This is ../info/lispref.info, produced by makeinfo version 4.0b from lispref/lispref.texi. INFO-DIR-SECTION XEmacs Editor START-INFO-DIR-ENTRY * Lispref: (lispref). XEmacs Lisp Reference Manual. END-INFO-DIR-ENTRY 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 XEmacs Lisp Reference Manual (for 19.14 and 20.0) v3.1, March 1996 XEmacs Lisp Reference Manual (for 19.15 and 20.1, 20.2, 20.3) v3.2, April, May, November 1997 XEmacs Lisp Reference Manual (for 21.0) v3.3, April 1998 Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995 Free Software Foundation, Inc. Copyright (C) 1994, 1995 Sun Microsystems, Inc. Copyright (C) 1995, 1996 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: 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 the global binding is unchanged; this means that the change is visible in that buffer alone. A variable may have buffer-local bindings in some buffers but not in others. The global binding is shared by all the buffers that don't have their own bindings. Thus, if you set the variable in a buffer that does not have a buffer-local binding for it, the new value is visible in all buffers except those with buffer-local bindings. (Here we are assuming that there are no `let'-style local bindings to complicate the issue.) 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. The usual way to make a buffer-local binding is with `make-local-variable', which is what major mode commands use. This affects just the current buffer; all other buffers (including those yet to be created) continue to share the global value. 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 global value of the variable as usual, but any `setq' creates a buffer-local binding for the current buffer. The new value is stored in the buffer-local binding, leaving the (default) global binding untouched. The global value can no longer be changed with `setq'; you need to use `setq-default' to do that. Local variables in a file you edit are also represented by buffer-local bindings for the buffer that holds the file within XEmacs. *Note Auto Major Mode::.  File: lispref.info, 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. This is because `let' does not distinguish between different kinds of bindings; it knows only which variable the binding was made for. *Please 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. The value returned is VARIABLE. - Function: local-variable-p variable buffer &optional after-set This returns `t' if VARIABLE is buffer-local in buffer BUFFER; else `nil'. If optional third arg AFTER-SET is non-`nil', return `t' if SYMBOL would be buffer-local after it is set, regardless of whether it is so presently. A `nil' value for BUFFER is _not_ the same as `(current-buffer)', but means "no buffer". Specifically: If BUFFER is `nil' and AFTER-SET is `nil', a return value of `t' indicates that the variable is one of the special built-in variables that is always buffer-local. (This includes `buffer-file-name', `buffer-read-only', `buffer-undo-list', and others.) If BUFFER is `nil' and AFTER-SET is `t', a return value of `t' indicates that the variable has had `make-variable-buffer-local' applied to it. - Function: buffer-local-variables &optional buffer This function returns a list describing the buffer-local variables in buffer BUFFER. It returns an association list (*note Association Lists::) in which each association contains one buffer-local variable and its value. When a buffer-local variable is void in BUFFER, then it appears directly in the resulting list. If BUFFER is omitted, the current buffer is used. (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 local variables. ;; This one is local and void: foobar ;; This one is local and nonvoid: (bind-me . 69)) Note that storing new values into the CDRs of cons cells in this list does _not_ change the 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 global (default) binding of VARIABLE becomes visible in this buffer. Usually this results in a change in the value of VARIABLE, since the global value is usually different from the buffer-local value just eliminated. If you kill the local binding of a variable that automatically becomes local when set, this makes the global value visible in the current buffer. However, if you set the variable again, that will once again create a 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', and the abbrev table to the value of `fundamental-mode-abbrev-table'. 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'. A 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: lispref.info, 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 except when specifically overridden. 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 local value. - Function: default-value symbol This function returns SYMBOL's default value. This is the value that is seen in buffers 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 value This sets the default value of SYMBOL to VALUE. It does not evaluate SYMBOL, but does evaluate VALUE. The value of the `setq-default' form is VALUE. 