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This is Info file ../../info/lispref.info, produced by Makeinfo-1.63 from the input file lispref.texi. Edition History: GNU Emacs Lisp Reference Manual Second Edition (v2.01), May 1993 GNU Emacs Lisp Reference Manual Further Revised (v2.02), August 1993 Lucid Emacs Lisp Reference Manual (for 19.10) First Edition, March 1994 XEmacs Lisp Programmer's Manual (for 19.12) Second Edition, April 1995 GNU Emacs Lisp Reference Manual v2.4, June 1995 XEmacs Lisp Programmer's Manual (for 19.13) Third Edition, July 1995 Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995 Free Software Foundation, Inc. Copyright (C) 1994, 1995 Sun Microsystems, Inc. Copyright (C) 1995 Amdahl Corporation. Copyright (C) 1995 Ben Wing. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Foundation. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the section entitled "GNU General Public License" is included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the section entitled "GNU General Public License" may be included in a translation approved by the Free Software Foundation instead of in the original English. File: lispref.info, Node: Handling Errors, Next: Error Symbols, Prev: Processing of Errors, Up: Errors Writing Code to Handle Errors ............................. The usual effect of signaling an error is to terminate the command that is running and return immediately to the XEmacs editor command loop. You can arrange to trap errors occurring in a part of your program by establishing an error handler, with the special form `condition-case'. A simple example looks like this: (condition-case nil (delete-file filename) (error nil)) This deletes the file named FILENAME, catching any error and returning `nil' if an error occurs. The second argument of `condition-case' is called the "protected form". (In the example above, the protected form is a call to `delete-file'.) The error handlers go into effect when this form begins execution and are deactivated when this form returns. They remain in effect for all the intervening time. In particular, they are in effect during the execution of functions called by this form, in their subroutines, and so on. This is a good thing, since, strictly speaking, errors can be signaled only by Lisp primitives (including `signal' and `error') called by the protected form, not by the protected form itself. The arguments after the protected form are handlers. Each handler lists one or more "condition names" (which are symbols) to specify which errors it will handle. The error symbol specified when an error is signaled also defines a list of condition names. A handler applies to an error if they have any condition names in common. In the example above, there is one handler, and it specifies one condition name, `error', which covers all errors. The search for an applicable handler checks all the established handlers starting with the most recently established one. Thus, if two nested `condition-case' forms offer to handle the same error, the inner of the two will actually handle it. When an error is handled, control returns to the handler. Before this happens, XEmacs unbinds all variable bindings made by binding constructs that are being exited and executes the cleanups of all `unwind-protect' forms that are exited. Once control arrives at the handler, the body of the handler is executed. After execution of the handler body, execution continues by returning from the `condition-case' form. Because the protected form is exited completely before execution of the handler, the handler cannot resume execution at the point of the error, nor can it examine variable bindings that were made within the protected form. All it can do is clean up and proceed. `condition-case' is often used to trap errors that are predictable, such as failure to open a file in a call to `insert-file-contents'. It is also used to trap errors that are totally unpredictable, such as when the program evaluates an expression read from the user. Error signaling and handling have some resemblance to `throw' and `catch', but they are entirely separate facilities. An error cannot be caught by a `catch', and a `throw' cannot be handled by an error handler (though using `throw' when there is no suitable `catch' signals an error that can be handled). - Special Form: condition-case VAR PROTECTED-FORM HANDLERS... This special form establishes the error handlers HANDLERS around the execution of PROTECTED-FORM. If PROTECTED-FORM executes without error, the value it returns becomes the value of the `condition-case' form; in this case, the `condition-case' has no effect. The `condition-case' form makes a difference when an error occurs during PROTECTED-FORM. Each of the HANDLERS is a list of the form `(CONDITIONS BODY...)'