Missing definitions for "<thing> equivalent to a function that <action>"

[anba]

Added in #1073

[ljharb]

This phrase was actually added in ES2015: http://ecma-international.org/ecma-262/6.0/#sec-promisereaction-records

I copied that phrasing in the "finally" spec, but afterwards, I filed #584, which refactored the existing (definitionless) usage to be strings, instead of callbacks, stored in internal slots.

I think it'd be fine to add a definition, but ES2015 didn't seem to need one - what definition would you suggest and where would it go?

[anba]

That seems like a different case, because in ES2015 it was only used in a non-normative context whereas now it used for normative text.

[ljharb]

Fair enough :-) What definition would you suggest and where would it go?

[anba]

The easiest fix is to create normal built-in functions à la:

Add:

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25.6.5.3.1.1 Return Completion Functions

A Return Completion function is an anonymous built-in function that has a [[Completion]] internal slot.

When a Return Completion function F is called, the following steps are taken:

1. Return Completion(F.[[Completion]]). 

The length property of a Return Completion function is 0.

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And then replace:

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7. Let valueThunk be equivalent to a function that returns value.

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With:

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7. Let stepsReturnCompletion be the algorithm steps defined in Return Completion Functions.
8. Let valueThunk be CreateBuiltinFunction(stepsReturnCompletion, « [[Completion]] »).
9. Set valueThunk.[[Completion]] to NormalCompletion(value).

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And replace:

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7. Let thrower be equivalent to a function that throws reason.

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With:

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7. Let stepsReturnCompletion be the algorithm steps defined in Return Completion Functions.
8. Let thrower be CreateBuiltinFunction(stepsReturnCompletion, « [[Completion]] »).
9. Set thrower.[[Completion]] to ThrowCompletion(reason).

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And if we don't want to repeat the three steps to create this new function, it's also possible to add a helper like:

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25.6.5.3.1.2 MakeReturnCompletionFunction(completionRecord)

The abstract operation MakeReturnCompletionFunction with argument completionRecord performs the following steps:

1. Let stepsReturnCompletion be the algorithm steps defined in Return Completion Functions.
2. Let F be CreateBuiltinFunction(stepsReturnCompletion, « [[Completion]] »).
3. Set F.[[Completion]] to completionRecord.
4. Return F.

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And then use it like:

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7. Let valueThunk be MakeReturnCompletionFunction(NormalCompletion(value)).

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7. Let thrower be MakeReturnCompletionFunction(ThrowCompletion(reason)).

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[domenic]

A more ambitious fix would be to tackle #933.

[arai-a]

Related to #2222
if the function is passed to Promise.prototype.then ("Then Finally Function"'s case), the function's [[Realm]] slot affects embedding behavior,
so it should be explicit about [[Realm]] slot.