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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