Whatever you may call them, Promises, Futures or even Deferred the concept is the same. You ask an asynchronous task to do something, and it gives you a promised result back instantly which you can then act upon.

Take this trivial example in javascript:

var results = searchTwitter("Starwars"); filterByAuthor("@darthvader1977", results); displayTweets(results);

Now wouldn’t that be great? Looks synchronous but behaves asynchronously? Sadly, I believe that this would require a language construct not native to most programming languages.

There are a few promise libraries written in Javascript that provide an API we can utilise on our previous example.

searchTwitter("Starwars"). then(filterBydarthvader1977). then(displayTweets);

An heres where I’m confused. Why is this better then doing:

searchTwitter("Starwars", function (results) { displayTweets(filterByAuthor("@darthvader1977", results)); });

Yes I guess, yes it does look prettier and more concise, and the API is explicit in its intent, but is that enough? This covers the Promises/A proposal as detailed on the CommonJS wiki. In my opinion pretty useless, at least in Javascript. From what I can tell, all of the proposals seem require the supply of a callback of some sort. This to me is interesting as I thought the whole idea behond promises was to get rid of the layers of nested callback, not just to reduce them.

So this leads me to an idea, which after a conversation with a collegue seems to make sense. So lets start with an example.

var tweets = searchTwitter("Startwars"); tweets.filterByAuthor("@darthvader1977"); tweets.display();

As you can see, we call method on the tweets variable. I’m assuming that the searchTwitter method instantly returns a tweets object, with each method queuing the call until the asynchronous call to searchTwitter completes, which then calls each method in turn in the order it was invoked.

Now I’m certain this isn’t a new idea, and there must be a library out there or a specification, can someone tell me where?