mooremachine

mooremachine

Introduction

It's widely known that if you want to sequence some series of asynchronous actions in node, you should use a library like vasync or async to do so -- they let you define a series of callbacks to be run in order, like this:

async.series([
    function (cb) { thing.once('something', cb) },
    function (cb) { anotherthing.get(key, function () { cb(null, blah); }); }
]);

This lets you define sequential actions with asynchronous functions. However, if you need more complex logic within this structure, this becomes rapidly limiting -- it is difficult, for example, to create a loop. You have to improvise one by nesting some form of loop within a series call and a second layer of callbacks.

Another problem comes if you want to define multiple ways to return from one of these functions in the async series -- e.g. if there is an error and result path that are separate:

    function (cb) {
        thing.once('error', cb);
        thing.once('success', function (result) { cb(null, result); });
    }

While one such additional path is manageable, things quickly become very complex.

Instead, let us think of each of the callbacks in such an async sequence as being states of a finite state machine. With async.series we are limited to defining only edges that progress forwards through the list. If, instead, we could define whatever edges we like, we could construct conditional logic and loops across async boundaries. If we had some way to "gang" the callbacks set up in each state together, they could all be disconnected at state exit and avoid the need for complex logic to deal with out-of-state events.

This library provides a framework for dealing with just such an async finite state machine.

Moore machines

A Moore machine (as opposed to a Mealy machine) is an FSM whose outputs depend solely on the present state, and not any other inputs. They are considered to be a simpler approach than the Mealy machine, and easier to reason about.

In our analogy, of course, our state machine does not have distinct outputs (since really we are using it to run arbitrary code). If we consider the FSM's "outputs" as the total set of side-effects it has on the program's state, however, we can interpret a Moore machine as being an FSM where code only runs on the entry to a new state, and all other events can only serve to cause state transitions.

Example

todo