This is Info file elisp, produced by Makeinfo version 1.68 from the input file elisp.texi. INFO-DIR-SECTION Editors START-INFO-DIR-ENTRY * Elisp: (elisp). The Emacs Lisp Reference Manual. END-INFO-DIR-ENTRY This version is the edition 2.5 of the GNU Emacs Lisp Reference Manual. It corresponds to Emacs Version 20.3 Published by the Free Software Foundation 59 Temple Place, Suite 330 Boston, MA 02111-1307 USA Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1998 Free Software Foundation, Inc. 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: elisp, Node: Defining Variables, Next: Tips for Defining, 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 the value should ever change. 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. - Special Form: defvar SYMBOL [VALUE [DOC-STRING]] This special form defines SYMBOL as a variable and can also initialize and document it. The definition informs a person reading your code that SYMBOL is used as a variable that might be set or changed. Note that SYMBOL is not evaluated; the symbol to be defined must appear explicitly in the `defvar'. If SYMBOL is void and VALUE is specified, `defvar' evaluates it and sets SYMBOL to the result. But if SYMBOL already has a value (i.e., it is not void), VALUE is not even evaluated, and SYMBOL's value remains unchanged. If VALUE is omitted, the value of SYMBOL is not changed in any case. If SYMBOL has a buffer-local binding in the current buffer, `defvar' operates on the default value, which is buffer-independent, not the current (buffer-local) binding. It sets the default value if the default value is void. *Note Buffer-Local Variables::. When you evaluate a top-level `defvar' form with `C-M-x' in Emacs Lisp mode (`eval-defun'), a special feature of `eval-defun' arranges to set the variable unconditionally, without testing whether its value is void. 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 Emacs 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 conveniently using the commands `set-variable' and `edit-options'. However, it is better to use `defcustom' instead of `defvar' for user option variables, so you can specify customization information. *Note Customization::. Here are some examples. This form defines `foo' but does not initialize it: (defvar foo) => foo This example initializes 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+ nil)' would get an error if it were evaluated, but since it is not evaluated, there is no error.) (defvar bar (1+ nil) "*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)) (if 'DOC-STRING (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 standard global value, established here, that should not be changed by the user or by other programs. Note that SYMBOL is not evaluated; the symbol to be defined must appear explicitly in the `defconst'. `defconst' always evaluates VALUE, and sets the value of SYMBOL to the result if VALUE is given. If SYMBOL does have a buffer-local binding in the current buffer, `defconst' sets the default value, not the buffer-local value. (But you should not be making buffer-local bindings for a symbol that is defined with `defconst'.) 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 to `interactive' (*note Using Interactive::.). However, this feature is largely obsoleted by `defcustom' (*note Customization::.). *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: elisp, Node: Tips for Defining, Next: Accessing Variables, Prev: Defining Variables, Up: Variables Tips for Defining Variables Robustly ==================================== 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) (if my-mode-map nil (let ((map (make-sparse-keymap))) (define-key my-mode-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) (if my-mode-map nil (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.  File: elisp, Node: Accessing Variables, Next: Setting Variables, Prev: Tips for Defining, 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 the current binding of SYMBOL is void.  File: elisp, 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" 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)))  File: elisp, Node: Variable Scoping, Next: Buffer-Local Variables, Prev: Setting Variables, Up: Variables Scoping Rules for Variable Bindings =================================== A given symbol `foo' can 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 all 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: elisp, 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 "free" 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 then call `binder', then the binding made in `binder' will be seen in `user': (defun foo (lose) (user)) * However, if we define `foo' as follows and then 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'. Emacs Lisp uses dynamic scoping because simple implementations of lexical scoping are slow. In addition, every Lisp system needs to offer dynamic scoping at least as an option; if lexical scoping is the norm, there must be a way to specify dynamic scoping instead for a particular variable. It might not be a bad thing for Emacs to offer both, but implementing it with dynamic scoping only was much easier.  