List to "Tree" in State Monad

StateTree.hs

module StateTree ( buildTree ) where

import Prelude hiding (lookup)
import Control.Monad.State
import Data.Map (Map, insert, insertWith, empty, lookup)

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 = Map String Item
root = Item { sId="0", pId="-1", value="root node", children=[] }

-- Make "state lines" for each Item in the list.
buildStatement :: Item -> State Table ()
buildStatement item =
  state $ \tree -> ((), (mergeNode tree item))

-- insert the item into the map and update the children if pId matches
mergeNode :: Table -> Item -> Table
mergeNode tree item =
  case lookup (pId item) tree of
    Just treeItem -> insert (sId item) item parentUpdated
    Nothing -> insert (sId item) item tree -- top root
  where
    parentUpdated = insertWith mergeToParent (pId item) item tree -- -> map updated
    mergeToParent child parent = parent { children =  children parent ++ [sId child] }

statements :: [] Item -> State Table ()
statements items =
  foldl folder initStatement items
  where initStatement = buildStatement root
        folder sts item = sts >>= (\_ -> buildStatement item)

buildTree :: ((), Table)
buildTree = runState (statements list) empty