map union record types from js

gistfile1.txt

type ofCst;
type unknownCstElement;
type cstChildrenDict = Js.Dict.t(list(unknownCstElement));

type cstNode = {
  name: string,
  children: cstChildrenDict,
};

type token;
type cstElement =
  | Token(token)
  | Node(cstNode);

type lexErrors;
type parseErrors;

type parseResult = {
  cst: cstNode,
  lexErrors,
  parseErrors,
};

let mapUnknownCst = (unknownCstElement: unknownCstElement) =>
  if (Js.typeof(Obj.magic(unknownCstElement)##tokenType) != "undefined") {
    Token(Obj.magic(unknownCstElement): token);
  } else {
    Node(Obj.magic(unknownCstElement): cstNode);
  };

[@bs.module "./parser/index.js"]
external parse: string => parseResult = "parse";