rest parameters

[Isiah Meadows]

Andy, look at my most recent post to this list. What do you think of that point?

https://mail.mozilla.org/pipermail/es-discuss/2015-October/044337.html

On Wed, Oct 7, 2015 at 4:04 AM, Andy Earnshaw <andyearnshaw at gmail.com> wrote:
> Damn, forgot the reply-all button the first time.  Including the original
> and reply here, plus my reply inline:
>
> On Tue, 6 Oct 2015 at 19:05 Tab Atkins Jr. <jackalmage at gmail.com> wrote:
>>
>> On Mon, Oct 5, 2015 at 6:33 AM, Andy Earnshaw <andyearnshaw at gmail.com>
>> wrote:
>> > On Sat, 3 Oct 2015 at 00:27 Tab Atkins Jr. <jackalmage at gmail.com> wrote:
>> >> A few examples of the ambiguity:
>> >>
>> >> function f1(...a, b, c) {print arguments}
>> >> f1(1) // ([], 1, undefined) or ([], undefined, 1) ?
>> >>
>> >> function f2(...a, b="default") {print arguments}
>> >> f2(1) // ([], 1) or ([1], "default") ?
>> >>
>> >> function f3(a, b, ...c, d, e) {print arguments)
>> >> f3(1,2,3) // (1, 2, [], 3, undefined) or (1, 2, [], undefined, 3) or
>> >> (1, undefined, [], 2, 3) or something else?
>> >
>> >
>> > I don't think these are as ambiguous as you say.  For the first and
>> > third
>> > case, the first possible result you mention in both cases seems pretty
>> > natural.  I don't think you could make an argument for the other cases,
>> > they
>> > don't really make any sense.
>>
>> Let me rename the arguments, then, to make it more obvious:
>>
>> function f1(...frontOfList, secondToLast, last) {print arguments;}
>> f1(1)
>> // (frontOfList=[], secondToLast=1, last=undefined)
>> // or (frontOfList=[], secondToLast=undefined, last=1)?
>>
>> It would be difficult to argue, given these argument names, that the
>> author intended the first result.  They clearly intended the second
>> result.
>
>
> Function arguments are always filled in from left to right, with the
> proposed exception in this case being that rest params don't swallow passed
> in arguments when arguments.length <= fn.length.  I don't think anyone
> coming from a background in writing code would expect the second result, any
> more than they would expect:
>
>     function f1 (first, last) { print arguments; }
>     f1(1);
>     // (first=undefined, last=1)
>
> Unless there's a precedent for this behaviour in a language I'm unaware of.
>
>>
>> This is what I mean by:
>>
>> > You can come up with answers to these questions.  What you can't do is
>> > come up with answers that are *obviously correct*.
>> ~TJ
>
>
>
> I don't know... the first result was "obviously correct" to me ;-).
> Admittedly, "obviously correct" can be subjective when defining language
> behaviour.
>
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-- 
Isiah Meadows