interp.ml

exception ImplementMe

type t = True | False | If of t * t * t | Int of int | Plus of t * t | Greater of t * t

let isval t =
  match t with
      True|False -> true
    | Int _ -> true
    | _ -> false

(* Problem 1a.  *)

exception NormalForm 

let rec step t =
	match t with
	| If(True, t2, t3) -> t2
	| If(False, t2, t3) -> t3
	| If(t1, t2, t3) -> If(step t1, t2, t3)
	| Plus (t1, t2) -> 
			(match (isval t1, isval t2) with
			| (false, _) -> Plus (step t1, t2)
			| (true, false) -> Plus (t1, step t2)
			| (true, true) ->
					(match (t1, t2) with
					| (Int i1, Int i2) -> Int(i1 + i2)
					| _ -> raise NormalForm))
	| Greater (t1, t2) ->
			(match (isval(t1), isval(t2)) with
			| (false, _) -> Greater (step t1, t2)
			| (true, false) -> Greater (t1, step t2)
			| (true, true) ->
					(match (t1, t2) with
					| (Int i1, Int i2) -> if i1 > i2 then True else False
					| _ -> raise NormalForm))
	| _ -> raise NormalForm

(* Problem 1b. *)
  
(*let rec eval t = raise ImplementMe*)

step True