A tail calling interpreter for Python (already landed in CPython)

This does NOT mean Python will get Tail Call Optimization, as Guido cannot be shown The Light, and has decided.

It is not an optimization ; it changes program semantics - converts programs that will run out of stack eventually regardless of the amount of available memory (and raise exceptions an the process, for example, which a program might rely on. Either way, semantics are changed)

It should only be called Tail Call Elimination.

By that standard, any optimization that changes scaling in any dimension changes semantics, which, well, I’m not saying its wrong, but I would say it is exactly what people looking for optimization want.

> By that standard, any optimization that changes scaling in any dimension changes semantics

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).

Modern C compilers are able to transform something like this:

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.

Amazing it can do that. How does it work?

That definitely does seem to change its semantics to me. I am not a c expert but this surely has problems the previous one doesn’t?

It does change the semantics if n is negative or large enough to cause an overflow. The challenge for the compiler is to somehow prove that neither of those things can happen.