. Here CONDITIONS is an error condition name to be handled, or a list of condition names; BODY is one or more Lisp expressions to be executed when this handler handles an error. Here are examples of handlers: (error nil) (arith-error (message "Division by zero")) ((arith-error file-error) (message "Either division by zero or failure to open a file")) Each error that occurs has an "error symbol" that describes what kind of error it is. The `error-conditions' property of this symbol is a list of condition names (*note Error Symbols::.). Emacs searches all the active `condition-case' forms for a handler that specifies one or more of these condition names; the innermost matching `condition-case' handles the error. Within this `condition-case', the first applicable handler handles the error. After executing the body of the handler, the `condition-case' returns normally, using the value of the last form in the handler body as the overall value. The argument VAR is a variable. `condition-case' does not bind this variable when executing the PROTECTED-FORM, only when it handles an error. At that time, it binds VAR locally to a list of the form `(ERROR-SYMBOL . DATA)', giving the particulars of the error. The handler can refer to this list to decide what to do. For example, if the error is for failure opening a file, the file name is the second element of DATA--the third element of VAR. If VAR is `nil', that means no variable is bound. Then the error symbol and associated data are not available to the handler. Here is an example of using `condition-case' to handle the error that results from dividing by zero. The handler prints out a warning message and returns a very large number. (defun safe-divide (dividend divisor) (condition-case err ;; Protected form. (/ dividend divisor) ;; The handler. (arith-error ; Condition. (princ (format "Arithmetic error: %s" err)) 1000000))) => safe-divide (safe-divide 5 0) -| Arithmetic error: (arith-error) => 1000000 The handler specifies condition name `arith-error' so that it will handle only division-by-zero errors. Other kinds of errors will not be handled, at least not by this `condition-case'. Thus, (safe-divide nil 3) error--> Wrong type argument: integer-or-marker-p, nil Here is a `condition-case' that catches all kinds of errors, including those signaled with `error': (setq baz 34) => 34 (condition-case err (if (eq baz 35) t ;; This is a call to the function `error'. (error "Rats! The variable %s was %s, not 35" 'baz baz)) ;; This is the handler; it is not a form. (error (princ (format "The error was: %s" err)) 2)) -| The error was: (error "Rats! The variable baz was 34, not 35") => 2 File: lispref.info, Node: Error Symbols, Prev: Handling Errors, Up: Errors Error Symbols and Condition Names ................................. When you signal an error, you specify an "error symbol" to specify the kind of error you have in mind. Each error has one and only one error symbol to categorize it. This is the finest classification of errors defined by the Emacs Lisp language. These narrow classifications are grouped into a hierarchy of wider classes called "error conditions", identified by "condition names". The narrowest such classes belong to the error symbols themselves: each error symbol is also a condition name. There are also condition names for more extensive classes, up to the condition name `error' which takes in all kinds of errors. Thus, each error has one or more condition names: `error', the error symbol if that is distinct from `error', and perhaps some intermediate classifications. In order for a symbol to be an error symbol, it must have an `error-conditions' property which gives a list of condition names. This list defines the conditions that this kind of error belongs to. (The error symbol itself, and the symbol `error', should always be members of this list.) Thus, the hierarchy of condition names is defined by the `error-conditions' properties of the error symbols. In addition to the `error-conditions' list, the error symbol should have an `error-message' property whose value is a string to be printed when that error is signaled but not handled. If the `error-message' property exists, but is not a string, the error message `peculiar error' is used. Here is how we define a new error symbol, `new-error': (put 'new-error 'error-conditions '(error my-own-errors new-error)) => (error my-own-errors new-error) (put 'new-error 'error-message "A new error") => "A new error" This error has three condition names: `new-error', the narrowest classification; `my-own-errors', which we imagine is a wider classification; and `error', which is the widest of all. The error string should start with a capital letter but it should not end with a period. This is for consistency with the rest of Emacs. Naturally, XEmacs will never signal `new-error' on its own; only an explicit call to `signal' (*note Signaling Errors::.) in your code can do this: (signal 'new-error '(x y)) error--> A new error: x, y This error can be handled through any of the three condition names. This example handles `new-error' and any other errors in the class `my-own-errors': (condition-case foo (bar nil t) (my-own-errors nil)) The significant way that errors are classified is by their condition names--the names used to match errors with handlers. An error symbol serves only as a convenient way to specify the