test.hs

import Data.Char (toLower)
import Data.List (sort)

isAlpha c = (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z')

lower = map (\w -> toLower w)

count t = length . filter (== t)

removeDups [] = []
removeDups (x:xs) = x : (filter (/= x) $ removeDups xs)

join sep [] = ""
join sep (x:xs) = x ++ sep ++ (join sep xs)

format sep = map (\x -> snd x ++ sep ++ (show $ fst x))

commonWords n cs = join "\n" $ format " : " $ take n . reverse . sort . removeDups $ zip (map (\w -> count w ws) ws) ws
    where ws = map (lower . filter isAlpha) $ words cs

main :: IO ()
main = do putStrLn $ show $ commonWords 3 "This, indeed, is a test input! Test input I said."