- Beginners - Haskell.org

Converting an imperative program to haskell
by Hein Hundal 29 Apr '10

29 Apr '10

Hi, I am new to Haskell. I programmed in imperative languages for many years and I have been using Mathematica for the last 15 years which mixes functional and imperative programming. I started learning Haskell about 2 months ago mostly by reading the first 13.5 chapters of Real World Haskell and doing about 50 of the Euler Problems. "The Evolution of a Haskell Programmer", the Haskell Wiki, and this listserv have also been helpful. I figured I should try a larger program in Haskell, so I am converting one of my Mathematica programs, a simulator for the card game Dominion, over to Haskell. Most of that is going well except for one patch of imperative code. My Haskell version of this code is ugly. I was hoping someone could recommend a better way to do it. I will paste a simplified version of the code below. If necessary, I can provide all the other code used for this simplified example (about 30 more lines of Haskell code, or an equivalent program in C++ (130 lines of code). Cheers, Hein -----------------Ugly Haskell Code------------------------- data LoopState = LSt GameState Strat Int Int Int [Int] Int Int Bool proc :: GameState -> Strat -> (Int, Int, GameState) proc gs' strat = let iA = 1 iTh = 0 i = 0 aA = replicate 20 0 iB = 1 iC = 0 bDone = False gs = apps ("<<<"++show(gs')++">>>") gs' lst = LSt gs strat iA iTh i aA iB iC bDone lst2 = until isDone procloop lst isDone (LSt gs strat iA iTh i aA iB iC bDone) = bDone output (LSt gs strat iA iTh i aA iB iC bDone) = (iB, iC, appd aA gs) in output lst2 procloop :: LoopState -> LoopState procloop (LSt gs' strat iA iTh i aA iB iC bDone) = do let iCd = stratchoose strat gs' iA iTh iB aA let gs = apps ("iB "++show iB++ "\n" ) gs' if iA<=0 || i>=20 || actcd gs <=0 || iCd == -1 then LSt gs strat iA iTh (i+1) aA iB iC True else if iCd == 1 then LSt gs strat iA (iTh+1) (i+1) aA iB iC False else let gs2 = delh iCd gs aA2 = append aA iCd (iA3, iC3, iB3, gs3, aA3) = doAct iA iC iB gs2 aA2 strat iCd (iA4, iC4, iB4, gs4, aA4) = doAct iA3 iC3 iB3 gs3 aA3 strat iCd in if iTh>0 then LSt gs4 strat iA4 (iTh-1) (i+1) aA4 iB4 iC4 False else LSt gs3 strat iA3 (iTh-1) (i+1) aA3 iB3 iC3 False ---------------end of Haskell code-------------------------------

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Iterating through a list of char...
by Jean-Nicolas Jolivet 29 Apr '10

29 Apr '10

Hi there! I'm trying to iterate through each character of a string (that part I can do!) however, I need to apply a transformation to each character...based on the previous character in the string! This is the part I have no clue how to do! I'm totally new to Haskell so I'm pretty sure I'm missing something obvious... I tried with list comprehensions...map... etc... but I can't figure out how I can access the previous character in my string in each "iteration".... to use simple pseudo code, what i need to do is: while i < my_string length: if my_string[i-1] == some_char: do something with my_string[i] else do something else with my_string[i] I'm using imperative programming here obviously since it's what I am familiar with...but any help as to how I could "translate" this to functional programming would be really appreciated! Jean-Nicolas Jolivet

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[Int] oddness with summing large lists
by Philip Scott 29 Apr '10

29 Apr '10

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

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Re: [Haskell-beginners] Re: Iterating through a list of char...
by Hein Hundal 29 Apr '10

29 Apr '10

Jean-Nicolas, Here is a variation on Corentin's solution: -- This function replaces every character that -- follows an 'a' with 'A' repA :: String -> String repA s = zipWith f (' ':s) s where f 'a' y = 'A' f x y = y Cheers, Hein > From: Jean-Nicolas Jolivet <jeannicolascocoa(a)gmail.com> > > I'm trying to iterate through each character of a string > (that part I > can do!) however, I need to apply a transformation to each > character...based on the previous character in the string! > This is the part I have no clue how to do! > [snip] > while i < my_string length: > if my_string[i-1] == some_char: > do something with > my_string[i] > else > do something else > with my_string[i]

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let (x',seed') = x seed
by Amy de Buitléir 29 Apr '10

29 Apr '10

I'm confused by the syntax this function definition I found in a tutorial: bind :: (a → StdGen → (b,StdGen)) → (StdGen → (a,StdGen)) → (StdGen → (b,StdGen)) bind f x seed = let (x',seed') = x seed in f x' seed' The part that confuses me is "let (x',seed') = x seed". The left side is a tuple, and the right side is two separate values, so how does the binding work? I gather from the context of the example (a tutorial on monads) that the value of x is bound to x', and the value of seed is bound to seed', so that "bind f" is a function that acts on a tuple and produces a new tuple. Is that correct? Thank you in advance to anyone who can de-confuse me.

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Feedback on google code jam practice
by Aur Saraf 29 Apr '10

29 Apr '10

Hello, New to this list, so hello all :-). I've uploaded practice solutions for google code jam questions here: http://github.com/SonOfLilit/codejam . As I'm still learning idiomatic Haskell, I'd love to get feedback on my code. How would /you/ do it? Thanks a lot, -- Aur

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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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