May 2011 - Haskell-Cafe

Trying to return a map from State monad
by michael rice 20 May '11

20 May '11

I'm not sure what's going on BELOW. Was working with mapAccumL earlier and decided to move over to State monad for more control. :m + Data.Map:m + Data.ListPrelude Data.List Data.Map> let f key new old = new ++ oldPrelude Data.List Data.Map> let moby2 = "Moby Dick is a great book. Moby Dick was a white whale."Prelude Data.List Data.Map> fst $ mapAccumL (\ (m,p) w -> ((insertWithKey' f p [w] m, (snd p, w)),"")) (singleton ("\n", "\n") [], ("\n","\n")) (words moby2)(fromList [(("\n","\n"),["Moby"]),(("\n","Moby"),["Dick"]),(("Dick","is"),["a"]),(("Dick","was"),["a"]),(("Moby","Dick"),["was","is"]),(("a","great"),["book."]),(("a","white"),["whale."]),(("book.","Moby"),["Dick"]),(("great","book."),["Moby"]),(("is","a"),["great"]),(("was","a"),["white"])],("white","whale.")) Michael BELOW=========================== import Control.Monad.Stateimport Data.Map type Prefix = (String,String)type GeneratorState = (Map Prefix [String],Prefix,[String]) non_word = "\n" f key new old = new ++ old buildMap :: GeneratorState (Map Prefix [String]) buildMap = do (mp,(pfx1,pfx2),all@(w1:words)) <- get if (Prelude.null all) then {- No more words. Return final map (adding non_word for prefix). -} return (insertWithKey' f (pfx1,pfx2) [non_word] mp) else {- Add word to map at prefix. Continue. -} put (insertWithKey' f (pfx1,pfx2) [w1] mp, (pfx2,w1), words) buildMap ========================== Prelude> :l markov3.hs[1 of 1] Compiling Main ( markov3.hs, interpreted ) markov3.hs:11:12: `GeneratorState' is applied to too many type arguments In the type signature for `buildMap': buildMap :: GeneratorState (Map Prefix [String])Failed, modules loaded: none.

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where is it appropriate to write library-related suggestions?
by Nicholas Tung 20 May '11

20 May '11

Hi all, I am curious if there is an avenue to make suggestions / comments about libraries? For example, "proc" in System.Process conflicts with the Arrows notation "proc", yielding confusing error messages. thanks, Nicholas — https://ntung.com — CS and Mathematics major @ UC Berkeley

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For Euler 25; What is the first term in the Fibonacci sequence to contain 1000 digits?; the following seems to work.
by KC 20 May '11

20 May '11

For Euler 25; What is the first term in the Fibonacci sequence to contain 1000 digits?; the following seems to work. -- For number of digits being 5 or more. fibNdigits :: Int -> Int fibNdigits nDigits = floor (((fromIntegral nDigits) - 1.0) / (logBase 10 phi)) + 2 where sq5 = sqrt 5 :: Double phi = (1 + sq5) / 2 -- -- Regards, KC

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Status of Haskell + Mac + GUIs & graphics
by Dummy 19 May '11

19 May '11

Hi, i am really happy about this lively discussion around Haskell (Mac) GUIs. I might hint to two other options: 1) Eclipse's SWT wraps the native platform's toolkits quite sucessfully. The platform-dependent bindings are mostly automatically generated, afaik. Some time ago i summarized my thoughts about extending SWT's infrastructure/code for Haskell here: http://www.reddit.com/r/haskell_proposals/comments/9w7nk/adjust_the_swt_bin… 2) Ruby's Shoes provides simple native wrappers as i've pointed out on Reddit: http://www.reddit.com/r/haskell/comments/he8pl/status_of_haskell_mac_guis_g… Bye, Lenny

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Haskell Weekly News: Issue 182
by Daniel Santa Cruz 19 May '11