File: elisp, 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 do the job 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: elisp, 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, which are variable-value pairs. At entry to a function or to a `let' form, we can push bindings onto 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 GNU 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) onto a stack, and storing the new 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: elisp, 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: elisp, Node: Buffer-Local Variables, Next: Frame-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. Emacs also supports additional, unusual kinds of variable binding: "buffer-local" bindings, which apply only in one buffer, and frame-local bindings, which apply only in one frame. Having different values for a variable in different buffers and/or frames is an important customization method. This section describes buffer-local bindings; for frame-local bindings, see the following section, *Note Frame-Local Variables::. (A few variables have bindings that are local to each terminal; see *Note Multiple Displays::.) * 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 buffer-local values.  File: elisp, Node: Intro to Buffer-Local, Next: Creating Buffer-Local, Up: Buffer-Local Variables Introduction to Buffer-Local Variables -------------------------------------- A buffer-local variable has a buffer-local binding associated with a particular buffer. The binding is in effect when that buffer is current; otherwise, it is not in effect. If you set the variable while a buffer-local binding is in effect, the new value goes in that binding, so its other bindings are unchanged. This means that the change is visible only in the buffer where you made it. The variable's ordinary binding, which is not associated with any specific buffer, is called the "default binding". In most cases, this is the global binding. A variable can have buffer-local bindings in some buffers but not in other buffers. The default binding is shared by all the buffers that don't have their own bindings for the variable. (This includes all newly created buffers.) If you set the variable in a buffer that does not have a buffer-local binding for it, this sets the default binding (assuming there are no frame-local bindings to complicate the matter), so the new value is visible in all the buffers that see the default binding. The most common use of buffer-local bindings is for major modes to change variables that control the behavior of commands. For example, C mode and Lisp mode both set the variable `paragraph-start' to specify that only blank lines separate paragraphs. They do this by making the variable buffer-local in the buffer that is being put into C mode or Lisp mode, and then setting it to the new value for that mode. *Note Major Modes::. The usual way to make a buffer-local binding is with `make-local-variable', which is what major mode commands typically use. This affects just the current buffer; all other buffers (including those yet to be created) will continue to share the default value unless they are explicitly given their own buffer-local bindings. A more powerful operation is to mark the variable as "automatically buffer-local" by calling `make-variable-buffer-local'. You can think of this as making the variable local in all buffers, even those yet to be created. More precisely, the effect is that setting the variable automatically makes the variable local to the current buffer if it is not already so. All buffers start out by sharing the default value of the variable as usual, but setting the variable creates a buffer-local binding for the current buffer. The new value is stored in the buffer-local binding, leaving the default binding untouched. This means that the default value cannot be changed with `setq' in any buffer; the only way to change it is with `setq-default'. *Warning:* When a variable has buffer-local values in one or more buffers, you can get Emacs very confused by binding the variable with `let', changing to a different current buffer in which a different binding is in effect, and then exiting the `let'. This can scramble the values of the buffer-local and default bindings. To preserve your sanity, avoid using a variable in that way. If you use `save-excursion' around each piece of code that changes