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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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guards
by kolli kolli 11 Nov '11

11 Nov '11

Hi,, I am new to use ..plz bare with me.. How do we write two conditions using guards suppose if we have a if condition if t1 t2 t3 typing :: TypingContext -> Term -> Maybe Type --the typing relation typing capGamma (Var x) = contextLookup x capGamma typing capGama Tru = Just(TypeBool) typing capGama Fls = Just(TypeBool) typing capGama Zero = Just(TypeBool) typing capGama (Succ t) = typing capGama t typing capGama (Pred t) = typing capGama t typing capGama (IsZero t) = typing capGama t typing capGama (If t1 t2 t3) | typing capGama t1 == TypeBool && typing capGama t2 == x = Just(x) | otherwise = Nothing where x = typing capGama t3 its giving me error Couldn't match expected type `Maybe Type' against inferred type `Type' In the second argument of `(==)', namely `TypeBool' In the first argument of `(&&)', namely `typing capGama t1 == TypeBool' In the expression: typing capGama t1 == TypeBool && typing capGama t2 == TypeBool how can i give two conditions in a single guard...

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Show a function?
by Costello, Roger L. 10 Nov '11

10 Nov '11

Hi Folks, Consider this function: true = (\x y -> x) When I type this at WinGHCi: true I would like WinGHCi to show this string: TRUE Is there a way to "show" a function? /Roger

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Search for docs on operator '<+>'.
by Allen S. Rout 10 Nov '11

10 Nov '11

Google seems to barf on '<+>'. I've found a haskell "list of operators", and it's not mentioned. Could some kind soul point me at documentation for this thing? - Allen S. Rout

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My Bitcoin miner
by Zhi-Qiang Lei 10 Nov '11

10 Nov '11

Hi, Learning Haskell, I write a Bitcoin miner for practice. Currently it's very basic(single thread, non-long-polling support). You are free and welcome to modify it. Best regards, Zhi-Qiang Lei zhiqiang.lei(a)gmail.com

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Help : foldl skipping one value in list
by Lev Broido 10 Nov '11

10 Nov '11

Hi I am implementing toy loop search algorithm in graph . I encountered strange problem -- it seems , that fold doesn't run on the whole list . Code is at http://hpaste.org/53776 Section in question is updVis function Problem occurs when _newStateMap_2 is being calculated Please advise. Thanks, Lev

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problem with lazy IO
by Dennis Raddle 10 Nov '11

10 Nov '11

I have the following code, which reads several thousand text files and creates some Map data. On Windows XP, it crapped out with the error "resource exhausted: too many open files." So I'm sure I need to make it stricter. I've never tried to do that before, so I'm not sure where the strictness is necessary. Maybe all I need is to switch to strict ByteStrings? -- Read a bunch of text files. The files are organized in pairs, where the file names in each pair are -- "a<number>.txt" and "f<fnumber>.txt". Example: "a00001.txt" and "f00001.txt". -- Each file is a list of float values specified in a certain text format (which happens to be -- an output format of the audio-related program Csound) -- -- Input: -- [(String,String)] : a list of pairs of file names -- String : directory in which the files exist readPvsFiles :: [(String,String)] -> String -> IO (Map Int (Map Float Float)) readPvsFiles filenames dir = do -- contents :: [(Int,Map Float Float)] -- where Int is the number in the file name. -- and Map Float Float is a map of numbers in the -- in the first file mapped to numbers in the second file contents <- mapM (oneFilePair dir) filenames return $ M.fromList contents -- Read one file pair. -- -- Input: -- String : directory of the files -- (String,String) : the two file names in the pair -- (Int,Map Float Float) : -- Int is the number in the file name. -- and Map Float Float is a map of numbers in the -- in the first file mapped to numbers in the second file oneFilePair :: String -> (String,String) -> IO (Int,Map Float Float) oneFilePair dir (ffile,afile) = do -- Read the float values from each file (which is a list of floats in -- a text format. fvalues <- readTableValues (dir ++ "/" ++ ffile) avalues <- readTableValues (dir ++ "/" ++ afile) -- t is the number in the filename. let t = read . take 6 . drop 1 $ ffile return (t, M.fromList $ zip fvalues avalues) -- Open the file via readFile, and parse all the text values in it. readTableValues :: String -> IO [Float] readTableValues s = do b <- readFile s let bs = lines b lenLine = head . drop 1 $ bs n = read (drop 6 lenLine) :: Int -- valueLines :: [String]. This is a list of the lines -- in the file that have the float values. One float -- value in text format per line. valueLines = take n . drop 25 $ bs return $ map read valueLines

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