On Thu, May 2, 2013 at 2:28 PM, Claus Reinke <[email protected]> wrote: >> Promise.of(value).then(cb) = cb(value) >> promise.then(Promise.of) = promise > > My interpretation of these laws for promises is that attaching a callback to > a resolved promise should execute that callback synchronously (though the > callback itself may create an asynchronous promise, introducing its own > delays).
It's not that the callback "may" create an async promise: it *must* create an async promise, if you want to reason about it in terms of the monad laws. The cb must have the signature "a -> Mb", where M in this case is Promise. If cb returns a non-promise value, then you're not following the monad laws, and you can't reason about monadic behavior. Assuming it does follow the monad laws properly, then the return value of cb is *always* accessible in the next tick only, regardless of whether it runs synchronously or not. If you're depending on side-effects, you're already stepping outside of what the monad abstraction is meant to contain, so slight differences like that are acceptable. ~TJ _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
