type symbol = string
type 'a env = symbol -> 'a

type expr =
    | TUnit                             (* () *)
    | TInt of int                       (* i *)
    | TAdd of expr * expr               (* e1+e2 *)
    | TSub of expr * expr               (* e1-e2 *)
    | TMul of expr * expr               (* e1*e2 *)
    | TDiv of expr * expr               (* e1/e2 *)
    | TIf  of expr * expr * expr        (* if e1 then e2 else e3 *)
    | TVar of symbol                    (* x *)
    | TLet of symbol * expr * expr      (* let x=e1 in e2 *)
    | TLetrec of (string * expr * expr) (* let rec f = ... f ... in ... *)
    | TApp of expr * expr               (* e1 e2 *)
    | TFun of symbol * expr             (* fun x -> e *)
    | TSeq of expr * expr               (* e1; e2 *)
    | TRef of expr                      (* ref e *)
    | TDeref of expr                    (* !e *)
    | TAffect of expr * expr            (* (e1 := e2) *)

type value =
    | VUnit
    | VInt of int
    | VFun of (value -> value)
    | VExp of expr