Error: Unbound record field Server.callback - Ocaml - webserver

Question

I am following a tutorial that explains how to make a simple web server in OCaml with lwt and Cohttp.
I have a _tags file that contains the following:
true: package(lwt), package(cohttp), package(cohttp.lwt)
And a webserver.ml:
open Lwt
open Cohttp
open Cohttp_lwt_unix
let make_server () =
let callback conn_id req body =
let uri = Request.uri req in
match Uri.path uri with
| "/" -> Server.respond_string ~status:`OK ~body:"hello!\n" ()
| _ -> Server.respond_string ~status:`Not_found ~body:"Route not found" ()
in
let conn_closed conn_id () = () in
Server.create { Server.callback; Server.conn_closed }
let _ =
Lwt_unix.run (make_server ())
Then, ocamlbuild -use-ocamlfind webserver.native triggers the following error:
Error: Unbound record field callback
Command exited with code 2.
If I change to: Server.create { callback; conn_closed } it will also trigger:
Error: Unbound record field callback
Command exited with code 2.
I am not sure how to solve this, so thanks in advance for looking into this.

Answer 1

Probably, you are using a very outdated tutorial, that was written for an old cohttp interface. You can try to look at the up-to-date tutorials in the upstream repository.
In your case, at least the following changes should be made, to compile the program:
You should use function Server.make to create an instance of a server;
The callback and conn_closed values should be passed as function parameters, not as a record, e.g.,
Server.make ~callback ~conn_closed ()
You should use function Server.create and pass a value, that was returned from function Server.make to create a server instance.
So, probably, the following should work:
open Lwt
open Cohttp
open Cohttp_lwt_unix
let make_server () =
let callback conn_id req body =
let uri = Request.uri req in
match Uri.path uri with
| "/" -> Server.respond_string ~status:`OK ~body:"hello!\n" ()
| _ -> Server.respond_string ~status:`Not_found ~body:"Route not found" ()
in
Server.create (Server.make ~callback ())
let _ =
Lwt_unix.run (make_server ())