gistfile1.hs

data StartsWithA a b = StartsWithA a (StartsWithB a b)
                     | EmptyA
                       deriving Show

data StartsWithB a b = StartsWithB b (StartsWithA a b)
                     | EmptyB
                       deriving Show

infixr 7 <<<
infixr 7 >>>
 
x >>> y = StartsWithA x y
x <<< y = StartsWithB x y

zipAB :: [a] -> [b] -> StartsWithA a b
zipAB (a : as) (b: bs) = a >>> (b <<< (zipAB as bs))
zipAB (a : as) _ = a >>> EmptyB
zipAB _ _ = EmptyA


main = putStrLn $ show $ (zipAB ["a", "b", "c"] [1, 2, 3], zipAB ["a", "b", "c"] [1, 2], zipAB ["a", "b"] [1, 2, 3])