- Beginners - Haskell.org

getRecursiveContents - example from `Real World Haskell'
by Johann Giwer 05 Dec '08

05 Dec '08

`Real World Haskell' is a great book. I really love it. When I tried an example from the 9th Chapter, I was a bit disappointed: *Main> f <- getRecursiveContents "/home/johann/" Heap exhausted; Current maximum heap size is 128000000 bytes (122 Mb); use `+RTS -M<size>' to increase it. The function lookes like this: getRecursiveContents :: FilePath -> IO [FilePath] getRecursiveContents topdir = do names <- getDirectoryContents topdir let properNames = filter (`notElem` [".", ".."]) names paths <- forM properNames $ \name -> do -- 1 let path = topdir </> name isDirectory <- doesDirectoryExist path if isDirectory then getRecursiveContents path else return [path] return (concat paths) -- 2 OK, I'm using a small machine and my home directory contains ~30,000 files. But that couldn't be the real problem. And even if this function is a small example it should work reliable. The programming language I know best (and this is meant relative -- I'm only a `would be programmer') is python. Python has good support for functional programming, but no builtin tail recursion. So my first idea about the bug in `getRecursiveContents' went in this direction. Two hours later I had worked out this solution: getRecursiveContents :: FilePath -> IO [FilePath] getRecursiveContents = getRecursiveContents' [] where getRecursiveContents' l p = E.handle (\_ -> return (p:l)) $ do -- 3 c <- getDirectoryContents p let c' = filter (`notElem` [".", ".."]) c x <- foldM (\l' p' -> getRecursiveContents' l' (p </> p')) l c' -- 4 return (x) Folding (4) and appending (3) would give less memory usage than mapping (1) and concatenation (2), I thought. This function worked well for small directory (for which the original one did, too). But tested with my home directory it went into an infinite loop. That led me to the actually problem: `doesDirectoryExist' also accepts symlinks to directories. Another hour later this was fixed: getRecursiveContents :: FilePath -> IO [FilePath] getRecursiveContents = getRecursiveContents' [] where getRecursiveContents' l p = do s <- getSymbolicLinkStatus p if isDirectory s then E.handle (\_ -> return (p:l)) $ do c <- getDirectoryContents p let c' = filter (`notElem` [".", ".."]) c x <- foldM (\l' p' -> getRecursiveContents' l' (p </> p')) l c' return (x) else return (p:l) Finally I fixed the original function (this only took about 30 min :-). The handle (5) catches errors caused by unreadable directories getRecursiveContents :: FilePath -> IO [FilePath] getRecursiveContents topdir = E.handle (\_ ->return [topdir]) $ do -- 5 names <- getDirectoryContents topdir let properNames = filter (`notElem` [".", ".."]) names paths <- forM properNames $ \name -> do let path = topdir </> name s <- getSymbolicLinkStatus path if isDirectory s then getRecursiveContents path else return [path] return (concat paths) The imports for all functions mentioned above are: import Control.Monad ( forM, filterM, foldM ) import qualified Control.Exception as E import System.Directory (doesDirectoryExist, getDirectoryContents) import System.FilePath ((</>)) import System.Posix (getSymbolicLinkStatus, isDirectory) Any suggestions about this solution are welcome. Johann

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Special session on Haskell language
by John Edward 04 Dec '08

04 Dec '08

Special session on Haskell language at MULTICONF-09 call for papers Special session on Haskell language will be held at the 2009 Multi Conference in Computer Science, Information Technology and Control systems and Computational Science and Computer Engineering (MULTICONF-09) (website: http://www.PromoteResearch.org) and it will be held during July 13-16 2009 in Orlando, FL, USA. We invite draft paper submissions. The event consists of the following conferences: · International Conference on Artificial Intelligence and Pattern Recognition (AIPR-09) · International Conference on Automation, Robotics and Control Systems (ARCS-09) · International Conference on Bioinformatics, Computational Biology, Genomics and Chemoinformatics (BCBGC-09) · International Conference on Enterprise Information Systems and Web Technologies (EISWT-09) · International Conference on High Performance Computing, Networking and Communication Systems (HPCNCS-09) · International Conference on Information Security and Privacy (ISP-09) · International Conference on Recent Advances in Information Technology and Applications (RAITA-09) · International Conference on Software Engineering Theory and Practice (SETP-09) · International Conference on Theory and Applications of Computational Science (TACS-09) · International Conference on Theoretical and Mathematical Foundations of Computer Science (TMFCS-09) The website http://www.PromoteResearch.org contains more details. Sincerely John Edward Publicity committee

