import Data.List (break)
import Data.Bifunctor (bimap) 

unlessNull _ [] = []
unlessNull f xs = f xs

untilDoThen cond l r = unlessNull (uncurry (<>) . bimap l r . break cond)

reverseWords = untilDoThen isSpace         reverse keepSpaces
keepSpaces   = untilDoThen (not . isSpace) id      reverseWords

revwords_ r w "" = r ++ w
revwords_ r w (' ':s) = revwords_ (r ++ w ++ " ") "" s
revwords_ r w (x:s) = revwords_ r (x:w) s

revwords = revwords_ "" ""