Stack (vector) OCaml

 module type  MY_STACK =
    sig
      type 'a t
      exception Empty of string
      val size : int
      val create : unit -> 'a t
      val increase : 'a t -> unit
      val push : 'a option * 'a t -> unit
      val top : 'a t -> 'a
      val pop : 'a t -> unit
      val isEmpty : 'a t -> bool
    end;;


  module MyStack : MY_STACK =
  struct
    type 'a t = {mutable n: int; mutable a: 'a option array;}
    exception Empty of string
    let size = 5
    let increase_factor = 2
    let create ()= {n = 0 ; a = Array.make size None}
    let increase s =
      let newSize = size * increase_factor  in
      let newArr = Array.make newSize None
      in begin
          for i = 0 to size-1 do
            newArr.(i) <- s.a.(i)
           done;
           size = newSize;
           s.a <- newArr
          end
   let push(e, s) =
    begin
      if s.n = size then  increase s;
      s.a.(s.n) <- e;
      s.n <- s.n + 1;
    end

  let top s = if s.n=0 then raise (Empty "module MyStack: top")
   else match s.a.(s.n-1) with
        Some elem -> elem
        | None -> failwith  "module MyStack: top (implementation error!!!)"
  let isEmpty s = s.n=0
  let pop s = if s.n = 0 then raise (Empty "module MyStack:pop")
          else s.n <- s.n-1

  end;;

  let mys = StackMutList.create();;

 let testStack = MyStack.create();;
 MyStack.isEmpty testStack;;
 MyStack.push(Some(1), testStack);;
 MyStack.push(Some(2), testStack);;
 MyStack.push(Some(3), testStack);;
 MyStack.push(Some(4), testStack);;
 MyStack.isEmpty testStack;;
 MyStack.top testStack;;
 MyStack.pop testStack;;
 MyStack.top testStack;;
 MyStack.pop testStack;;
 MyStack.top testStack;;
 MyStack.pop testStack;;
 MyStack.top testStack;;
 MyStack.pop testStack;;
 MyStack.isEmpty testStack;;