calc.ml

module StringMap = Map.Make (String);;

open StringMap;;

type op =
  | Add 
  | Sub
  | Mult
  | Div;;

type expr = 
  | Lit of int
  | BOp of expr * op * expr
  | Var of string
  
let evalop op l r = 
  match op with
  | Add -> l + r
  | Sub -> l - r
  | Mult -> l * r
  | Div -> l / r;;
  
let rec interpret e m =
  match e with
  | Lit n -> n 
  | BOp (l, op, r) -> evalop op (interpret l m) (interpret r m)
  | Var str -> find str m;;

let rec compile e =
  match e with
  | Lit n -> fun m -> n
  | BOp (l, op, r) -> 
      let
        cl = compile l and cop = evalop op and cr = compile r in 
      fun m -> cop (cl m) (cr m)
  | Var str -> find str;;

let x = Var "x";;
let e = BOp ((BOp (x, Mult, (Lit 4))), Add, (BOp (x, Mult, (Lit 2))));;
let m = add "x" 1 empty;;

(interpret e m, compile e m);;