[Haskell-beginners] I have created an ugly Haskell program..

[Michael Mossey]

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 :)
> 
>