February 2010 - Haskell-Cafe

Using ShowS or Difference Lists
by Mark Spezzano 08 Feb '10

08 Feb '10

Hi, Just wondering whether I can use ShowS or tupling or Difference Lists to speed up the following code? It's basic text processing. It takes in a list of Lines where each Line is a list of Words and intersperses " " between them then concatenates them into a longer String. Note that there is a recursive call and the ++ operator. Thanks Mark -- Function: joinLines -- Joins the Words within Lines together with whitespace and newline characters -- Argument: Lines to pad with whitespace and newlines -- Evaluate: The processed and concatenated String joinLines :: [Line] -> String joinLines (l:[]) = concat (intersperse " " l) joinLines (l:ls) = (concat (intersperse " " l)) ++ ('\n':joinLines ls)

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Links to darcs.haskel.org in the haskellwiki
by Henk-Jan van Tuyl 08 Feb '10

08 Feb '10

L.S., There are a lot of links in the haskellwiki that point to projects at darcs.haskel.org; I hope that anyone who moves a project, looks the links up and updates them. An example of a page with several obsolete links is http://www.haskell.org/haskellwiki/Applications_and_libraries/Web_programmi… Validity of links can automatically be checked with the linkchecker: http://validator.w3.org/checklink I have done quite a lot of updating of links in the past, I hope more people are willing to check and update pages that haven't been touched for a while. Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html --

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Spelling checker exercise
by Matthew Phillips 08 Feb '10

08 Feb '10

Hello all, sorry to bring up an old chestnut, but I’m trying to improve my Haskell-fu by writing a small program, and chose Peter Norvig’s spelling checker as my exercise material (http://norvig.com/spell-correct.html) While I’ve gotten it working, and found it quite illuminating, I also found it to to be very slow. And then I discovered that, of course, others have been here well before me ([1] and [2]). Those discussions were very interesting, but the code they refer to is mostly not available, so the most I’ve gotten out of it so far is that: (a) I should be using strict left folds and strict Map insertions for the word frequency map (this shaved off about a second: ~5s -> ~4s for a single word on my 2.4GHz MacBook Pro, GHC 6.10.4) (b) I should probably be using ByteString’s (c) Using Set’s for the edit permutations probably isn’t worth it (although I found using plain lists made it about a second slower) (b) is difficult because I’ve used matching patterns plus list comprehensions to generate the potential edits, and I really like how elegantly it pans out that way. Because ByteString’s are not lists, I can’t see a way to keep the current structure and use them. The code is at [3] (link to version at time of post). Profiling [4] shows: $ ./spelling becuase +RTS -p becuase -> because $ cat spelling.prof total time = 4.02 secs (201 ticks @ 20 ms) total alloc = 1,544,257,792 bytes (excludes profiling overheads) COST CENTRE MODULE %time %alloc train Main 52.7 19.7 readFile Main 28.9 8.6 wordsBy Main 10.9 49.5 toLower Main 7.0 21.8 ... So it appears that “train" (building the freq map) and “readFile” in “nwords" are the places to hone. I will look at using Bloom Filters or Trie’s instead of Data.Map, but I wonder if readFile should be taking nearly %30 of the run time, even for a 6MB file? Sorry to dump such a long post on the list — I’ll understand if no one can be bothered rehashing this. But, in summary I’d like to know: (a) how could I use ByteString’s for this to speed up I/O and reduce memory usage without losing the nice readability? (b) should readFile be so slow? (c) any other tips Possibly all my questions could be answered if someone has the code from the old posts. Cheers, Matthew. [1]: http://haskell.markmail.org/search/?q=norvig%20spelling#query:norvig%20spel… [2]: http://thread.gmane.org/gmane.comp.lang.haskell.cafe/21780 [3]: http://github.com/scramjet/spelling/blob/44fd336ef4f62d49c7087dbc1ffb9c009b… [4]: http://github.com/scramjet/spelling/blob/f5146b24b77443c22975ff69fb91aa922c…

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Interpreting profiling results
by Gregory Collins 08 Feb '10

08 Feb '10

Hello all, A friend and I are trying to benchmark some network server code which runs fine on my OSX laptop (~7500 req/s) but which is >10x slower on a much faster 8-core Xeon Linux box. The profiling run on Linux looks like this: individual inherited COST CENTRE MODULE no. entries %time %alloc %time %alloc MAIN MAIN 1 0 95.7 0.2 100.0 100.0 ... How should we interpret this result? MAIN doesn't seem to correspond to any user code so we're wondering where the time is going. G. -- Gregory Collins <greg(a)gregorycollins.net>

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cabal install time
by Ozgur Akgun 08 Feb '10

08 Feb '10

I hate sending such messages to cafe, but... When I try to install "time" I get the following error: > sudo cabal install time ld: duplicate symbol ___hscore_d_name in /Users/username/.cabal/lib/unix-2.4.0.0/ghc-6.10.4/libHSunix-2.4.0.0.a(dirUtils.o) and /Library/Frameworks/GHC.framework/Versions/610/usr/lib/ghc-6.10.4/base-4.1.0.0/libHSbase-4.1.0.0.a(PrelIOUtils.o) UsingSnow Leopard with the following versions of ghc and cabal > ghc --version The Glorious Glasgow Haskell Compilation System, version 6.10.4 > cabal --version cabal-install version 0.6.4 using version 1.6.0.3 of the Cabal library Thanks for any help. Best, -- Ozgur Akgun

