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Normative: Add initializer callback for side effects #165

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merged 7 commits into from
Nov 8, 2018
Merged

Normative: Add initializer callback for side effects #165

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9f6d669
Normative: Add `kind: "initializer"` for side effects
littledan Oct 30, 2018
5bba880
Fix most of the errors from review
littledan Nov 2, 2018
14b8919
Normative: Throw an exception on a missing initializer
littledan Nov 2, 2018
9c3c148
Normative: Permit prototype and static initializers
littledan Nov 4, 2018
77543ec
Normative: Throw a TypeError if an initializer returns undefined
littledan Nov 5, 2018
77e1922
Documentation for initializers
littledan Nov 5, 2018
9acf8f5
Add Oxford commas
littledan Nov 5, 2018
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69 changes: 57 additions & 12 deletions spec.html
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Original file line number Diff line number Diff line change
Expand Up @@ -159,7 +159,7 @@ <h1>The ElementDescriptor Specification Type</h1>
<tr> <th>Field Name</th> <th>Value</th> </tr>
</thead>
<tbody>
<tr> <td>[[Kind]]</td> <td>One of `"method"` or `"field"`</td> </tr>
<tr> <td>[[Kind]]</td> <td>One of `"method"`, `"field"` or `"initializer"`</td> </tr>
<tr> <td>[[Key]]</td> <td>A Property Key or %PrivateName% object</td> </tr>
<tr> <td>[[Descriptor]]</td> <td>A Property Descriptor</td> </tr>
<tr> <td>[[Placement]]</td> <td>One of `"static"`, `"prototype"`, or `"own"`</td> </tr>
Expand Down Expand Up @@ -188,6 +188,16 @@ <h1>The ElementDescriptor Specification Type</h1>
<li>_element_.[[Descriptor]].[[Configurable]] is *false*.</li>
</ul>
</li>
<li>
If _element_.[[Kind]] is `"initializer", then
<ul>
<li>_element_.[[Key]] absent.</li>
<li>_element_.[[Descriptor]] is absent.</li>
<li>_element_.[[Placement]] is `"own"`.</li>
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We might consider allowing "static" as the decorator way of doing https://github.com/tc39/proposal-class-static-block

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I agree; static should be allowed here since they also can have initializers

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I don't see this as a way towards a static block. This is just a way that a decorator can add extra code to run. But decorators are already able to run code at the end of when a class is executing, in the finisher callback. We could allow this out of consistency (it would have slightly different timing from finishers), but I don't see a use case.

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A use case is the same as for "own" - being able to invoke a setter on a superclass (in this case, on a superclass's constructor that had, say, static set foo(v) {})

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@ljharb What would be the problem with using a finisher for that?

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Certainly you could use a finisher for that - but then you'd be forced to use a class-level decorator to change the semantics of a specific field, so instead of:

class C {
  @useSet static a = x;
  static b = y;
  @useSet static c = z;
}

you'd need to have:

@useSet('a')
@useSet('c')
class C {
  static a = x;
  static b = y;
  static c = z;
}

or:

@useSet('a', 'c')
class C {
  static a = x;
  static b = y;
  static c = z;
}

both of which are decidedly less ergonomic and elegant.

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I don't see why that would be the case. You can use a finisher on a static field declaration too.

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@ljharb, wait, but you can use finisher with a field or a method decorator. It is not required to use class-level decorators to activate finishers.

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ahh, that wasn't clear to me. In that case - instead of this "initializer" approach, is there a reason not to allow finishers on "own" fields, to provide an "instance finisher"?

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@ljharb yeah, I came to the same idea in my comment below :)

<li>_element_.[[Initializer]] is present.</li>
<li>_element_.[[Decorators]] is absent.</li>
</ul>
</li>
</ul>
</p>
</emu-clause>
Expand Down Expand Up @@ -408,6 +418,27 @@ <h1>CoalesceClassElements ( _elements_ )</h1>
<emu-note>In the case of public class elements, coalescing corresponds in semantics to ValidateAndApplyPropertyDescriptor. Note that this algorithm only coalesces method and accessor declarations, and it leaves field declarations as is.</emu-note>
</emu-clause>

