Andreas Rossberg wrote:
On 11 July 2012 17:20, Russell Leggett <[email protected] <mailto:[email protected]>> wrote:On Wed, Jul 11, 2012 at 7:14 AM, Andreas Rossberg <[email protected] <mailto:[email protected]>> wrote: AFAICS, '?' on a variable itself would always be redundant, because a variable pattern is irrefutable anyway. So you're saying that even this should match in a refutable pattern: let [a,b,c] = [1,2]; I would expect that to fail, especially in a hypothetical pattern matching construct.That would fail because the array pattern is refutable. What I meant is something else, namely that there is no difference between these:let x = ... let ?x = ... or these: let {x: x, y: y} = ... let {x: ?x, y: ?y} = ...Pattern matching recursively decomposes the RHS and matches a (sub)value against the respective (sub)pattern of the LHS. Once you reach a variable, that submatch is unconditional, so a '?' doesn't change anything.
Thanks -- this recovers my forgotten reason for advocating prefix-? on property names, not binding identifiers (which occupy the value positions in object literals). I think it addresses Allen's objection about why ? is not available for all bindings, simple identifiers too. It is part of the pattern grammar for property names only.
For arrays, the just-so story of prefix-? having an implicit "0", "1", etc. name after it works well enough -- there's no difficulty computing indexes statically.
Russell asked about 'length' -- per a previous thread, I thought we agreed that one [[Get]] of 'length' would be done before matching if and only if the array literal contains a spread (covering rest, the dual for a pattern of spread in the array literal corresponding to the pattern).
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