How a command happens: an effect that may produce a message, None for effects that only act on the
outside world. It cannot fail: an expected failure becomes a message, e.g. with Effect.match, and a
defect ends the run. update returns commands as data; only runCommand turns one into an execution.
How a command happens: an effect that may produce a message,
Nonefor effects that only act on the outside world. It cannot fail: an expected failure becomes a message, e.g. withEffect.match, and a defect ends the run.updatereturns commands as data; onlyrunCommandturns one into an execution.