<emu-clause id="initialize-public-instance-elements" aoid="InitializeInstanceFields">
<h1>InitializeInstanceElements ( _O_, _constructor_ )</h1>

<emu-alg>
1. Assert: Type ( _O_ ) is Object.
1. Assert: Assert _constructor_ is an ECMAScript function object.
1. Let _elements_ be the value of _F_'s [[Elements]] internal slot.
1. For each item _element_ in order from _elements_,
1. If _element_.[[Placement]] is `"own"` and _element_.[[Kind]] is `"method"`,
1. Perform ? DefineClassElement(_O_, _element_).
1. For each item _element_ in order from _elements_,
1. If _element_.[[Placement]] is `"own"` and _element_.[[Kind]] is `"field"`,
1. Assert: _element_.[[Descriptor]] does not have a [[Value]], [[Get]] or [[Set]] slot.
1. Perform ? DefineClassElement(_O_, _element_).
1. <ins>If _element_.[[Kind]] is `"initializer"`,</ins>
1. <ins>Assert: _element_.[[Placement]] is `"own"`.</ins>
1. <ins>Perform ? Call(_element_.[[Initializer]], _O_).</ins>
1. Return.
</emu-alg>
</emu-clause>

</emu-clause>


Expand Down Expand Up @@ -563,7 +594,7 @@ <h1>Element Descriptors</h1>
<p>An <dfn>element descriptor</dfn> describes an element of a class or object literal and has the following shape:</p>
<pre><code class=typescript>
interface ElementDescriptor {
kind: "method" or "field"
kind: "method", "initializer" or "field"
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Suggested change
kind: "method", "initializer" or "field"
kind: "method", "initializer", or "field"

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Done

key: String, Symbol or Private Name,
placement: "static", "prototype", or "own"
descriptor: PropertyDescriptor,
Expand Down Expand Up @@ -689,7 +720,7 @@ <h1>DecorateConstructor ( _elements_, _decorators_ )</h1>
1. Append _elementsAndFinisher_.[[Finisher]] to _finishers_.
1. If _elementsAndFinisher_.[[Elements]] is not *undefined*,
1. Set _elements_ to the concatenation of _elementsAndFinisher_.[[Elements]] and _privateElements_.
1. If there are two class elements _a_ and _b_ in _elements_ such that _a_.[[Key]] is _b_.[[Key]] and _a_.[[Placement]] is _b_.[[Placement]], throw a *TypeError* exception.
1. If there are two class elements _a_ and _b_ in _elements_ such that _a_.[[Kind]] is not `"initializer"` and _b_.[[Kind]] is not `"initializer"`, _a_.[[Key]] is _b_.[[Key]], and _a_.[[Placement]] is _b_.[[Placement]], throw a *TypeError* exception.
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@nicolo-ribaudo nicolo-ribaudo Nov 2, 2018 •
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Can this be made more readable? (I don't know if it valid from a spec-grammar point of view)

1. If there are two class elements _a_ and _b_ in _elements_ such that all the following conditions are true:
  1. _a_.[[Kind]] is not `"initializer"`
  1. _b_.[[Kind]] is not `"initializer"`
  1. _a_.[[Key]] is _b_.[[Key]]
  1. _a_.[[Placement]] is _b_.[[Placement]]
1. Then throw a *TypeError* exception.

