Example of a specialization pattern in Elixir

spec.exs

# Just an example of specialization in Elixir.
#
# I'm not sure if this pattern is a good practice in Elixir or
# it's too much "OOP-like" and it would be more Elixirific to
# use protocols. But using protocols to do the same thing would
# probably lead to more boilerplate code (explicitly define all
# funX in every protocol implementation).
#


# general base module
defmodule ImplBase.State do
  defstruct base_data: nil
end

defmodule ImplBase do
  alias ImplBase.State

  defmacro __using__([]) do
    quote do
      def fun1(state) do
        #
        # NOTE: this is problematic - we DEMAND that all implementations' state data
        #       allow Acces.get(data,:base_data)
        #
        #       Or we could use some custom protocol to access base_data here but it's
        #       almost the same situation...
        #
        IO.puts "fun1 - ImplBase implementation. data=#{inspect state.base_data}"
        state
      end
      def fun2(state) do
        IO.puts "fun2 - ImplBase implementation. data=#{inspect state.base_data}"
        state
      end
      defoverridable [fun1: 1, fun2: 1]
    end
  end

  @doc "Initialize ImplBase data"
  def init do
    %State{base_data: :some_base_data}
  end
end

# specific module no 1
#   - implementation without using any processes
defmodule Impl1.State do
  defstruct base_data: nil, impl1_data: nil
end

defmodule Impl1 do
  alias Impl1.State
  use ImplBase

  @doc "Initialize Impl1 data"
  def init do
    %State{base_data: ImplBase.init, impl1_data: :some_impl1_data}
  end

  def fun1(s=%State{}) do
    IO.puts "fun1 - Impl1. data=#{inspect s}"
    s
  end
end

# specific module no 2
#   - implementation using Agent to store data
#
# NOTE: we have little problem here - we must use one data structure
#       as state wich allows Access.get(s,:base_data) and another
#       datastructure to be fed into Agent...
defmodule Impl2.State do
  defstruct impl2_data: nil
end

defmodule Impl2 do
  alias Impl2.State
  use ImplBase

  @doc "Initialize Impl2 data"
  def init do
    {:ok,a} = Agent.start_link fn -> %State{impl2_data: :some_impl2_data} end
    %{base_data: ImplBase.init, agent: a}
  end

  def fun1(s=%{agent: a}) do
    data = Agent.get a, fn x -> x end
    IO.puts "fun1 - Impl2. data=#{inspect data}"
    s
  end
end

#
# main
#

Impl1.init
|> Impl1.fun1
|> Impl1.fun2

Impl2.init
|> Impl2.fun1
|> Impl2.fun2