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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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defining expression
by John Moore 07 Feb '10

07 Feb '10

Hi, This gives me the explaination that it expecting a Expression but it been giving a IO b can some explain whats going on please. import Maybe data Expression = Val Double | Add Expression Expression | Subtract Expression Expression | Multiply Expression Expression | Divide Expression Expression | Var String | Let String Expression Expression deriving Show demo1 = (Add(Multiply(Divide(Subtract(Val 25)(Val 5))(Val 10))(Val 7))(Val 30)) type Dict =[(String,Expression)] emptyDict :: Dict emptyDict = [] addEntry :: String->Expression ->Dict -> Dict addEntry n e d = (n,e): d lookupEntry :: String -> Dict -> Maybe Expression lookupEntry n [] = Nothing lookupEntry n (x:xs) = if (n == k) then (Just v) else lookupEntry n xs where (k,v) = x evalStep :: Dict -> Expression -> Expression evalStep d(Val x)= (Val x) evalStep d(Add x y) = case x of (Val a) -> case y of (Val b) -> Val (a+b) left -> Add x (evalStep d y) right -> Add (evalStep d x)y evalStep d(Subtract x y) = case x of (Val a) -> case y of (Val b) -> Val (a-b) left -> Subtract x (evalStep d y) right -> Subtract (evalStep d x)y evalStep d(Multiply x y) = case x of (Val a) -> case y of (Val b) -> Val (a*b) left -> Multiply x (evalStep d y) right -> Multiply (evalStep d x)y evalStep d (Divide x y) = case x of (Val a) -> case y of (Val b) -> Val (a/b) left -> Divide x (evalStep d y) right -> Divide (evalStep d x)y evalStep d (Let n e1 e2) = do putStrLn ("Adding definition for "++n) v <- evalStep d (addEntry n e1 d) e2 putStrLn ("Removing definition for "++n) return v evaluate :: Dict-> [Expression] -> Expression -> IO () evaluate d(x:xs) e = do putStrLn (show e) putStrLn "Do another step (y/n) or rollback (r)? :" c <- getLine case c of "y" -> let e'= (evalStep d e)in evaluate d (e:x:xs) e'-- build up history "r" -> case (x:xs) of (x:xs)-> evaluate d xs x []-> do { putStrLn "Empty" ;evaluate d(x:xs) e } "n" -> putStrLn $ "Ok you said no :" ++ c John

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Installing hmatrix on OS X
by Duncan Mortimer 07 Feb '10

07 Feb '10

Hi all, I'm trying to install the easyVision library on OS X, following the notes at http://mit.edu/harold/Public/easyVisionNotes.html. However, despite a few hours web trawling and hacking away with various cabal options, I can't get "hmatrix" to install. I'm using OS X 10.5.8 with GHC 6.10.4. I installed gsl using macports, as suggested in the notes above, but cabal doesn't seem to be able to find it. The error I'm getting is: [13:22:35 dmorti hmatrix-0.8.2.0]$ cabal install hmatrix Resolving dependencies... [1 of 1] Compiling Main ( /var/folders/05/05Ltg9vxE+a1hR422nbuE++++TI/-Tmp-/hmatrix-0.8.2.05810/hmatrix-0.8.2.0/Setup.lhs, /var/folders/05/05Ltg9vxE+a1hR422nbuE++++TI/-Tmp-/hmatrix-0.8.2.05810/hmatrix-0.8.2.0/dist/setup/Main.o ) Linking /var/folders/05/05Ltg9vxE+a1hR422nbuE++++TI/-Tmp-/hmatrix-0.8.2.05810/hmatrix-0.8.2.0/dist/setup/setup ... Configuring hmatrix-0.8.2.0... Checking foreign libraries... FAIL *** Sorry, I can't link GSL. *** Please make sure that the appropriate -dev packages are installed. *** You can also specify the required libraries using *** cabal install hmatrix --configure-option=link:lib1,lib2,lib3,etc. setup: Package hmatrix-0.8.2.0 can't be built on this system. cabal: Error: some packages failed to install: hmatrix-0.8.2.0 failed during the building phase. The exception was: exit: ExitFailure 1 It appears that others have also had this error, but have been able to fix it up by including "--extra-lib-dirs" and "--extra-include-dirs" options, or using "ld-options: -framework vecLib". I've tried both of these approaches, but still get the same errors. Can anyone offer any suggestions? Thanks very much for your help, Duncan Mortimer

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