October 2010 - Beginners

dealing with chained functions
by Alec Benzer 20 Apr '11

20 Apr '11

Is there a generally accepted "better" way of dealing with the following: say I have three functions that all rely on output from each other. Ie, we get raw input, which one function processes. another function then requires the output from this function and produces some other output, which a third function then requires. would it be better to implement the functions like this: f1 :: a -> b f2 :: b -> c f3 :: c -> d final :: a -> d or like this: f1 :: a -> b f2 :: a -> c f3 :: a -> d Ie, in the latter set, f2 calls f1 on its input, and then proceeds to process it. f3 calls f2 on its input, etc. In the former case, final would just chain all three functions together itself (or I might just do it in main or somewhere else that's appropriate) Are one of these methods generally considered a better practice than the other, specifically in a functional context? My gut tells me that the former method looks more appropriate, but I'm not sure if it'd really be the accepted practice regardless of context, or if one might use either of these methods depending on the specifics of what they're doing. Or if it's just a matter of taste/opinion?

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editing a list
by Jonathan Phillips 07 Nov '10

07 Nov '10

I have a list (more precisely a string) which I'm trying to recurse through, editing sections as I come to it. It's obvious how to do it with a for loop, but I'm doing it all wrong in haskell, as this simple task is running mind-numbingly slowly. addOneNum cycles through each line, and when it gets to character 23, it calls changeNo, which edits the string. The problem is clearly the adding on to the end of the string, but the only other way I can see of doing it is writing the 'new string' backwards and then reverse it at the end. This looks like it would make the function even less legible than it already is though. addOneNum :: String -> String -> Integer -> String addOneNum x [] _ = x addOneNum x (y:ys) n = case n of 23 -> changeNo x (y:ys) n _ -> case y of '\n' -> addOneNum (x ++ [y]) ys 1 _ -> addOneNum (x ++ [y]) ys (n+1) changeNo :: String -> String -> Integer -> String changeNo v (w:x:y:z:xs) n = case length(show((read([w,x,y,z])::Integer) + 1)) of 0 -> addOneNum (v ++ " ") xs (n+4) 1 -> addOneNum (v ++ show((read([w,x,y,z])::Integer) + 1) ++ " ") xs (n+4) 2 -> addOneNum (v ++ show((read([w,x,y,z])::Integer) + 1) ++ " ") xs (n+4) 3 -> addOneNum (v ++ show((read([w,x,y,z])::Integer) + 1) ++ " ") xs (n+4) 4 -> addOneNum (v ++ show((read([w,x,y,z])::Integer) + 1)) xs (n+4) _ -> error "asdf" changeNo w _ n = error "changeNo error" Also, while I'm at it, it looks like I want to be using an as-pattern (or some C-style #define thing): changeNo v (w:x:y:z:xs) n = case length(s@(show((read([w,x,y,z])::Integer) + 1))) of 0 -> addOneNum (v ++ " ") xs (n+4) 1 -> addOneNum (v ++ s ++ " ") xs (n+4) 2 -> addOneNum (v ++ s ++ " ") xs (n+4) 3 -> addOneNum (v ++ s ++ " ") xs (n+4) 4 -> addOneNum (v ++ s) xs (n+4) _ -> error "asdf" but that gives me a 'not in scope' error. Any suggestions? Cheers, PhiJ

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error when unregistering a package
by Ashish Agarwal 28 Oct '10

28 Oct '10

I always have trouble with conflicting packages when I do a fresh install of the Haskell Platform on an Ubuntu machine. Last time, I unregistered lots of packages, and let the necessary ones get recompiled. That fixed things. However, this time I'm having trouble with the unregister step. For many packages I get an error about a file not existing, e.g. $ sudo ghc-pkg unregister QuickCheck ghc-pkg: /var/lib/ghc-6.12.1/package.conf.d/QuickCheck-2.1.0.3-d4ae7d01dc2504e1785e18cd555e91ed.conf: removeLink: does not exist (No such file or directory) However, for many other packages, it works fine. Anyone know how to unregister a package when you get this error?

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Help-me read file
by Gilmara Pompelli 27 Oct '10

27 Oct '10

Hello I have a file with 100 lists, with 100 ints. I have to read the file and apply the map and sort functions on lists. II did it to read file: learquivo :: FilePath -> IO ([[Int]]) learquivo s = do conteudo <- readFile s return (read conteudo) But now applying the sort, the error appears: <interactive>:1:5: Couldn't match expected type `[a]' against inferred type `IO [[Int]]' how can I fix? Thanks

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Best practice for catching pure errors in IO?
by John Smith 27 Oct '10

27 Oct '10

I have a pure value which may throw errors when it is evaluated in an IO monad. My code to handle it looks like this: temp <- try $ evaluate value case temp of Left (error::ErrorCall) -> scoldTheUserAndTryAgain Right correct -> doSomething Can this be done without the temporary variable, or with less plumbing? I tried lifting either, but GHC was fussing about ambiguous types, appending a signature to a pattern match is no longer allowed(?), and it didn't look that much cleaner.

