April 2010 - Beginners

Re: [Haskell-beginners] [Int] oddness with summing large lists
by Philip Scott 28 Apr '10

28 Apr '10

Hi Scott, Thanks for the reply, > I can't really do too much with it because I don't have any swap on > this machine, but I definately don't get > 0 for your first example. > ghci quits rather ungracefully when I do that :) It is a silly thing to be doing, granted - just wondering if what I saw was expected behavior. Perhaps it is the OS (Windows 7) killing the process for using to much memory - but I would've thought I would get a stack overflow before that happens. Ho hum! - Philip > On Wed, Apr 28, 2010 at 4:32 PM, Philip Scott > <haskell-beginners(a)foo.me.uk> wrote: > >> Hi all, >> >> I kind of stumbled into this while doing something else - but I thought it >> was worth posting since I have never actually managed to crash ghci before >> :) >> >> Prelude> let d = [1..1000000000000] :: [Int] >> Prelude> sum d >> 0 >> >> 0? I mean, I might have expected an integer overflow, but 0? >> >> And now the really odd part... take away one zero >> >> Prelude> let d = [1..100000000000] :: [Int] >> Prelude> sum d >> < ghci crashes and quits> >> >> A slightly more reasonable number.. >> >> Prelude> let d = [1..10000000] :: [Int] >> Prelude> sum d >> *** Exception: stack overflow >> >> At least I can appreciate what is going on in this one :) >> >> I'm aware that this is a silly way to sum from 1 to n, but I am at a loss to >> explain the above behavior (Except the stack overflow, that is fair enough). >> I'm also aware that Int is a machine Int, not a clever infinite haskell Int. >> >> Any ideas? >> >> All the best, >> >> Phil >> _______________________________________________ >> Beginners mailing list >> Beginners(a)haskell.org >> http://www.haskell.org/mailman/listinfo/beginners >> >> >

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Final call for participation: Summer school "Applied Functional Programming", Aug 16-27, Utrecht NL, Application deadline: May 1
by Andres Loeh 27 Apr '10

27 Apr '10

[Feel free to advertise the school to others who might be interested.] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Utrecht Summer School in Computer Science: Applied Functional Programming August 16-27, 2010 Utrecht, The Netherlands ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The summer school is intended as a first or an advanced course on Haskell, consisting of lectures and projects to be solved by the participants, working in teams. We mainly target bachelor and master students, but others who are interested are also welcome to apply. The deadline for applications is *** Saturday, May 1, 2010 ***. More information: http://www.cs.uu.nl/wiki/USCS2010 If you have any questions, feel free to contact me. Cheers, Andres -- Andres Loeh, Universiteit Utrecht mailto:andres@cs.uu.nl mailto:mail@andres-loeh.de http://www.andres-loeh.de

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Trying to compile my first program that imports another program
by Mitchell Kaplan 26 Apr '10

26 Apr '10

Hi, I created (with help) a function to test for prime numbers. It worked well enough for now in ghci. ---------------- f x n y | n>y = True | rem x n == 0 = False | otherwise = f x (n+1) y primeQ x = f x 2 y where y = floor(sqrt(fromIntegral x)) --------------- I then wanted to create object code so that I could import it. It seemed that I had to precede the above with the 2 lines: ---------------- module Prime where ---------------- I ran: ghc -c prime.hs, and created prime.o and prime.hi. Next, I wanted to write a program to import and use this function. I wrote: ------------ module Main () where import Prime main = primeQ 123 ------------ I tried to compile this with: ghc -o test Main.hs prime.o I got the following error: Main.hs:5:0: Couldn't match expected type 'IO t' against inferred type 'Bool' In the expression: main When checking the type of the function 'main' ---------------- First I'd like a hint as to what I need to do to make this work. It's pretty obvious that I don't know what I'm doing with regard to types. Also, I have no idea if I have to name this module Main, but when I didn't the compiler complained about that. In the function that I think I had to re-write to make object code, I wound up with 2 where statements, which worries me. I'd really appreciate any help in getting me unraveled. Mitchell

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University project - weird problem
by Renato dos Santos Leal 25 Apr '10