19 May '11

Welcome to issue 180 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of May 8 to 14, 2011. Announcements Eric Kow released the second edition of Parallel Haskell Digest [http://goo.gl/OXGIw] Don Stewart sent in a proposal from the Haskell.org committee to incorporate Haskell.org [http://goo.gl/X1j8i] Lazycat Manatee has released a new version of Manatee [http://goo.gl/vVTgy] Quotes of the Week * Richard O'Keefe: [On iterators and generators] This being Haskell, I'm expecting to see Cogenerator... * luqui: Haskell is a DSL for writing monad tutorials. * mattmight: Coq is a tool for turning ordinary programs into POPL papers. * kmc: most CS courses will teach you four kinds of Java * geheimdienst: haskell makes hard things easy and easy things a strong lax monoidal endofunctor ... * spoolio: Haskell programmers know the cost of nothing, the type of everything, and might know the value of a few things later. * catamorphism: FP is related to FP in the same way that UML is related to OOP. * geheimdienst: * geheimdienst will from now on use a function carpeTiem aliased to getCurrentTime * edwardk: [on data-accessor] However, its choice of internal representation makes me throw up in my mouth a little bit Top Reddit Stories * Fancy function graphs! Domain: pnyf.inf.elte.hu, Score: 47, Comments: 21 On Reddit: http://goo.gl/ibZO1 Original: http://goo.gl/24hQ6 * Online SVG figures with Haskell Domain: self.haskell, Score: 46, Comments: 11 On Reddit: http://goo.gl/jgxk4 * Erlang-in-Haskell: an implementation of an Erlang-like distributed computing framework for Haskell Domain: hackage.haskell.org, Score: 41, Comments: 2 On Reddit: http://goo.gl/yMMRG Original: http://goo.gl/eq6s3 * Why Eager Languages Don't Have Products and Lazy Languages Don't Have Sums Domain: james-iry.blogspot.com, Score: 38, Comments: 35 On Reddit: http://goo.gl/ngpNe Original: http://goo.gl/2O6yq * Introducing JsonGrammar Domain: martijn.van.steenbergen.nl, Score: 34, Comments: 2 On Reddit: http://goo.gl/wiGI9 Original: http://goo.gl/ns20k * The reasons I don't write all my code in Haskell Domain: codeflow.wordpress.com, Score: 33, Comments: 34 On Reddit: http://goo.gl/HMZ8c Original: http://goo.gl/hyuSa * Experimental GHC with GHCJS built in and Cabal support for JavaScript files Domain: article.gmane.org, Score: 30, Comments: 3 On Reddit: http://goo.gl/8zzZT Original: http://goo.gl/MQMvN * Two fun bits of Haskell teaching news Domain: serpentine.com.nyud.net, Score: 28, Comments: 3 On Reddit: http://goo.gl/JQUtF Original: http://goo.gl/mYz1q * Themes on Streams Domain: byorgey.wordpress.com, Score: 26, Comments: 1 On Reddit: http://goo.gl/ywP1K Original: http://goo.gl/ywPnl * CmdArgs - Fighting the GHC Optimiser Domain: neilmitchell.blogspot.com, Score: 20, Comments: 15 On Reddit: http://goo.gl/0WThF Original: http://goo.gl/CgCgz Top StackOverflow Questions * Safe execution of untrusted Haskell code votes: 19, answers: 2 http://goo.gl/9eHuc * When do I use ByteString and when do I not? votes: 16, answers: 2 http://goo.gl/vVQig * Why can I not make String an instance of a typeclass? votes: 14, answers: 3 http://goo.gl/Pn8DS * Android application in haskell votes: 11, answers: 1 http://goo.gl/UfnA1 * What are the most important abstractions in Haskell? Monads? Applicatives? votes: 9, answers: 3 http://goo.gl/RaXVQ About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz

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ANNOUNCE: levmar-1.0
by Bas van Dijk 19 May '11

19 May '11

Dear all, I released levmar-1.0, an implementation of the Levenberg-Marquardt minimization algorithm that can be used for least squares, curve fitting and nonlinear programming. The most important change in this release is the use of vectors and matrices from the hmatrix package (which uses the high performance vector package). An application at work became 25 times faster when switching to the new levmar! I'm not sure this major speedup can be entirely attributed to the switch from lists to vectors. I also switched the FFI types from CDouble to Double in the internal bindings-levmar package. This made the expensive realToFrac conversions from CDouble to Double unnecessary which I believe had a big impact. API Documentation: http://hackage.haskell.org/package/levmar-1.0 I also switched the repository from darcs to git and hosted it on github: https://github.com/basvandijk/levmar/ Regards, Bas

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ANNOUNCE: Haskell Communities and Activities Report (20th ed., May 2011)
by Janis Voigtländer 19 May '11

