Comment by KPGv2

Nota bene, then is not a monad because the implementation of then implies map is isomorphic to flatmap. This is because `then` turns the return value of a callback into a flat promise, even if the callback itself returns a promise.

That is to say, then checks the type of the return value and then takes on map or flatmap behavior depending on whether the return value of the callback is a Promise or not.