25 Apr '10

I've got a university project that demands me to do a program that receive a .c file and analyze its syntax using haskell. There are just a few things that I have to analyze: literal strings, identifiers (in the program: identificadores), constants (constantes), operators (operadores) and reserverd words(palavras reservadas) There are two major problems in the program: (1) I've got this guard in le_bloco: | x `elem` listS = do separador (x:xs) but it doesn't seem to work. Every time I enable it I recieve this in execution time (after calling verifica) ERROR - Cannot find "show" function for: *** Expression : verifica *** Of type : IO a So I've made one workaround that prints the separator but stops the program...I guess the problem is doing the recursivity (2) My second problem is: when I have one identifier or keyword alone in a line or it's the last element of it it just won't print my coment! this is the function: pPCouI (x:xs) z | membroPC z = do{ putStr (z ++ " <palavra chave>\n") ; le_bloco (x:xs)} | otherwise = do{putStr z ; putStr " <identificador>\n" ; le_bloco (x:xs)} *Please help me solving those problems as soon as possible!* Here is the whole program: listO = ['+', '-', '*', '/', '%', '^', '=', '>', '<', '.', '|', '&', '!', '~'] listS = [';', '{', '(', ')', '}', '[', ']', ','] listC = ['0','1'..'9'] listCF = listC ++ ['.'] listA = listO ++ listS ++ [' '] listPC = ["auto","double","int","struct","break","else","long","switch","case", "enum","register","typedef","char","extern","return","union","const", "float","short","unsigned","continue","for","signed","void","default", "goto","sizeof","volatile","do","if","static","while"] verifica = do putStr ("Favor visualizar o codigo para ver os bugs e erros do programa\n") putStr ("Digite o nome do arquivo de entrada: ") arqent <- getLine texto <- readFile arqent le_bloco texto le_bloco (x:xs) | x `elem` listO = do operador (x:xs) | x `elem` listC = do cnum (x:xs) -- | x `elem` listS = do separador (x:xs) | x `elem` listS = do{ putStr[x] ; putStr " <separador>\n" } | x == '"' = litstr (xs) | x /= ' ' = pchave (x:xs) [] | x == ' ' = le_bloco xs | otherwise = do { putStr "Outro\n" ; le_bloco xs } separador (x:xs) | x `elem` listS = do{ putStr [x] ; separador xs} | otherwise = do{ putStr " <separador>\n" ; le_bloco xs} cnum (x:xs) | x `elem` listCF = do{ putChar x ; cnum xs} | otherwise = do{ putStr " <cte. numerica>\n" ; le_bloco (x:xs)} operador (x:xs) | x `elem` listO = do{ putChar x ; operador xs} | otherwise = do{ putStr " <operador>\n" ; le_bloco (x:xs)} litstr (x:xs) | x /= '"' = do{ putChar x ; litstr xs} | otherwise = do{ putStr " <literal string>\n" ; le_bloco xs} pchave (x:xs) ys | x `notElem` listA = pchave xs (ys++[x]) | otherwise = pPCouI (x:xs) ys pPCouI (x:xs) z | membroPC z = do{ putStr (z ++ " <palavra chave>\n") ; le_bloco (x:xs)} | otherwise = do{putStr z ; putStr " <identificador>\n" ; le_bloco (x:xs)} membroPC x | x `elem` listPC = True | otherwise = False I'm sorry for the bad english, it's been a while since the last time i used it =) Ah, I'm just starting to learn Haskell, first time i've seen it was like a month ago so pretend that I know nothing

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Problems with Data types a la carte
by Rodrigo Geraldo 23 Apr '10

23 Apr '10

Hi ! I've tried to implement the expression interpreter present in Data types a la carte, putting the new "cases" of my data type in a separate module. When I'm trying to create a simple expression like this: x :: Expr (Val :+: Add :+: Mult) x = (val 2000 `mult` val 399) `plus` (val 3) I'm getting the following type error that I can't solve: src/Main.hs:25:4: No instance for (Add :<: Mult) arising from a use of `plus' at src/Main.hs:25:4-43 Possible fix: add an instance declaration for (Add :<: Mult) In the expression: (val 2000 `mult` val 399) `plus` (val 3) In the definition of `x': x = (val 2000 `mult` val 399) `plus` (val 3) src/Main.hs:25:5: No instance for (Val :<: Mult) arising from a use of `val' at src/Main.hs:25:5-12 Possible fix: add an instance declaration for (Val :<: Mult) In the first argument of `mult', namely `val 2000' In the first argument of `plus', namely `(val 2000 `mult` val 399)' In the expression: (val 2000 `mult` val 399) `plus` (val 3) I've attached the source code... Regards Rodrigo

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Generating Random Float
by Giorgio Stefanoni 23 Apr '10

