Generator issue: exceptions while initializing arguments

Brendan Eich brendan at
Sat Sep 8 15:20:39 PDT 2012

SpiderMonkey (Firefox 15 and newer has default parameters):

js> function f(a = g) { function g(){}; return a; }
js> f()
function g(){}

So function g is hoisted and a defaults to it, as expected.

The trivial generator variant, i.e., what you get by putting a * after 
function and before f, above:

js> function* f(a = g) { function g() {}; return a; }
js> var it = f();
function g(){}

(note well, not yet implemented in SpiderMonkey) should behave the same, 
or crazypants.


Brendan Eich wrote:
> Brendan Eich wrote:
>>>  However, the semantics of parameter default value initialization 
>>> can result in exceptions and more closely links argument 
>>> instantiations with the actual activation of the generator function. 
>>> You can't really build the parameter bindings without performing 
>>> declaration instantiation for the function.
>> Agreed, that's absolutely true (no "really" about it -- or rather, 
>> "for real" ;-). 
> And (I should have written), if your point was that we must bind all 
> var and top level function/let/const declarations before the implicit 
> yield; inserted "after the opening brace", I don't see a problem. 
> Hoisted functions with usable initial values, hoisted vars with 
> undefined initial values, and temporal dead zones for the let and 
> const declarations that are at top (generator body) level. Right?
> /be
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