Universal Server ("My favorite Erlang Program")

uni.erl

% Read more:
% - https://joearms.github.io/published/2013-11-21-My-favorite-erlang-program.html
% - https://ferd.ca/my-favorite-erlang-container.html
%
% How to run:
% $ erl
% > c(uni).
% {ok,uni}
% > uni:test().
% 30414093201713378043612608166064768844377641568960512000000000000

-module(uni).
-export([test/0]).

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

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).