macros_answer.ex

defmodule Macros do
  defmacro or_default(expr, default) do
    quote do
      try do
        unquote(expr)
      rescue
        e ->
          unquote(default)
      end
    end
  end
end


defmodule MacrosTest do
  use ExUnit.Case

  test "macro expansion works properly" do
    require Macros
    assert 42 == Macros.or_default(100/0, 30 + 12)
    assert 42 == Macros.or_default(30 + 12, flunk("this should not happen"))
  end

end