When I started writing macros that define named functions with def, I didn’t find any blog posts etc. that were devoted to the topic. Are there any? Should I write one? There are, I think, some special gotchas that beginners should be able to find all in one place.

I’m referring to macros like one I recently wrote:

  defmacro defaults_may_be_overridden(pairs) when is_list(pairs) do
    for pair <- pairs, do: Here.def_optional(pair)
  end

  def def_optional({name_atom, default}) do
    function_name = {name_atom, [], nil}
    quote do
      def unquote(function_name),
        do: get_env(unquote(name_atom), unquote(default))
    end
  end

That’s used to create a bunch of configuration getters functions while omitting unnecessary words:

  defaults_may_be_overridden(
    reload_timeout: 150,
    logging_enabled: true,
    reload_callback: constantly(:irrelevant_return_value),
    load_first: false,
    src_dirs: []
  )

There are issues like:

• defmacrop may surprise you because it doesn’t create macros available at function-definition scope.
• Converting between function name ASTs and atoms is something you have to learn.
• If you want to write a defXXX macro, you may want to handle guards, and the AST for guards was surprising to me.