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What causes <<loop>>?
by Martin Hofmann 03 Dec '08

03 Dec '08

I've already posted this mail on haskell-cafe, but apparently the subject suggested a too simple question, so I try it here again. I am picking up a discussion with the same topic from haskell-users on 8th November. Thunks with reference on themselves was mentioned as main reason for <<loop>>. > A safe recursive definition would be > let x = Foo (x+1) > However, if you leave out the constructor, > let x = x + 1 > you get a <<loop>> (or a deadlock). > Are there any other reasons? I am trying to debug monadic code which stores state information in a record maintaining several Data.Maps, but in vain so far. A state is modified/changed in several steps by a compound function i.e. changeA $ changeB $ changeC state Could this also lead to a deadlock? If so, can I prevent this using CPS? If the state transformation is the main purpose/computation of the monad at all, is it better to use CPS? Is code using mtl, records or Data.Map more prone to <<loop>> or does this not matter at all. How can I identify self-referencing thunks more easily? Is there any rule of thumb to prevent <<loop>>? Are there any less obvious newbie mistakes causing <<loop> then the one above? Sorry, a lot of questions at once, but I am kind of helpless because I can't find any reason of my <<loop>>, so any comments, experience, and tips are highly appreciated. Thanks, Martin

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ANNOUNCE: Haskell Communities and Activities Report (15th ed., November 2008)
by Janis Voigtlaender 03 Dec '08

03 Dec '08

On behalf of the many, many contributors, I am pleased to announce that the Haskell Communities and Activities Report (15th edition, November 2008) http://www.haskell.org/communities/ is now available from the Haskell Communities home page in PDF and HTML formats. Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. I hope you will find it as interesting a read as I did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline ;-) to the call. The editor collects all the contributions into a single report and feeds that back to the community. When I try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ---------------------------------------- End of April 2009: target deadline for contributions to the May 2009 edition of the HC&A Report ---------------------------------------- Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editor for a simple e-mail reminder in April (you could point me to them as well, and I can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Janis Voigtlaender <hcar at haskell.org> -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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Memoization
by abdullah abdul Khadir 01 Dec '08

