Example of Lwt thread-local storage values

test.ml

(* Build with:
    ocamlbuild -use-ocamlfind -pkg lwt.unix -tag annot test.native
 *)

let id_key = Lwt.new_key ()

let log fmt =
  let tid =
    match Lwt.get id_key with
    | None -> "<unknown>"
    | Some id -> id
  in
  Printf.(kprintf (fun (msg:string) -> printf "[%s] %s\n%!" tid msg) fmt)

let in_thread ?id fn =
  Lwt.with_value id_key id fn

let () = Lwt_unix.run (
  let open Lwt in
  log "main function starts";
  in_thread ~id:"thread 1" (fun () -> log "hello");
  in_thread ~id:"thread 1" (fun () -> log "world");
  let rec looper n () =
    match n with
    | 0 -> return_unit
    | n ->
       log "looper %d" n;
       Lwt_unix.sleep (Random.float 1.0)
       >>= looper (n-1)
  in
  join [ 
    in_thread ~id:"foo" (looper 10);
    in_thread ~id:"bar" (looper 5);
    looper 7 ()
  ]
)