Clarification regarding Completion Records
jmdyck at ibiblio.org
Fri Sep 23 16:51:52 UTC 2016
On 16-09-23 10:24 AM, Felix Kling wrote:
> I read these paragraphs over and over again ;) Maybe I just need someone to
> explain it to me in different words.
> So to apply these to my case:
> - `ResolveBinding` is an *"abstract operation"* so it returns a completion
> record, whose value is a *"Reference"*, even though completion records are
> only supposed to have language values as value (6.2.2) (this still confuses me)
The spec is not consistent in this area. That is, these statements are not
* Every call to an abstract operation returns a completion record.
* Every completion record's [[Value]] is a language value or ~empty~.
* Some abstract operations (e.g. ResolveBinding) need to convey values to
the caller that are neither language values nor ~empty~.
(See https://github.com/tc39/ecma262/issues/496 for more examples, and for
possible ways of resolving the inconsistency.)
> - In `Let exprRef be the result of evaluating Expression`, `exprRef` is a
> Reference, the completion record obtained by *"evaluating Expression"* was
> implicitly unwrapped, according to 126.96.36.199: *"Any reference to a Completion
> Record value that is in a context that does not explicitly require a complete
> Completion Record value is equivalent to an explicit reference to the
> `[[Value]] field of the Completion Record value unless the Completion Record
> is an abrupt completion."`
> Am I right so far?
Not entirely. Here's another way to look at it. If the implicit rule of
188.8.131.52 were made explicit, we would have something like:
Let exprRef be maybe-unwrap(the result of evaluating Expression).
where maybe-unwrap(cr) is a shorthand for:
* cr, if cr is an abrupt completion, or
* cr.[[Value]] if cr is a normal completion.
So (theoretically), for any given execution of this algorithm step, `exprRef`
is either a Reference or an abrupt completion.
In this case, things also work if you ignore 184.108.40.206 (so that `exprRef` is
always a Completion Record), because the subsequent call to GetValue() starts
with a call to ReturnIfAbrupt(), which accomplishes the unwrapping.
> Then, does the sentence
>> The algorithms of this specification often implicitly return Completion
>> Records whose `[[Type]]` is `normal`.
> mean that
> 1. Algorithms may or may not return a completion record (i.e. some do and some
> 2. Algorithms *always* return a completion record, but it's not always
> *explicitly* stated in the algorithm.
Statement 2 would result in a similar inconsistency to the one noted above
(just replacing "abstract operation" with "algorithm").
Statement 1 doesn't have that problem, but it still has the problem of fitting
into the world in which the 'abstract operation' inconsistency exists.
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