February 2010 - Beginners

Simplifying code
by edgar klerks 10 Feb '10

10 Feb '10

Hi All, I wrote a program which permutates a list words with some rules. For example if I feed it a list like: banana waterloo vraag It gives back the list: banana b@nana ban@na b@n@na banan@ b@nan@ ban@n@ b@n@n@ waterloo wa+erloo water|oo waterl0o etc However I have the feeling I am doing things to complicated. I am still a beginner. Would someone like to help me simplify somethings. If you think this is inappropriate please state also. I am not offended then. I understand you are offering your spare time to help me. The first thing I don't get is this. I recognize some things could be rewritten with a bind operator (because of the concat $ fmap), but I am puzzled how: mutateWords :: [Char] -> [[Char]] mutateWords word = nub.concat $ fmap snd <$> fmap unzip <$> ( foldr(\x z -> let char = snd x nm = number word lst = fst x in (insertAt char nm <$> lst) : z ) [[]] $ mw word ) Here is the full code: import Data.List import System import System.IO import Control.Applicative ---CONFIG section leat = ['s' ==> '$', 't' ==> '+', 'l' ==> '|', 'o' ==> '0','e' ==> '3', 'a' ==> '@', 'v' ==> '^'] leata = fst.unzip $ leat leatb = snd.unzip $ leat -- Perl like assoc lists infixl 1 ==> a ==> b = (a, b) -- Flipped fmap sometimes nicer infixl 4 <$$> xs <$$> f = f <$> xs -- first I need to find the positions of the mutatable charachters. findPositions :: [Char] -> [[Int]] findPositions xs = take (length index) $ index <*> [xs] where index = elemIndices <$> leata -- And generate all subsequences findSubSeq :: [Char] -> [[[Int]]] findSubSeq = fmap subsequences <$> findPositions -- Only change elements which needs to be changed insertAt :: Char -> [(Int, Char)] -> [Int] -> [(Int,Char)] insertAt c xs ps = xs <$$> (\x -> if (fst x) `elem` ps then (fst x , c) else x ) -- Couples character to mutable positions mw word = (findSubSeq word) `zip` leatb number = zip [0..] mutateWords :: [Char] -> [[Char]] mutateWords word = nub.concat $ fmap snd <$> fmap unzip <$> ( foldr(\x z -> let char = snd x nm = number word lst = fst x in (insertAt char nm <$> lst) : z ) [[]] $ mw word ) generateAll :: [Char] -> [[Char]] generateAll word = g lea $ mutateWords word where g 0 words = words g n words = g (n - 1) (nub $ words >>= mutateWords ) lea = length leata main = do filename <- getArgs wordlist <- readFile $ filename !! 0 let a = (words wordlist) >>= generateAll mapM_ putStrLn a -- Flatliner ICT Service, Email: Edgar.klerks(a)gmail.com, Tel: +31727851429 Fax: +31848363080 Skype: edgar.klerks Website: flatlinerict.nl Adres: Koelmalaan 258, 1813JD, Alkmaar Nederland

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Is there a an include statement equivalence in Haskell
by Andrew Smith 10 Feb '10

10 Feb '10

Hi I have a few lines of Haskell which I'd like to be pulled into my Haskell program, similar to the #include in C/C++, or Latex. I don't want to just paste the code permanently using editor, or pass it through say M4, nor build my own module. At the moment its a couple of lines I've used in three or four similar programs. As a Haskell beginner, is my thinking still stuck in the imperative coding style era ? -- Andrew

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Re: Simplifying code
by edgar klerks 10 Feb '10

10 Feb '10

He Daniel. I have some trouble understanding what you are saying here: >* -- first I need to find the positions of the mutatable charachters. * No, you don't need to do that, it's in general more efficient to not care about positions when dealing with lists. >* findPositions :: [Char] -> [[Int]] *>* findPositions xs = take (length index) $ index <*> [xs] *>* where index = elemIndices <$> leata * [f1, ..., fm] <*> [x1, ..., xn] produces a list of length m*n, so length (index <*> [xs]) == length index * length [xs] == length index ~> remove "take (length index) $" About this piece of code: fmap snd <$> fmap unzip <$> p Prelude Control.Applicative> let p = [[[(1,2),(3,4),(4,5)]]] Prelude Control.Applicative> fmap snd <$> fmap unzip <$> p [[[2,4,5]]] Prelude Control.Applicative> But yeah, it is ridiculously complicated. And I made it up by trial and error and looking at the types. I have a better approach now. I am now building a tree and then walk trough it to collect all the permutations. Thanks for your help and effort! Edgar

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Reading pixels in Haskell SDL
by han 09 Feb '10

09 Feb '10

I'm using SDL (http://hackage.haskell.org/packages/archive/SDL/0.5.9/doc/html/Graphics-UI-…) As far as I know, in the original SDL for C, reading and writing pixels on a surface is done by accessing pointer + y * width + x. In Pygame, since Python has no such thing as pointer, they have get_at and set_at function for this task. I thought there would be something equivalent in Haskell SDL, but couldn't find any. For writing pixels on the Surface, I've found a couple of functions: Graphics.UI.SDL.Video.fillRect (http://hackage.haskell.org/packages/archive/SDL/0.5.9/doc/html/Graphics-UI-…) Graphics.UI.SDL.Primitives.pixel (http://hackage.haskell.org/packages/archive/SDL-gfx/0.5.3/doc/html/Graphics…) ... However, I can't find one that reads. Graphics.UI.SDL.Types.surfaceGetPixels (http://hackage.haskell.org/packages/archive/SDL/0.5.9/doc/html/Graphics-UI-…) looks like the one, but I have no idea how I can use it, since it returns Pixels which is defined as: data PixelsData type Pixels = Ptr PixelsData I want to read a pixel at (x, y) of a Surface. Does anyone know how?

