book example: https://github.com/realworldocaml/examples/blob/master/code/async/echo.ml

echo.ml

open Core
open Async

(* Copy data from the reader to the writer, using the provided buffer
   as scratch space *)
let rec copy_blocks buffer r w =
  Reader.read r buffer
  >>= function
  | `Eof -> return ()
  | `Ok bytes_read ->
    Writer.write w (Bytes.to_string buffer) ~len:bytes_read;
    Writer.flushed w
    >>= fun () ->
    copy_blocks buffer r w

(* part 1 *)
(** Starts a TCP server, which listens on the specified port, invoking
    copy_blocks every time a client connects. *)
let run () =
  let host_and_port =
    Tcp.Server.create
      ~on_handler_error:`Raise
      (Tcp.Where_to_listen.of_port 8765)
      (fun _addr r w ->
         let buffer = Bytes.create (16 * 1024) in
         copy_blocks buffer r w)
  in
  ignore (host_and_port : (Socket.Address.Inet.t, int) Tcp.Server.t Deferred.t)

(* part 2 *)
(* Call [run], and then start the scheduler *)
let () =
  run ();
  never_returns (Scheduler.go ())

The async docs landing page instructs, "Read more in Chapter 18 of Real World OCaml," perhaps suggesting the book should be used as a reference. This is potential point of frustration for many readers of the book as the code examples in the book will not run and it's not clear how to diagnose the issue.

I just found Real World OCaml v2. I see the updated echo server example has the same changes as above 🏆

So I guess I've been reading the wrong version of the book! I'll read through v2 and who knows, maybe I'll be able to help find some fixes later on.