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 'local) => local (setq local 'value-in-foo) => value-in-foo (setq-default local 'new-default) => new-default local => value-in-foo (default-value 'local) => new-default ;; In (the new) buffer `bar': local => new-default (default-value 'local) => new-default (setq local 'another-default) => another-default (default-value 'local) => another-default ;; Back in buffer `foo': local => value-in-foo (default-value 'local) => another-default - Function: set-default symbol value This function is like `setq-default', except that SYMBOL is evaluated. (set-default (car '(a b c)) 23) => 23 (default-value 'a) => 23  File: lispref.info, Node: Variable Aliases, Prev: Buffer-Local Variables, Up: Variables Variable Aliases ================ You can define a variable as an "alias" for another. Any time you reference the former variable, the current value of the latter is returned. Any time you change the value of the former variable, the value of the latter is actually changed. This is useful in cases where you want to rename a variable but still make old code work (*note Obsoleteness::). - Function: defvaralias variable alias This function defines VARIABLE as an alias for ALIAS. Thenceforth, any operations performed on VARIABLE will actually be performed on ALIAS. Both VARIABLE and ALIAS should be symbols. If ALIAS is `nil', remove any aliases for VARIABLE. ALIAS can itself be aliased, and the chain of variable aliases will be followed appropriately. If VARIABLE already has a value, this value will be shadowed until the alias is removed, at which point it will be restored. Currently VARIABLE cannot be a built-in variable, a variable that has a buffer-local value in any buffer, or the symbols `nil' or `t'. - Function: variable-alias variable &optional follow-past-lisp-magic If VARIABLE is aliased to another variable, this function returns that variable. VARIABLE should be a symbol. If VARIABLE is not aliased, this function returns `nil'. - Function: indirect-variable object &optional follow-past-lisp-magic This function returns the variable at the end of OBJECT's variable-alias chain. If OBJECT is a symbol, follow all variable aliases and return the final (non-aliased) symbol. If OBJECT is not a symbol, just return it. Signal a `cyclic-variable-indirection' error if there is a loop in the variable chain of symbols.  File: lispref.info, 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: lispref.info, 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 XEmacs Lisp and for other function-like objects. "function" In XEmacs 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 that a function is a primitives is 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 Lisp Primitives: (internals)Writing Lisp 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. "compiled function" A "compiled function" is a function that has been compiled by the byte compiler. *Note Compiled-Function Type::. - 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: compiled-function-p object This function returns `t' if OBJECT is a compiled function. For example: (compiled-function-p (symbol-function 'next-line)) => t  File: lispref.info, 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 XEmacs 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.  File: lispref.info, Node: Lambda Components, Next: Simple Lambda, Up: Lambda Expressions Components of a Lambda Expression --------------------------------- A function written in Lisp (a "lambda expression") is a list that looks like this: (lambda (ARG-VARIABLES...) [DOCUMENTATION-STRING] [INTERACTIVE-DECLARATION] BODY-FORMS...) The first element of a lambda expression is always the symbol `lambda'. This indicates that the list represents a function. The reason functions are defined to start with `lambda' is so that other lists, intended for other uses, will not accidentally be valid as functions. The second element is a list of symbols-the argument variable names. This is called the "lambda list". When a Lisp function is called, the argument values are matched up against the variables in the lambda list, which are given local bindings with the values provided. *Note Local Variables::. The documentation string is a Lisp string object placed within the function definition to describe the function for the XEmacs help facilities. *Note Function Documentation::. The interactive declaration is a list of the form `(interactive CODE-STRING)'. This declares how to provide arguments if the function is used interactively. Functions with this declaration are called "commands"; they can be called using `M-x' or bound to a key. Functions not intended to be called in this way should not have interactive declarations. *Note Defining Commands::, for how to write an interactive declaration. The rest of the elements are the "body" of the function: the Lisp code to do the work of the function (or, as a Lisp programmer would say, "a list of Lisp forms to evaluate"). The value returned by the function is the value returned by the last element of the body.  