Now that you understand
of what goes into programming a major mode, you may decide you want
to customize an existing one. Luckily, in most cases, you
don't have to worry about changing any
mode's existing Lisp code to do this; you may not
even have to look at the code. All Emacs major modes have
"hooks" for letting you add your
own code to them. Appropriately, these are called
mode-hooks. Every built-in major mode in Emacs
has a mode hook called
mode-name is the name of the mode or the
function that invokes it. For example, C mode has c-mode-hook, shell mode has shell-mode-hook, etc.
What exactly is a hook? It is a variable whose value is some Lisp code to run when the mode is invoked. When you invoke a mode, you run a Lisp function that typically does many things (e.g., sets up key bindings for special commands, creates buffers and local variables, etc.); the last thing a mode-invoking function usually does is run the mode's hook if it exists. Thus, hooks are "positioned" to give you a chance to override anything the mode's code may have set up. For example, any key bindings you define override the mode's default bindings.
We saw earlier that Lisp code can be used as the value of a Lisp variable; this use comes in handy when you create hooks. Before we show you exactly how to create a hook, we need to introduce yet another Lisp primitive function: lambda. lambda is very much like defun in that it is used to define functions; ...