intended error message and list of condition names. It would be cumbersome to give `signal' a list of condition names rather than one error symbol. By contrast, using only error symbols without condition names would seriously decrease the power of `condition-case'. Condition names make it possible to categorize errors at various levels of generality when you write an error handler. Using error symbols alone would eliminate all but the narrowest level of classification. *Note Standard Errors::, for a list of all the standard error symbols and their conditions. File: lispref.info, Node: Cleanups, Prev: Errors, Up: Nonlocal Exits Cleaning Up from Nonlocal Exits ------------------------------- The `unwind-protect' construct is essential whenever you temporarily put a data structure in an inconsistent state; it permits you to ensure the data are consistent in the event of an error or throw. - Special Form: unwind-protect BODY CLEANUP-FORMS... `unwind-protect' executes the BODY with a guarantee that the CLEANUP-FORMS will be evaluated if control leaves BODY, no matter how that happens. The BODY may complete normally, or execute a `throw' out of the `unwind-protect', or cause an error; in all cases, the CLEANUP-FORMS will be evaluated. If the BODY forms finish normally, `unwind-protect' returns the value of the last BODY form, after it evaluates the CLEANUP-FORMS. If the BODY forms do not finish, `unwind-protect' does not return any value in the normal sense. Only the BODY is actually protected by the `unwind-protect'. If any of the CLEANUP-FORMS themselves exits nonlocally (e.g., via a `throw' or an error), `unwind-protect' is *not* guaranteed to evaluate the rest of them. If the failure of one of the CLEANUP-FORMS has the potential to cause trouble, then protect it with another `unwind-protect' around that form. The number of currently active `unwind-protect' forms counts, together with the number of local variable bindings, against the limit `max-specpdl-size' (*note Local Variables::.). For example, here we make an invisible buffer for temporary use, and make sure to kill it before finishing: (save-excursion (let ((buffer (get-buffer-create " *temp*"))) (set-buffer buffer) (unwind-protect BODY (kill-buffer buffer)))) You might think that we could just as well write `(kill-buffer (current-buffer))' and dispense with the variable `buffer'. However, the way shown above is safer, if BODY happens to get an error after switching to a different buffer! (Alternatively, you could write another `save-excursion' around the body, to ensure that the temporary buffer becomes current in time to kill it.) Here is an actual example taken from the file `ftp.el'. It creates a process (*note Processes::.) to try to establish a connection to a remote machine. As the function `ftp-login' is highly susceptible to numerous problems that the writer of the function cannot anticipate, it is protected with a form that guarantees deletion of the process in the event of failure. Otherwise, XEmacs might fill up with useless subprocesses. (let ((win nil)) (unwind-protect (progn (setq process (ftp-setup-buffer host file)) (if (setq win (ftp-login process host user password)) (message "Logged in") (error "Ftp login failed"))) (or win (and process (delete-process process))))) This example actually has a small bug: if the user types `C-g' to quit, and the quit happens immediately after the function `ftp-setup-buffer' returns but before the variable `process' is set, the process will not be killed. There is no easy way to fix this bug, but at least it is very unlikely. Here is another example which uses `unwind-protect' to make sure to kill a temporary buffer. In this example, the value returned by `unwind-protect' is used. (defun shell-command-string (cmd) "Return the output of the shell command CMD, as a string." (save-excursion (set-buffer (generate-new-buffer " OS*cmd")) (shell-command cmd t) (unwind-protect (buffer-string) (kill-buffer (current-buffer))))) File: lispref.info, Node: Variables, Next: Functions, Prev: Control Structures, Up: Top Variables ********* A "variable" is a name used in a program to stand for a value. Nearly all programming languages have variables of some sort. In the text of a Lisp program, variables are written using the syntax for symbols. In Lisp, unlike most programming languages, programs are represented primarily as Lisp objects and only secondarily as text. The Lisp objects used for variables are symbols: the symbol name is the variable name, and the variable's value is stored in the value cell of the symbol. The use of a symbol as a variable is independent of its use as a function name. *Note Symbol Components::. The Lisp objects that constitute a Lisp program determine the textual form of the program--it is simply the read syntax for those Lisp objects. This is why, for example, a variable in a textual Lisp program is written using the read syntax for the symbol that represents the variable. * Menu: * Global Variables:: Variable values that exist permanently, everywhere. * Constant Variables:: Certain "variables" have