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11.6 Tips for Defining Variables Robustly

When you define a variable whose value is a function, or a list of functions, use a name that ends in ‘-function’ or ‘-functions’, respectively.

There are several other variable name conventions; here is a complete list:

...-hook
The variable is a normal hook (see Hooks).
...-function
The value is a function.
...-functions
The value is a list of functions.
...-form
The value is a form (an expression).
...-forms
The value is a list of forms (expressions).
...-predicate
The value is a predicate—a function of one argument that returns non-nil for success and nil for failure.
...-flag
The value is significant only as to whether it is nil or not. Since such variables often end up acquiring more values over time, this convention is not strongly recommended.
...-program
The value is a program name.
...-command
The value is a whole shell command.
...-switches
The value specifies options for a command.

When you define a variable, always consider whether you should mark it as safe or risky; see File Local Variables.

When defining and initializing a variable that holds a complicated value (such as a keymap with bindings in it), it's best to put the entire computation of the value into the defvar, like this:

     (defvar my-mode-map
       (let ((map (make-sparse-keymap)))
         (define-key map "\C-c\C-a" 'my-command)
         ...
         map)
       docstring)

This method has several benefits. First, if the user quits while loading the file, the variable is either still uninitialized or initialized properly, never in-between. If it is still uninitialized, reloading the file will initialize it properly. Second, reloading the file once the variable is initialized will not alter it; that is important if the user has run hooks to alter part of the contents (such as, to rebind keys). Third, evaluating the defvar form with C-M-x will reinitialize the map completely.

Putting so much code in the defvar form has one disadvantage: it puts the documentation string far away from the line which names the variable. Here's a safe way to avoid that:

     (defvar my-mode-map nil
       docstring)
     (unless my-mode-map
       (let ((map (make-sparse-keymap)))
         (define-key map "\C-c\C-a" 'my-command)
         ...
         (setq my-mode-map map)))

This has all the same advantages as putting the initialization inside the defvar, except that you must type C-M-x twice, once on each form, if you do want to reinitialize the variable.