Macros in guard clauses...

when_macro.ex

defmodule MyGuards do

  defmacro is_kid age do
    quote do: unquote(age) < 12
  end

  defmacro is_adult age do
    quote do: unquote(age) > 18
  end

end
# order of module matters .....
defmodule Hello do

  import MyGuards

  def hello(name,age) when is_kid(age) do
    IO.puts "Hello Kid #{name}"
  end

  def hello(name,age) when is_adult(age) do
    IO.puts "Hello Mister #{name}"
  end
  
  def hello(name,age) do
    IO.puts "Hello Youth #{name}"
  end

end