What, specifically, should be included in theme_name_setup()?

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Nothing. A theme_name_setup() function should not even exist.

  1. The name is too vague. What does setup mean? This is also the root of your question, because such a name tells us nothing about what the function does. Could be anything or nothing at all. Technically, themes don’t even need a functions.php. But they work, the setup happens by WordPress.

  2. It violates the principle of single responsibility. This term comes from object-oriented programming, but the idea applies to all good code, it is even the core of the UNIX philosophy: Do one thing and do it well.
    This leads us to the next problem.

  3. It impairs interoperability. Consider the following function:

    add_action( 'after_setup_theme', 'theme_name_setup' );

function theme_name_setup() {
    require 'widgets.php';
    require 'comment-enhancements.php';
    require 'javascript.php';
}

    What do I have to do to load only widgets.php and javascript.php in my child theme? I have to unhook the broad function and repeat parts of your code. And then I pray you will never rename, combine or split files in an update of your parent theme.

To express this in a more positive way:

  • Give each class and function a name that tells the reader what it does. If you cannot come up with a good, precise name, this code does probably too much.

  • You can use the same hook with multiple callbacks. This is the point of the action/filter API. Use it.

  • Try to let each function return an useful value, so you can test and debug it separately. load_theme_textdomain() for example returns TRUE when a file was found, FALSE otherwise. If you are using a separate function to load the translation, you can use that return value.

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