Hour 24: Installing Emacs Add-Ons: Hooks

Sams Teach Yourself Emacs in 24 Hours

Hour 24: Installing Emacs Add-Ons

Sections in this Hour:
Simple and Complex Add-Ons	Controlling Options	Summary
Installing an Add-On	Hooks	Q&A
Loading Techniques	Adapting Functions	Exercises
Filenames and Interpreters	A Sampling of Emacs Packages

Hooks

Hooks are often provided for add-on Lisp files. Hooks are aptly named--they are functions that an Emacs process can "hook" into in order for a user to change the default behavior. An example might make this clearer:


(add-hook 'text-mode-hook 'turn-on-auto-fill)

By default, Emacs doesn't automatically wrap lines in text- mode. Adding the preceding lines to your .emacs file causes the function turn-on-auto-fill to be automatically loaded along with text-mode. When this hook is active, lines are automatically wrapped as you type. To try out a hook without including it in your .emacs file, type the line (complete with parentheses) into the Scratch buffer and then type C-x C-e, which is the keybinding for Evaluate LastS-Expression. The hook is then active until you exit Emacs.

Hooks are also used when you want to load two modes simultaneously. For example, if you want outline-minor-mode to be loaded along with AucTeX when you are editing TeX files, the following line is needed in your .emacs file:


(add-hook 'LaTeX-mode-hook '(lambda () (outline-minor-mode1)))

Using the lambda statement is a method of creating a nameless function (see Hour 22, "Learning Lisp Basics"); in this case, it causes outline-minor-mode to be loaded along with AucTeX's LaTeX mode. You didn't need to use lambda in the earlier examples because turn-on-auto-fill is already a function, whereas outline-minor-mode is a mode. In order for outline-minor-mode to be used in an add-hook statement, it is necessary to wrap it in a function definition.

Hooks can also be used to choose between several available options. CC-mode is a major mode that is used for editing C and C++ source-code files. The mode can be set to use one of several different coding styles with the following statement:


(add-hook 'c-mode-hook 
          ' (lambda () 
              (c-set-style "gnu")))

Many modes have their own built-in hooks, which you normally don't want to disable. The add-hook statement leaves existing hooks active while adding an additional hook.

Binding Keys With Hooks

Hook statements are often used to change the keybindings of a mode, most of which have predefined keymaps. Everyone has different editing habits and needs, so the most convenient and comfortable keys differ from user to user. Using mode-hooks ensures that the new keybindings only take effect when the particular mode is in use. Following are some examples:


(add-hook 'notes-mode-hook 
  ' (lambda () 
    (define-key notes-mode-map [f5] 'notes-follow-prev-link) 
    (define-key notes-mode-map [f6] 'notes-follow-next-link)))

This example changes two multiple-key bindings to an adjacent pair of unused function keys. This speeds up two often-used commands in notes-mode. This mode is briefly described at the end of the hour:


  (setq mail-mode-hook 
    ' (lambda () 
     (define-key mail-mode-map "\C-cm" 'mail-send-and-exit) 
     (define-key mail-mode-map "\C-cc" 'mail-cc) 
     ('turn-on-auto-fill) 
     (setq fill-column 65)))

In this example, two keybindings have been changed to shorter key sequences, auto-fill has been turned on, and the fill column has been narrowed to 65. Mail-mode doesn't have a predefined hook, so the setq statement is used to create one. Any mail-mode hook statements that follow this one in the .emacs file can use the add-hook function rather than setq.

Some add-ons have special provisions for changing the keybindings. For example, Folding-mode includes a function called fold-default-keys-function, which simplifies the alteration of keybindings. A function such as this in your .emacs file changes just one of the keybindings:


defun my-fold-bind-keys () 
 "My favorite fold key settings" 
 ;;  - I want the default ones, but I want fold-whole-buffer and 
 ;;    fold-open-buffer to be close together, instead of the 
 ;;    default 
 ;;  - read them all. 
 ;; 
 (fold-default-keys) 
 ;;  - first disable old bind and define new one. 
 ;; 
 (define-key folding-mode-map "\C-c\C-w" nil) 
 (define-key folding-mode-map "\C-c\C-p" 'fold-whole-buffer) 
 )

This function first sets the keymap to the default with the (fold-default-keys) statement. Next, the default keybinding for fold-whole-buffer is disabled and a new binding (C-c C-p) is substituted.

The following line assigns the new my-fold-bind-keys function as the new default:


setq fold-default-keys-function 'my-fold-bind-keys)

Sams Teach Yourself Emacs in 24 Hours

Contents Index

Hour 24: Installing Emacs Add-Ons

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Sections in this Hour:
Simple and Complex Add-Ons	Controlling Options	Summary
Installing an Add-On	Hooks	Q&A
Loading Techniques	Adapting Functions	Exercises
Filenames and Interpreters	A Sampling of Emacs Packages