- Haskell-Cafe - Haskell.org

newbie:: getting random Ints
by Peter Rooney 04 Apr '02

04 Apr '02

hello all, total newbie to haskell, armed with "the craft of functional programming". i can't seem to generate random numbers. i have read this: http://www.haskell.org/onlinelibrary/random.html and searched the web and archives, and read thru Random.hs (mostly over my head), but have been unable to get any combination of getStdRandom randomR, etc. to work. even the example in the 98 report, import Random rollDice :: IO Int rollDice = getStdRandom (randomR (1,6)) gets me: Main> rollDice Main> after loading the file, which makes me think i'm missing something! TCFP has an example of how to do it yourself, but i can see that my needs are more than met by Random.hs. my various attempts all look a lot like this: Main> getStdGen (random 5) ERROR - Type error in application *** Expression : getStdGen (random 5) *** Term : getStdGen *** Type : IO StdGen *** Does not match : a -> b could someone post an example of how to generate random integers within a range? tia, peter

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Re: hashmap withdrawal and poor haskell style
by Long Goodbye 04 Apr '02

04 Apr '02

module Main where main = do content <- getContents let starstat = oneline content alphabet = ['a'..'z'] count ch = length . filter (==ch) oneline str ch = [ch] ++ " " ++ stars (count ch str) stars x = take x ['*','*'..] sequence $ map (putStrLn . starstat) alphabet Just another example ... I thought you might like an example close to your original code _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp.

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hashmap withdrawal and poor haskell style
by Michal Wallace 04 Apr '02

04 Apr '02

Hello everyone, I just wrote my first haskell program. I started with a simple python program and tried to see if I could port it to haskell. The program reads text from stdin and prints out a histogram of all the letters: """ alphabet = 'abcdefghjiklmnopqrstuvwxyz' def letter_count(lines): res = {} for line in lines: for char in line.lower(): if char not in alphabet: continue if res.has_key(char): res[char] += 1 else: res[char] = 1 return res if __name__=="__main__": import sys hist = letter_count(sys.stdin) for letter in alphabet: print letter, "*" * hist.get(letter, 0) """ Kid stuff, but it took me about 10 hours to port it... :) Here's what I came up with: """ module Main where alphabet = "abcdefghijklmnopqrstuvwxyz" count ch str = length [c | c <- str , c == ch] hist str = [count letter str | letter <- alphabet] oneline ch str = [ch] ++ " " ++ stars (count ch str) stars x = if x == 0 then "" else "*" ++ stars ( x - 1 ) report str ch = do putStrLn ( oneline ch str ) loop f (h:t) = if t == [] then f h else do f h loop f t main = do content <- getContents let rpt letter = report content letter loop rpt alphabet """ Other than ignoring upper case letters, and being really really slow, it seems to work fine in hugs.... One thing I really missed was a hash / dictionary. I tried for about an hour to use Assoc following the examples from PLEAC: http://pleac.sourceforge.net/pleac_haskell/hashes.html ... But I never got it working: :> module Main where :> import Assoc (empty) :> main :: IO() :> main = do line <- getContents :> let w = length line :> count:: AssocDefault String Int :> count = w.foldl (\a s -> a.update s (+1)) empty :> print x -> ERROR "alphahist.hs":6 - Undefined type constructor "AssocDefault" (I couldn't get Assoc to work either, even with "Import Assoc (Assoc)" though I looked in the Assoc.hs file and could see it in there.. This was the most frustrating part of the experiment) Can anyone point me in the right direction here? Also, I'd really like to here anyone's thoughts on the code I have above, especially concercing what I could have done better. :) Thanks! Cheers, - Michal http://www.sabren.net/ sabren(a)manifestation.com ------------------------------------------------------------ Give your ideas the perfect home: http://www.cornerhost.com/ cvs - weblogs - php - linux shell - perl/python/cgi - java ------------------------------------------------------------

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Re: hashmap withdrawal and poor haskell style
by Yoann Padioleau 03 Apr '02

