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]
;;

I think I get the point.

# List.iter;;
- : ('a -> unit) -> 'a list -> unit = <fun>
# let f x = ();;
val f : 'a -> unit = <fun>

then

# List.iter f;;
- : '_weak1 list -> unit = <fun>

The f is not binding any type to x until we pass a 'a list.

---

There are 2 solutions to solve this by my knowledge:

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

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

and thank @yawaramin helps

- let () = List.iter (fun s ->
+ let () = List.iter (fun (s: Student.t) ->
  Printf.printf "Hello, %s\n" s.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]
;;