard, a library which supports a card-like data structures and uses associated types to provide shuffling, dealing, and other facilities. It's general enough to support many different types of playing cards; it currently comes with the common "French Deck" (4-suit, 13 card deck that is very common in the US) implemented and an example cribbage scoring application. SVGFonts 0.1. Tillmann Vogt [24]announced his first Haskell library, [25]SVGFonts 0.1, which parses the relatively unknown SVG Font format to produce outlines of characters. The big advantage of this format is that it is XML, which means easy parsing and manipulating. network-bytestring 0.1.2. Johan Tibell [26]announced a new release of [27]network-bytestring, a Haskell library for fast socket I/O using ByteStrings. New in this release is support for scatter/gather I/O (also known as vectored I/O). Scatter/gather I/O provides more efficient I/O by using one system call to send several separate pieces of data and by avoiding unnecessary copying. Jobs Lecturer in Computer Science, University of Leicester. Roy L. Crole [28]announced an opening for a lectureship at the University of Leicester. The successful candidate will have a strong or promising research record in computer science, with a background in formal foundations (either algorithms and complexity, or semantics of programming or modelling languages), and will be able to contribute to undergraduate and postgraduate teaching and supervision in software engineering. Blog noise [29]Haskell news from the [30]blogosphere. * Mark Wassell: [31]Edge Detection in Haskell. * Sean Leather: [32]Latest on the incremental fold and attributes. * Sean Leather: [33]Haskell mode for Vim on a Mac. * Sean Leather: [34]Incremental attributes. * >>> Human Constraints: [35]Haskell Type Hackery. * LHC Team: [36]A new beginning.. * London Haskell Users Group: [37]Don Stewart: Engineering Large Projects in Haskell: A Decade of Haskell at Galois. * Ulisses Costa: [38]More Hylomorphisms in Haskell. * Ulisses Costa: [39]Anamorphisms in Haskell. * London Haskell Users Group: [40]Next Meeting: Don Stewart from Galois. * LHC Team: [41]Hello world!. * mightybyte: [42]Adding Authentication to the Blog App. * Leon Smith: [43]Polynomial multiplication. * Joe Fredette: [44]Card games, scorekeeping, and ... Associated Datatypes?. * mightybyte: [45]Basic Happstack Blog App. * mightybyte: [46]A Standalone Auth Framework for Happstack. * Mads Lindstrøm: [47]Introducing WxGeneric. * happstack.com: [48]2 new projects that use Happstack. * >>> Brant Carlson: [49]Lightning Modeling in Haskell. * Leon Smith: [50]Lloyd Allison's Corecursive Queues. About the Haskell Weekly News New editions are posted to [51]the Haskell mailing list as well as to [52]the Haskell Sequence and [53]Planet Haskell. [54]RSS is also available, and headlines appear on [55]haskell.org. To help create new editions of this newsletter, please see the information on [56]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [57]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.general/17080 3. http://code.haskell.org/~arossato/xmobar/ 4. http://article.gmane.org/gmane.comp.lang.haskell.general/17072 5. http://haskell.org/haskell-symposium/2009/ 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/56772 7. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hmatrix-static 8. http://www.hmatrix.googlepages.com/ 9. http://article.gmane.org/gmane.comp.lang.haskell.cafe/56495 10. http://projects.haskell.org/haskellmode-vim/ 11. http://article.gmane.org/gmane.comp.lang.haskell.cafe/56761 12. http://haskell.org/haskellwiki/SafeConcurrent 13. http://article.gmane.org/gmane.comp.lang.haskell.cafe/56758 14. http://haskell.org/haskellwiki/Hac5 15. http://article.gmane.org/gmane.comp.lang.haskell.cafe/56747 16. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Yogurt 17. http://article.gmane.org/gmane.comp.lang.haskell.cafe/56731 18. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/elerea 19. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/elerea-examples 20. http://article.gmane.org/gmane.comp.lang.haskell.cafe/56639 21. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/tree%2Dmonad 22. http://article.gmane.org/gmane.comp.lang.haskell.cafe/56573 23. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HCard 24. http://article.gmane.org/gmane.comp.lang.haskell.cafe/56444 25. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/SVGFonts 26. http://article.gmane.org/gmane.comp.lang.haskell.cafe/56374 27. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/network-bytestri… 28. http://article.gmane.org/gmane.comp.lang.haskell.general/17066 29. http://planet.haskell.org/ 30. http://haskell.org/haskellwiki/Blog_articles 31. http://www.poundstone.org/blog/?p=137 32. http://feedproxy.google.com/~r/splonderzoek/~3/KWqNCgsK1ww/latest-on-increm… 33. http://feedproxy.google.com/~r/splonderzoek/~3/0QNaK4byWhg/haskell-mode-for… 34. http://feedproxy.google.com/~r/splonderzoek/~3/4B0bHoeTCeQ/incremental-attr… 35. http://blog.human-constraints.com/?p=49 36. http://lhc-compiler.blogspot.com/2009/04/new-beginning.html 37. http://www.londonhug.net/2009/04/10/don-stewart-engineering-large-projects-… 38. http://ulissesaraujo.wordpress.com/2009/04/09/more-hylomorphisms-in-haskell/ 39. http://ulissesaraujo.wordpress.com/2009/04/08/anamorphisms-in-haskell/ 40. http://www.londonhug.net/2009/04/09/next-meeting-don-stewart-from-galois/ 41. http://lhc-compiler.blogspot.com/2009/04/hello-world.html 42. http://softwaresimply.blogspot.com/2009/04/adding-authentication-to-blog-ap… 43. http://blog.melding-monads.com/2009/04/07/polynomial-multiplication/ 44. http://lowlymath.net/2009/card-games-scorekeeping-and-associated-datatypes/ 45. http://softwaresimply.blogspot.com/2009/04/basic-happstack-blog-app.html 46. http://softwaresimply.blogspot.com/2009/04/standalone-auth-framework-for-ha… 47. http://lindstroem.wordpress.com/2008/05/03/introducing-wxgeneric/ 48. http://blog.happstack.com/2009/04/06/2-new-projects-that-use-happstack/ 49. http://bayern.stanford.edu/~brant/lightning/ 50. http://blog.melding-monads.com/2009/03/09/lloyd-allisons-corecursive-queues/ 51. http://www.haskell.org/mailman/listinfo/haskell 52. http://sequence.complete.org/ 53. http://planet.haskell.org/ 54. http://sequence.complete.org/node/feed 55. http://haskell.org/ 56. http://haskell.org/haskellwiki/HWN 57. http://code.haskell.org/~byorgey/code/hwn/

