Книга: Learning GNU Emacs, 3rd Edition

11.5.1 Components of a Major Mode

11.5.1 Components of a Major Mode

A major mode has various components that integrate it into Emacs. Some are:

• The symbol that is the name of the function that implements the mode

• The name of the mode that appears in the mode line in parentheses

• The local keymap that defines key bindings for commands in the mode

• Variables and constants known only within the Lisp code for the mode

• The special buffer the mode may use

Let's deal with these in order. The mode symbol is set by assigning the name of the function that implements the mode to the global variable major-mode, as in:

(setq major-mode 'calc-mode)

Similarly, the mode name is set by assigning an appropriate string to the global variable mode-name, as in:

(setq mode-name "Calculator")

The local keymap is defined using functions discussed in Chapter 10. In the case of the calculator mode, there is only one key sequence to bind (C-j), so we use a special form of the make-keymap command called make-sparse-keymap that is more efficient with a small number of key bindings. To use a keymap as the local map of a mode, we call the function use-local-map, as in:

(use-local-map calc-mode-map)

As we just saw, variables can be defined by using setq to assign a value to them, or by using let to define local variables within a function. The more "official" way to define variables is the defvar function, which allows documentation for the variable to be integrated into online help facilities such as C-h v (for describe-variable). The format is the following:

(defvar varname initial-value "description of the variable")

A variation on this is defconst, with which you can define constant values (that never change). For example:

(defconst calc-operator-regexp "[-+*/%]"
  "Regular expression for recognizing operators.")

defines the regular expression to be used in searching for arithmetic operators. As you will see, we use the calc- as a prefix for the names of all functions, variables, and constants that we define for the calculator mode. Other modes use this convention; for example, all names in C++ mode begin with c++-. Using this convention is a good idea because it helps avoid potential name clashes with the thousands of other functions, variables, and so on in Emacs.

Making variables local to the mode is also desirable so that they are known only within a buffer that is running the mode.[80] To do this, use the make-local-variable function, as in:

(make-local-variable 'calc-stack)

Notice that the name of the variable, not its value, is needed; therefore a single quote precedes the variable name, turning it into a symbol.

Finally, various major modes use special buffers that are not attached to files. For example, the C-x C-b (for list-buffers) command creates a buffer called *Buffer List*. To create a buffer in a new window, use the pop-to-buffer function, as in:

(pop-to-buffer "*Calc*")

There are a couple of useful variations on pop-to-buffer. We won't use them in our mode example, but they are handy in other circumstances.

switch-to-buffer

Same as the C-x b command covered in Chapter 4; can also be used with a buffer name argument in Lisp.

set-buffer

Used only within Lisp code to designate the buffer used for editing; the best function to use for creating a temporary "work" buffer within a Lisp function.

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