list to tree v2

v2.hs

data Item = Item {
  sId :: String,
  pId :: String,
  value :: [Char],
  children :: [Item]
} deriving (Show)

list :: [Item]
list = [ 
  Item { sId="1", pId="0", value="1", children=[] }, 
  Item { sId="1-1", pId="1", value="1-1", children=[] }, 
  Item { sId="1-2", pId="1", value="1-2", children=[] }, 
  Item { sId="2", pId="0", value="2", children=[] }, 
  Item { sId="2-1", pId="2", value="2-1", children=[] }, 
  Item { sId="2-2", pId="2", value="2-2", children=[] }]

type Table = M.Map String Item
  
rootNode = Item { sId="0", pId="-1", value="root node", children=[] }
initTable = M.insert "0" rootNode M.empty

toTable :: [Item] -> Table
toTable xs = foldl (\acc x -> M.insert (sId x) x acc ) initTable xs

foo = evalState (numberTree list) (toTable list)

-- 進入點 --
numberTree :: [Item] -> State Table (State Table Item)
numberTree xs = do
  a <- numberNode xs
  return a
  
  where
  
  -- State s a
  numberNode :: [Item] -> State Table (State Table Item)
  -- numberNode [] = Nil
  numberNode (x:xs) = do
    table <- get
    newTable <- return (nNode x table)
    put newTable
    return (numberNode xs)

  nNode :: Item -> Table -> Table 
  nNode x table =
    let parent = fromJust (M.lookup (pId x) table)
    in (update parent x )
    where 
    update :: Item -> Item -> Table
    update parent current = 
      let a = parent { children = (sId current) : children parent }
          b = M.adjust (\x -> a) (sId parent) table
      in b  

    message itm tbl msg = 
      itm { value = "Table= " ++ msg ++ (show tbl)}