March 24 meeting notes

# Waldemar Horwat (8 years ago)

Here are my notes from today's meeting.

Waldemar

DaveH's presentation on using generators to write asynchronous code.

How do you compose generators? yield*

Waldemar: Given yield*, writing base-case trivial generators that don't yield becomes useful but its syntax is a problem. Generators should not be distinguished from functions by the presence of a yield statement. Cormack: Why can't yield* work on both generators and functions, doing a type test? Dave, Waldemar: Flattening using type testing like this leads to trouble. What if the return type of the generator/function is a generator? Deferred issue about Next API throwing vs. multiple methods.

Moved generators to harmony

Proxy handler parameter: Why not use "this"? Crossing levels; interferes with inheritance of handler object.

Brendan: delete doesn't need a receiver parameter because it only works on own properties. It will still get a proxy parameter. Bug on Tom's slide: set needs a receiver parameter. get and set need a proxy parameter.

Completion values source statements are currently statically uncomputable: eval("1; if (...) 2;") can produce a completion value of either 1 or 2 eval("1; while (...) 2;") ditto Dave: Propose making the above examples return either 2 or undefined. That fixes behavior for #-functions: #(...) {1; if (...) 2;} #(...) {1; while (...) 2;}

eval("3;;;") would still produce a completion value of 3, not undefined.

Refutable matching and switch extensions: Multiple objections to syntax chosen for pattern-matching switch: colon vs. no colon after default clause, need for blocks, etc. Refutable matching doesn't retrofit into imperative switch syntax well. Waldemar: Refutable matching is half-baked at this point, with too many syntactic and semantic problems. Not clear it's worth its added complexity.

The refutable matching wiki has the following consequences on irrefutable matching: Pattern [x,y]: Matched to [3,4], produces x=3, y=4. Matched to [3,4,5], produces x=3, y=4. Matched to [3], produces x=undefined, y=undefined. (wiki spec bug.) Pattern [..., x, y]: Matched to [3,4], produces x=3, y=4. Matched to [3], looks up negative array indices. (wiki spec bug.)

Pattern [x,y] behaves like [x,y,...] for refutable matching. (wiki spec bug.) Can't match on zero-length arrays. (wiki spec bug?)

Lucas: Feature value should overcome complexity costs. Waldemar: if guards introduce unknown syntactic and semantic complexity Waldemar: where can you use refutable matching outside of switch/match statements and perhaps catch guards? switch/match statements are too heavyweight and differ too much from irrefutable matching assignment; catching doesn't really need destructuring but benefits from conditions. The typical usage (as in Perl) is to use them in if statements: if (pattern =~ expr) {we have matched!}

catch({z,w} if z < w): OK, but then you can't get to the entire exception object from the catch clause. catch(z if z instanceof T): Useful

Waldemar: Refutable matching should integrate trademarking to be compelling. Concern about backing ourselves into a corner by implementing irrefutable pattern matching in catch guards that will later preclude refutable matching. Brendan's example: catch({x, y}) would succeed on {x:3} now but fail later if we change to refutable pattern matching.

#-functions: Alex, Erik: No longer support #-functions with a different semantics from functions. Brendan's proposal: #(a,b) {a + b} // Inherits "this" only lexically #(this | a,b) {...} // behaves like regular functions with respect to "this" Alex: Make function objects soft-bind "this" Dave, Waldemar, Brendan: Object to soft-binding "this". Allen: We should respect the intent for "this" binding, as expressed by whoever is defining the function (as opposed to whoever is calling it). Alex: Debating soft-bind use case of users wanting to override "this" in a function bound with Function.bind. Waldemar: We're discussing raw closures here, not functions produced by Function.bind. Lively debate.

Discussion on what #(x | ...) would mean. Brendan: it would bind "x" instead of "this" with the caller-supplied this-value. "this" would then be unconditionally inherited from the enclosing scope. #(self = this | a, b): Default value for "self" is "this". Waldemar: Don't like defaulting here. How would you call such a function indirectly via Function.apply without supplying a value for "self"? Cormack: Use | syntax for regular functions? Luke: User confusion with two slightly different ways of doing the same thing (function vs. #). Users will then want to use #-functions for constructors, etc. Alex: How do you express functions in terms of #-functions? If we do #-functions, does that eliminate the "const foo(x,y) {...}" shorthand for const function definition syntax? Brendan: Yes. Alex: Willing to consider #-functions with a different semantics from functions. Waldemar: Don't want implicit "return" of completion value in #-functions due to leakage and inversion of behavior vs. functions.

Records: To sort field names or not to sort: Brendan, DaveH: Nor sorting may be cheaper Waldemar: Don't like hidden state. A canonical order such as sorting (as currently on wiki) is much simpler conceptually. When specifying behavior of various methods that create records, no need to be careful about property order. Philosophical discussion: Can one make a language simpler by adding features? Yes, over the long term, if new simple features obviate old complex features and people forget the old way of doing things. DaveH: Fear of Structured Clone between web-workers. How does toString work on records? Duplicate property names in #{p: v, p: v}? Error. Duplicate property names between explicitly specified properties and properties introduced by ... in #{p:v, ... rest}? ... wins? === is elementwise.

Tuples: Brendan: Does anyone here really hate records and tuples? Lucas: Likes dicts (mutable, but otherwise identical to records) in preference to records. Allen: Not much benefit to dicts over some way to provide null for the prototype. Waldemar: No, a null prototype is not sufficient. Even if you don't have a prototype, in lots of contexts the language will still look for a toString property in your object and call it if it finds it. Thus you're not safe using a null-prototype object to store arbitrarily named properties. Dicts or tuples avoid that problem because they're a distinct primitive type. Debate about weak vs. strong keys. Dave: If you have a size-querying method, you can't have weak keys without becoming nondeterministic.

# Waldemar Horwat (7 years ago)

Here are my rough notes from today's meeting.

Waldemar


IPR discussion Intel changed their ECMAScript patent declaration to RANDZ. Now they wonder why no one else made a RANDZ declaration. Istvan described the history. Mozilla is also unhappy with the current state of affairs.  Even though this instance turned out well, it shows the potential for problems. Lots more IPR discussion

Rick Hudson, Stephan Herhut:  River Trail proposal Proposal will appear on wiki shortly. Deterministic, except for things such as array reduction order if the reduction operation is nonassociative. Parallel arrays are immutable. Various parallel methods take kernel functions to operate on subcomponents. MarkM: Are the requirements on the kernel functions to allow them to be optimized well-defined? Rick: Yes

var a = new ParallelArray(...) var b = a.map(function(val) {return val+1;}); Allen: This can be done today sequentially by replacing ParallelArray with Array.

var b = a.combine(function(i) {return this.get(i)+1;}); var sum = a.reduce(function(a, b) {return a+b;});

Competitors: OpenCL: GPUs do poor job of context-switching. WebCL: Too many things undefined. Browser-provided webworkers (task parallelism).

Waldemar: Can the kernels do nested parallel operations, such as what's needed for a matrix multiply? Rick: Yes Waldemar: What data types can you use? Rick: This builds on the typed array work.

Some desire for not having too many array times.  Can we unify ParallelArray with Array? DaveH: No. Holes alone cause too much of a semantic dissonance. Waldemar: Would like to unify ParallelArray with typed arrays. DaveH: No, because ParallelArrays can hold objects and typed arrays can't. Waldemar: Why not?

Discussion back to knowing which kernels can be optimized. DaveH, MarkM: Nondeterministic performance degradation is preferable to nondenterministic refusal to run the code.  This leaves implementations space to grow. What about throwing exceptions only for functions that can never be optimized because they mutate state?

Waldemar:  Is this optimized?  (Note that there are several different issues here.) let y = ...; function f(x) {return x+y;} a.map(f)

Note that merely reading mutable state is not necessarily a cause for deoptimization because parallel functions don't run in parallel with other code, so that state stays fixed for the duration of the parallel operation.

Allen: Concerned about every closure carrying along sufficient information to do the kind of abstract interpretation needed to optimize it as a ParallelArray kernel. Allen's issue summary:

  • Do we want to do this?
  • If so, how do we structure the activity (separate standard or part of ESnext or ESnextnext)?
  • Data parallelism or parallelism in general? Rick: Our goal is to get it into browsers. Debate about whether to do this for ES6 or ES7. Brendan, DaveH: Let's just do the work.  The browsers can ship it when it's ready, regardless of when the standard becomes official.  Need to get new features user-tested anyway. Structurally this will part of the mainline ECMAScript work (es-discuss + meetings), not in separate meetings as was done with internationalization.

Allen's spec status.

Olivier:  Concerns about latency issues related to module fetches blocking.  Multiple script tags can fetch their scripts concurrently; modules have latency problems such as: <script src=A.js> <script src=B.js> <script src=C.js>

vs. <script src=C.js>

where C.js is: module A at "A.js" module B at "B.js" // use A and B

Alex:  Have modules block document.write.

Long debate about asynchronous loading approaches.

Olivier:  To get better latency, you can make your first module load simple, but after that you'll need to use the AMD syntax again. DaveH: Change the HTML semantics. Alex: Evaluate the outer module asynchronously in script blocks such as the one below, because there is no non-module code after the module: <script>

module M {   module A at "A.js"   ... } </script>

Olivier: This will error out if followed out by the otherwise correct: <script>

M.foo(); </script>

DaveH proposal: Bifurcate grammar so that scripts (whether in <script>

tags or in files included from those) cannot contain static module load statements, not even in syntactically nested modules.  Modules can include such statements but can only be parsed via dynamic loads. <script>

System.load("interp.js", function(m) {...}); </script>

interp.js contains: module stack = "stack.js"; export function evaluate(...) {...}

stack.js contains: export class Stack {...}

The body of an eval would be treated like a script body, not a module body.  This avoids the tarpit of dealing with synchronous i/o in eval.

For-of design variants: Variant 1: import keys from "@iter" for let k of keys(o)

Variant 2: for own (let k in o)

Current way: Object.keys(o).forEach(function(...){...});

Picked variant 1.

DaveH: The people want Array.of and Array.from and don't want to wait for the ... syntax. Array.of(a, b, c) ≡ [a, b, c] Array.from(a) ≡ [... a]

Brendan: Disallow initializer in: for (var i = x in o) MarkM: OK, as long as Brendan is the first penguin through the ice hole. Mozilla will take it out.

Brendan's arrow proposal: ArrowFormalParams_opt -> Block_opt

ArrowFormalParams_opt => AssignmentExpr

ArrowFormalParams: (FPL) (this Initialiser_opt, FPL)  // Makes this dynamic (this Initialiser_opt)       // Makes this dynamic Identifier

Brendan:  -> and => both use static this.

DaveH:    -> defaults to dynamic this, => defaults to static this

Debate about Tennent Correspondence Principle and to what extent this complies.

ArrowFormalParams is actually parsed as the cover grammar: (CommaExpression) with later semantic restrictions on what the CommaExpression can actually contain.

Waldemar:  A lot of problems that are individually nonfatal but too much in combination.

  1. Two different arrows is confusing.  Will keep using the wrong one and get weird results.  Examples:   -> {a:b}  // intended to return an object literal, instead has a

labeled expression   -> {}     // intended to return an empty object, instead returns undefined   => {}     // intended to return undefined, instead returns an empty object

  1. Having both dynamic and static this options is overkill.
  2. TCP hazards
  3. Cover grammar introduces a lot of spec complexity.
  4. Different 'this' defaults in DaveH's variant of the proposal

Wild debate. Poll of who objects to which problem in the proposal: 1.  AWB, MM, AR, AR, LH, DC, WH, DH 2.  AWB, MM, AR, AR, LH, BE, DC, WH, DH 3.  AR, AR, LH, DC, BE 5.  AWB, MM, AR, AR, LH, BE, DC, WH, DH

If we choose only one arrow, which is it?  Flow of debate assumed it's =>, with a do expression used to place statements there if needed.

Luke, Waldemar:  Use only one arrow, but allow either a block or an expression there.  If it starts with a {, it's always a block. MarkM: What about => function foo(){}?

No issue there; it's a function expression.  Only => { starts a statement.

MarkM:  => assignment cannot be TCP due to yield.

Debate over meaning of TCP.

Some people have changed camps.  The ones above are from the original poll. Confusion about the meaning of poll polarity on item 3. New results: WH, AWB, MM:  object to 1, 2, 5; want TCP BE, DH, AR, AR, LH, OH:  object to 1, 2, 5; don't want TCP DC:  object to 2, 5; don't want TCP

DC switched to middle camp. AWB, WH, MM prefer to keep TCP but are ok with switching to middle camp.

Consensus on:

  • Have only one arrow, namely =>
  • this is always static.  No provision for dynamic this.
  • Other than the treatment of this, the function behaves like a normal function.  return, break, etc. behave as though the function body were in a function(...){...}.

To be discussed later: The thing that comes after the arrow:

  1. Always an expression (possibly a do expression)?
  2. A block if it starts with {, otherwise an expression.
# Brendan Eich (7 years ago)

Waldemar Horwat wrote:

Consensus on:

  • Have only one arrow, namely =>
  • this is always static. No provision for dynamic this.
  • Other than the treatment of this, the function behaves like a normal function. return, break, etc. behave as though the function body were in a function(...){...}.

To be discussed later: The thing that comes after the arrow:

  1. Always an expression (possibly a do expression)?
  2. A block if it starts with {, otherwise an expression.

See strawman:arrow_function_syntax

# Domenic Denicola (7 years ago)

Great to gain consensus indeed. This looks just about as nice as it can be.

Minor question: There are no examples of "very empty" arrow functions, e.g. => or => 5. And from what I can tell reading the grammar , they are not allowed. Is this necessary? CoffeeScript's allowance of -> (expr) gives some nice-looking code, for example see 1.

