No more modes?

Brendan Eich brendan at
Thu Oct 14 10:14:21 PDT 2010

On Oct 14, 2010, at 9:21 AM, Mark S. Miller wrote:

> I can we why it seems that I was implying that, but I'm making more modular arguments than that.

I appreciate modular arguments and expect no less from you :-). But however modular, the bigger issues I keep harping on remain. Still, this is a good separate topic so I'll add something.

> This issue *by itself* suggests that a harmony opt-in should be managed by a prologue / pragma at the beginning of a Program production[1], rather than an attribute on a script tag, so it can follow the same opt-in logic as "use strict" and apply to eval code as well.

Maybe, but ES5 direct eval from strict code uses strict mode in the eval'ed program. No directive or pragma in that eval'ed program's source string (passed as the direct eval's argument) is needed.

So the directive may be a good idea (more below), but it is not necessary by the design of ES5 strict mode and direct eval. The same seems to apply to Harmony opt-in.

> Thus,
>     e2(' "use harmony"; var module = 1');
> could be illegal on browsers supporting harmony. And such evaled code would also not have the global object at the bottom of their scope chain. This in-language switch makes more sense to me than a markup-based switch such as <script ...>, and would allow the switch to be recognized in non-browser environments such as commonjs.

The problem remains syntax errors in top-level script. Developers are generally not going to write Harmony script if the only way to load it is via XHR, or else to encode it with escapes and so on in string literals fed to eval.

It's not an "either/or", though: script versioning a la RFC4329 and a "use harmony" pragma could both be worth having.

But if you only have "use harmony", then you cannot use it in script tags without inevitable syntax errors in old browsers on the new syntax that follows the pragma. This means you have to eval, and this is both onerous for developers and expensive (and different from script-loading) at runtime.

> I will address the more general "more modes" and compatibility direction questions for later messages.

Please, let's get to this. Otherwise more-modular arguments about "use harmony" (which is still a mode, I note!) do not help resolve the crucial issues: fallback content selection for old browsers.


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