some generator issues

Brendan Eich brendan at
Mon Apr 23 08:39:33 PDT 2007

On Apr 22, 2007, at 9:04 PM, Yuh-Ruey Chen wrote:

> Alternatively, we could make and gen.send() special in that
> they are already prebound to gen (e.g. still
> results in gen.send()).

This is a possibility, but it would change the |this| binding inside  
the generator function from its initial value, captured when the  
generator function was called to create a geneator-iterator.

The JS1.7 native methods for generators (next, send, etc.) throw if  
called on the wrong type of |this|, so it's conceivable that ES4  
could bind these methods to their generator-iterator and avoid such  
type errors. But what you propose above requires re-binding |this| in  
the saved execution context for the generator function, which is kept  
inside the generator-iterator.

> Personally, I'd rather not do this - pretty much
> all builtin classes have unbound prototype methods rather than bound
> instance methods.

Not all built-in classes have unbound prototype methods, and for good  
reason in certain cases. String and Array methods are intentionally  
generic, apart from to*String/valueOf -- those throw type errors if  

Class methods in ES4 are bound, so extracting a reference to one  
forms a binding with the object from which the method ref was  
extracted, supplying that object as |this| in subsequent calls via  
the ref.

But the idea for generator next and send that the saved |this|, from  
the original activation of the generator function, would be replaced  
by the generator-iterator, is interesting. It provides the  
convenience Yuh-Ruey is seeking. On the other hand, it overwrites the  
|this| captured in the generator call. I think that's a fatal  
(barely) flaw.

So still looking for a slick way to propagate the generator-iterator...


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