values that never change. * Local Variables:: Variable values that exist only temporarily. * Void Variables:: Symbols that lack values. * Defining Variables:: A definition says a symbol is used as a variable. * Accessing Variables:: Examining values of variables whose names are known only at run time. * Setting Variables:: Storing new values in variables. * Variable Scoping:: How Lisp chooses among local and global values. * Buffer-Local Variables:: Variable values in effect only in one buffer. File: lispref.info, Node: Global Variables, Next: Constant Variables, Up: Variables Global Variables ================ The simplest way to use a variable is "globally". This means that the variable has just one value at a time, and this value is in effect (at least for the moment) throughout the Lisp system. The value remains in effect until you specify a new one. When a new value replaces the old one, no trace of the old value remains in the variable. You specify a value for a symbol with `setq'. For example, (setq x '(a b)) gives the variable `x' the value `(a b)'. Note that `setq' does not evaluate its first argument, the name of the variable, but it does evaluate the second argument, the new value. Once the variable has a value, you can refer to it by using the symbol by itself as an expression. Thus, x => (a b) assuming the `setq' form shown above has already been executed. If you do another `setq', the new value replaces the old one: x => (a b) (setq x 4) => 4 x => 4 File: lispref.info, Node: Constant Variables, Next: Local Variables, Prev: Global Variables, Up: Variables Variables That Never Change =========================== Emacs Lisp has two special symbols, `nil' and `t', that always evaluate to themselves. These symbols cannot be rebound, nor can their value cells be changed. An attempt to change the value of `nil' or `t' signals a `setting-constant' error. nil == 'nil => nil (setq nil 500) error--> Attempt to set constant symbol: nil File: lispref.info, Node: Local Variables, Next: Void Variables, Prev: Constant Variables, Up: Variables Local Variables =============== Global variables have values that last until explicitly superseded with new values. Sometimes it is useful to create variable values that exist temporarily--only while within a certain part of the program. These values are called "local", and the variables so used are called "local variables". For example, when a function is called, its argument variables receive new local values that last until the function exits. The `let' special form explicitly establishes new local values for specified variables; these last until exit from the `let' form. Establishing a local value saves away the previous value (or lack of one) of the variable. When the life span of the local value is over, the previous value is restored. In the mean time, we say that the previous value is "shadowed" and "not visible". Both global and local values may be shadowed (*note Scope::.). If you set a variable (such as with `setq') while it is local, this replaces the local value; it does not alter the global value, or previous local values that are shadowed. To model this behavior, we speak of a "local binding" of the variable as well as a local value. The local binding is a conceptual place that holds a local value. Entry to a function, or a special form such as `let', creates the local binding; exit from the function or from the `let' removes the local binding. As long as the local binding lasts, the variable's value is stored within it. Use of `setq' or `set' while there is a local binding stores a different value into the local binding; it does not create a new binding. We also speak of the "global binding", which is where (conceptually) the global value is kept. A variable can have more than one local binding at a time (for example, if there are nested `let' forms that bind it). In such a case, the most recently created local binding that still exists is the "current binding" of the variable. (This is called "dynamic scoping"; see *Note Variable Scoping::.) If there are no local bindings, the variable's global binding is its current binding. We also call the current binding the "most-local existing binding", for emphasis. Ordinary evaluation of a symbol always returns the value of its current binding. The special forms `let' and `let*' exist to create local bindings. - Special Form: let (BINDINGS...) FORMS... This special form binds variables according to BINDINGS and then evaluates all of the FORMS in textual order. The `let'-form returns the value of the last form in FORMS. Each of the BINDINGS is either (i) a symbol, in which case that symbol is bound to `nil'; or (ii) a list of the form `(SYMBOL VALUE-FORM)', in which case SYMBOL is bound to the result of evaluating VALUE-FORM. If VALUE-FORM is omitted, `nil' is used. All of the VALUE-FORMs in BINDINGS are evaluated in the order they appear and *before* any of the symbols are bound. Here is an example of this: `Z' is bound to the old value of `Y', which is 2, not the new value, 1. (setq Y 2) => 2 (let ((Y 1) (Z Y)) (list Y