gistfile1.scala

  val username: Option[String] = for {
    parsed <- requestJson.parseOption   // Parse the JSON.
    jsonObject <- parsed.obj            // Get the JSON as a JsonObject instance.
    userIDJson <- jsonObject("userid")  // Get the "userid" field from the JsonObject.
    userID <- userIDJson.string         // Get the value of the "userid" field.
    user <- lookupUser(userID)          // Get an instance of User for the user ID.
  } yield user.username