"use subset" introductory material

Brendan Eich brendan at mozilla.org
Mon Jun 30 23:31:08 PDT 2008

On Jun 30, 2008, at 9:08 PM, Mark S. Miller wrote:

> [+es4-discuss]
> On Mon, Jun 30, 2008 at 7:37 PM, Maciej Stachowiak <mjs at apple.com>  
> wrote:
>> JSON be handled with a generic subset mechanism? I expect not,  
>> since a
>> pragma inside the JSON source in the form of an initial quoted  
>> string would
>> be (a) invalid JSON and (b) ineffective as a way to validate  
>> incoming JSON,
>> since malicious alleged JSON would not use such a pragma.
> Whether or not it's a good idea, given "use subset JSON" as a
> recognized/enforced subset directive, one could trivially implement
> JSON.stringify(str) in terms of
>     eval('"use subset JSON"; (' + str + ')')

That's nothing like how JSON parsing is implemented in Mozilla. If  
the idea is to add a mode to the ES parser, then I'm worried about  
missed exclusion tests and false economies in a hacked up JS parser  
trying to serve two (or more) masters.

JSON is defined by http://www.ietf.org/rfc/rfc4627.txt -- not by  
ES1-3 or any future spec. It's a "subset" of Python and other  
languages -- it's more accurately its own language. It's better off  
with ts own parser implementation, unit tests, etc. -- browsers want  
this for application/json handling anyway (no pragma or restrictive  
API mode required).

Given that JSON.stringify is a proposed extension in ES3.1 (and was  
slated to be in ES4, after we rejected the old json.org API), why  
does the above trivial (except for possibly non-trivial risks in  
subsetting a real JS parser) re-implementation via eval matter?

>> I do think JSON should be supported natively, but it does not seem  
>> at all
>> analogous to strict mode / cautious subset.
> I think I agree. In any case, I agree that JSON is not by itself a
> compelling case for "use subset X". My point is only that JSON is a
> huge counter-example to Brendan's statement that "profiled (subsetted)
> standards are meaningless to harmful on the web".

JSON is not huge, and that's one point in favor of keeping it  
separate from ES futures. It is not defined as a subset in any ES spec.

It's also not an intentional, new-in-the-last-month, paper-spec-only  
subset of JavaScript -- it is a subset after the fact. As Doug has  
written, he "discovered" it. Inventing new, multiple, as-yet-unused  
subsets for ES3.1 -- and not implementing any of them in any  
experimental-to-beta released browser, especially not in IE8 -- is a  
bad idea. It will cause general and widespread opposition to any  
attempt to standardize such a ES3.1 this year.

At least OOXML and E4X (to name two Ecma standards of mixed repute)  
each had one implementation -- however buggy or deficient some have  
argued those specs and their single implementations were. ES3.1 has  
none, not even a buggy work-in-progress reference implementation.

My point is to recall the original "ES3 + reality" anti-mission-creep  
goal for ES3, which you among others espoused. Right now it's on a  
road to completion at the same time frame as a cut-down ES4, which  
will make for a busy 2009 -- assuming its supporters actually  
demonstrate it in several testable, interoperating implementations.

> JSON was defined as
> an enforced subset of JavaScript, and it has been extraordinarily
> helpful to the web.

Except where people used JS parsers naively. Which is one variation  
on a theme that you are still playing in advocating "use subset JSON".


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