Run sgml-pretty-print when opening an xml file in emacs?

Question

I can currently use sgml-pretty-print to pretty print an xml file in emacs, but it's a manual process:

1. M-<
2. C-space
3. M->
4. M-x sgml-pretty-print

I'd like this to happen automatically (or at least have some option to do so). I'm new to emacs/elisp, and do not understand how:

1. emacs knows what code to run when you open a file (does this start in files.el?)
2. If you wanted to override that code with your own, how to do that

Answer 1

This should do the trick for you:

(add-hook 'find-file-hook 'my-sgml-find-file-hook)
(defun my-sgml-find-file-hook ()
  "run sgml pretty-print on the file when it's opened (if it's sgml)"
  (when (eq major-mode 'sgml-mode)
    (sgml-pretty-print (point-min) (point-max))))

The key pieces of information are the find-file-hook, point-min (-max), and major-mode.

If you want to learn more about elisp, you can take a look at this question, which gives some pointers on how to figure things out.

Answer 2

A slightly simpler alternative to Trey Jackson's answer. Just add this to your ~/.emacs file:

(add-hook 'sgml-mode-hook #'(lambda ()
  (sgml-pretty-print (point-min) (point-max))))