Programming Phoenix

Managing State with Processes

Functional programs are stateless, but we still need to be able to manage state. In Elixir, we use concurrent processes and recursion to handle this task. That may sound counterintuitive, but let’s take a look at how it works with a simple program.

Let’s create a Counter server that counts up or down. Create a lib/rumbl/counter.ex file and key this in:

otp/listings/rumbl/lib/rumbl/counter.ex

defmodule Rumbl.Counter do

  def inc(pid), do: send(pid, :inc)

  def dec(pid), do: send(pid, :dec)

  def val(pid, timeout \\ 5000) do 
    ref = make_ref()
    send(pid, {:val, self(), ref})
    receive do
      {^ref, val} -> val
    after timeout -> exit(:timeout)
    end
  end ...