ECMAScript Harmony

# Brendan Eich (9 years ago)

It's no secret that the JavaScript standards body, Ecma's Technical
Committee 39, has been split for over a year, with some members
favoring ES4, a major fourth edition to ECMA-262, and others
advocating ES3.1 based on the existing ECMA-262 Edition 3 (ES3)
specification. Now, I'm happy to report, the split is over.

The Ecma TC39 meeting in Oslo at the end of July was very productive,
and if we keep working together, it will be seen as seminal when we
look back in a couple of years. Before this meeting, I worked with
John Neumann, TC39 chair, and ES3.1 and ES4 principals, especially
Lars Hansen (Adobe), Mark Miller (Google), and Allen Wirfs-Brock
(Microsoft), to unify the committee around shared values and a common
roadmap. This message is my attempt to announce the main result of
the meeting, which I've labeled "Harmony".

Executive Summary

The committee has resolved in favor of these tasks and conclusions:

  1. Focus work on ES3.1 with full collaboration of all parties, and
    target two interoperable implementations by early next year.
  2. Collaborate on the next step beyond ES3.1, which will include
    syntactic extensions but which will be more modest than ES4 in both
    semantic and syntactic innovation.
  3. Some ES4 proposals have been deemed unsound for the Web, and are
    off the table for good: packages, namespaces and early binding. This
    conclusion is key to Harmony.
  4. Other goals and ideas from ES4 are being rephrased to keep
    consensus in the committee; these include a notion of classes based
    on existing ES3 concepts combined with proposed ES3.1 extensions.

Detailed Statement

A split committee is good for no one and nothing, least of all any
language specs that might come out of it. Harmony was my proposal
based on this premise, but it also required (at least on the part of
key ES4 folks) intentionally dropping namespaces.

This is good news for everyone, both those who favor smaller changes
to the language and those who advocate ongoing evolution that
requires new syntax if not new semantics. It does mean that some of
the ideas going back to the first ES4 proposals in 1999, implemented
variously in JScript.NET and ActionScript, won't make it into any ES
standard. But the benefit is collaboration on unified successor
specifications to follow ES3, starting with ES3.1 and continuing
after it with larger changes and improved specification techniques.

One of the use-cases for namespaces in ES4 was early binding (use
namespace intrinsic), both for performance and for programmer
comprehension -- no chance of runtime name binding disagreeing with
any earlier binding. But early binding in any dynamic code loading
scenario like the web requires a prioritization or reservation
mechanism to avoid early versus late binding conflicts.

Plus, as some JS implementors have noted with concern, multiple open
namespaces impose runtime cost unless an implementation works
significantly harder.

For these reasons, namespaces and early binding (like packages before
them, this past April) must go. This is final, they are not even a
future possibility. To achieve harmony, we have to focus not only on
nearer term improvements -- on "what's in" or what could be in -- we
must also strive to agree on what's out.

Once namespaces and early binding are out, classes can desugar to
lambda-coding + Object.freeze and friends from ES3.1. There's no need
for new runtime semantics to model what we talked about in Oslo as a
harmonized class proposal (I will publish wiki pages shortly to show
what was discussed).

We talked about desugaring classes in some detail in Oslo. During
these exchanges, we discussed several separable issues, including
classes, inheritance, like patterns, and type annotations. I'll avoid
writing more here, except to note that there were clear axes of
disagreement and agreement, grounds for hope that the committee could
reach consensus on some of these ideas, and general preference for
starting with the simplest proposals and keeping consensus as we go.

We may add runtime helpers if lambda-coding is too obscure for the
main audience of the spec, namely implementors who aim to achieve
interoperation, but who may not be lambda-coding gurus. But we will
try to avoid extending the runtime semantic model of the 3.1 spec, as
a discipline to guard against complexity.

One possible semantic addition to fill a notorious gap in the
language, which I sketched with able help from Mark Miller: a way to
generate new Name objects that do not equate as property identifiers
to any string. I also showed some sugar, but that is secondary at
this point. Many were in favor of this new Name object idea.

There remain challenges, in particular getting off of the untestable
and increasingly unwieldy ES1-3.x spec formalism. I heard some
generally agree, and no one demur, about the ES4 approach of using an
SML + self-hosted built-ins reference implementation (RI).

We are going to look into stripping the RI of namespaces and early
binding (which it uses to ensure normative self-hosted behavior, not
susceptible to "user code" modifying the meaning of built-ins),
simplifying it to implement ES3.1plus or minus (self-hosted built-ins
may require a bit more magic). More on that effort soon.

