In what ways does the following "eval" regularity break?

Mark S. Miller erights at google.com
Wed Oct 29 23:23:42 PDT 2008


The intent of ES3's eval operator is clearly that it at least
approximate the following result:

For certain triples of source code text:
    <some code a> <some code b> <some code c>
is equivalent to
    <some code a> eval("<some code b>") <some code c>

By placing quotes around <some code b>, I am not suggesting literally
surrounding it with quotes, but rather turning it into a string
literal whose value is the original code string.

Certainly, we understand that <some code b> must be balanced and must
either be an expression, a statement, a block, or a function body.

Even with these restrictions, there are some cases that don't work:

    function foo() {eval('return 3;');}
    foo();
    SyntaxError on line 1: return not in function


This is all intriguingly similar to our previous discussions of
Tennent Correspondence (TC), to whit:

The intent of ES3's function abstraction is clearly that it at least
approximate lambda abstraction. If it actually provided proper lambda
abstraction, we'd have TC (though ES is better than the vast majority
of other languages in this regard). By understanding the ways in which
ES3's function violates TC, we come to a better understanding of how
to use it well, and of what hazards to watch out for. Our discussions
on this list have really helped my understanding of function.
Likewise, with eval, if we understand better in what ways it violates
the regularity it approximates so well, we'll understand better the
hazards to watch out for in the occasional uses we encounter.

A case I'm particularly interested in is when <some code b> is the
body of a function. Other than the return issue illustrated above, in
what other ways does ES3's eval() violate this regularity? One place
to look is the difference between the Program production + the notion
of eval context on the one hand vs the function body production + the
notion of function context on the other hand. In ES3.1, we allow the
"use strict" directive only at the beginning of Program units. If one
tried to get the effect of turning strictness on for an individual
function body by evaling a Program string beginning with "use strict"
that otherwise contained the function's original code, performance
aside, and other than the above "return" issue, how else might one get
into trouble?

In a similar vein, in what ways might

    new Function("<some code b>")
differ in meaning from
    function(){eval("<some code b>");}

The obvious one: In the latter, <source code b> can refer to
non-global variable that are in scope at eval's site. What else?

-- 
    Cheers,
    --MarkM


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