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 _______________________________________________ es-discuss mailing list es-discuss@mozilla.org https://mail.mozilla.org/listinfo/es-discuss