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read
by Russ Abbott 27 Oct '10

27 Oct '10

Hi, I'm having a problem with the read function. I thought it took a string and converted it into a value (if there is a conversion). But it doesn't seem to work for me on GHCi Prelude> read "123" *** Exception: Prelude.read: no parse Prelude> read (show 123) *** Exception: Prelude.read: no parse There must be something trivial that I'm missing. Thanks. *-- Russ Abbott*

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Yampa
by Luca Ciciriello 27 Oct '10

27 Oct '10

Hi All. I've started to use Haskell regularly in my job in industry. My job is to develop software for UAVs (Unmanned Aerial Vehicle). In particular I want to use Yampa to handle the sensor gimbal of the vehicle. Is there someone with some kind of experience in the industrial use of Yampa? Where can I find (if exists...) documentation to guide me in using Yampa in the real world? Thanks for any answer. Luca.

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Need help with code duplication (types and typeclasses)
by Edward Turpin 26 Oct '10

26 Oct '10

I can't seem to get my head around types and typeclasses (as well as many other things in Haskell). Here's the following code I need help with. In module 1: //////////////////////// Start of Code ////////////////////////////// data NewtonRow = NewtonRow { iteration :: Integer, xn :: Double, fxn :: Double, errEst :: Double } instance Show NewtonRow where show row = show (iteration row) ++ "\t" ++ showEFloat (Just 8) (xn row) "\t" ++ showEFloat (Just 2) (fxn row) "\t" ++ showEFloat (Just 2) (errEst row) "\n" newtype NewtonTable = NewtonTable { getNewtonTable :: [NewtonRow] } instance Show NewtonTable where show table = "n\t" ++ "xn\t\t\t" ++ "fxn\t\t" ++ "xn - xn-1\n" ++ concatMap show (getNewtonTable table) instance Monoid NewtonTable where mempty = NewtonTable [] table1 `mappend` table2 = NewtonTable $ (getNewtonTable table1) ++ (getNewtonTable table2) cons :: NewtonRow -> NewtonTable -> NewtonTable row `cons` table = NewtonTable (row : getNewtonTable table) ////////////////////// End of Code //////////////////////////////// In module 2: ///////////////// Start of more Code /////////////////////////////// data BisectRow = BisectRow { iteration :: Integer, start :: Double, end :: Double, middle :: Double, deviation :: Double, fm :: Double } instance Show BisectRow where show row = show (iteration row) ++ "\t" ++ showEFloat (Just 4) (start row) "\t" ++ showEFloat (Just 4) (end row) "\t" ++ showEFloat (Just 4) (middle row) "\t" ++ showEFloat (Just 4) (deviation row) "\t" ++ showEFloat (Just 4) (fm row) "\n" newtype BisectTable = BisectTable { getBisectTable :: [BisectRow] } instance Show BisectTable where show table = "n\t" ++ "start\t\t" ++ "end\t\t" ++ "middle\t" ++ "error\t\t" ++ "f(middle)\n" ++ concatMap show (getBisectTable table) instance Monoid BisectTable where mempty = BisectTable [] table1 `mappend` table2 = BisectTable $ (getBisectTable table1) ++ (getBisectTable table2) cons :: BisectRow -> BisectTable -> BisectTable row `cons` table = BisectTable (row : getBisectTable table) ////////////////////// End of code ///////////////////////////////// There is obviously duplication going on, but I can't figure out how to get rid of it. "BisectTable" is essentially the same as "NewtonTable" except that it has a different number of columns, different column headers (in the "show" functions), and different row elements. "BisectRow" and "NewtonRow" are also somewhat similar. And the "cons" functions in each module pretty much do the same thing. I'm just trying to learn Haskell, so as far as I know I might be doing ass-backwards. Can anybody give any suggestions?

