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]
;;

ohh, I get confused again when I meet the problem as following:

type student = { id: int; name: string; };;

let bob = { id = 1; name = "Bob"};;
(* val bob : student = {id = 1; name = "Bob"} *);;

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

let () = List.iter (fun s ->
  Printf.printf "Hello, %s\n" s.name
) [bob]
;;

It can be compiled and run as expected… why?

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]
;;