A coroutine-ish solution is ultimately how Kmett did it too. What makes it tricky is when you do it over Sum types. You actually don't know how long either side

A coroutine-ish solution is ultimately how Kmett did it too. What makes it tricky is when you do it over Sum types. You actually don't know how long either side of the sum will take to compute, so you need to execute them speculatively. You actually have to be able to yield for one of several coroutines.