ecma262 VS proposal-ptc-syntax

These aren't valid JavaScript (@@iterator would throw an error). They are actually internal Symbols used in JavaScript. They are used to implement features like iteration, instanceOf, and such internally. They actually might get removed or changed

Note that in previous editions, the ECMAScript® Language Specification used the term "lexical environment" before it decided to rename it to "Environment Record" so you might encounter this term in other definitions and tutorials.

For a new specification to be written, you need two things, a_ technical committee_, and a standard. The standard specification for JavaScript is called ECMA-262, and the technical committee is Technical Committee-39(TC39).

4. Proposed something else [ https://github.com/tc39/proposal-ptc-syntax ]

While apple is against Syntactic tail calls, they’re mainly just opposed to versions of it that would remove/unrequire the tail-call optimisation they already do: https://github.com/tc39/ecma262/issues/535

For the version of it that is backwards compatible, they wouldn’t need to do anything other than recognise it as valid syntax. Their main concern is that it "could add confusion with very little benefit."

The spec for STC has a critique of PTC:

- performance

- developer tools

- Error.stack

- cross-realm tail calls

- developer intent

See: https://github.com/tc39/proposal-ptc-syntax#issues-with-ptc

Apple's 2016 response as to why they won't implement STC is here: https://github.com/tc39/ecma262/issues/535

- STC is part of the spec and will take too long to change.

- Now that they've implemented support for PTC, they don't want to regress web pages that rely on it.

- They don't want to discourage vendors from implementing PTC by agreeing to STC.

- They don't want to introduce confusion.

Some of these arguments about confusion and delays seem wrong hindsight, since on every point things would have been better if they'd just agreed to the compromise of STC.

- It would have been part of the spec years ago

- STC would have had a clear way for web pages to know when tail calls could be relied on (and PTC would have been optional)

- Other vendors didn't implement PTC in any case, despite no agreement on STC

- There's even more confusion as things are now

The proposal of "syntactic tail calls" to provide an explicit syntax for tail calls, co-championed by committee members from Mozilla (responsible for SpiderMonkey, the engine of Firefox) and Microsoft, was a response to these concerns. However, this proposal is now listed among the TC39's inactive proposals, possibly due to diminished interest, which may stem from the infrequent use of tail recursive functions in JavaScript.

This is not actually about Tail Call Optimisation, which is more flexible and optional matter of optimisation, but about Proper Tail Calls, which are actually part of the ECMAScript 6 specification (over implementer objections)—in strict mode, calls in tail position must not create additional stack frames. This is the last piece of ECMAScript 6 that most engines haven’t implemented, because it’s rather controversial: it actually causes some performance problems, and makes debugging harder, and may have security issues (in 2016, Mozilla declared it impossible to implement across realm boundaries due to their security model).

https://github.com/tc39/proposal-ptc-syntax has a lot of useful information about it all, and a proposal to make it explicit in syntax, such as with `return continue …`.

(Fun terminology problems here. The term TCO is commonly used for PTC, and PTC is very close to being a subset of TCO, but the mandatory stack frame elision which ruins debugging feels to me like it falls outside of TCO. In various situations, debuggers will mark things like “stack frame omitted” when they’ve optimised one out of existence, but you can generally compile things differently, or something like that, to prevent this. But with PTC, it feels like the engine is kinda not even allowed to know that a stack frames may be absent. So I say PTC and TCO are a little distinct, though PTC is mostly just a subset of TCO. Reminds me of the terminology of tree-shaking versus dead code removal—where the former is essentially a subset of the latter, but that the effects are just slightly different, though I’d say it’s more slight in that case than this.)

4. Proposed something else [ https://github.com/tc39/proposal-ptc-syntax ]

While apple is against Syntactic tail calls, they’re mainly just opposed to versions of it that would remove/unrequire the tail-call optimisation they already do: https://github.com/tc39/ecma262/issues/535

For the version of it that is backwards compatible, they wouldn’t need to do anything other than recognise it as valid syntax. Their main concern is that it "could add confusion with very little benefit."

The spec for STC has a critique of PTC:

- performance

- developer tools

- Error.stack

- cross-realm tail calls

- developer intent

See: https://github.com/tc39/proposal-ptc-syntax#issues-with-ptc

Apple's 2016 response as to why they won't implement STC is here: https://github.com/tc39/ecma262/issues/535

- STC is part of the spec and will take too long to change.

- Now that they've implemented support for PTC, they don't want to regress web pages that rely on it.

- They don't want to discourage vendors from implementing PTC by agreeing to STC.

- They don't want to introduce confusion.

Some of these arguments about confusion and delays seem wrong hindsight, since on every point things would have been better if they'd just agreed to the compromise of STC.

- It would have been part of the spec years ago

- STC would have had a clear way for web pages to know when tail calls could be relied on (and PTC would have been optional)

- Other vendors didn't implement PTC in any case, despite no agreement on STC

- There's even more confusion as things are now

You can see a direct example of this with Proper Tail Calls (PTC). It was added to the ECMAScript spec in 2015 as part of es6, but as of today - 7 years later - only Safari has shipped it*. As a result it is effectively not a thing in JavaScript, and the followup proposal meant to address issues with PTC ("Syntactic Tail Calls") has been basically ignored because PTC is already in the spec.

V8 implemented tail call optimization in the past, and the V8 team backed the TC39 proposal for syntactic tail calls (where you'd write return continue func() to make the use of TCO explicit). In Node 6 and 7 we could use them with the flag --harmony-tailcalls. The feature was removed from Node 8 after that proposal didn't go anywhere, but it's interesting, and shows some interest.