- Beginners - Haskell.org

plugins package
by chomp 24 Aug '08

24 Aug '08

Hello I am having troubles bulding the plugins package : manu:~/Desktop/tmp manu$ cabal install plugins Resolving dependencies... cabal: cannot configure plugins-1.2. It requires ghc >=6.8 There is no available version of ghc that satisfies >=6.8 I am on a Mac PPC (OS X 10.4) with ghc-6.8.2 and Cabal-1.4.0.1 so the error message is puzzling... Can anyone help ? I'd like to install Lambdabot and plugins is one of the dependencies... Thanks Manu

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Running Yi 0.4.3
by Niels Aan de Brugh 23 Aug '08

23 Aug '08

Hi there, I'm having some trouble installing and running Yi 0.4.3. I just compiled and installed Yi, after disabling parsec-3.0.0 and recompiling regex-tdfa first (Yi requires a 2.x version, regex takes whatever it can get, including 3.0.0). Here's a short excerpt from my console (newlines inserted for readability). niels(~/src/yi-0.4.3): sudo runhaskell Setup.hs install Installing: /usr/local/lib/yi-0.4.3/ghc-6.8.2 Installing: /usr/local/bin Registering yi-0.4.3... Reading package info from "dist/installed-pkg-config" ... done. Saving old package config file... done. Writing new package config file... done. niels(~): mkdir .yi niels(~): cd .yi/ niels(~/.yi): cp /home/niels/src/yi-0.4.3/examples/yi.hs . niels(~/.yi): yi Launching custom yi: "/home/niels/.yi/yi-i386-linux" yi: exception :: System.Glib.GError.GError niels(~/.yi): yi --frontend=pango Launching custom yi: "/home/niels/.yi/yi-i386-linux" yi-i386-linux: exception :: System.Glib.GError.GError niels(~/.yi): yi --frontend=gtk Launching custom yi: "/home/niels/.yi/yi-i386-linux" yi-i386-linux: exception :: System.Glib.GError.GError niels(~/.yi): ghc-pkg list /usr/lib/ghc-6.8.2/package.conf: (Cabal-1.2.3.0), Cabal-1.4.0.1, (Cabal-1.5.1), Crypto-4.1.0, HTTP-3001.0.4, HUnit-1.2.0.0, HaXml-1.13.3, QuickCheck-1.1.0.0, Stream-0.2.2, X11-1.4.1, array-0.1.0.0, arrows-0.3, base-3.0.1.0, binary-0.4.1, bitset-0.6, bloomfilter-1.2.1, bytestring-0.9.0.1, cairo-0.9.12.1, cairo-0.9.13, containers-0.1.0.1, directory-1.0.0.0, fgl-5.4.1.1, filepath-1.1.0.0, fingertree-0.0, gconf-0.9.12.1, gconf-0.9.13, (ghc-6.8.2), glade-0.9.12.1, glade-0.9.13, glib-0.9.12.1, glib-0.9.13, gnomevfs-0.9.13, gstreamer-0.9.13, gtk-0.9.12.1, gtk-0.9.13, gtkglext-0.9.12.1, gtkglext-0.9.13, haddock-2.1.0, haskell98-1.0.1.0, hpc-0.5.0.0, html-1.0.1.1, hxt-8.0.0, mtl-1.1.0.0, network-2.1.0.0, old-locale-1.0.0.0, old-time-1.0.0.0, packedstring-0.1.0.0, pandoc-0.46, parallel-1.0.0.0, parsec-2.1.0.0, pretty-1.0.0.0, process-1.0.0.0, pureMD5-0.2.3, random-1.0.0.0, readline-1.0.1.0, regex-base-0.72.0.1, regex-base-0.93.1, regex-compat-0.71.0.1, regex-compat-0.91, regex-posix-0.72.0.2, regex-posix-0.93.1, regex-tdfa-0.94, rts-1.0, soegtk-0.9.13, sourceview-0.9.12.1, sourceview-0.9.13, svgcairo-0.9.13, tagsoup-0.6, template-haskell-2.2.0.0, time-1.1.2.0, unix-2.3.0.0, utf8-string-0.3.1, vty-3.0.1, wx-0.10.3, wxcore-0.10.3, xhtml-3000.0.2.1, xmonad-0.6, xmonad-contrib-0.6, yi-0.4.3 niels(~/.yi): dpkg -l | grep libghc6-gtk ii libghc6-gtk-dev 0.9.12.1-1ubuntu2 A GUI library for Haskell (Gtk2Hs) -- GTK+ b ii libghc6-gtkglext-dev 0.9.12.1-1ubuntu2 A GUI library for Haskell (Gtk2Hs) -- gtkgle I'm running a recent version of Ubuntu Linux and GHC 6.8.2. Does anyone know what mistake I'm making? I'd really like to replace Vim for Yi for Haskell source file editing some day but up until now this is actually the first time I got Yi to compile. Regards, Niels

