Why?

unbound_record_field_name.ml

module Student = struct
  type t = { id: int; name: string; }
  let init (id, name) = { id = id; name = name; }
end
;;

let alice = Student.init(0, "Alice")
(* val alice : Student.t = {Student.id = 0; name = "Alice"} *);;

let () = Printf.printf "Hello, %s\n" alice.name
;;

let () = List.iter (fun s ->
  Printf.printf "Hello, %s\n" s.name
  (*                            ^^^^  *)
  (* Error: Unbound record field name *)
) [alice]
;;

OK. According to the module section of Real World OCaml, we need open the module signature.

We can expose it after module definition

module Student = struct
  type t = { id: int; name: string; }
  let init (id, name) = { id = id; name = name; }
end
;;

+ open Student;;

Or, expose it in minimum

- let () = List.iter (fun s ->
+ let () = let open Student in List.iter (fun s ->
  Printf.printf "Hello, %s\n" s.name
) [alice]
;;