chess.ex

defmodule M do
  defmacro common_fields(extra \\ []) do
    [color: nil, id: nil] ++ extra
  end
end

defmodule Pawn do
  import M
  defstruct common_fields
end

defmodule King do
  import M
  defstruct common_fields([moved: false])
end

defprotocol Move do
  def valid_moves(piece)
end

defimpl Move, for: Pawn do
  def valid_moves(_) do
    IO.puts "I am a pawn"
  end
end

defimpl Move, for: King do
  def valid_moves(_) do
    IO.puts "I am a king"
  end
end

defmodule T do
  def test do
    Move.valid_moves(%Pawn{})
    Move.valid_moves(%King{})
  end
end

T.test