April 2002 - Haskell-Cafe

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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