Chapter 22

Exercise: MacrosAndCodeEvaluation-1

defmodule My do
  defmacro myunless(condition, clauses) do
    do_clause = Keyword.get(clauses, :do, nil)
    else_clause = Keyword.get(clauses, :else, nil)

    quote do
      if unquote(condition) do
        unquote(else_clause)
      else
        unquote(do_clause)
      end
    end
  end
end

defmodule MyTest do
  require My

  My.myunless(1 == 2, do: IO.puts("1 != 2"), else: IO.puts("1 == 2"))
end

Exercise: MacrosAndCodeEvaluation-2

defmodule Times do
  defmacro times_n(n) do
    quote do
      def unquote(String.to_atom("times_#{n}"))(m) do
        unquote(n) * m
      end
    end
  end
end

defmodule Test do
  require Times
  Times.times_n(3)
  Times.times_n(4)
end

12 = Test.times_3(4)
20 = Test.times_4(5)

Exercise: MacrosAndCodeEvaluation-3