September 2007 - Haskell-Cafe

Can somebody give any advice for beginners?
by clisper 11 Sep '07

11 Sep '07

haskell is greate but i don't know how to start. 2007-09-11 clisper

11 10

Re: [Haskell-cafe] haskell and reflection
by Reinier Lamers 11 Sep '07

11 Sep '07

Op 11-sep-2007, om 18:43 heeft Greg Meredith het volgende geschreven: > Thanks for these comments. i wouldn't judge Haskell solely on the > basis of whether it embraced reflection as an organizing > computational principle or as a toolbox for programmers. Clearly, > you can get very far without it. And, it may be that higher-order > functional gives you enough of the 'programs that build programs' > capability that 80% of the practical benefits of reflection are > covered -- without having to take on the extra level of complexity > that reflection adds to typing. i was really just seeking information. Template Haskell [1] is a system that lets you write programs that get executed at *compile time*, and that produce parts of the Haskell program to be compiled by manipulating a representation of the program as structured data. It's a form of reflection restricted to compile time, if you'd ask me. Regards, Reinier [1] http://www.haskell.org/haskellwiki/Template_Haskell

1 0

Comments and/or Criticisms
by PR Stanley 10 Sep '07

10 Sep '07

Hi Any comments and/or criticisms would be most appreciated: --count the occurrences of char in string countC :: Char -> [Char] -> Int countC x xs = sum [1 | c <- xs, c == x] --count occurrences of chars in string countCS :: [Char] -> [(Char, Int)] countCS xs = [(x, (countC x xs)) | x <- [' '..'z'], (countC x xs) > 0] Can anyone come up with a better alternative? Thanks, Paul

8 8

Custom unary operator extension?
by Peter Verswyvelen 10 Sep '07

10 Sep '07

I find it unfortunate that one can't (I guess) define custom unary operators in Haskell. Is this correct? If so, is this just because eg (* 100) declares a function that partially applies the * operator, so this syntax disallows unary operators? Could this be fixed by introducing a different syntax for partial operator application? E.g. (? * 100) instead of just (*100)? This would also fix the ambiguity between (-10) as being either the number minus 10 or the "subtract by 10' function. Cheers, Peter

7 14

turning an imperative loop to Haskell
by Axel Gerstenberger 10 Sep '07

10 Sep '07

Hi all, I am completely stuck with a problem involving a loop construct from imperative programming, that I want to translate to Haskell code. The problem goes as follows: Based on a value u_n, I can calculate a new value u_{n+1} with a function f(u). u_n was calculated from u_{n-1} and so on down to some initial value u_0. So far it looks like a standard recursion to me. The main goal is to write each result u_{n+1} to a file or screen. The problem arises (for me), when u is an array of doubles or a complex data object instead of a simple Double/Integer value, so that I can store only maybe the last or the two last steps in memory. I never need older values. In C, I would write a for loop, calculate the new u and write it to the file. Then I update the old values to the new ones and do the next step in the for loop. Here is what I did in Haskell: I create an infinite list and tried to print the n-th value to the screen/file. But it always calculates all values in the list "all_results", before it starts printing values to screen. On the other side, the function f is called exactly 50 times as the loop suggests. The result is correct, however, it would prohibitive much memory for more complex data and more steps. Can anyone help in explaining me, how I can print to screen and still keep only the last needed values in memory? I can only find imperative solutions, but maybe it is an imperative problem anyway.? Thanks for your time. Best Axel My approach: module Main where import System.IO import Text.Printf main :: IO () main = do let all_results1 = take 20000 $ step [1] --print $ length all_results1 -- BTW: if not commented out, -- all values of all_results -- are already -- calculated here loop [1..50] $ \i -> do let x = all_results1!!i putStrLn $ show i ++ " " ++ show x -- create an infinite list with values u_{n+1} ++ [u_n,u_{n-1},...,u_1] -- where u_{n+1} = f (u_n) step history = case history of [] -> error "no start values" xs -> xs ++ (step [ f (head $ reverse (xs) )]) f u = u + 1 + (sqrt u) -- some arbitrary complex function -- copied from some blog, not sure if this is a good way loop ns stuff = mapM_ stuff ns

12 17

Where would I find fromInt?
by PR Stanley 10 Sep '07

10 Sep '07

Hi Where would I find the fromInt function, please? Better still, would anyone know how to write a func for converting int to float and vice versa? Thanks, Paul

