Tree labeling using State monad

Tree.elm

module Tree
    exposing
        ( Tree(Node)
        , NodeId
        , CollapsibleTree
        , makeTree
        )

--Using http://package.elm-lang.org/packages/folkertdev/elm-state/2.1.0

import State exposing (State)
import State


type Tree a
    = Node a (List (Tree a))


type alias NodeId =
    Int


type alias CollapsibleTree a =
    Tree ( a, Bool, NodeId )


type alias IdGen a =
    State NodeId a


makeTree : Tree a -> CollapsibleTree a
makeTree tree =
    State.finalValue 0 (labelTree tree)


labelTree : Tree a -> IdGen (CollapsibleTree a)
labelTree (Node root children) =
    State.map2
        (\nid collapsibleChildren -> Node ( root, False, nid ) collapsibleChildren)
        newNodeId
        (State.traverse labelTree children)


newNodeId : IdGen NodeId
newNodeId =
    State.modify (\x -> x + 1) |> State.andThen (\_ -> State.get)