http://blog.dantup.com/2013/09/functional-programming-challenge-words-with-indexes/

Main.hs

module Main where

import Data.Char
import Data.List


main :: IO ()
main = print $ wordIndexes "Hello test  World"

wordIndexes :: String -> [(Int, String)]
wordIndexes s = white 0 s
  where
    white _ [] = []
    white i xs = let (w, xs') = span isSpace xs
                 in word (i + length w) xs'
    word _ [] = []
    word i xs = let (w, xs') = break isSpace xs
                in (i, w) : white (i + length w) xs'