Pretty Printing

let kupo_pp fmt s = Format.fprintf fmt "%s kupo" s;;
#install_printer kupo_pp;;

let h = "Hello"
(* val h : stirng = Hello kupo *)

(* First expose it in signature *)
module type Stack = sig
  type 'a t
  exception Empty
  val empty : 'a t
  val is_empty : 'a t -> bool
  val push : 'a -> 'a t -> 'a t
  val peek : 'a t -> 'a
  val pop : 'a t -> 'a t
  val size : 'a t -> int
  val pp :
    (Format.formatter -> 'a -> unit) -> Format.formatter -> 'a t -> unit
end

(* Then implement it *)
module ListStack : Stack = struct
  type 'a t = 'a list
  exception Empty
  let empty = []
  let is_empty = function [] -> true | _ -> false
  let push x s = x :: s
  let peek = function [] -> raise Empty | x :: _ -> x
  let pop = function [] -> raise Empty | _ :: s -> s
  let size = List.length
  let pp pp_val fmt s =
    let open Format in
    let pp_break fmt () = fprintf fmt "@," in
    fprintf fmt "@[<v 0>top of stack";
    if s <> [] then fprintf fmt "@,";
    pp_print_list ~pp_sep:pp_break pp_val fmt s;
    fprintf fmt "@,bottom of stack@]"
end

(* Install printer *)
#install_printer ListStack.pp;;

(* Then it will display as we want *)

ListStack.empty
(**
- : 'a ListStack.t = top of stack
                     bottom of stack
					 *)

ListStack.(empty |> push 1 |> push 2)
(**
- : int ListStack.t = top of stack
                      2
                      1
                      bottom of stack
					  *)