If we split the server side to public API and handle_call callbacks this allows us to change both in tandem easily. In addition clients do not need to know the internal structure of the server.
e.g.
defmodule Server do
use GenServerdef init_context(), do %Server{}
def send_update(pid, context), do GenServer.call(pid, {:send_update, context})
At this point the client doesn’t need to know anything about context, it doesn’t need to know how send_update works.
send_update might perform some pre-checks before calling the Server, it might chain several other GenServer.cast/call before calling the service. It might change the call to GenServer.call(pid, {:update, %{context | callee_ts: DateTime.utc_now}})
This style of programming is very old and widely used by C/Ada programs to hide implementation details.
