Wanted to understand what Plug.Router does to get a "conn" available, so reimplemented it (without peeking).

reimplementing_plug_router.exs

defmodule Toy.GetContext do
  def send_resp(conn, code, text) do
    IO.puts "Response! conn: #{conn}, code: #{code}, text: #{text}"
  end

  def conn do
    "the conn"
  end
end

defmodule Toy.PlugRouter do
  defmacro __using__(_opts) do
    quote do
      import Toy.PlugRouter
    end
  end

  defmacro get(_path, do: block) do
    quote do
      import Toy.GetContext
      unquote(block)
    end
  end
end

defmodule Toy.Router do
  use Toy.PlugRouter

  get "/hello" do
    send_resp(conn, 200, "Hi!")
  end
end

Previously: Reimplementing Ecto.Migration

@henrik Close! Plug makes use of the var! macro for injecting a variable into your route context: https://github.com/elixir-lang/plug/blob/master/lib/plug/router.ex#L360