On Fri, Aug 9, 2013 at 6:40 PM, Brendan Eich <[email protected]> wrote:
> On Aug 9, 2013, at 3:32 PM, Rick Waldron <[email protected]> wrote: > > On Fri, Aug 9, 2013 at 5:54 PM, Axel Rauschmayer <[email protected]> wrote: > >> AFAICT, there is no current consensus on whether destructuring assignment >> is refutable by default or not: >> >> https://github.com/rwldrn/tc39-notes/blob/master/es6/2013-07/july-23.md#44-consider-deferring-es6-refutable-matching >> >> Could we make destructuring assignment fail soft and introduce a marker >> for refutable parts of a pattern (e.g. a prefix `!`): >> >> let { !a: foo, b: bar } = { a: 1 }; // ok: foo = 1, b = undefined >> let { !a: foo, b: bar } = { }; // exception >> >> This would have one advantage: It would be possible to bring this >> operator to parameter declarations (allowing one to declaratively specify a >> minimum arity). Then destructuring assignment and parameter handling would >> work very similarly. >> >> function bla(!mandatoryArg, optionalArg1, optionalArg2 = 123) { >> ... >> } >> >> > > While I agree this is interesting and should be explored further, I reject > the proposal to add more meaning to the "!" character. Given this proposal, > "!" would sometimes mean "not" or "negate" (as in it's current form) and > sometimes mean "a required thing". Meanwhile, "refute" is a synonym for > "negate", which is the opposite of what you're claiming the semantics would > be, ie. irrefutable, "impossible to omit" > > > No. > > > or "impossible to deny [a value for this identifier binding]". > > > > No, refutable is what Axel means. He wants matching, so a failure or > refutation to cause fall through to next match. What we have in JS now with > o.p and in ES6 with let {p} = o; is irrefutable -- no way to mismatch, if > the property is missing you get undefined. > > So ! Is not too far off the mark. Recall that the late proposal was to > unify patterns used now in destructuring and later in matching by requiring > ? for irrefutability and making lack of ? mean "refutable". > My argument was specifically about the current meaning of the ascii exclamation "!" and that assigning it an additional context-based meaning that's quite the opposite of the current unary operator meaning, isn't a proposal that I would support. This is stated with no regard for previous refutable matching proposals. Rick
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