Subset relation (was: RE: ES3.1 Draft: 11 June 2008 version available)

[Brendan Eich]

On Jun 16, 2008, at 10:48 PM, Mark S. Miller wrote:

> On Mon, Jun 16, 2008 at 10:19 PM, Brendan Eich  
> <brendan at mozilla.org> wrote:
>> I am not going to repeat what I wrote at an earlier point in this  
>> thread (13
>> June at 10:24) -- you didn't reply to what I wrote then. Did that  
>> message
>> not reach you?
>
> Are you referring to
> <https://mail.mozilla.org/pipermail/es3.x-discuss/2008-June/ 
> 000108.html>?

Yes.

> It was the closest match I could find. I responded to this message.
> What remaining point in this message do you feel still needs to be
> addressed? I'm not being difficult. I just reread this message and
> couldn't spot it.

You replied only to the bit about reformed with, where I wrote:

> "Reformed with" was an attempt to restore lexical scope by exact  
> type annotation. That's what people voted down, not the ES1-3  
> "with" statement.


but that was not the main point (since reformed with was withdrawn),  
it was just setting the record straight ("reformed with", not "strict  
with" -- and the fact that it was voted down with red on the  
spreadsheet does not argue against plain old "with" being allowed in  
strict mode).

The main point was that (a) 'with' is widespread and popular;  
therefore (b) strict mode that bans 'with' could fail to be used.


>> The question isn't whether an existing statement is "good enough",  
>> it's
>> whether a strict mode that bans it is "usable enough".
>
> A strict mode which doesn't ban is clearly not.

Why "clearly"? Usability depends on users and "ergonomics". Something  
about 'with' is usable enough that users persist in writing programs  
using it. These users say (when they speak up coherently at all) that  
'with' makes the language more convenient. Well-known JS hackers say  
this, to me even, and get annoyed by nagging such as was found in  
older Firefoxes (console warnings about "deprecated with").


> If you get rid of "with", then the static analysis rule in ES4 becomes
> very simple: all free variables in a program (script, compilation
> unit, whatever) are global references, to be looked up as properties
> of that program's global object, whether or not those properties are
> present.

That allows lexical-reference typos through to run-time, where they  
become errors -- that is not something the old, withdrawn strict mode  
in ES4 allowed.

It's true that 'with' prevents analysis from deciding where a free  
name will be found, but with the old strict mode, that's actually a  
useful escape hatch. Otherwise (no 'with') the name would have to  
resolve to a compiler-visible global definition, or you would have to  
reach for eval.


>> This old notion of strict mode was to be an optional feature, at the
>> implementation's discretion. We dropped it in favor of 'use  
>> strict' a la
>> Perl -- "use good taste and sanity".
>
> And is "with" either in good taste or sane?

I avoid 'with', but I try not to confuse my taste with others' tastes  
(plural), or with necessity. Why is it necessary for strict mode to  
ban 'with'?


> The global object makes the contents of the global scope unknown. But
> it does not ambiguate which variable name occurences are to be
> interpreted as references into this global scope. Without "with", ES4
> strict scopes would be statically analyzable. I'm surprised you're
> willing to give that up.

As I wrote previously, all browser implementations have to support  
'with' and deoptimize code in its body. There's no savings to be had  
in rejecting it from strict mode, and doing so takes a tiny bit of  
extra code. On the other hand, such a strict mode may be less used  
than 'with', because of 'with' perduring.

Is 'with' any worse than eval, for the purposes of the analysis you  
have in mind, if you already allow the "operator" form of eval in  
strict mode?


>> so is kicking 'with' out of
>> strict mode worth it, especially if it impairs adoption of "use  
>> strict"?
>
> Yes. Otherwise I don't see the point of "use strict".

Can you define the point of "use strict" other than by appealing to  
taste?

/be
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mail.mozilla.org/pipermail/es-discuss/attachments/20080616/f3533959/attachment-0002.html