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purely functional (non-monadic) alternatives for Haskell animations in _The Haskell School of Expression_?
by Benjamin L.Russell 21 Aug '08

21 Aug '08

Recently, I mentioned Paul Hudak's _The Haskell School of Expression_ (see http://www.haskell.org/soe/) as an example of using multimedia examples to motivate learning programming in another education-related programming mailing list, plt-scheme, in a thread there entitled "More pedagogic stuff" (see http://list.cs.brown.edu/pipermail/plt-scheme/2008-August/026441.html) In reply, a respondent, Matthias Felleisen, wrote the following (see http://list.cs.brown.edu/pipermail/plt-scheme/2008-August/026445.html) >P.S. Yes, I have read Paul's book and I think Haskell has something >to it. >You may be surprised to learn, however, that world.ss animations are >purely functional while Haskell animations (in Paul's book) are actually >quasi-imperative. That is, they are using monads and carry the >imperativeness >on their sleeves. In response to this claim, does anybody know how to rewrite Hudak's SOE animations so that they do not use monads and are "purely functional?" -- Benjamin L. Russell

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Reading input
by Barry Burd 21 Aug '08

21 Aug '08

In the last line of this program, I get the following error message (from ghci)... Couldn't match expected type `Int' against inferred type `Char' Expected type: [Int] Inferred type: String In the first argument of `median', namely `lst' In the expression: median lst Failed, modules loaded: none. Here's the program... module Main where import IO lessThan :: Int -> [Int] -> [Int] lessThan x lst = filter (< x) lst greaterThan :: Int -> [Int] -> [Int] greaterThan x lst = filter (> x) lst numLessThan :: Int -> [Int] -> Int numLessThan x lst = length (lessThan x lst) numGreaterThan :: Int -> [Int] -> Int numGreaterThan x lst = length (greaterThan x lst) numLessGreater :: Int -> [Int] -> (Int, Int) numLessGreater x lst = (numLessThan x lst, numGreaterThan x lst) isMedian :: (Int, Int) -> Bool isMedian (x, y) = x == y medians :: [Int] -> [Int] medians lst = [x | x <- lst, isMedian (numLessGreater x lst)] median :: [Int] -> Int median lst = case medians lst of -- All the values in the result are the same, we just pick the first one x:xs -> x main = do putStr "Enter a list: " lst <- getLine median lst I'm sure this is because Haskell isn't automatically changing a String to a List of numbers. But how can I do this? Thanks.

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ghci interactive behaviour.
by Venkat Ramanan 20 Aug '08

20 Aug '08

Hi all, I'm just beginning to learn haskell. I see some strange behaviour in ghci. Don't know if it is related to readline. Behaviour-1: Lets say after we enter an expression, we just hit "Enter" 3 times to insert 3 empty lines. To get back the expression, we need to do <C-p> ( or up-arrow ) 3 times. In other words, the empty lines seem to be stored in the history. Bash ( and tcsh ) seem to save only non-empty lines. Behaviour-2: It doesn't seem to do pruning of duplicates. If the same expression is entered twice, both of them are stored in the history instead of just one. Bash uses environmental variables (HISTCONTROL and HISTIGNORE ) to modify this behavior. Is there a way to get saner behaviour like in bash? I'm using ghc-6.8.2 which comes with ubuntu 8.04. Thanks, Venkat.

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Absolute beginner's questions...
by Barry Burd 19 Aug '08

19 Aug '08

I'm just getting started with Haskell. I've written the following simple program: module Med where -- lst = [5, 3, 7, 8, 0, 2, 4, 1, 6] lst = [7, 3, 1, 2, 0, 9, 8] numLessThan x = length([y | y <- lst, y < x]) numGreaterThan x = length([y | y <- lst, y > x]) median = [ x | x <- lst, numLessThan x == numGreaterThan x] I realize that this code isn't very robust. My questions are as follows: - I remember running this code with some values of lst (containing an odd number of unique integer values) and getting the wrong answer. Is that possible? I can't replicate the results because I don't remember what values of lst didn't work. - How can I add a line or two to display intermediate results (in order to debug the code if necessary)? I've tried adding putTraceMsg but I keep getting parse errors. I probably haven't fully assimilated the syntax of functional programming yet. Thanks.

