There still has to be some weird way to call cofunctions from regular functions. Changing the single definition of a function from "def" to "codef" means revisiting all the sites which call that function in the body of regular functions, and pushing the change up the stack as you mentioned.
Yes, but when using generators as coroutines, I believe that invoking a coroutine from a non-coroutine will be a relatively rare thing to do. Essentially you only do it when starting a new coroutine, and most of the time it can be hidden inside whatever library you're using to schedule your coroutines.
In each of my scheduler examples, there is only one place where this happens. It's not particularly weird, either -- just a matter of wrapping costart() around it, which is a normal function, no magic involved.
I think this is the wrong direction. But, if you want to head that way, why not make calling a cofunction from a function also transparent, and exhaust the iterator when the function is called?
Because this is almost always the *wrong* thing to do. The cofunction you're calling is expecting to be able to suspend the whole stack of calls right back up to the trampoline, and by automatically exhausting it you're preventing it from being able to do so.
Calling a cofunction from a non-cofunction is overwhelmingly likely to be an error, and should be reported as such. For cases where you really do want to exhaust it, a function could be provided for that purpose, but you should have to make a conscious decision to use it.
Again, marking the points at which your function could be suspended is a very important feature, in my mind.
I'm still very far from convinced about that. Or at least I'm not convinced that the benefits of such awareness justify the maintenance cost of keeping the call markers up to date in the face of program changes.
Also, consider that if cocall is made to work on both ordinary functions and cofunctions, there is nothing to stop you from simply marking *every* call with cocall just on the offchance. People being basically lazy, I can well imagine someone doing this, and then they've lost any suspendability-awareness benefit that the call markers might bring.
Even if they don't go to that extreme, there is nothing to ensure that cocall markers are removed when no longer necessary, so redundant cocalls are likely to accumulate over time, to give misleading indications to future maintainers.