(* Environments implemented using association lists (from CAML library) *)

module Env : Env = 
struct
  type 'a env = Env of (string * 'a) list
  exception Bad_id
  let empty = Env []
  let extend (Env env) v x = Env ((v,x)::env)
  let lookup (Env env) v =
    try List.assoc v env
    with Not_found -> raise Bad_id
end