To explain further, the `receive` keyword is a bit like a switch statement (but actually a pattern matcher) for incoming messages. Here they made a new server

To explain further, the `receive` keyword is a bit like a switch statement (but actually a pattern matcher) for incoming messages. Here they made a new server that takes an return process (From) and a number (N). The exclamation point sends the result back to the return process. factorial_server() -> receive {From, N} -> From ! factorial(N), factorial_server() end. factorial(0) -> 1; factorial(N) -> N * factorial(N-1). This code then spawns the server, sends a message to that server to become a factorial server, then tells that server to send it back a message with the factorial of 50. It then specifies it's own message listener that takes whatever it receives and returns it. test() -> Pid = spawn(fun universal_server/0), Pid ! {become, fun factorial_server/0}, Pid ! {self(), 50}, receive X -> X end. A couple of the major advantages of Erlang its distributed parallel nature, and also hot code update. Which happens in `Pid ! {become, fun factorial_server/0}` where it overides the receive loop of universal_server with that of the factorial_server. Though I think proper hot code update doesn't work like this