TC39 General on 2024-09-03

08:31	<Michael Ficarra>	A Coq Mechanization of JavaScript Regular Expression Semantics: https://dl.acm.org/doi/10.1145/3674666
13:58	<jmdyck>	In 6.1, they acknowledge the distinction between main-body and annex B regexes, but don't explicitly say which they're mechanizing (as far as I could see). I'm pretty sure it's just main-body.
20:10	<nicolo-ribaudo>	Do the two modes have different features, or just different parsing rules?
20:10	<nicolo-ribaudo>	These grammars do not agree on all inputs: for instance, the standard grammar rejects ], whereas the legacy grammar allows it and treats it as a valid regex matching the character ‘]’. Given this, we left parsing out of our mechanization.
20:42	<jmdyck>	It's mostly syntax, but it isn't just syntax, some of the pseudocode is different too, so it makes a difference (to the resulting mechanization) which you pick.
20:43	<Richard Gibson>	Do the two modes have different features, or just different parsing rules? just different parsing rules AFAICT, but the interpretation of various `\c…` in various possible locations gets really intricate
20:44	<jmdyck>	For research purposes, it makes sense for them to stick to the main-body, but I think it's at least worth saying so.
20:51	<snek>	i experimented a bit with an nfa vm for engine262 for verifying optimizations, cool to see this approach as well
22:58	<bakkot>	I kind of want to propose a `Math.argmax(it, fn)` which is like `(it, fn) => Iterator.from(it).reduce((a, b) => f(a) > f(b) ? a : b)`, but apparently Python's (rather numpy's) `argmax` gives you the index of the maximum item rather actually taking a function.
22:59	<bakkot>	possible alternatives: the `fn` arg is optional, and it just gives you the index if omitted (which is useless if the argument isn't indexable, but whatever) call it `maxBy`, which is a natural enough name (and also apparently already exists in lodash, neat)
23:00	<bakkot>	also, I want to suggest that both this and a hypothetical `sortBy` would allow the function to return any of { number, bigint, string }, and do the natural comparison within each type, but throw if the function ever returns two different types