Bigger question: It sounds like TCP was sacrificed in favor of maximal minimalism, which makes sense. But, is this strawman friendly toward future TCP endeavors, perhaps in ES.next.next? For example, if do expressions were specced in the way discussed previously, could putting one to the right of the => result in TCP semantics?

# Brendan Eich (7 years ago)

Domenic Denicola wrote:

Minor question: There are no examples of "very empty" arrow functions, e.g. => or => 5. And from what I can tell reading the grammar , they are not allowed.

They were allowed in the grammar I wrote -- see the wiki history. Several TC39ers reacted badly to them yesterday and rather than wreck consensus over them, I took 'em out. Perhaps we could add them back with more time.

Bigger question: It sounds like TCP was sacrificed in favor of maximal minimalism, which makes sense. But, is this strawman friendly toward future TCP endeavors, perhaps in ES.next.next? For example, if do expressions were specced in the way discussed previously, could putting one to the right of the => result in TCP semantics?

There was a lengthy discussion of what "TCP" means. JS has statements as well as expressions, it's the C curse by way of "make it look like Java". So adding a TCP lambda form now, however good for macros, compiler writers, and certain users of the language, is likely to create confusion.

The biggest worry has been the "completion value leak". This is why I've argued block-lambdas are better for a pure TCP form than anything built on function's body plan or "clade". Yesterday,

Luke made a stronger point: TCP 'return' means async callbacks (e.g. promise when functions) must be written ML-style, with return value in tail position. Callbacks can be lengthy, so writing and if-else nest with a variable holding the r.v. to achieve this is awkward and users will prefer early returns (so the calllback's spine is the normal execution path). But TCP will make these wrong-return bugs.

In general I sensed a new consensus: it's too late to wedge TCP into JS, at least under any "___ function syntax". Allen held out hope for block-lambdas later but others (Mark and I at least) view them as effectively ruled out by arrows.

Dave wrote the equivalence he wants for "TCP" (an overloaded TLA, we've talked past one another by abusing it):

e ~= do { e; }

where e is an expression and not a statement. So lexical |this| is still important (lack of it is an ongoing source of bugs). And we ban 'arguments' in arrow bodies.

We may add a pure TCP form such as block-lambdas later, but with arrows "in", I give future lambda-ish additions very long odds. Don't want two things.

The deeper conclusion is that the C curse in JS, and millions of users learning JS-as-it-is over 15 years, in particular using early return, weigh heavily against a late TCP lambda form. Certainly against one in function's clade.

# Russell Leggett (7 years ago)

Any discussion on maximal minimal classes? Just curious of the status.

# Andreas Rossberg (7 years ago)

On 29 March 2012 16:11, Brendan Eich <brendan at mozilla.org> wrote:

There was a lengthy discussion of what "TCP" means. JS has statements as well as expressions, it's the C curse by way of "make it look like Java". So adding a TCP lambda form now, however good for macros, compiler writers, and certain users of the language, is likely to create confusion.

For me, the biggest blow against TCP lambda forms in general was Mark's observation regarding the incompatibility with 'yield' that you mention in the other post. If the TCP does not cover the whole language, then that vastly reduces lambda's utility as a general abstraction mechanism for compiler or macro writers. It means that you cannot generally replace a 'for' loop with a 'forEach' method in a generator. That makes it look like a rather dead born idea.

(Of course, you can turn that around as an argument against generators in their current form, because it just demonstrates that they are not properly compositional. But short of adopting more general continuation support, which nobody wants, there doesn't seem to be much of an alternative.)

The biggest worry has been the "completion value leak". This is why I've

argued block-lambdas are better for a pure TCP form than anything built on function's body plan or "clade". Yesterday,

Luke made a stronger point: TCP 'return' means async callbacks (e.g. promise when functions) must be written ML-style, with return value in tail position. Callbacks can be lengthy, so writing and if-else nest with a variable holding the r.v. to achieve this is awkward and users will prefer early returns (so the calllback's spine is the normal execution path). But TCP will make these wrong-return bugs.

I actually think this is a less severe problem. Ignoring the issue of potential confusion for users (which is real), the main thing lacking there in terms of expressiveness is some labelled variant of 'return', analogous to break and continue. If we were so inclined, I'm sure we could come up with something usable on that front.

# Waldemar Horwat (7 years ago)

On Wed, Mar 28, 2012 at 11:47 PM, Erik Corry <erik.corry at gmail.com> wrote:

2012/3/29 Waldemar Horwat <waldemar at google.com>:

Wild debate. Poll of who objects to which problem in the proposal: 1.  AWB, MM, AR, AR, LH, DC, WH, DH 2.  AWB, MM, AR, AR, LH, BE, DC, WH, DH 3.  AR, AR, LH, DC, BE 5.  AWB, MM, AR, AR, LH, BE, DC, WH, DH

Given that there were two ARs it is good that Andreas always agreed with the other one!  :-)

AR = Andreas Rossberg AR = Alex Russell

Waldemar
# David Herman (7 years ago)

On Mar 29, 2012, at 6:23 AM, Domenic Denicola wrote:

Bigger question: It sounds like TCP was sacrificed in favor of maximal minimalism, which makes sense.

No, maximal minimalism is not our universal principle for ES6. It's important for classes, for various reasons, but it's not our approach to everything. TCP was sacrificed primarily because the legacy of C causes most people to expect return, break, and continue to expect to be local to one function only, so it seems to cause tons of confusion to break this expectation.

But, is this strawman friendly toward future TCP endeavors, perhaps in ES.next.next?

Perhaps. But I wouldn't hold my breath; I make almost no predictions that far into the future.

For example, if do expressions were specced in the way discussed previously, could putting one to the right of the => result in TCP semantics?

Definitely not. The do-expressions would not magically reach outside of their function. The point of do-expressions is that they are totally compositional; they don't introduce any new implicit changes to the meaning of break/continue/return. And since => functions will bind return and disable break/continue, there's no way do-expressions could get around that.

# Waldemar Horwat (7 years ago)

For me it was a tradeoff. I prefer consistency. The treatment of 'this' was done TCP-style, so I'd have preferred for the other language constructs to also behave TCP-style. However, practical gotchas begin to form:

  • yield cannot be done TCP-style within the framework of what we're doing.

  • what should break, continue, and return do if they're done TCP-style? They'd throw some exception across to unwind the stack to the destination of the break, continue, or return. That exception could be intercepted by try/finally blocks inside functions dynamically on the stack in between the source and target function, which then brings up the questions of what it would reflect as, etc. (If you just make it bypass catch and finally clauses, you create even worse problems.)

    Waldemar

# Brendan Eich (7 years ago)

Andreas Rossberg wrote:

On 29 March 2012 16:11, Brendan Eich <brendan at mozilla.org <mailto:brendan at mozilla.org>> wrote:

Yesterday, Luke made a stronger point: TCP 'return' means async
callbacks (e.g. promise when functions) must be written ML-style,
with return value in tail position. Callbacks can be lengthy, so
writing and if-else nest with a variable holding the r.v. to
achieve this is awkward and users will prefer early returns (so
the calllback's spine is the normal execution path). But TCP will
make these wrong-return bugs.

I actually think this is a less severe problem. Ignoring the issue of potential confusion for users (which is real), the main thing lacking there in terms of expressiveness is some labelled variant of 'return', analogous to break and continue. If we were so inclined, I'm sure we could come up with something usable on that front.

Dave did:

strawman:return_to_label

It did not seem to be headed for consensus and inclusion in harmony:proposals when we last discussed it.

# Brendan Eich (7 years ago)

Russell Leggett wrote:

Any discussion on maximal minimal classes? Just curious of the status.

Was not on yesterday's agenda.

# Russell Leggett (7 years ago)

On Thu, Mar 29, 2012 at 3:13 PM, Brendan Eich <brendan at mozilla.org> wrote:

Russell Leggett wrote:

Any discussion on maximal minimal classes? Just curious of the status.

Was not on yesterday's agenda.

Thanks. I'll just keep an eye open.

# Andreas Rossberg (7 years ago)

On 29 March 2012 20:35, Brendan Eich <brendan at mozilla.org> wrote:

Andreas Rossberg wrote:

On 29 March 2012 16:11, Brendan Eich <brendan at mozilla.org <mailto:

brendan at mozilla.org>> wrote:

Yesterday, Luke made a stronger point: TCP 'return' means async callbacks (e.g. promise when functions) must be written ML-style, with return value in tail position. Callbacks can be lengthy, so writing and if-else nest with a variable holding the r.v. to achieve this is awkward and users will prefer early returns (so the calllback's spine is the normal execution path). But TCP will make these wrong-return bugs.

I actually think this is a less severe problem. Ignoring the issue of potential confusion for users (which is real), the main thing lacking there in terms of expressiveness is some labelled variant of 'return', analogous to break and continue. If we were so inclined, I'm sure we could come up with something usable on that front.

Dave did:

**doku.php?id=strawman:return_**to_labelstrawman:return_to_label

It did not seem to be headed for consensus and inclusion in harmony:proposals when we last discussed it.

Interesting. But this is different from what I had in mind. It is more like a generalisation of 'break', while I was thinking of something like:

function f(o) { o.forEach(function g(x) { if (...) return 0 from f; let x2 = x.map(function h(y) { if (...) return from g return y*2 // returns from h }) ... }) return 27 }

That is, just a way for referring to the specific function (or arrow function, if we had a syntax for naming them) that the return is supposed to be associated with.

(I suppose Dave's proposal can encode that, but it would result in less natural and significantly more verbose code.)

# Brendan Eich (7 years ago)

Andreas Rossberg wrote:

Dave did:

http://wiki.ecmascript.org/doku.php?id=strawman:return_to_label

It did not seem to be headed for consensus and inclusion in
harmony:proposals when we last discussed it.

Interesting. But this is different from what I had in mind. It is more like a generalisation of 'break', while I was thinking of something like:

function f(o) { o.forEach(function g(x) { if (...) return 0 from f; let x2 = x.map(function h(y) { if (...) return from g return y*2 // returns from h }) ... }) return 27 }

That is, just a way for referring to the specific function (or arrow function, if we had a syntax for naming them) that the return is supposed to be associated with.

Nice. This gives functions some of the oomph of block-lambdas (see gist.github.com/1677893 from @wavded [Marc Harter]). However:

  • Requires function declarations, or seems to -- too restrictive (vs. arrows as block-lambda replacement in the gist above)?

  • Waldemar's point about try/finally standing in the way of a forced return. Perhaps this is "you get what you ask for".

Still, very pretty syntax, nice contextual 'from' keyword linkage.

(I suppose Dave's proposal can encode that, but it would result in less natural and significantly more verbose code.)

(Yes, it's escape continuations for JS, so it can do more but you have to say more.)

# Brendan Eich (7 years ago)

Andreas Rossberg wrote:

For me, the biggest blow against TCP lambda forms in general was Mark's observation regarding the incompatibility with 'yield' that you mention in the other post.

It seems 'yield' may be a red herring. We all forgot that it is contextually defined as a keyword only in generator functions, which have a distinguished head (* after function or before method name).

Arrows cannot be generator functions, so yield is not reserved in arrow bodies. It need not be illegal, though. But probably it's best to ban just to avoid confusion:

function yield(e) { return 42; }

function* gen() { yield (v) => yield(v); yield 99; }

The yield call after the =>, if not an error as the arrow function

syntax proposal has it now, would call the outer yield function.

# Andreas Rossberg (7 years ago)

On 29 March 2012 21:26, Brendan Eich <brendan at mozilla.org> wrote:

Nice. This gives functions some of the oomph of block-lambdas (see gist.github.com/**1677893, gist.github.com/1677893 from @wavded [Marc Harter]). However:

  • Requires function declarations, or seems to -- too restrictive (vs. arrows as block-lambda replacement in the gist above)?

I was thinking, it should be possible to extend arrow syntax to allow an optional name? As in:

f(n) => n==0 ? 1 : x * f(n-1)

As far as I can see, this does not introduce any ambiguity and could still be parsed by cover grammar. And as the example shows, would be useful regardless of return (although not necessarily worthwhile).

  • Waldemar's point about try/finally standing in the way of a forced

return. Perhaps this is "you get what you ask for".

I wouldn't make it a catchable exception. Finally blocks are executed, but catch is orthogonal.

Anyway, just thinking out loud. I'm not proposing any of this for the time being.

# Mark S. Miller (7 years ago)

In order to avoid contributing to the list of "exceptions to TCP for non-rejected programs", we need to reject "yield" in these positions. And it is upwards compatible since arrow functions are new. And as you observe, less confusing.

With such a "yield" prohibited, AFAICT the exceptions in non-rejected programs are "return" and the open question about "var". Did we miss any others?

# Brendan Eich (7 years ago)

Andreas Rossberg wrote:

I was thinking, it should be possible to extend arrow syntax to allow an optional name? As in:

f(n) => n==0 ? 1 : x * f(n-1)

I know you're not proposing, but this would have to be a separate proposal. I fear adding it to arrow function syntax will lose consensus. For one thing, the binding forms all have a keyword in front (even formal parameter and catch variable bindings do). This doesn't.

Currently arrow functions occur as an alternate AssignmentExpression right-hand side. They are expressions. What you sketched would have to be produced at statement level to be a declaration, but then it lacks a prefix keyword and so is harder to see (or perhaps just "inconsistent" in some way that is foolish to worry about?). Not sure, but there's a smell...

# Andreas Rossberg (7 years ago)

On 29 March 2012 22:26, Brendan Eich <brendan at mozilla.org> wrote:

Andreas Rossberg wrote:

For me, the biggest blow against TCP lambda forms in general was Mark's observation regarding the incompatibility with 'yield' that you mention in the other post.