File: lispref.info, Node: Simple Lambda, Next: Argument List, Prev: Lambda Components, Up: Lambda Expressions A Simple Lambda-Expression Example ---------------------------------- Consider for example the following function: (lambda (a b c) (+ a b c)) We can call this function by writing it as the CAR of an expression, like this: ((lambda (a b c) (+ a b c)) 1 2 3) This call evaluates the body of the lambda expression with the variable `a' bound to 1, `b' bound to 2, and `c' bound to 3. Evaluation of the body adds these three numbers, producing the result 6; therefore, this call to the function returns the value 6. Note that the arguments can be the results of other function calls, as in this example: ((lambda (a b c) (+ a b c)) 1 (* 2 3) (- 5 4)) This evaluates the arguments `1', `(* 2 3)', and `(- 5 4)' from left to right. Then it applies the lambda expression to the argument values 1, 6 and 1 to produce the value 8. It is not often useful to write a lambda expression as the CAR of a form in this way. You can get the same result, of making local variables and giving them values, using the special form `let' (*note Local Variables::). And `let' is clearer and easier to use. In practice, lambda expressions are either stored as the function definitions of symbols, to produce named functions, or passed as arguments to other functions (*note Anonymous Functions::). However, calls to explicit lambda expressions were very useful in the old days of Lisp, before the special form `let' was invented. At that time, they were the only way to bind and initialize local variables.  File: lispref.info, Node: Argument List, Next: Function Documentation, Prev: Simple Lambda, Up: Lambda Expressions Advanced Features of Argument Lists ----------------------------------- Our simple sample function, `(lambda (a b c) (+ a b c))', specifies three argument variables, so it must be called with three arguments: if you try to call it with only two arguments or four arguments, you get a `wrong-number-of-arguments' error. It is often convenient to write a function that allows certain arguments to be omitted. For example, the function `substring' accepts three arguments--a string, the start index and the end index--but the third argument defaults to the LENGTH of the string if you omit it. It is also convenient for certain functions to accept an indefinite number of arguments, as the functions `list' and `+' do. To specify optional arguments that may be omitted when a function is called, simply include the keyword `&optional' before the optional arguments. To specify a list of zero or more extra arguments, include the keyword `&rest' before one final argument. Thus, the complete syntax for an argument list is as follows: (REQUIRED-VARS... [&optional OPTIONAL-VARS...] [&rest REST-VAR]) The square brackets indicate that the `&optional' and `&rest' clauses, and the variables that follow them, are optional. A call to the function requires one actual argument for each of the REQUIRED-VARS. There may be actual arguments for zero or more of the OPTIONAL-VARS, and there cannot be any actual arguments beyond that unless the lambda list uses `&rest'. In that case, there may be any number of extra actual arguments. If actual arguments for the optional and rest variables are omitted, then they always default to `nil'. There is no way for the function to distinguish between an explicit argument of `nil' and an omitted argument. However, the body of the function is free to consider `nil' an abbreviation for some other meaningful value. This is what `substring' does; `nil' as the third argument to `substring' means to use the length of the string supplied. Common Lisp note: Common Lisp allows the function to specify what default value to use when an optional argument is omitted; XEmacs Lisp always uses `nil'. For example, an argument list that looks like this: (a b &optional c d &rest e) binds `a' and `b' to the first two actual arguments, which are required. If one or two more arguments are provided, `c' and `d' are bound to them respectively; any arguments after the first four are collected into a list and `e' is bound to that list. If there are only two arguments, `c' is `nil'; if two or three arguments, `d' is `nil'; if four arguments or fewer, `e' is `nil'. There is no way to have required arguments following optional ones--it would not make sense. To see why this must be so, suppose that `c' in the example were optional and `d' were required. Suppose three actual arguments are given; which variable would the third argument be for? Similarly, it makes no sense to have any more arguments (either required or optional) after a `&rest' argument. Here are some examples of argument lists and proper calls: ((lambda (n) (1+ n)) ; One required: 1) ; requires exactly one argument. => 2 ((lambda (n &optional n1) ; One required and one optional: (if n1 (+ n n1) (1+ n))) ; 1 or 2 arguments. 