Seting a ref to the result of a Lwt_io.read_line in a chain of bound threads

Question

I'm creating a chat server, and I have a function that handles login. There exists a preset ref called nick and a preset input stream imp. My code is as follows:

let handle_login nr (inp,outp) = 
Lwt_io.printl "<Enter your 'nick'name>" >>= Lwt.return(nick := (Lwt_io.read_line inp))

However, this code give me the error:

Error: This expression has type string Lwt.t
       but an expression was expected of type string.

I know that the following code does work:

let handle_login nr (inp,outp) = 
Lwt_io.printl "<Enter your 'nick'name>" >>= Lwt.return(nick := "Jane")

In short, I don't know how to assign vars to values obtained from threads.

Answer 1

I'm not very familiar with Lwt, but if it works like any other monad I would think this should work:

let handle_login nr (inp, outp) = 
  Lwt_io.printl "<Enter your 'nick'name>"
  >>= fun () -> Lwt_io.read_line inp
  >>= fun str -> Lwt.return (nick := str)

But I have to point out as well that mutating shared state from async code is a disaster waiting to happen. That you can do it certainly doesn't mean you should.