On Fri, Feb 14, 2014 at 7:40 AM, Allen Wirfs-Brock <[email protected]>wrote:
[...]
>
> 1) Extend the ES5 semantics to include the new declaration forms. For
> example:
>
> (function() {
> eval("let answer=42");
> console.log(answer); // 42
> })();
>
> 2) Use the strict mode binding semantics if the eval code directly
> contains any of the new declaration forms:
>
> (function() {
> eval("
> var answer=42;
> let forceSeprateEnvironment = true;
> ");
> console.log(answer); // ReferenceError: answer is not defined
> })();
>
> 3) Combination. use ES5 non-strict binding semantics for var/function
> declarations but place let/const/class bindings into a per eval environment:
>
> (function() {
> eval("
> var answer=42;
> let localToEval = true;
> ");
> console.log(answer); // 42
> console.log(localToEval); // ReferenceError: localToEval is not
> defined
> )();
>
>
> It would certainly be possible to specify #1, but I don't like it. Other
> than for the global environment it would be cleaner if the new block
> scope-able declarations were never dynamically added to the environment.
>
> I think either #2 or #3 is plausible. #2 is a simpler story but
> introduces a refactoring hazard. If you have some existing eval code that
> defines some "global" functions or variables, then simply adding a
> let/const/class declaration to the eval code ruins those global
> declarations.
>
> I prefer the simplicity of #2, but I also worry about the WTF impact it
> might have on evolving existing code.
>
> Can we get away with #2, or are we going to have to go with #3? Are there
> other alternatives?
>
I actually prefer #3. Given only knowledge of ES5 and of the rest of ES6, I
find it least surprising. "var"s hoist out of blocks. In non-strict code,
"function"s leak out of blocks in ways that are hard to explain. I can
understand non-strict direct eval as being block-like, in that "var" and
"function" leak out of them, but all the reliably block-local declarations
stay within the direct eval.
Also, I buy the refactoring issue. It's like the problem with micro-modes:
bizarre and unexpected non-local influences.
--
Cheers,
--MarkM
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