If this change proves to be really disruptive to add-on authors, I wonder if we
could just make "let" behave like "var" for add-ons? The JS tokenizer could
just substitute var for let when parsing code from an add-on compartment. (Bug
1030420 will put add-on code in a separate compartment and it's ready to land
soon.) I think that would mostly mitigate the concerns Jeff raised in the
previous thread about having to test two configurations.
-Bill
----- Original Message -----
> From: "Shu-yu Guo" <s...@mozilla.com>
> To: "Chris Peterson" <cpeter...@mozilla.com>
> Cc: dev-platform@lists.mozilla.org
> Sent: Wednesday, September 17, 2014 5:26:57 PM
> Subject: Re: ES6 lexical temporal dead zone has landed on central
>
> Well, SM's 'let' extension never really let you redeclare let bindings. What
> happened was that it was too much work to parse function body-level lets as
> actual lets, and so they were parsed as vars, thus allowing
> "redeclarations".
>
> Keep in mind that vars aren't *really* redeclared. When actual semantics is
> closer to erasure semantics: just pretend the second var isn't there. That
> is, consider
>
> var x = 42;
> var f = function () { print(x); }
> var x = 43;
> var g = function () { print(x); }
>
> f and g above are closing over the *same* binding of x.
>
> I would bet that Rust lets you actually redeclare (viz. shadow with a new
> binding in the same scope) , such that if you had the equivalent code in
> Rust above, f and g would close over *different* bindings.
>
> So having lets behave like vars in that respect is probably going to lead to
> the same crappiness that we have with vars now. Why they didn't allow
> shadowing with a new binding? I don't know, it would be nice -- but perhaps
> was too big a break, and that it would behave strangely, or at least
> surprisingly, with hoisting semantics.
>
> ----- Original Message -----
> From: "Chris Peterson" <cpeter...@mozilla.com>
> To: dev-platform@lists.mozilla.org
> Sent: Wednesday, September 17, 2014 4:37:17 PM
> Subject: Re: ES6 lexical temporal dead zone has landed on central
>
> On 9/15/14 4:43 PM, Shu-yu Guo wrote:
> > If you work with JS that contains `let` bindings, you may start
> > encountering
> > the following two errors:
> >
> > 1. TypeError: redeclaration of variable foo
> >
> > To fix, rename the variable or remove the extra `let` if you are
> > assigning to an already-bound variable.
> >
> > These are static errors. You may pass your JS through the syntax
> > checker
> > in the SpiderMonkey shell (-c) to detect them.
>
> Much of the `let` fallout being reported is from variable
> redeclarations. For compatibility, perhaps TC39 should reconsider
> whether `let` redeclarations are worthy of being static errors.
>
> JS allows you to redeclare vars. Rust allows you to redeclare variables
> with `let` (even changing the type!). SpiderMonkey's non-standard JS1.8
> allowed you to redeclare variables with `let` (until Shu made it ES6
> compatible). Maybe variable redeclarations are not such a big problem
> for JS developers.
>
> chris
>
>
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