It is presumably useful when delivering a promise to know if the delivery was successful or not (where it might be unsuccessful if it was already delivered, perhaps on another thread).
The {{deliver}} function seems to currently communicate this by returning a truthy value (the promise itself) on success and a falsy value ({{nil}}) on failure. If this is intentional, the docstring should say so so that users can comfortably rely on it.
In CLJ-1038 Rich elected for the docstring to not describe the return value; I'm not sure if that was a reluctance to fully specify the return value (promise vs {{nil}}) even if partially describing it (truthy vs falsy) would be okay.
