gistfile1.hs

data Tree a = Node a [Tree a]
            | Leaf a 

build :: Monad m => (a -> m [a]) -> a -> m (Tree a)
build get_sons a = do sons <- get_sons a
                      case sons of
                        []  -> return $ Leaf a
                        lst -> do trees <- sequence $ map (build get_sons) lst
                                  return $ Node a trees