OCaml variable, which keeps it value between function calls

Question

Is there a way in OCaml for a variable inside a function to keep its value between function calls? It should work like Pythons default argument, which is a reference to the same object in every function call or the function should rather yield and not explicitly return a value. The effect should be the following (if the function was to return natural numbers):

foo ();;
0
foo ();;
1

Answer 1

Yes this is possible. You need to define a local ref outside of the closure and access its value and modify it every time the closure is used like so:

let foo =
  (* local variable x *)
  let x = ref 0 in
  (* the closure that will be named foo *)
  fun () -> let r = !x in
            x := r+1; r