Rationale of indirect eval and its subtle features

Dmitry A. Soshnikov dmitry.soshnikov at gmail.com
Sat Feb 19 13:54:09 PST 2011


On 19.02.2011 21:23, Allen Wirfs-Brock wrote:
> The distinction between a direct eval and an eval first appears in 
> Ecma-262, Edition 2 which said: "If value of the *eval *property is 
> used in any way other than a direct call (that is, other than by the 
> explicit use of its name as an /Identifier /which is the 
> /MemberExpression /in a /CallExpression/), or if the *eval *property 
> is assigned to, an *EvalError *exception may be thrown."
>
> Note that the ES2 definition left support for indirect eval as an 
> implementation option.  Some browsers tried to implement throwing on 
> EvalError but they discovered that this was incompatible with the web. 
>  So they implemented globally scoped indirect eval which proved to be 
> compatible with the web.  By the time of the drafting of ES5 the 
> situations was that that some browsers implemented indirect and direct 
> evals identically and some browsers implemented globally scoped 
> indirect eval. No browser made use of the specification permitted 
> EvalError.
>

Yeah, I see. So this is all about the lexical (static) scope. I.e. the 
closured lexical environment of the built-in `eval` is the global 
environment, therefore it has access only for the global bindings, right?

It seems also explains that the `Function`'s code is always evaluated in 
the global context -- it's in contrast with direct `eval`'s which isn't 
a function call at all. Is it so (regarding `Function`)?

> For ES5 we wanted to eliminate any implementation options from the 
> eval spec and to make it compatible with the web.  Globally scoped 
> indirect eval served was the solution we choose. The rest of the 
> details including the treatment of strict code follow from that decision.
>

Yep, it's clear.

> Also note that the term  "indirect eval" is never used in the 
> normative parts of the ES5 specification.  It only distinguishes 
> "direct calls to eval" as the special case.  The term "indirect eval" 
> is only used in Annex D.
>

Yes, though, exactly the term "indirect" became the most popular in this 
case.

>
>
>>> On 18.02.2011 20:04, Allen Wirfs-Brock wrote:
>>>> Here is another way to think about it.
>>>>
>>>> The built-in function object that is the initial value of the 
>>>> global  "eval"  (defined in 15.1.2.1) is a non-strict function. 
>>>> Hence anything it does defaults to "non-strict" and it uses the 
>>>> global environment as the VariableEnvironment for the code it 
>>>> evaluates.  However, if the argument code contains an use strict 
>>>> directive, then it uses a new child environment of the global 
>>>> environment as the VariableEnvironment.   This is all specific 
>>>> defined behavior of the built-in eval function object and is 
>>>> completely independent of who calls the function or how the caller 
>>>> accessed the function.
>>>>
>>>
>>> Yes, it's clear how it looks/implemented _technically_, however 
>>> doesn't explain a practical rationale. So at the moment I have the 
>>> following conclusion and understanding (please correct me if I'm 
>>> mistaken):
>>>
>>> An indirect `eval` introduced into ES spec not because of some/any 
>>> security reason (what I mentioned earlier in my explanations), and 
>>> even not because of some _practical_ reasons, but just because of 
>>> _complexity of the implementation_. I.e. _initially_ indirect `eval` 
>>> is related with the implementation level only (only after that JS 
>>> devs adapted this _technique_ for already mentioned getting of the 
>>> global object in strict mode).
>>>
>>> Is this explanation correct?
>
> It's not how I would state it.  As a general design principle the 
> behavior of any function, when called should only depend upon the 
> environment it was defined in and the arguments that are explicitly 
> passed to it.

Yes, as I mentioned -- a normal lexical scope implementation.

>  eval, as a built-in function, is defined within the global 
> environment and it has no explicit arguments that expose the scope of 
> its caller.  Hence, it only is allowed to manipulate the global scope.
>

Yes, only if an implementation can handle its invocation differently. I 
see that you say that built-in `eval` is just a simple function which 
obey the same lexical scoping rules and therefore all the identifiers 
lookup is made in statically saved environment.

On the other hand, why the implementation cannot treat the `eval`'s 
activation differently? Why not to save its original lexical environment 
(that is, global), set it to the environment of the caller and after the 
evaluation again restore it?


