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(* test data *)

let answer = Div (Div (Con 1972, Con 2), Con 23);; (* 42 *)

let error = Div(Con 1, Con 0);; (* Exception: Division_by_zero. *)

(* term type *)

type term = Con of int | Div of term * term ;;



(* primitive evaluator *)

let rec eval = function Con a -> a
            | Div (t, u) -> eval(t) / eval(u) ;;


(* with exception handling *)
  
type eval_exception = string;;

type 'a m  = Raise of eval_exception | Return of 'a;;

let rec eval x = match x with 
  | Con a      -> Return a
  | Div (t, u) -> match eval(t) with
                  | Raise e -> Raise e
                  | Return b -> match eval(u) with
                                | Raise e  -> Raise e
                                | Return c -> if c = 0 then Raise ("divided by my zero") 
                                                       else Return ( b / c) ;;
  
(* with state *)
  
(* without explicitly type declearation *)
let rec eval term x = match (term, x)  with 
  | ((Con a),  x)   -> (a, x)
  | (Div (t, u), x) -> let (b,y) = (eval t x)  in
                       let (c,z) = (eval u y) in
                       (b / c, z + 1)  ;;



type state = int ;;
let rec eval (exp:term) (x:state) : int * state  = match (exp, x)  with 
  | ((Con a),  x)   -> (a, x)
  | (Div (t, u), x) -> let (b,y) = (eval t x)  in
                       let (c,z) = (eval u y) in
                       (b / c, z + 1) ;;
  

(* which one is correct in above three declearration? *) 
type 'a m = 'a * state;;
type 'a m = state -> 'a * state;;
type 'a m = 'a -> 'a * state;;
type ('a, state) m = 'a * state;;

(* only this one is right *)
type m 'a = state -> 'a * state;;

let rec eval term : int m  = match term with 
  | (Con a)  -> fun x ->  (a, x)
  | (Div (t, u)) -> fun x -> 
                      let (b,y) = eval(t)(x) in
                      let (c,z) = eval(u)(y) in
                    (b / c, z + 1);;
  
  
(* output *)
( * somethig is wrong with this) 
  
(* helper function for pretty print *)
let rec showterm  = function Con a -> "Con " ^ (string_of_int a)
                            | Div (x, y) -> "Div (" ^ showterm(x) ^ ","  ^ showterm(y) ^ " )" ;;


let line t a = (showterm t) ^ " =  " ^ (string_of_int a) ^ " => " ;;

let rec eval term  = match term with 
  | (Con a)  -> (line (Con a) a, a)
  | (Div (t, u)) -> let (x,a) = eval t in
                    let (y,b) = eval u in
                    (x ^ y ^ (line (Div (t, u)) (a/b)), a/b);;


(* with type * *)
type output_step = string ;;
type m 'a = output_step * 'a ;;

let rec eval term : int m  = match term with 
  | (Con a)  -> (line (Con a) a, a)
  | (Div (t, u)) -> let (x,a) = eval t in
                    let (y,b) = eval u in
                    (x ^ y ^ (line (Div (t, u)) (a/b)), a/b);;