Generator issue: exceptions while initializing arguments

Brendan Eich brendan at
Fri Sep 14 19:12:47 PDT 2012

Kevin Smith wrote:
> What I'm questioning now is the idea that, in generators, the implicit 
> first yield should occur *before the defaults are evaluated*.  I don't 
> think that's a tenable position.  Is there an implementation problem 
> with inserting the implicit first yield *after* the defaults are 
> evaluated but *before* the rest of the function body?

Not particularly, and this was where I landed at first.

In the SpiderMonkey implementation, at first, the bytecode emitter emits 
the implicit yield opcode (JSOP_GENERATOR) before all other generated 
code. But since then some conditional opcodes are emitted even before 
the implicit yield:

Other implementations may generate different intermediate or target 
forms from source, but I think the choice of where to put the implicit 
yield can be made either way without hardship.

Other implementors should weigh in.


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