On Wed, Aug 11, 2010 at 11:31 PM, Greg Ewing <greg.ewing@canterbury.ac.nz> wrote:
ghazel@gmail.com wrote:
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.
I'm with ghazel on this one. Long, long ago I used a system that effectively used implicit cocalls. It was a threading OS with non-preemptive scheduling, so instead of locking you'd just refrain from calling any one of the (very few) syscalls that would allow another thread to run. This worked fine when we just got started, but as we started building more powerful abstractions, a common bug was making a call to some abstraction which behind your back, sometimes, perhaps inside more abstraction, would make a syscall. This was nightmarish to debug (especially since it could happen that the offending abstraction was maintained by someone else and had just evolved to make its first syscall). So, coming back to this, I think I am on the side of explicitly marking cocalls. But whether it's better to use cocall or yield-from, I don't know. -- --Guido van Rossum (python.org/~guido)