This does NOT mean Python will get Tail Call Optimization, as Guido cannot be shown The Light, and has decided.
replies(4):
It should only be called Tail Call Elimination.
That doesn't follow. This isn't like going from driving a car to flying an airplane. It's like going from driving a car to just teleporting instantly. (Except it's about space rather than time.)
It's a difference in degree (optimization), yes, but by a factor of infinity (O(n) overhead to 0 overhead). At that point it's not unreasonable to consider it a difference in kind (semantics).
for (int i = 0; i < n; i++) a += i;
To:
a += n * (n+1) / 2;
Is this an optimisation or a change in program semantics? I've never heard anyone call it anything slse than an optimisation.
> Is this an optimisation or a change in program semantics?
Note that I specifically said something can be both an optimization and a change in semantics. It's not either-or.
However, it all depends on how the program semantics are defined. They are defined by the language specifications. Which means that in your example, it's by definition not a semantic change, because it occurs under the as-if rule, which says that optimizations are allowed as long as they don't affect program semantics. In fact, I'm not sure it's even possible to write a program that would be guaranteed to distinguish them based purely on the language standard. Whereas with tail recursion it's trivial to write a program that will crash without tail recursion but run arbitrarily long with it.
We do have at least one optimization that is permitted despite being prohibited by the as-if rule: return-value optimization (RVO). People certainly consider that a change in semantics, as well as an optimization.
With this kind of "benign" change, all programs that worked before still work, and some that didn't work before now work. I would argue this is a good thing.