Processes and explicit messages

The idea

Enter with anonymous functions and tuple patterns understood. Elixir processes are lightweight execution units with independent mailboxes. spawn starts a process and returns its PID. send delivers a message asynchronously, while receive selects a matching message from the current process’s mailbox. A process does not share mutable variables with its caller, so coordination uses messages rather than updates to a common variable.

self returns the PID of the process executing it. Capture the parent PID before spawning when a worker must reply to its caller; calling self inside the worker would name the worker instead. Tagged tuples identify the kind of result being delivered. Production request-response designs also need timeouts and correlation identifiers when several replies may overlap. This chapter uses one worker and one explicitly matched reply.

parent = self()
spawn(fn -> send(parent, {:result, 6 * 7}) end)
receive do
  {:result, value} -> IO.inspect(value)
end

42

The worker sends its result to the captured parent PID. The parent waits for the matching tuple before printing.