Strict mode eval

Andreas Rossberg rossberg at google.com
Fri May 13 02:05:13 PDT 2011


On 13 May 2011 01:50, Mark S. Miller <erights at google.com> wrote:
>> Assume that
>>
>> - we distinguish two variants of the eval function, strict and
>> non-strict -- let's call these values EVAL_s and EVAL_ns.
>> - initially (in global scope), the identifier `eval' is bound to EVAL_ns.
>> - in a strict mode scope it will be considered rebound to EVAL_s
>> instead (unless it has already been shadowed by user code anyway).
>>
>> (In addition, at least in strict mode, the only calls to `eval' that
>> are considered _direct_ calls would be those where `eval' statically
>> refers to the initial binding or one of the implicit strict-mode
>> rebindings -- i.e., where it has not been shadowed by the user.)
>
> I think the core insight here is good, and had it been made in time, could
> have led to a better semantics than what we adopted into ES5. I like the
> idea that ' "use strict";' effectively inserts a
> DeclarativeEnvironmentRecord binding 'eval' to EVAL_s, though I'd put this
> record on the stack at the strict/non-strict boundary rather than just above
> the global object.

Yes, my previous description of "shadowing" `eval' at the point of
"use strict" was meant to describe just that.

> Even better, since 'eval' cannot be rebound by ES5/strict, ES-next, or SES
> code, and since "eval(str)" is effectively a special form anyway, why not
> remove the dynamic "and if 'eval' is bound to the original global eval
> function" condition from direct eval? Why not just treat "eval(str)" as a
> direct eval special form independent of what 'eval' is bound to in that
> scope?

That's what I tried to suggest in the parenthesized paragraph above,
and it was the reason for my question to Allan.

The difficulty in ES5 would be that scoping is not really static --
not even in strict-mode code, which might still be surrounded by
non-script scopes shadowing `eval' dynamically (esp `with'). But for
Harmony it'd be nice.

Thanks,
/Andreas


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