December 2008 - Beginners

A type level programming question
by Levi Stephen 21 Dec '08

21 Dec '08

Hi, I was looking at Data.Param.FSVec from parameterized-data [1]. The method to retrieve an element from a FSVec has the following type: (!) :: (Pos s, Nat i, i :<: s) => FSVec s a -> i -> a I was wondering if it was possible to write a function of type: elementAt :: FSVec s a -> Int -> a that called the above function, throwing an error if the index was out of bounds. e.g., elementAt v idx | idx < 0 || idx > length v = error "Out of bounds" | otherwise = .... But, I can't figure out how to implement the otherwise part. In particular I can't work out how to call the (!) function with the i :<: s class constraint satisfied. Thanks, Levi

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MVar and Par ..
by Mozhgan 16 Dec '08

16 Dec '08

Hi .. Hope you are doing well . I've just joined this group coz I am really a beginner and have no idea that what I have to do ! Recently, I am struggling to do some simple experiment with haskell language about parallelism and wrong answers that we can get while using a shared variable . I tried to write a simple program, for example calculationg 'n=n+1' few times.And then I tried to do it in parallel by using 'par' and 'pseq' . The aim was to get the wrong answer because we have to share a variable here,and without using 'MVar' function we will get the wrong answer for the calculation . I don't know how to write it in parallel in order to get a wrong answer when we don't use MVar,because we have a shared variable here. I read about MVars as well,but also I don't know how to combine MVar and Par together to get the program to work. I wrote this : module Main where f :: Int -> Int -> Int f i n = g 1 i n where g x i n | x <= i = g (x+1) i (n+1) | otherwise = n main :: IO () main = do putStrLn "starting..." let r = f 10 5 putStrLn (show r) putStrLn "finished" I want to make to work in parallel by using 'Par'.And also use MVar for this simple example to work. All of the example about MVar are a little bit complicated and I couldn't figure it that how can I write one,the same ! Can any one help me with this ? I want a simple example that I can feel the need of MVar when I run my program in parallel and while I am using a shared variable. Regards; Mozhgan

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unix-2.3.1.0 failed during the building phase
by Renick Bell 16 Dec '08

16 Dec '08

I was trying to install this package and received this error: [a lot of stuff] Preprocessing library unix-2.3.1.0... Building unix-2.3.1.0... [ 1 of 21] Compiling System.Posix.Signals ( System/Posix/Signals.hs, dist/build/System/Posix/Signals.o ) [ 2 of 21] Compiling System.Posix.Signals.Exts ( dist/build/System/Posix/Signals/Exts.hs, dist/build/System/Posix/Signals/Exts.o ) [ 3 of 21] Compiling System.Posix.User ( dist/build/System/Posix/User.hs, dist/build/System/Posix/User.o ) [ 4 of 21] Compiling System.Posix.Unistd ( dist/build/System/Posix/Unistd.hs, dist/build/System/Posix/Unistd.o ) [ 5 of 21] Compiling System.Posix.Time ( dist/build/System/Posix/Time.hs, dist/build/System/Posix/Time.o ) [ 6 of 21] Compiling System.Posix.Resource ( dist/build/System/Posix/Resource.hs, dist/build/System/Posix/Resource.o ) [ 7 of 21] Compiling System.Posix.Process.Internals ( System/Posix/Process/Internals.hs, dist/build/System/Posix/Process/Internals.o ) [ 8 of 21] Compiling System.Posix.Error ( System/Posix/Error.hs, dist/build/System/Posix/Error.o ) [ 9 of 21] Compiling System.Posix.Files ( dist/build/System/Posix/Files.hs, dist/build/System/Posix/Files.o ) System/Posix/Files.hsc:504:42: Ambiguous occurrence `c_rename' It could refer to either `System.Posix.Files.c_rename', defined at System/Posix/Files.hsc:507:3 or `System.Posix.Internals.c_rename', imported from System.Posix.Internals at System/Posix/Files.hsc:93:0-28 cabal: Error: some packages failed to install: unix-2.3.1.0 failed during the building phase. The exception was: exit: ExitFailure 1 Being a beginner, I'm not sure what to do here... -- Renick Bell http://the3rd2nd.com

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Problems with one of my first examples
by Jeff C. Britton 15 Dec '08