1 2) => 3 ((lambda (n &rest ns) ; One required and one rest: (+ n (apply '+ ns))) ; 1 or more arguments. 1 2 3 4 5) => 15  File: lispref.info, Node: Function Documentation, Prev: Argument List, Up: Lambda Expressions Documentation Strings of Functions ---------------------------------- A lambda expression may optionally have a "documentation string" just after the lambda list. This string does not affect execution of the function; it is a kind of comment, but a systematized comment which actually appears inside the Lisp world and can be used by the XEmacs help facilities. *Note Documentation::, for how the DOCUMENTATION-STRING is accessed. It is a good idea to provide documentation strings for all the functions in your program, even those that are only called from within your program. Documentation strings are like comments, except that they are easier to access. The first line of the documentation string should stand on its own, because `apropos' displays just this first line. It should consist of one or two complete sentences that summarize the function's purpose. The start of the documentation string is usually indented in the source file, but since these spaces come before the starting double-quote, they are not part of the string. Some people make a practice of indenting any additional lines of the string so that the text lines up in the program source. _This is a mistake._ The indentation of the following lines is inside the string; what looks nice in the source code will look ugly when displayed by the help commands. You may wonder how the documentation string could be optional, since there are required components of the function that follow it (the body). Since evaluation of a string returns that string, without any side effects, it has no effect if it is not the last form in the body. Thus, in practice, there is no confusion between the first form of the body and the documentation string; if the only body form is a string then it serves both as the return value and as the documentation.  File: lispref.info, Node: Function Names, Next: Defining Functions, Prev: Lambda Expressions, Up: Functions Naming a Function ================= In most computer languages, every function has a name; the idea of a function without a name is nonsensical. In Lisp, a function in the strictest sense has no name. It is simply a list whose first element is `lambda', or a primitive subr-object. However, a symbol can serve as the name of a function. This happens when you put the function in the symbol's "function cell" (*note Symbol Components::). Then the symbol itself becomes a valid, callable function, equivalent to the list or subr-object that its function cell refers to. The contents of the function cell are also called the symbol's "function definition". The procedure of using a symbol's function definition in place of the symbol is called "symbol function indirection"; see *Note Function Indirection::. In practice, nearly all functions are given names in this way and referred to through their names. For example, the symbol `car' works as a function and does what it does because the primitive subr-object `#' is stored in its function cell. We give functions names because it is convenient to refer to them by their names in Lisp expressions. For primitive subr-objects such as `#', names are the only way you can refer to them: there is no read syntax for such objects. For functions written in Lisp, the name is more convenient to use in a call than an explicit lambda expression. Also, a function with a name can refer to itself--it can be recursive. Writing the function's name in its own definition is much more convenient than making the function definition point to itself (something that is not impossible but that has various disadvantages in practice). We often identify functions with the symbols used to name them. For example, we often speak of "the function `car'", not distinguishing between the symbol `car' and the primitive subr-object that is its function definition. For most purposes, there is no need to distinguish. Even so, keep in mind that a function need not have a unique name. While a given function object _usually_ appears in the function cell of only one symbol, this is just a matter of convenience. It is easy to store it in several symbols using `fset'; then each of the symbols is equally well a name for the same function. A symbol used as a function name may also be used as a variable; these two uses of a symbol are independent and do not conflict.  