Clarification regarding Completion Records

Felix Kling list at felix-kling.de
Fri Sep 23 14:24:55 UTC 2016


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)

- As per your quote, `Return ?ResolveBinding(...)` simply returns that 
completion record.

- 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 6.2.2.2: *"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?


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 don't).
2. Algorithms *always* return a completion record, but it's not always 
*explicitly* stated in the algorithm.


Felix

On 2016-09-23 03:32, Bergi wrote:
> Felix Kling wrote:
> 
>> I need some clarification around completion records.
> 
> You'll want to have a look at the algorithmic conventions
> (http://www.ecma-international.org/ecma-262/7.0/#sec-algorithm-conventions)
> and the implicit coercion of completion values
> (http://www.ecma-international.org/ecma-262/7.0/#sec-implicit-completion-values):
> 
> | Calls to abstract operations return Completion Records.
> 
> | The algorithms of this specification often implicitly return
> | Completion Records whose [[Type]] is normal. Unless it is otherwise
> | obvious from the context, an algorithm statement that returns a value
> | that is not a Completion Record, such as:
> | > Return "Infinity".
> | means the same thing as:
> | > Return NormalCompletion("Infinity").
> |
> | However, if the value expression of a “return” statement is a
> | Completion Record construction literal, the resulting Completion
> | Record is returned. If the value expression is a call to an abstract
> | operation, the “return” statement simply returns the Completion
> | Record produced by the abstract operation.
> 
> Yes, it's a bit sloppy, but makes the spec much more readable. It's
> already technical enough, so this does formally apply some common
> sense :-)
> 
> kind regards,
>  Bergi
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