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This is Info file ../../info/lispref.info, produced by Makeinfo version 1.68 from the input file lispref.texi. Edition History: GNU Emacs Lisp Reference Manual Second Edition (v2.01), May 1993 GNU Emacs Lisp Reference Manual Further Revised (v2.02), August 1993 Lucid Emacs Lisp Reference Manual (for 19.10) First Edition, March 1994 XEmacs Lisp Programmer's Manual (for 19.12) Second Edition, April 1995 GNU Emacs Lisp Reference Manual v2.4, June 1995 XEmacs Lisp Programmer's Manual (for 19.13) Third Edition, July 1995 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) v3.2, April, May 1997 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: 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 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 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 `#<subr car>' 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 `#<subr car>', 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: #<subr and> 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. - Function: 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 three such functions; `mapcar' and `mapconcat', which scan a list, are described here. For the third mapping function, `mapatoms', see *Note Creating Symbols::. - Function: mapcar FUNCTION SEQUENCE `mapcar' applies FUNCTION to each element of SEQUENCE in turn, and returns a list of the results. The argument SEQUENCE may be a list, a 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. (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 the 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 `s1' a function definition which is another symbol `s2'; then `s1' serves as an alias for whatever definition `s2' 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)) => #<subr 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)) => #<subr car> ;; 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. File: lispref.info, Node: Inline Functions, Next: Related Topics, Prev: Function Cells, Up: Functions Inline Functions ================ You can define an "inline function" by using `defsubst' instead of `defun'. An inline function works just like an ordinary function except for one thing: when you compile a call to the function, the function's definition is open-coded into the caller. Making a function inline makes explicit calls run faster. But it also has disadvantages. For one thing, it reduces flexibility; if you change the definition of the function, calls already inlined still use the old definition until you recompile them. Since the flexibility of redefining functions is an important feature of XEmacs, you should not make a function inline unless its speed is really crucial. Another disadvantage is that making a large function inline can increase the size of compiled code both in files and in memory. Since the speed advantage of inline functions is greatest for small functions, you generally should not make large functions inline. It's possible to define a macro to expand into the same code that an inline function would execute. But the macro would have a limitation: you can use it only explicitly--a macro cannot be called with `apply', `mapcar' and so on. Also, it takes some work to convert an ordinary function into a macro. (*Note Macros::.) To convert it into an inline function is very easy; simply replace `defun' with `defsubst'. Since each argument of an inline function is evaluated exactly once, you needn't worry about how many times the body uses the arguments, as you do for macros. (*Note Argument Evaluation::.) Inline functions can be used and open-coded later on in the same file, following the definition, just like macros. File: lispref.info, Node: Related Topics, Prev: Inline Functions, Up: Functions Other Topics Related to Functions ================================= Here is a table of several functions that do things related to function calling and function definitions. They are documented elsewhere, but we provide cross references here. `apply' See *Note Calling Functions::. `autoload' See *Note Autoload::. `call-interactively' See *Note Interactive Call::. `commandp' See *Note Interactive Call::. `documentation' See *Note Accessing Documentation::. `eval' See *Note Eval::. `funcall' See *Note Calling Functions::. `ignore' See *Note Calling Functions::. `indirect-function' See *Note Function Indirection::. `interactive' See *Note Using Interactive::. `interactive-p' See *Note Interactive Call::. `mapatoms' See *Note Creating Symbols::. `mapcar' See *Note Mapping Functions::. `mapconcat' See *Note Mapping Functions::. `undefined' See *Note Key Lookup::. File: lispref.info, Node: Macros, Next: Loading, Prev: Functions, Up: Top Macros ****** "Macros" enable you to define new control constructs and other language features. A macro is defined much like a function, but instead of telling how to compute a value, it tells how to compute another Lisp expression which will in turn compute the value. We call this expression the "expansion" of the macro. Macros can do this because they operate on the unevaluated expressions for the arguments, not on the argument values as functions do. They can therefore construct an expansion containing these argument expressions or parts of them. If you are using a macro to do something an ordinary function could do, just for the sake of speed, consider using an inline function instead. *Note Inline Functions::. * Menu: * Simple Macro:: A basic example. * Expansion:: How, when and why macros are expanded. * Compiling Macros:: How macros are expanded by the compiler. * Defining Macros:: How to write a macro definition. * Backquote:: Easier construction of list structure. * Problems with Macros:: Don't evaluate the macro arguments too many times. Don't hide the user's variables. File: lispref.info, Node: Simple Macro, Next: Expansion, Up: Macros A Simple Example of a Macro =========================== Suppose we would like to define a Lisp construct to increment a variable value, much like the `++' operator in C. We would like to write `(inc x)' and have the effect of `(setq x (1+ x))'. Here's a macro definition that does the job: (defmacro inc (var) (list 'setq var (list '1+ var))) When this is called with `(inc x)', the argument `var' has the value `x'--*not* the *value* of `x'. The body of the macro uses this to construct the expansion, which is `(setq x (1+ x))'. Once the macro definition returns this expansion, Lisp proceeds to evaluate it, thus incrementing `x'. File: lispref.info, Node: Expansion, Next: Compiling Macros, Prev: Simple Macro, Up: Macros Expansion of a Macro Call ========================= A macro call looks just like a function call in that it is a list which starts with the name of the macro. The rest of the elements of the list are the arguments of the macro. Evaluation of the macro call begins like evaluation of a function call except for one crucial difference: the macro arguments are the actual expressions appearing in the macro call. They are not evaluated before they are given to the macro definition. By contrast, the arguments of a function are results of evaluating the elements of the function call list. Having obtained the arguments, Lisp invokes the macro definition just as a function is invoked. The argument variables of the macro are bound to the argument values from the macro call, or to a list of them in the case of a `&rest' argument. And the macro body executes and returns its value just as a function body does. The second crucial difference between macros and functions is that the value returned by the macro body is not the value of the macro call. Instead, it is an alternate expression for computing that value, also known as the "expansion" of the macro. The Lisp interpreter proceeds to evaluate the expansion as soon as it comes back from the macro. Since the expansion is evaluated in the normal manner, it may contain calls to other macros. It may even be a call to the same macro, though this is unusual. You can see the expansion of a given macro call by calling `macroexpand'. - Function: macroexpand FORM &optional ENVIRONMENT This function expands FORM, if it is a macro call. If the result is another macro call, it is expanded in turn, until something which is not a macro call results. That is the value returned by `macroexpand'. If FORM is not a macro call to begin with, it is returned as given. Note that `macroexpand' does not look at the subexpressions of FORM (although some macro definitions may do so). Even if they are macro calls themselves, `macroexpand' does not expand them. The function `macroexpand' does not expand calls to inline functions. Normally there is no need for that, since a call to an inline function is no harder to understand than a call to an ordinary function. If ENVIRONMENT is provided, it specifies an alist of macro definitions that shadow the currently defined macros. Byte compilation uses this feature. (defmacro inc (var) (list 'setq var (list '1+ var))) => inc (macroexpand '(inc r)) => (setq r (1+ r)) (defmacro inc2 (var1 var2) (list 'progn (list 'inc var1) (list 'inc var2))) => inc2 (macroexpand '(inc2 r s)) => (progn (inc r) (inc s)) ; `inc' not expanded here. File: lispref.info, Node: Compiling Macros, Next: Defining Macros, Prev: Expansion, Up: Macros Macros and Byte Compilation =========================== You might ask why we take the trouble to compute an expansion for a macro and then evaluate the expansion. Why not have the macro body produce the desired results directly? The reason has to do with compilation. When a macro call appears in a Lisp program being compiled, the Lisp compiler calls the macro definition just as the interpreter would, and receives an expansion. But instead of evaluating this expansion, it compiles the expansion as if it had appeared directly in the program. As a result, the compiled code produces the value and side effects intended for the macro, but executes at full compiled speed. This would not work if the macro body computed the value and side effects itself--they would be computed at compile time, which is not useful. In order for compilation of macro calls to work, the macros must be defined in Lisp when the calls to them are compiled. The compiler has a special feature to help you do this: if a file being compiled contains a `defmacro' form, the macro is defined temporarily for the rest of the compilation of that file. To use this feature, you must define the macro in the same file where it is used and before its first use. Byte-compiling a file executes any `require' calls at top-level in the file. This is in case the file needs the required packages for proper compilation. One way to ensure that necessary macro definitions are available during compilation is to require the files that define them (*note Named Features::.). To avoid loading the macro definition files when someone *runs* the compiled program, write `eval-when-compile' around the `require' calls (*note Eval During Compile::.). File: lispref.info, Node: Defining Macros, Next: Backquote, Prev: Compiling Macros, Up: Macros Defining Macros =============== A Lisp macro is a list whose CAR is `macro'. Its CDR should be a function; expansion of the macro works by applying the function (with `apply') to the list of unevaluated argument-expressions from the macro call. It is possible to use an anonymous Lisp macro just like an anonymous function, but this is never done, because it does not make sense to pass an anonymous macro to functionals such as `mapcar'. In practice, all Lisp macros have names, and they are usually defined with the special form `defmacro'. - Special Form: defmacro NAME ARGUMENT-LIST BODY-FORMS... `defmacro' defines the symbol NAME as a macro that looks like this: (macro lambda ARGUMENT-LIST . BODY-FORMS) This macro object is stored in the function cell of NAME. The value returned by evaluating the `defmacro' form is NAME, but usually we ignore this value. The shape and meaning of ARGUMENT-LIST is the same as in a function, and the keywords `&rest' and `&optional' may be used (*note Argument List::.). Macros may have a documentation string, but any `interactive' declaration is ignored since macros cannot be called interactively.