15 Dec '08

Hello, I have started reading "Yet Another Haskell Tutorial" by Hal Daum´e III which can be found here http://www.cs.utah.edu/~hal/docs/daume02yaht.pdf One of the early examples in section 3.8 pg. 35 is this askForWords = do putStrLn "Please enter a word:" word <- getLine if word == "" then return [] else do rest <- askForWords return (word : rest) I want to print the returned list and everything I try fails. I have tried the following: printList l = if length l >= 1 then do putStrLn (head l) printList (tail l) else putStrLn("") f = printList askForWords and I get Expression : printList askForWords *** Term : askForWords *** Type : IO [[Char]] *** Does not match : [[Char]] ************************************* The exercise right below this asks for a very slight modification to read numbers instead. However, I am confused about how to convert strings to numbers. If I type in the hugs interactive console read "5" + 3 --> 8 -- ok perfect However read "5" gives ERROR - Unresolved overloading *** Type : Read a => a *** Expression : read "5" Yet page 33 of the tutorial has the following code: doGuessing num = do putStrLn "Enter your guess:" guess <- getLine let guessNum = read guess -- ok in let stmt, but not at repl prompt? Anyway I take the info that has been presented and create this function: askForNumbers = do hSetBuffering stdin LineBuffering putStrLn "Give me a number (or 0 to stop)" numStr <- getLine let num = read numStr if num == 0 then return [] else do rest <- askForNumbers return (num : rest) However, when I try to use it, like say map sqrt askForNumbers ERROR - Type error in application *** Expression : map sqrt askForNumbers *** Term : askForNumbers *** Type : IO [Integer] *** Does not match : [a] ********************************************************* Is there a way to write printList to handle Strings or numbers? Or should I write printList (map show askForNumbers) Thanks, Jeff

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Existential data types
by Michael Snoyman 15 Dec '08

15 Dec '08

Hi everyone, I believe I have come to the conclusion that what I would like to do is impossible, but I would like to have that confirmed. I would basically like to be able to have a heterogeneous list without boxing everything in a data type. Below is the sample code, with the code I would like to use commented out. I'm I missing something, or does Haskell simply not support what I am trying for? {-# LANGUAGE ExistentialQuantification #-} data Foo = Foo String class IFoo a where toFoo :: a -> Foo instance IFoo Foo where toFoo = id data A = A String instance IFoo A where toFoo (A a) = Foo a data B = B Int instance IFoo B where toFoo (B b) = Foo $ show b data FooBox = forall t. IFoo t => FooBox t instance IFoo FooBox where toFoo (FooBox f) = toFoo f myPrint :: IFoo t => [(String, t)] -> IO () myPrint = mapM_ myPrint' myPrint' :: IFoo t => (String, t) -> IO () myPrint' (key, value) = let (Foo foo) = toFoo value in putStrLn $ key ++ ": " ++ foo {- What I'd like to do: main = myPrint [ ("one", Foo "1") , ("two", A "2") ] -} main = myPrint [ ("one", FooBox $ Foo "1") , ("two", FooBox $ A "2") ] ---- Thanks

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gtk2hs timeoutSeq
by Norbert Wojtowicz 15 Dec '08

15 Dec '08

Hello, I'm trying to use a sequence of timeoutAdd's in a gtk2hs app. By sequence, I mean one (IO Bool) must return as False before the next begins: timeoutSeq [ (fn1, 100), (fn2, 200) ] fn1 will keep running every 100, until it returns False. Then fn2 will get a timeoutAdd 200, and so on... First attempt (works fine, I think): timeoutSeq :: [(IO Bool, Int)] -> IO () timeoutSeq [] = return () timeoutSeq ((fn,n):rest) = do timeoutAdd (aux fn rest) n return () where aux :: IO Bool -> [(IO Bool, Int)] -> IO Bool aux fn [] = fn aux fn lst = do r <- fn case r of True -> return True False -> do timeoutSeq lst return False Now, I'd like to write a timeoutDep (fn1, n1) (fn2, n2) where fn1 and fn2 are both running at their respective rates, and fn1 will run only as long as fn2 is running. The use case for this is eg: Run fn1 often and fn2 less often, but I only care fn1 to be running as long as fn2 is still valid. timeoutDep (fn,n) (fn2,n2) = do hdl <- timeoutAdd fn n timeoutSeq [(aux fn2 hdl, n2), (return False, 100)] where aux fn2 hdl = do r <- fn2 case r of True -> return True False -> do timeoutRemove hdl return False This implementation sort of works, except it exits immiediately (and as IO ()), so I can't for example do this: timeoutSeq [((timeoutDep (print "a" >> return True, 100) (print "b" >> return True, 100)), 100) , (print "c" >> return False, 100)] Ths silly example should keep printing 'a' and 'b' w/o ever printing 'c'. Any ideas or suggestions?