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Simplifying code
by edgar klerks 09 Feb '10

09 Feb '10

Thanks for all your examples. These are helping me really much. I have an imperative background (which hinders me), but I have the feeling functional programming has the future and I find it really fun to do. I will produce my own real functional version tomorrow and post it (maybe the day after that). It is 1:00 so i got to go to bed. Thanks for all your help! (If this message is not placed under the thread can someone explain how I do this from firefox. I have made my default mail handler gmail in firefox, but many times the post doesn't appear under the thread. I understand this is an annoyance.) With kind regards, Edgar

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Re: [Haskell-beginners] define action getInt like getLine
by kane96＠gmx.de 09 Feb '10

09 Feb '10

ahhh, I didn't know that it's so simple with read :) -------- Original-Nachricht -------- > Datum: Mon, 8 Feb 2010 22:26:59 +0000 > Von: Stephen Tetley <stephen.tetley(a)gmail.com> > An: kane96(a)gmx.de > Betreff: Re: [Haskell-beginners] define action getInt like getLine > Hello > > Upthread Daniel Fisher posted this one > > getInt :: IO Int > getInt = fmap read getLine > > If you check the type of read > > *GHCi> :t read > read :: (Read a) => String -> a > > 'reads' is polymorphic it will read anything from a String - well > within reason, anything that is an instance of the Read type class. > The "(Read a) => " part of the type signature indicates this > constraint - that "a" ( the return type of the function read) must be > an instance of Read. > > As I wrote, you can easily derive an instance of Read by adding Read > to the deriving clause of your data type, so the next thing to do is > write a function swapping as few parts from Daniel's example as > possible: > > -- Here you have to make a change as you want a Month rather than an > Int, so swap the ??? for what you want... > getMonth :: IO ??? > > -- The function definition can stay the same - none of the components > in Daniel's definition actually depended on reading an Int: > getMonth = fmap read getLine > > > > On 8 February 2010 21:43, <kane96(a)gmx.de> wrote: > > but I have to write the action > > readMonth :: IO Month > > on my own and still don't have an idea how to do it > > > > -- GRATIS für alle GMX-Mitglieder: Die maxdome Movie-FLAT! Jetzt freischalten unter http://portal.gmx.net/de/go/maxdome01

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Filtering lists
by legajid＠free.fr 09 Feb '10

09 Feb '10

Hi, i get a .csv file with two different record formats (too bad). The first one, identified by a "A" in the second field, contains 4 fields. The second one, identified by a "B", contains 10 fields. In order to put them in lists, i use lines then break to get a list of lists. Then i want to filter this list, one for the A list, the other one for the B list. Unfortunately, the pattern for the A type contains 4 fields; when a type B list occurs (10 fields), i get a message concerning the pattern : Non-exhaustive patterns in lambda How can i separate the two record formats? Thanks, Didier

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(Integral b, RealFrac b) to Int
by han 09 Feb '10

09 Feb '10

I have this code: line (sx, sy) (tx, ty) | abs (sx - tx) > abs (sy - ty) = zip xs [(sy + (sy - ty) * x / xd) | x <- [0 .. xd]] | otherwise = zip [(sx + (sx - tx) * y / yd) | y <- [0 .. yd]] ys where xs = target sx tx ys = target sy ty xd = abs (sx - tx) yd = abs (sy - ty) It currently has the inferred type signature of (Integral b, RealFrac b) => (b, b) -> (b, b) -> [(b, b)] and I want it to be (Int, Int) -> (Int, Int) -> [(Int, Int)] which, when coerced, causes an error. Any idea?

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define action getInt like getLine
by kane96＠gmx.de 09 Feb '10

09 Feb '10

Hi, how can I write an action getInt :: IO Int that works like getLine :: IO String but for Int instead of String. I know that I can read the input with getLine and then convert it by using read, but don't know how to write it as an action. I tried getInt :: IO Int getInt read <- getLine but that doesn't work. -- Jetzt kostenlos herunterladen: Internet Explorer 8 und Mozilla Firefox 3.5 - sicherer, schneller und einfacher! http://portal.gmx.net/de/go/atbrowser

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Problems with System-Eval-Haskell
by Joe Fox 08 Feb '10

08 Feb '10

Hi I am fairly new to haskell , I was trying to use Eval function in haskell but its giving me a problem , even the example code given in * http://www.cse.unsw.edu.au/~dons/hs-plugins/html/System-Eval-Haskell.html*<http://www.cse.unsw.edu.au/%7Edons/hs-plugins/html/System-Eval-Haskell.html> do i <- eval "1 + 6 :: Int" [] :: IO (Maybe Int) when (isJust i) $ putStrLn (show (fromJust i)) is not working for me , it gives following error on the commandline: Warning: -Onot is deprecated: Use -O0 instead and doesnt print 7 , i guess that is what is expected to be printed. Can any one point me in the right way how to use eval function ? Thanks Joe

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