On Nov 28, 2013, at 10:40 AM, Brendan Eich wrote:
> Kevin Smith wrote:
>>
>> function*(a = yield/b/g) {
>> a = yield/b/g;
>> }
>>
>>
>> Why allow yield as an identifier in parameter defaults of generators?
>> Wouldn't it be simpler just to disallow there as well?
>
> When proposing an irregularity, you need a better reason than "why allow?"
>
> Parameter default values pave a cowpath of the form
>
> function f(a) {
> a = a || default_a;
> ...
> }
>
> (yes, falsy test and all).
>
> The new form does not desugar that way, of course. But defaults are evaluated
> on each activation (contrast with Python, the mutable value default shared
> singleton side-channel). So why shouldn't yield in a default work? There
> could be a reason, but we don't have it yet.
>
> Allen may have a thought.
'yield' is already disallowed in generator function default initializer
expressions (although I was reviewing the latest spec. revision yesterday and
there may still be still bugs I have to fix in that regard).
The reason it isn't allowed is that the generator object is not yet
instantiated and active at the point where a default value initializer would be
evaluated. (note you yield from a invocation on the generator object, not the
generator function that creates a generator object) .
Allen
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