type 'a t = { mutable c : 'a list }
exception Empty
let create () = { c = [] }
let clear s = s.c <- []
let copy s = { c = s.c }
let push x s = s.c <- x :: s.c
let pop s =
  match s.c with
    hd::tl -> s.c <- tl; hd
  | []     -> raise Empty
let top s =
  match s.c with
    hd::_ -> hd
  | []     -> raise Empty
let is_empty s = (s.c = [])
let length s = List.length s.c
let iter f s = List.iter f s.c

And, here is the interface file, stack.mli:

type 'a t
exception Empty
val create : unit -> 'a t
val push : 'a -> 'a t -> unit
val pop : 'a t -> 'a
val top : 'a t -> 'a
val clear : 'a t -> unit
val copy : 'a t -> 'a t
val is_empty : 'a t -> bool
val length : 'a t -> int
val iter : ('a -> unit) -> 'a t -> unit