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

Usually, you create a record corresponding to a state of the automata, and you have another type for the event triggering the transition to another state. In the state record, you have a map to find, for each event, the new state.

Let's suppose your transitions are triggered by strings:

type event = string

module EventMap = Map.Make(struct
    type t = event
    let compare = compare
  end)

type state = {
  state_info : ...; (* the content of that state, id, comment, etc. *)
  mutable state_transitions : state EventMap.t;
}

let next_state current_state event =
  try
    EventMap.find event current_state.state_transitions
  with Not_found -> current_state

Here, I supposed that unknown events stay on the same state, but you could have an error state in the record...