What is message passing?

Message passing is how Erlang processes exchange information, since they share no memory. The ! operator sends a message asynchronously into the target process's mailbox, and receive pulls a matching message out of the caller's own mailbox.

Pid ! {self(), {add, 2, 3}},
receive
    {ok, Result} -> Result
after 5000 ->
    timeout
end.

Sending never blocks the sender, even if the receiving process is busy — the message just waits in the mailbox. receive scans the mailbox for the first message matching one of its patterns, skipping over (but not discarding) messages that don't match, and can specify an after clause to avoid waiting forever.

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