pub fn desired_size(
  of controller: DefaultController(a),
) -> Result(Int, Nil)

pub fn enqueue(
  in controller: DefaultController(a),
  chunk chunk: a,
) -> Result(Nil, String)

pub fn error(
  controller: DefaultController(a),
  reason reason: b,
) -> Result(Nil, String)

pub fn terminate(
  controller: DefaultController(a),
) -> Result(Nil, String)