One of the big differences between Erlang and Elixir is indeed that Elixir allows you to write macros in the language itself, using quote / unquote constructs just like lisps.

This makes it easy to generate code on compile time, which is then executed in runtime without any performance penalty.

A large part of Elixir itself is actually implemented as macros. For instance, the "unless" construct:

  defmacro unless(condition, do: do_clause, else: else_clause) do
    quote do
      if(unquote(condition), do: unquote(else_clause), else: unquote(do_clause))
    end
  end

(code simplified for clarity)

Imho You shouldn't really write macros, they are there for libraries to make it easy to do the right thing with less boilerplate.