> > > This seems like something that can be deferred to ES7. >> > > Which "this"? .chain is deferred to ES7, and in V8 experimentally.
Deferring `chain` is, if not future-hostile to `chain`, then "future-irritant". The way Promises are currently spec'd, the return value of the `then` callback is itself recursively unwrapped using `then`, rather than the more appropriate single-unwrapping of `chain`. Single unwrapping of the output side is consistent with the AP2 design principal that the provider does not determine the unwrapping policy. You cannot consistently implement AP2 without `chain` from the get-go. It is the primitive unwrapping function which `then` should be calling.
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