3 2

simple parsing (parsec) intro
by Tim Newsham 10 Sep '07

10 Sep '07

I wrote a small intro about how to write a parser in haskell. Its basically about parsec and how it works, but its written without directly referencing parsec and aimed towards beginners (basically wrote it for some friends). Any feedback is appreciated. http://www.thenewsh.com/%7Enewsham/formal/parse/parser.lhs Tim Newsham http://www.thenewsh.com/~newsham/

2 1

Second Request for code review - Knuth Morris Pratt for Data.Sequence
by Justin Bailey 10 Sep '07

10 Sep '07

Using the code developed for ByteStrings by myself, Christ Kuklewicz and Daniel Fischer, I've implemented Knuth-Morris-Pratt substring searching on Data.Sequence "Seq" values. Attached you'll find the library in kmp.zip.safe. The algorithm is implemented in the module Data.Sequence.KMP. At the root, SpeedTest.hs can be compiled on Windows with the "prof_compile.bat" file included (you'll need to install the "regex-dfa" and "regex-base" packages from Hackage to build). SpeedTest searches for a known value in the 7 MB file "endo.dna" (which can be downloaded from http://www.icfpcontest.org/endo.zip) using several different algorithms and methods: strict and lazy bytestrings, regular expressions, and the KMP algorithm for "Seq" values. SpeedTest is pretty fast but I worry about its space usage. It may just be the nature of Seq values, but I cannot get it to run in constant space when I think it should be. I'm especially interested in help here. All comments and feedback are welcome. Since the zip file includes a darcs context file, feel free to send patches. Justin

1 0

WinHugs shortcut keys
by Peter Verswyvelen 10 Sep '07

10 Sep '07

Winhugs seems to have some shortcut keys, like F5 for run. It does not seem to have shortcut keys for reload, edit, etc... These would be really handy. It is possible to customize the shortcuts assigned to the commands? If not, can it be build from source? Thanks, Peter

2 3

Installation of GLUT package
by Ronald Guida 10 Sep '07

10 Sep '07

About a month ago, I asked about how to install freeglut and get GHC to recognize it. I was side-tracked; I finally decided to try to get it working, and I'm just stuck. I have GHC 6.6.1 and freeglut 2.4.0. Freeglut successfully compiled "out of the box" in MS Visual Studio.Net 2003 on my Windows XP box. GHC won't recognize freeglut; I found out that I need to recompile the GLUT package to get things working. Today I downloaded the GLUT package from hackage.haskell.org and attempted to install it. The installation blows up. According to Cabal/How_to_install_a_Cabal_package, I need to download the package, unzip it, and then run the following three commands. 1. runghc Setup.hs configure 2. runghc Setup.hs build 3. runghc Setup.hs install The first command, "runghc Setup.hs configure", fails, reporting that it can't find "sh". Naturally, I'm on a Windows box, so I don't have the Unix shell on my system. So now I'm stuck... * Is there an easy way to work around the absence of "sh"? * Is there an easy way to get "sh" on my Windows box? * <joke> GHC was available pre-compiled for Windows. Is there an easy way to get someone else to compile the GLUT package for me? :-) </joke> * Is it feasible to walk past a cut and "upgrade" to Linux? Most Windows programs can be replaced by open-source equivalents, but my sticky issue is that there's no Linux version of iTunes. -- Ron PS, Here's the output of configure. Do I really need all this stuff just for the GLUT package? C:\GLUT\GLUT-2.0>runghc Setup.hs configure Configuring GLUT-2.0... configure: Dependency base-any: using base-2.1.1 configure: Dependency OpenGL-any: using OpenGL-2.2.1 configure: Using install prefix: C:\Program Files configure: Binaries installed in: C:\Program Files\Haskell\bin configure: Libraries installed in: C:\Program Files\Haskell\GLUT-2.0\ghc-6.6.1 configure: Private binaries installed in: C:\Program Files\GLUT-2.0 configure: Data files installed in: C:\Program Files\Common Files\GLUT-2.0 configure: Using compiler: C:\ghc\ghc-6.6.1\bin\ghc.exe configure: Compiler flavor: GHC configure: Compiler version: 6.6.1 configure: Using package tool: C:\ghc\ghc-6.6.1\bin\ghc-pkg.exe configure: Using ar found on system at: C:\ghc\ghc-6.6.1\bin\ar.exe configure: No haddock found configure: No pfesetup found configure: No ranlib found configure: Using runghc found on system at: C:\ghc\ghc-6.6.1\bin\runghc.exe configure: No runhugs found configure: No happy found configure: No alex found configure: Using hsc2hs: C:\ghc\ghc-6.6.1\bin\hsc2hs.exe configure: No c2hs found configure: No cpphs found configure: No greencard found Setup.hs: Cannot find: sh

5 12