riak_backend.ex

defmodule Beamie.Store.RiakHTTP do
  def put(bucket, key, value) do
    IO.puts "storing value #{inspect value} with key #{key} in bucket '#{bucket}'"
    put_req(bucket, key, value)
  end

  def get(bucket, key) do
    IO.puts "retrieving value for key #{key} from bucket '#{bucket}'"
    get_req(bucket, key)
  end

  #...
end

store.ex

  defp prepare_code_store_backend() do
    {:ok, backend} = :application.get_env(:beamie, :code_store_backend)
    Beamie.Store.Generator.gen_module(backend)
  end

# ...

defmodule Beamie.Store.Generator do
  # create a module at runtime that will dispatch to the chosen backend
  def gen_module(backend) do
    Module.create(Beamie.Store, quote do
      @backend unquote(backend)

      def put(bucket, key, value) do
        apply(@backend, :put, [bucket, key, value])
      end

      def get(bucket, key) do
        apply(@backend, :get, [bucket, key])
      end
    end)
  end
end