File: lispref.info, Node: Defining Functions, Next: Calling Functions, Prev: Function Names, Up: Functions Defining Functions ================== We usually give a name to a function when it is first created. This is called "defining a function", and it is done with the `defun' special form. - Special Form: defun name argument-list body-forms `defun' is the usual way to define new Lisp functions. It defines the symbol NAME as a function that looks like this: (lambda ARGUMENT-LIST . BODY-FORMS) `defun' stores this lambda expression in the function cell of NAME. It returns the value NAME, but usually we ignore this value. As described previously (*note Lambda Expressions::), ARGUMENT-LIST is a list of argument names and may include the keywords `&optional' and `&rest'. Also, the first two forms in BODY-FORMS may be a documentation string and an interactive declaration. There is no conflict if the same symbol NAME is also used as a variable, since the symbol's value cell is independent of the function cell. *Note Symbol Components::. Here are some examples: (defun foo () 5) => foo (foo) => 5 (defun bar (a &optional b &rest c) (list a b c)) => bar (bar 1 2 3 4 5) => (1 2 (3 4 5)) (bar 1) => (1 nil nil) (bar) error--> Wrong number of arguments. (defun capitalize-backwards () "Upcase the last letter of a word." (interactive) (backward-word 1) (forward-word 1) (backward-char 1) (capitalize-word 1)) => capitalize-backwards Be careful not to redefine existing functions unintentionally. `defun' redefines even primitive functions such as `car' without any hesitation or notification. Redefining a function already defined is often done deliberately, and there is no way to distinguish deliberate redefinition from unintentional redefinition. - Function: define-function name definition - Function: defalias name definition These equivalent special forms define the symbol NAME as a function, with definition DEFINITION (which can be any valid Lisp function). The proper place to use `define-function' or `defalias' is where a specific function name is being defined--especially where that name appears explicitly in the source file being loaded. This is because `define-function' and `defalias' record which file defined the function, just like `defun'. (*note Unloading::). By contrast, in programs that manipulate function definitions for other purposes, it is better to use `fset', which does not keep such records. See also `defsubst', which defines a function like `defun' and tells the Lisp compiler to open-code it. *Note Inline Functions::.  File: lispref.info, Node: Calling Functions, Next: Mapping Functions, Prev: Defining Functions, Up: Functions Calling Functions ================= Defining functions is only half the battle. Functions don't do anything until you "call" them, i.e., tell them to run. Calling a function is also known as "invocation". The most common way of invoking a function is by evaluating a list. For example, evaluating the list `(concat "a" "b")' calls the function `concat' with arguments `"a"' and `"b"'. *Note Evaluation::, for a description of evaluation. When you write a list as an expression in your program, the function name is part of the program. This means that you choose which function to call, and how many arguments to give it, when you write the program. Usually that's just what you want. Occasionally you need to decide at run time which function to call. To do that, use the functions `funcall' and `apply'. - Function: funcall function &rest arguments `funcall' calls FUNCTION with ARGUMENTS, and returns whatever FUNCTION returns. Since `funcall' is a function, all of its arguments, including FUNCTION, are evaluated before `funcall' is called. This means that you can use any expression to obtain the function to be called. It also means that `funcall' does not see the expressions you write for the ARGUMENTS, only their values. These values are _not_ evaluated a second time in the act of calling FUNCTION; `funcall' enters the normal procedure for calling a function at the place where the arguments have already been evaluated. The argument FUNCTION must be either a Lisp function or a primitive function. Special forms and macros are not allowed, because they make sense only when given the "unevaluated" argument expressions. `funcall' cannot provide these because, as we saw above, it never knows them in the first place. (setq f 'list) => list (funcall f 'x 'y 'z) => (x y z) (funcall f 'x 'y '(z)) => (x y (z)) (funcall 'and t nil) error--> Invalid function: # Compare these example with the examples of `apply'. - Function: apply function &rest arguments `apply' calls FUNCTION with ARGUMENTS, just like `funcall' but with one difference: the last of ARGUMENTS is a list of arguments to give to FUNCTION, rather than a single argument. We also say that `apply' "spreads" this list so that each individual element becomes an argument. `apply' returns the result of calling FUNCTION. As with `funcall', FUNCTION must either be a Lisp function or a primitive function; special forms and macros do not make sense in `apply'. (setq f 'list) => list (apply f 'x 'y 'z) error--> Wrong type argument: listp, z (apply '+ 1 2 '(3 4)) => 10 (apply '+ '(1 2 3 4)) => 10 (apply 'append '((a b c) nil (x y z) nil)) => (a b c x y z) For an interesting example of using `apply', see the description of `mapcar', in *Note Mapping Functions::. It is common for Lisp functions to accept functions as arguments or find them in data structures (especially in hook variables and property lists) and call them using `funcall' or `apply'. Functions that accept function arguments are often called "functionals". Sometimes, when you call a functional, it is useful to supply a no-op function as the argument. Here are two different kinds of no-op function: - Function: identity arg This function returns ARG and has no side effects. - Command: ignore &rest args This function ignores any arguments and returns `nil'.  