Jason Orendorff wrote:
I think Allen is proposing a scheme where in code like
function f(a1=EXPR1, a2=EXPR2, a3=EXPR3) { ... }
f();
first EXPR1 is evaluated in a scope including only a1,
then EXPR2 is evaluated in a scope that contains a1 and a2,
then EXPR3 is evaluated in a scope that contains a1, a2, and a3,
then a new environment is created for the function body, its vars and
local functions.
Actually Allen was just reminding me of a TC39 meeting's agreement to
bind formals from the left, as if by let* (from Scheme) -- and a1 would
not be bound in EXPR1's scope, so any a1 used in EXPR1 would be from an
outer scope, etc.
This is without precedent in JS.
Since Kevin was really getting at where the implicit yield goes in
function* f(a1 = E1, ~~~ aN = EN) {
~~~
}
where ~~~ is meta-ellipsis, and he agreed the activation scope for all
parameter default expressions is simpler (and thanks to both of you for
clarifying and separating issues here), we're left with the implicit
yield placement issue.
I wonder whether we can't shed light on this issue by looking at the
thinnest basis case of a generator with an observable default parameter:
let n = 0;
function *gen(a = ++n) {}
for (let x of gen()) {}
assert(n === 1);
Does this help?
/be
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