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Re: [Haskell-beginners] Why does forkIO also create native threads?
by Peter Verswyvelen 13 Apr '09

13 Apr '09

It seems that it's not the forkIO that is creating native threads (using GHC 6.10.1, -O flag) For example, the following code only has 5 threads on my Windows box, while I fork 10 lightweight threads import System.IO import Control.Concurrent import Control.Monad entry :: Char -> IO() entry c = do forever $ putChar c main = do hSetBuffering stdout NoBuffering mapM_ (forkIO . entry) ['0'..'9'] putStrLn "all forked; press ENTER to quit" getLine However, things can get bizarre. When we change the entry function into entry :: Char -> IO() entry c = do putChar c threadDelay maxBound Then on my machine 13 native threads *are* created. This might make sense because its waiting forever, but native threads are also created for the following code: entry :: Char -> IO() entry c = do putChar c forever $ threadDelay 0 This feels like overkill, so it seems threadDelay always forks a new native thread, although this is not documented. We can also get into the dark corners of GHC's lightweight thread scheduling: a thread switch only happens when memory is allocated on the GC head (but this is documented in the API) So the following code will not work, the runtime seems to get stuck in an infinite loop after the first thread is created. entry :: Char -> IO() entry c = do putChar c forever $ return () In artificial cases like this you can use yield entry :: Char -> IO() entry c = do putChar c forever yield On Mon, Apr 13, 2009 at 10:37 AM, Lych <lych77(a)gmail.com> wrote: > I wrote a simple ECHO server like this: > ---------------------------------------------------- > import Network > import System.IO > import Control.Concurrent > > service cs = do > ln <- hGetContents cs > hPutStr cs ln > service cs > > > acceptloop ls = do > (cs, host, port) <- accept ls > hSetBuffering cs LineBuffering > forkIO (service cs) > acceptloop ls > > main = do > ls <- listenOn (PortNumber 10061) > acceptloop ls > ---------------------------------------------------- > > And tested it with a client that initiates as many parallel > connections as possible. The number of connections can just reach to > 1000+ on my machine, but I thought it should be more, since forkIO > generates lightweight threads. I also examined the number of threads > in this process with some tool and it proved there was one native > thread per connection. When I replaced the 'forkIO' in the program > with 'forkOS', I got an exactly similar result. Have I thought > something wrong about forkIO? > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners >

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