On Nov 11, 2011, at 3:17 PM, David Herman wrote: > 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.
This is a really good point. It's the kind of consistency (among two pattern kinds and two use-cases) that we need to attend to, not a foolish consistency to always [[Get]] 'length' (or not). I change my preferred answer from [no, no] or [no, _] to [no, yes]. /be > > 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 [email protected] https://mail.mozilla.org/listinfo/es-discuss
