August 2009 - Haskell-Cafe

Another thanks for inspiration to RWH - Bloom Filters
by Günther Schmidt 31 Aug '09

31 Aug '09

Hi all, My app has to use rather large amounts of static data in order to run. I have been using an SQLite database to hold this data so far. The largest chunk of data was a table with roughly 80k records. I only make use of this table to see if a particular key is present. Reading RWH over from time to time I came accross the Bloom Filter and finally realized that this would be a much better solution. So I could insert all the keys in the table into a suitable bloom filter instead and the just query the bloom filter instead of the database. I haven't figured out yet how to write a "literal" representation of the "filled" bloom filter yet though and could do with some tipps. I'm using the bloomfilter package from hackage. Günther

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ANN: moe html combinator
by Jinjing Wang 30 Aug '09

30 Aug '09

hand written html is fine, except for the closing tags: what if html' - do head' - do meta [http_equiv "Content-Type", content "text/html; charset-utf-8"] (/) title' - str "my title" link [rel "icon", _type "image/png", href "panda_icon.png"] (/) body' - do div [_class "container"] - do str "hello world" produces <html> <head> <meta http-equiv="Content-Type" content="text/html; charset-utf-8"> </meta> <title> my title </title> <link rel="icon" type="image/png" href="panda_icon.png"> </link> </head> <body> <div class="container"> hello world </div> </body> </html> That's moe. There's also extra dsl sugar / flavors for your convienence, including (currently available) markdown and kawaii. It's a new library, tries to bring some haskell fun back to web programming, so .. enjoy. - moe: http://hackage.haskell.org/package/moe -- jinjing

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[Haskell Cafe] ASCII Graphics Library wanted
by Paul Sujkov 30 Aug '09

30 Aug '09

Hi, is there any (by any means) portable ASCII-Graphics library for Haskell, such as NCurses or AALib? I see HaHa library on Hackage, but is it the only existing library for such a purpose (version 0.2 alerts me a bit)? Or maybe someone has any experience in developing some rogue-like gams in Haskell? :) -- Regards, Paul Sujkov

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Re: [Haskell-cafe] GUI library
by Michael Mossey 30 Aug '09

30 Aug '09

Hi Jean-Denis, Thanks for the information. Do you know how WxHaskell fits my needs? For example, does it have good docs and examples for a beginner? Does it have the ability to draw lines and characters on a surface? Does it have a type of "canvas" which usually refers to an optimized drawing surface? Thanks, Mike Jean-Denis Koeck wrote: > I began writing a commercial app with a GUI using Gtk2hs, > but it looked ugly on Windows. I'm switching to WxHaskell. > > 2009/8/29 Michael Mossey <mpm(a)alumni.caltech.edu > <mailto:mpm@alumni.caltech.edu>> > > I want to choose a GUI library for my project. Some background: I'm > a beginner to functional programming and have been working through > Haskell books for a few months now. I'm not just learning Haskell > for s**ts and giggles; my purpose is to write > music-composition-related code; in particular, I want to write a > graphical musical score editor. (Why write my own editor, you may > ask? Because I want to fully integrate it with > computer-assisted-composition algorithms that I plan to write, also > in Haskell.) I decided to use Haskell for its great features as a > functional programming language. > > Regarding a choice of GUI library, I want these factors: > > - it needs to provide at a minimum a drawing surface, a place I can > draw lines and insert characters, in addition to all the standard > widgets and layout capabilities we have to come to expect from a GUI > library. > > - This is a Windows application. > > - it needs to be non-confusing for an intermediate-beginner > Haskeller. Hopefully good documentation and examples will exist on > the web. > > - It might be nice to have advanced graphics capability such as Qt > provides, things like antialiasied shapes, and a canvas with > efficient refresh (refereshes only the area that was exposed, and if > your canvas items are only primitives, it can do refreshes from > within C++ (no need to touch your Haskell code at all). However I'm > wondering if qtHaskell fits my criteria "well-documented" and "lots > of examples aimed at beginners". > > Thanks, > Mike > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org <mailto:Haskell-Cafe@haskell.org> > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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How to use dyre relaunch/restore State of Gtk2hs Object?
by Andy Stewart 30 Aug '09

