On Sep 13, 2013, at 9:58 AM, Andreas Rossberg <rossb...@google.com> wrote:
> On 13 September 2013 18:39, Oliver Hunt <oli...@apple.com> wrote:
>> The problem with temporal dead zones is that they lead to weird behaviour in
>> edge cases, and almost all of them should be syntactically identifiable as
>> errors up front. The problem is that you can only _almost_ get syntax
>> checked behaviour upfront because eval.
>
> There must be a misunderstanding -- eval is not relevant to the issue
> that TDZ is addressing. The issue is mutual recursion between
> bindings that can perform arbitrary (higher-order) computations. In
> that situation, it is generally impossible to ensure the absence of
> errors without a type system. And even with one, it is tedious to the
> degree that it is arguably impractical. So, as long as JavaScript
> allows that (and I don't see how we can change that), TDZ is the
> best/only we can do.
>
> What "weird edge case" do you have in mind? I cannot imagine anything
> that isn't even weirder without TDZ.
>
It isn't eval doing anything magic here - you can syntactically identify and
use of not yet live properties, except eval(), e.g.
function f1(a=b, b=...) ... // can syntax error easily
function f2(a=function () {return b}, b=...) ... // again you can syntax error,
or you can be clever and say we're not calling it and not error
function f3(a=function () {return b}(), b=...) ... // syntax error as we can
prove you're going to use b
function f4(a=eval("b"), b=...) ... // eval makes things opaque
Personally I don't believe TDZ's are a good solution - they essentially add
another null/undefined value internally. They make things more complex for
developers.
I certainly don't get what the TDZ win is for default parameters, and example
would help me reason about it.
> /Andreas
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