Example of using real guard clauses in your own code with Elixir macros. Inspired by `plug :foo when action in [:create, :update]`. Also see https://gist.github.com/henrik/d21251bcf5d569e16ca9 for "fake" guard clauses that allow any condition/function to be used.

say_when_real_guards.exs

defmodule Lab do
  defmacro say({:when, _, [message, condition]}) do
    {result, _} = Code.eval_quoted(quote do
      case true do
        true when unquote(condition) -> true
        true -> false
      end
    end)

    if result do
      quote do: IO.puts unquote(message)
    end
  end
end

defmodule Run do
  import Lab

  def run do
    say "won't happen" when 1 > 2
    say "hello" when 2 > 1
    #say "not valid as a guard clause" when String.include?("hej", "ho")
  end
end

Run.run