February 2003 - Haskell-Cafe

Why no findM ? simple Cat revisited
by Ahn Ki-yung 12 Nov '03

12 Nov '03

Simple Cat (revisitied) \begin{code} import IO findM f [] = return Nothing findM f (x:xs) = do { v <- x; if f v then return (Just v) else findM f xs } isLeft (Left _) = True isLeft _ = False main = findM (isLeft) (hCat stdin) where hCat h = try (hGetLine h) : hCat h \end{code} This is my answer for the question of my own, which is posted a couple There are mapM, filterM in the Haskell 98 Standard Library. But why no findM there ? As you can see from simple cat, it seems quite useful. I think fildM should be added to the module Monad. -- Ahn Ki-yung

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splatterhunks
by William Lee Irwin III 22 Apr '03

22 Apr '03

-- This program takes patch filenames as arguments and splits the patch -- files up on a hunk-by-hunk basis. I have found this useful for Linux -- kernel development, and perhaps others might find it useful in -- similar contexts where patches are the primary method of communication. module Main where import System import List main = getArgs >>= mapM_ splatterHunkFile splatterHunkFile file = mapM_ (\(f,ss) -> writeFile f (unlines ss)) . zip (map (((reverse . takeWhile (/= '/') . reverse $ file)++).("-"++).show) [1..]) . splatterHunks =<< readFile file splatterHunks ss = map (map unlines) $ zipWith (\s -> map (s:)) (map head . (splitList "diff") $ lines ss) (map (splitList "@@") . (splitList "diff") $ lines ss) splitList prefix ss = map ($ss) $ zipWith (\m n -> (if n >= 0 then take (n - m) else id) . drop m) (map fst . filter (\(n,s) -> isPrefixOf prefix s) $ zip [0..] ss) (tail (map fst . filter (\(n,s) -> isPrefixOf prefix s) $ zip [0..] ss) ++ [-1])

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comonads, io
by peter＠premise.demon.co.uk 06 Apr '03

06 Apr '03

I came across an interesting paper "Codata and comonads in Haskell" by Richard Kieburtz, and some other work by Alberto Pardo on comonads. Kieburtz proposes an OI comonad, as an alternative to (or something alongside?) the IO monad. Just for reasons of symmetry, it's natural to wonder if they aren't as pervasive in functional programs as monads. Frustratingly, I can't seem to grasp the intuition. I'm aware that the state-monad s->(s,a) has a dual state-in-context comonad (s,s->a). How does the side-effect manifest itself? The (ineffective) hints the authors give revolve around the idea that side-effects "derive" from the context of a program. Any other hints? Are comonads especially appropriate for programming systems that react to changes in their environment? Is an object of OI type something like a program to react to events? I can't make the slightest sense of Kieburtz's OI co-monad, with commands like coGetChar :: OI Handle -> Char. Peter Hancock

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debug with GHC FFI?
by paul＠theV.net 06 Mar '03

06 Mar '03

I am using Green Card to make some FFI modules with GHC, but sometimes it segfaults in the middle of nowhere, leaving no sensible stack trace in gdb. I wonder what is the proper way to debug FFI modules written in C? Trying to add a "-g" flag doesn't seem help at all... Regards, .paul.

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Re:
by wr 28 Feb '03

28 Feb '03

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Hugs' graphics lib
by Daniel Jian 27 Feb '03

27 Feb '03

Hi all, I have just installed Hugs to my system. And then I also compiled its drawing library. My question is how can I include that library in my code? I know this is rather silly a question, but I have tried to play around with the environment variable in .bashrc, and I have added:-P$HOME/local/graphics-2.0.4/lib/x11 to the HUGSFLAGS. But Hugs still complains that it cannot include the lib. I am just wondering, can I make an executable file using hugs? How can I do that (if I can)? Many thanks! Blessings, Daniel Jian __________________________________________________ Do you Yahoo!? Yahoo! Tax Center - forms, calculators, tips, more http://taxes.yahoo.com/

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Buenas noches !!!
by Olga Lucia Quintero Fonseca 24 Feb '03

24 Feb '03

Buenas noches, quisiera consultarles sobre lo siguiente, tengo estas dos funciones -- Calcula Raiz Wn wn n = cos c :+ (-1*sin c) where c=2*pi/n twiddlefactor r k = concat [(map (^i) [(wn rs)^i | i<-[0..s-1]])| i<-[0..r-1]] where rs = (r^k) ; s = (r^(k-1)) la dificultad que tengo es que twiddlefactor no me acepta enteros compila pero no me acepta los parametros, sin embargo si la defino como --Calcula vector de Twiddle Factor twiddlefactor r s rs= concat [(map (^i) diag) | i<-[0..r-1]] where diag = [(wn rs)^i | i<-[0..s-1]] no tengo problema, lo que sucede es que posteriormente esta definicion me es inconveniente pues el valor de rs es r^k y lo necesito en un programa recursivo, que me esta dando inconvenientes precisamente por que debo estar reenviando el valor r^k. Agradezco la ayuda que puedan darme

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Re: Simple problem from Hudak's SOE
by C.Reinke 24 Feb '03

24 Feb '03

> I checked out the type of "snd" in hugs and basically > figured out what it does, but I was wondering where I could find the > definition of this function. Is it possible to look at it in hugs? If your hugs is configured correctly to use an editor, simply type ":find snd". This underappreciated feature is one reason why you want to set the editor in hugs!-) http://cvs.haskell.org/Hugs/pages/hugsman/started.html hth, Claus

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Debugging haskell
by Sengan.Baring-Gould＠nsc.com 24 Feb '03

24 Feb '03

http://www.catb.org/~esr/writings/taoup/html/ch01s06.html states that "debugging often occupies three-quarters or more of development time". I don't think that is my experience in Haskell... more like 1/4 at most. I was wondering what others felt. Sengan

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Simple problem from Hudak's SOE
by M. Parker 24 Feb '03

24 Feb '03

I'm a real newbie to Haskell, and I'm having trouble with a particular problem dealing with higher-order functions. Exercise 5.9 in Hudak's "School of Expression" asks us to write a function, "makeChange," s.t. it makes change for a given amount using coins in a coin supply (represented by a list of decreasing integers). For example, make Change 99 [5,1] ==> [19,4] This chapter is about higher order functions, so I'm assuming he wants us to compute the result using the higher order functions defined in the chapter (map, foldl, foldr). I devised two solutions: {-Solution 1-} makeChange money coinList = zipWith div (scanl mod money coinList) coinList {-Solution 2-} makeChange' money (coin:coins) = let money' = money `mod` coin numCoins = money `div` coin in (numCoins: makeChange' money' coins) makeChange' 0 _ = [] makeChange' _ [] = [] However, my problem is that neither solution uses the higher-order functions defined in the chapter. So is it possible to solve this problem using map and fold? Furthermore, Hudak makes the case that we should strive to find the higher-order solutions instead of the recursive ones because the latter leads to clearer and more concise solutions. Although solution 1 is more concise, I feel like Solution 2 is clearer to me than Solution 1, but maybe this is just because I'm new to haskell and higher order functions. It just seems like its easier to understand the actual algorithm in solution 2 than in solution 1. Thanks, Matt Parker University of North Texas undergrad http://www.cs.unt.edu

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