01 Dec '08

Hi, I am a student and we had an assignment in Haskell. The question, was given a string of the form "1-2+3*5-7+4-6+3" i.e., any sequence of integers as well as some operators between them we had to find a maximum possible value for the expression as well as the expression itself . So for maxval "1-2+3*5-7+4-6+3" it is (76,"(1-((2+3)*((5-(7+4))-(6+3))))"). The function we had to write was maxval :: String -> (Int,String). For further details on the question, have a look at our sir's web page here<http://www.cmi.ac.in/%7Emadhavan/courses/programming08/assignment6.txt>. I solved the question, we had to use memoization, and submitted the solution. It is given below. Now the problem is I am just wondering if it can be solved in a better manner. Translation : Is there some way in Haskell to do it in a more simpler way and as well as to reduce the number of lines of the program. > {- > > --------------------------------------------------------------------------------------------------------------------------- > *********************** module Memo.hs > ****************************************************** > > --------------------------------------------------------------------------------------------------------------------------- > -} > module > Memo(Table,emptytable,memofind,memolookup,memoupdate,memoupdateArray) where > > data (Eq a) => Table a b c = T [(a,a,(b,c),(b,c))] > deriving (Eq,Show) > > emptytable :: (Eq a) => (Table a b c) > emptytable = T [] > > memofind :: (Eq a) => (Table a b c) ->(a,a)-> Bool > memofind (T []) _ = False > memofind (T ((y,z,(v1,s1),(v2,s2)):l)) x > | x == (y,z) = True > | otherwise = memofind (T l) x > > memolookup :: (Eq a) => (Table a b c) -> (a,a) -> ((b,c),(b,c)) > memolookup (T ((y,z,(v1,s1),(v2,s2)):l)) x > | x == (y,z) = ((v1,s1),(v2,s2)) > | otherwise = memolookup (T l) x > > memoupdate :: (Eq a) => (Table a b c) -> (a,a,(b,c),(b,c)) -> (Table a b c) > memoupdate (T l) x = T (x:l) > > memoupdateArray :: (Eq a) => (Table a b c) -> [(a,a,(b,c),(b,c))] -> (Table > a b c) > memoupdateArray t [] = t > memoupdateArray t (x:xs) = memoupdate (memoupdateArray t xs) x > > {- > > --------------------------------------------------------------------------------------------------------------------------- > ***********************End of module > Memo.hs*********************************************** > > --------------------------------------------------------------------------------------------------------------------------- > -} > > > {- > > --------------------------------------------------------------------------------------------------------------------------- > ******************The actual program , > assign-6.hs****************************************** > > --------------------------------------------------------------------------------------------------------------------------- > -} > > minArray :: [(Int,String)] -> (Int,String) > minArray ((x,expr):[]) = (x,expr) > minArray ((x,expr):l) |((min x (fst (minArray l))) ==x) = (x,expr) > |otherwise = (minArray l) > > maxArray :: [(Int,String)] -> (Int,String) > maxArray ((x,expr):[]) = (x,expr) > maxArray ((x,expr):l) |((max x (fst (minArray l))) ==x) = (x,expr) > |otherwise = (minArray l) > > import Memo > import Char > > type Tuple = (Int,Int,(Int,String),(Int,String)) > > maxval :: String ->(Int, String) > maxval expr = snd (memolookup (buildmemo expr 1 emptytable) (1,length > expr)) > > initmemo :: (String) -> (Table Int Int String) > initmemo expr = (memoupdateArray (emptytable) > [(i,i,(toInt(expr!!(i-1)),[expr!!(i-1)]), > (toInt(expr!!(i-1)),[expr!!(i-1)]))|i<-[1..length > expr],j<-[0,1],i `mod` 2 ==1]) > > buildmemo :: (String)->Int -> (Table Int Int String)-> (Table Int Int > String) > buildmemo expr col memo | (col > length expr) = memo > | (col == 1) = buildmemo expr 3 (memoupdateArray > (emptytable) > [(i,i,(toInt(expr!!(i-1)),[expr!!(i-1)]), > > (toInt(expr!!(i-1)),[expr!!(i-1)]))|i<-[1..length expr],i `mod` 2 ==1]) > | otherwise = buildmemo expr (col+2) (memoupdateArray > (memo) (createList expr memo (1,col))) > > createList :: String-> (Table Int Int String) -> (Int,Int) -> [Tuple] > createList expr memo (i,j) | j > (length expr) = [] > | otherwise = (i,j,min_expr,max_expr):(createList expr > memo (i+2,j+2)) > where > min_expr = minArray [x | (x,y) <- list] > max_expr = maxArray [y | (x,y) <- list] > list = [(compute memo (i,k) (k+2,j) > (expr!!k))|k<-[i..j-2],k `mod` 2 ==1] > > compute :: (Table Int Int > String)->(Int,Int)->(Int,Int)->Char->((Int,String),(Int,String)) > compute memo (x1,x2) (y1,y2) op |op == '+' = > ((min1+min2,"("++min1_expr++"+"++min2_expr++")"), > > (max1+max2,"("++max1_expr++"+"++max2_expr++")")) > |op == '-' = > ((min1-max2,"("++min1_expr++"-"++max2_expr++")"), > > (max1-min2,"("++max1_expr++"-"++min2_expr++")")) > |op == '*' = (minArray xs,maxArray xs) > where > xs = [(min1*min2,"("++min1_expr++"*"++min2_expr++")"), > (min1*max2,"("++min1_expr++"*"++max2_expr++")"), > (max1*min2,"("++max1_expr++"*"++min2_expr++")"), > > (max1*max2,"("++max1_expr++"*"++max2_expr++")")] > ((min1,min1_expr),(max1,max1_expr)) = (memolookup memo > (x1,x2)) > ((min2,min2_expr),(max2,max2_expr)) = (memolookup memo > (y1,y2)) > > minArray :: [(Int,String)] -> (Int,String) > minArray ((x,expr):[]) = (x,expr) > minArray ((x,expr):l) |((min x (fst (minArray l))) ==x) = (x,expr) > |otherwise = (minArray l) > > maxArray :: [(Int,String)] -> (Int,String) > maxArray ((x,expr):[]) = (x,expr) > maxArray ((x,expr):l) |((max x (fst (maxArray l))) ==x) = (x,expr) > |otherwise = (maxArray l) > > toInt :: Char -> Int > toInt x = ord x - ord '0' > > {- > > --------------------------------------------------------------------------------------------------------------------------- > ***********************End of program > assign-6.hs******************************************* > > --------------------------------------------------------------------------------------------------------------------------- > -}