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Good idea, done

1. Return the Record { [[Elements]]: _elements_, [[Finishers]]: _finishers_ }.
</emu-alg>
</emu-clause>
Expand Down Expand Up @@ -744,12 +775,14 @@ <h1>FromElementDescriptor ( _element_ )</h1>
1. Let _desc_ be PropertyDescriptor{ [[Value]]: `"Descriptor"`, [[Writable]]: *false*, [[Enumerable]]: *false*, [[Configurable]]: *true* }.
1. Perform ! DefinePropertyOrThrow(_obj_, @@toStringTag, _desc_).
1. Perform ! CreateDataPropertyOrThrow(_obj_, `"kind"`, _element_.[[Kind]]).
1. Let _key_ be _element_.[[Key]].
1. If _key_ is a Private Name, set _key_ to ? PrivateNameObject(_key_).
1. Perform ! CreateDataPropertyOrThrow(_obj_, `"key"`, _key_).
1. If _element_.[[Kind]] is `"method"` or `"field"`,
1. Let _key_ be _element_.[[Key]].
1. If _key_ is a Private Name, set _key_ to ? PrivateNameObject(_key_).
1. Perform ! CreateDataPropertyOrThrow(_obj_, `"key"`, _key_).
1. Perform ! CreateDataPropertyOrThrow(_obj_, `"placement"`, _element_.[[Placement]]).
1. Perform ! CreateDataPropertyOrThrow(_obj_, `"descriptor"`, ! FromPropertyDescriptor(_element_.[[Descriptor]])).
1. If _element_.[[Kind]] is `"field"`,
1. If _element_.[[Kind]] is `"method"` or `"field"`,
1. Perform ! CreateDataPropertyOrThrow(_obj_, `"descriptor"`, ! FromPropertyDescriptor(_element_.[[Descriptor]])).
1. If _element_.[[Kind]] is `"field"` or `"initializer"`,
1. Let _initializer_ be _element_.[[Initializer]].
1. If _initializer_ is ~empty~, set _initializer_ to *undefined*.
1. Perform ! CreateDataPropertyOrThrow(_obj_, `"initializer"`, _initializer_).
Expand Down Expand Up @@ -784,23 +817,35 @@ <h1>ToElementDescriptor ( _elementObject_ )</h1>
1. If _kind_ is not one of `"method"` or `"field"`, throw a *TypeError* exception.
1. Let _key_ be ? Get(_elementObject_, `"key"`).
1. If _key_ has a [[PrivateName]] internal slot, set _key_ to _key_.[[PrivateName]].
1. If _kind_ is `"initializer"`,
1. If _key_ is not *undefined*, throw a *TypeError* exception.
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1. Otherwise, set _key_ to ? ToPropertyKey(_key_).
1. Let _placement_ be ? ToString(? Get(_elementObject_, `"placement"`)).
1. If _placement_ is not one of `"static"`, `"prototype"`, or `"own"`, throw a *TypeError* exception.
1. Let _descriptor_ be ? ToPropertyDescriptor(? Get(_elementObject_, `"descriptor"`)).
1. If _kind_ is `"initializer"`,
1. If _placement_ is not `"own"`, throw a *TypeError* exception.
1. Let _descriptor_ be ? Get(_elementObject_, `"descriptor"`)
1. If _kind_ is `"initializer"`,
1. If _descriptor_ is not *undefined*, throw a *TypeError* exception.
1. Otherwise,
1. Set _descriptor_ to ? ToPropertyDescriptor(_descriptor_).
1. Let _initializer_ be ? Get(_elementObject_, `"initializer"`).
1. Let _elements_ be ? Get(_elementObject_, `"elements"`).
1. If _elements_ is not *undefined*, throw a *TypeError* exception.
1. If _kind_ not `"field"`,
1. If _kind_ not `"field"` or `"initializer"`,
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Typo: "If kind is not"

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Done

1. If _initializer_ is not *undefined*, throw a *TypeError* exception.
1. If _key_ is a Private Name,
1. If _descriptor_.[[Enumerable]] is *true*, throw a *TypeError* exception.
1. If _descriptor_.[[Configurable]] is *true*, throw a *TypeError* exception.
1. If _placement_ is `"prototype"`, throw a *TypeError* exception.
1. If _kind_ is `"field"`,
1. If _descriptor_ has a [[Get]], [[Set]] or [[Value]] internal slot, throw a *TypeError* exception.
1. Let _element_ be the ElementDescriptor { [[Kind]]: _kind_, [[Key]]: _key_, [[Placement]]: _placement_, [[Descriptor]]: _descriptor_ }.
1. If _kind_ is `"field"`, set _element_.[[Initializer]] to _initializer_.
1. Let _element_ be the ElementDescriptor { [[Kind]]: _kind_, [[Placement]]: _placement_ }.
1. If _kind_ is `"method"` or `"field",
1. Set _element_.[[Key]] to _key_.
1. Set _element_.[[Descriptor]] to _descriptor_.
1. If _kind_ is `"field"` or `"initializer"`,
1. Set _element_.[[Initializer]] to _initializer_.
1. Return _element_.
</emu-alg>
</emu-clause>
Expand Down