Regarding transformations of returns, the initial implementation added an anonymous callback argument and converted returns into using it. But that takes the option of using the callback out of the programmer's hands, and is entirely non-obvious for someone calling the function (the number of parameters required is one more than those listed in the function definition)

I plan to write a follow-up shortly that discusses possible approaches of doing this sort of thing, and your suggestion is one of the ideas I'm going to explore. Thanks for reading & commenting :)