> Direct eval, is the "odd duck".  There is no normal function mechanism 
> that permits its behavior.   Rather than extending the semantics of 
> function calls in general (any call may be an indirect eval call) we 
> special case the handling of direct eval in 10.4.2 step 2.
>

Hm.. and this is the answer on my previous question. No special 
semantics with replacing saved lexical environment.

> Implementation impact is something we're alway considering, but it 
> isn't needed to justify this behavior.  If you are going to treat eval 
> as a normal function object then it needs to be callable as such.
>

Yes, it's clear now -- the lexical scope and a common functions' 
behavior -- that's the main reason.

>
>>>
>>>> An "indirect eval" is any invocation of the above function.
>>>>
>>>
>>> This is a subtle case, thanks (taking into account your following 
>>> explanation of with the inlining the code to the call site). So, 
>>> _abstractly_ (? -- I can't say precisely whether it's _technically_ 
>>> though, since I'm not the implementer), if _syntactically_ we use 
>>> this form:
>>>
>>> eval(...)
>>>
>>> it isn't even an `eval` call, but just an inlining. I can imagine it 
>>> abstractly as (with providing a "sandbox" environment):
>>>
>>> (function foo() {
>>>   eval("var x = 10;")
>>> })();
>>>
>>> is _abstractly_ desugars (inline) into:
>>>
>>> (function foo() {
>>>   (function __directEval(configurableBindings?) {
>>>     var x = 10;
>>>   })(true);
>>> })();
>>>
>>
>> Note: this abstract representation doesn't explain also dynamic 
>> eval's string. i.e. when some variable should be determined at 
>> runtime, when the environment is formed:
>>
>> (function foo(x) {
>>   eval("var x = " + x + ";")
>> })(10);
>>
>> How here the inlining can work?
>
> The declaration instantiation rules get in the way of these simple 
> inlining models.  Consider this:
> function outer() {
>    var x=0;
>    (function () {
>       x=1;
>       var x=2}
>     )();
>    return x;   //returns 0
> }
>
> compared to
>
> function outer() {
>    var x=0;
>    (function () {
>       x=1;
>      eval(" var x=2");}
>     )();
>    return x;   //returns 1
> }
>

Yeah, I know that the "hoisting" breaks my simplified model of `eval`'s 
inlining, but I wanted just to imagine how it is made. If to exclude the 
"hoisting" (i.e. all definitions are made sequentially in runtime), then 
direct `eval`'s desugars quite normally.

Thanks, again Allen, I think at this step it became completely clear. 
(probably I'll write a small blog-post note with explaining all this 
suble cases and especially why Function("...") in contrast with (direct) 
`eval` executes in the global scope).

Dmitry.