to a different current buffer, you will not have this problem (*note Excursions::.). Here is an example of what to avoid: (setq foo 'b) (set-buffer "a") (make-local-variable 'foo) (setq foo 'a) (let ((foo 'temp)) (set-buffer "b") BODY...) foo => 'a ; The old buffer-local value from buffer `a' ; is now the default value. (set-buffer "a") foo => 'temp ; The local `let' value that should be gone ; is now the buffer-local value in buffer `a'. But `save-excursion' as shown here avoids the problem: (let ((foo 'temp)) (save-excursion (set-buffer "b") BODY...)) Note that references to `foo' in BODY access the buffer-local binding of buffer `b'. When a file specifies local variable values, these become buffer-local values when you visit the file. *Note File Variables: (emacs)File Variables.  File: elisp, Node: Creating Buffer-Local, Next: Default Value, Prev: Intro to Buffer-Local, Up: Buffer-Local Variables Creating and Deleting Buffer-Local Bindings ------------------------------------------- - Command: make-local-variable VARIABLE This function creates a buffer-local binding in the current buffer for VARIABLE (a symbol). Other buffers are not affected. The value returned is VARIABLE. The buffer-local value of VARIABLE starts out as the same value VARIABLE previously had. If VARIABLE was void, it remains void. ;; In buffer `b1': (setq foo 5) ; Affects all buffers. => 5 (make-local-variable 'foo) ; Now it is local in `b1'. => foo foo ; That did not change => 5 ; the value. (setq foo 6) ; Change the value => 6 ; in `b1'. foo => 6 ;; In buffer `b2', the value hasn't changed. (save-excursion (set-buffer "b2") foo) => 5 Making a variable buffer-local within a `let'-binding for that variable does not work reliably, unless the buffer in which you do this is not current either on entry to or exit from the `let'. This is because `let' does not distinguish between different kinds of bindings; it knows only which variable the binding was made for. If the variable is terminal-local, this function signals an error. Such variables cannot have buffer-local bindings as well. *Note Multiple Displays::. *Note:* do not use `make-local-variable' for a hook variable. Instead, use `make-local-hook'. *Note Hooks::. - Command: make-variable-buffer-local VARIABLE This function marks VARIABLE (a symbol) automatically buffer-local, so that any subsequent attempt to set it will make it local to the current buffer at the time. A peculiar wrinkle of this feature is that binding the variable (with `let' or other binding constructs) does not create a buffer-local binding for it. Only setting the variable (with `set' or `setq') does so. The value returned is VARIABLE. *Warning:* Don't assume that you should use `make-variable-buffer-local' for user-option variables, simply because users *might* want to customize them differently in different buffers. Users can make any variable local, when they wish to. It is better to leave the choice to them. The time to use `make-variable-buffer-local' is when it is crucial that no two buffers ever share the same binding. For example, when a variable is used for internal purposes in a Lisp program which depends on having separate values in separate buffers, then using `make-variable-buffer-local' can be the best solution. - Function: local-variable-p VARIABLE &optional BUFFER This returns `t' if VARIABLE is buffer-local in buffer BUFFER (which defaults to the current buffer); otherwise, `nil'. - Function: buffer-local-variables &optional BUFFER This function returns a list describing the buffer-local variables in buffer BUFFER. (If BUFFER is omitted, the current buffer is used.) It returns an association list (*note Association Lists::.) in which each element contains one buffer-local variable and its value. However, when a variable's buffer-local binding in BUFFER is void, then the variable appears directly in the resulting list. (make-local-variable 'foobar) (makunbound 'foobar) (make-local-variable 'bind-me) (setq bind-me 69) (setq lcl (buffer-local-variables)) ;; First, built-in variables local in all buffers: => ((mark-active . nil) (buffer-undo-list . nil) (mode-name . "Fundamental") ... ;; Next, non-built-in buffer-local variables. ;; This one is buffer-local and void: foobar ;; This one is buffer-local and nonvoid: (bind-me . 