function.sent beta equivalence?

Brendan Eich brendan at
Wed Jun 24 21:17:37 UTC 2015

Hrm, the meeting notes don't quite capture it 
is one view; my fault for not editing after), but we have discussed 
passing function.sent into the sub-generator in the past. Ben Newman 
first raised the idea the other year, but we finally all came to see at 
the last (May) meeting that there's no good way to do it in general.

When you write a delegating generator that does nothing else, it's 
tempting to want function.sent delegation. But in general the yield* 
could want a different first-next param than what was passed down, 
especially when the delegator looks like this:

   function *g2(...args) { prolog code here; maybe-return yield* 
g(...args); optional epilog here }

See the discussion from the May meeting, I'll cite it here:"""

MM: But it's passing something from before when yield * is evaluated.

DH: I'm beginning to think that this is the wrong path (passing in via yield *).

AWB: You could create some kind of wrapper if you wanted to pass in the 
first value to the subgenerator.

MM: Libraries could do this.

yield  *  wrap(g,;

MM: "wrap" returns an iterator which wraps the generator which primes 
the subgenerator with the supplied value.

Does this make sense? Sorry again for sparse notes.


Jason Orendorff wrote:
> Quick question about function.sent:
> As of ES6, there's this parallel between functions and generators:
>      // how to wrap a function
>      // f2 is equivalent to f, if f doesn't use `this`.
>      function f(...) { ... }
>      function f2(...args) { return f(...args); }
>      // how to wrap a generator
>      // g2 is equivalent to g, if g doesn't use `this`.
>      function* g(...) { ... }
>      function* g2(...args) { return yield* g(...args); }
> That is, λx.f(x) = f, a sort of beta-equivalence rule, and for
> generators, you just need to add `yield*` to get an analogous rule. My
> understanding is that this is one reason we have `yield*`. (Writing a
> wrapping function/generator that also covers `this` is left as an easy
> exercise.)
> Given function.sent, is there still a way to "wrap" a generator?
> -j
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