19 May '11

On behalf of all the contributors, I am pleased to announce that the Haskell Communities and Activities Report (20th edition, May 2011) is now available in PDF and HTML formats: http://haskell.org/communities/05-2011/report.pdf http://haskell.org/communities/05-2011/html/report.html Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. I hope you will find it as interesting a read as I did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline ;-) to the call. The editor collects all the contributions into a single report and feeds that back to the community. When I try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ========================================== End of October 2011: target deadline for contributions to the November 2011 edition of the HC&A Report ========================================== Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editor for a simple e-mail reminder in October (you could point me to them as well, and I can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Janis Voigtlaender <hcar at haskell.org>

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GPL, LGPL, and GHC Linking
by Jon Kristensen 19 May '11

19 May '11

Hello everyone! I just want to let you know that I have written a blog post about my thoughts about the issue of GHC linking and the General Public Licenses. I welcome any feedback or discussions around it. You can find the post at http://www.jonkristensen.com/gpl-lgpl-and-ghc-linking/. Warm regards, Jon Kristensen

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Open CV or alternate image processing library for Haskell on windows?
by Gregory Guthrie 19 May '11

19 May '11

I wanted to look into using Haskell for an introductory Image Processing class, but the main package used for such things (OpenCV) does not appear to be available for windows systems. Is there some other good option for image processing in Haskell, or has anyone ported openCV to a windows Leksah environment? -------------------------------------------

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Re: [Haskell-cafe] ANNOUNCE: iterIO-0.1 - iteratee-based IO with pipe operators
by Simon Marlow 19 May '11

19 May '11

On 12/05/2011 16:04, David Mazieres expires 2011-08-10 PDT wrote: > At Thu, 12 May 2011 09:57:13 +0100, > Simon Marlow wrote: >> >>> So to answer my own question from earlier, I did a bit of >>> benchmarking, and it seems that on my machine (a 2.4 GHz Intel Xeon >>> 3060, running linux 2.6.38), I get the following costs: >>> >>> 9 ns - return () :: IO () -- baseline (meaningless in itself) >>> 13 ns - unsafeUnmask $ return () -- with interrupts enabled >>> 18 ns - unsafeUnmask $ return () -- inside a mask_ >>> >>> 13 ns - ffi -- a null FFI call (getpid cached by libc) >>> 18 ns - unsafeUnmask ffi -- with interrupts enabled >>> 22 ns - unsafeUnmask ffi -- inside a mask_ >> >> Those are lower than I was expecting, but look plausible. There's room >> for improvement too (by inlining some or all of unsafeUnmask#). > > Do you mean inline unsafeUnmask, or unmaskAsyncExceptions#? I tried > inlining unsafeUnmask by writing my own version and giving it the > INLINE pragma, and it didn't affect performance at all. Right, I meant inlining unmaskAsyncExceptions#, which would require compiler support. >> However, the general case of unsafeUnmask E, where E is something more >> complex than return (), will be more expensive because a new closure for >> E has to be created. e.g. try "return x" instead of "return ()", and >> try to make sure that the closure has to be created once per >> unsafeUnmask, not lifted out and shared. > > Okay. I'm surprised by getpid example wasn't already stressing this, > but, indeed, I see a tiny difference with the following code: > > ffi>>= return . (1 +) -- where ffi calls getpid > > 13 ns - no unmasking > 20 ns - unsafeUnmask when not inside _mask > 25 ns - unsafeUnmask when benchmark loop in inside one big _mask > > So now we're talking about 28 cycles or something instead of 22. > Still not a huge deal. Ok, sounds reasonable. >> There are no locks here, thanks to the message-passing implementation we >> use for throwTo between processors. > > Okay, that sounds good. So then there is no guarantee about ordering > of throwTo exceptions? That seems like a good thing since there are > other mechanisms for synchronization. What kind of ordering guarantee did you have in mind? We do guarantee that in throwTo t e1 throwTo t e2 Thread t will receive e1 before e2 (obviously, because throwTo is synchronous and only returns when the exception has been raised). Pending exceptions are processed in LIFO order (for no good reason other than performance), so there's no kind of fairness guarantee of the kind you get with MVars. One thread doing throwTo can be starved by others. I don't think that's a serious problem. Cheers, Simon

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