A Process is a long-lived function, that has access to a mailbox to receive messages.

Here's how we can create a process to print out the message from the last tutorial:

Riot.run @@ fun () ->
  let open Riot in
  let pid = spawn (fun () -> print_endline "Hello, Joe!") in

Riot has a spawn function that can be used to create a new process. Riot processes are cheap, and Riot programs can have millions of processes. They are not like Operating System processes (or threads), and are closer to green-threads or fibers.

spawn takes a unit -> unit function as an input, and will give us back a pid. A Pid is a Process Identifier. Pids are unique during the execution of a program and can be used to send messages to processes, to check if they are still alive, and to terminate them.

  let pid = spawn (fun () -> print_endline "Hello, Joe!") in

Inside of a process, we can get the pid of the process by calling self (). A Pid can also be pretty-printed with Pid.pp but it is not serializable.

  let pid = spawn (fun () -> Format.printf "Hello, %a!" Pid.pp (self ())) in

A common scenario is waiting for a number of pids to terminate. For this Riot offers a wait_pids function that will return after all the pids are finished.

Be mindful that if the pids do not terminate, this function will get the caller process stuck in that wait loop. We will see later in this tutorial more flexible mechanisms for detecting when other processes terminate.

  wait_pids [pid]

And as before, if we want the runtime to finish, we should call shutdown ().

• the next step introduces you to communicating processes and Message passing