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Info file elisp, produced by Makeinfo, -*- Text -*- from input file elisp.texi. This file documents GNU Emacs Lisp. This is edition 1.03 of the GNU Emacs Lisp Reference Manual, for Emacs Version 18. Published by the Free Software Foundation, 675 Massachusetts Avenue, Cambridge, MA 02139 USA Copyright (C) 1990 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. 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. 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 y (1+ x)) ; before 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 be set. The most-local existing binding of the variable is the binding that is set; shadowed bindings are not affected. If SYMBOL is not actually a symbol, a `wrong-type-argument' error is signaled. (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 Logically speaking, `set' is a more fundamental primitive that `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 needed only when the choice of variable to be set is made at run time. For example, the command `set-variable', which reads a variable name from the user and then sets the variable, needs to use `set'. Common Lisp note: in Common Lisp, `set' always changes the symbol's special value, ignoring any lexical bindings. In Emacs Lisp, all variables and all bindings are special, so `set' always affects the most local existing binding. File: elisp, Node: Variable Scoping, Next: Buffer-Local Variables, Prev: Setting Variables, Up: Variables Scoping Rules for Variable Bindings =================================== A given symbol `foo' may have several local variable bindings, established at different places in the Lisp program, as well as a global binding. The most recently established binding takes precedence over the others. Local bindings in Emacs Lisp have "indefinite scope" and "dynamic extent". "Scope" refers to *where* textually in the source code the binding can be accessed. Indefinite scope means that any part of the program can potentially access the variable binding. "Extent" refers to *when*, as the program is executing, the binding exists. Dynamic extent means that the binding lasts as long as the activation of the construct that established it. The combination of dynamic extent and indefinite scope is called "dynamic scoping". By contrast, most programming languages use "lexical scoping", in which references to a local variable must be textually within the function or block that binds the variable. Common Lisp note: variables declared "special" in Common Lisp are dynamically scoped like variables in Emacs Lisp. * Menu: * Scope:: Scope means where in the program a value is visible. Comparison with other languages. * Extent:: Extent means how long in time a value exists. * Impl of Scope:: Two ways to implement dynamic scoping. * Using Scoping:: How to use dynamic scoping carefully and avoid problems. File: elisp, Node: Scope, Next: Extent, Prev: Variable Scoping, Up: Variable Scoping Scope ----- Emacs Lisp uses "indefinite scope" for local variable bindings. This means that any function anywhere in the program text might access a given binding of a variable. Consider the following function definitions: (defun binder (x) ; `x' is bound in `binder'. (foo 5)) ; `foo' is some other function. (defun user () ; `x' is used in `user'. (list x)) In a lexically scoped language, the binding of `x' from `binder' would never be accessible in `user', because `user' is not textually contained within the function `binder'. However, in dynamically scoped Emacs Lisp, `user' may or may not refer to the binding of `x' established in `binder', depending on circumstances: * If we call `user' directly without calling `binder' at all, then whatever binding of `x' is found, it cannot come from `binder'. * If we define `foo' as follows and call `binder', then the binding made in `binder' will be seen in `user': (defun foo (lose) (user)) * If we define `foo' as follows and call `binder', then the binding made in `binder' *will not* be seen in `user': (defun foo (x) (user)) Here, when `foo' is called by `binder', it binds `x'. (The binding in `foo' is said to "shadow" the one made in `binder'.) Therefore, `user' will access the `x' bound by `foo' instead of the one bound by `binder'. File: 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 work as intended in Emacs Lisp, because after the call to `make-add' exits, the variable `n' is no longer bound to the actual argument 2. (defun make-add (n) (function (lambda (m) (+ n m)))) ; Return a function. => make-add (fset 'add2 (make-add 2)) ; Define function `add2' with `(make-add 2)'. => (lambda (m) (+ n m)) (add2 4) ; Try to add 2 to 4. error--> Symbol's value as variable is void: n 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: variable-value pairs. At entry to a function or to a `let' form, we can push bindings on the stack for the arguments or local variables created there. We can pop those bindings from the stack at exit from the binding construct. We can find the value of a variable by searching the stack from top to bottom for a binding for that variable; the value from that binding is the value of the variable. To set the variable, we search for the current binding, then store the new value into that binding. As you can see, a function's bindings remain in effect as long as it continues execution, even during its calls to other functions. That is why we say the extent of the binding is dynamic. And any other function can refer to the bindings, if it uses the same variables while the bindings are in effect. That is why we say the scope is indefinite. The actual implementation of variable scoping in 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. When a new local binding is created, the local value is stored in the value cell, and the old value (belonging to a previous binding) is pushed on a stack. When a binding is eliminated, the old value is popped off the stack and stored in 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. File: elisp, Node: Buffer-Local Variables, Prev: Variable Scoping, Up: Variables Buffer-Local Variables ====================== Global and local variable bindings are found in most programming languages in one form or another. Emacs also supports another, unusual kind of variable binding: "buffer-local" bindings, which apply only to one buffer. Emacs Lisp is meant for programming editing commands, and having different values for a variable in different buffers is an important customization method. * Menu: * Intro to Buffer-Local:: Introduction and concepts. * Creating Buffer-Local:: Creating and destroying buffer-local bindings. * Default Value:: The default value is seen in buffers that don't have their own local values. File: elisp, Node: Intro to Buffer-Local, Next: Creating Buffer-Local, Prev: Buffer-Local Variables, 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. When a variable has 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'. The best way to preserve your sanity is to avoid such situations. If you use `save-excursion' around each piece of code that changes to a different current buffer, you will not have this problem. Here is an example of incorrect code: (setq foo 'b) (set-buffer "a") (make-local-variable 'foo) (setq foo 'a) (let ((foo 'temp)) (set-buffer "b") ...) foo => 'a ; The old buffer-local value from buffer `a' ; is now the default value. (set-buffer "a") foo => 'temp ; The local 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") ...)) Local variables in a file you edit are also represented by buffer-local bindings for the buffer that holds the file within Emacs. *Note Auto Major Mode::. File: elisp, Node: Creating Buffer-Local, Next: Default Value, Prev: Intro to Buffer-Local, Up: Buffer-Local Variables Creating and Destroying Buffer-local Bindings --------------------------------------------- * Command: make-local-variable SYMBOL This function creates a buffer-local binding for SYMBOL in the current buffer. Other buffers are not affected. The value returned is SYMBOL. The buffer-local value of SYMBOL starts out as the same value SYMBOL previously had. ;; 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 the value. => 5 (setq foo 6) ; Change the value in `b1'. => 6 foo => 6 ;; In buffer `b2', the value hasn't changed. (save-excursion (set-buffer "b2") foo) => 5 * Command: make-variable-buffer-local SYMBOL This function marks SYMBOL automatically buffer-local, so that any attempt to set it will make it local to the current buffer at the time. The value returned is SYMBOL. * Function: buffer-local-variables &optional BUFFER This function tells you what the buffer-local variables are in buffer BUFFER. It returns an association list (*note Association Lists::.) in which each association contains one buffer-local variable and its value. If BUFFER is omitted, the current buffer is used. (setq lcl (buffer-local-variables)) => ((fill-column . 