Oct 25, 2012

Playing with futures (and async interfaces in general)

One of the main feedback points I’ve gotten on cl-async is the fact that it uses CPS (continuation apssing style), which essentially means that asynchronous operations do not return values, but instead are given callbacks which receive the values computed by the operation as arguments.

The biggest problem I see with CPS is not so much the style (I do a lot of JS programming and honestly it doesn’t bug me too much), but the fact that if a function works synchronously, converting it to work asynchronously suddenly means that every other function that calls it must not be converted to CPS. This is, thanks to Common Lisp’s lack of continuation support, impossible to get around, as far as I can tell. I know of cl-cont and a few other libraries, but they don’t fix the fundmental problem.

The syntax can be changed around to make it prettier, but currently there is no way to convert a synchronous function to being asynchronous without rewriting every function that calls it in CPS. How horrible.


A future is an object that may recieve a value later. One or more callbacks can be attached to it, and these callbacks will be called when the future’s value is done computing. This makes the syntax for async operations a bit more flexible and natural, but also allows some nice opportunities for syntax wrapping via macros that’s a lot cleaner than code walkers (like cl-cont).

Here is a comment on Reddit about futures, with a great example. along with some great discussion about the implementation and how it works. Basically, a value from deep down in a bunch of nested callbacks can be attained by being smart about how futures are binding to each other.

I’ve implemented this type of future in the future branch of cl-async, and plan on using it for building drivers.

cl-async syntax

I’ve decided that although I’d like to include my future implementation in the master branch of cl-async once I know it’s stable, I will not be implementing futures in cl-async itself, but rather have futures be the standard for creating drivers.

The reason behind this is that futures are very value-based (like most drivers) and the cl-async library is very stream/multi-value based. It doesn’t really make sense to implement futures in TCP, for instance, and re-trigger a future whenever new data comes in. Futures also don’t make sense for servers, and would add uneeded complexity to delayed functions. I feel there is no part of cl-async that would benefit from having integrated futures.

I’ve also decided that I don’t want to include cl-cont or any other code walker. I have nothing against them and I think the goals are admirable, but they don’t solve the real problem cl-async has: the impossibility of blocking the current operation while processing other events on the same thread, and continuing the original operation once it completes (ie, continuations).

While some might cringe at CPS, I feel that cl-async has an easy, understandable API and if someone wants to wrap it in a syntax layer, be my guest, but know that the root problem is still there: you will never be able to call functions using CPS without converting all functions using it to be CPS, either by hand or with a code walker.

Such is our plight.