Not sure I like the `become` keyword. Seems bizarre - someone encountering this word in code for the first time would have no idea what it's doing.
Why don't they just use `tailcall`? That would make it's obvious what it's doing because we've been using the term for nearly half a century, and the entire literature on the subject uses the term "tail call".
Even better would be to just automatically insert a tail call - like every other language that has supported tail calls for decades - provided the callee has the same signature as the caller. If it's undesirable because we want a stack trace, then instead have some keyword or attribute to suppress the tail call - such as `no_tail`, `nontail` or `donttail`.
Requiring tail calls to be marked will basically mean the optimization will be underutilized. Other than having a stack trace for debugging, there's basically no reason not to have the optimization on by default.
Rust does allow tail call optimization. But that's LLVM's decision to optimize tail calls on a case-by-case basis. An explicit syntax to denote tail calls would be the difference between tail call optimization and guaranteed tall call elimination, which is important because if you're writing a tail-recursive function then it's pretty trivial to blow the stack at any moderate recursion depth unless you can guarantee the elimination.
As for why it's not trivial for Rust to do this by default, consider the question of what should happen in the case of local destructors, which in an ordinary function would be called after `return myfunc()` returns, but in a tail-recursive function would need to be called beforehand. The proposals for `become` tend to handle this by making it a compiler error to have any locals with destructors in scope at the point of the tail-call, further motivating the explicit syntax.
> Even better would be to just automatically insert a tail call - like every other language that has supported tail calls for decades - provided the callee has the same signature as the caller.
I've used Scala for many years and concluded this was a mistake. It makes it too easy to accidentally rely on an optimisation and then break your code much later with a seemingly innocuous change. Better to make it explicit.
Sure, but if you're relying on the TCO, you kind of have to actually state that, regardless? (At least if you want to avoid accidental regressions.)
I don't see any way around having to state your intent (as a programmer) for this. It's just a semantic difference in behavior unless your Abstract Machine allows for just ignoring the possibility of stack overflow entirely.
> I don't see any way around having to state your intent (as a programmer) for this.
I want a way to state that my intent is to not rely on the TCO. Even a second explicit annotation for "do not TCO this function" would be better than what Scala currently does (but frankly TCO is surprising enough to debug that I think it should be opt-in rather than opt-out).
> I want a way to state that my intent is to not rely on the TCO.
How would that work? You want a nontail keyword to indicate that intent explicitly?
I guess I could imagine a scenario where you want to de-optimize a TCO into a non-TC... but I mean... that's got to be rare enough to just not bother with?
EDIT: Also, this is the exact opposite of your comment which I replied to. Make up your mind on what you want and we can maybe find a way to achieve it
> How would that work? You want a nontail keyword to indicate that intent explicitly?
Either that, or to not have TCO applied if I don't set a keyword (which I'd prefer).
> I guess I could imagine a scenario where you want to de-optimize a TCO into a non-TC... but I mean... that's got to be rare enough to just not bother with?
Sometimes I know that a given function will eventually need to be non-TC, in which case I want a way to make it non-TCO now. More commonly a junior team member just hasn't thought about TC-ness at all, in which case I'd rather the function not be TCOed and fail-fast than be TCOed until it isn't.
> EDIT: Also, this is the exact opposite of your comment which I replied to.
No it isn't. It's the detail of how it happens. If you use Scala at scale with a team that includes non-experts it will happen to you.
As far as the name of the keyword: anyone who knows what "tail call" means will figure out "become" pretty quickly. If they don't get it from context clues, someone will just have to tell them, "oh, it's a tail call", and the confusion will dissolve, because "become" is really not a bad word for what happens in a tail call. (This is obviously less important than the implementation issues kibwen handled.)
I'm generally pretty conservative about keywords. But it changes the semantics of the return, so it makes sense to change the word used in that position.