ES3.1 standardizes getters and setters that were first implemented at
Mozilla and copied by Apple and Opera. More such de-facto
standardization is on the table for a successor edition in the
harmonized committee.

I heard good agreement on low-hanging "de-facto standard" fruit,
particularly let as the new var, to match block-scoped const as still
proposed (IIRC) in 3.1. Also some favorable comments about simple
desugarings such as expression closures and destructuring assignment,
and other changes in JS1.7 and 1.8 that do not require new runtime
semantic models.

Obviously, these require new syntax, which is appropriate for a major
post-3.1 "ES-harmony" edition. Syntax is user interface, there's no
reason to avoid improving it. What's more, the intersection semantics
of extended ES3 implementations conflict and choke off backward- compatible semantics for syntax that may even parse in all top four
browsers (e.g., functions in blocks).

Both the appropriateness of new syntax, and the need to make
incompatible (with ES3 extensions) semantic changes, motivate opt-in
versioning of harmonized successor edition. I believe that past
concerns about opt-in versioning requiring server file suffix to MIME
type mapping maintenance were assuaged (browsers in practice, and
HTML5 + RFC 4329, do not consider server-sent Content-Type -- the web
page author can write version parameters directly in script tag type
attributes).

Some expressed interest in an in-language pragma to select version;
this would require immediate version change during parsing. It's a
topic for future discussions.

The main point, as important as cutting namespaces in my view, is
that the committee has a vision for extending the language
syntactically, not trying to fit new semantics entirely within some
combination of existing "three of four top browsers" syntax and
standard library extensions.

As Waldemar Horwat (Google) said on the final day, the meeting was
seminal, and one of the most productive in a long while. Much work
remains on 3.1 and Harmony, but we are now on a good footing to make
progress as a single committee.

# Brendan Eich (9 years ago)

In light of Harmony, and the recurrent over- and under-cross-posting,
I'd like to merge the es3.x-discuss at mozilla.org and es4- discuss at mozilla.org lists into es-discuss at mozilla.org. The old
archives will remain available via the web, and the old aliases will
forward to the new list. Any objections?

# Douglas Crockford (9 years ago)

Brendan Eich wrote:

In light of Harmony, and the recurrent over- and under-cross-posting,
I'd like to merge the es3.x-discuss at mozilla.org and es4- discuss at mozilla.org lists into es-discuss at mozilla.org. The old
archives will remain available via the web, and the old aliases will
forward to the new list. Any objections?

Yes. I'd like to keep ES3.x to discuss ES3.1 business until we get that finalized.

# Brendan Eich (9 years ago)

On Aug 13, 2008, at 2:36 PM, Douglas Crockford wrote:

Brendan Eich wrote:

In light of Harmony, and the recurrent over- and under-cross-posting, I'd like to merge the es3.x-discuss at mozilla.org and es4- discuss at mozilla.org lists into es-discuss at mozilla.org. The old archives will remain available via the web, and the old aliases will forward to the new list. Any objections?

Yes. I'd like to keep ES3.x to discuss ES3.1 business until we get
that finalized.

I think Allen Wirfs-Brock was in favor of merging the lists, but we
could just rename es4-discuss to es-discuss. Is it worth dropping
that 4?

# P T Withington (9 years ago)

[Trimmed recipients]

On 2008-08-13, at 17:26EDT, Brendan Eich wrote:

One possible semantic addition to fill a notorious gap in the language, which I sketched with able help from Mark Miller: a way to generate new Name objects that do not equate as property identifiers to any string. I also showed some sugar, but that is secondary at this point. Many were in favor of this new Name object idea.

Is this "Name object" what we old Lispers would call a SYMBOL?

# Brendan Eich (9 years ago)

On Aug 13, 2008, at 2:50 PM, P T Withington wrote:

[Trimmed recipients]

On 2008-08-13, at 17:26EDT, Brendan Eich wrote:

One possible semantic addition to fill a notorious gap in the language, which I sketched with able help from Mark Miller: a way to generate new Name objects that do not equate as property identifiers to any string. I also showed some sugar, but that is secondary at this point. Many were in favor of this new Name object idea.

Is this "Name object" what we old Lispers would call a SYMBOL?

I said "gensym" in Oslo and had it in a draft, but kids today do not
appreciate how l33t it is.

The inspiration came from several sources, and one idea for sugar
came from PLT-Scheme's define-local-member-name.

