- Beginners - Haskell.org

Defining 'words' in terms of 'span'
by Roger Whittaker 17 Mar '10

17 Mar '10

I found some exam papers linked from this page: http://www.cs.chalmers.se/Cs/Grundutb/Kurser/d1pt/d1pta/external.html And I have been trying some questions from them. I'm currently baffled by question 2(b) on this one: http://www.cs.chalmers.se/Cs/Grundutb/Kurser/d1pt/d1pta/tenta2000-04.ps A word is a sequence of alphabetic characters, which you can recognise using the standard function isAlpha :: Char -> Bool Using span, define a function words :: String -> [String] which finds a list of the words occurring in a string. For example, words "Now is the winter of our discontent!" == ["Now","is","the","winter","of","our","discontent"] words "2+3" == [] words "1 by 1" == ["by"] Can anyone give me a clue how to start? -- ======================== Roger Whittaker roger(a)disruptive.org.uk http://disruptive.org.uk ========================

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about installation of Haskell
by love_pku 17 Mar '10

17 Mar '10

Dear Sir or Madam:: Sorry to trouble you! I am a beginner of Haskell and I just run into problems to install it. So I hope if you can tell me which version of Haskell should I use for Ubuntu 8.04 operating system.. Thank you very much! Best wishes, Yours juan

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generating by mapping
by Szilveszter Juhos 16 Mar '10

16 Mar '10

Hi, I have something like: data Stuff = Stuff { aNum :: Int , anStr :: String } deriving (Show) let first = Stuff 123 "qwe" let second = Stuff 321 "asd" print first print second and works fine. What is the right "map" statement if I want to generate these stuff using lists like: let strs = [ "qwe", "asd", "zxc" ] let nums = [ 123, 321, 345] ? Cheers: Szilva

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Reading in the contents of a variable number of files
by Tim Perry 16 Mar '10

16 Mar '10

I need to read in and then combine a variable number of files. I am trying to figure out how to get the contents of all the files into an array. let fileContentsArray = getData ["dataFile1.txt", "dataFile2.txt"] getData :: [String] -> [String] getData [] = [] getData (f:files) = do fileData <- readFile f fileData : (getData files) I figured I could use readFile to open the files. But, I need to use <- to get the data out of the IO Action and into a value. I thought I could write a function like the following, but I'm getting no-where. I suspect I'm missing something basic about IO in Haskell. Can anybody point me to a good resource.... Thanks, Tim

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Parsec 3.0
by Tim Perry 15 Mar '10

15 Mar '10

When I install Parsec using Cabal, I get version 2.1.0.1. Is there a way to get version 3.1? Or do I have to build it by hand? Thanks, Tim C:\Documents and Settings\perry\haskell>cabal upgrade parsec --reinstall Resolving dependencies... Configuring parsec-2.1.0.1... Preprocessing library parsec-2.1.0.1... Building parsec-2.1.0.1... ....

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Struct-like syntax issues
by Sergey Mironov 14 Mar '10

14 Mar '10

Hi. I have definitions like data MyData = MyData { a :: A, b :: B, c :: C, d :: D, ... z :: Z } makeMyData :: A -> B -> MyData makeMyData a' b' = MyData { a = a', b = b', c = default_c, ... z = default_z } The goal is to use it like myData1 = set_x x' . set_m m' . set_c c' $ makeMyData my_a my_b myData2 = set_z z' . $ makeMyData my_a my_b But. Writing set_* functions is a problem - MyData is loong and things like set_d :: D -> MyData -> MyData set_d val struct = struct{d=val} will form that boilerplate. Is there any _right_ way of doing it? myData3 = (makeMyData my_a my_b) {x=x', z=z'} approach is not working.. -- Thanks, Sergey

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Randomly selecting a data constructor
by Evgenij Merenkov 14 Mar '10

14 Mar '10

Hello. I have the following data and it's constructors: FigType = TR1 | TR2 | TR3 | TR4 | SQ | L1 | L2 | Z1 | Z2 | Z3 | Z4 | NoFigure I need to randomly return one of them in a function. The obvious solution to me, is simply using a random number generator and case statement, where each value generated is associated with each data constructor. Like this: import System.Random randomFigType :: IO FigType randomFigure = do x <- randomRIO (1::Int,11::Int) return $ case x of 1 -> TR1 2 -> TR2 3 -> TR3 e t.c. But is there a more direct way to do this? _________________________________________________________________ Hotmail: Trusted email with Microsoft’s powerful SPAM protection. https://signup.live.com/signup.aspx?id=60969

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homogeneous value list
by Luca Ciciriello 14 Mar '10

14 Mar '10

Hi. I know, this is a very basic question but I've had some trouble with it. I've a list and I want to write a function isHomogeneous :: [String] -> Bool returning True is all the elements of the list are equals and False otherwise. In my first ugly test I've used a list-comprehension method, but I'm sure that exists a more elegant/short way to obtain the same result. Thanks in advance for any answer. Luca _________________________________________________________________ Tell us your greatest, weirdest and funniest Hotmail stories http://clk.atdmt.com/UKM/go/195013117/direct/01/

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Time interval calculation
by legajid 14 Mar '10

14 Mar '10

Hi, my following code should show time before executing listeprem, then time after execution. import System.Time gettime :: IO ClockTime gettime = getClockTime main=do t1d <- gettime let t1=last (listeprem 5000) t1f <- gettime putStrLn ("Methode 1 : " ++ show t1d) putStrLn (" " ++ show t1) putStrLn (" " ++ show t1f) Looking at the screen, t1d is displayed then, after a few seconds, t1 and t1f. But, t1d and t1f are equal. It seems like if t1d and t1f where calculated at start of procedure, before we need calculating t1 for putStrLn. How can i have t1f evaluated after t1, so i can calculate time elapsed for calculation of t1? Thanks, Didier

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Primes calculation
by legajid 13 Mar '10

13 Mar '10

Hi, i'm trying to write a function to find all primes from 2 to N. My idea is : take the first number (2) try to find whether it's a multiple of one of all existing primes ([] at first) add 2 to the list of primes take the following number (3) find if multiple of existing primes ([2]) add 3 to the list of primes take the following number (4) find if multiple of existing primes ([2, 3]) do not add 4 to the list of primes ... take the following number (8) find if multiple of existing primes ([2, 3, 5, 7]) do not add 8 to the list of primes take the following number (9) find if multiple of existing primes ([2, 3, 5, 7]) do not add 9 to the list of primes (multiple of 3) and so on... So, i would like a function like : f (x : xs) = g x : f xs g would return x if x is prime, [] otherwise But g would use the preceding value of f (list of primes before the calculation for x) that is a result of g itself. f needs g that needs f : what's wrong with my mind ? Perhaps i am still under control of imperative programming ? Thanks for your help, Didier.

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