Example of using "guard clauses" in your own code with Elixir macros. Inspired by `plug :foo when action in [:create, :update]`. This implementation accepts any types of conditions, not just the subset allowed in actual guard clauses. But also see https://gist.github.com/henrik/68f136f9916165e0defb for an implementation with real guard clauses.

say_when.exs

defmodule Lab do
  defmacro say({:when, _, [message, condition]}) do
    {result, _} = Code.eval_quoted(condition)

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

defmodule Run do
  import Lab

  def run do
    say "won't happen" when 1 > 2
    say "hello" when 2 > 1
  end
end

Run.run

The Plug implementation can be found hereabouts: https://github.com/elixir-lang/plug/blob/8dc8b4052a3bf32f7a75b0be89283a074c038088/lib/plug/builder.ex#L247 It uses real guard clauses via macros (meaning e.g. you're limited to certain functions).