30 Aug '09

Hi all. I try to use dyre (http://hackage.haskell.org/packages/archive/dyre/0.7.2/doc/html/Config-Dyre…) to relaunch/restore State of TextView to make TextView's content can't lost when main program reboot. Below is source code for example: ------------------------------> source code start <------------------------------ module DyreExample where import Graphics.UI.Gtk hiding (get) import qualified Config.Dyre as Dyre import Config.Dyre.Relaunch import Config.Dyre.Compile import System.IO import Event import Test import Data.Binary import qualified Graphics.UI.Gtk.Gdk.Events as E data Config = Config { message :: String, errorMsg :: Maybe String } defaultConfig :: Config defaultConfig = Config "Dyre Example v0.1" Nothing showError :: Config -> String -> Config showError cfg msg = cfg { errorMsg = Just msg } realMain Config{message = message, errorMsg = errorMsg } = do initGUI textView <- textViewNew tn <- restoreTextState testName tv <- restoreBinaryState textView putStrLn tn rootWindow <- windowNew rootWindow `onDestroy` mainQuit windowFullscreen rootWindow -- rootWindow `containerAdd` textView rootWindow `containerAdd` tv widgetShowAll rootWindow rootWindow `onKeyPress` (\event -> dyreKeyTest event textView) mainGUI dyreExample = Dyre.wrapMain $ Dyre.defaultParams { Dyre.projectName = "Main" , Dyre.configDir = Just (return "/home/andy/Projects/Haskell/dyre/") , Dyre.cacheDir = Just (return "/test/Download/cache/") , Dyre.realMain = realMain , Dyre.showError = showError } dyreMainParams = Dyre.defaultParams { Dyre.projectName = "Main" , Dyre.configDir = Just (return "/home/andy/Projects/Haskell/dyre/") , Dyre.cacheDir = Just (return "/test/Download/cache/") , Dyre.realMain = realMain , Dyre.showError = showError } dyreTestParams = Dyre.defaultParams {Dyre.projectName = "Test" ,Dyre.configDir = Just (return "/home/andy/Projects/Haskell/dyre/")} dyreKeyTest :: E.Event -> TextView -> IO Bool dyreKeyTest ev textView = do case eventTransform ev of Nothing -> return False Just e -> do let eventName = eventGetName e case eventName of "M-m" -> do out2 <- customCompile dyreMainParams putStrLn $ show out2 relaunchWithTextState testName Nothing relaunchWithBinaryState textView Nothing return True _ -> return False -- instance Binary TextView where -- put a = put a -- get = get ------------------------------> source code end <------------------------------ On the surface, just add TextView's Binary instance can fix problem. But TextView is Haskell binding (by gtk2hs) for C struct in GTK+ library, i wonder how to write Binary instance for TextView. My aim is write GUI program (through gtk2hs) will keep GUI State when main program recompile/reboot (like XMonad). Example, above code, i add TextView widget in Window, and type "I love Haskell" in it, when this program recompile/reboot itself, the content "I love Haskell" still display in TextView widget, and not empty. So any ideas or suggestions? Thanks! -- Andy

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Problem with monadic formlets
by Colin Paul Adams 30 Aug '09

30 Aug '09

I'm trying to validate user input against a database (using HaskellDB, but that doesn't seem to be the problem, as replacing the database monadic code with return True gives the same problem. This is part of my code: register :: Database -> XForm Registration --register db = Registration <$> pure_user <*> passConfirmed register db = Registration <$> (user db) <*> passConfirmed user :: Database -> XForm String user db = pure_user `F.checkM` F.ensureM valid error where valid name = do let q = do t <- table user_table restrict (t!user_name .==. constant name) return t rs <- query db q return $ null rs error = "Username already exists in the database!" pure_user :: XForm String pure_user = input `F.check` F.ensure valid error where input = "Username" `label` F.input Nothing valid = (>= 3) . length error = "Username must be three characters or longer." passConfirmed :: XForm String passConfirmed = fst <$> passwords `F.check` F.ensure equal error where passwords = (,) <$> pass "Password" <*> pass "Password (confirm)" equal (a, b) = a == b error = "The entered passwords do not match!" pass :: String -> XForm String pass caption = input `F.check` F.ensure valid error where input = caption `label` F.password Nothing valid = (>=6) . length error = "Password must be six characters or longer." If I uncomment the commented line, and comment out the line after it (in register), then everything works as expected. However, using it as it is, one of the calls to pass gets the user's name for validation (and consequently either fails if the user name is only 5 characters, or the comparison of the two passwords fail (unless I type the user name as the password). I thought the applicative style meant the effects did not influence one another, but here there is clear contamination. What am i doing wrong? -- Colin Adams Preston Lancashire

