// identificatori
type ide = string

type aexp =
  | AEint of int
  | AEplus of (aexp * aexp)
  | AEminus of (aexp * aexp)
  | AElet of (ide * aexp * aexp)
  | AEide of ide

let rec aexp_to_string : aexp -> string =
  fun e ->
    match e with
      AEint i -> sprintf "%d" i
      | AEplus (e,f) -> sprintf "(%s + %s)" (aexp_to_string e) (aexp_to_string f)
      | AEminus (e,f) -> sprintf "(%s - %s)" (aexp_to_string e) (aexp_to_string f)
      | AElet (x, e, f) ->
        Printf.sprintf "(let %s = %s in %s)" x (aexp_to_string e) (aexp_to_string f)
      | AEide (x) -> x

// error handling
let unbound_identifier_error ide =
  failwith (Printf.sprintf "unbound identifier %s" ide)

// semantic domains
type eval = int
type dval = eval // denotable and expressible values coincide

let eval_to_string : eval -> string =
  fun e -> Printf.sprintf "%d" e

// ambiente
type env = ide -> dval

let empty = fun v -> unbound_identifier_error v