env.sml

(*Used when trying to look up a name that doesn't exist*)
exception NameNotBound;

(*Maps a name as a string to its value as a given type*)
type 'a Env = string -> 'a;

(*Creates an empty Env which raises NameNotBound if var name a does not exist in Env*)
fun env_new () : 'a Env = fn x => raise NameNotBound;
 
(*Looks up the value of a given name in the given Env
  Raises NameNotBound exception if name not found in the given Env*)
fun env_lookup (e : 'a Env) (x : string) : 'a = e x;

(*Adds a new element with the given name and value to the given Env*)
fun env_bind (e : 'a Env) (a : string) (b : 'a) : 'a Env = 
	fn (x : string) => if (x = a) then b
						else e x;