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Haskell Weekly News: Issue 149 - February 08, 2010
by jfredett＠gmail.com 08 Feb '10

08 Feb '10

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20100208 Issue 149 - February 08, 2010 --------------------------------------------------------------------------- Welcome to issue 149 of HWN, a newsletter covering developments in the [1]Haskell community. Hello Haskellers, this week's HWN was delayed a bit in the hopes of making it a bit more substantial. I hate putting up thin HWNs, but of course this must occasionally happen. We have several new CFPs for workshops this week, a new benchmarking package, and some fun quotes. Till next week, Haskellers, your Haskell Weekly News! Announcements Call for Papers: Haskell Symposium 2010. Jeremy.Gibbons [2]announced a call for papers for the 2010 ACM SIGPLAN Haskell Symposium 2010 in Baltimore, Maryland; on 30th September. 2nd CfP: LOPSTR 2010. Temur Kutsia [3]announced a second call for papers for the 20th International Symposium on Logic-Based Program Synthesis and Transformation, being held in Hagenberg, Austria, July 23-25, 2010 (co-located with PPDP 2010). PAR 2010: First CFP. Koen Claessen [4]announced a first call for papers for PAR'10, the Workshop on Partiality and Recursion in Interactive Theorem Provers. data-ordlist-0.2. Leon Smith [5]announced a new release of ordlist, including a change to the module name and bug fixes. progression-0.1. Neil Brown [6]announced Progression, a metalibrary which consolidates various existing tools for Haskell optimization (notably Criterion). HList darcs repo. Oleg He who inhabits all types, but is not _|_, has [7]announced a new darcs repo for HList (and OOHaskell) on community.haskell.org. Discussion a beginner question: decorate-op-undecorate. Aran Donohue [8]asked about how to write a function which can 'inspect' inside a datatype. Translation of Haskell type classes. Enrique Martin [9]talked about some experiments he's done with type classes, and asked some questions regarding optimization related to them. Category Theory woes. Mark Spezzano [10]asked about resources for learning about category theory. Blog noise [11]Haskell news from the [12]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Dan Piponi (sigfpe): [13]The Categorification of the Naturals. * Roman Cheplyaka: [14]hledger. * Luke Palmer: [15]Associative Alpha Blending. * Neil Brown: [16]Progression: Supporting Optimisation in Haskell. * Holumbus: [17]Hayoo! Webservice API. * apfelmus: [18]The Operational Monad Tutorial. * Dan Piponi (sigfpe): [19]Tagging Monad Transformer Layers. * Darcs: [20]darcs weekly news #53. * JP Moresmau: [21]EclipseFP 1.110 released, with debugging support. Quotes of the Week * lispy|web: This curses binding appears to be terminally broken * lispy: I did, 'cabal install mage' and it complains about curses * lament: Just use fix to find the least funny joke * copumpkin: A monad is just a lax functor from a terminal bicategory, duh. fuck that monoid in category of endofunctors shit About the Haskell Weekly News New editions are posted to [22]the Haskell mailing list as well as to [23]the Haskell Sequence and [24]Planet Haskell. [25]RSS is also available, and headlines appear on [26]haskell.org. To help create new editions of this newsletter, please see the information on [27]how to contribute. Send stories to jfredett . at . gmail . dot . com. The darcs repository is available at darcs get [28]http://patch-tag.com/r/jfredett/HWN2/pullrepo HWN2 . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.general/17778 3. http://article.gmane.org/gmane.comp.lang.haskell.general/17770 4. http://article.gmane.org/gmane.comp.lang.haskell.general/17769 5. http://article.gmane.org/gmane.comp.lang.haskell.cafe/70035 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/69929 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/69898 8. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/69984 9. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/69914 10. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/69803 11. http://planet.haskell.org/ 12. http://haskell.org/haskellwiki/Blog_articles 13. http://blog.sigfpe.com/2010/02/decategorification-of-naturals.html 14. http://ro-che.blogspot.com/2010/02/hledger.html 15. http://lukepalmer.wordpress.com/2010/02/05/associative-alpha-blending/ 16. http://chplib.wordpress.com/2010/02/04/progression-supporting-optimisation-… 17. http://holumbus.fh-wedel.de/blog/?p=33 18. http://apfelmus.nfshost.com/articles/operational-monad.html 19. http://blog.sigfpe.com/2010/02/tagging-monad-transformer-layers.html 20. http://blog.darcs.net/2010/02/darcs-weekly-news-53.html 21. http://jpmoresmau.blogspot.com/2010/02/eclipsefp-1110-released-with-debuggi… 22. http://www.haskell.org/mailman/listinfo/haskell 23. http://sequence.complete.org/ 24. http://planet.haskell.org/ 25. http://sequence.complete.org/node/feed 26. http://haskell.org/ 27. http://haskell.org/haskellwiki/HWN 28. http://patch-tag.com/r/jfredett/HWN2/pullrepo%20HWN2

