domenic at domenicdenicola.com (2013-12-10T01:25:55.792Z)
`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) .
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