03 Apr '02

Remi Turk <remi(a)abcweb.nl> writes: > > -------------------- > module Main where > > import Char > import FiniteMap > > printCount :: (Char, Integer) -> IO () > printCount (letter, count) > =3D putStrLn $ letter : replicate count '*' > > countLetters :: String -> [(Char, Integer)] > countLetters letters > =3D fmToList $ addListToFM_C (+) emptyFM $ zip letters (repeat 1) > > main =3D getContents >>=3D mapM_ printCount . countLetters . filter isAl= > pha this is not exactly the same semantic than the python version, for example if 'z' is not in the contents, then you will not display 'z' (without star) and you will not respect also the order of the letter (i think) > -------------------- > > Happy Hacking > > Remi > > --=20 > See the light and feel my warm desire, > Run through your veins like the evening sun > It will live but no eyes will see it, > I'll bless your name before I die. > > Key fingerprint =3D CC90 A1BA CF6D 891C 5B88 C543 6C5F C469 8F20 70F4 > > --wRRV7LY7NUeQGEoC > Content-Type: application/pgp-signature > Content-Disposition: inline > > -----BEGIN PGP SIGNATURE----- > Version: GnuPG v1.0.6 (GNU/Linux) > Comment: For info see http://www.gnupg.org > > iD8DBQE8qzpFbF/EaY8gcPQRAm8DAJ44i0avXmnavtDu+OOy1mdrBL88HwCfWsx+ > GFJUUd1zm1wV8CwfqU2TN0k= > =X3DP > -----END PGP SIGNATURE----- > > --wRRV7LY7NUeQGEoC-- > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Yoann Padioleau, INSA de Rennes, France, Opinions expressed here are only mine. Je n'écris qu'à titre personnel. **____ Get Free. Be Smart. Simply use Linux and Free Software. ____**

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03 Apr '02

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Re: hashmap withdrawal and poor haskell style
by Yoann Padioleau 03 Apr '02

03 Apr '02

Michal Wallace <sabren(a)manifestation.com> writes: > module Main where > alphabet = "abcdefghijklmnopqrstuvwxyz" in haskell you can do alphabet = ['a'..'z'] > count ch str = length [c | c <- str , c == ch] can do count c s = length (filter (c ==) s) or more cryptic: count c = length . (filter (c==)) > hist str = [count letter str | letter <- alphabet] > oneline ch str = [ch] ++ " " ++ stars (count ch str) > stars x = if x == 0 > then "" > else "*" ++ stars ( x - 1 ) stars n = replicate n '*' > report str ch = do putStrLn ( oneline ch str ) > loop f (h:t) = if t == [] > then f h > else do f h > loop f t you can use mapM_ main = do content <- getContents mapM_ (\a -> report content a) alphabet > main = do content <- getContents > let rpt letter = report content letter > loop rpt alphabet > """ you dont choose the more efficient strategy cos you parse 26 times the contents of the file, it is better to do (as you did in python) parse 1 and remember the number of occurence. > > Other than ignoring upper case letters, and being really > really slow, it seems to work fine in hugs.... > > One thing I really missed was a hash / dictionary. I > tried for about an hour to use Assoc following the > examples from PLEAC: > > http://pleac.sourceforge.net/pleac_haskell/hashes.html > > > ... But I never got it working: that's because i dont really use standard haskell library, i redefine a library in a file named Common.hs where there is a constructor AssocDefault. So if you want use AssocDefault, they you have to do: module Main where import Prelude hiding (($),(^),(.),(!!),map,take,lookup,.... AS in the example in the pleac section* import Common Put your code here. so it becomes: #!/usr/bin/runhugs -98 module Main where import Prelude hiding (($),(^),(.),(!!),map,take,lookup,drop,splitAt,reverse,filter,takeWhile,dropWhile,null,foldl,length) import Common main = do s <- getContents s.downcase .foldl(\h c -> if isAlpha c then h.update c (+1) else h) (empty::AssocDefault Char Int) .(\h -> ['a'..'z'].each (\c -> putStrLn (c^replicate (h!c) '*'))) or in more standard haskell, i will do: #!/usr/bin/runhugs -98 module Main where import Prelude hiding ((.)) (.) o f = f o update k f xs = xs.map (\ (k2,v) -> if k == k2 then (k,f v) else (k2,v)) each:: (e -> IO ()) -> [e] -> IO () each = mapM_ main :: IO () main = do s <- getContents s.map toLower .foldl(\h c -> if isAlpha c then h.update c (+1) else h) [(c, 0::Int) | c <- ['a'..'z']] .each (\ (c,n) -> putStrLn (show c ++ (replicate n '*' ))) > > > :> module Main where > :> import Assoc (empty) > :> main :: IO() > :> main = do line <- getContents > :> let w = length line > :> count:: AssocDefault String Int > :> count = w.foldl (\a s -> a.update s (+1)) empty > :> print x > > -> ERROR "alphahist.hs":6 - Undefined type constructor "AssocDefault" that's because AssocDefault is a constructor that standard haskell does not define. > > Also, I'd really like to here anyone's thoughts on the code > I have above, especially concercing what I could have done > better. :) > > Thanks! > > Cheers, > > - Michal http://www.sabren.net/ sabren(a)manifestation.com > ------------------------------------------------------------ > Give your ideas the perfect home: http://www.cornerhost.com/ > cvs - weblogs - php - linux shell - perl/python/cgi - java > ------------------------------------------------------------ > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Yoann Padioleau, INSA de Rennes, France, Opinions expressed here are only mine. Je n'écris qu'à titre personnel. **____ Get Free. Be Smart. Simply use Linux and Free Software. ____**