>>
>>> From this position it looks just an optimization technique. And if 
>>> we do this:
>>>
>>> var myEval = eval;
>>> myEval("var x = 10;");
>>>
>>> we already can't inline the `eval` at parsing stage (?), right? And 
>>> if we'd to pass the information about strictness of the surrounding 
>>> context, we just should pass additional argument the `myEval`, yep? 
>>> Though, I don't see which issue this passing of this additional 
>>> argument can lead (which by the way?).
>>>
>>>> A "direct eval" is essentially a special syntactic from in the ES5 
>>>> language with its own specific semantics. Some people have talked 
>>>> about it as the "eval" operator.  A direct eval does not call the 
>>>> built-in eval function object.  Instead, it has its own slightly 
>>>> different semantics that can be directly implemented at the call site.
>>>
>>> Yes, it was new for me (and for any JS dev I guess). And how it 
>>> looks I tried to describe above (is that understanding is correct?).
>>>
>>>>  These semantics include access to the immediately surrounding 
>>>> LexicalEnvironment and the propagation of strictness into the eval 
>>>> code.
>>>>
>>>
>>> Yep, if to accept the inheriting of the strictness by the inner 
>>> functions (with which I desugared the eval inlining).
>>>
>>> Thanks again, Allen, it seems clear now. But the main thing which I 
>>> found out from it, is that _initially_ it's _not about 
>>> JS-programming_, it's just an _optimization technique_ for the 
>>> _implementers_.
>>>
>>> Dmitry.
>>>
>>>> Allen
>>>>
>>>>
>>>> On Feb 18, 2011, at 4:52 AM, Dmitry A. Soshnikov wrote:
>>>>
>>>>> Have no idea what with mail sever again, there is no this message 
>>>>> there (links in message?). The original question is below:
>>>>>
>>>>> On 18.02.2011 11:51, Dmitry A. Soshnikov wrote:
>>>>>> Hello,
>>>>>>
>>>>>> As is known, indirect `eval` evaluates in the global context 
>>>>>> (this note is not for the TC-39 group of course, but for all 
>>>>>> other who read this list). But what is the big rationale of that 
>>>>>> indirect eval is allowed to create a global binding even 
>>>>>> _regardless_ the fact that the global code is _strict_?
>>>>>>
>>>>>> I.e.:
>>>>>>
>>>>>> "use strict";
>>>>>>
>>>>>> eval("var x = 10;"); // direct
>>>>>> this.eval("var y = 20;"); // indirect
>>>>>>
>>>>>> console.log(typeof x, typeof y); // "undefined", "number"
>>>>>>
>>>>>> The only _practical_ rationale of the indirect `eval` regarding 
>>>>>> the strict mode is that via its call it's possible to get the 
>>>>>> global object via `this` value from any place (since just 
>>>>>> returning `this` won't work as it's set to `undefined` in a 
>>>>>> simple function call):
>>>>>>
>>>>>> var global = (function () { return this; })(); // ES3, non-strict 
>>>>>> ES5 way
>>>>>>
>>>>>> var global = ("indirect", eval)("this"); // strict ES5
>>>>>>
>>>>>> Which else practical/theoretical/academical rationales of the 
>>>>>> indirect eval are? And what again the rationale that it's allowed 
>>>>>> to create the global binding (i.e. not in the "sandbox" 
>>>>>> environment, but reusing `VariableEnvironment` of the "caller" -- 
>>>>>> not a direct caller though, but the adjusted -- the global 
>>>>>> scope)? Could you clarify this, I want to give a more concrete 
>>>>>> explanations for my readers.
>>>>>>
>>>>>> P.S.:
>>>>>>
>>>>>> Just recently Allen brought a good addition (well, a reminder 
>>>>>> even better to say, since I also wanted to add it ;) to mention 
>>>>>> this subtle case with indirect eval and it's ability to affect 
>>>>>> the global environment regardless the strict mode: 
>>>>>> http://dmitrysoshnikov.com/ecmascript/es5-chapter-2-strict-mode/#comment-5831
>>>>>>
>>>>>> It's without doubts a very needed addition and as is said -- a 
>>>>>> subtle case, but still -- what are the exact rationales for that? 
>>>>>> I mentioned this case in Allen's blog before, and though that 
>>>>>> this is a bug of the spec -- 
>>>>>> http://www.wirfs-brock.com/allen/posts/39#comment-46
>>>>>>
>>>>>> Allen answered that this is related with complexity of 
>>>>>> implementation, here:
>>>>>>
>>>>>> "An /indirect/ eval called from strict code is strict only if the 
>>>>>> eval code explicit contains an “use strict;” directive. It does 
>>>>>> not “inherit” the strictness of its caller.
>>>>>>
>>>>>> Every call is potentially an indirect eval call. If the 
>>>>>> strictness of indirect evals depended upon the strictness of the 
>>>>>> caller then we would have to pass the strictness of the caller 
>>>>>> through every call. That would require either an extra implicit 
>>>>>> parameter on every call or the ability to a callee to examine the 
>>>>>> call stack to to determine the strictness of its caller. We don’t 
>>>>>> want to force this on implementations."
>>>>>>
>>>>>> The basic idea is seems clear -- we need to pass a flag that the 
>>>>>> code of an inner function (from which indirect `eval` was called) 
>>>>>> is strict, but if the _global_ code is strict -- and indirect 
>>>>>> `eval` always evaluates in the global context, and moreover, if 
>>>>>> the global code is strict that means _any_ inner function (except 
>>>>>> those which are created via `Function` constructor and have no 
>>>>>> own strictness) is automatically also strict:
>>>>>>
>>>>>> "use strict";
>>>>>>
>>>>>> (function foo() {
>>>>>>   // this code is also strict
>>>>>>   ("indirect", eval)("var x = 10;");
>>>>>> })();
>>>>>>
>>>>>> console.log(x); // 10
>>>>>>
>>>>>> is it also required to pass special flags to handle this case?
>>>>>>
>>>>>> Thanks,
>>>>>> Dmitry.
>>>>>
>>>>> _______________________________________________
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>>>>> https://mail.mozilla.org/listinfo/es-discuss
>>>>
>>>
>>
>

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