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

Roger Whittaker wrote:

Thanks for the suggestions - I think the idea was that you had to use a recursive definition in terms of span.

I eventually did this, but didn't feel it was at all nice or elegant:

import Data.Char

mywords :: [Char] -> [[Char]]

mywords l | ((snd (span isAlpha l)) == "" && (fst (span isAlpha l)) == "") = [] | ((snd (span isAlpha l)) == "" && (fst (span isAlpha l)) /= "") = [fst (span isAlpha l)] | (fst (span isAlpha l)) == "" = mywords (tail (snd (span isAlpha l))) | otherwise = [fst (span isAlpha l)] ++ mywords (tail (snd (span isAlpha l)))

You can perform a few simple steps to improve this definition. First of all, introduce a variable for the the common span isAlpha part and use pattern matching instead of fst and snd . mywords xs | r == "" && l == "" = [] | r == "" && l /= "" = [l] | l == "" = mywords (tail r) | otherwise = [l] ++ mywords (tail r) where (l,r) = span isAlpha xs There, much better already. Then, you can group the four different cases, noting that they are actually independent of each other: the result is always a concatenation of either [] or [l] with either [] or mywords (tail r) mywords xs = y ++ ys where (l,r) = span isAlpha xs y = if l == "" then [] else [l] ys = if r == "" then [] else mywords (tail r) Regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com