Late to the party, but I've brought more booze.
On Nov 5, 2011, at 2:41 PM, Brendan Eich wrote:
> We have:
>
> 1. Should an array pattern always query 'length'?
>
> 2. If the answer to (1) is "no", then should ... in an array pattern query
> 'length'?
>
> On reflection and at this point in the thread, with your reply in mind, my
> prefs in order: [no, yes], [no, no]. In no case do I favor [yes]. I'm
> refutably matching [no, _] :-P.
I feel strongly that the appropriate semantics is [no, yes].
Here's my reasoning. Arrays are a multi-purpose data structure in JS. Sometimes
they are used for fixed-size tuples, and sometimes they are used for dynamic
length arrays. (Similarly, objects are used both for fixed-size records and for
dynamic size dictionaries.)
When you use a fixed-length tuple in JS, you do not query the .length property.
When you use a dynamic-length array, you do.
When you use a fixed-size record in JS, you do not use object enumeration. When
you use a dynamic-size dictionary in JS, you do.
Destructuring is meant to provide elegant syntax for all of these use cases.
The syntax of [] destructuring is for fixed-length tuples if there is no
ellipsis, and for dynamic-length arrays if there is an ellipsis. That's what
the ellipsis is good for; distinguishing the case where you know statically how
many elements you expect from the case where you don't.
More concretely, here's the rough desugaring I expect. I'll use 〰〰 as
meta-ellipsis (thanks, Unicode!). I'll just specify the special case where each
element is an identifier. It's straightforward to generalize to arbitrary
nested destructuring patterns and hole patterns.
A pattern of the form
[a0, a1, 〰〰, ak]
desugars to
a0 = %v[0];
a1 = %v[1];
〰〰
ak = %v[k];
A pattern of the form
[a0, a1, 〰〰, ak, ...r]
desugars to
a0 = %v[0];
a1 = %v[1];
〰〰
ak = %v[k];
let %length = %v.length;
r = [ %v[i] for i of [k+1, 〰〰, %length - 1] if (i in %v) ];
This can be generalized further to allow a fixed number of patterns *after* the
ellipsis as well:
A pattern of the form
[a0, a1, 〰〰, ak, ...r, bn, bn-1, 〰〰, b0]
desugars to
a0 = %v[0];
a1 = %v[1];
〰〰
ak = %v[k];
let %length = %v.length;
r = [ %v[i] for i of [k+1, 〰〰, %length - n - 2] if (i in %v) ];
bn = %v[%length - n - 1];
bn-1 = %v[%length - (n - 1) - 1];
〰〰
b0 = %v[%length - 0 - 1];
Dave
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