File: lispref.info, Node: Mapping Functions, Next: Anonymous Functions, Prev: Calling Functions, Up: Functions Mapping Functions ================= A "mapping function" applies a given function to each element of a list or other collection. XEmacs Lisp has several such functions; `mapcar' and `mapconcat', which scan a list, are described here. *Note Creating Symbols::, for the function `mapatoms' which maps over the symbols in an obarray. Mapping functions should never modify the sequence being mapped over. The results are unpredictable. - Function: mapcar function sequence `mapcar' applies FUNCTION to each element of SEQUENCE in turn, and returns a list of the results. The argument SEQUENCE can be any kind of sequence; that is, a list, a vector, a bit vector, or a string. The result is always a list. The length of the result is the same as the length of SEQUENCE. For example: (mapcar 'car '((a b) (c d) (e f))) => (a c e) (mapcar '1+ [1 2 3]) => (2 3 4) (mapcar 'char-to-string "abc") => ("a" "b" "c") ;; Call each function in `my-hooks'. (mapcar 'funcall my-hooks) (defun mapcar* (f &rest args) "Apply FUNCTION to successive cars of all ARGS. Return the list of results." ;; If no list is exhausted, (if (not (memq 'nil args)) ;; apply function to CARs. (cons (apply f (mapcar 'car args)) (apply 'mapcar* f ;; Recurse for rest of elements. (mapcar 'cdr args))))) (mapcar* 'cons '(a b c) '(1 2 3 4)) => ((a . 1) (b . 2) (c . 3)) - Function: mapconcat function sequence separator `mapconcat' applies FUNCTION to each element of SEQUENCE: the results, which must be strings, are concatenated. Between each pair of result strings, `mapconcat' inserts the string SEPARATOR. Usually SEPARATOR contains a space or comma or other suitable punctuation. The argument FUNCTION must be a function that can take one argument and return a string. The argument SEQUENCE can be any kind of sequence; that is, a list, a vector, a bit vector, or a string. (mapconcat 'symbol-name '(The cat in the hat) " ") => "The cat in the hat" (mapconcat (function (lambda (x) (format "%c" (1+ x)))) "HAL-8000" "") => "IBM.9111"  File: lispref.info, Node: Anonymous Functions, Next: Function Cells, Prev: Mapping Functions, Up: Functions Anonymous Functions =================== In Lisp, a function is a list that starts with `lambda', a byte-code function compiled from such a list, or alternatively a primitive subr-object; names are "extra". Although usually functions are defined with `defun' and given names at the same time, it is occasionally more concise to use an explicit lambda expression--an anonymous function. Such a list is valid wherever a function name is. Any method of creating such a list makes a valid function. Even this: (setq silly (append '(lambda (x)) (list (list '+ (* 3 4) 'x)))) => (lambda (x) (+ 12 x)) This computes a list that looks like `(lambda (x) (+ 12 x))' and makes it the value (_not_ the function definition!) of `silly'. Here is how we might call this function: (funcall silly 1) => 13 (It does _not_ work to write `(silly 1)', because this function is not the _function definition_ of `silly'. We have not given `silly' any function definition, just a value as a variable.) Most of the time, anonymous functions are constants that appear in your program. For example, you might want to pass one as an argument to the function `mapcar', which applies any given function to each element of a list. Here we pass an anonymous function that multiplies a number by two: (defun double-each (list) (mapcar '(lambda (x) (* 2 x)) list)) => double-each (double-each '(2 11)) => (4 22) In such cases, we usually use the special form `function' instead of simple quotation to quote the anonymous function. - Special Form: function function-object This special form returns FUNCTION-OBJECT without evaluating it. In this, it is equivalent to `quote'. However, it serves as a note to the XEmacs Lisp compiler that FUNCTION-OBJECT is intended to be used only as a function, and therefore can safely be compiled. Contrast this with `quote', in *Note Quoting::. Using `function' instead of `quote' makes a difference inside a function or macro that you are going to compile. For example: (defun double-each (list) (mapcar (function (lambda (x) (* 2 x))) list)) => double-each (double-each '(2 11)) => (4 22) If this definition of `double-each' is compiled, the anonymous function is compiled as well. By contrast, in the previous definition where ordinary `quote' is used, the argument passed to `mapcar' is the precise list shown: (lambda (x) (* x 2)) The Lisp compiler cannot assume this list is a function, even though it looks like one, since it does not know what `mapcar' does with the list. Perhaps `mapcar' will check that the CAR of the third element is the symbol `*'! The advantage of `function' is that it tells the compiler to go ahead and compile the constant function. We sometimes write `function' instead of `quote' when quoting the name of a function, but this usage is just a sort of comment. (function SYMBOL) == (quote SYMBOL) == 'SYMBOL See `documentation' in *Note Accessing Documentation::, for a realistic example using `function' and an anonymous function.  File: lispref.info, Node: Function Cells, Next: Inline Functions, Prev: Anonymous Functions, Up: Functions Accessing Function Cell Contents ================================ The "function definition" of a symbol is the object stored in the function cell of the symbol. The functions described here access, test, and set the function cell of symbols. See also the function `indirect-function' in *Note Function Indirection::. - Function: symbol-function symbol This returns the object in the function cell of SYMBOL. If the symbol's function cell is void, a `void-function' error is signaled. This function does not check that the returned object is a legitimate function. (defun bar (n) (+ n 2)) => bar (symbol-function 'bar) => (lambda (n) (+ n 2)) (fset 'baz 'bar) => bar (symbol-function 'baz) => bar If you have never given a symbol any function definition, we say that that symbol's function cell is "void". In other words, the function cell does not have any Lisp object in it. If you try to call such a symbol as a function, it signals a `void-function' error. Note that void is not the same as `nil' or the symbol `void'. The symbols `nil' and `void' are Lisp objects, and can be stored into a function cell just as any other object can be (and they can be valid functions if you define them in turn with `defun'). A void function cell contains no object whatsoever. You can test the voidness of a symbol's function definition with `fboundp'. After you have given a symbol a function definition, you can make it void once more using `fmakunbound'. - Function: fboundp symbol This function returns `t' if SYMBOL has an object in its function cell, `nil' otherwise. It does not check that the object is a legitimate function. - Function: fmakunbound symbol This function makes SYMBOL's function cell void, so that a subsequent attempt to access this cell will cause a `void-function' error. (See also `makunbound', in *Note Local Variables::.) (defun foo (x) x) => x (foo 1) =>1 (fmakunbound 'foo) => x (foo 1) error--> Symbol's function definition is void: foo - Function: fset symbol object This function stores OBJECT in the function cell of SYMBOL. The result is OBJECT. Normally OBJECT should be a function or the name of a function, but this is not checked. There are three normal uses of this function: * Copying one symbol's function definition to another. (In other words, making an alternate name for a function.) * Giving a symbol a function definition that is not a list and therefore cannot be made with `defun'. For example, you can use `fset' to give a symbol SYMBOL1 a function definition which is another symbol SYMBOL2; then SYMBOL1 serves as an alias for whatever definition SYMBOL2 presently has. * In constructs for defining or altering functions. If `defun' were not a primitive, it could be written in Lisp (as a macro) using `fset'. Here are examples of the first two uses: ;; Give `first' the same definition `car' has. (fset 'first (symbol-function 'car)) => # (first '(1 2 3)) => 1 ;; Make the symbol `car' the function definition of `xfirst'. (fset 'xfirst 'car) => car (xfirst '(1 2 3)) => 1 (symbol-function 'xfirst) => car (symbol-function (symbol-function 'xfirst)) => # ;; Define a named keyboard macro. (fset 'kill-two-lines "\^u2\^k") => "\^u2\^k" See also the related functions `define-function' and `defalias', in *Note Defining Functions::. When writing a function that extends a previously defined function, the following idiom is sometimes used: (fset 'old-foo (symbol-function 'foo)) (defun foo () "Just like old-foo, except more so." (old-foo) (more-so)) This does not work properly if `foo' has been defined to autoload. In such a case, when `foo' calls `old-foo', Lisp attempts to define `old-foo' by loading a file. Since this presumably defines `foo' rather than `old-foo', it does not produce the proper results. The only way to avoid this problem is to make sure the file is loaded before moving aside the old definition of `foo'. But it is unmodular and unclean, in any case, for a Lisp file to redefine a function defined elsewhere.