On 11/25/2013 04:48 PM, Brendan Eich wrote:
Brendan Eich wrote:
Kevin Smith wrote:
This makes for wtfjs additions, but they all seem non-wtf on
reflection (or did to us when Waldemar threw them up on a
whiteboard last week). By non-wtf, I mean anyone who groks that
yield is reserved only in function* can work them out.
The star after function really helps. ES5's "use strict" directive
prologue in the body applying to its left (even in ES5 --
duplicate formals are a strict error) is goofy.
Agree on all counts, but not quite understanding yet.
Say I'm parsing this, and the token stream is paused at the "#":
function(a = # yield
I assume that we're not un-reserving yield in strict mode. That means that I
don't know whether to treat `yield` as an identifier or reserved word until I
get to that goofy prologue.
Ouch, you're right. We can't handle this without backtracking. Waldemar should
weigh in.
Well, we can handle it. We know due to lack of * after function that yield, whether reserved (due
to "use strict"; in body prologue) or not, can't be yield-the-operator. So it's either an
identifier (no "use strict";) or a reserved word (and an error due to lack of * after
function).
So we parse it as an identifier, just as we parse duplicate formal parameters. Then if we
see "use strict", we must post-process the parse tree and throw an error. Kind
of a shame, but there it is.
At least reserving 'let' in ES5 strict did some good!
/be
For another example of why keying off generator/non-generator instead of strict
mode for the parsing of yield is the right thing to do:
function*(a = yield/b/g) {
a = yield/b/g;
}
One of these is a regexp. The other is a couple divisions.
Get this wrong and you can introduce security problems.
Waldemar
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