# Allen Wirfs-Brock (9 years ago)

I think Allen Wirfs-Brock was in favor of merging the lists, but we could just rename es4-discuss to es-discuss. Is it worth dropping that 4?

I think that "es-discuss" is a lot more "future proof" and now is as good a time to change as any.

As I've already mentioned to Brendan, I think it may be useful in the long term to separate detailed discussion of the "next" revision from longer term or speculative discussions. However, for now, we need to rebuild a single community and most of the ES3.x-discuss items are getting (or should be) cross posted to es4-discuss anyway. Given that, I think we should just merge the lists and if we find that 3.1 and post 3.1 items are getting intermingled in a confusing manner we can always split it out again. I suppose one way to preserve that option is for everyone to just stop posting to ES3.x-discuss and only post the new es-discuss.

# Maciej Stachowiak (9 years ago)

On Aug 13, 2008, at 2:30 PM, Brendan Eich wrote:

In light of Harmony, and the recurrent over- and under-cross-posting, I'd like to merge the es3.x-discuss at mozilla.org and es4- discuss at mozilla.org lists into es-discuss at mozilla.org. The old archives will remain available via the web, and the old aliases will forward to the new list. Any objections?

I support this move.

# Peter Michaux (9 years ago)

On Wed, Aug 13, 2008 at 2:26 PM, Brendan Eich <brendan at mozilla.org> wrote:

[snip]

We talked about desugaring classes in some detail in Oslo. During these exchanges, we discussed several separable issues, including classes, inheritance, like patterns, and type annotations. I'll avoid writing more here,

Is there more to read elsewhere? I'd like to know concretely what "desugaring classes" means.

[snip]

Thanks, Peter

# Kris Zyp (9 years ago)

We talked about desugaring classes in some detail in Oslo. During these exchanges, we discussed several separable issues, including classes, inheritance, like patterns, and type annotations. I'll avoid writing more here,

Is there more to read elsewhere? I'd like to know concretely what "desugaring classes" means.

I would too, I assume this is based on Mark's proposal for Classes as Sugar (www.mail-archive.com/es4-discuss@mozilla.org/msg01624.html). However, Mark's proposal seems pretty vague in the area of typing, so I am curious how this would work as well. Would there be new functionality/semantics for defining types on properties (via defineProperty in ES3.1, sugared in ES4) with some type of object structures like I suggested in mail.mozilla.org/pipermail/es4-discuss/2008-July/003321.html?

Kris

# Peter Michaux (9 years ago)

On Wed, Aug 13, 2008 at 8:26 PM, Kris Zyp <kris at sitepen.com> wrote:

We talked about desugaring classes in some detail in Oslo. During these exchanges, we discussed several separable issues, including classes, inheritance, like patterns, and type annotations. I'll avoid writing more here,

Is there more to read elsewhere? I'd like to know concretely what "desugaring classes" means.

I would too, I assume this is based on Mark's proposal for Classes as Sugar (www.mail-archive.com/es4-discuss@mozilla.org/msg01624.html).

Thanks for the link.

Mark's proposal looks like something that could perhaps be handled with macros. What I've thought would be a great way to add classes to ECMAScript is to give ECMAScript macros and then let folks build their own class macro libraries. That is essentially what has happened now but the wrapping of the prototype-based system and the lack of macros has lead to "ugly" code. Folks are going to continue with their own wrapping of the built-in prototype system anyway even if there is one sugar wrapping included in the language. Different situations require different OOP features. Some situations more complex than others. For example, if the built-in sugar doesn't have mixins and someone needs mixins then they will need to wrap exactly the same prototype system that Mark's proposal wraps.

This brings up an interesting post from what is getting close to a year ago.

On Nov 12, 2007 Brendan Eich wrote:

On Nov 12, 2007 YR Chen wrote:

Macros have been proposed and discussed before - they're somewhere on the wiki. AFAIK, they've been deferred to a future ES5.

Researchers on the list may be willing to speak to macros with more authority than I have. I'll just say that apart from the fun with C- like syntax (which can be handled; we're close to specifying standard ASTs for ES4), sound hygiene theory is still a research topic. I expect good results from academia in a year or so, and hope that TG1 will take advantage of them.

In the mean time, I think it would be very wrong to defer syntactic conveniences until after macros are done. With macros, conveniences that de-sugar in ES4 could be re-specified (and implemented at the same time) cheaply, for sure -- but users deserve convenient syntax sooner than "after macros".

