March 2012 - Beginners

Processing a list of files the Haskell way
by Michael Schober 22 Mar '12

22 Mar '12

Hi everyone, I'm currently trying to solve a problem in which I have to process a long list of files, more specifically I want to compute MD5 checksums for all files. I have code which lists me all the files and holds it in the following data structure: data DirTree = FileNode FilePath | DirNode FilePath [DirTree] I tried the following: -- calculates MD5 sums for all files in a dirtree addChecksums :: DirTree -> IO [(DirTree,MD5Digest)] addChecksums dir = addChecksums' [dir] where addChecksums' :: [DirTree] -> IO [(DirTree,MD5Digest)] addChecksums' [] = return [] addChecksums' (f@(FileNode fp):re) = do bytes <- BL.readFile fp rest <- addChecksums' re return ((f,md5 bytes):rest) addChecksums' ((DirNode fp filelist):re) = do efiles <- addChecksums' filelist rest <- addChecksums' re return $ efiles ++ rest This works fine, but only for a small number of files. If I try it on a big directory tree, the memory gets junked up and it aborts with an error message telling me that there are too many open files. So I guess, I have to sequentialize the code a little bit more. But at the same time, I want to keep it as functional as possible and I don't want to write C-like code. What would be the Haskell way to do something like this? Thanks for all the input, Michael

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Libexiv2, cabal and C++
by edgar klerks 21 Mar '12

21 Mar '12

Dear Haskellers, I wrote a simple FFI binding to libExiv2 ( https://bitbucket.org/eklerks/libexiv2bindings/src) which is a library for reading exif tags from image files. I want to use cabal as build system, but I don't have a clue how to use it in this case. Normally it works like a charm. But because I have to first create a c-wrapper library around the C++ code, it becomes very complicated. IDoes anyone have some good resources I can look into? With kind regards, Edgar Klerks

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State monad and destructive updates
by Rohit Garg 19 Mar '12

19 Mar '12

Hi, As I have been trying to learn the monads a bit more, I have come to realize that State monad doesn't actually represent state. It just provides a convenient glue for threading state. Is that correct? If so, then for performance using state monad is as good as not using it at all. Is there a way to have state with destructive update? Thanks, -- Rohit Garg http://rpg-314.blogspot.com/ Graduate Student Applied and Engineering Physics Cornell University

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When a return type constrained by class
by Haisheng Wu 18 Mar '12

18 Mar '12

Hi there, Do you have any idea why the following code is not valid? class Guess g where makeGuess :: g -> String instance Guess GuessLetter where makeGuess = ... instance Guess GuessWord where makeGuess = ... *-- | This function is nod valid * nextGuess :: Guess g => String -> g nextGuess = ... Thank you. -Haisheng

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gui pkgs
by Andy 18 Mar '12

18 Mar '12

Hi I'm trying to create a calculator in Haskell, something like the one in the enclosed image. To program the gui , I thought i might use a toolkit such as wxHaskell to create the buttons and frame etc.. I followed all the instructions on the wxHaskell homepage and everything seemed to install properly. Next I tried to use it with ghci, but to no avail. It seems you have to register wxHaskell with ghci by using a command like <ghci -package wx> but that crashes . Every tutorial I read didn't work. Does anyone have a clear set of instructions,that does work? Also, is there a better toolkit for doing guis than wxHaskell? I tried Glade, but it's so bug-ridden I couldn't get anything done. I need help.

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Re: [Haskell-beginners] Finite self-referential list hangs
by tcollin371 18 Mar '12

18 Mar '12

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Finite self-referential list hangs.
by tcollin371 18 Mar '12

18 Mar '12

I am trying to generate a search tree for a logic puzzle using a list that generates itself by using earlier states to derive later states. The search tree is finite, but when it reaches the end, it hangs and while the list doesn't grow, it doesn't finish. I've boiled the problem down to a much simpler list as follows: tryThis :: [Int] tryThis = 3 : [ x | x <- tryThis' tryThis] where tryThis' :: [Int] -> [Int] tryThis' [] = [] tryThis' (x : xs) = result' x ++ tryThis' xs where result' :: Int -> [Int] result' 3 = [2, 2, 2] result' 2 = [1, 1] result' 1 = [0] result' 0 = [] When I load this into GHCi and type tryThis, it prints a list of all of the numbers I would expect, then hangs. Does the self-generating technique not work with finite lists? Shouldn't tryThis' just generate a final empty list that would terminate this? How can this be written to successfully terminate? Thank you for any help. -TC

