interleave :: [[a]] -> [[a]]
interleave lists = foldl (\l x -> l >>= (interleave' x)) [[]] lists
    where interleave' :: [a] -> [a] -> [[a]]
          interleave' l1 [] = [l1]
          interleave' [] l2 = [l2]
          interleave' l1@(h1:t1) l2@(h2:t2) = fmap (h1:) (interleave' t1 l2) ++ fmap (h2:) (interleave' l1 t2)