Intermediate Emacs Hacking

Customizing Emacs is important to get the most out of it. You can change the way it operates to reflect your way of doing things, which in turn makes you more efficient. Just as custom-built boots fit better than factory boots, a customized Emacs fits you better than standard, off-the-shelf Emacs.

Emacs changes can be session-specific or permanent. You can customize Emacs directly by executing commands in the mini-buffer or by modifying variables using the set-variable command. These changes are volatile, meaning you lose them when you end your session. To make permanent changes, you can create or modify an init file. Emacs examines several init files when it loads. Probably the easiest way to customize is to edit .emacs, stored in your home directory. Before loading your .emacs file, Emacs loads default.el from your library path. Emacs also looks in its load path for site-start.el, which system administrators may use to provide site-wide customization. Alternatively, you can make permanent Emacs changes through the Customize menu (Options→Customize Emacs in version 21.1 and up). This gives you a GUI-based front end for modifying your .emacs file.

Like most GUI front ends, Customize is not as powerful as hacking the text configuration file, but it is easier to use. Fire up the Customization group, and you notice that Emacs builds the menus on the fly. That way it's always up to date (Figure 1).

Figure 1. The Top-Level Customization Menu

To navigate Customize's tree structure, either point to a button and press Return or click on the button. Full Emacs searching also works in the Customize buffers. Each level in the tree is represented in a buffer, and you can manipulate Customize buffers as usual. For example, when you are done with a level, kill the buffer.

I'd like to turn on the PHP speedbar whenever I enter PHP mode, for example, when I visit a PHP file. To do this, I follow the menu tree to PHP mode customization (Figure 2). As you can see, I've toggled the state but haven't yet saved my changes. I can set it only for this session, or I can save the change for future sessions. When I do the latter, Emacs edits my .emacs file. You can verify the change by searching for the variable name in .emacs.

Figure 2. Setting the Speedbar to Turn On in PHP Mode

If you aren't sure of the name of a variable or where to find it in Customize's vast tree, you can use regular expression searches on variable names and their contents. If you want to change how Emacs prints, you could search on the regex “print” and Emacs would build a custom Customize menu for you. See the menu under Options→Customize Emacs for these and other options.

You also can modify Emacs' behavior by editing ~/.emacs directly. This is a good way to add function and insert bulk customizations you may learn about from other people. For example, C mode can be modified by setting variables that affect how it operates.

Many modes are customized on a per-buffer basis. This means you write a short function that sets the variables and set that function to be executed whenever Emacs enters the mode. The outline of it is:

(defun rays_c_mode ()
  "ray's c/c++ mode hook"
  (message "Loading Ray's C mode...")
  ...
  (message "Loading Ray's C mode... Done")
  )
(add-hook 'c-mode-common-hook 'rays_c_mode)

The Lisp function defun defines a function, in this case rays_C_mode. The function takes no parameters; it prints out messages only for the user's benefit. The last line adds the function rays_c_mode to C mode's mode hook, that is, a list of functions executed whenever Emacs enters C mode. You can see more of Ray's C mode in my personal .emacs file (see Resources).

It is customary to name the variables for a particular mode by prepending the mode's name to them. To see variables associated with a particular mode, then, make a regex search on the variable name, with M-X apropos-variable. For C mode, we eliminate a lot of false hits with the regex “^c-”. When Emacs returns a list of the results, move to that buffer and press Return on any variable that interests you for more information.

To find out what else you can search on, try M-X apropos-command RET apropos RET. "apropos-zippy"? I'll let you examine that one.

A function can, of course, call other functions. This possibility is one of several things that make editing your .emacs file and programming in Emacs Lisp more powerful than using Customize.