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How to calculate de number of digits of an integer? (was: Is logBase right?)
by Roberto López 30 Aug '09

30 Aug '09

Ok. I wonder if someone could help me with this problem... I want to calculate the number of digits of a positive integer. I was thinking of ... numDigits n = truncate (logBase 10 n) + 1 But (logBase 10 1000) = 2.9999999999999996 so numDigits 1000 = 2. Maybe adding a small amount numDigits n = truncate ((logBase 10 n) + 0.0000000000000005) + 1 Prelude> numDigits 100 3 Prelude> numDigits 1000 4 Prelude> numDigits 10000 5 Prelude> numDigits 10000 5 Prelude> numDigits 100000 6 Prelude> numDigits 1000000 7 Prelude> numDigits 10000000 8 Prelude> numDigits 100000000 9 Prelude> numDigits 1000000000 9 <---- This is wrong!!!! Prelude> numDigits 10000000000 11 Is there a reliable way to calculate the number of digits by means of logBase? Regards!

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Re: [Haskell-cafe] How to calculate de number of digits of an integer?
by Henning Thielemann 29 Aug '09

29 Aug '09

Bulat Ziganshin schrieb: > Hello Henning, > > Tuesday, August 25, 2009, 7:01:24 PM, you wrote: > >> I hope that 'show' will not need quadratic time but will employ a more >> efficient algorithm > > yes, you are right I thought a little about it. If I had to implement that in GMP it could be done quite fast in many cases: Count the number of bits, say it is 'k' and multiply with logBase 10 2. If 2^k and 2^(k+1)-1 have the same number of decimal digits, we are done. Otherwise we have to process some of the most significant bits. If the number is between n*2^k and (n+1)*2^k-1 and both bounds have the same number of decimal digits (logBase 10 n + k * logBase 10 2), we are also done. Only for numbers close to powers of 10 we have to process the whole integer.

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Re: [Haskell-cafe] How to calculate the number of digits of an integer? (was: Is logBase right?)
by Uwe Hollerbach 29 Aug '09

29 Aug '09

Ouch! That is indeed an improvement... I don't recall all the details of this codelet, but I think I got the seed off the net somewhere (perhaps this list?), and it might well have been better originally. So, brightly brightly and with beauty, I probably executed a verschlimmbesserung. After a year and a half, I find I still have almost no intuition about performance issues in haskell... guess I have to practice more. Uwe On 8/29/09, Bertram Felgenhauer <bertram.felgenhauer(a)googlemail.com> wrote: > Uwe Hollerbach wrote: >> Here's my version... maybe not as elegant as some, but it seems to >> work. For base 2 (or 2^k), it's probably possible to make this even >> more efficient by just walking along the integer as stored in memory, >> but that difference probably won't show up until at least tens of >> thousands of digits. >> >> Uwe >> >> ilogb :: Integer -> Integer -> Integer >> ilogb b n | n < 0 = ilogb b (- n) >> | n < b = 0 >> | otherwise = (up 1) - 1 >> where up a = if n < (b ^ a) >> then bin (quot a 2) a >> else up (2*a) >> bin lo hi = if (hi - lo) <= 1 >> then hi >> else let av = quot (lo + hi) 2 >> in if n < (b ^ av) >> then bin lo av >> else bin av hi > > We can streamline this algorithm, avoiding the repeated iterated squaring > of the base that (^) does: > > -- numDigits b n | n < 0 = 1 + numDigits b (-n) > numDigits b n = 1 + fst (ilog b n) where > ilog b n > | n < b = (0, n) > | otherwise = let (e, r) = ilog (b*b) n > in if r < b then (2*e, r) else (2*e+1, r `div` b) > > It's a worthwhile optimization, as timings on n = 2^1000000 show: > > Prelude T> length (show n) > 301030 > (0.48 secs, 17531388 bytes) > Prelude T> numDigits 10 n > 301030 > (0.10 secs, 4233728 bytes) > Prelude T> ilogb 10 n > 301029 > (1.00 secs, 43026552 bytes) > > (Code compiled with -O2, but the interpreted version is just as fast; the > bulk of the time is spent in gmp anyway.) > > Regards, > > Bertram > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Hackage rankings : August 2009
by Don Stewart 29 Aug '09

29 Aug '09

Monthly statistics on the most popular Haskell applications and libraries on Hackage. August 2009 edition now up: http://donsbot.wordpress.com/2009/08/29/haskell-popularity-rankings-septemb… -- Don

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