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Sender: Ivan Gotovchits <lambda.q.q@gmail.com>
From: Ivan Gotovchits <ivg@ieee.org>
To: caml-list@inria.fr
Date: Thu, 31 Jan 2013 09:37:29 +0400
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Subject: [Caml-list] [lwt] multiplexing several threads


Dear list,

I'm stuck with a simple problem when using lwt. I have two lwt
threads. One on them reads commands from user (using Lwt_read_line),
another reads events from system. A need to multiplex them. Speaking in
ocaml, I have three functions:

  type state    (* program state to process *)
  type command  (* command to be executed (issued either by user or by
                   system event) *)
  
  val read_from_user: unit -> command Lwt.t
  val read_event: unit -> command Lwt.t
  val process: state -> command -> state Lwt.t

 
And a need to write a function loop

  val loop: state -> state Lwt.t

accepting some state, taking a command from user or an event from system
(whichever comes first) and passing it to `process' function, and doing
it in a cycle, until explicitly ordered to stop (with Exit command). 

The following solution

   let rec loop sc =
     let cmd = read_from_user () <?> read_event () in
     cmd >>= (process state)

   and process sc = function
     | Exit -> return sc
     | cmd  -> (* modify sc*) loop sc

   let r = Lwt_main.run (loop state)

doesn't work when the event thread wakes up first. In that case the next
step of the loop will call Lwt_read_line.read_line before the previous
invocation finishes and it breaks user input.

Please, help me to find a feasible solution to my problem!

-- 
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