69)) Note that storing new values into the CDRs of cons cells in this list does *not* change the buffer-local values of the variables. - Command: kill-local-variable VARIABLE This function deletes the buffer-local binding (if any) for VARIABLE (a symbol) in the current buffer. As a result, the default binding of VARIABLE becomes visible in this buffer. This typically results in a change in the value of VARIABLE, since the default value is usually different from the buffer-local value just eliminated. If you kill the buffer-local binding of a variable that automatically becomes buffer-local when set, this makes the default value visible in the current buffer. However, if you set the variable again, that will once again create a buffer-local binding for it. `kill-local-variable' returns VARIABLE. This function is a command because it is sometimes useful to kill one buffer-local variable interactively, just as it is useful to create buffer-local variables interactively. - Function: kill-all-local-variables This function eliminates all the buffer-local variable bindings of the current buffer except for variables marked as "permanent". As a result, the buffer will see the default values of most variables. This function also resets certain other information pertaining to the buffer: it sets the local keymap to `nil', the syntax table to the value of `(standard-syntax-table)', the case table to `(standard-case-table)', and the abbrev table to the value of `fundamental-mode-abbrev-table'. The very first thing this function does is run the normal hook `change-major-mode-hook' (see below). Every major mode command begins by calling this function, which has the effect of switching to Fundamental mode and erasing most of the effects of the previous major mode. To ensure that this does its job, the variables that major modes set should not be marked permanent. `kill-all-local-variables' returns `nil'. - Variable: change-major-mode-hook The function `kill-all-local-variables' runs this normal hook before it does anything else. This gives major modes a way to arrange for something special to be done if the user switches to a different major mode. For best results, make this variable buffer-local, so that it will disappear after doing its job and will not interfere with the subsequent major mode. *Note Hooks::. A buffer-local variable is "permanent" if the variable name (a symbol) has a `permanent-local' property that is non-`nil'. Permanent locals are appropriate for data pertaining to where the file came from or how to save it, rather than with how to edit the contents.  File: elisp, Node: Default Value, Prev: Creating Buffer-Local, Up: Buffer-Local Variables The Default Value of a Buffer-Local Variable -------------------------------------------- The global value of a variable with buffer-local bindings is also called the "default" value, because it is the value that is in effect whenever neither the current buffer nor the selected frame has its own binding for the variable. The functions `default-value' and `setq-default' access and change a variable's default value regardless of whether the current buffer has a buffer-local binding. For example, you could use `setq-default' to change the default setting of `paragraph-start' for most buffers; and this would work even when you are in a C or Lisp mode buffer that has a buffer-local value for this variable. The special forms `defvar' and `defconst' also set the default value (if they set the variable at all), rather than any buffer-local or frame-local value. - Function: default-value SYMBOL This function returns SYMBOL's default value. This is the value that is seen in buffers and frames that do not have their own values for this variable. If SYMBOL is not buffer-local, this is equivalent to `symbol-value' (*note Accessing Variables::.). - Function: default-boundp SYMBOL The function `default-boundp' tells you whether SYMBOL's default value is nonvoid. If `(default-boundp 'foo)' returns `nil', then `(default-value 'foo)' would get an error. `default-boundp' is to `default-value' as `boundp' is to `symbol-value'. - Special Form: setq-default [SYMBOL FORM]... This special form gives each SYMBOL a new default value, which is the result of evaluating the corresponding FORM. It does not evaluate SYMBOL, but does evaluate FORM. The value of the `setq-default' form is the value of the last FORM. If a SYMBOL is not buffer-local for the current buffer, and is not marked automatically buffer-local, `setq-default' has the same effect as `setq'. If SYMBOL is buffer-local for the current buffer, then this changes the value that other buffers will see (as long as they don't have a buffer-local value), but not the value that the current buffer sees. ;; In buffer `foo': (make-local-variable 'buffer-local) => buffer-local (setq buffer-local 'value-in-foo) => value-in-foo (setq-default buffer-local 'new-default) => new-default buffer-local => value-in-foo (default-value 'buffer-local) => new-default ;; In (the new) buffer `bar': buffer-local => new-default (default-value 'buffer-local) => new-default (setq buffer-local 'another-default) => another-default (default-value 'buffer-local) => another-default ;; Back in buffer `foo': buffer-local => value-in-foo (default-value 'buffer-local) => another-default - Function: set-default SYMBOL VALUE This function is like `setq-default', except that SYMBOL is an ordinary evaluated argument. (set-default (car '(a b c)) 23) => 23 (default-value 'a) => 23  File: elisp, Node: Frame-Local Variables, Next: Future Local Variables, Prev: Buffer-Local Variables, Up: Variables Frame-Local Variables ===================== Just as variables can have buffer-local bindings, they can also have frame-local bindings. These bindings belong to one frame, and are in effect when that frame is selected. Frame-local bindings are actually frame parameters: you create a frame-local binding in a specific frame by calling `modify-frame-parameters' and specifying the variable name as the parameter name. To enable frame-local bindings for a certain variable, call the function `make-variable-frame-local'. - Command: make-variable-frame-local VARIABLE Enable the use of frame-local bindings for VARIABLE. This does not in itself create any frame-local bindings for the variable; however, if some frame already has a value for VARIABLE as a frame parameter, that value automatically becomes a frame-local binding. If the variable is terminal-local, this function signals an error, because such variables cannot have frame-local bindings as well. *Note Multiple Displays::. A few variables that are implemented specially in Emacs can be (and usually are) buffer-local, but can never be frame-local. Buffer-local bindings take precedence over frame-local bindings. Thus, consider a variable `foo': if the current buffer has a buffer-local binding for `foo', that binding is active; otherwise, if the selected frame has a frame-local binding for `foo', that binding is active; otherwise, the default binding of `foo' is active. Here is an example. First we prepare a few bindings for `foo': (setq f1 (selected-frame)) (make-variable-frame-local 'foo) ;; Make a buffer-local binding for `foo' in `b1'. (set-buffer (get-buffer-create "b1")) (make-local-variable 'foo) (setq foo '(b 1)) ;; Make a frame-local binding for `foo' in a new frame. ;; Store that frame in `f2'. (setq f2 (make-frame)) (modify-frame-parameters f2 '((foo . (f 2)))) Now we examine `foo' in various contexts. Whenever the buffer `b1' is current, its buffer-local binding is in effect, regardless of the selected frame: (select-frame f1) (set-buffer (get-buffer-create "b1")) foo => (b 1) (select-frame f2) (set-buffer (get-buffer-create "b1")) foo => (b 1) Otherwise, the frame gets a chance to provide the binding; when frame `f2' is selected, its frame-local binding is in effect: (select-frame f2) (set-buffer (get-buffer "*scratch*")) foo => (f 2) When neither the current buffer nor the selected frame provides a binding, the default binding is used: (select-frame f1) (set-buffer (get-buffer "*scratch*")) foo => nil When the active binding of a variable is a frame-local binding, setting the variable changes that binding. You can observe the result with `frame-parameters': (select-frame f2) (set-buffer (get-buffer "*scratch*")) (setq foo 'nobody) (assq 'foo (frame-parameters f2)) => (foo . nobody)  File: elisp, Node: Future Local Variables, Prev: Frame-Local Variables, Up: Variables Possible Future Local Variables =============================== We have considered the idea of bindings that are local to a category of frames--for example, all color frames, or all frames with dark backgrounds. We have not implemented them because it is not clear that this feature is really useful. You can get more or less the same results by adding a function to `after-make-frame-hook', set up to define a particular frame parameter according to the appropriate conditions for each frame. It would also be possible to implement window-local bindings. We don't know of many situations where they would be useful, and it seems that indirect buffers (*note Indirect Buffers::.) with buffer-local bindings offer a way to handle these situations more robustly. If sufficient application is found for either of these two kinds of local bindings, we will provide it in a subsequent Emacs version.  File: elisp, Node: Functions, Next: Macros, Prev: Variables, Up: Top Functions ********* A Lisp program is composed mainly of Lisp functions. This chapter explains what functions are, how they accept arguments, and how to define them. * Menu: * What Is a Function:: Lisp functions vs. primitives; terminology. * Lambda Expressions:: How functions are expressed as Lisp objects. * Function Names:: A symbol can serve as the name of a function. * Defining Functions:: Lisp expressions for defining functions. * Calling Functions:: How to use an existing function. * Mapping Functions:: Applying a function to each element of a list, etc. * Anonymous Functions:: Lambda expressions are functions with no names. * Function Cells:: Accessing or setting the function definition of a symbol. * Inline Functions:: Defining functions that the compiler will open code. * Related Topics:: Cross-references to specific Lisp primitives that have a special bearing on how functions work.  