Copyright (C) 2000-2012 |
GNU Info (elisp)Setting VariablesHow to Alter a Variable Value ============================= The usual way to change the value of a variable is with the special form `setq'. When you need to compute the choice of variable at run time, use the function `set'. - Special Form: setq [symbol form]... This special form is the most common method of changing a variable's value. Each SYMBOL is given a new value, which is the result of evaluating the corresponding FORM. The most-local existing binding of the symbol is changed. `setq' does not evaluate SYMBOL; it sets the symbol that you write. We say that this argument is "automatically quoted". The `q' in `setq' stands for "quoted." The value of the `setq' form is the value of the last FORM. (setq x (1+ 2)) => 3 x ; `x' now has a global value. => 3 (let ((x 5)) (setq x 6) ; The local binding of `x' is set. x) => 6 x ; The global value is unchanged. => 3 Note that the first FORM is evaluated, then the first SYMBOL is set, then the second FORM is evaluated, then the second SYMBOL is set, and so on: (setq x 10 ; Notice that `x' is set before y (1+ x)) ; the value of `y' is computed. => 11 - Function: set symbol value This function sets SYMBOL's value to VALUE, then returns VALUE. Since `set' is a function, the expression written for SYMBOL is evaluated to obtain the symbol to set. The most-local existing binding of the variable is the binding that is set; shadowed bindings are not affected. (set one 1) error--> Symbol's value as variable is void: one (set 'one 1) => 1 (set 'two 'one) => one (set two 2) ; `two' evaluates to symbol `one'. => 2 one ; So it is `one' that was set. => 2 (let ((one 1)) ; This binding of `one' is set, (set 'one 3) ; not the global value. one) => 3 one => 2 If SYMBOL is not actually a symbol, a `wrong-type-argument' error is signaled. (set '(x y) 'z) error--> Wrong type argument: symbolp, (x y) Logically speaking, `set' is a more fundamental primitive than `setq'. Any use of `setq' can be trivially rewritten to use `set'; `setq' could even be defined as a macro, given the availability of `set'. However, `set' itself is rarely used; beginners hardly need to know about it. It is useful only for choosing at run time which variable to set. For example, the command `set-variable', which reads a variable name from the user and then sets the variable, needs to use `set'. Common Lisp note: In Common Lisp, `set' always changes the symbol's "special" or dynamic value, ignoring any lexical bindings. In Emacs Lisp, all variables and all bindings are dynamic, so `set' always affects the most local existing binding. One other function for setting a variable is designed to add an element to a list if it is not already present in the list. - Function: add-to-list symbol element This function sets the variable SYMBOL by consing ELEMENT onto the old value, if ELEMENT is not already a member of that value. It returns the resulting list, whether updated or not. The value of SYMBOL had better be a list already before the call. The argument SYMBOL is not implicitly quoted; `add-to-list' is an ordinary function, like `set' and unlike `setq'. Quote the argument yourself if that is what you want. Here's a scenario showing how to use `add-to-list': (setq foo '(a b)) => (a b) (add-to-list 'foo 'c) ;; Add `c'. => (c a b) (add-to-list 'foo 'b) ;; No effect. => (c a b) foo ;; `foo' was changed. => (c a b) An equivalent expression for `(add-to-list 'VAR VALUE)' is this: (or (member VALUE VAR) (setq VAR (cons VALUE VAR))) |