Syntax Errors in utop when using Sexplib and Fieldslib

[Gabriel Jaldon]

I'm following Real World OCaml and some of the code provided raise Syntax Errors. Below are some of the code:

```

let custom_to_sexp = <:sexp_of<float * string list * int>>;;

```

- The syntax error are on the characters `<:`

```

module Logon = struct

  type t =

    { session_id: string;

      time: Time.t;

      user: string;

      credentials: string;

    }

  with fields

end

```

- syntax error is on `with`

I'm using utop version 1.19.3 and OCaml 4.03.0.

My ~/.ocamlinit file looks like:

```

(* Added by OPAM. *)

let () =

  try Topdirs.dir_directory (Sys.getenv "OCAML_TOPLEVEL_PATH")

  with Not_found -> ()

;;

#use "topfind";;

#thread;;

(* #camlp4o;; *)

#require "ppx_jane,core.top";;

#require "async";;

#require "core_extended";;

open Core.Std;;

```

How to make those syntax extensions work with utop?

[Yaron Minsky]

If you're using a recent version of Core, we've made a rather major
switch from using camlp4 to using PPX. We really need to get RWO
updated to match, which we're working on.

Anyway, it looks like you have roughly the right things in your
ocamlinit. You should be able to write:

let custom_to_sexpo = [%sexp_of: float * string list * int]

instead, and that should work. For the Logon, you should be able to write:

module Logon = struct

type t = { ... }
[@@deriving fields]
end

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[Gabriel Jaldon]

Thanks a lot for the response, Yaron! Apart from some outdated content, the book is great! I'm looking forward to the update.