January 2009 - Beginners

gtk2hs, inputAdd and forked process
by Johann Giwer 15 Jan '09

15 Jan '09

Some times ago I wrote a frontend for mpg123 using python and gtk2. Now I'm trying to do the same in haskell, but my attempt failed in a very early stage. I like to start a subprocess and read its output via 'inputAdd'. The (simplified) code looks like this: module Main (Main.main) where import System.IO.UTF8 hiding (putStr, putStrLn, print) import qualified Control.Exception as E import System.Glib.MainLoop import System.Environment import System.Posix.Process import System.Posix.IO import System.Posix.Types import Graphics.UI.Gtk main :: IO () main = do name <- getProgName argv <- getArgs case argv of [file] -> play file _ -> putStrLn $ "Usage: " ++ name ++ " MP3 FILE" play :: FilePath -> IO () play file = do initGUI (r0,w0) <- createPipe (r1,w1) <- createPipe pid <- forkProcess $ do closeFd w0 closeFd r1 dupTo r0 stdInput dupTo w1 stdOutput executeFile "mpg123" True ["--output", "alsa", "--remote"] Nothing closeFd w1 closeFd r0 window <- windowNew onDestroy window mainQuit button <- buttonNew set button [ buttonLabel := "Play" ] onClicked button $ do w <- fdToHandle w0 E.handle ( \e -> print e ) $ do hPutStrLn w ( "L " ++ file ) return () set window [ containerChild := button ] widgetShowAll window inputAdd (fromIntegral r1) [ IOIn,IOPri ] priorityHigh ( readData r1 ) mainGUI readData :: Fd -> IO Bool readData h = do E.handle ( \e -> print e >> return True ) $ do (s,_) <- fdRead h 1 putStr s return False When loading the file in ghci, I get half a second of sound and a couple of status message lines. Compiled with ghc, this program gives absolute no output (So threading seams to be the problem?). In a next step, I wrapped every IO action in the main process in a call of 'forkIO' and used MVars for the file name and descriptors, but that doesn't help. Does anybody have experience with 'inputAdd' and forked processes? Thanks in advance -Johann

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Odd behavior from WinHugs
by David Schonberger 14 Jan '09

14 Jan '09

Hi all. I'm attempting exercise 4.6 from Daume's Yet Another Haskell Tutorial: Exercise 4.6 Write a datatype Tuple which can hold one, two, three or four elements, depending on the constructor (that is, there should be four constructors, one for each number of arguments). Also provide functions tuple1 through tuple4 which take a tuple and return Just the value in that position, or Nothing if the number is invalid (i.e., you ask for the tuple4 on a tuple holding only two elements) For starters, I began with the following, stored in a file called MyTuple.hs: data myTuple a b c d = my4Tuple a b c d | my3Tuple a b c | my2Tuple a b | my1Tuple a deriving (Show) When I load this into WinHugs I get the following error: file:.\MyTuple.hs:1 - Syntax error in data declaration (unexpected symbol "a") However I get the same error if I reduce it to a single data constructor, in an effort to create just a quadruple: data myTuple a b c d = my4Tuple a b c d What is even more odd is that I have the following code in a file called Pair.hs: {-data Pair a b = Pair a b deriving (Show)pairFst (Pair x y) = xpairSnd (Pair x y) = y data Quadruple a b c d = Quadruple a a b b deriving (Show)fstPairofQuad (Quadruple a b c d) = [a,b]sndPairofQuad (Quadruple a b c d) = [c,d]-} data Quad a b c d = QuadFour a b c d | QuadThree a b c | QuadTwo a b | QuadOne a deriving (Show) {-fstPairofQuad (QuadFour a b c d) = (a,b)mdlPairofQuad (QuadFour a b c d) = (b,c)sndPairofQuad (QuadFour a b c d) = (c,d)fstLstofQuad (QuadFour a b c d) = (a,d)-} Whether I just keep Quad visible in the file or I take away both pairs of comment braces, when I load that file into WinHugs it works fine, and I have no trouble constructing a Quad or using any of the QuadFour utility fcns (which, admittedly are not what is called for in the exercise... but that's probably besides the point). What in the world might be causing the error in the MyTuple file? Thanks, David

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The problem with Monads...
by Rafael Gustavo da Cunha Pereira Pinto 14 Jan '09

