Clarify the destructuring syntax

[Allen Wirfs-Brock]

On Apr 11, 2014, at 5:22 AM, André Bargull wrote:

> On 4/11/2014 2:02 PM, Егор Николаев wrote:
>> @André Bargull
>> Are there any reasons for these restrictions? It confuses me. If I can
>> for-of over string, so why can't I destructuring it?
> 
> I'd say it's mostly a language designer decision and it may still change before the final specification is published. I'd encourage you to file bug reports at https://bugs.ecmascript.org/ and/or post to this mailing list if you find any issues with the current specification draft. That way Allen (the spec editor, cc-ed) and other TC-39 members are notified and can take appropriate actions.

This was discusses fairly extensively in TC39 meetings and on this list.  The final consensus was that non objects should not be destructurable (ie, ToObject is not applied).  One argument against allowing non-objects is that it might be future-hostile to anticipated future proposal to  turn destructuring into  more general pattern patching,

This is the decision that is reflected in the ES6 draft starting with rev17. As it stand right now, there is no advocacy within TC39 for reopening this decision. 

Allen






> 
> 
> - André
> 
> 
>> 
>> 
>> On Fri, Apr 11, 2014 at 3:10 PM, André Bargull <andre.bargull at udo.edu
>> <mailto:andre.bargull at udo.edu>> wrote:
>> 
>>>    Hi Erop,
>>> 
>>> 
>>>    On Fri, Apr 11, 2014 at 12:35 PM, Егор Николаев <termi1uc1 at gmail.com  <https://mail.mozilla.org/listinfo/es-discuss>> wrote:
>>> 
>>>    >/  1. Should the AssignmentExpression of DestructuringAssignment always to be
>>>    /
>>>    >/the Object type? />/```javascript />/let {length} = "123";
>>>    />/assert(length, 3); />/``` />/Is this valid? />// Yes.
>> 
>>    No.
>> 
>>    13.2.1.4 Runtime Semantics: Evaluation,
>>    LexicalBinding : BindingPattern Initializer, step 4
>> 
>>    https://people.mozilla.org/~jorendorff/es6-draft.html#sec-let-and-const-declarations-runtime-semantics-evaluation
>> 
>> 
>> 
>>>    >/
>>>    />/  If it is:
>>>    />/  2. Should the result of Get(obj, name) always be the Object type if
>>>    />/  DestructuringAssignmentTarget is an ObjectLiteral or an ArrayLiteral?
>>>    />/  According the spec 12.14.5.4 step 4.b this expression is invalid:
>>>    />/  ```javascript
>>>    />/  let {text: {length}} = {text: "123"};
>>>    />/  assert(length, 3);
>>>    />/  ```
>>>    />/
>>>    /
>>>    Yes.
>> 
>>    No.
>> 
>>    13.2.3.7 Runtime Semantics: KeyedBindingInitialization
>>    BindingElement : BindingPattern Initializer_opt, step 4
>> 
>>    https://people.mozilla.org/~jorendorff/es6-draft.html#sec-runtime-semantics-keyedbindinginitialization
>> 
>> 
>>>    You can test this in the web console in a Nightly or Aurora build of
>>>    Firefox.
>> 
>>    You can test this in traceur or my test implementation
>>    [https://github.com/anba/es6draft].  ;-)
>> 
>>    Maybe the final draft will revert this restriction, it was
>>    originally introduced in rev17. It is somewhat annoying, especially
>>    for the string type, because the restriction also applies to spread
>>    array elements and spread calls. For example `[..."abc"]` must now
>>    be written as `[...new String("abc")]` to get the array of code points.
>> 
>>    - André
>> 
>> 
>