Composable event-based state machines

Trying it out

npm run start

What is this?

This is a start at a framework for writing highly-composable state machines. While it was initially inspired by Dan Abramov's excellent article on algebraic effects, the end result looks and acts more like a framework for composing lightweight, anonymous state machines.

Each event responder in a machine can (and should be) pure, returning a description of the effect it wants to have on the system. This effect can be a reduction of the state machine from its previous state to a new state, a message to be sent back into the machine, or the output of a number of effect combinators (like Parallel for running multiple effects, or Try for running an effect with an error handler).

This project was written for two primary reasons. One, as an experiment in writing more-composable D3 code, as most code written in D3 (purely in D3, ignoring combination with composable frameworks like React) is composed by putting one piece of code before/after another. Two, because I wanted to be able to specify event responders that responded to more than one event. For instance, an responder that activates on every set of 10 subsequent sensor readings, if you wanted to monitor a running average. Or, an event responder that activates only when a player says "yes" to going on that kill-10-goblins quest, but has in the past already killed 50 goblins and said no to the elf princess Sharlene. My intuition here was that a multi-event responder could be curried into a series of single-event responders, each one modifying the system to respond to the next event in sequence.

Status

First draft (working and complete-for-now). Currently displays a WASD-controllable swarm and exposes a few demonstrative keystrokes (try hitting Enter a few times, or pressing P).

When was it written?

August 2019 -> now

Next steps