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[Haskell Cabal] Failed to compil on install twidge
by Gabriel Delattre 17 Mar '12

17 Mar '12

Hi ! I'm new to Haskell and Cabal. I try to install twidge. I get this tone of tone of warning. It is a fresh install of Haskell from the package (pkg) $cabal install twidge ^ 5 warnings generated. [1 of 7] Compiling Codec.Binary.UTF8.String ( Codec/Binary/UTF8/String.hs, dist/build/Codec/Binary/UTF8/String.o ) Codec/Binary/UTF8/String.hs:153:5: parse error (possibly incorrect indentation) cabal: Error: some packages failed to install: ConfigFile-1.1.1 depends on text-0.11.1.13 which failed to install. HSH-2.0.4 depends on text-0.11.1.13 which failed to install. HUnit-1.2.4.2 failed during the building phase. The exception was: ExitFailure 1 HaXml-1.13.3 failed during the building phase. The exception was: ExitFailure 1 MissingH-1.1.1.0 depends on text-0.11.1.13 which failed to install. SHA-1.5.0.0 depends on binary-0.5.1.0 which failed to install. binary-0.5.1.0 failed during the building phase. The exception was: ExitFailure 1 curl-1.3.7 failed during the building phase. The exception was: ExitFailure 1 hoauth-0.2.3 depends on utf8-string-0.3.7 which failed to install. hslogger-1.1.5 depends on text-0.11.1.13 which failed to install. network-2.3.0.11 depends on text-0.11.1.13 which failed to install. parsec-3.1.2 depends on text-0.11.1.13 which failed to install. text-0.11.1.13 failed during the building phase. The exception was: ExitFailure 1 twidge-1.0.6.1 depends on utf8-string-0.3.7 which failed to install. utf8-string-0.3.7 failed during the building phase. The exception was: ExitFailure 1 Also the ton of warning are very long I can not copy paste them here (lines of lines of error/warning) So please find them there : http://pastebin.com/txVTRw1U Gabriel

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Uhc and JavaScript, is it usable?
by Nathan Hüsken 16 Mar '12

16 Mar '12

Hi, I trying to inform myself about the uhc haskell -> js compiler but I can find very little information. Is it mature enough to be used? Can an interactive application using the canvas for animations be made with it? Is webgl possible? Thanks! Nathan -- Diese Nachricht wurde von meinem Android-Mobiltelefon mit K-9 Mail gesendet.

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Strange gchi behavior?
by Olwe Melwasul 15 Mar '12

15 Mar '12

I'm working my way through Real World Haskell and am in Chapter 3. On page 56 there is a discussion called "Record Syntax" and it gives this data constructor: data Customer = Customer { customerID :: CustomerID , customerName :: String , customerAddress :: Address } deriving (Show) where CustomerID and Address were defined (and loaded) before. But when I try to put in data, I get these errors *Main> :load BookStore.hs [1 of 1] Compiling Main ( BookStore.hs, interpreted ) Ok, modules loaded: Main. *Main> customer1 = Customer 271828 "J.R. Hacker" ["255 Syntax Ct", "Milpitas, CA 95134", "USA"] <interactive>:1:11: parse error on input `=' *Main> <interactive>:1:18: parse error (possibly incorrect indentation) *Main> <interactive>:1:21: parse error on input `,' *Main> <interactive>:1:6: parse error on input `]' *Main> If I put the whole data add on one line, I still get *Main> customer1 = Customer 271828 "J.R. Hacker" ["255 Syntax Ct", "Milpitas, CA 95134", "USA"] <interactive>:1:11: parse error on input `=' *Main> I'm on Ubuntu 11.10, using Emacs23, haskell-mode, running 7.0.4. Any ideas what's going wrong? The book gives this alternate version: data Customer = Customer Int String [String] deriving (Show) customerID :: Customer -> Int customerID (Customer id _ _) = id customerName :: Customer -> String customerName (Customer _ name _) = name customerAddress :: Customer -> [String] customerAddress (Customer _ _ address) = address ...but it produces the same errors. Olwe

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