(elisp)Tips for Defining
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 (Note: 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 "good" arguments and `nil' for "bad" arguments. `...-flag' The value is significant only as to whether it is `nil' or not. `...-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 "risky"; see Note: 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. But be careful not to write the code like this: (defvar my-mode-map nil DOCSTRING) (unless my-mode-map (setq my-mode-map (make-sparse-keymap)) (define-key my-mode-map "\C-c\C-a" 'my-command) ...) This code sets the variable, then alters it, but it does so in more than one step. If the user quits just after the `setq', that leaves the variable neither correctly initialized nor void nor `nil'. Once that happens, reloading the file will not initialize the variable; it will remain incomplete.
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