Convert list to tree using State monad, in a suck less manner 😅

v3.hs

module Bar () where
import Data.Maybe
import Control.Monad.State
import qualified Data.Map as M

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 = evalState (numberTree list) (toTable list)
test1 = mapM_ print entry -- 印出 table 內容
test2 = fromJust $ M.lookup "2" entry -- 查找 "2" 這個 item 看它 children = ["2-1","2-2"]

-- 進入點 --
numberTree :: [Item] -> State Table Table
numberTree (x:xs) =
  numberNode x
  >>= \z -> if (length xs) /= 0 then (numberTree xs) else get
  >>= \table -> return table
  where
    numberNode :: Item -> State Table ()
    numberNode x = do
      table <- get
      newTable <- return (nNode x table) -- a == s
      put newTable
      return ()
    
    nNode :: Item -> Table -> Table
    nNode x table = 
      M.adjust 
        (\item -> item { children = (children item) ++ [(sId x)] }) 
        (pId x) 
        table