May 15, 2012
CLOS FSM with MOP Sauce

Common Lisp in the desire to be as cool as possible includes in its specification the Common Lisp Object System, or CLOS, which itself can be introspected and altered in great detail using the MetaObject Protocol, or MOP, as described in The Art of the Metaobject Protocol. Unfortunately, MOP didn’t make it into the ANSI standard, but most implementations include MOP as it is described in the book, and a compatibility package :closer-mop (available in Quicklisp) makes using the symbols described seamless between implementations.

One of the features of MOP is the ability to make an instance of a CLOS class funcallable, that is allow a class instance to be a valid first argument to funcall. This behavior in a lot of ways can resemble the traditional method model from other languages, but that’s not how I intend use it here.

The Idea

I’m going to show and tell an implementation of a generic finite state machine that uses the MOP concepts of funcallable-standard-object and funcallable-standard-class to marshal the flow of incoming events to the machine and the concept of CLOS generic method dispatch to handle the execution of transition handlers for any given state of the machine.

These features will allow us to build a structure that lets us focus solely on the problem at hand and defer features like event data binding, state-dependent method selection and unexpected state handling entirely to the language without pushing the boundaries of any specific feature.

The Design

The design we’re going for is such that we can define a class with a state slot that will hold :keyword name of a state. We’ll make instances of this class funcallable so that when we make an instance we will be able to simply (funcall fsm-instance fsm-evemt) repeatedly and have the machine dispatch to the correct event, perform any logic, and transition to the next state based on the input.

We would be able to query the state of the machine with (state fsm-instance) and receive a keyword, and we should be able to drive events into the machine until we’re in a desired or unexpected state. Any attempt to feed the machine an event while the machine is in an invalid state should result in an error.

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