The game's code, as a module that answers for each piece of compiled code by name.

Names are the ones GameMaker's compiler gives:

• gml_Object_<object>_<Event>_<subtype> — an object's event (Cauldron2D.GameMaker.Event.handler_name/3)
• gml_Room_<room>_Create — a room's creation code
• gml_RoomCC_<room>_<n>_Create — the creation code of the nth instance, counting from 0, among the room's instances that have creation code

handler/1 returns a function of the world, the instance running the code (self) and, for collision events, the other instance's id (nil otherwise), returning the world. Room creation code runs with self nil. nil from handler/1 means the game has no code there and nothing runs.

defmodule MyGame.Code do
  @behaviour Cauldron2D.GameMaker.Code

  @impl true
  def handler("gml_Object_Ball_Step_0"), do: &MyGame.Ball.step/3
  def handler(_name), do: nil
end