February 2010 - Beginners

Maximum length of a GHCi command?
by Kwanghoon Choi 02 Mar '10

02 Mar '10

Dear Haskeller, I have a little issue in using GHCi as follows: === Prelude> length "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa" <interactive>:1:254: lexical error in string/character literal at character '\EOT' === It seems to me that GHCi doesn't allow me to type more than some specified length of a command. Is this interpretation correct? How can I make it longer? This issue is quite important at least to me. My tool is just a simple Haskell program. I don't like to add any extra interactive environment, but I like to just use GHCi as an interactive environment for my tool. I strongly believe that this ides is pervasive among many haskellers. Thanks in advance for your help/feedback in advance. Kwanghoon

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Small question
by edgar klerks 01 Mar '10

01 Mar '10

Hi there, I have created a function mkc: mkc :: (a -> b) -> a -> m b mkc f = (\x -> return $ f x) which works wonderful in statements like: number <- many1 digit >>= mkc(read) But somehow I think this function should already exist, it is some kind of lifting. If it exist where can I find it? Or is there a reason, why it isn't in prelude, because it is more elegant way to solve this problem. Thanks in advance, Edgar

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Parse error in pattern
by Florian Duret 28 Feb '10

28 Feb '10

Hello, I try to set up a verification on the number of arguments given to my program, but keep on getting "Parse error in pattern" Here is what my code looks like: main :: IO() main = do -- On commence par ouvrir le fichier SAC en mode binaire argsList <- getArgs if (length (argsList) == 0) then do putStrLn $ "No filename given to the program.\n $ ProgramName file.sac" return () else sacFile1 <- openBinaryFile fileToOpen ReadMode ghci complains, and tells "Parse error in pattern", indicating the 'if' line number. Can you please help ? Thank you very much, Florian

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Haskell and OpenGL
by Tyler Hayes 26 Feb '10

26 Feb '10

For anyone comfortable with OpenGl in Haskell, what is the equivalent for glutTimerFunc? Is it addTimerCallback? If so, how does it work--I've tried it and I can't seem to get it to work... It's really odd that there are straightforward implementations of callbacks for display, window resize, keyboard/mouse, etc., but not for something as common as a timer callback. Thanks, Tyler

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search a tree
by John Moore 26 Feb '10

26 Feb '10

Hi All, I have an expression which contains several different let statements let x = 4*5 and let Y = 6+4 for example. What I want to do is traverse the tree and just return the variables. In other words I just want it to return a list of all the variables in a tree. I have a function for Var String. So would it be something like given a String -> [String] or is this expression ->String (gives back a string) Whats the best way to check or compare? Jon

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Y-combinator
by Jon Harrop 25 Feb '10

25 Feb '10

OCaml can do this: $ ocaml -rectypes Objective Caml version 3.11.1 # let fix f = (fun x -> f(x x)) (fun x y -> f(x x) y);; val fix : (('a -> 'b) -> 'a -> 'b) -> 'a -> 'b = <fun> # fix (fun f -> function 0 -> 1 | n -> n*f(n-1)) 10;; - : int = 3628800 Haskell 98 doesn't seem to be able to type check that definition of the y-combinator directly. Is there a Haskell extension that would let it handle this? -- Dr Jon Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/?e

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State Transformer Confusion
by Chris Pettitt 25 Feb '10

25 Feb '10

Hello Haskell-Beginners, I'm having a fair amount of difficulty understanding the ST monad. In my basic experimentation I'm finding that I'm getting more heap allocated with STUArray than with a regular Array, which is not what I would expect. I've attached a sample program that attempts to sum a list of Ints into an array of length 1, using STUArray and UArray. The idea is based loosely on http://www.haskell.org/haskellwiki/Monad/ST, but using an array instead of an STRef. I compile using these arguments: ghc -fforce-recomp -O2 -prof -auto-all -caf-all --make sttest.hs When I run the ST-based function (sumInPlace1), I get the following output: ./sttest +RTS -sstderr 1784293664 240,944,212 bytes allocated in the heap 35,448 bytes copied during GC 4,052 bytes maximum residency (1 sample(s)) 12,332 bytes maximum slop 1 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 459 collections, 0 parallel, 0.00s, 0.00s elapsed Generation 1: 1 collections, 0 parallel, 0.00s, 0.00s elapsed INIT time 0.00s ( 0.00s elapsed) MUT time 0.30s ( 0.31s elapsed) GC time 0.00s ( 0.00s elapsed) RP time 0.00s ( 0.00s elapsed) PROF time 0.00s ( 0.00s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 0.30s ( 0.31s elapsed) %GC time 0.8% (1.2% elapsed) Alloc rate 804,910,126 bytes per MUT second Productivity 99.0% of total user, 96.5% of total elapsed While with the Array based function (sumInPlace2), I get the following output: ./sttest +RTS -sstderr 1784293664 173,627,572 bytes allocated in the heap 22,596 bytes copied during GC 3,920 bytes maximum residency (1 sample(s)) 12,464 bytes maximum slop 1 MB total memory in use (0 MB lost due to fragmentation) Generation 0: 333 collections, 0 parallel, 0.00s, 0.00s elapsed Generation 1: 1 collections, 0 parallel, 0.00s, 0.00s elapsed INIT time 0.00s ( 0.00s elapsed) MUT time 0.14s ( 0.15s elapsed) GC time 0.00s ( 0.00s elapsed) RP time 0.00s ( 0.00s elapsed) PROF time 0.00s ( 0.00s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 0.15s ( 0.15s elapsed) %GC time 1.3% (1.9% elapsed) Alloc rate 1,200,569,571 bytes per MUT second Productivity 98.3% of total user, 94.7% of total elapsed One additional point of confusion for me: when I run either function with +RTS -hc and use hp2ps I get an empty graph. I've seen these tools work quite well before, so I suspect I'm doing something wrong now. Thanks, Chris

