On Nov 10, 2013, at 11:44 AM, Mark S. Miller wrote:
> Hi Brendan and Allen, you are both responding only to my point #1. I consider
> my point #2 --- cognitive load of the reader on encountering a rarely seen
> construct --- to be the more significant criticism here.
Hence my choice of using # which is more suggestive to the meaning. It seems
suggestive enough that once somebody was initially exposed to it they would
understand it when they say it.
>
> Note that I am not saying that the meanings of all syntactic constructs have
> to be guessable with no guidance. But the rarely used ones do.
>
> 3) Awkward syntax for some rarely used cases is the price of good syntax for
> common cases.
But sometimes syntax to provide access to basic information or functionality
that is rarely used. Arguably 'arguments' already provides such a hook but we
are trying to generally discourage use of 'arguments' because of its other
undesirable characteristics.
A different alternative would be via a new default bind. We could specify that
every function has an implicit const bind for 'actualArgCount' (unless there is
a different explicit declaration for that name).
As for rarity. I've been in enough long discussions with Web API folks on the
public-script-coord list and on various WebIDL bugs to feel that this usage to
define over-loads isn't that rare in the DOM API world. It would be nice for
ES to have a story for handling such definitions other than falling back to
using 'arguments'
Allen
>
>
>
>
> On Sun, Nov 10, 2013 at 11:08 AM, Brendan Eich <bren...@mozilla.com> wrote:
> On the other hand, we are this close in ES6 to relieveing people from having
> to use arguments at all.
>
> I like the intention behind Allen's idea. To defend the concrete syntax a
> bit, one would argue that only the final formal parameter may be prefixed
> with #, and we are free to use # in other ways elsewhere.
>
> If # should be kept reserved even in this context, then perhaps something like
>
> function f(args: x, y, z) {
> // use x, y, z freely (they could be patterns)
> switch (args.length) {
> case 1: ...
> case 2: ...
> // etc.
> }
> }
>
> The "label" names a rest parameter for the entire parameter list.
>
> /be
> Mark S. Miller <mailto:erig...@google.com>
> November 10, 2013 6:52 PM
>
> 1) "#" is one of the few ascii characters we haven't used up yet. Let's not
> use it up on a minor convenience.
>
> 2) Introducing new syntax for use cases that are expected to be frequent,
> e.g., classes, often pays for its weight. The frequency makes it likely that
> readers will understand what it is the vast majority of times they encounter
> it. New syntax that gets used once in a blue moon can only be justified if
> the pain of doing without in those blue-moon cases is huge. In this case, it
> isn't. For those rare occasions when they see it, readers are much more
> likely to guess what "arguments.length" mean that this funny newfangled "#"
> thing.
>
> Code is read much more often than it is written -- at least code that
> matters. Brevity is useful as a metric only to the degree that it serves as a
> proxy for cognitive load on the reader.
>
>
>
>
>
>
> --
> Cheers,
> --MarkM
>
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> Allen Wirfs-Brock <mailto:al...@wirfs-brock.com>
> November 10, 2013 7:12 PM
>
> One of the the few remaining uses of a function's 'arguments' binding is to
> determine the actual number of passed arguments. This is necessary in some
> overloading scenarios where a function has different behavior when an
> argument is completely absent then it has when undefined (or any other
> default value) is explicitly passed in that parameter position. That
> situation occurs in a number of DOM APIs and even a few ES library functions.
>
> For example(see https://bugs.ecmascript.org/show_bug.cgi?id=1877 ),
> Array.prototype.splice returns different results for:
> [1,2,3].splice()
> and
> [1,2,3].splice(undefined)
>
> The natural ES6 declaration for a splice function is:
>
> function splice(start, deleteCount, ...items) {...
>
> but if you write it this way then within the body you have to have a test
> like:
>
> if (arguments.length == 0) {...
>
> to implement the correct web-compatable behavior.
>
> Or, alternatively you could declare the functions as:
>
> function splice(...actualArgs) {
> let [start, stop, ...item] = actualArgs;
> ...
> if (actualArgs.length == 0) {...
>
> So, to implement a Web-compaable version of splice you either have to use
> 'arguments' to determine the actual number of passed objects or you need to
> declare it with a bogus parameter pattern and use explicit or implicit
> destructuring to parse out the positional parameters.
>
> One way around this dilemma would be to provide a syntactic affordance for
> determing the actual argument count. For example, one possibility would be
> to allow the last item of any formal parameter list to be an item of the
> syntactic form:
>
> ActualArgumentCount : '#' BindingIdentifier
>
> So, the declaration for splice could then be:
>
> function splice(start, deleteCount, ...items, #argCount) {
> ...
> if (argCount == 0) {...
>
> Thoughts?
>
> Allen
>
>
> _______________________________________________
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> https://mail.mozilla.org/listinfo/es-discuss
>
>
>
> --
> Cheers,
> --MarkM
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