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:[email protected]>
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:[email protected]>
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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