- Beginners - Haskell.org

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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Problem building packages
by emmanuel.delaborde 11 Aug '08

11 Aug '08

Hello, I am trying to build HAppS-Util with cabal-install it fails with the following message: could not execute: /Library/Frameworks/GHC.framework/Versions/608/usr/ lib/ghc-6.8.3/ghc-asm What can I do to circumvent this ? Thanks E ----------------------------------------------------------------------------------------------- This e-mail (and any attachments) is confidential and may contain personal views which are not the views of Cimex Media Ltd and any affiliated companies, unless specifically stated. It is intended for the use of the individual or group to whom it is addressed. If you have received it in error, please delete it from your system, do not use, copy or disclose the information in any way nor act in reliance on it and please notify postmaster(a)cimex.com A company registered in England Wales. Company Number 03765711 Registered Office : The Olde Bakehouse, 156 Watling Street East, Towcester, Northants NN12 6DB This email was scanned by Postini, the leading provider in Managed Email Security.

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Factorials using foldx
by Paul Johnston 08 Aug '08

08 Aug '08

Hi (First question so be gentle)! Just started to look at Haskell and I was wondering about various versions of calculating factorials. If I have facr 0 = 1 facr n = foldr (*) 1 [1..n] facl 0 = 1 facl n = foldl (*) 1 [1..n] Is there any difference in efficiency, I remember reading it is better to count down than to count up but left to right or right to left :-) Cheers Paul Paul Johnston Humanities Development Team Room 2.12 Bridgeford Street Building Manchester University Tel 0161 275 1396 Mail Paul.Johnston(a)manchester.ac.uk Web http://web-1.humanities.manchester.ac.uk/prjs/mcasspj/ How to shoot yourself in the foot Unix % ls foot.c foot.h foot.o toe.d toe.o %rm* .o rm:.o no such file or directory

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Representing a subset relationship
by Levi Stephen 04 Aug '08

04 Aug '08

Hi, I was wondering what the best way to represent a 'subset' type relationship would be. The use case I'm looking at is where the subset would be a user selection, so a filter would not be appropriate. For example, say I have something like: type Order = [OrderItem] type OrderItem = OrderItem { qty :: Int, product: Product } Now I'd like the user to be able to select a number of items (e.g., priority shipping, gift wrapping) One possible way I can see is to add extra fields to the OrderItem record, but this seems like it would not be easy to extend. It doesn't seem like using [Product] would be appropriate as if the Product is updated it will need to be updated in both the OrderItem list and in the subset. Another approach would be to use a ProductIdType in place of Product, then look up the actual Product as needed. Are there other (and better) ways to do this? Thanks, Levi

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Pattern Matching
by Alex Watt 03 Aug '08

03 Aug '08

Hi there, The best way I can explain what I'd like to do is show an example, so here goes: same :: a -> a -> Bool same x x = True same _ _ = False Is there any way to do this in Haskell? The actual reason I'd like to do this is something slightly more complicated. I want to match against a list of elements, like so: foo [x,x,x] = ... Thanks for any help. - Alex

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