14 Jan '09

Last night I was thinking on what makes monads so hard to take, and came to a conclusion: the lack of a guided tour on the implemented monads. Let's take the Writer monad documentation: all it says is: Inspired by the paper "Functional Programming with Overloading and Higher-Order Polymorphism", Mark P Jones (http://web.cecs.pdx.edu/~mpj/pubs/springschool.html) Advanced School of Functional Programming, 1995. SO WHAT? The best approach is the Part II of the "All About Monads" tutorial. There you have the almost ideal approach, except that the examples are just thrown there, with no step-by-step explanation. Of course one could copy, paste and run it, but this gives pretty much a "is it right?' feeling. Questions like "if a Reader is an application, why don't use a regular function instead?" or "what bind means for a State monad?". I will try to work on a "Part II" extended version on my vacations... maybe a WikiMonad... or MonadPedia... :-) After all, it is my duty as a haskell noob to write another monad tutorial! :D Cheers! -- Rafael Gustavo da Cunha Pereira Pinto Electronic Engineer, MSc.

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First request for references
by Alan Cameron 13 Jan '09

13 Jan '09

Hi, I stumbled across Haskell while researching 'functional programming'. I have done some initial reading and have the following questions: 1. Are there any references to real-world applications using this language? I ask because most of the articles I have found seem to be of an extreme academic nature. I want to be sure that my time will not be wasted delving deeper. 2. Are there any tutorials with full code included which I can use on the HUGS system I have installed? I have found a few "A Gentle Introduction to Haskell 98" which assumes far too much previous knowledge to be immediately useful to me. Thanks for any replies. Alan Cameron

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Referring to several constructors at once
by Tomer Libal 12 Jan '09

12 Jan '09

Hello, I am trying to define a logical calculus using one data type for the rules. There are three types of rules and each has 0,1 or 2 assumptions (rule, rule1 and rule2 below). I have defined all the rules as different constructors so the difference between the types according to 0,1 or 2 assumptions is very weak. I would still like to be able to group the rules in types according to the number of assumptions in order to use pattern matching. Is there a simple way to do that or another way I should implement the data type such that I can refer to the rules both according to their number of assumptions and according to their type? 27 data Rule = Axiom {lowseq :: Sequent} 28 | WeakeningL {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 29 | WeakeningR {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 30 | ContractionL {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 31 | ContractionR {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 32 | PermutationL {rule :: Rule, lowseq :: Sequent} 33 | PermutationR {rule :: Rule, lowseq :: Sequent} 34 | Mix {rule1 :: Rule, rule2 :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 35 | NotL {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 36 | NotR {rule :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 37 | AndL {rule1 :: Rule, rule2 :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 38 | AndR {rule1 :: Rule, rule2 :: Rule, lowseq :: Sequent, foccur :: FormulaOccur} 39 deriving (Eq, Show) Thanks, Tomer

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untilM and scanM
by Jan Snajder 11 Jan '09

11 Jan '09

Hi, is there a reason why there is no monadic version of "until" in the Haskell libraries? It would be defined as follows: untilM :: (Monad m) => (a -> Bool) -> (a -> m a) -> a -> m a untilM p f x | p x = return x | otherwise = f x >>= untilM p f The same applies to scanM, also not part of the libraries: scanM :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m [a] scanM f q [] = return [q] scanM f q (x:xs) = do q2 <- f q x qs <- scanM f q2 xs return (q:qs) I often find myself in need for these. To me these seem idiomatic enough to be included in the library. But since they is not, I guess there must be another, more idiomatic way to do this. Thank you, Jan

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typeOf raises Error
by Frank Schwidom 11 Jan '09

11 Jan '09

Hi, if im calling (on the console) "typeOf 1" after importing Data.Typeable, i will gain "Integer", as i expected, but if i do so in code which is to compile, then there raises an error ------------ import Data.Typeable main= print (typeOf 1) ------------ $ ghc -c DT.hs $ ghc -c DT.hs DT.hs:4:15: Ambiguous type variable `t' in the constraints: `Num t' arising from the literal `1' at DT.hs:4:15 `Typeable t' arising from a use of `typeOf' at DT.hs:4:8-15 Probable fix: add a type signature that fixes these type variable(s) what can i do, to solve this problem? Regards