It seems 'yield' may be a red herring. We all forgot that it is contextually defined as a keyword only in generator functions, which have a distinguished head (* after function or before method name).

Well, in the same way 'return' is contextually defined as a keyword in ordinary functions. ;)

Arrows cannot be generator functions, so yield is not reserved in arrow

bodies.

Wait, that isn't the issue. The issue is that if 'yield' does not work, then control-flow abstractions (which are the main motivation for TCP lambdas) won't generally work, because they could not be used in generators. Just to give a stupid example, consider:

function* squares(a) { for (int i = 0; i < a.length; ++i) yield a[i]*a[i]; }

This cannot be turned into:

function* squares(a) { a.forEach(x => yield x*x); }

To me, that seems like a complete roadblock wrt the primary motivation for TCP lambdas.

# Andreas Rossberg (7 years ago)

On 29 March 2012 22:37, Brendan Eich <brendan at mozilla.org> wrote:

Andreas Rossberg wrote:

I was thinking, it should be possible to extend arrow syntax to allow an optional name? As in:

f(n) => n==0 ? 1 : x * f(n-1)

I know you're not proposing, but this would have to be a separate proposal.

I fear adding it to arrow function syntax will lose consensus.

Oh yes, definitely. Just thought experiments...

For one thing, the binding forms all have a keyword in front (even formal parameter and catch variable bindings do). This doesn't.

Not quite: parameters in arrow functions don't either.

Currently arrow functions occur as an alternate AssignmentExpression

right-hand side. They are expressions. What you sketched would have to be produced at statement level to be a declaration, but then it lacks a prefix keyword and so is harder to see (or perhaps just "inconsistent" in some way that is foolish to worry about?).

Ah, no, I was thinking of this as still being purely an expression, not extending its meaning to be a declaration. That is, the function name is only visible in the body. I completely agree that all declaration statements should start with a keyword.

# Brendan Eich (7 years ago)

Andreas Rossberg wrote:

Ah, no, I was thinking of this as still being purely an expression, not extending its meaning to be a declaration. That is, the function name is only visible in the body. I completely agree that all declaration statements should start with a keyword.

Ok, I'm with you. Named function expressions suggest allowing named arrow expressions. But I'm not going to update the proposal unless there is decisive consensus on adding named arrow function expressions

# Brendan Eich (7 years ago)

Andreas Rossberg wrote:

To me, that seems like a complete roadblock wrt the primary motivation for TCP lambdas. From yesterday's meeting, it seems one of many. Too many, and perhaps it's the clearest roadblock as you suggest, but altogether and due to many fallen stones, the road is blocked.

# Brendan Eich (7 years ago)

Mark S. Miller wrote:

With such a "yield" prohibited, AFAICT the exceptions in non-rejected programs are "return" and the open question about "var". Did we miss any others?

Not that I know of, but why is 'var' at issue? An arrow function is still a function, and function body scopes (hoisted) var bindings. If arrows bind 'return' why not 'var' too? Better to avoid diverging from non-arrow functions than try to minimize TCP exceptions when there are any exceptions at all, I say.

# Waldemar Horwat (7 years ago)

Rough notes from today's meeting.

Waldemar

ES 5.1 quirks in defining globals when the global object's prototype already contains a property with the same name: ecmascript#78 Waldemar: What if the prototype contains a getter or setter? Doesn't matter for the purpose of defining globals. Agreed on the solution proposed in the bug.

John: Do we have a process for tracking ES5.1 errata? Allen: Yes. John: WIll try to get an official errata on the ECMA site.

Strict function caller leak: ecmascript#310 We should correct this in the errata. MarkM might have more comments but he's not here.

Luke: 15.10.4.1 Regexp.source on empty regexps on all browsers returns the empty string instead of (?:), which is incompatible with the spec text. That was a change to the spec in ES5 but browsers didn't obey it. Allen, Waldemar: This isn't a spec bug. It was an intentional change. Luke: It breaks prototype.js. DaveH: If this is the only breakage, change the spec to insert the regexp source into /(?: and )/ instead of / and /. Waldemar: It's not the only problem. Escaping of slashes is also a problem here. Luke: Browsers diverge here. Firefox is the only browser that does escaping. Debated. No consensus.

David Fugate: Test262 update slideshow Microsoft sources now include ECMA copyright boilerplate instead of Microsoft's. MarkM will do likewise for Google sources. 19 new bugs 26 bugs resolved Prepopulated bug reports International402 mini-suite framework now (quietly) live, with one fake test Bill Ticehurst will replace David Fugate's role on Test262. Development focus will shift to ES6 tests. Allen: Section numbers will change in ES6, which will impact test suite organization.

Norbert: Internationalization Final draft won't make it for June. Will try for December. Allen reviewed half of the spec and had lots of substantial comments. There are quite a number of fixes that need to be made to the spec. Discussion about implementations and overspecification vs. underspecification.

MarkM: Quick follow-up on TCP correspondence from yesterday. What should var do? MarkM proposed that var be statically rejected if it would cross a closure boundary. Waldemar: Since we gave up on TCP, => should work just like a function

except for 'this'. Discussion cut off by Luke, since it looks like it won't be a quick one.

Discussion of hypot, hypot2. hypot is the square root of the sum of squares and takes either two or three arguments. hypot2 is the sum of squares and takes either two or three arguments. Waldemar: How is hypot2 better than just doing xx + yy? Luke: It's just ergonomics. General reluctance about the hypot2 name because it looks like the 2 means two arguments (as in atan2). Some debate about other function names (hypotSq? sumOfSquares?). MarkM: How is hypot better than sqrt(xx + yy)? It's potentially more efficient and more accurate. It is widespread in numeric libraries. Consensus: hypot will support just two or three arguments. hypot2 dropped. Waldemar, MarkM: Why not one or zero arguments? It would be 0 for zero arguments and abs for one argument. Allen, DaveH: If you pass one argument to hypot, you'll get NaN. Luke: It's not variadic. Waldemar: Why isn't it variadic? Luke: 2 or 3 is the 99% use case. Waldemar: 2 or 3 arguments is the 99% use case for max. Waldemar: If it's not variadic and takes only 2 or 3 arguments, you'll get silent mistakes. If you pass in four arguments, you'll get the hypot of the first three, and the last one will be silently ignored. That's bad. Luke: Will go back to the experts to explore implementing variadic hypot.

Add parseInt and parseFloat to Number, matching isNaN and isFinite? Use the name Number.parse instead of Number.parseFloat? Should these functions be specified as independent functions from the existing global parseInt and parseFloat, or should they be the same function objects? Consensus: Not duplicting parseInt and parseFloat into Number. May consider doing a Number.parse in the future.

Math.cbrt cube root: Handles negatives, more accurate than pow. Approved.

Naming: Math.sign vs. Math.signum? Sticking with Math.sign.

Should the spec try to enforce accuracy? No. None of the existing math libraries spec accuracy.

MIN_VALUE is 0 on ARM implementations because they don't represent denorms. This is a severe spec violation. However, apparently turning off denorms is a major power savings on ARM. Allen, Waldemar: MIN_VALUE must be a nonzero value. If a platform doesn't represent denorms, it should make MIN_VALUE be the smallest positive normalized number on that platform.

Number.toInteger: Just does the internal ToInteger. Not the same as floor. Renamed it to toInt.

Count leading zeros: Number.clz. Consensus that we want it. Should we include "32" in the name? Waldemar: "32" not needed in name. None of the other bit operators include "32" in the name. MarkM: Prefers clz32 but doesn't feel strongly.

Hex floating point literals: Waldemar: Other languages include these things. They're rarely used but when you want one, you really want one. Use cases are similar to that of hex literals. Will explore adding them. MarkM: 0x3.p1 currently evaluates to undefined. This would be a breaking change. Waldemar: Not clear anyone would notice. How did other languages deal with this?

Relocated and rescheduled meetings: May 21-23 Mozilla, Mountain View (May 21 is internationalization) Sep 25-26 Northeastern, Boston The July and November meetings are the same are previously scheduled.

DaveH's presentation about module loaders Discussion about cross-origin problems and modeling iframes About loader providing a function to define the entire set of built-ins: Allen: A lot of intrinsics refer to the built-ins. The above function defines the ones that ECMAScript knows about. What about other, implementation-specific ones? Have the environment provide a way for making environments that include such things? Luke: Objections related to WebIDL [I didn't follow the logic] Discussion about the role ECMAScript should play in the hierarchy of web infrastructure.

Waldemar: Is there any notion of a parent environment, other than whatever environment intrinsics get the built-ins from? (quietly hoping the answer is no) DaveH: No. Waldemar: What about the dynamic craziness related to inner and outer window objects? DaveH: Will need extra hooks for that.

Waldemar: What creates new intrinsic worlds? DaveH: Creating a Loader does, as in: l = new Loader(System, {intrinsics: null}); // Can also specify existing intrinsics to share a world a = l.eval("[]"); Object.getPrototypeOf(a) != Array.prototype b = l.eval("[]"); Object.getPrototypeOf(b) != Array.prototype Object.getPrototypeOf(b) === Object.getPrototypeOf(a)

Allen: Can DefineBuiltIns be called any time? DaveH: It can be called on any object at any time. Don't want to get now into esoterica of what happens if there are existing properties, setters, etc.

DaveH: Imperative module replacement example: @websockets exists as version 1. An implementation wants to improve it to v2: import "@websockets" as ws; System.set("@websockets", polyfills(ws));

Waldemar: What can you provide to the intrinsics parameter of the Loader constructor, other than the previously covered examples of null and another Loader? DaveH: Nothing else is allowed there. Waldemar, Luke: Then we'll need a notion of a loader type just like we have a notion of an array type. At some point we'll need to say what happens when you pass an object that inherits from a Loader, a proxied Loader, etc.

Module syntax alternatives: module x at "foo.js" module x = "foo.js" module "foo.js" as x module x at "foo.js" Don't have time to discuss these now.

Unicode presentation In some places ES5.1 treats unicode characters as arbitrary 16-bit chunks. In other places it has special provisions for surrogate pairs. For much text processing it doesn't matter; either works. The trouble points are:

  • Supplementary characters within source code identifiers
  • Regular expressions
  • String comparison
  • Case conversion

Norbert's alternatives:

  1. UTF-32 strings
  2. UTF-32 or UTF-16 strings Waldemar: Either of these would be nightmares because they'd provide two different ways of encoding the same supplementary character, and by Murphy's Law you'd get the wrong one at the most inconvenient time. Wasted a lot of time in Perl which has this problem. A far better solution would be to keep representation as always UTF-16 and just fix the functions to understand UTF-16 better. Olivier: For ECMAScript to adopt the UTF-32 model, you'd need an amazingly compelling reason that would blow all other ones out of the water. Consensus: No one wants UTF-32 strings.

Norbert's favored third option: Stick with UTF-16, change functions to understand UTF-16. MarkM: Careful about breaking compatibility! Change functions or create new ones?

Regular expressions: /u mode that matches via UTF-16 code points instead of code units. /./ would match a code point; supplementaries can be in ranges. Clear that this must be done via a mode; this would break too much stuff without a mode such as /u.

Waldemar: Why not graphemes? Nebojša: The Unicode folks tried it; it became too difficult.

Case conversion UTF-16 fixes are uncontroversial? Waldemar: Not so. When designing ES3 we intentionally disabled some Unicode case conversions to avoid nasty surprises. For example, we wanted /[a-z]/i to match only the ASCII letters. Had we allowed the true Unicode conversions, this would also match the non-ASCII Turkish dotless lower-case i (ı) or upper-case dotted I(İ). There were similar issues around ß expanding into SS. How will we deal with situations like this with /u?

Make /u be the little red switch for:

  1. Unicode code point semantics
  2. Unicode based \d\D\w\W\b\B
  3. Unicode case folding
  4. Remove some/all identity escapes to allow \p, \X, \N
  5. Don't match web reality?

Can we do only 1+3+4+5, without 2? Waldemar: That makes no sense. Given 3, you shouldn't expect \w to match the same characters as in non-/u mode because, at the least, /\w/i will match ı or İ. Consensus on doing 1+3+4+5. 2 needs discussion.

< and > will work as they do now on strings -- compare code units as

unsigned 16-bit integers. trim: There are currently no supplementary white space characters, so we can redefine it to support supplementary characters without breaking anything. toLowerCase and toUpperCase: Safari already converts supplementary characters with apparently no ill effects. Consensus on redefining these to work properly on supplementary characters.

Base the spec on Unicode 5.1. Implementations will be permitted to support later versions if they choose.

String.fromCodePoint(cp0, cp1, ...) This accepts integers between 0 and 0x10FFFF. If the integer is in the surrogate range, it will produce an unpaired surrogate.

String.prototype.codePointAt(pos) Here pos is the code unit position. If it's the second surrogate of a pair or an unpaired starting surrogate, it will return the code unit of the surrogate.

String.prototype.[iterator] Should return code point strings (of length 1 or 2), not numbers.

Waldemar: Would also want an iterator for graphemes. DaveH: The default iterator should return code point substrings.

Code point escape: "\u{20BB7}" === "\uD842\uDFB7" V8 doesn't currently throw a syntax error for \u{, but that will get fixed. Exclude 0xD800-0xDFFF? No.

