Tobie Langel wrote:
On Sep 10, 2012, at 9:48 PM, Jason Orendorff<[email protected]> wrote:
On Mon, Sep 10, 2012 at 12:50 PM, Kevin Smith<[email protected]> wrote:
function f(x=EXPR1, y=EXPR2) { BODY }
===>
function f(x, y) {
if (x === void 0) x = EXPR1;
if (y === void 0) y = EXPR2;
BODY
}
I'm not so sure - the desugaring above would mean that default expressions
would have visibility across curly-brace boundaries, which I find to be
quite surprising.
It is surprising. It could even bite unwary programmers. But in what
scope do you propose to evaluate default-param expressions?
In their lexical scope.
Which one? If you mean let*, so that
function f(a = a, b = b*a) { return [a, b]; }
var a = 42;
console.log(f()) // [42, NaN]
console.log(f(1)) // [1, NaN]
console.log(f(1,2)) // [1, 2]
works, that's one "lexical scope" approach. But it is novel to JS and
seen nowhere else in the language (in particular, there's no orthogonal
analogue of let* in Scheme).
If you mean let with temporal dead zone (a kind of let rec, or let with
hoisting and no read before initialization), then the reads of a and b
above before they are initialized for f() and f(1) would throw.
What's more, as Allen reminded me tonight, "lexical scope" can be made
to work with magic arguments objects, which alias the lexical bindings
through spec and implementation magical back doors. But this violates
the spirit of lexical scope. Do you really want to claim formal
parameters have lexical scope given such junk as:
function f(a, b = arguments[0]) { arguments[0] = 99; return [a, b]; }
console.log(f(1)) // [99, 1]
console.log(f(1, 2)) // [99, 2]
?
We could forbid arguments from being used to mean f's activation's
arguments object in parameter default values. But that still leaves
arguments[0] = 99 setting "lexically scoped" x to 99. This is required
by backward compatibility, and I argue it makes x var-like, not let-like
or lexically scoped -- even though one could work magic via arguments
object getters and setters, in the spec.
Implementations want let and anything let-like to be optimized assuming
no such aliasing. I think users want that too, for bug reduction and
readability.
/be
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