Re: [Haskell-beginners] Re: Defining 'words' in terms of 'span'

Am Dienstag 16 März 2010 21:54:24 schrieb Tim Attwood:

"Roger Whittaker" wrote in message news:20100316170815.GA19787@disruptive.org.uk...

...

I found some exam papers linked from this page: http://www.cs.chalmers.se/Cs/Grundutb/Kurser/d1pt/d1pta/external.html

And I have been trying some questions from them.

I'm currently baffled by question 2(b) on this one: http://www.cs.chalmers.se/Cs/Grundutb/Kurser/d1pt/d1pta/tenta2000-04.p s

A word is a sequence of alphabetic characters, which you can recognise using the standard function

isAlpha :: Char -> Bool

Using span, define a function words :: String -> [String] which finds a list of the words occurring in a string. For example,

words "Now is the winter of our discontent!" == ["Now","is","the","winter","of","our","discontent"]

words "2+3" == []

words "1 by 1" == ["by"]

Can anyone give me a clue how to start?

words' = filter (all isAlpha) . words

Prelude> words' "Now is the winter of our discontent!" ["Now","is","the","winter","of","our"] Maybe you thought of words'' = map (filter isAlpha) . words , but that doesn't do the required thing for e.g. "oops,forgot the space after the comma!"

...but since they specifically want span in the answer I'd guess they want some sort of recursive function

Sure. At the bottom, such a function must use recursion. The question is, can you employ some combinators like fold*, or do you have to recur explicitly.

words' s = wGen s [] where wGen s a = let (n,ws) = span (\x -> not (isAlpha x)) s (w,ss) = span (isAlpha) ws in case (w=="",ss="") of True, _ -> reverse a False, True -> reverse (w:a) False, False -> wGen ss (w:a)

something ugly like that maybe.

More like words' s = case span isAlpha (dropWhile (not . isAlpha) s) of ("",_) -> [] (w,rest) -> w:words' rest