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Installing Gtk2hs
by Colin Paul Adams 30 Nov '08

30 Nov '08

The install instructions seem very straight-forward, but they are not working for me. I did: ./configure --enable-docs --disable-deprecated make su -c'make install' but the demos won't compile (they can't find the libraries). checking the stderr from make shows: ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: mtl make: *** Deleting file `package.conf.inplace' package.conf.inplace: openBinaryFile: does not exist (No such file or directory) package.conf.inplace: openBinaryFile: does not exist (No such file or directory) ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: base ghc-pkg: Unversioned dependencies found: mtl make[1]: *** Deleting file `package.conf.inplace' package.conf.inplace: openBinaryFile: does not exist (No such file or directory) package.conf.inplace: openBinaryFile: does not exist (No such file or directory) on the commandline: Warning: -fffi is deprecated: use -XForeignFunctionInterface or pragma {-# LANGUAGE ForeignFunctionInterface#-} instead package.conf.inplace: openBinaryFile: does not exist (No such file or directory) make[1]: *** [glib/System/Glib.o] Error 1 make: *** [all] Error 2 My environment is Fedora 10 (64-bit). Ghc 6.10.1. Any suggestions? -- Colin Adams Preston Lancashire

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In-place lazy I/O
by Alexander Dunlap 30 Nov '08

30 Nov '08

Hi all, Suppose my program has one or more persistent files that it reads at or near the beginning of its execution and writes at or near the end. Thus, an extremely simplified model of my program could be > main = do > h <- openFile "some-file" ReadMode > c <- hGetContents h > w <- openFile "some-file" WriteMode > hPutStr w (f c) where f is some arbitrary function. My question is how to do this. It seems like the data from h won't necessarily be forced until f is called when it is written, and there is no guarantee that all of the data will even be used by f until it is written. Thus, I will get a file-locking error when trying to write the file. Do I have to rig things so that I know f will consume all of its input? Is it better to use strict I/O? Is there a better idiom for this entirely? Thanks for all of your help. Alex

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Help in Haskell
by Andre Paulo Machado 29 Nov '08

29 Nov '08

Hi folks..... Somebody could help me, please? Please, I would like a help in haskell programming, because I have with following question: I need to create a list of strings, like as a graph. See: initiaList = [ ("BSB,"SSA"), ("CNF","SSA"), ("CNF","GIG"), ("CNF","GRU"), ("GIG","CNF"), ("GIG","GRU"), ("GRU","BSB"), ("GRU","GIG"), ("GRU","CNF"), ("SSA","CNF") ] With this list above, I will create a function that returns a path with max number of links: example: maxLink = 4 (result until 4) command: myFunction "BSB" "GRU" 4 result: 1st Option: BSB -> SSA -> CNF -> GRU 2nd Option: BSB -> SSA -> CNF -> GIG -> GRU ---------------------------------------------------------------------------- command: myFunction "BSB" "GRU" 3 result: 1st Option: BSB -> SSA -> CNF -> GRU ---------------------------------------------------------------------------- How do this? I'm waiting from you, thank you so much. Andre

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Recommended GUI library?
by Colin Paul Adams 29 Nov '08

29 Nov '08

Looking at the wiki, it says that none of the high-level libraries are in a really good state for production usage (i.e. they are still all reasearch-level). What would people recommend? I'd really prefer to use a high-level library. -- Colin Adams Preston Lancashire

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Possible to update Haskell syntax
by cm 27 Nov '08

27 Nov '08

I'm very interested in learning Haskell, but one frustration is that lists always require commas. As an example [1,2,3,4] is a valid expression, but [1 2 3 4] is not. In this ideal world, another example would be that [(fun1 3 4) 7] would be a valid expression and be a two-element list equivalent to [fun1 3 4, 7]. If what I am suggesting is syntactically possible and doesn't break more advanced features of the language that I (as a beginner) am unaware of, I'm interested in patching the open source for my own use (as well as others who might want it).

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