Another solution here. The inspiration is to try to use Data.Map's fromListWith to do the main work. Notice that you can "decapitate" the useless head of each list with the single line dropWhile (not . both) . M.toAscList $ neated import Control.Arrow import qualified Data.Map as M data Combine v = LeftOnly v | RightOnly v | BothOfThem v v deriving (Show) cmb :: Combine a -> Combine a -> Combine a cmb (LeftOnly x) (RightOnly y) = BothOfThem x y cmb (RightOnly y) (LeftOnly x) = BothOfThem x y both (_,(BothOfThem _ _ )) = True both _ = False chain _ last2 ((t,LeftOnly v):xs) = (t,(v,last2)) : chain v last2 xs chain last1 _ ((t,RightOnly v):xs) = (t,(last1,v)) : chain last1 v xs chain _ _ ((t,BothOfThem v w):xs) = (t,(v,w)) : chain v w xs chain _ _ [] = [] specialZip d1 d2 = let neated = M.fromListWith cmb $ map (second LeftOnly) d1 ++ map (second RightOnly) d2 dr = dropWhile (not . both) . M.toAscList $ neated in chain "" "" dr Philip Scott wrote:
Michael Mossey wrote:
Function you seek is 'specialZip' below. 'fluff' and 'decapitate' are helpers. Not extensively tested.
Thanks Michael, that looks much better than mine :)