[Haskell-beginners] Re: Defining 'words' in terms of 'span'
timothyea at comcast.net
Tue Mar 16 16:54:24 EDT 2010
"Roger Whittaker" <roger at disruptive.org.uk> wrote in message
news:20100316170815.GA19787 at disruptive.org.uk...
>I found some exam papers linked from this page:
> And I have been trying some questions from them.
> I'm currently baffled by question 2(b) on this one:
> 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
...but since they specifically want span in the answer I'd guess they want
some sort of recursive function
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.
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