Z)) => (1 2) - Special Form: let* (BINDINGS...) FORMS... This special form is like `let', but it binds each variable right after computing its local value, before computing the local value for the next variable. Therefore, an expression in BINDINGS can reasonably refer to the preceding symbols bound in this `let*' form. Compare the following example with the example above for `let'. (setq Y 2) => 2 (let* ((Y 1) (Z Y)) ; Use the just-established value of `Y'. (list Y Z)) => (1 1) Here is a complete list of the other facilities that create local bindings: * Function calls (*note Functions::.). * Macro calls (*note Macros::.). * `condition-case' (*note Errors::.). Variables can also have buffer-local bindings (*note Buffer-Local Variables::.). These kinds of bindings work somewhat like ordinary local bindings, but they are localized depending on "where" you are in Emacs, rather than localized in time. - Variable: max-specpdl-size This variable defines the limit on the total number of local variable bindings and `unwind-protect' cleanups (*note Nonlocal Exits::.) that are allowed before signaling an error (with data `"Variable binding depth exceeds max-specpdl-size"'). This limit, with the associated error when it is exceeded, is one way that Lisp avoids infinite recursion on an ill-defined function. The default value is 600. `max-lisp-eval-depth' provides another limit on depth of nesting. *Note Eval::. File: lispref.info, Node: Void Variables, Next: Defining Variables, Prev: Local Variables, Up: Variables When a Variable is "Void" ========================= If you have never given a symbol any value as a global variable, we say that that symbol's global value is "void". In other words, the symbol's value cell does not have any Lisp object in it. If you try to evaluate the symbol, you get a `void-variable' error rather than a value. Note that a value of `nil' is not the same as void. The symbol `nil' is a Lisp object and can be the value of a variable just as any other object can be; but it is *a value*. A void variable does not have any value. After you have given a variable a value, you can make it void once more using `makunbound'. - Function: makunbound SYMBOL This function makes the current binding of SYMBOL void. Subsequent attempts to use this symbol's value as a variable will signal the error `void-variable', unless or until you set it again. `makunbound' returns SYMBOL. (makunbound 'x) ; Make the global value ; of `x' void. => x x error--> Symbol's value as variable is void: x If SYMBOL is locally bound, `makunbound' affects the most local existing binding. This is the only way a symbol can have a void local binding, since all the constructs that create local bindings create them with values. In this case, the voidness lasts at most as long as the binding does; when the binding is removed due to exit from the construct that made it, the previous or global binding is reexposed as usual, and the variable is no longer void unless the newly reexposed binding was void all along. (setq x 1) ; Put a value in the global binding. => 1 (let ((x 2)) ; Locally bind it. (makunbound 'x) ; Void the local binding. x) error--> Symbol's value as variable is void: x x ; The global binding is unchanged. => 1 (let ((x 2)) ; Locally bind it. (let ((x 3)) ; And again. (makunbound 'x) ; Void the innermost-local binding. x)) ; And refer: it's void. error--> Symbol's value as variable is void: x (let ((x 2)) (let ((x 3)) (makunbound 'x)) ; Void inner binding, then remove it. x) ; Now outer `let' binding is visible. => 2 A variable that has been made void with `makunbound' is indistinguishable from one that has never received a value and has always been void. You can use the function `boundp' to test whether a variable is currently void. - Function: boundp VARIABLE `boundp' returns `t' if VARIABLE (a symbol) is not void; more precisely, if its current binding is not void. It returns `nil' otherwise. (boundp 'abracadabra) ; Starts out void. => nil (let ((abracadabra 5)) ; Locally bind it. (boundp 'abracadabra)) => t (boundp 'abracadabra) ; Still globally void. => nil (setq abracadabra 5) ; Make it globally nonvoid. => 5 (boundp 'abracadabra) => t File: lispref.info, Node: Defining Variables, Next: Accessing Variables, Prev: Void Variables, Up: Variables Defining Global Variables ========================= You may announce your intention to use a symbol as a global variable with a "variable definition": a special form, either `defconst' or `defvar'. In Emacs Lisp, definitions serve three purposes. First, they inform people who read the code that certain symbols are *intended* to be used a certain way (as variables). Second, they inform the Lisp system of these things, supplying a value and documentation. Third, they provide information to utilities such as `etags' and `make-docfile', which create data bases of the functions and variables in a program. The difference between `defconst' and `defvar' is primarily a matter of intent, serving to inform human readers of whether programs will change the variable. Emacs Lisp does not restrict the ways