Maximally minimal classes: Luke: These aren't good enough to be a net win. Waldemar: These don't address the hard problems we need to solve. Concerned about both future-hostility (making it cumbersome for future classes to declare, say, object layout without cumbersome syntax by taking over, say, const syntax) and putting developers into a quandry -- if they want to do anything more sophisticated, they'll need to refactor their code base away from these classes. Unless one choice is clearly superior, having two choices (traditional and extended object literals) is better than having three (traditional, extended object literals, and minimal classes). Minimal classes currently don't carry their weight over extended object literals. Perhaps they can evolve into something that carries its weight, but it would be more than just bikeshedding. Alex: We need to do something. Debated without resolution.

Discussion about events/asynchrony/observers.

# Allen Wirfs-Brock (7 years ago)

I don't think the report on maximally minimal classes fully reflections the discussion:

Maximally minimal classes:

Alex and Allen initiated the discussion as a status up-date to TC-39.. We pointed out that this proposal had recently been extensively discussed on es-discuss and that it appear to have considerable support from most of the participants in that discussion.

Luke: These aren't good enough to be a net win.

I'm not sure whether this is an exact quote. Luke certainly did raise the issue of whether classes, as defined by this proposal, added enough functionality to ES to justify the inherent complexity of a new feature.

Allen and Alex reiterated that this proposal is only trying to address the most common class definition use cases but in a way that allows for future extensions to address a broader range of use cases. These is significant value in what the proposal provides even if it doesn't do everything any might want.

dherman stated he has some minor design issues he wants to further discuss, but that overall the level of functionality in this proposal was useful and would be a positive addition. He supports it.

Waldemar: These don't address the hard problems we need to solve. Concerned about both future-hostility (making it cumbersome for future classes to declare, say, object layout without cumbersome syntax by taking over, say, const syntax) and putting developers into a quandry

We discussed this concern quite a bit and did not identify any specify ways in which the current proposal would block future extensions. Waldemar was asked to provide specific examples if he comes up with any. Allen pointed out that future syntactic additions can also enforce new semantics. For example addition of a per instance state declarations and a "const" keyword to the constructor declaration could cause ad hoc this.property assignments to be statically rejected, if that was a desired semantics.

-- if they want to do anything more sophisticated, they'll need to refactor their code base away from these classes. Unless one choice is clearly superior, having two choices (traditional and extended object literals) is better than having three (traditional, extended object literals, and minimal classes). Minimal classes currently don't carry their weight over extended object literals. Perhaps they can evolve into something that carries its weight, but it would be more than just bikeshedding.

The above is a statement of Waldemar's opinion. Other opinions expressed in the discussion aren't record in the original notes.

Alex: We need to do something.

Allen and Alex also expressed that it is unlikely that any class proposal that significantly goes beyond will be accepted for ES6.

Debated without resolution.

In summary:

Waldemar should identify any specific ways that the syntax or semantics of this proposal would be future hostile.

Waldemar, Luke, and MarkM expressed varying levels of concern as to whether the user benefit of the proposal was sufficient to justify its inclusion. In order to resolve this question, both sides of the issue really need to provide better supporting evidence for the next meeting.

# Mathias Bynens (7 years ago)

Code point escape: "\u{20BB7}" === "\uD842\uDFB7" V8 doesn't currently throw a syntax error for \u{, but that will get fixed. Exclude 0xD800-0xDFFF?  No.

What will be the maximum number of hex digits allowed in \u{ABCDE}? The strawman[1] mentioned up to 8 hex digits, but it seems like 6 hex digits should be enough. This has been discussed here before[2].

[1] strawman:support_full_unicode_in_strings [2] esdiscuss/2012-January/019943

# Claus Reinke (7 years ago)

DaveH's presentation about module loaders

Shouldn't any newly designed async loading APIs support promises? For ES proposals, promises seem to be burried in the concurrency strawman.

DaveH: Imperative module replacement example: @websockets exists as version 1. An implementation wants to improve it to v2: import "@websockets" as ws; System.set("@websockets", polyfills(ws));

That looks a bit like fiddling with Object.prototype for polyfills. Is that the recommended approach, and how does it interact with the static aspects of modules? Wouldn't it be more honest to use conditional loading, falling back to dynamic module handling?

Module syntax alternatives: module x at "foo.js" module x = "foo.js" module "foo.js" as x module x at "foo.js"

module x from "foo.js"

Claus

# Herby Vojčík (7 years ago)

Claus Reinke wrote:

Module syntax alternatives: module x at "foo.js" module x = "foo.js" module "foo.js" as x module x at "foo.js"

module x from "foo.js"

module x "foo.js"

# Norbert Lindenberg (7 years ago)

Maximum 6 hex digits, maximum value 0x10FFFF.

Norbert

# Waldemar Horwat (7 years ago)

On 03/29/2012 08:30 PM, Allen Wirfs-Brock wrote:

I don't think the report on maximally minimal classes fully reflections the discussion:

Maximally minimal classes:

Alex and Allen initiated the discussion as a status up-date to TC-39.. We pointed out that this proposal had recently been extensively discussed on es-discuss and that it appear to have considerable support from most of the participants in that discussion.

Luke: These aren't good enough to be a net win.

I'm not sure whether this is an exact quote.

It is.

Luke certainly did raise the issue of whether classes, as defined by this proposal, added enough functionality to ES to justify the inherent complexity of a new feature.

Allen and Alex reiterated that this proposal is only trying to address the most common class definition use cases but in a way that allows for future extensions to address a broader range of use cases. These is significant value in what the proposal provides even if it doesn't do everything any might want.

dherman stated he has some minor design issues he wants to further discuss, but that overall the level of functionality in this proposal was useful and would be a positive addition. He supports it.

Waldemar: These don't address the hard problems we need to solve. Concerned about both future-hostility (making it cumbersome for future classes to declare, say, object layout without cumbersome syntax by taking over, say, const syntax) and putting developers into a quandry

We discussed this concern quite a bit and did not identify any specify ways in which the current proposal would block future extensions. Waldemar was asked to provide specific examples if he comes up with any. Allen pointed out that future syntactic additions can also enforce new semantics. For example addition of a per instance state declarations and a "const" keyword to the constructor declaration could cause ad hoc this.property assignments to be statically rejected, if that was a desired semantics.

-- if they want to do anything more sophisticated, they'll need to refactor their code base away from these classes. Unless one choice is clearly superior, having two choices (traditional and extended object literals) is better than having three (traditional, extended object literals, and minimal classes). Minimal classes currently don't carry their weight over extended object literals. Perhaps they can evolve into something that carries its weight, but it would be more than just bikeshedding.

The above is a statement of Waldemar's opinion. Other opinions expressed in the discussion aren't record in the original notes.

Yes, I marked it as such. Most of the other opinions had already been eloquently expressed on es-discuss.

Alex: We need to do something.

Allen and Alex also expressed that it is unlikely that any class proposal that significantly goes beyond will be accepted for ES6.

I don't remember that. Probably just missed it, but there wasn't much discussion of that option.

Debated without resolution.

In summary:

Waldemar should identify any specific ways that the syntax or semantics of this proposal would be future hostile.

Waldemar, Luke, and MarkM expressed varying levels of concern as to whether the user benefit of the proposal was sufficient to justify its inclusion. In order to resolve this question, both sides of the issue really need to provide better supporting evidence for the next meeting.

Allen

Thank you for the corrections. It's sometimes hard for me to participate and take notes at the same time, and I welcome corrections.

 Waldemar
# Allen Wirfs-Brock (7 years ago)

On Mar 30, 2012, at 3:50 PM, Waldemar Horwat wrote:

Thank you for the corrections. It's sometimes hard for me to participate and take notes at the same time, and I welcome corrections.

Waldemar

they are much appreciated. I don't volunteer as note taker for that exact reason.

# David Herman (7 years ago)

On Mar 30, 2012, at 7:42 AM, Claus Reinke wrote:

DaveH's presentation about module loaders

Shouldn't any newly designed async loading APIs support promises?

No, for the very simple reason that promises have not been standardized and won't be in ES6.

For ES proposals, promises seem to be burried in the concurrency strawman.

It's not being considered for ES6.

DaveH: Imperative module replacement example: @websockets exists as version 1. An implementation wants to improve it to v2: import "@websockets" as ws; System.set("@websockets", polyfills(ws));

That looks a bit like fiddling with Object.prototype for polyfills. Is that the recommended approach, and how does it interact with the static aspects of modules?

It Just Works. :) JS is a multi-staged language. One stage can modify the module table, and then later scripts will be compiled in the context of that new module table. The module itself is immutable.

Wouldn't it be more honest to use conditional loading, falling back to dynamic module handling?

You can certainly do that too. But the idea of polyfills is that client code (including third-party libraries you may be using that you don't want to or can't change) should be able to be written to recent versions of a standard API, and the polyfill makes sure to back-fill any gaps in older environments.

Module syntax alternatives: module x at "foo.js" module x = "foo.js" module "foo.js" as x module x at "foo.js"

module x from "foo.js"

As I've said before, this uses "from" inconsistently -- it muddles the difference between binding a name to an external module (module x from "foo.js") and binding a name to a specific export of an external module (import x from "foo.js").

# Brendan Eich (7 years ago)

David Herman wrote:

As I've said before, this uses "from" inconsistently -- it muddles the difference between binding a name to an external module (module x from "foo.js") and binding a name to a specific export of an external module (import x from "foo.js").

Another point of view: "from" says go get it if you don't have it. "module" says the module body is what you get. "import" says bind exports from the designated module in the current block scope.

In this light "from" can't be read by itself, you have to look for the leading "module" or "import". But maybe that's better than using something static or passive such as "at" instead?

# Claus Reinke (7 years ago)

DaveH's presentation about module loaders Shouldn't any newly designed async loading APIs support promises?

No, for the very simple reason that promises have not been standardized and won't be in ES6.

Understood. Though this leaves us with:

  • use of module loaders could profit from promises (eg, importing several modules in parallel, waiting for all of them to be available, or mixing module with other promise-based dependencies)

  • promises are in wide-spread use, are not standardized in ES, and do not seem to converge on a single implementation/API; as long as ES specs couldn't profit from promises, that was not as much of a problem

For the special case of modules, one could add off-spec support for a currently popular promise library, to avoid further divergence.

It Just Works. :) JS is a multi-staged language. One stage can modify the module table, and then later scripts will be compiled in the context of that new module table. The module itself is immutable.

So, a set only influences later dynamic loads? And the latest set before load wins? And, since "static" import is synchronous (same stage), any "static" construct textually after the set, but in the same stage, refer to the old module table, while the set-effect is implicitly pending?

Is the multi-stage aspect spelled out somewhere? In particular, I would have expected to be able to use "static" module constructs in module loader callbacks:

// stage n System.load("m", m => // "static" checks for stage n+1 at load-time import {f,g} from m; // stage n+1 f(1,2); )

but ImportDeclaration seems limited to Program and ModuleBody.

Wouldn't it be more honest to use conditional loading, falling back to dynamic module handling?

You can certainly do that too. But the idea of polyfills is that client code (including third-party libraries you may be using that you don't want to or can't change) should be able to be written to recent versions of a standard API, and the polyfill makes sure to back-fill any gaps in older environments.

I keep forgetting that parameterized modules are not yet on the table. If imports were module parameters, one could pass in patched imports to unchanged importers, without having to assign to the module table (fewer side-effects).

Module syntax alternatives: module x at "foo.js" module x = "foo.js" module "foo.js" as x module x at "foo.js"

module x from "foo.js"

As I've said before, this uses "from" inconsistently -- it muddles the difference between binding a name to an external module (module x from "foo.js") and binding a name to a specific export of an external module (import x from "foo.js").

"from" on its own doesn't do much, it is a modifier; "module" and "import" are the operative words. "module x from source" indicates a non-inline module, to be fetched from source, "import x from module" indicates a non-inline declaration, imported from module.

Since module declarations are just declarations, one might also consider 'import m from "foo.js"', but that wouldn't make a syntactical distinction for non-inline module declarations.

Claus

# Erik Arvidsson (4 years ago)

March 25 2015 Meeting Notes

Brian Terlson (BT), Allen Wirfs-Brock (AWB), John Neumann (JN), Jeff Morrison (JM), Sebastian Markbage (SM), Yehuda Katz (YK), Dave Herman (DH), Alan Schmitt (AS), Lee Byron (LB), Domenic Denicola (DD), Kevin Smith (KS), Andreas Rossberg (ARB), Brendan Eich (BE), Erik Arvidsson (EA), Adam Klein (AK), Jordan Harband (JHD), Mark Miller (MM), Istvan Sebestyen (IS), Jafar Husain (JH), Rick Waldron (RW)

6(iv) A Declarative Alternative to toMethod (Allen Wirfs-Brock)

AWB presents allenwb/ESideas/blob/master/dcltomethod.md.

Object.assign with 2nd arg object literal has a super hazard.

toMethod was previous workaround, but easy to forget and hard to use with object literals.

YK notes toMethod still valuable addition to meta-programming API.

AWB detials further issue of deep-clone vs. shallow-, unresolved (esp. viz .prototype).

Discussion of whether we can avoid toMethod entirely, not just for this use-case. MM hopes so, YK dashes hope by assertion -- to be continued later.

AWB observes that object literals and classes handle super and other contextual forms fine, by being special forms affording sound [[HomeObject]] initialization opportunity. Therefore proposes mixin contextual-keyword operator to extend object literal special form for this use-case.

NB: mixin is postfix operator with what looks like an object literal after it -- not binary operator.

MM: I suggested to AWB that the operator should be named mixin=, so it resembles the assignment operators (+=, *=, etc.).

All object literal syntax on right of mixin is allowed, except for __proto__.

mixin does [[DefineOwnProperty]] based on object literal contents (unlike Object.assign which uses [[Set]]).

Name abstraction of object literal for later param to Object.assign (still permitted, a bug if super used) doesn't work:

    let mixins = {...}; Object.assign(target, mixins); // how to do this
with mixin operator?