23 Apr '10

Hi guys, I'm working with Haskell for a little project at the university and I'm stuck with a problem. I'm trying to generate a random number in the interval [0.0,1.0]. It is straightforward for me to implement the function on ints: rand :: IO Int rand = do r <- newStdGen let (x1, r2) = randomR (0,1) r print x1 return x1 However, if I try to generate a Float instead, I always get an Arithmetic Overflow error that I can not understand. Code: rand :: IO Float rand = do r <- newStdGen let (x1, r2) = randomR (0.0::Float,1.0::Float) r print x1 return x1 Do you have any clue why this happens? Thanks in advance, -- Giorgio

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eve / did = 0.talktalktalk...
by Alexander.Vladislav.Popov 22 Apr '10

22 Apr '10

Dear Everebody. Hepl me please to parallelize (parallel computing of evedidtalk function) the rebus: -- | eve / did = 0.talktalktalk... ten :: Integral a => [a] ten = [0..9] infixr 7 /: (/:) :: (Integral a) => [a] -> [a] -> [a] (/:) [] _ = [0] (/:) _ [] = [] (/:) x y = coldiv (getInteger x) (getInteger y) getInteger :: (Num a) => [a] -> a getInteger = foldl ((+) . (*10)) 0 coldiv :: (Integral a) => a -> a -> [a] coldiv a b = q : if r == 0 then [] else coldiv (r * 10) b where (q, r) = a `quotRem` b evedidtalk = [ ([e, v, e], [d, i, d], [t, a, l, k]) | e <- ten, v <- ten, v /= e, d <- ten, d /= e, d /= v, i <- ten, i /= e, i /= v, i /= d, t <- ten, t /= e, t /= v, t /= d, t /= i, a <- ten, a /= e, a /= v, a /= d, a /= i, a /= t, l <- ten, l /= e, l /= v, l /= d, l /= i, l /= t, l /= a, k <- ten, k /= e, k /= v, k /= d, k /= i, k /= t, k /= a, k /= l, take 9 ([e, v, e] /: [d, i, d]) == [0, t, a, l, k, t, a, l, k] ] Sincerely, Alexander

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sunrise sunset
by Markus Böhm 22 Apr '10

22 Apr '10

Is there any Haskell package/impl already that calculates sunrise and sunset for a given location (via lat/lon)? -- Mike

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cabal re-configure not recognized
by Ashish Agarwal 21 Apr '10

21 Apr '10

When changing a cabal file, if one runs 'cabal build' immediately, a message states that you must re-configure. So I run 'cabal configure' and then again try 'cabal build', but I get the same message again. The problem goes away if I wait about a minute. So I suspect there is an issue with the file modification time being ahead of what cabal thinks the time is. Has anyone experienced this problem, and know what might be the fix? Thank you. The issue occurs only on my Ubuntu machine. I do not have this problem with the same code on Mac OS X.

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Creating an exe file
by legajid 21 Apr '10

21 Apr '10

Hi, After running interpreted programs, i want to create an exe file; in a first time, just for my local machine. Later shall i try to install on other machines ... To do this, i wrote a batch file : d: cd d:\prog\lang\haskell set shs=%1 ghc %shs% -o%shs:hs=exe% del %shs:hs=hi% del %shs:hs=o% pause where %1 is my hs file full path. Example of a run : D:\>cd d:\prog\lang\haskell D:\PROG\lang\haskell>set shs=E:\didier\haskell\cases.hs D:\PROG\lang\haskell>ghc E:\didier\haskell\cases.hs -oE:\didier\haskell\cases.exe D:\PROG\lang\haskell>del E:\didier\haskell\cases.hi D:\PROG\lang\haskell>del E:\didier\haskell\cases.o D:\PROG\lang\haskell>pause Appuyez sur une touche pour continuer... It's ok with some modules i have to import, like import Data.List import Data.Maybe But, when using wxhaskell, i get a lot of errors, although i include import Graphics.UI.WXCore import Graphics.UI.WX here is a small list of errors : E:\didier\haskell\graphe.o:fake:(.data+0x2f0): undefined reference to `wxcorezm0zi11zi1zi2_GraphicsziUIziWXCoreziDialogs_infoDialog_closure' E:\didier\haskell\graphe.o:fake:(.data+0x300): undefined reference to `wxzm0zi11zi1zi2_GraphicsziUIziWXziDraw_zdf4_closure' E:\didier\haskell\graphe.o:fake:(.data+0x314): undefined reference to `wxzm0zi11zi1zi2_GraphicsziUIziWXziDraw_zdf4_closure' what should the correct ghc command line be, in order to get a successful compilation ? Thanks for help, Didier Running ghc 6.10.4 on Windows XP SP2, with wxhaskell-0.11.1.2

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