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Please, help me for myFirstCode
by Sok H. Chang 26 Oct '10

26 Oct '10

Hello, everyone. I need help for my first code. I write this code for extracting one sentence from text file. The code is ----------------------------------------------------------------------------------------------------------------------- import System import System.Random main = do myHeader <- readFile "C:\\Documents and Settings\\myHeader.txt" putStrLn $ myExtractSentence myHeader myExtractSentence file = ary !! ranNoInt where ranNoInt =<< ranNo -- *mark ranNo = randomRIO (0, n-1) n = length ary ary = lines file ---------------------------------------------------------------------------------------------------------------------- There are two questions. 1) I've got following error message. Haskell>ghc --make Test01.hs -o Test [1 of 1] Compiling Main ( Test01.hs, Test01.o ) Test01.hs:10:9: parse error (possibly incorrect indentation) How can fix it? 2) Is it OK at "*mark" line? Thank you. S. CHANG

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Directory statistics
by Alexander.Vladislav.Popov 26 Oct '10

26 Oct '10

Hi, haskellers. I’ve written a program to assess directory utilization in the selected path. It displays on the console a list indicating directory size and path. The list is sorted by descending size. I need some criticism and some models I could follow to write a more concise and expressive program. Thanks in advance. Alexander. > {-# LANGUAGE ScopedTypeVariables #-} > module Main (main) where > import Control.Exception (SomeException, finally, bracket, handle) > import Control.Monad > import Data.List > import System.Directory > import System.Environment > import System.IO > import Text.Printf > main :: IO () > main = do > args <- getArgs > if null args > then > putStr "Displays directory utilization in the selected path.\nUsage: dirstat <path>\n" > else > mapM_ ds3 args It safely returns a file size. If an error occurs during file opening, it will return 0. > filesize :: FilePath -> IO Integer > filesize path = (withFile path ReadMode hFileSize) `catch` const (return 0) Get size of a directory. > ds :: FilePath -> IO Integer > ds path = do > contents <- getDirectoryContents path `catch` const (return []) > let visibles = getVisible contents > let path' = clrSlash path > a <- (liftM sum) $ sequence $ map (\p -> filesize (path' `mkpath` p)) visibles -- size of a current dir > (liftM ((+a) . sum)) $ mapM (\p -> ds (path' `mkpath` p)) visibles -- current + children Returns a list of pairs: (file size, path) > ds2 :: FilePath -> IO [(Integer, FilePath)] > ds2 path = do > contents <- getDirectoryContents path > let visibles = getVisible contents > let path' = clrSlash path > let paths = map (\p -> path' `mkpath` p) visibles > let pairs = map (\p -> (ds p, p)) paths > a <- sequenceFst pairs > return $ (reverse . sort . filter (\e -> fst e > 0)) a Compare it to the function *sequence :: (Monad m) => [m a] -> m [a]* > sequenceFst :: (Monad m) => [(m t, t1)] -> m [(t, t1)] > {-# INLINE sequenceFst #-} > sequenceFst ms = foldr k (return []) ms > where > k (ms, p) m' = do { s <- ms; xs <- m'; return ((s,p):xs) } Driver > ds3 :: FilePath -> IO () > ds3 path = do > s <- ds2 path > prn s > where > prn [] = return () > prn (s:ss) = (putStr . showDir) s >> prn ss Auxiliary > skipDots = (`notElem` [".", ".."]) > getVisible = filter skipDots > mkpath p1 p2 = p1 ++ "/" ++ p2 > clrSlash = reverse . dropWhile (\c -> c =='/' || c == '\\') . reverse Displays information about a directory. > showDir :: (Integer, String) -> String > showDir (s,p) = printVolume s 3 ++ "\t" ++ show p ++ "\n" *Main> shred "1234567890" ["123","456","789","0"] > shred [] = [] > shred ss = (take 3 ss) : (shred (drop 3 ss)) *Main> prettyNum "1234567890" "1 234 567 890" > prettyNum = concat . intersperse " " . (map reverse) . reverse . shred . reverse *Main> units (1024*1024*1024) [1073741824,1048576,1024,1,0] > units :: Integer -> [Integer] > units 0 = [0] > units x = x : units' x > where > units' 0 = [] > units' x = y : units' y > where > y = round (fromIntegral x / 1024) Just SI prefixes > prefix = ["B", "K", "M", "G", "T", "P", "E", "Z", "Y"] *Main> tagged (1024*1024*1024) [("1 073 741 824","B"),("1 048 576","K"),("1 024","M"),("1","G"),("0","T")] > tagged x = (map (prettyNum . show) $ units x) `zip` prefix *Main> printVolume (1024*1024*1024) 5 1 024 M > printVolume x width = printf "%*s %s" width (fst one) (snd one) > where > one = head $ dropWhile (\p -> length (fst p) > width) $ tagged x

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