Use an arrow:

    let mixins = obj => obj mixin {...}; mixins(target);

Some inconveniences with classes (see URL). Solution is mixin class {...} extended special form. Throws if left-hand side is not a constructor (spec IsConstructor test returns false). Class body to right cannot include constructor. Gets non-enumerability of methods, static methods, right.

DH: question of user expectation of what's expected "moving" method from object to object. this as dynamically bound, lexically bound for arrows, understood; ditto lexically scoped upvars. Supposes we would have preferred super to be implicit parameter, akin to this, but we didn't do that. JS made method extraction easy, so it's common -- super as distinct from this goes against grain. So extracting a method with rebound super still wants toMethod -- mixin doesn't help.

AWB: "if we had been braver, maybe we could have made super be dynamically bound... but we didn't." (supposes JITs would have optimized away unnecessary super-params) DH: right, too late -- and who knows if it would have worked... but we cannot dismiss toMethod use-case for single-method extraction, even with cloning issues. DD/EA: agree, cannot desugar to existing functions not expressed as method of mixin right-part literal forms.

MM: no matter what we do, we can't make ES5-ish method-extracting code continue to work with super added somewhere in the method body DH: This is a regression of expressiveness. MM: Old library code that does mixins the old way is ok if used only in ES5-ish way. YK: If toMethod available, people will write patches to fix such library code.

Group debates exact method-extraction expressiveness/safety regression, mourns the loss. Some general sense that we should not throw toMethod baby out with bath-water.

ARB: this proposal is very imperative. If we want provide high-level support for mixins, especially with suggestive syntax, then it should be declarative, e.g. like traits. e.g. Scala class Foo extends Bar with Baz... or early ES6 class proposals DH: real problem with syntax that hides mutation, misuses mixin. AWB: can bikeshed operator name. DD: mixin is already variously defined by ecosystem, wrong word here.

MM: This is mid-level abstraction, should we do it or provide only high-level traits as ARB suggests? plus toMethod as low-level -- if high- and low- without mid-, would we need this mid-level proposal? AWB: "a lot of stuff in JS happens at mid-level." MM: if high-level needs language extension, doubts mid-level; if mid-level enables self-hosted high-level, may be ok.

DD: if mixin misnomer not used, then this would probably be uncontroversial. (BE: not uncontroversial with ARB) DH: been down design path of fake object literals (with triangle), hard to avoid kooky outcomes. DD: really need some syntax for [[DefineOwnProperty]], must avoid connoting "assignment" that runs setters. MM: if we used := value then value would not be property descriptor, so := ain't right either.

BE: beware justifying more kludges because JS is kludgey. There i said it!

MM: how do you write the trait to be mixed in? Lambda-abstracted form (obj => obj mixin {...}) pleasant.

AWB: dares to write :={...} instead of mixin {...}, DH calls it the barkeep operator. :={

DH: is this about 1. a collection of properties from which 2. to mutate the target? Need something connoting that double meaning.

YK: re: MM's lambda-abstracted point, can linearize mixins along prototype chain and get super chaining to work. YK shows Ember's Mixin.create example (LINK NEEDED) demonstrating this. Distinct from AWB's proposal, more about traits in JS than about define-properties-on-target mid-level. DD: back when Chrome Canary had toMethod, I was able to use it to create these kind of mixin-proto-chain things: jsbin.com/fepudi/2/edit?html

YK: my Ember-based lambda-abstracted class-extends expression is a bit too much mechanism BE: or ceremony? YK: see gist.github.com/wycats/f79fe019d4bf29177b6c part-way down. MM: I like it, what's the problem? YK: wouldn't it be better to support high-level mixin syntax:

mixin TextSupport {...}
class MyComponent extends EmberComponent with TextSupport {...}

MM: don't need more syntax YK: indeed, could just have

class MyComponent extends mixin(EmberComponent, TextSupport) {...}

YK: I'm personally ok with this.

EA/DH: want happy-path "blessed syntax". DH: but let's not rush to design it here!

MM: with arrow-based lambda-abstraction + YK's class extends chaining for linearized super-preserving mixins, I don't want mixin. Chaining multiple super calls is winning here.

AWB: still leaves a mid-level gap, for define-these-properties-on-this-target. MM: is cost of plugging this hole worth the benefit, given new syntax very-high costs as noted? AWB: we need a stage 0 proposal for at least some two of three {low,mid,high}-level ideas.

BE: did we not agree that low-level toMethod is needed. Group: not in light of new news.

DH: advocates toMethod as analogous to bind AWB: not so, clone issue MM: not so, more important: bind does not allow this rebinding, toMethod allows generating new super bindings in new cloned methods If you could write function with free super that would throw without toMethod, then ok -- but we don't have that If you were constrained by new syntax to write a factory of super-bound methods, then ok too. YK: some extra boilerplate.

AWB: want to use concise method syntax since it handles super correctly, somehow thus the idea of target mixin { method() {...} }.

MM: difference between method-with-unbound-super and lamba-abstracted class-extends mixin/trait chaining: latter does not imply toMethod plausible to have low- and high-level that cover the space without AWB's mixin mid-level. MM: kind of want function-bearing-unbound-super to be not callable. AWB/YK/BE: function* vs. function precedent, but don't want more sigils/function-suffix-punctuators

MM: sketch of special form that allows unbound super:

function (super) foo (x, y) { ...super ...x ...y }

Factory of methods with unbound super uses. Trying for low-level principled alternative to toMethod.

AWB: alternative I prefer would leave function with unbound super uncallable, add Reflect.toMethod API for people to build libraries. BE: syntax for low-level is unusual, really prefer API high-level as ARB suggested (Scala) would be new syntax MM: lambda-abstracted class-extends relieves me of wanting anything like that high-form BE: true, although new users and mass-market programmers still "WAT" at the boilerplate but it's not that bad here (vs., e.g., the module pattern)

KS: painful if unbound-super in functions allowed, because define-properties-on-target requires long-hand unconcise-method syntax ARB: why not allow function f(super, a, b) {...} and (in concise method position) m(super, x, y) {...} instead? MM: Oh! YK: probably few can track our discussion at this point!

YK: want to avoid "harsh end of life" outcome for ES5-ish libraries.

KS: if we had not bound super in concise methods in object literals, then we could use concise methods without wrong-super fear. BE: is ARB's leading super parameter dead? YK: it's easy to forget and kind of weird DH: It's like default-final, you have to opt out too much and people forget.

AWB: need some stage 0 proposals, at two of three levels. YK: not sure we want Babel implementing this yet. YK: Babel should not implement stage 0, wait for stage 1 or later. Actually use stage-N as flag. EA: Babel can do whatever they want DH: We should care, because ecosystem effects, premature de-facto standards

Agreement that Babel (and other compilers) need to flag early-stage stuff, not expose prematurely, message well.

6(v) Additional Meta-Properties in ES7 (Allen Wirfs-Brock)

(allenwb/ESideas/blob/master/ES7MetaProps.md )

AWB: [presents slides (link?)]

AWB: function.callee: refers to the currently-executing function

MM: What's the use case for function.callee?

AWB: Referring to anonymous functions, or arrow functions

MM: Just assign it a binding in the containing scope. Say, a let binding.

AWB: Lots of requests to support callee, even though there are these other ways of supporting it.

JH: New syntax should start "10000 points down"

MM: Possible use case: referring to a concise method from inside the method's body

DH: Or inline event handlers in HTML

YK: I really hope people don't use it in that case

EA: Even for addEventListener, some people want this, since it's common to just pass the function as an argument without naming it

BE: You could have concise methods bring their name into scope, but that runs afoil of the same issue with dual class bindings in ES6 (immutable in the class body, mutable in the containing scope)

DH: There seems to be a disconnect between this committee's confusion over why anyone would need this feature and emphatic requests from the community for its inclusion

YK: The biggest use-case is callback-based APIs where the function needs to use its own identity to refer to itself, say to call removeEventListener

DH: That sounds like the best argument so far. Maybe the problem is the name? It sounds fancy, but it's statically always this function

MM: How does function.callee interact with arrow functions? It seems like it must refer to the enclosing function, just as this does

DH: Agree

DD: Disagree

DH: Explaining why arrow functions are treated specially: arrow is not syntactic sugar for function, they are a different kind of thing, as close to TCP as possible, unlike concise methods, which are "close to" sugar over name: function() {}

DD: But concise methods are not just sugar: see super bindings

YK: People who have tried to replace all functions with arrow functions find that they do not behave the same

MM: The analogy for arrow functions is to blocks, not functions

JH: Is "block" really a good metaphor, given that ES5 did not have any block scoping?

MM: That's the best I've been able to come up with

YK: It's really close to blocks in Ruby

BE: "function" is much too long. Maybe there's another keyword we could use? "function" is both too long, and confusing due to arrows.

DH: Do we even want this to refer to arrow functions, syntax aside? I say no.

DD: But if the main use case is event listeners, then that doesn't help

DH: Major problem is that referring to the arrow function is a refactoring hazard (e.g., moving it inside a forEach will result in a different answer). But maybe the argument is that it's a reflection feature, and reflection features are already refactoring hazards.

YK: There are two kinds of callback-based APIs in JS: synchronous and asynchronous. In the sync case, you want TCP. But for the async case, you don't.

JM: The explanation to the average programmer can't be "because TCP"; people won't understand that

MM: Before arrow functions, programmers constantly used this inside a callback and expected it to refer to the this of the containing function. The reason I explain arrow functions as blocks is that it's just "I want to run this code as a callback", with all bindings identical.

JM: I think the problem is just that I've tried to explain TCP to people

YK: Don't try to explain it that way. Maybe the problem is that we only have fat-arrow (designed for the synchronous callback case), but people want something that also works in the async case (which could have been thin-arrow).

DD: Rethinking, due to the forEach case. Maybe I do want function.callee to refer to the outer function.

YK: I think we need two things: one that refers to "this function" (including arrows), and one that does not include arrows.

ARB: Where does that end? Why not add more levels of function?

YK: We already have these two levels that have different behavior (arrow and non-arrow)

DD: Is it fair to say the argument is between 0 and 2 ways of refererring to a callee?

MM: Do we have any quantification of the need for this field?

YK: It used to be a big deal for me in ES5, but I got over it

DD: My problem with having 2 ways is it adds a cognitive burden whenever I want to use this thing.

YK: I agree that it's a cognitive burden to have 2, and that that may point towards having 0

AWB: I'd make an argument for 1: it refers to the innermost function, whatever kind of function it is (including arrows)

MM: This whole discussion is the result of an anti-pattern, that there's a requirement to refer to function identity to interact with, e.g., addEventListener, instead of having it return a token

YK: addEventListener is a void function, so it could be fixed in DOM

DH: Someone would have to do that

DD: setTimeout already returns a token, and in Node it's the right thing, rather than a number like on the web

MM: Caja actually wraps setTimeout and has never run into compatibility problems (jQuery works fine)

...moving on with AWB's slides...

AWB: function.count to find out how many arguments were actually passed.

ARB: Why are we working on this now? We don't have much experience with how people are using ES6 in the wild.

DH: Now is a good time to explore this space. In strict mode, we got away from using magically-scoped variables for introspecting on certain kinds of things. new.target provides the blueprint for a new way to expose these things, so it seems reasonable to consider adding such things with this new syntax.

AWB: ES6 provides lots of new ways to pass arguments, but doesn't provide an easy way to answer the question "how many arguments were passed"? This is useful for overloading based on the number of arguments. Using ...args and destructuring is a pain.

DH: Why not function.length?

AWB: To avoid confusion with the length property of Function instances

ARB: Don't we have the same issue with arrows again?

AWB: Yes.

DH: We could make each decision at a local maxima, and end up with a completely confusing set of cases. Another option would be to have function.callee be a record, with other properties hanging off of it to ensure consistency between the different properties.

DH: The two concepts are "nearest enclosing function" and "nearest enclosing callable thing"

AWB: next up, function.arguments: the actual argument values

MM: How is this different from arguments

AWB: You get a fresh array each time you ask for it

DH: Why?

DD: It could be a frozen array and always return the same one

MM: I still don't understand what advantage this has over arguments?

AWB: It's a real array

YK: Use case is to name individual arguments, and then pass ...args to some other function

MM: But why couldn't you use arguments for that case?

AWB: The difference is that it's a real array. Also, we're walking the line of whether arguments is considered deprecated.

DD: And it works in arrow functions.

DH: I agree with AWB's vision that arguments is deprecated, and that's the argument that makes function.arguments make sense

MM: What is the evil of the strict arguments object such that we would want to deprecated that and not deprecate this?

DH: Strict arguments is TCP-violating

MM: So is function.arguments

DH: I admit it's not a strong argument

JH: How bad is using ...args and having to use destructuring on the next line?

EA: You lose documentation of what the actual arguments are expected to be.

DH: Options are you use one of 3: 1. ...args and destructuring (bad for documentation) 2. named params and ...args (have to do math) 3. arguments

AWB: But won't arguments.length materialize the arguments object?

BE: Not in Spidermonkey, or in V8

YK: If we're going to go with arguments, then we need to stop saying it's deprecated

DH: So which parts of arguments are still deprecated? We all agree that arguments.callee, arguments.caller, and Function.arguments are deprecated.

YK: Indexing arguments still seems like a bad practice in ES6. Spread is OK, though.

DH: That is not a coherent position.

YK: The thing that's not OK is treating arguments as first-class value

DH: That seems like a very fine distinction

DD: Can we pop the stack here?