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Re: [Haskell-cafe] a beginner question: decorate-op-undecorate
by John Lato 08 Feb '10

08 Feb '10

Hi Stephen, > From: Stephen Tetley <stephen.tetley(a)gmail.com> > > Hi John > > I'm not sure about making Binding polymorphic to get Functor, > Traversable, Foldable... > > While I think you're correct that partitionEithers might not be a > useful example to draw from in this case, I'd assume that Binding > would be part of a larger syntax-tree, thus there might not be a > appropriate single leaf to make the tree polymorphic on. Felipe > Lessa's point - to use Uniplate or one of the Generics packages - > might be a better candidate for implementing traversals. I did not consider that, and hadn't seen any other replies when I first replied. If true, I would agree that one of the Generics packages would be the best option. Regardless, I think this is a valid approach. By thinking of data in terms of Functor, Traversable, etc., one can recognize these patterns. Even if Binding is part of a larger syntax tree and can't reasonably be made polymorphic, creating an fmap-like function to do what the OP requested is IMHO simple and idiomatic Haskell. Using Generics then simplifies the implementation. Cheers, John

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Number formatting
by Günther Schmidt 08 Feb '10

08 Feb '10

Hello, is there a function which will format a number with separators, something that will turn 14509.8674 into 14,509.86? Günther

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GUI programming
by Victor Nazarov 08 Feb '10

08 Feb '10

Hello, I've been writing some GUI application with Gtk2hs. It's an interpreter for lambda-calculus and combinatory logic, it's GPL and if you interested I can share it with cafe. The problem is that the GUI code has become very ugly and I'm tempted to rewrite it totally. I've been looking forward to the FRP stuff, but I've never seen a single definition of the term. Conal Eliot's "denotational programming" is too general to be definition. I want to try Grapefruit, but I got totally lost when I see arrow notation. I consider more lightweight and more imperative approach, something closer to CSP (Communicating Secuential Processes) then FRP. I've just crafted some sample program to illustrate my idea. The behaviour is a monad and it's IO monad so you can do any IO (Gtk2hs) programming you wish. The differences is that you don't attach static event handlers and tries to determine what to do dependent on application state. You attach and detach handlers as much as possible. Behaviour looks like a process that can stop execution and wait for some GUI event. When event arrived it continues execution. Do you see this approach viable. There are steel some details to emerge: * How to wait for several events * How to handle IO exceptions Here is the code: {-# LANGUAGE ExistentialQuantification #-} module Main where import Data.IORef import System.Glib import Graphics.UI.Gtk import Control.Monad.Trans type Event obj = IO () -> IO (ConnectId obj) data Behaviour a = forall b. BBind (Behaviour b) (b -> Behaviour a) | BIO (IO a) | forall obj. GObjectClass obj => BWaitEvent (Event obj) (Behaviour a) instance Monad Behaviour where action >>= generator = BBind action generator return a = BIO (return a) instance MonadIO Behaviour where liftIO action = BIO action runBehaviour :: Behaviour a -> IO a runBehaviour (BBind (BWaitEvent event after) f) = runBehaviour (BWaitEvent event (after >>= f)) runBehaviour (BBind (BIO a) f) = a >>= \x -> runBehaviour (f x) runBehaviour (BBind (BBind a f) g) = runBehaviour (a >>= (\x -> f x >>= g)) runBehaviour (BIO a) = a runBehaviour (BWaitEvent event after) = do sigIdRef <- newIORef (error "You can't access sigIdRef before signal is connected") sigId <- event $ do sigId <- readIORef sigIdRef signalDisconnect sigId runBehaviour after return () writeIORef sigIdRef sigId return (error "You can't expect result from behaviour") waitEvent :: GObjectClass obj => Event obj -> Behaviour () waitEvent event = BWaitEvent event (return ()) main :: IO () main = do initGUI window <- windowNew onDestroy window mainQuit set window [windowTitle := "Hello World"] button <- buttonNew let buttonB label = do liftIO $ set button [buttonLabel := label] waitEvent (onClicked button) buttonB (label ++ "*") runBehaviour (buttonB "*") set window [containerChild := button] widgetShowAll window mainGUI -- Victor Nazarov

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cabal fun (not)
by Johannes Waldmann 08 Feb '10

08 Feb '10

while trying to live with different versions of packages, I found cabal-install's behaviour strange because: you can "cabal install" a package without any complaint but the next "ghc-pkg list" is all red because of conflicts. and it's too late then - there's no way to get back. even if you re-install some package, it seems it is getting a different hashcode, so it's not recognized. if you were doing "cabal install --global" for some ghc package (e.g. old-locale), then ghc seems completely hosed and you've got to re-install the compiler. well that's not a big deal by itself but afterwards you have to re-install all packages ... so please please please have "cabal install" fail with some error message if (that is, before) the install would break anything. - J. $ ghc --version The Glorious Glasgow Haskell Compilation System, version 6.12.1 $ cabal --version cabal-install version 0.8.0 using version 1.8.0.2 of the Cabal library

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