Joe Armstrong's favorite Erlang program (from http://joearms.github.io/2013/11/21/My-favorite-erlang-program.html)

demo1.erl

-module(demo1).

-export([test/0]).

universal_server() ->
    receive
       {become, F} ->
           F()
    end.

factorial_server() ->
    receive
       {From, N} ->
           From ! factorial(N),
           factorial_server()
    end.

factorial(0) -> 1;
factorial(N) -> N * factorial(N-1).

test() ->
    Pid = spawn(fun universal_server/0),
    Pid ! {become, fun factorial_server/0},
    Pid ! {self(), 50},
    receive
        X -> X
    end.