Given the time lines, will hygienic macros now be ready before or in time for ES4?

Perhaps classes don't need a hygienic macro system and hygienic macros can come later.

[snip]

Peter

# Garrett Smith (9 years ago)

On Wed, Aug 13, 2008 at 2:26 PM, Brendan Eich <brendan at mozilla.org> wrote:

It's no secret that the JavaScript standards body, Ecma's Technical Committee 39, has been split for over a year, with some members favoring ES4, a major fourth edition to ECMA-262, and others advocating ES3.1 based on the existing ECMA-262 Edition 3 (ES3) specification. Now, I'm happy to report, the split is over.

That is good to know.

A split committee is good for no one and nothing,

It led to contrasting ideas.

There remain challenges, in particular getting off of the untestable and increasingly unwieldy ES1-3.x spec formalism.

Unit tests:

  • help developer understanding
  • provide examples for discussion
  • provide conformance suite for implementors
  • define boundaries on what is and what is not supported*

The tests could use a custom test runner or use an existing one. It would be useful if the testrunner could be run in a browser.

It would be helpful to have code comments in the tests, indicating exactly which rules of the spec are being tested, so that the reader can quickly go to that section of the spec (spec in HTML).

Real code examples in the spec would be useful, too. I can easily see a bookmarklet used to capture and execute the example selection of the spec.

  • defining "not supported" could be as simple as having an assertion test that should be ignored, and should be marked ignore, along with a comment as to why the test is ignored (use of Host object, et c). Ignored tests can be a useful form of documentation.

As Waldemar Horwat (Google) said on the final day, the meeting was seminal,

I like it.

Garrett

# Peter Michaux (9 years ago)

On Fri, Aug 15, 2008 at 1:40 PM, Garrett Smith <dhtmlkitchen at gmail.com> wrote:

[snip]

There remain challenges, in particular getting off of the untestable and increasingly unwieldy ES1-3.x spec formalism.

Unit tests:

  • help developer understanding
  • provide examples for discussion
  • provide conformance suite for implementors
  • define boundaries on what is and what is not supported*

The tests could use a custom test runner or use an existing one. It would be useful if the testrunner could be run in a browser.

It would be helpful to have code comments in the tests, indicating exactly which rules of the spec are being tested, so that the reader can quickly go to that section of the spec (spec in HTML).

Real code examples in the spec would be useful, too. I can easily see a bookmarklet used to capture and execute the example selection of the spec.

This seems like "literate programming" to me where the program is a set of unit tests.

[snip]

Peter

# Yuh-Ruey Chen (9 years ago)

It seems like slashdot has a story on this, and as usual, it's completely misleading and sensationalist, and anti-MS (without mentioning any other ES4 dissenters) to boot:

tech.slashdot.org/article.pl?sid=08/08/16/1552227

sigh...

# Maciej Stachowiak (9 years ago)

On Aug 16, 2008, at 10:19 PM, Yuh-Ruey Chen wrote:

It seems like slashdot has a story on this, and as usual, it's completely misleading and sensationalist, and anti-MS (without mentioning any other ES4 dissenters) to boot:

tech.slashdot.org/article.pl?sid=08/08/16/1552227

It's hard to control stories like this, except by repeating what we
think is the most sensible point of view, i.e. that peace and unity in
the world of ECMAScript is within sight and is most definitely a good
thing.

# Brendan Eich (9 years ago)

To quote Sally from "You're a Good Man, Charlie Brown" ("My New
Philosophy"): "Why are you telling me?!"

That's my new philosophy about such chatter. Slashdot is a sewer.
Didn't Digg replace it? Or maybe reddit. Yawn.

# Scott Elcomb (9 years ago)

On Sun, Aug 17, 2008 at 1:51 PM, Brendan Eich <brendan at mozilla.org> wrote:

To quote Sally from "You're a Good Man, Charlie Brown" ("My New Philosophy"): "Why are you telling me?!"

That's my new philosophy about such chatter. Slashdot is a sewer. Didn't Digg replace it? Or maybe reddit. Yawn.

Hmm. While I'm not sure Slashdot, Reddit or other social sites are sewers just yet (even if they're getting close for me) I couldn't agree more with Brendan on the "You're a Good Man."

My first post to this list (pre-combination) was a direct result of Slashdot conversation.

Even though I was really looking forward to ES4, I believe that now is a very appropriate time for communities to harmonize.

My $0.02. Ok, back to lurking...

  • Scott.