proposal-ptc-syntax

Safari has tail-call optimisation. We don’t have it elsewhere mostly because Google: 1. Agreed to implement it [it’s in ES6] 2. Implemented and shipped it behind a flag 3. Unshipped it. 4. Proposed something else [ https://github.com/tc39/proposal-ptc-syntax ] Prior to that, Firefox had proposed a carve out for cross-realm calls, but then they didn’t bother implementing anything. 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 ignore the 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

I agree, syntactic tail calls seems like a great compromise if automatic tail calls are too risky. I'd love to have them in the language. Here are some strawman syntax examples from the Syntactic Tail Calls proposal. Return Continue function factorial(n, acc = 1) { if (n === 1) { return acc; } return continue factorial(n - 1, acc * n) } let factorial = (n, acc = 1) => continue n == 1 ? acc : factorial(n - 1, acc * n); // or, if continue is an expression form: let factorial = (n, acc = 1) => n == 1 ? acc : continue factorial(n - 1, acc * n); Function sigil // # sigil, though it's already 'claimed' by private state. #function() { /* all calls in tail position are tail calls */ } // Note that it's hard to decide how to readably sigil arrow functions. // This is probably most

from the linked PTC proposal in the article (https://github.com/tc39/proposal-ptc-syntax): > Unfortunately, the TC39 process at this time did not require heavy implementation involvement and so while many implementers were skeptical, the feature was included and standardized as part of ES6.