On Fri, Mar 3, 2017 at 10:10 AM, Abe Dillon <abedillon@gmail.com> wrote:
I really think the whole "lazy" idea is misguided. If it's possible for the interpreter to determine automatically when it needs to force evaluation of a lazy expression or statement, then why not make *all* expressions and statements lazy by default? I think it's pretty clear when to force evaluation: 1) when the result is used in a control flow statement/expression 2) when the result is output (file, network, or other I/O) and 3) evaluate all pending lazy code before releasing the GIL. At that point, why not make lazy evaluation an implicit feature of the language, like the garbage collector.
4) When the evaluation will have side effects. Making everything lazy is fine when you can guarantee that there's no visible effect (modulo performance) of evaluating things in a different order, or not evaluating some of them at all. In Python, that can't be guaranteed, so universal laziness is way too dangerous. Think of all the problems people have with getting their heads around multithreading, and consider that this is basically going to take your code and turn it into a bunch of threads, and join on those threads only when there's a reason to. Debugging becomes highly non-deterministic, because adding a quick little 'print' call to see what's going on might force evaluation a little sooner, which means X happens before Y, but without that print, Y happens before X... aeons of fun. No, if laziness is added to Python, it *must* be under programmer control. ChrisA