Going functional with elixir & otp

kv.exs

defmodule KV do
  @moduledoc """
  Key value store.

  ## Examples

  Basic usage

      iex> {:ok, pid} = KV.start_link()
      iex> KV.put(pid, "foo", :bar)
      iex> KV.get(pid, "foo")

  What if key is missing?

      iex> {:ok, pid} = KV.start_link()
      iex> KV.get(pid, "foo")
      nil

  Remember to `flush()` after yourself to see the messages ;-)
  """

  def start_link do
    Task.start_link(fn -> loop(%{}) end)
  end

  def put(pid, key, value) do
    send(pid, {:put, key, value})
  end

  def get(pid, key) do
    send(pid, {:get, key, self()})
  end

  defp loop(map) do
    receive do
      {:get, key, caller} ->
        send(caller, Map.get(map, key))
        loop(map)

      {:put, key, value} ->
        loop(Map.put(map, key, value))
    end
  end
end

stack.exs

defmodule Stack.Client do
  def pop(server) do
    GenServer.call(server, :pop)
  end

  def push(server, value) do
    GenServer.cast(server, {:push, value})
  end
end

defmodule Stack.Server do
  use GenServer

  def start_link(opts) do
    GenServer.start_link(__MODULE__, :ok, opts)
  end

  def init(:ok) do
    {:ok, []}
  end

  def handle_call(:pop, _from, [head | tail]) do
    {:reply, head, tail}
  end

  def handle_cast({:push, value}, stack) do
    {:noreply, [value | stack]}
  end
end