Problem7

-- One more solution for
-- http://www.haskell.org/haskellwiki/99_questions/Solutions/7
-- that works for infinite input too (i.e. it's lazy in its input)

data NestedList a = Elem a | List [NestedList a]

get1st :: NestedList a -> (Maybe a, Maybe (NestedList a))
get1st (Elem e)         = (Just e, Nothing)
get1st (List [])        = (Nothing, Nothing)
get1st (List (l : ls))  = let (e, l') = get1st l
                              tail = maybe (List ls)
                                         (\h -> List (h : ls)) l'
                          in maybe (get1st tail)
                                   (\e' -> (Just e', Just tail)) e

flatten :: NestedList a -> [a]
flatten nl = let (a1, as) = get1st nl
             in maybe [] (: maybe [] flatten as) a1

main = putStrLn . show . (take 10) . flatten $
            List $ map Elem [1..]

            --List [List [List [], Elem 4], List [Elem 3, Elem 2], Elem 1]
            --(List [] :: NestedList Int)
            --(Elem 1)