November 2011 - Beginners

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Haskell-Platform are both on arch, GHC is still on 7.0.3 and Haskell Platform is an orphaned package that was flagged out of date about 2 months ago.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 17 Nov '11

17 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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subRegex https? with anchor href tags
by Shakthi Kannan 12 Nov '11

12 Nov '11

Hi, I am trying to replace every occurrence of http://URL or https://URL with: <a href="http://URL"> http://URL </a> <a href="https://URL"> https://URL </a> respectively in a text file. I tried with Text.Regex with an example: Prelude> let s = "Google website is http://www.google.com in US" Prelude> import Text.Regex Prelude Text.Regex> let s3 = subRegex (mkRegex "https?:[^\\s\n\r]+") s "<a href=\"\\1\"></a>" Prelude Text.Regex> s3 "Google website is <a href=\"*** Exception: Ix{Int}.index: Index (1) out of range ((0,0)) Prelude Text.Regex> I then tried a simpler example: Prelude Text.Regex> subRegex (mkRegex "e") "hello" "\\1" "h*** Exception: Ix{Int}.index: Index (1) out of range ((0,0)) What could I be missing? I am using GHCi 6.12.3 on Fedora 14. Appreciate any help in this regard. Thanks, SK -- Shakthi Kannan http://www.shakthimaan.com

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Couple of problems with leksah
by Peter Hall 12 Nov '11

12 Nov '11

I didn't get any response on the Leksah forum, so I hope it's ok to ask here. First problem, I keep seeing this in the error log: > Setup: You need to re-run the 'configure' command. The version of Cabal being > used has changed (was Cabal-1.10.2.0, now Cabal-1.10.1.0). Additionally the > compiler is different (was ghc-6.12, now ghc-7.0) which is probably the cause > of the problem. When I run configure, it just prints: > Resolving dependencies... > Configuring pokercalc-0.0.1... But it doesn't fix anything. The other problem is when I try to update cabal: > cabal install leksah Resolving dependencies... cabal: cannot configure haddock-2.9.4. It requires ghc >=7.2 && <7.4 There is no available version of ghc that satisfies >=7.2 && <7.4 I'm using Haskell Platform 2011.2.0.1 for Mac OS X 10.6 Any ideas? Peter

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