smlsharp: read a json-represented list

list.json

{
  "type": "cons",
  "head": 1,
  "tail": {
    "type": "cons",
    "head": 42,
    "tail": {
      "type": "nil"
    }
  }
}

main.sml

(*
type Dlist = { type: 'nil' } | { type: 'cons', head: number, tail: Dlist }
*)
type dlist = { "type": string }
type dcons = { "type": string, head: int, tail: dlist Dynamic.dyn }

val j = Dynamic.fromJsonFile "list.json";

val L = _dynamic j as dlist Dynamic.dyn;

fun dlistToList (x : dlist Dynamic.dyn) =
    case #"type" (Dynamic.view x) of
        "nil" => []
      | "cons" =>
        let
          val xs = _dynamic x as dcons
        in
          #head xs :: dlistToList (#tail xs)
        end