AK: The question about whether we can do indexing seems separable from whether to call the array-like thing arguments or function.arguments

DH: It's important to get clear on what to say about function.arguments, and what to say about deprecation

BE: I think it's fair to say we don't want to spend all afternoon on this

...break...

Initial Value Passed to first call of a Generator next function

function * gen(a)  {

}

var g = gen(1); // this?
g.next(1); // this?

AWB: function.next: the current yield result

MM: Of all of these, this is the one I find compelling

AWB: These don't have to all be in a package. We can do one of these without the others

MM: I think we should do that with this proposal and move it through the process

YK: Can someone state the compelling use-case?

JH: A good usecase is a lexer. You need to get access to the first character that was passed to next().

...next slide...

AWB: function.thisGenerator: current generator instance

MM: If you want this, you can wrap your generator function with another function, bind the generator in the function scope, and then you can refer to that from within the generator function.

YK: But what's the use case?

MM: That's a separate question, as the above suggestion allows referring to the generator if there are use cases

BE: Is this something people asked for? It doesn't seem that important.

...next slide...

AWB: export.name: current module identifier

DH: Do we need export.name if we have import from this?

DD: I thought it was import from this module

DH: Don't like compound keywords

YK: I'm still opposed to import from this.

AWB: Other proposals: import.this, import.meta

DH: Let's discuss separately.

...end of slide deck...

AWB: So I'd like to put all these at stage 0.

MM: I propose we make function.next stage 0 and leave the rest.

DD: I got the impression that we rejected the rest.

AWB: That's not my impression

MM: What does stage 0 require?

DD: Only being not categorically rejected

[discussion of how to handle organization of stage 0 proposals; how to make it clearer which stage 0 things we'll do and which we won't]

MM: So can we list these as rejected proposals and bring them back later if we change our mind?

AWB: I think that would be a mistake. That seems too strong for these.

Conclusion/ Resolution

  • function.next to be split out into its own proposal
  • AWB to keep the rest alive for now at stage 0

Report on 402

(Rick Waldron)

RW: Complete, barring any editorial bugs.

  • Has been reviewed by Norbert Lindenberg and Andrée Bargul.
  • Assembled a whole team to read the extensions to the spec.
  • Added 402 repo to Github. With same proposal pipeline as 262.

Making a motion for acceptance/ratification/whatnot of 402 2nd edition... forward to the ecma assembly, subject to the rf opt out...

Unanimous approval to forward this to TC39 rftg.

TC39 vote 6 in favor. 1 abstains (JH, Netflix)

IS: Shall 402 be submitted for ISO fast-track? Or leave it ECMA-only like the first edition?

JN: Send it to ISO and see what they say

Conclusion/ Resolution

  • Submit to Ecma GA
  • Submit to ISO

6(vi) Function Bind and Private Fields Redux (Kevin Smith)

zenparsing/es-function-bind, zenparsing/es-private-fields

KS: [presents slides (link?)]

KS: Abstract references recap.

KS: Problems with combining the various use cases. Instead, provide two different syntaxes, one for function binding and one for private state.

KS: Part One: Function Bind via ::

function f() { return this.x }
let bound = ({ x: 100 })::f;
bound(); // -> 100

DH: This could be really nice in that you don't have to use bind anymore

KS: :: doesn't take care of all of bind: only binds this, not any arguments

ARB: Syntax somewhat in conflict with :: in C++, inverts the meaning of '::' vs '.'

AWB: Prior to ES6 there was a lot of confusion about what this means. With ES6 classes (and arrow functions) we made the story a lot cleaner. Maybe it is better to let this cool down a bit and see how things turn out in a few years.

MM: This is only an argument against the infix operator.

DD: Also, the example uses function instead of being a method in class.

KS: Unary Function Bind

::console.log  // -> console.log.bind(console)

AWB: This seems perfectly reasonable

MM: We could do the prefix notation without the infix notation

YK: What's the pedagogy here? When should programmers use ::? MM: If you're calling a method, use dot. If you're extracting a method, use :: YK: Why didn't we fix this with classes? MM: That's water under the bridge.

KS: Back to the infix operator. If the function is immediately called, then I'd like to be able to desugar to call:

obj::f();
// -> f.call(obj);

DD: What about with new?

KS: Don't do this desugaring with new

AWB: [back to the unary operator] I like this because it's an operation on a reference

MM: Can you write other expressions on the right-hand-side that evaluate to a reference?

AWB: DH's prior proposal allowed it

MM: The case I can think of is a variable reference, inside a with, that's resolved to an object property

AWB: No, that won't work in this case because it's not [...the right kind of reference?...]. The base is an object environment record in this case.

KS: Can this be a syntax error if it's not a property lookup?

DH: Why not require it to be a member reference?

MM: I don't see any reason not to statically constrain this to property lookup, either dot or square bracket

AWB: Some concerns, will consider offline

KS: [new slide: Bind, The Big Picture] the infix operator is an alternative to the adapter pattern or extension methods.

MM: this should really be passed as the first argument.

KS: The implicit this is really the first argument.

DH: Worried about the additional overhead this syntax adds to the language [referring to the infix operator]. Having to make the decision between . and :: seems problematic.

JH: And in C# you always use .

YK: The alternative is you use an adapter first, and then . the rest of the way to the right

JH: Yes, you can do this with an adapter pattern, but then you need to build an adapter layer and decide how to do it.

BE: The infix operator seems attractive for extension methods

JH: I agree that it's sensitive to add new syntax, but this has been done in other languages and people adapt to it

DH: The way forward seems to be to get large-scale feedback about using this [maybe from Babel users?]

DD: React developers using Babel are already using the existing :: implementation, and filed bugs when KS changed the semantics

BE: Agree with DH, need more data to see how users like it

EA: What AWB said earlier about the use of this rings true, ES6 just changed the uses of this (with classes and arrows)

[lots of discussion of other tokens: ->, .:, .?]

Conclusion on bind

  • Get more feedback from users of Babel

Part 2 of KS presentation: Private Fields

KS: Preface: this conflicts somewhat with decorators, in that it makes use of @ for its syntax. Will focus on the semantics in this presentation.

KS: [slides]

JM: Is there something essential to private fields that makes it important that private fields are created at construct time and guaranteed to all be present on instances?

MM: It's important because you'd like to maintain invariants in your implementation, invariants you don't get anyway for public fields [due to them being public]

KS: Private fields should not write through the prototype chain [example from slides]

AWB: This just fails because the imposter object doesn't have the private field, nothing to do with prototypes

KS: Private fields should also not read through the prototype chain

AWB: It seems like that just falls out of the fact that the field isn't present on the receiver

JM: Trying to find common ground between private fields and initializers for instance properties

KS: Two main options around initialization: either initialize all private fields to undefined, then initialize them one by one, or run all initializers first, then create and write all fields at the same time. In the latter case, referring to this in the initializer expressions must be disallowed.

[lots of discussion about initialization, resulting in punting and moving on to discussion of how these things work after initialization]

KS: [presenting spec on github] zenparsing/es-private-fields

[discussion of the use of PrivateMap in the spec language]

MM: The PrivateMap is not reified in the spec text, which should avoid any complaints related to WeakMaps and the transposed representation

KS: Initialization: current spec text takes the "batching" approach discussed previously

AWB: Initialization of private slots involves walking through the inheritance hiearchy, gathering private field requirements at each level, and that gives us the necessary information for allocation. Then there's a staged initialization, where at each base class, the initializers end up running before super returns.

YK: Worried about return-override from super constructors. You have to add the private fields to whatever object is returned from super

ARB: Can we get away from the details of the initialization and step up to finish KS's presentation?

KS: Brand checks pass once initializers have run for a particular level of the inheritance hierarchy.

YK: Due to return override, you can't simply walk the inheritance hierarchy to gather the set of private fields.

AWB: It's important to me that the private fields are atomically allocated across the whole inheritance hierarchy.

BE: But you can't do that with return override.

BE: We could decide that return override is incompatible with these private fields.

BE: Or we could just figure out a way to let private fields be added each time super() returns

AWB: How do these things interact with proxies?

MM: You get a type error if you try to look up the the field on something that fails the brand check, and proxies fail the brand check.

MM: Another advantage to this proposal is that you can still add private fields to frozen objects.

--- MM branches off to talk about initializers, asking for them to initialize instead of assign, which would allow data-dependent const fields

[discussion of possible constructor syntaxes for initialization]

YK: Worried that moving declaration of instance properties (private and public) into constructor would break decorators

MM: But then how do you decorate fields whose initialization is data-dependent?

YK: That's not a case that comes up very often.

DD strawman:

class Point {
  constructor(x, y) {
    private @x = x;
    public y = y;
  }

  get x() {
    return this. at x;
  }
}

Presentation continued

# Erik Arvidsson (4 years ago)

March 26 2015 Meeting Notes

Brian Terlson (BT), Allen Wirfs-Brock (AWB), John Neumann (JN), Jeff Morrison (JM), Sebastian Markbage (SM), Yehuda Katz (YK), Dave Herman (DH), Alan Schmitt (AS), Lee Byron (LB), Domenic Denicola (DD), Kevin Smith (KS), Andreas Rossberg (ARB), Brendan Eich (BE), Erik Arvidsson (EA), Adam Klein (AK), Jordan Harband (JHD), Mark Miller (MM), Istvan Sebestyen (IS), Jafar Husain (JH), Rick Waldron (RW)

Private state continued

KS: the "nested stuff" part of the private state implementation might be more controversial, but please note that it's separable.

KS: Modified the V8 self hosted Promise to see what it would look like with "private state"

AWB: the most interesting use of private state is in the typed array hierarchy, especially in terms of subclassing

KS: presents zenparsing/es-private-fields/blob/master/examples/promise-before.js

KS: After: zenparsing/es-private-fields/blob/master/examples/promise-after.js

ARB: The implicit this in @name is not something we have done before in JS.

BE: CoffeeScript set the precedence. Are you suggesting that you always have to write this. at name?

AWB: The semantics does not change.

DD: The name is in a lexical scope

AWB: We could do the same with non private. That would not be a good idea.

JM: should there be symmetry between private and public properties?

BE: it's part of our job to make the ergonomics sweet and competitive.

KS: moving on to helper functions. If the helper function itself needs access to the private state, where can I put it? I can't put it outside the class anymore (like in promise-before.js), because then it doesn't have lexical access to the private state. So that seems to want to push that function declaration of the helper function into the class body. In the example here I'm mixing my proposals (with this::_resolve(x)), but you could have done it just as a function.

ARB: Might be cleaner to introduce @name methods. Both static and prototype methods.

JM: three alternatives here. What you have here, a lexically confined closure. Or, a static method that is "private". A third way is a private instance method (on the prototype).

KS: so you're talking about this kind of setup

class C {
  @x() {
    C. at x();
  }
  static @x() {

  }
}

I tried to go down this path. It seems weird that I have to do this.constructor. at y() (or C. at y()) to call theymethod fromx`.

AWB/ARB/others: no, that seems perfectly fine to me.

BE/AWB: The @x method is on the prototype.

MM: The visibility is that it is only visible inside the class body.

AK: I don't think this. at x() means the right thing, because we didn't want private field access to go through prototype chains.

(general acceptance that this is a deep issue)

MM: I now think they should be on the instance

DD: on the instance is bad ... memory-inefficient ...

BE: implementation doesn't have to work that way, even if semantics do

DD: I see ... so since they're private, we can lock them down enough that such an optimization is not observable

EA: These methods do not need to be properties anywhere.

MM: even in the weak map explanation, it's not like the closure pattern with separate-method-per-instance; it's a separate weak map per instance, but each of those weak maps points to the same function object.

(discussion of how this is related to vtables)

ARB: hold on, if this is private, why do you need a vtable at all... they're completely static

AWB: well ... what if you reify them ...

MM: reifying them is not what we're considering today; that'll be a much bigger fight.

ARB: it's basically a private scope.

MM: Compile time symbol table.

AWB: When you start to combine subclasses the tables makes things simpler because you can combine these tables.

ARB: not for methods ... for private methods it completely doesn't matter

BE: yes, it boils away. When we're talking about vtable it's purely about future things like interfaces

ARB: doesn't even matter for interfaces...

EA: what about call and apply? this. at x.call(otherThis, ...)

ARB: I think you can do that. And that's part of why I don't think we should allow a shorthand that omits this.

AWB: why would you ever do that?

DD: can you use super inside private methods?

MM: The home object for a @m() {} method needs to be the prototype (C.prototype) and similarly the constructor for a static at-method.

JM: Would it make more sense to think of these as functions that are set as instance properties during the instance property initialization.

MM: We want class private

ARB/MM: The --weak-- map explanation vs the private symbol is that the map case the function is associated with the object ....???

AWB: Or the vtable explanation.

EA: What if you had C.

class C {
  @a;

  m() {
    let somethingElse = C;  // <--
    somethingElse. at x();

  }
  @x() {
    C. at x();
  }
  static @x() {

  }
}

ARB: that is a very good point; I think it means we can't have the same name as both static and instance.

MM: It would force internal polymorphism; it's not good to pay the cost of this when it's essentially accidental.

ARB: given a random object o and you do o. at x on it, you have no way of knowing which type of private that is referencing statically (i.e. instance method private or static method private or instance data private), and that defeats the point.

MM/ARB: Do not allow them to have the same name.

DD: now this is bizarre. They act too different from public methods/public static methods.

EA: Kevin's original (nested declarations) idea is seeming more attractive after this exercise.

KS: (presents original nested declarations idea)

class C {
  @a;
  function xHelper(obj) {
    obj. at a;
  }

  m() {
    xHelper(this);
  }
}

(general admiration)

ARB: Weird about this is that it looks like a public declaration.

AWB: In this case you are not talking about dispatch.

