- Beginners - Haskell.org

Compile single file within cabal-dev environment
by Petar Radosevic 20 Nov '11

20 Nov '11

It seems so trivial, but I haven't been able to do it. I'm trying to compile a single .hs file whose dependencies are available in a cabal-dev environment. For example, at my website repo [1] you can see that I have a ``wunki.hs`` which uses Hakyll [2] to create my website. I have installed Hakyll with cabal-dev. But now I want to ``ghc --make wunki.hs`` with that cabal-dev environment. Is this possible? [1]: https://github.com/wunki/www.wunki.org [2]: https://github.com/jaspervdj/hakyll -- Petar Radošević, Programmer wunki.org | @wunki

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cabal can't find stdc++ headers or library?
by Eric Lasian 20 Nov '11

20 Nov '11

Hi all, When trying to install double-conversion, I'm getting the following errors. $ cabal install double-conversion Resolving dependencies... Warning: double-conversion.cabal: Ignoring unknown section type: test-suite Configuring double-conversion-0.2.0.3... cabal: Missing dependency on a foreign library: * Missing C library: stdc++ This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. cabal: Error: some packages failed to install: double-conversion-0.2.0.3 failed during the configure step. The exception was: ExitFailure 1 So it seems to me that everything is in the expected place: the headers are in /usr/include/c++/4.* and the library itself is in /usr/lib. Explicitly forcing cabal to search these directories (with --extra-include-dirs and --extra-lib-dirs as suggested) doesn't help. Any ideas? version info: cabal-install version 0.8.0, using version 1.8.0.2 of the Cabal library, The Glorious Glasgow Haskell Compilation System, version 7.2.1 Thanks, Eric

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Int V.S. Word32
by Haisheng Wu 20 Nov '11

20 Nov '11

Hello, I got great performance difference for the following code if I used type `Int` rather than `Data.Word.Word32`. Anyone can help to explain why such difference? Thanks a lot. -Simon module Main where import Data.Word main :: IO () main = print $ p14 p14 = maximum [ (startChain n 0, n) | n <- [2..1000000] ] startChain :: Word32 -> Int -> Int startChain 1 count = count + 1 startChain n count = startChain (intTransform n) (count+1) intTransform :: Word32 -> Word32 intTransform n | even n = n `div` 2 | otherwise = 3 * n + 1

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haskell-platform on arch
by Michael Xavier 18 Nov '11

18 Nov '11

Hi Haskell Beginners, I was wondering if any experienced Haskellers used arch. I noticed today that while GHC and Haskell-Platform are both on arch, GHC is still on 7.0.3 and Haskell Platform is an orphaned package that was flagged out of date about 2 months ago. Does anyone know what we can do about this? I don't have any experience maintaining arch packages but I figured someone on here may know someone who would want to be the package maintainer. It may help other beginners who code on Arch, since Arch is supposed to be about having the freshest packages. -- Michael Xavier http://www.michaelxavier.net LinkedIn <http://www.linkedin.com/pub/michael-xavier/13/b02/a26>

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testing testing
by Lee Render 18 Nov '11

18 Nov '11

test thanks

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Questions on set up on OSX
by Sophie 17 Nov '11

17 Nov '11

I have the basic install, ghci working, going through the Real World book, and have some set up questions: - Is there any REPL which will take (close to) full Haskell syntax, including function definitions? - I use TextMate and Intelli-J, but they don't seem Haskell friendly. The Haskell TextMate bundle seems very confused about indenting, tabs vs. spaces, etc, and I could not even find a code formatter / highlighted for Intelli-J. What environments would you recommend to work with Haskell on OSX? If Emacs, which one? If Eclipse ... I suppose if I have to... I tried Leksah but it seems too buggy even under simple window resizing. Thanks for any pointers!! Sophie

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(no subject)
by Benjamin L. Russell 16 Nov '11

16 Nov '11

http://elikat.com/blogs/wp-content/plugins/index1.php

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(no subject)
by Alia 14 Nov '11

14 Nov '11

http://nuevaesperanzaparatodos.com/modules/mod_wdbanners/site.php?html1

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Combinations of choosing n items from a list
by Peter Hall 14 Nov '11

14 Nov '11

Hi all, This function took me a long time to write, getting my head around the double recursion. It returns a list of all possible sub-sets of a given length from a list. I have a couple of questions. 1. Can it be improved for efficiency or style? 2. Is there a library that already has this and other related functions? I assumed there would be but I couldn't find it on hoogle. combinations :: Int -> [a] -> [[a]] combinations _ [] = [] combinations 0 _ = [] combinations 1 x = map (:[]) x combinations n (x:xs) = (map (x:) $ combinations (n-1) xs) ++ combinations n xs e.g. > combinations 3 [1..5] [[1,2,3],[1,2,4],[1,2,5],[1,3,4],[1,3,5],[1,4,5],[2,3,4],[2,3,5],[2,4,5],[3,4,5]] Thanks, Peter

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Using Maybe monad with Zipper
by Hugo Ferreira 14 Nov '11

14 Nov '11

Hello, I picked up code that originally had the following function: leftCursor :: Z.Zipper a -> Maybe a leftCursor z = if Z.beginp z then Nothing else Z.safeCursor $ Z.left z However now I need a functions that returns all three elements to the left of the zipper. I then considered using composition and came up with the following: left :: Maybe (Z.Zipper a) -> Maybe (Z.Zipper a) left (Just z) = if Z.beginp z then Nothing else Just $ Z.left z left Nothing = Nothing cursor :: Maybe (Z.Zipper a) -> Maybe a cursor (Just z) = Z.safeCursor z cursor Nothing = Nothing which I then used as follows: instPrev1Of3TagRule :: POSTags -> Maybe TransformationRule instPrev1Of3TagRule z = do (_, _, prev1) <- leftCursor z (_, _, prev2) <- (cursor . left . return) z (_, _, prev3) <- (cursor . left . left . return) z (_, correct, incorrect) <- Z.safeCursor z return $ Prev1Of3TagRule (Replacement incorrect correct) prev1 prev2 prev3 But I was not happy with this for two reasons: 1. I was repeating work needlessly. I should pick up the last zipper and just move the cursor again once for the next character. 2. I was not taking advantage of the Maybe Monad So I finally changed the above to: left :: Maybe (Z.Zipper a) -> Maybe (Z.Zipper a, a) left (Just z) = if Z.beginp z then Nothing else Just $ (Z.left z, Z.cursor z) left Nothing = Nothing instPrev1Of3TagRule :: POSTags -> Maybe TransformationRule instPrev1Of3TagRule z = do (z1,(_, _, prev1)) <- (left . return) z (z2, (_, _, prev2)) <- (left . return) z1 (_z3, (_, _, prev3)) <- (left . return) z2 (_, correct, incorrect) <- Z.safeCursor z return $ Prev1Of3TagRule (Replacement incorrect correct) prev1 prev2 prev3 So my question is, is this correct? Can the above code be made simpler/cleaner? Somehow the double use of Maybe seems wrong. TIA, Hugo F.

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