in which a variable can be used based on `defconst' or `defvar' declarations. However, it does make a difference for initialization: `defconst' unconditionally initializes the variable, while `defvar' initializes it only if it is void. One would expect user option variables to be defined with `defconst', since programs do not change them. Unfortunately, this has bad results if the definition is in a library that is not preloaded: `defconst' would override any prior value when the library is loaded. Users would like to be able to set user options in their init files, and override the default values given in the definitions. For this reason, user options must be defined with `defvar'. - Special Form: defvar SYMBOL [VALUE [DOC-STRING]] This special form defines SYMBOL as a value and initializes it. The definition informs a person reading your code that SYMBOL is used as a variable that programs are likely to set or change. It is also used for all user option variables except in the preloaded parts of XEmacs. Note that SYMBOL is not evaluated; the symbol to be defined must appear explicitly in the `defvar'. If SYMBOL already has a value (i.e., it is not void), VALUE is not even evaluated, and SYMBOL's value remains unchanged. If SYMBOL is void and VALUE is specified, `defvar' evaluates it and sets SYMBOL to the result. (If VALUE is omitted, the value of SYMBOL is not changed in any case.) When you evaluate a top-level `defvar' form with `C-M-x' in Emacs Lisp mode (`eval-defun'), a special feature of `eval-defun' evaluates it as a `defconst'. The purpose of this is to make sure the variable's value is reinitialized, when you ask for it specifically. If SYMBOL has a buffer-local binding in the current buffer, `defvar' sets the default value, not the local value. *Note Buffer-Local Variables::. If the DOC-STRING argument appears, it specifies the documentation for the variable. (This opportunity to specify documentation is one of the main benefits of defining the variable.) The documentation is stored in the symbol's `variable-documentation' property. The XEmacs help functions (*note Documentation::.) look for this property. If the first character of DOC-STRING is `*', it means that this variable is considered a user option. This lets users set the variable conventiently using the commands `set-variable' and `edit-options'. For example, this form defines `foo' but does not set its value: (defvar foo) => foo The following example sets the value of `bar' to `23', and gives it a documentation string: (defvar bar 23 "The normal weight of a bar.") => bar The following form changes the documentation string for `bar', making it a user option, but does not change the value, since `bar' already has a value. (The addition `(1+ 23)' is not even performed.) (defvar bar (1+ 23) "*The normal weight of a bar.") => bar bar => 23 Here is an equivalent expression for the `defvar' special form: (defvar SYMBOL VALUE DOC-STRING) == (progn (if (not (boundp 'SYMBOL)) (setq SYMBOL VALUE)) (put 'SYMBOL 'variable-documentation 'DOC-STRING) 'SYMBOL) The `defvar' form returns SYMBOL, but it is normally used at top level in a file where its value does not matter. - Special Form: defconst SYMBOL [VALUE [DOC-STRING]] This special form defines SYMBOL as a value and initializes it. It informs a person reading your code that SYMBOL has a global value, established here, that will not normally be changed or locally bound by the execution of the program. The user, however, may be welcome to change it. Note that SYMBOL is not evaluated; the symbol to be defined must appear explicitly in the `defconst'. `defconst' always evaluates VALUE and sets the global value of SYMBOL to the result, provided VALUE is given. If SYMBOL has a buffer-local binding in the current buffer, `defconst' sets the default value, not the local value. *Please note:* Don't use `defconst' for user option variables in libraries that are not standardly preloaded. The user should be able to specify a value for such a variable in the `.emacs' file, so that it will be in effect if and when the library is loaded later. Here, `pi' is a constant that presumably ought not to be changed by anyone (attempts by the Indiana State Legislature notwithstanding). As the second form illustrates, however, this is only advisory. (defconst pi 3.1415 "Pi to five places.") => pi (setq pi 3) => pi pi => 3 - Function: user-variable-p VARIABLE This function returns `t' if VARIABLE is a user option--a variable intended to be set by the user for customization--and `nil' otherwise. (Variables other than user options exist for the internal purposes of Lisp programs, and users need not know about them.) User option variables are distinguished from other variables by the first character of the `variable-documentation' property. If the property exists and is a string, and its first character is `*', then the variable is a user option. If a user option variable has a `variable-interactive' property, the `set-variable' command uses that value to control reading the new value for the variable. The property's value is used as