reimplementing_ecto_migration.exs

# TODO:
# Runner like https://github.com/elixir-lang/ecto/blob/master/lib/ecto/migration.ex (does it have state?)

defmodule Ecto.Runner do
  def start_command({:create, table}) do
    IO.puts "create table: #{table}"
  end

  def subcommand({:add, column, type}) do
    IO.puts "add column: #{column}(#{type})"
  end

  def end_command do
    IO.puts "done with something"
  end
end

defmodule Ecto.Migration do
  alias Ecto.Runner

  defmodule Create do
    def add(column, type) do
      Runner.subcommand({:add, column, type})
    end
  end

  defmacro __using__(_opts) do
    quote do
      import unquote(__MODULE__)
    end
  end

  defmacro create(object, do: block) do
    {:table, _, [name]} = object

    quote do
      fn ->
        import Create
        Runner.start_command({:create, unquote(name)})
        unquote(block)
        Runner.end_command
      end.()
    end
  end
end

defmodule MyMigration do
  use Ecto.Migration

  def change do
    create table(:foo) do
      add :name, :string
    end
  end
end

MyMigration.change

Actually, "add" was available within the entire "change" function, so I fixed that with a fn -> … end.() to scope the import.