Elixir Agent - a Lightweight GenServer

If you come from Erlang and are familiar with the GenServer behaviour you may be interested in the Agent set of methods that ship with Elixir. They provide a lightweight mechanic to save and retrieve state. Here are two functionally equivalent modules, one written as a GenServer and the other with Agent.

defmodule NumberGenerator do

  use GenServer

 
  def start_link do

    GenServer.start_link(__MODULE__, 1)

  end

 
  def next_number(pid) do

    GenServer.call(pid, :next_number)

  end

 
  def handle_call(:next_number, number) do

    {:reply, number, number + 1}

  end

end

defmodule NumberGenerator do

  def start_link do

    Agent.start_link(fn -> 1 end)

  end

 
  def next_number(pid) do

    Agent.get_and_update(pid, fn num -> {num, num + 1} end)

  end

end

Pretty nice how much you can shrink the code; and for things as trivial as this you probably don’t even need to have a module at all! To be fair I used the anonymous function version of Agent; which makes it seem a lot smaller. Here is the same one again with non-anonymous functions.

defmodule NumberGenerator do

  def start_link do

    Agent.start_link(__MODULE__, :init, [])

  end

 
  def next_number(pid) do

    Agent.get_and_update(pid, __MODULE__, :fetch_and_advance, [])

  end

 
  def init, do: 1

  def fetch_and_advance(number), do: {number, number + 1}

end

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