File: elisp, Node: What Is a Function, Next: Lambda Expressions, Up: Functions What Is a Function? =================== In a general sense, a function is a rule for carrying on a computation given several values called "arguments". The result of the computation is called the value of the function. The computation can also have side effects: lasting changes in the values of variables or the contents of data structures. Here are important terms for functions in Emacs Lisp and for other function-like objects. "function" In Emacs Lisp, a "function" is anything that can be applied to arguments in a Lisp program. In some cases, we use it more specifically to mean a function written in Lisp. Special forms and macros are not functions. "primitive" A "primitive" is a function callable from Lisp that is written in C, such as `car' or `append'. These functions are also called "built-in" functions or "subrs". (Special forms are also considered primitives.) Usually the reason we implement a function as a primitive is either because it is fundamental, because it provides a low-level interface to operating system services, or because it needs to run fast. Primitives can be modified or added only by changing the C sources and recompiling the editor. See *Note Writing Emacs Primitives::. "lambda expression" A "lambda expression" is a function written in Lisp. These are described in the following section. *Note Lambda Expressions::. "special form" A "special form" is a primitive that is like a function but does not evaluate all of its arguments in the usual way. It may evaluate only some of the arguments, or may evaluate them in an unusual order, or several times. Many special forms are described in *Note Control Structures::. "macro" A "macro" is a construct defined in Lisp by the programmer. It differs from a function in that it translates a Lisp expression that you write into an equivalent expression to be evaluated instead of the original expression. Macros enable Lisp programmers to do the sorts of things that special forms can do. *Note Macros::, for how to define and use macros. "command" A "command" is an object that `command-execute' can invoke; it is a possible definition for a key sequence. Some functions are commands; a function written in Lisp is a command if it contains an interactive declaration (*note Defining Commands::.). Such a function can be called from Lisp expressions like other functions; in this case, the fact that the function is a command makes no difference. Keyboard macros (strings and vectors) are commands also, even though they are not functions. A symbol is a command if its function definition is a command; such symbols can be invoked with `M-x'. The symbol is a function as well if the definition is a function. *Note Command Overview::. "keystroke command" A "keystroke command" is a command that is bound to a key sequence (typically one to three keystrokes). The distinction is made here merely to avoid confusion with the meaning of "command" in non-Emacs editors; for Lisp programs, the distinction is normally unimportant. "byte-code function" A "byte-code function" is a function that has been compiled by the byte compiler. *Note Byte-Code Type::. - Function: functionp OBJECT This function returns `t' if OBJECT is any kind of function, or a special form or macro. - Function: subrp OBJECT This function returns `t' if OBJECT is a built-in function (i.e., a Lisp primitive). (subrp 'message) ; `message' is a symbol, => nil ; not a subr object. (subrp (symbol-function 'message)) => t - Function: byte-code-function-p OBJECT This function returns `t' if OBJECT is a byte-code function. For example: (byte-code-function-p (symbol-function 'next-line)) => t  File: elisp, Node: Lambda Expressions, Next: Function Names, Prev: What Is a Function, Up: Functions Lambda Expressions ================== A function written in Lisp is a list that looks like this: (lambda (ARG-VARIABLES...) [DOCUMENTATION-STRING] [INTERACTIVE-DECLARATION] BODY-FORMS...) Such a list is called a "lambda expression". In Emacs Lisp, it actually is valid as an expression--it evaluates to itself. In some other Lisp dialects, a lambda expression is not a valid expression at all. In either case, its main use is not to be evaluated as an expression, but to be called as a function. * Menu: * Lambda Components:: The parts of a lambda expression. * Simple Lambda:: A simple example. * Argument List:: Details and special features of argument lists. * Function Documentation:: How to put documentation in a function.