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Problem with GHCI.
by Chimpion Chimpion 24 Feb '10

24 Feb '10

Could anyone tell me why this might be happening? I was trying to add some modules to the GHCi(:d +...). Couldn't match expected type `String -> IO String' against inferred type `()' In the expression: () In the expression: () :: String -> IO String In the definition of `__cmCompileExpr': __cmCompileExpr = () :: String -> IO String Thank you.

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GHC 6.12, -fwarn-unused-do-bind and Text.Printf
by Iustin Pop 23 Feb '10

23 Feb '10

Hi there, I'm trying to upgrade some code to ghc 6.12 but under -Wall, the new warning about unused do binds is triggering on Text.Printf.printf usage. This code: import Text.Printf main :: IO () main = do printf "Hello, world\n" printf "Hello the second time\n" When run with runhaskell -Wall gives: Warning: A do-notation statement discarded a result of type GHC.Prim.Any. Suppress this warning by saying "_ <- printf "Hello, world\n"", or by using the flag -fno-warn-unused-do-bind. Now, I'm confused why such a simple usage of printf triggers this warning; enforcing the first printf to type IO () fixes the warning, but it seems strange to no longer be able to simply write printf. Anyone knows a workaround, or can enlighten me what I'm doing wrong? A side question would be why we have "instance PrintfType (IO a)" instead of "instance PrintfType (IO ())"; what use is there for that generic type? Can printf return anything else than IO () (when used under IO at all)? thanks, iustin

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Call for papers: MULTICONF-10, Orlando, USA, July 2010
by John Edward 22 Feb '10

22 Feb '10

It would be highly appreciated if you could share this announcement with your colleagues, students and individuals whose research is in computer science, computer engineering, information science and related areas. Call for papers: MULTICONF-10, Orlando, USA, July 2010 The 2010 multi-conference (MULTICONF-10) (website: http://www.PromoteResearch.org ) will be held during July 12-14, 2010 in Orlando, Florida, USA. The primary goal of MULTICONF is to promote research and developmental activities in computer science, information technology, control engineering, and related fields. Another goal is to promote the dissemination of research to a multidisciplinary audience and to facilitate communication among researchers, developers, practitioners in different fields. The following conferences are planned to be organized as part of MULTICONF-10. • International Conference on Artificial Intelligence and Pattern Recognition (AIPR-10) • International Conference on Automation, Robotics and Control Systems (ARCS-10) • International Conference on Bioinformatics, Computational Biology, Genomics and Chemoinformatics (BCBGC-10) • International Conference on Computer Communications and Networks (CCN-10) • International Conference on Enterprise Information Systems and Web Technologies (EISWT-10) • International Conference on High Performance Computing Systems (HPCS-10) • International Conference on Information Security and Privacy (ISP-10) • International Conference on Image and Video Processing and Computer Vision (IVPCV-10) • International Conference on Software Engineering Theory and Practice (SETP-10) • International Conference on Theoretical and Mathematical Foundations of Computer Science (TMFCS-10) MULTICONF-10 will be held at Imperial Swan Hotel and Suites. It is a full-service resort that puts you in the middle of the fun! Located 1/2 block south of the famed International Drive, the hotel is just minutes from great entertainment like Walt Disney World® Resort, Universal Studios and Sea World Orlando. Guests can enjoy free scheduled transportation to these theme parks, as well as spacious accommodations, outdoor pools and on-site dining — all situated on 10 tropically landscaped acres. Here, guests can experience a full-service resort with discount hotel pricing in Orlando. Please see the website http://www.PromoteResearch.org for more details. Sincerely John Edward

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