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 languag
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...