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Is there a way to use images as parameters to Haskell functions?
by Benjamin L.Russell 18 Aug '08

18 Aug '08

Recently, I came across a new book for beginner-level programmers that uses the idea of images as parameters to functions, and was wondering whether there is any way to do this in Haskell (they use PLT Scheme in DrScheme): How to Design Programs/2e: Prolog - Section 2 - Arithmetic and Arithmetic: http://www.ccs.neu.edu/home/matthias/HtDP/Prologue/book-Z-H-4.html > Before we show you how to do some ``real'' programming, let's discuss one more kind of data to spice things up: > >> <rocket-s.jpg> At the above prompt, the actual image of <rocket-s.jpg> is inserted, not the textual reference to the image. I.e., there is a picture at the prompt, instead of text. Further, these images can be used as parameters, similarly to strings, in functions; viz.: >One important library -- world.ss -- supports operations for computing the width and height of an image: > >(* (image-width ) > (image-height )) > >Once you have added a library to your program, clicking Run gives you > > 1200 > >because that's the area of a 30 x 40 image. The idea of using images as parameters to functions in teaching programming to beginners could be useful in motivating learning Haskell as well, and should be implementable as such. Does anybody know of any similar tool for Haskell? -- Benjamin L. Russell

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Ruby Quiz Solution #15 - won't work after compilation
by Alex Watt 14 Aug '08

14 Aug '08

If I call "main" in ghci, my program runs as it should, but if I try compiling it, and then running the result of that compilation, i.e.: $ ghc -o animal_game game.hs $ ./animal_game unexpected things occur, particularly "getChar" does not work correctly, and if I take a route that doesn't lead to getChar needing to be called, the program stops running at the end of 2nd run despite your response to the do-you-want-to-play-again-question. $ ./animal_game .... Do you want to play again? (y/n) y .... Do you want to play again? (y/n) y Thanks for playing! $ The source code is here: http://rafb.net/p/7fYlEV72.html I don't know how long that will stay up there, so let me know if it's gone and I'll repaste or find a better alternative. What would also be nice is any comments on the actual code; as a begginner, it's nice to get some feedback, and I might as well also ask the following question. Originally I wanted to the types to be as such: data Animal = Animal String data Question a b = Question String a b -- where a is a Question or an Animal, as is b The problem is I wanted methods that had types that were able to accept an argument of type Animal or Question, and for it to act appropriately on that type, I tried adding them to common type classes (a new one I created called 'Askable' which contained a method 'ask'), but I ran into a whole bunch of problems. Is there any easy way to do it?

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Haskell in K-12 education?
by Benjamin L.Russell 13 Aug '08

13 Aug '08

Does anybody know of any uses of Haskell in teaching programming in K-12 education? Recently, I read an interesting essay entitled "Why Computer Science Doesn’t Matter" (see http://www.ccs.neu.edu/home/matthias/essays/not-matter.html) by Matthias Felleisen and Shriram Krishnamurthi, on how using multimedia examples in teaching programming in K-12 education could potentially elevate programming to a Fourth R ("'rogramming"), in addition to the current Three Rs ("reading," "’riting," and "’ritmetic"). They suggest tying together programming and mathematics, using the idea that "modern arithmetic and algebra doesn’t have to be about numbers," but that "[i]t may involve images, strings, symbols, Cartesian points, and other forms of “objects.”" Their discussion reminded me of _The Haskell School of Expression_ (see http://www.haskell.org/soe/) by Paul Hudak, which also uses multimedia examples to motivate programming, but at a more advanced (college) level. Currently, DrScheme (see http://www.drscheme.org/) (based on Scheme) and Squeak (see http://www.squeak.org/) (based on a dialect of Smalltalk) are two programming environments that are often used in K-12 schooling to teach programming, but I see no reason that Haskell could not fulfill this role as well. Does anybody know of any books/tools for using Haskell as a tool to teach programming in K-12 education? -- Benjamin L. Russell

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SOEGraphics
by Simon Parry 13 Aug '08

13 Aug '08

Hello, apologies if this is the wrong forum. I'm having trouble getting going with the SOEGraphics library (I'm trying to work my way through The Haskell School of Expression book). I'm running GHC 6.8.3 using emacs on a windows XP machine. I'm getting a 'Could not find module `GraphicsUtils'' load error when I import SOEGraphics but I can see the file in the correct folder (which is included in my .emacs file)? Any suggestions? thanks Simon Parry

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