server4.erl

-module(server4).
-export([start/2, rpc/2, swap_code/2]).

start(Name, Mod) ->
  register(Name, spawn(fun() -> loop(Name,Mod,Mod:init()) end)).

swap_code(Name, Mod) -> rpc(Name, {swap_code, Mod}).

rpc(Name, Request) ->
  Name ! {self(), Request},
  receive
    {Name, crash} -> exit(rpc);
    {Name, ok, Response} -> Response
  end.
  
loop(Name, Mod, OldState) ->
  receive
    {From, {swap_code, NewCallbackMod}} ->
      From ! {Name, ok, ack},
      loop(Name, NewCallbackMod, OldState);
    {From, Request} ->
      try Mod:handle(Request, OldState) of
        {Response, NewState} ->
          From ! {Name, ok, Response},
          loop(Name, Mod, NewState)
      catch
      _: Why ->
        log_the_error(Name, Request, Why),
        From ! {Name, crash},
        loop(Name, Mod, OldState)
      end
  end.

log_the_error(Name, Request, Why) ->
  io:format("Server ~p request ~p ~n"
            "caused exception ~p~n",
            [Name, Request, Why]).