On Feb 1, 2012, at 2:13 PM, Brendan Eich wrote:
> Allen Wirfs-Brock wrote:
>> Another loose end.
>>
>> With the addition of for-of, for-in reverts back to always iterating over
>> the property keys of an object as it does not support the generalized
>> iteration protocols. An implication of this is that using a de-structuring
>> pattern as the iteration variable has very limited utility:
>>
>> for (let [c] in obj) print(c); //print the first character of
>> each of obj's enumerable property names
>>
>> for (const {length:len} in obj) print(len); print the length
>> of each of obj's enumerable property names
>>
>>
>> Given this lack of utility, why should we allow de-structuring in this
>> context?
>
> These are not totally silly examples. I say when in doubt, let language
> regularity win.
Well, they're at least 98% silly and these were the only even semi-plausable
example I could think of.
>
> We are also thereby future-friendly in case some evolution of property keys
> becomes even more structured. I know, unlikely, but again: regularity when in
> doubt.
In which case, it would be easy enough to allow them in the future. It's always
easer to relax a restriction then it is to add one.
>
>> Eliminating it arguably increases language complexity by introducing in a
>> bit more grammar irregularity. On the other hand, eliminating useless
>> functionality can be seen as as simplification.
>
> Not useless, your own examples show use-cases.
>
>> I'm leaning towards banning destructing in for-in.
>
> Too nannyish!
I don't really see the consistency in trying to ban the initializer in
for (var k=42 in obj) ;
because it is a "awful, lazy-grammar-resue error". While adding
for (var [c] in obj) ;
I don't actually feel too strongly either way and it is actually more work for
me to disallow the destructuring pattern. However, I am trying to find some
basis for consistency in the design choices we make.
Allen
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