ARB: at least it should not be function, maybe private xHelper() { }

KS: the concern is that if you use private then people would think it's a private method...

BE: private function xHelper() { } might help

DD: Not intuitive to programmers coming from other languages. People already use external helper functions.

KS: Feedback has not been too positive. The external helper function is used in Python for example.

ARB: fear that most programmers will want to use method syntax and will make helpers public just to get that

Immutable Records, Tuples, Maps and Sets

(Sebastian Markbage and Lee Byron)

sebmarkbage/ecmascript-immutable-data-structures

LB: ImmutableMap reutrns a new Map when you mutate them.

LB: Wants value semantics. Especially for == and ===.

MM: As well as Object.is.

BE: "NaNomali"

LB: Wants deep equality so that a Map of Maps or vector/records works too.

LB: Wants to work across realms and shared memory for workers.

LB: Based on Typed Objects.

Wants new syntax.

const xy = #{x: 1, y: 2};
const xyz = #{...xy, z: 3};

const xy = #[x, y];
const xyz = #[...xy, z];

LB: Record is the realm specific constructor/wrapper (like String)

BE: For value types we didn't want the wrappers.

JHD: What happens if you pass these primitives into the Object function? (like Object('string'))

LB: Works the same. It creates a wrapper of the correct type.

EA: Are the keys always sorted?

LB: Wants to allow implementations to do whatever what they want but...

MM: I think you are doing the right thing since then the comparison is cleaner.

AWB: Module namespace exports are sorted with the default comparison.

ARB: When you implement immutable maps you do not use hash tables but ordered trees.

MM: You cannot define an order for opaque object.

MM: Can an immutable map refernce a non mutable object?

ARB: You can specifiy that keys have to be deeply immutable.

LB: Would like to have same keys as in ordinary maps.

ARB: Then how would you implement this?

LB: You would use a hash function/method and a trie. The hash is optional. That is why we want it implementation dependent.

LB: Would have to define what order to use.

ARB: That means undeterministic behavior.

LB: In immutable.js the order is part of the equality.

MM: If you want you could canoninicalize the order before comparing.

LB: Most people do not depend on the order.

MM: You could do insertion order and make === be equal but make Object.is return false if the order is not the same.

ARB: Another option is to use value order where possible but use insertion order otherwise.

MM: You could use hash consing.

ARB: Does not help for ordering.

MM: Need to make sure that you do not use the real physical address, due to mutually suspicious code in the same realm. Need to be unguessable or use the hash to achieve information about code it should not know about.

YK: Maybe you want lower level primitives so that user code can implement this.

LB: No. We need to define the semantics so that VM can optimize this.

LB: A VM can use shared memory. It can use low level structs and memory to achieve.

ARB: A VM can have efficient per object hash codes.

JH: Also syntax.

AWB: Per object hash code is the big missing building block that we do not want to provide.

EA: It is not possible to make a user exposed hash code that is going to be as efficient as the internal hash code which might just be the address of the object.

MM: For ordering you might get away with a not very good pseudo random generator.

LB: It seems simpler and good enough to just stick with the insertion order for immutable maps.

MM: Implied cost. The precise equality would have to take the ordering into consideration.

LB: Batteries included philosophy.

AWB: You can still have a standard library that can be implemented in JS.

LB: Libraries can always provide more data structures.

AWB: I don't think we should provide new libraries without providing the primitives that allows these to be implemented in user code.

BE: What primitives are missing?

LB: Assuming we have typed objects and value types, value semantics

MM: Cryptographically safe pseudo-random numbers as a hash code.

DH: Sharing immutable objects across workers.

MM: Cryptographic pseudo random number generators.

MM: The number has to be large because collision is fatal

LB: Another issue is that we are creating a new value type for every record.

Conclusion/Resolution

  • Identify the requirements.
  • Progress report at a future meeting.

Composition Functions

(Jafar presents slides)

(Lots of discussion. Notes lost to a grue.)

Conclusion/Resolution

  • Wide agreemeent that async functions are worth generalizing
  • Wide agreement that promises are the dominant use case and should be the default
  • Tricky problems with hoisting semantics, need more time to woodshed that problem
  • async/await is wanted/needed so we should urgently figure out if this can ever be made to work

(discussion about wtf woodshed means - behind woodshed = to kill, to/inside woodshed = to spank)

Additional export __ from statements

(Lee Byron presents leebyron/ecmascript-more-export-from )

General agreement, suggestion to add Babel transpilation and fill out spec text, bring back at next committee meeting for a fast-track through stages.

64-bit math

(Brendan Eich presents gist.github.com/BrendanEich/4294d5c212a6d2254703)

Moves to stage

# Brendan Eich (4 years ago)

March 24 2015 Meeting Notes

Brian Terlson (BT), Allen Wirfs-Brock (AWB), John Neumann (JN), Jeff Morrison (JM), Sebastian Markbage (SM), Yehuda Katz (YK), Dave Herman (DH), Alan Schmitt (AS), Lee Byron (LB), Domenic Denicola (DD), Kevin Smith (KS), Andreas Rossberg (ARB), Brendan Eich (BE), Erik Arvidsson (EA), Adam Klein (AK), Jordan Harband (JHD), Mark Miller (MM), Istvan Sebestyen (IS), Jafar Husain (JH), Rick Waldron (RW)

4 ECMA-262 6th Edition

(AWB)

AWB: (presents slides)

AWB: release candidates are intended to be complete subject to bugs so everyone could review them. RC1 was the review draft for the RF opt-out period.

AWB: bug count fluctuates toward zero and back up; minor technical or editorial issues. Waldemar found some small issues in the grammar (unnecessary parametrization, Unicode regexp grammar, ...). Bugs that come up between now and June will be fixed as practical, or deferred to ES7 if not.

AWB: any last minute issues!?

BE: what about the things that are backward-incompatible?

AWB: e.g. Brian has some issues around the function name property

BT: i.e., the assignment of names when assigning a function to a var declaration

AWB: EA has an issue around [[DefineOwnProperty]] on module namespace objects...

BE: built-in prototypes?

AWB: in RC1 it became pretty clear that at least making Array.prototype not-an-array would break at least some peoples' code. So for RC1 I reverted Array.prototype back to an Array exotic object. There was similar discussion about RegExp.prototype. For RC1 I did an experiment with some hacks that I thought would maybe give us a middle path. But that got reverted back.

BE: not sure there's any profit in trying to change it in the spec.

BT: that's what IE has: RegExp.prototype is an ordinary object; the only prototypes that are not ordinary objects are Function.prototype and Array.prototype.

BE: what about Object.prototype.toString on those prototypes? [Actually I was asking what RegExp.prototype.toString() returns. /be]

BT: it gives [object Object]. We haven't come across any code yet that breaks because of these changes, and to put that in perspective we have 3 or 4 sites that break due to the function name thing.

Module Namespace Exotic Objects

DH: I think there was a minor issue with the module namespace objects...

EA: my concern here is that we're introducing a new exotic object.

DH: that has been my intention for fully six years. (For some value of six.)

DD: I think the wiki said "Object.create(null) with getters"...

DH: yeah you're right, the exoticness is not the higher-order bit.

AWB: let me run through the things you can do with a MNS exotic object. A get; no set; a get with a symbol; enumerate; has-own (name or symbol); and that's pretty much it. Everything else fails (false return or throw). So e.g. Object.getOwnPropertyDescriptor will throw on one of these.

EA: a solution is to simply return a data property descriptor. Then code that uses Object.getOwnPropertyDescriptor, e.g. for mixins, will work.

AWB: the reason I made it throw is that the MOP lets you define things that are neither data properties nor accessors, and this is one of those cases, and the MOP lets you throw in those cases, so we could.

DH: it is self-evidently possible to write the spec as it's written now, but the question is whether we should.

MM: I would find it more useful to not throw.

AWB: even though it's not really writable?

YK: it is writable, but it's not writable by you.

AWB: I worry that we're establishing a precedent that we're going to make [[GetOwnProperty]] behave that way.

DH: there's a lot of code out there that has expectations about objects you give it, and if you build an object that breaks those expectations, then that particular class of object becomes rounded up and treated as toxic and people say to never use these objects, or alternately it starts to poison the well of these common operations, which is even worse. For example we did a similar thing with enumerate. Module objects ought to behave as close to normal objects as possible. It's a much narrower diversion to have an object which says it's writable but you can't write to it, than one that throws when you try to get a descriptor for its properties. On top of that, there is precedent for property writes quietly failing...

MM: you could write something with an accessor that has that behavior, but it's not normal... non-writable properties normally throw in strict, fail silently in sloppy.

AWB: being a data property means that if you get the descriptor you get the value of it. That means in [[GetOwnProperty]] I have to access the value, which could fail because of TDZ.

DH: this is the MOP. TDZ is part of the core semantics. How do we reflect TDZ in the MOP. We could throw, or we could have some special representation of TDZ state in the MOP (e.g. undefined). A throw seems better at this stage, in the case where you hit TDZ.

ARB: what happens when you do a [[Get]] in the TDZ?

DH: it throws

YK: why is it not just a getter

ARB/MM/AWB: then you have to reify the getter, and it's annoying to implement

Conclusion/Resolution

  • Change module namespace exotic object [[GetOwnProperty]] to reflect as a writable data property.
  • [[Set]] continues to throw in strict mode.

Function Name Issue

BT: we found several sites that are broken by the new function.name algorithm. One site we found is using function objects as maps, and so doing e.g. if (func[key]) { ... }. Since ES6 defines new semantics for func.name, this breaks the site. We have enough information such that we're not going to ship this. We could perhaps only infer for assignments to let and const, not var.

AWB: we infer lots of places...

BT: we've only seen problems for assignments to var.

MM: so if we removed it from var everything would work?

BT: yes, but it seems a bit hasty to make that change without testing...

MM: at what point in the process can changes like this not go in.

JN/AWB: we're there now.

DD: it's not a big deal if we have a different function.name algorithm in ES7.

DH: no, really!? If we have something that pretty clearly needs to be fixed, we should be able to change right up until the GA votes.

AWB: the GA needs the time to review it...

DH: in a 1300-page spec, a small change like this is not going to change anyone's opinion about ES6.

AWB: it's a process thing. This is about a feature, and it's either in or not in.

DH: the feature is in. I'm saying this is at the level of bugfix.

ARB: in Chrome every function has a name property....

BT: the problem is that it switches from falsy to truthy when it becomes a non-empty string.

BE: why are we changing anonymous function names?

MM: we're not changing anonymous function names, we're changing which functions are anonymous :)

ARB: which is one of these webpages?

BT: cheezburger.com, the comments won't load.

BE: stop ship bug, then!

ARB: cheeseburger.com?

AK: no, cheezburger.com.

DH: we should at least have a GitHub errata page where we can enumerate these things...

DD: once the great tooling revolution arrives, there will be a live GitHub version of the spec that implementers consult.

DH: yeah, but until the Great Leap Forward arrives, we need something in place.

YK: let's put errata in tc39/ecma262.

MM: having this be an errata-managed issue seems fine to me.

BE: what about a new property, instead of .name?

AWB: on Twitter somebody suggested it should be a symbol

MM: we're standardizing the de-facto precedent. The reasonable backoff suggested by the data is to not infer the data from the var assignment.

DD: two options. Make the change now with no var; or, leave the spec as is, let Microsoft accumulate more data to see if no-var-assignment is sufficient; if so then use errata to officially recommend those.

MM: let the ES6 spec and Microsoft's implementation diverge, and use that to figure out exactly what the right fix is.

EA: we should either do full inferencing or revert to ES5 semantics.

MM: what about concise methods?

EA: that's fine; they're new syntax

AWB: removing name inference would be a big deal.

BT: there are going to be more issues like this; errata management seems like the way to go.

AWB: indeed, the nature of the process is that we don't get this level of information until we're at this point in the process, with implementations.

BT: honestly, this issue is small compared to sloppy mode const semantics, which we've found issues with as well.

(general consternation)

MM: did we have a resolution for the class bindings issue raised on list?

AWB: probably leave as status quo...

DD: what about making them const?

AWB: that was the other possibility...

YK: there was a reason we didn't do that... I remember feeling strongly at the time.

DH: global?

YK: in the global space, people want to replace things that are global.

MM: I retreat...

AWB: OK, so, I don't think we should keep fishing on "is there anything else we should change"?

Conclusion/Resolution (for function.name issue)

  • Leave the spec as-is
  • Let Microsoft experiment with more web-compatible versions of the algorithm, e.g. not-for-var
  • Have a public place to track and post errata regarding this issue, until the Great Tooling Revolution of 2015 arrives

Let's Ship This Thing

JN: we need two votes; we vote to refer the spec to ourselves, then we vote to approve the spec.

(general confusion, slowly turning to incredulity and then acceptance)

AWB: (presents slide titled "A Motion")

DH: (joking) I filed a patent for the Window object. It is a very unique design.

MM: (joking) if you get it granted, don't license it to anybody.

AWB: we intend to approve the final draft here, but conditional upon completion of the RF opt-out period.

JN: (conducts a vote)

the TC39 RFTG has approved referring ES2015 to the TC39 TC (unanimous)

the TC39 TC has approved referring ES2015 to the Ecma GA (unanimous: Google, Microsoft, Mozilla, jQuery, Facebook, Inria)

JN: one last vote! should we recommend to the GA that they refer ES2015 to the GA for fast-track approval. During ES5, the Japanese national body of the ISO recommended a large number of changes which we incorporated as ES5.1. If we refer this document to ISO for fast-track, then we can assume Japan and others will make comments. Those comments will be approved.

AWB: this is the complication of this fast-track stuff. "Fast" means "about a year." If we start this down the fast-track, it will interfere with ES7; about the time we exit the fast-track process for ES6, we will also have ES7.