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First request for references
by Alan Cameron 10 Jan '09

10 Jan '09

Hi, I stumbled across Haskell while researching 'functional programming'. I have done some initial reading and have the following questions: 1. Are there any references to real-world applications using this language? I ask because most of the articles I have found seem to be of an extreme academic nature. I want to be sure that my time will not be wasted delving deeper. 2. Are there any tutorials with full code included which I can use on the HUGS system I have installed? I have found a few "A Gentle Introduction to Haskell 98" which assumes far too much previous knowledge to be immediately useful to me. Thanks for any replies. Alan Cameron

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how to print a floating-point number?
by Sergei Winitzki 08 Jan '09

08 Jan '09

Subject: how to print a floating-point number? hi, I am very new to Haskell. I am trying to benchmark the CPU time needed for a computation, and I can't figure out how to print a floating-point number. My code is as follows, and I expected it to work: import System.CPUTime main = do let result = some_computation print result time <- getCPUTime -- this is an Integer that needs to be divided by 1e12 to get time in seconds print (time / 1.0e12) -- I want this to print a floating-point number But this does not compile. Error message: No instance for (Fractional Integer) arising from use of `/' at fact1.hs:18:15-28 Possible fix: add an instance declaration for (Fractional Integer) In the first argument of `print', namely `(time1 / 1.0e12)' I thought this should work because e.g. 12 / 7 evaluates to 1.7142857142857142 in ghci. I understand this is some problem with types, but surely it is fixed not by adding any instance declarations but perhaps by using some Prelude or Numeric function. But which one? I tried everything I could find in the documentation: showFloat, adding ::Float everywhere, adding fromIntegral, etc.etc. - nothing works. All the books and the tutorials I looked at seem to discuss at length such nice things as Fibonacci numbers and recursive factorial functions rather than a practical problem like this. help will be much appreciated! Sergei

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Some confusion still with a YAHT example
by David Schonberger 08 Jan '09

08 Jan '09

Hello all. This is my first post and I'm essentially a rank beginner with Haskell, having used it briefly some 7 years ago. Glad to be back but struggling with syntax and understanding error messages. This post actually ties to a prior post, http://www.haskell.org/pipermail/beginners/2008-December/000583.html My troubles are with exercise 3.10, p45 of from HDIII's "Yet Another Haskell Tutorial" (YAHT). The exercise states: Write a program that will repeatedly ask the user for numbers until she types in zero, at which point it will tell her the sum of all the numbers, the product of all the numbers, and, for each number, its factorial. For instance, a session might look like: Note that the sample session accompanying the exercise suggests a session such as: Give me a number (or 0 to stop): 5 Give me a number (or 0 to stop): 8 Give me a number (or 0 to stop): 2 Give me a number (or 0 to stop): 0 The sum is 15 The product is 80 5 factorial is 120 8 factorial is 40320 2 factorial is 2 The following code handles the sum and products pieces--I built the module incrementally--but fails on the factorial part. In fact my current code, if it worked, would only output something like: The sum is 15 The product is 80 120 40320 2 But I'm not even getting that much. Here's the code: --begin code module AskForNumbers whereimport IO askForNums = do putStrLn "Enter a pos int or 0 to end: " numStr <- getLine let num = read numStr if num == 0 then return [] else do rest <- askForNums return (num:rest) listFactorial l = if length l == 0 then return 1 else do fact (head l) listFactorial (tail l) fact n = if n == 0 then return 1 else return foldr (*) 1 [1..n] f = do nums <- askForNums putStr ("Sum is " ++ (show (foldr (+) 0 nums)) ++ "\n") putStr ("Product is " ++ (show (foldr (*) 1 nums)) ++ "\n") listFactorial nums --end code Here is the error msg I get when I load into WinHugs (Sept 2006 version): ERROR file:.\AskForNumbers.hs:22 - Ambiguous type signature in inferred type *** ambiguous type : (Num a, Num [a], Monad ((->) [b]), Num c, Num (b -> [a] -> [a]), Enum a, Monad ((->) (c -> c -> c))) => a -> [b] -> [a] *** assigned to : fact Ambiguous type assigned to fact. Ok. Not sure what to make of that or how to correct it. I though I was passing fact an Integer, since I think nums is a list of Integers, since the sum and product lines in f work ok. Help? Thanks. David

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