convert list to tree structure

v1.hs

data Item = Item {
  sId :: String,
  pId :: String,
  value :: [Char],
  children :: [String]
} 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

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

entry = numberTree list (toTable list)
r1 = mapM_ print entry -- 印出 table 內容
r2 = M.lookup "2-1" entry -- 查找 "2-1" 這個 item

numberTree xs table = do
  foldl (\a x -> nNode x a) table 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