on 2009-03-27 05:17 P.J. Eby said the following:
At 04:08 PM 3/27/2009 +1300, Greg Ewing wrote:
You can't expect to improve something like that by
stuffing yield-from into the existing framework, because
the point of yield-from is to render the framework
itself unnecessary.
But it doesn't. You still need *something* that processes the yielded
values, since practical frameworks have various things to yield "to" -
i/o, time, mouse clicks, whatever. Correctly dealing with the call
stack part is tedious to implement, sure, but it's not really the focal
point of a microthreading framework.
I can chime in here with a use case, if an unusual one. I implemented
just such a framework based on generator syntax for my thesis work to
model the behaviour of software agents as a collection of interacting
activities (microprocesses). The top layer is based on Twisted (similar
to its _inlineCallbacks) and different schedulers decide on what to do
with yielded values.
This is really very similar to Philip Eby's code, the main difference
being that one uses a generic function yield_to as an extension point
and the other one uses (subclasses of) Deferreds.
You can handle the call stack in the Deferred-based case just as
clumsily as in the other :-).
And in my system, the "call stack" (i.e. the hierarchy of active
microprocesses) and how it can be manipulated by the agent is actually
the interesting part.
Usually, you need to have some way to control which microthreads are
actually to be executing, vs. the ones that are waiting for a particular
time, an I/O operation, or some other sort of event. None of that stuff
goes away just by taking care of the call stack.
Yes. However, the valuable addition that an explicit yield from syntax
would provide for my use case is a way to explicitly distinguish between
subgenerators just for the sake of refactoring code vs. sub-"processes".
I could remove quite some duplication from my current code.
Additionally, as noted in the PEP, it would open the path for
optimisations of the refactoring cases.
I also think that a separation of handling the generator call stack and
handling yielded values improves the situation for scheduling/trampoline
authors conceptually.
Just my 0.02€
cheers,
stefan
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