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Stack overflow, but hard to understand
by Michael Mossey 21 Oct '09

21 Oct '09

Okay, beginner encountering first major bug. I'm getting a stack overflow on a program that uses a lot of laziness to construct and modify lists---but the overflow happens even if I take just one element from the final list! The overflow seems to have started when I added the functions 'gauss' and 'gaussList' below, but I am at a loss to know why---these functions work perfectly in test cases and can even generate a long list. The following program is distilled (with a lot of work!) down to the essence of what triggers the problem. import Control.Monad import Control.Monad.Random import Data.List -- Here I put a moderate-sized list of filenames which just generic -- text or code. The longer the list, -- the more likely to overflow stack. Which is curious, because if IO were -- lazy here, it shouldn't be reading more than the first character of the -- first file. Methinks a hint to the problem. fileList = ... data TextGroup = Single Char Float deriving (Show) textGroup_addDelay :: TextGroup -> Float -> TextGroup textGroup_addDelay (Single c del) del2 = Single c (del+del2) mkTextGroup :: Char -> Rand StdGen TextGroup mkTextGroup c = liftM (Single c) gauss -- Here's the kinda weird thing I'm doing with Rand. I want to -- use fromList as the first computation to randomly choose -- a second Rand computation. It turns out if you replace -- this funkiness with -- gauss = getRandomR (0,1) -- the whole thing works. gaussList :: Rand StdGen (Rand StdGen Float) gaussList = fromList [ (getRandomR( -1.0 ,-0.8 ), 2) , (getRandomR( -0.8 ,-0.6 ), 2) , (getRandomR( -0.6 ,-0.4 ), 4) , (getRandomR( -0.4 ,-0.2 ), 8) , (getRandomR( -0.2 , 0.0 ), 12) , (getRandomR( 0.0 , 0.2 ), 12) , (getRandomR( 0.2 , 0.4 ), 8) , (getRandomR( 0.4 , 0.6 ), 4) , (getRandomR( 0.6 , 0.8 ), 2) , (getRandomR( 0.8 , 1.0 ), 2) ] gauss :: Rand StdGen Float gauss = do m <- gaussList m main = do gen0 <- newStdGen bufs <- mapM readFile fileList let buf = concat bufs let tgs = evalRand (do orig <- mapM mkTextGroup buf addBreaks orig) gen0 writeFile "output.ank" (show $ take 1 tgs)

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haskell on snow leopard
by Bernardas Jankauskas 20 Oct '09

20 Oct '09

Hello, For two past weeks I have been trying to get Haskell working on my macbook pro with Snow Leopard, but unfortunately I was unable to get it. There are some solutions on the internet, but these are mostly useful when you already have ghci and haskell platform installed on your computer, while I can't even get it installed. I have Xcode 3.1 installed, but it seems that it is not the case. Maybe you could recommend any solution to this problem? Sincerely, Bernardas Jankauskas P.S.: I have attached two screenshots of what occurs for me when I am trying to install haskell platform and ghci.

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Re: \x -> x < 0.5 && x > -0.5
by Stephen Tetley 20 Oct '09

20 Oct '09

Christian Maeder wrote: > Hoogle did not find a function of type: > > (b -> b -> b) -> (a -> b) -> (a -> b) -> a -> b > > or (b -> c -> d) -> (a -> b) -> (a -> c) -> a -> d > > But maybe such a function is worth being added to Data.Function. Hello Christian This is the big Phi or S' combinator. I would certainly second its addition to Data.Function. sprime p q r s = p (q s) (r s) Best wishes Stephen

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new locale?
by Robert Wills 19 Oct '09

19 Oct '09

Hello, http://hackage.haskell.org/package/old-locale-1.0.0.1 says "This package provides the old locale library. For new code, the new locale library is recommended." Does anyone know where I would find the new library? Thanks, Rob

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parsing upto n items with parsec
by Ashish Agarwal 19 Oct '09

19 Oct '09

Hi. I'm just learning Parsec and Haskell. It is a great library, and I see how "many" lets you parse 0 or more items, "many1" parses 1 or more items, and "count" parses exactly n items. However, there is no combinator that parses between m and n items. What would be the best implementation for this?

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Re: \x -> x < 0.5 && x > -0.5
by Jordan Cooper 19 Oct '09

19 Oct '09

Whoa... how on earth does this work? How does it interpret the sections as Reader monads? > That's a job for the reader monad. > > > Lambda Fu, form 53 - silent reader of truth > > import Control.Monad > import Control.Monad.Reader > > filter (liftM2 (&&) (< 0.5) (> -0.5)) xs > > > > Regards, > apfelmus

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edit-compile-test loop
by Tom Doris 19 Oct '09

19 Oct '09

Hi I'd like to know what the typical edit-compile-test loop looks like with the Haskell platform; that is, in C++ this would be edit, run make to compile everything in the project into libraries and executables, then run an executable test suite. I'm confused as to how people work on larger projects in Haskell - do you work on a single module and load it into ghci to test as you develop, then compile the entire package and run a test suite? Or do you generally only use ghci for prototyping and not when in the middle of proper development? Or do you compile the package and load that into ghci? I'd like to know as I'm starting to work on patches for some hackage packages which have proper cabal builds etc., and want to follow the correct (and efficient!) convention. Thanks Tom

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map question
by Joost Kremers 18 Oct '09

18 Oct '09

Hi all, I've just started learning Haskell and while experimenting with map a bit, I ran into something I don't understand. The following commands do what I'd expect: Prelude> map (+ 1) [1,2,3,4] [2,3,4,5] Prelude> map (* 2) [1,2,3,4] [2,4,6,8] Prelude> map (/ 2) [1,2,3,4] [0.5,1.0,1.5,2.0] Prelude> map (2 /) [1,2,3,4] [2.0,1.0,0.6666666666666666,0.5] But I can't seem to find a way to get map to substract 1 from all members of the list. The following form is the only one that works, but it doesn't give the result I'd expect: Prelude> map ((-) 1) [1,2,3,4] [0,-1,-2,-3] I know I can use an anonymous function, but I'm just trying to understand the result here... I'd appreciate any hints to help me graps this. TIA Joost -- Joost Kremers, PhD University of Frankfurt Institute for Cognitive Linguistics Grüneburgplatz 1 60629 Frankfurt am Main, Germany

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count function
by John Moore 18 Oct '09

18 Oct '09

Hi, Trying to get aprogram to count all the lines or words from a file. Not sure about the syntax. Probably missing some. Here's what I have so far any help in the right direction would be great. import System.IO main :: IO () main = do incom <- openFile "file.txt" ReadMode otcom <- openFile "prob.txt" WriteMode fCount incom otcom hClose incom hClose otcom fCount incomh otcomh = do eof <- hIsEof incomh if eof then return() else do c <- hGetCount incom it tells me parse error on = John

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I/O
by John Moore 18 Oct '09

18 Oct '09

Hi, Could someone please tell me how to check this program. The problem is where do I find the output? I laod this into ghc and it comes up OK modules loaded: Main gchi> How do I now check if it works or see the result. Program below (taken from real world haskell) -- file: ch07/toupper-imp.hs import System.IO import Data.Char(toUpper) main :: IO () main = do inh <- openFile "quux.txt" ReadMode outh <- openFile "output.txt" WriteMode mainloop inh outh hClose inh hClose outh mainloop :: Handle -> Handle -> IO () mainloop inh outh = do ineof <- hIsEOF inh if ineof then return () else do inpStr <- hGetLine inh hPutStrLn outh (map toUpper inpStr) mainloop inh outh John

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