Syntax error using "with fields" in OCaml

Question

I'm working through Real World OCaml and I'm getting a syntax error on the following bit of code:

`# module Logon = struct
    type t =
      { session_id: string;
        time: Time.t;
        user: string;
        credentials: string;
      }
    with fields
  end;;

On running, Utop underlines the word "with" and throws a syntax error. I've tried similar, more simple examples on my own and get the same error. Any thoughts on what's going on?

EDIT: Omitted "}" added.

Answer 1

For ocaml 4.01.0 : In utop : #require "fieldslib.syntax";; is solving the issue. (... do not forget to run opam install fieldslib).

For recent ocaml : (got some hints here ).

opam install ppx_jane fieldslib

#require  "ppx_jane";;
#require "fieldslib";; 
module Logon = struct
    type t =
      { session_id: string;
        time: Time.t;
        user: string;
        credentials: string;
      }
    [@@deriving fields]
  end;;