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Re: Ex 9.9 in Paul Hudak's book
by Lennart Augustsson 03 Apr '02

03 Apr '02

> What I would like to know is how the 'fix' function could be used > to find the fixed point of a function like ( \n -> 2*n ). The 'fix' function does compute a (the least) fixed point of ( \n -> 2*n ). The least fixed point is bottom. -- Lennart

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Re: sort inefficiency
by Dylan Thurston 03 Apr '02

03 Apr '02

On Wed, Apr 03, 2002 at 09:35:51AM +0400, Serge D. Mechveliani wrote: > The Standard library specifies only the map related to the name > `sort'. This map can be described, for example, via sort-by-insertion > program. > And the algorithm choice is a matter of each particular > implementation. Implementation has right to change the algorithm. Reading this, it occurred to me that if you're very picky the implementation probably isn't allowed to pick the algorithm: you need to assume that '<' is actually a total order to have much leeway at all. (Suppose, e.g., that comparing two particular elements yields an exception.) It seems to me this is a problem with providing code as specification: you probably fix the details more than you want. Best, Dylan Thurston

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guards
by Cagdas Ozgenc 29 Mar '02

29 Mar '02

Greetings. Is it requried that the type used in guards always Bool? It seems to me that the Haskell language is dependent to Bool type, and Bool type is depending to the Haskell language causing a circular dependency problem. Of course this is not a practical issue. Is there a theoretical explanation? Thanks for taking time.

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Re: Little problem
by Jorge Adriano 27 Mar '02

27 Mar '02

[moving to haskell-cafe] > Hello > I have little problem. I want to solve this equation > > f(n-1) = f(n)+n > f(6)=6 This is a simple recursive definition.. By the way, are you sure you didn't meant: f(n-1)=f(n)+(n-1) > => f(2)=? Not exactly sure what you're after here. You could simply do: f(2) =f(3)+3 =f(4)+4+3 =f(5)+5+4+3 =f(6)+6+5+4+3 =6+6+5+4+3 =24 anyway it's simple to see, or even prove by induction, that f(n) = (\sum_{i=n+1}^{7} i) -1 = (n+8)(7-n)/2 - 1 If I'm rigth and you're formula is suposed to be f(n-1) = f(n)+n it is even easier. J.A.

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