How to represent a simple finite state machine in Ocaml?

Question

I have written some state machine in C++ and Java but never in a functional language like Ocaml

Problem is I don't know if I can just adapt code from the object languages versions, since in Ocaml records and variants are more powerful than class;

So, I need an event-driven finite state machine (hierarchical like in UML), easily configurable

Could someone experienced in the field post a simple sample of that ? Just to avoid the most common traps

thanks :)

EDIT 16/03 : Is it possible to do it without mutable state ? And I'd like to encapsulate it properly under the name "FSM", should I choose a module or a class ?

Answer 1

It depends on how you have to operate the FSM, e.g., if you need to be able to store its state and continue later, or if you just want to execute it immediately. In the latter case, it's trivial to do it as a bunch of tail-recursive functions.

For example, assume the regexp C((A|B)*CD)* -- the following mutually recursive functions are a direct implementation of the respective FSM that recognises a list matching this regexp (if I didn't make any mistake :) ):

type alphabet = A | B | C | D

let rec s1 = function
  | C :: rest -> s2 rest
  | _ -> false

and s2 = function
  | [] -> true
  | (A | B) :: rest -> s2 rest
  | C :: rest -> s3 rest
  | _ -> false

and s3 = function
  | D :: rest -> s2 rest
  | _ -> false

Every function corresponds to exactly one state of the automaton and implements its transition function. Applying s1 : alphabet list -> bool will run the FSM on the argument.

PS: Note how this is an application demonstrating the benefit and elegance of tail call optimization...