March 2010 - Beginners

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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Issues With Install Script in Haskell Platform 2009.2.0.2
by aditya siram 13 Mar '10

13 Mar '10

Hi all, I am running Ubuntu Karmic Kameleon and GHC 6.10.4 and am having issues with the Haskell Platform 2009.2.0.2 source installer. The 'configure' and 'make' step work fine, but 'make install' errors with: scripts/install.sh Installing mtl-1.1.0.2... Error: The mtl-1.1.0.2/Setup script does not exist or cannot be run And indeed mtl-1.1.0.2/Setup does not exist. It appears that the 'make' step builds some packages and not others. The 'Setup' executable is only in the following package directories: ./network-2.2.1.4/Setup ./editline-0.2.1.0/Setup ./alex-2.3.1/Setup ./happy-1.18.4/Setup ./haskell-platform-2009.2.0.2/Setup ./fgl-5.4.2.2/Setup ./cgi-3001.1.7.1/Setup ./cabal-install-0.6.2/Setup ./HTTP-4000.0.6/Setup ./GLUT-2.1.1.2/Setup ./OpenGL-2.2.1.1/Setup ./zlib-0.5.0.0/Setup ./time-1.1.2.4/Setup Here is the current contents of my global and local packages: Global Packages: Cabal-1.6.0.3, HUnit-1.2.0.3, QuickCheck-1.2.0.0, array-0.2.0.0, base-3.0.3.1, base-4.1.0.0, bytestring-0.9.1.4, containers-0.2.0.1, directory-1.0.0.3, (dph-base-0.3), (dph-par-0.3), (dph-prim-interface-0.3), (dph-prim-par-0.3), (dph-prim-seq-0.3), (dph-seq-0.3), extensible-exceptions-0.1.1.0, filepath-1.1.0.2, (ghc-6.10.4), ghc-prim-0.1.0.0, haddock-2.4.2, haskell-src-1.0.1.3, haskell98-1.0.1.0, hpc-0.5.0.3, html-1.0.1.2, integer-0.1.0.1, mtl-1.1.0.2, network-2.2.1.2, old-locale-1.0.0.1, old-time-1.0.0.2, packedstring-0.1.0.1, parallel-1.1.0.1, parsec-2.1.0.1, pretty-1.0.1.0, process-1.0.1.1, random-1.0.0.1, regex-base-0.72.0.2, regex-compat-0.71.0.1, regex-posix-0.72.0.3, rts-1.0, stm-2.1.1.2, syb-0.1.0.1, template-haskell-2.3.0.1, time-1.1.4, unix-2.3.2.0, xhtml-3000.2.0.1 Local Packages: haskelldb-0.13, haskelldb-hsql-0.10, haskelldb-hsql-sqlite3-0.10, hsql-1.8.1, hsql-sqlite3-1.8.1

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Issues Installing haskelldb-hsql-sqlite3
by aditya siram 12 Mar '10

12 Mar '10

Hi all, I am having an issue installing haskelldb-hsql-sqlite3. When I do a 'cabal-install install haskelldb-hsql-sqlite3', it tries to rebuild 'directory-1.0.0.3' even though it is available and gives the following error: Building directory-1.0.0.3... System/Directory.hs:81:7: Could not find module `Control.Exception.Base': it is a member of the hidden package `base' Use -v to see a list of the files searched for. Any feedback is appreciated. -deech

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error with cabal install readline
by Ashish Agarwal 11 Mar '10

11 Mar '10

I'm on a Mac, and trying to install readline through cabal. I installed the readline library through macports and thus tried the command below. However, readline is still not found. Are there other options I can try? Thank you. $ cabal install readline --extra-include-dirs=/opt/local/include/ --extra-lib-dirs=/opt/local/lib/ Resolving dependencies... Configuring readline-1.0.1.0... checking for gcc... gcc checking for C compiler default output file name... a.out checking whether the C compiler works... yes checking whether we are cross compiling... no checking for suffix of executables... checking for suffix of object files... o checking whether we are using the GNU C compiler... yes checking whether gcc accepts -g... yes checking for gcc option to accept ISO C89... none needed checking for GNUreadline.framework... checking for readline... no checking for tputs in -lncurses... yes checking for readline in -lreadline... yes checking for rl_readline_version... yes checking for rl_begin_undo_group... no configure: error: readline not found, so this package cannot be built See `config.log' for more details. cabal: Error: some packages failed to install: readline-1.0.1.0 failed during the configure step. The exception was: exit: ExitFailure 1

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Library source
by Nicolas Couture-Grenier 10 Mar '10

10 Mar '10

Where can I find the haskell sources of the standard library? I searched for all files ending in .hs in my haskell folder and found none.

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