MM: we do need to figure out how to get this working, but the entire thing is worth it if for no other reason than the high-quality contributions of that one reviewer at the ISO. I don't know how else to engage him.

(discussion of new text vs. old text, will reviewers look at the deltas)

DD: there may be sections that are untouched, but they are very few.

AWB: there may be sentences that are untouched...

IS: it would be very unkind not to refer this to them. My suggestion would be that we should go ahead for the fast-track but it should be a conditional go-ahead on myself and John sitting down with ISO management and discussing a way to find a solution to integrate the different cycles. It would be ugly if there was a large delta between ISO's version (~ES5) and ours.

JN: it exacerbates the system right now. We have a four-year window between ES5 and ES6 and the plan is to have one-year windows. ISO may choose to wait until ES8 or ES9 to fast-track, bypassing ES7. They want to pick up the massive changes between ES5 and ES6. So I think it's important to do it this time and less important to do it next time.

DD: couldn't we just do this because it's good but not spend much time worrying about it?

AWB: unfortunately it's not that simple. The manpower to publish documents is not free; it would take away resources from feature work etc.

MM: Allen what's your opinion?

AWB: I think we need to do ISO. There is significance involved in that.

MM: so going forward with our new process do we do ISO every year?

AWB: I don't know, I only did it once and it took a year.

JN: it could take a longer this time, fast-track has changed.

AWB: should we do a partition standard?

JN: we don't want to go down that road...

AWB: maybe it would help make this maintainable over time?

JN: I'm not opposed, but it needs to be thought out...

AWB: maybe a partition standard could be moved through the ISO process independently...

IS: there should be minimal market confusion because most people download the Ecma version, not the ISO version...

DD: most people download the version on Jason Orendorff's personal web page...

IS: download counts support many more Ecma downloads than ISO downloads ("much much much much smaller")

AWB: that is probably why it is hard to get people in this room excited about ISO...

JN: so I think what we want to do is fast-track this year, but future fast-tracks would be questionable.

JN: so, let's vote (per company): do we approve submission to ISO for fast-track.

approve: Google, Microsoft, Mozilla, jQuery, Facebook, Inria, Netflix

opposed: (none)

abstain: jQuery

IS: ECMA 402 can be an ECMA-only standard according to Patrick. ECMA-only standards can be used as normative standards, so it's up to us whether we want to fast-track. Doesn't seem critical. ECMA-404 is the more interesting one.

AWB: Rick isn't here - are we also going to approve 402 right now?

YK: Is 402 aligned with ES6?

AWB: I hope so, ES6 normatively references it.

YK: Will ECMA be upset if we normatively reference a non-normative spec?

AWB: Yes.

AWB: Istvan, do we need a opt-out period for ECMA-402.

IS: Yes.

AWB: My understanding is that Rick has created the final draft and it has been available for a month for people to review.

IS: It would be nice if we could approve at this meeting, as far as I know this was the intention of the editor. So seems like we have a small problem.

AWB: Check with Rick. If people here take the time to look at the draft, by the end of the week we could vote to refer 402 to GA.

IS: We can't reference 402 edition 1 from ES6?

AWB: No.

Conclusion/Resolution

  • Everyone to review or do what is necessary to vote to pass along 402 on Thursday.

What to do with compact profile (ECMA-327) and E4X (ECMA-357)

AWB: These are still active standards. E4X still gets downloads. We can withdraw the standards.

BE: ActionScript 3 in Flash implements E4X.

IS: We had this on the agenda when we did ES5. At that time the point was that even if it was out of date, if implementers wanted to implement the old one they could. Adobe may have wanted this. 357 is often downloaded. But I don't know how serious this is.

AWB: E4X is not aligned with ES6 so it seems misleading for it to be out there.

IS: Some people may have used it with edition 3.

AWB: We had this conversation 5 years ago. Things have evolved. Maybe adobe still cares, or maybe they don't.

BE: It can be withdrawn without breaking other standards.

IS: We approved ECMA404 before we had the RF agreement. Should we start an opt-out period for ECMA404?

Conclusion/Resolution

  • Istvan to follow up with Adobe and the CC to see if they would object to withdrawing E4X

ECMA404 RF?

IS: We approved ECMA404 before we had the RF agreement. Should we start an opt-out period for ECMA404?

AWB: Can we retroactively make ECMA-404 Royalty Free?

IS: Yes

AWB: We should have an opt-out period then.

Conclusion/Resolution

  • Start an opt-out period.

Compact profile

DH: People who are interested in this can start doing work. Once there is actual demand work will happen. We shouldn't just make standards because someone might use it.

AWB: I think there is consensus to kill 327.

Conclusion/Resolution

  • Istvan to follow up with Adobe and the CC to see if they would object to withdrawing the compact profile.

Fast track ECMA-404

IS: The actual standard document is not ready, maybe ready for this afternooon. Then we can publish it for fast track.

Conclusion/Resolution

  • Revisit tomorrow.

6.1 ES6 Class Properties

(Jeff Morrison)

gist.github.com/jeffmo/054df782c05639da2adb

class Counter extends ReactComponent {
constructor(props) {
super(props);
state = {count: this.props.initialCount};
}
}
Counter.propTypes = {initialCount: React.PropTypes.numer};

JM: React ends up with some boiler plate code because it wants to add an own instance property.

class Counter extends ReactComponent {
static propTypes = {initialCount: React.PropTypes.numer};
state = {count: this.props.initialCount};

JM: Issues with storing instance values on the prototype. Reference types shared between instances.

JM: Transpose initializer into the constructor has issues with the scope.

JM: Wrap each initializer in its own closure from within the class-body environment.

JM: Store the closure as an 'initializer' value in a property descriptor on the prototype.

MM: Hold on. Adding another field in a property descriptor is very heavy handed and has to go through the whole proxy design.

YK: This is needed by other proposals. Not in the MOP though. Opposed to add new features that are not reflected.

AWB: Need to define when this happens. End of super()?

JM: Wants to move passed where the initializer is stored.

JM: Last step of construct. Immediately after calling super()

AWB: Inconsistent. In a non derived you cannot access the own instance field.

JM: That is by design. Sorry, it is the other way around. The initializer is executed in the base class before the constructor body is entered.

AWB: It is important to create properties up front (with an unitialized state or undefined).

YK: It is important to put these on the prototype because it allows reflection.

AWB: By putting these on the prototype they are visible to the application. If these have privileges then those privileges will leak.

DH: Not convinced that this is important. The important part is that we need to store the initializer behavior somewhere.

MM: Is it configurable?

DH: Not fundamental. JS assignment creates the configurable writable enumerable.

DH: With decorators one can create non writable etc.

MM: I would think that non configurable would be good so that the shape is guaranteed.

DH: For backwards compat we should stick with the old behavior.

AWB: This is about creating the instance shape. What new thing is this getting into. It provides, prior to construction, information about the shape.

DH: When we provide new syntax it is tempting to alter the semantics instead of following the patterns used today.

AWB: It is possible that the shape is all about private state and this is all about classic prototype oop.

MM: Using a descriptor is very heavy handed.

YK: I want it to be pervasive so a descriptor is good.

MM: If you put it in the descriptor.

FM: You can use any object as a prototype.

YK: And if a class constructor uses that as a prototype then you can get own instance properties.

AWB: What are the implications if every descriptor needs to have this new property?

AWB: Are you assuming that the current design with property descriptor is a good model to extend for reflection mechanism.

MM: The concern I have is that there is a property with the same name on the prototype but its purpose is not the same.

MM: What you want is something whose value is the initializer function.

DH: What you want is some association with the name and the initializer function

MM: What you want is that your initializer function returns a descriptor. Then the decorator can operate on that descriptor.

DH: Some way to associate the initializer. You can use a symbol.

DD: The decorators already act differently. prototype or on the constructor.

YK: Wants to operator on more than the value. Same way for methods as for inst props.

DH: API like method decorators.

sidetable.myinstprop = {value: func () {}, configurable: ..., ...};

YK: It is important to distinguish these from value properties. For example there migh be a use for creating own instance methods. Therefore using initializer instead of value is important.

AWB: When I create lots of objects I don't want to have to pay for all these internal MOP.

YK: The reified methods only come into play if you use reflection and decorators.

AWB: What if someone modifies these descriptors.

EA: It is important that the descriptor of these own instance properties is immutable, or at that you cannot change these afterwards because then the constructor function has to change its behavior after the fact.

Conclusion/Resolution

  • Stage 0
  • Do not use descriptors for initializer state.

6.2 Decorators

class {
@readonly name() { }
}

function readonly(prototype, name, desc) {
desc.writable = false;
return desc;
}
Object.defineProperty(Person.prototype, 'name',
readonly(Person.prototype, 'name', {
enumerable: false,
configurable: true,
writable: true,
value: <closure>
}));

MM: When defineProperty runs, does Person.prototype have an undecorated name property?

YK: No. The thing that you pass to the decorator looks like a descriptor, but is just an object.

AWB: We need to think about what the built in ones should be.

YK: Things that update descriptors seem like good candidates.

bit.ly/ember-js-classes

Examples of things that Ember might do with decorators.

YK: Shows example of more complex decorator usage:

class Person {
firstName = “…”;
lastName = “…”;
@concat('firstName', 'lastName', ' ') fullName;
@concat('lastName', 'firstName', ', ') formalName; }

MM: Can create a read-only getter from a declared property

AWB: Do you have a more plausible use case for more complex decorators

YK: (Shows ember gist. Shows @service which does dependency injection.)

AWB: Why would I use a decorator instead of a property initializer?

YK: More complex

JM: Decorators run once per class

AWB: This is an own property?

YK: The decorator would attach a more complex getter. @service does a lazy lookup.

DD: When you return a descriptor does it return an own property?

YK: ?

MM: Since this is integrated into TypeScript, how does TypeScript type the decorator method?

YK: Another way to ask question is I would prefer a more immutable API so that I know what the type is. Decorators themselves are typed, but they can mutate.

MM: It's a functional transformation of a descriptor. There's nothing being mutated on.

ARB: This is reified as a first class value. How can you track that without a full blown dependent type system?

YK: (Shows examples of decorators with no mutating effect).

DH: ARB is saying that the set of properties that end of being in the class ends up depending on what happens with these decorators. Mutation is a red herring. The issue is computably determining the class type. You're saying that there is an expressive subset of this system for which this is not a problem.

ARB: To know what to reject you'd have to know the returned value at type-check time.

JM: ARB is saying that the value is dependent on runtime. Enumerable true and enumerable false produce very different things.

ARB: Hard to forsee the implications, because annotations will want to spread.

YK: We should consider them on a case-by-case basis, but I am conservative. I'm not proposing targeting general declarations. Use case is that with class syntax we aren't able to do things we could with object literals.

MM: Are you proposing that you can decorate fields in an object literal?

YK: I don't consider it fundamental but there is a symmetry concern.

ARB: Still missing the big picture. Don't you ever want to decorate parameters and other things?

YK: These are things we might want to consider.

ARB: Take modules, it seems like people will want to do some metaprogramming by annotating imports.

YK: Proposal is addressing an expressive hole introduced by ES6. There is not a way to do higher order things with methods.

DH: We're targeting the class syntax but we need to focus on the whole and not the part. The broad category is metaprogramming. What is our model? In Scheme, it's static. In Ruby, it's dynamic. We should do a zoom out and look at what metaprogramming might mean for the language.

ARB: (Objects to saying that JS implies dynamic metaprogramming.)

DH: (Mentions objection to staging.) With a first class class system, a lot of meta- programming stuff you just do at runtime. If you've gone down the path of dynamic metaprogramming, then it's probably best to keep going that way. ARB: I think class expressions are a red herring.

DH: I just don't believe that we should do staging.

BE: staging not seriously proposed but noted as possible accidental outcome of generalizing decorators across first- and second-class parts of ES6. We want a principled approach that's not just for classes.

AWB: Say you had a decorator that was enumerable or non-enumerable...

YK: Users will want to apply descriptor decorators to object literals as well.

YK: Don't see why we should restrict class features to static

DH: Javascript is fundamentally a dynamic language and we shouldn't try to make it static

ARB: The dynamic stuff will remain, but we'll have these conflicts.

ARB: Original question was what problem are we solving?

DH: Need a more general system for "twiddling" knobs.

ARB: My question is how does this generalize?

DH: Current instinct is that first class things (classes, object literals) are easy to apply this to, but for second class things we'll fall off a complexity cliff. Agree that we should look at the bigger picture.

ARB: readonly is an example that we might want to have a more direct way.

YK: If readonly were built-in and you imported it, then the engine could do smarter things.

DH: Suggests stage 1, but audit the space which may lead to changes.

Conclusion

  • Stage 1, YK will audit the space.

6.3 ReverseIterable interface

(Lee Byron)

Proposal: leebyron/ecmascript-reverse-iterator

LB: [presents slides] (found in github repo)

[lots of discussion about how to extend iterators/iterables]

Conclusion

  • More work to be done in breakouts to figure out path forward from ES6 for iteration.

(--Format, for use as a reference--)

Agenda Item Number and Topic (eg. 4.1 ES6 End-game schedule review)

(Name of presenter or presenters)

RW: blah blah blah

  • List item 1
  • List item 2
// some code
// some monospaced data

Conclusion/Resolution

  • Resolution point 1
  • Resolution point 2
# Brendan Eich (4 years ago)

Erik Arvidsson wrote:

(Brendan Eich presents gist.github.com/BrendanEich/4294d5c212a6d2254703)

Moves to stage

"1" was lost from end of line here. I'll have a github repo instead of a gist for this shortly.