if it were the argument to `interactive'. *Warning:* If the `defconst' and `defvar' special forms are used while the variable has a local binding, they set the local binding's value; the global binding is not changed. This is not what we really want. To prevent it, use these special forms at top level in a file, where normally no local binding is in effect, and make sure to load the file before making a local binding for the variable. File: lispref.info, Node: Accessing Variables, Next: Setting Variables, Prev: Defining Variables, Up: Variables Accessing Variable Values ========================= The usual way to reference a variable is to write the symbol which names it (*note Symbol Forms::.). This requires you to specify the variable name when you write the program. Usually that is exactly what you want to do. Occasionally you need to choose at run time which variable to reference; then you can use `symbol-value'. - Function: symbol-value SYMBOL This function returns the value of SYMBOL. This is the value in the innermost local binding of the symbol, or its global value if it has no local bindings. (setq abracadabra 5) => 5 (setq foo 9) => 9 ;; Here the symbol `abracadabra' ;; is the symbol whose value is examined. (let ((abracadabra 'foo)) (symbol-value 'abracadabra)) => foo ;; Here the value of `abracadabra', ;; which is `foo', ;; is the symbol whose value is examined. (let ((abracadabra 'foo)) (symbol-value abracadabra)) => 9 (symbol-value 'abracadabra) => 5 A `void-variable' error is signaled if SYMBOL has neither a local binding nor a global value. File: lispref.info, Node: Setting Variables, Next: Variable Scoping, Prev: Accessing Variables, Up: Variables How 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 value, ignoring any lexical bindings. In Emacs Lisp, all variables and all bindings are (in effect) special, 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))) File: lispref.info, Node: Variable Scoping, Next: Buffer-Local Variables, Prev: Setting Variables, Up: Variables Scoping Rules for Variable Bindings =================================== A given symbol `foo' may have several local variable bindings, established at different places in the Lisp program, as well as a global binding. The most recently established binding takes precedence over the others. Local bindings in Emacs Lisp have "indefinite scope" and "dynamic extent". "Scope" refers to *where* textually in the source code the binding can be accessed. Indefinite scope means that any part of the program can potentially access the variable binding. "Extent" refers to *when*, as the program is executing, the binding exists. Dynamic extent means that the binding lasts as long as the activation of the construct that established it. The combination of dynamic extent and indefinite scope is called "dynamic scoping". By contrast, most programming languages use "lexical scoping", in which references to a local variable must be located textually within the function or block that binds the variable. Common Lisp note: Variables declared "special" in Common Lisp are dynamically scoped, like variables in Emacs Lisp. * Menu: * Scope:: Scope means where in the program a value is visible. Comparison with other languages. * Extent:: Extent means how long in time a value exists. * Impl of Scope:: Two ways to implement dynamic scoping. * Using Scoping:: How to use dynamic scoping carefully and avoid problems. File: lispref.info, Node: Scope, Next: Extent, Up: Variable Scoping Scope ----- Emacs Lisp uses "indefinite scope" for local variable bindings. This means that any function anywhere in the program text might access a given binding of a variable. Consider the following function definitions: (defun binder (x) ; `x' is bound in `binder'. (foo 5)) ; `foo' is some other function. (defun user () ; `x' is used in `user'. (list x)) In a lexically scoped language, the binding of `x' in `binder' would never be accessible in `user', because `user' is not textually contained within the function `binder'. However, in dynamically scoped Emacs Lisp, `user' may or may not refer to the binding of `x' established in `binder', depending on circumstances: * If we call `user' directly without calling `binder' at all, then whatever binding of `x' is found, it cannot come from `binder'. * If we define `foo' as follows and call `binder', then the binding made in `binder' will be seen in `user': (defun foo (lose) (user)) * If we define `foo' as follows and call `binder', then the binding made in `binder' *will not* be seen in `user': (defun foo (x) (user)) Here, when `foo' is called by `binder', it binds `x'. (The binding in `foo' is said to "shadow" the one made in `binder'.) Therefore, `user' will access the `x' bound by `foo' instead of the one bound by `binder'. File: lispref.info, Node: Extent, Next: Impl of Scope, Prev: Scope, Up: Variable Scoping Extent ------ "Extent" refers to the time during program execution that a variable name is valid. In Emacs Lisp, a variable is valid only while the form that bound it is executing. This is called "dynamic extent". "Local" or "automatic" variables in most languages, including C and Pascal, have dynamic extent. One alternative to dynamic extent is "indefinite extent". This means that a variable binding can live on past the exit from the form that made the binding. Common Lisp and Scheme, for example, support this, but Emacs Lisp does not. To illustrate this, the function below, `make-add', returns a function that purports to add N to its own argument M. This would work in Common Lisp, but it does not work as intended in Emacs Lisp, because after the call to `make-add' exits, the variable `n' is no longer bound to the actual argument 2. (defun make-add (n) (function (lambda (m) (+ n m)))) ; Return a function. => make-add (fset 'add2 (make-add 2)) ; Define function `add2' ; with `(make-add 2)'. => (lambda (m) (+ n m)) (add2 4) ; Try to add 2 to 4. error--> Symbol's value as variable is void: n Some Lisp dialects have "closures", objects that are like functions but record additional variable bindings. Emacs Lisp does not have closures. File: lispref.info, Node: Impl of Scope, Next: Using Scoping, Prev: Extent, Up: Variable Scoping Implementation of Dynamic Scoping --------------------------------- A simple sample implementation (which is not how Emacs Lisp actually works) may help you understand dynamic binding. This technique is called "deep binding" and was used in early Lisp systems. Suppose there is a stack of bindings: variable-value pairs. At entry to a function or to a `let' form, we can push bindings on the stack for the arguments or local variables created there. We can pop those bindings from the stack at exit from the binding construct. We can find the value of a variable by searching the stack from top to bottom for a binding for that variable; the value from that binding is the value of the variable. To set the variable, we search for the current binding, then store the new value into that binding. As you can see, a function's bindings remain in effect as long as it continues execution, even during its calls to other functions. That is why we say the extent of the binding is dynamic. And any other function can refer to the bindings, if it uses the same variables while the bindings are in effect. That is why we say the scope is indefinite. The actual implementation of variable scoping in Emacs Lisp uses a technique called "shallow binding". Each variable has a standard place in which its current value is always found--the value cell of the symbol. In shallow binding, setting the variable works by storing a value in the value cell. Creating a new binding works by pushing the old value (belonging to a previous binding) on a stack, and storing the local value in the value cell. Eliminating a binding works by popping the old value off the stack, into the value cell. We use shallow binding because it has the same results as deep binding, but runs faster, since there is never a need to search for a binding. File: lispref.info, Node: Using Scoping, Prev: Impl of Scope, Up: Variable Scoping Proper Use of Dynamic Scoping ----------------------------- Binding a variable in one function and using it in another is a powerful technique, but if used without restraint, it can make programs hard to understand. There are two clean ways to use this technique: * Use or bind the variable only in a few related functions, written close together in one file. Such a variable is used for communication within one program. You should write comments to inform other programmers that they can see all uses of the variable before them, and to advise them not to add uses elsewhere. * Give the variable a well-defined, documented meaning, and make all appropriate functions refer to it (but not bind it or set it) wherever that meaning is relevant. For example, the variable `case-fold-search' is defined as "non-`nil' means ignore case when searching"; various search and replace functions refer to it directly or through their subroutines, but do not bind or set it. Then you can bind the variable in other programs, knowing reliably what the effect will be. In either case, you should define the variable with `defvar'. This helps other people understand your program by telling them to look for inter-function usage. It also avoids a warning from the byte compiler. Choose the variable's name to avoid name conflicts--don't use short names like `x'. File: lispref.info, Node: Buffer-Local Variables, Prev: Variable Scoping, Up: Variables Buffer-Local Variables ====================== Global and local variable bindings are found in most programming languages in one form or another. XEmacs also supports another, unusual kind of variable binding: "buffer-local" bindings, which apply only to one buffer. Emacs Lisp is meant for programming editing commands, and having different values for a variable in different buffers is an important customization method. * Menu: * Intro to Buffer-Local:: Introduction and concepts. * Creating Buffer-Local:: Creating and destroying buffer-local bindings. * Default Value:: The default value is seen in buffers that don't have their own local values.