UnNesting.hs

UnNesting.hs

data Item a = Atom a
            | List [Item a]
  deriving (Eq, Show)

flatten :: Item a -> [a]
flatten (Atom x)  = [x]
flatten (List xs) = concat $ map flatten xs

flatten' :: Item a -> [a]
flatten' (Atom x)  = [x]
flatten' (List xs) = helper [] (reverse xs)
  where
    helper acc []            = acc
    helper acc (Atom x:xs)   = helper (x:acc) xs
    helper acc (List xs':xs) = helper (helper acc (reverse xs')) xs

main = print $ flatten $
         List [ Atom 'a'
              , List [ Atom 'b'
                     , Atom 'c']
              , Atom 'd'
              , Atom 'e'
              , List [ Atom 'f'
                     , Atom 'g'
                     , List [ Atom 'h'
                            , Atom 'i'
                            , Atom 'j' ]
                     , List [ Atom 'k' ]
                     , List [ Atom 'l'
                            , List [ Atom 'm'
                                   , Atom 'n'] ]
                            , Atom 'o' ]
              , Atom 'p'
              , Atom 'q' ]