- From: Domenic Denicola <domenic@domenicdenicola.com>
- Date: Thu, 2 May 2013 19:31:40 +0000
- To: Tab Atkins Jr. <jackalmage@gmail.com>, "public-script-coord@w3.org" <public-script-coord@w3.org>
- Message-ID: <B4AE8F4E86E26C47AC407D49872F6F9F7EF6C91B@SN2PRD0510MB360.namprd05.prod.outlook.>
DOMFuture has made no such switch. Some variable names were changed, but nothing more. ________________________________ From: Tab Atkins Jr.<mailto:jackalmage@gmail.com> Sent: ý5/ý2/ý2013 14:37 To: public-script-coord@w3.org<mailto:public-script-coord@w3.org> Subject: Promises: Auto-assimilating thenables returned by .then() callbacks: yay/nay? Now that DOM Futures have moved to accept semantics ("monad semantics") when a .then() callback returns a Future, there's only one remaining issue that I want to try and get agreement over. I am explicitly asking for feedback from Domenic and others who have experience in the Promises/A+ standardization effort. More echo-chamber from us in the "monad camp" won't be particularly helpful. ^_^ The last issue is automatic assimilation of thenables return by .then() callbacks. To be clear, by "thenable" I mean "any object with a .then() method, which is not a native/branded promise". I believe that thenables returned by a .then() callback should be treated as "plain values", and we should add an explicit assimilation function that does the job of converting a thenable into a native promise. My reasoning is that this is necessary for predictability. "Thenable" is a structural type - it applies to everything with a .then() method, regardless of whether it was *intended* to be a promise-like or not. We already have at least one explicit example of a library returning objects with a .then() method that aren't promise-like: the Casper.js library. I'm sure there are more, and will be more as long as we don't mess this up. If we auto-assimilate, it means that anytime we want to return the value of some function, if we're not sure that the return value is a primitive or a native promise, we have to use Future.accept() to guard it against the possibility of accidental and erroneous assimilation. (Wrapping the value in an accepted promise triggers the monad semantics, which are safe regardless of what the inner value is.) So, I propose that we instead expose an explicit assimilation function that takes a promise-like and returns a native promise. .then() callbacks then have a simple two-case semantics, which only check to see if the return value is a native/branded promise, and otherwise treat it as a plain value. This makes promise semantics predictable and simple, at the minor cost that when crossing library boundaries, you have to manually assimilate. (I think this is acceptable, as we already do it in other places. For example, jQuery objects are basically DOM Element-likes, but the DOM methods don't try and use them automatically. Instead, jQuery lets you get at the native Element backing them, for when you need to step back into DOM land. This would be similar.) ~TJ
Received on Thursday, 2 May 2013 19:32:41 UTC