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Re: [Haskell] Re: Help : A problem with IO
by abdullah abdul Khadir 15 Dec '08

15 Dec '08

Thanks all, I got it working finally. What did i learn ? a) I need to put a do after else for more than one instruction (?) b) All similar type of questions are to be redirected to haskell-beginner and haskell-cafe Points noted. Thank you once again, Abdullah Abdul Khadir On Wed, Nov 26, 2008 at 10:56 PM, wman <666wman(a)gmail.com> wrote: > if it should really read whole line, it must try to somehow chain the > values into the string / list of chars (using (:) ) > > getMyLine :: IO [Char] > getMyLine = do > c <- getChar > if (c == '\n') > then return "" -- imho [] looks nicer ;-) > else do > rest <- getMyLine > return $ c : rest > > -- or > > getMyLine :: IO [Char] > getMyLine = do > c <- getChar > if (c == '\n') > then return "" -- imho [] looks nicer ;-) > else getMyLine >>= return . (c:) > > -- or even shorter and still equivalent ;-) > > getMyLine :: IO [Char] > getMyLine = getChar >>= (\c -> if (c == '\n') then return [] else fmap > (c:) getMyLine) > > > > 2008/11/26 abdullah abdul Khadir <abdullah.ak2002(a)gmail.com> > >> Hi, >> >> The function getMyLine written by me is intended for getting a >> complete string from the standard input. >> >> import IO >> >> getMyLine :: IO [Char] >> getMyLine = do >> c <- getChar >> if(c == '\n') >> then return "" >> else cs <- getMyLine >> return [c] >> >> However I keep getting the following error: >> >> io.hs:14:2: Parse error in pattern >> Failed, modules loaded: none. >> >> I fail to understand what the error is. I tried out various things >> such as changing the alignment and so on to no avail. The following program, >> however, compiled successfully though it has the same structure as the >> previous program. >> >> checkd :: IO Bool >> checkd = do >> c <- getChar >> if(c=='d') >> then return True >> else return False >> >> Prelude> :load ./io.hs >> [1 of 1] Compiling Main ( io.hs, interpreted ) >> Ok, modules loaded: Main. >> *Main> checkd >> d >> True >> >> >> _______________________________________________ >> Haskell mailing list >> Haskell(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell >> >> >

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Bit arithmetic in Haskell
by Artyom Shalkhakov 09 Dec '08

09 Dec '08

Hello, I'm trying to do some bit arithmetic. Here's the function: > import Data.Bits > import Data.Word > > g :: Word32 -> [Word32] > g x = [(x `shiftR` 24) .&. 0xFF, > (x `shiftR` 16) .&. 0xFF, > (x `shiftR` 8) .&. 0xFF, > x .&. 0xFF] This function should give bytes for the given number, like this: g 255 -> [0,0,0,255] g 256 -> [0,0,1,255] g 65535 -> [0,0,255,255] In reality, I get very strange answers. Can anybody help me please? Regards, Artyom Shalkhakov.

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Wrapping random
by Torsten Otto 09 Dec '08

09 Dec '08

Hi all, I teach a high school class in Computer Science. The current programming goal is to implement chat-bots, and we're using Haskell of course. Now one of my students had the seemingly easy idea of having the bot answer with a random sentence if it doesn't have "good" answer. Random in Haskell has its problems. I understand why you can't just call a function as you would in Java. I'm not firm enough with monads myself (and certainly don't want to go there in the class beyond I/O) so I'm calling for help here: Is there a way to wrap the generation of random numbers so that for the students it works like a function? We have this working: > import System.Random > main = > do randomNumber <- randomRIO (1::Int,2) > print (randomAnswer randomNumber) > randomAnswer r > | (r == 1) = "Nope!" > | (r == 2) = "Absolutely!" > | otherwise = "Error!" Now, how can we use it for something like this: >findAnswer [] = "h" >findAnswer (x:xs) > | (z == "unknown") = findAnswer xs > | otherwise = z > where z = findWord x lexikon where instead of getting "h" we'd like to call a function that would give us one of the strings out of randomAnswer. (findAnswer looks through a list [(keyword,response)]. I've looked at realworldhaskell and the wikibook among other sources, but I can't manage to piece anything useful together. How do I manage to get something of type IO to represent itself as a String? Any help would be greatly appreciated. Regards, Torsten Otto

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HDBC commit problem
by Moritz Tacke 09 Dec '08

09 Dec '08

Hi! I am running into problems while using HDBC with the sqlite3 backend. The HDBC "commit" statement frequently results in exceptions with the sqlite3 errorcode 1, "SQL logic error or no database". As far as I know, and as far as the HDBC docs tell me, "commit" simply writes all yet open transactions onto the disk. How can it fail? The HDBC documetation mentions the prepare- and execute-steps as the ones which might throw exceptions - what are the problems a call to "commit" might encounter? I do not suspect SQL errors to be the cause - those should lead to earlier exceptions - and the database is definitly present and opened, as select calls following the failing commit statments succeed. Greetings!

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