75) (case-fold-search . t) ... (mark-ring #<marker at 5454 in buffers.texi>) (require-final-newline . t)) Note that storing new values into the CDRs of the elements in this list will *not* change the local values of the variables. * Command: kill-local-variable SYMBOL This function deletes the buffer-local binding (if any) for SYMBOL in the current buffer. As a result, the global (default) binding of SYMBOL becomes visible in this buffer. Usually this results in a change in the value of SYMBOL, since the global value is usually different from the buffer-local value just eliminated. It is possible to kill the local binding of a variable that automatically becomes local when set. This causes the variable to show its global value in the current buffer. However, if you set the variable again, this will once again create a local value. `kill-local-variable' returns SYMBOL. * Function: kill-all-local-variables This function eliminates all the buffer-local variable bindings of the current buffer. As a result, the buffer will see the default values of all variables. This function also resets certain other information pertaining to the buffer: its local keymap is set to `nil', its syntax table is set to the value of `standard-syntax-table', and its abbrev table is set 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. `kill-all-local-variables' returns `nil'. 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 except when specifically overridden. The functions `default-value' and `setq-default' allow you to access and change the 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 which has a buffer-local value for this variable. * 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::.). * Special Form: setq-default SYMBOL VALUE This sets the default value of SYMBOL to VALUE. SYMBOL is not evaluated, but VALUE is. 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, this 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: 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. * 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, Prev: Functions, 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 will 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.) Primitives provide the lowest-level interfaces to editing functions or operating system services, or in a few cases they perform important operations more quickly than a Lisp program could. 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. *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. Strings 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 programmers, the distinction is normally unimportant. * 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 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", even though it is not an expression at all, for historical reasons.) * 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: elisp, Node: Lambda Components, Next: Simple Lambda, Prev: Lambda Expressions, 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 argument variable names (symbols). 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 an actual string that serves to describe the function for the Emacs 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: elisp, 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) The body of this lambda expression is evaluated 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)) Here all the arguments `1', `(* 2 3)', and `(- 5 4)' are evaluated, left to right. Then the lambda expression is applied 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: elisp, 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 will 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 end of the string if you omit it. It is also convenient for certain functions to accept an indefinite number of arguments, as the functions `and' 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 more actual arguments than these unless `&rest' exists. 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'. 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 means to use the length of the string supplied. There is no way for the function to distinguish between an explicit argument of `nil' and an omitted argument. Common Lisp note: Common Lisp allows the function to specify what default values will be used when an optional argument is omitted; GNU Emacs 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. If three actual arguments are given; then 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: elisp, 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 Emacs help facilities. *Note Documentation::, for how the DOCUMENTATION-STRING is accessed. It is a good idea to provide documentation strings for all commands, and for all other functions in your program that users of your program should know about; internal functions might as well have only comments, since comments don't take up any room when your program is loaded. 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, 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. *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: elisp, 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". 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 more convenient to refer to them by their names in other functions. 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). Functions are often identified 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 very 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: elisp, Node: Defining Functions, Next: Calling Functions, Prev: Function Names, Up: Functions Defining Named 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) This lambda expression is stored in the function cell of NAME. The value returned by evaluating the `defun' form is 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. Note that the same symbol NAME may also be used as a global variable, since the value cell is independent of the function cell. 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 () "This function makes the last letter of a word upper case." (interactive) (backward-word 1) (forward-word 1) (backward-char 1) (capitalize-word 1)) => capitalize-backwards Be careful not to redefine existing functions unintentionally. `defun' will redefine 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. File: elisp, 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. This process 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'. *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 the choice of which function to call is made when you write the program. Usually that's just what you want. Occasionally you need to decide at run time which function to call. Then you can 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 this example with that 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 this list is "appended" to the other arguments. `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) An interesting example of using `apply' is found in the description of `mapcar'; see the following section. 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 such a function, 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: elisp, 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. Emacs 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. The results are made into a `nil'-terminated list. 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, until one ends. Return the list of results." (if (not (memq 'nil args)) ; If no list is exhausted, (cons (apply f (mapcar 'car args)) ; Apply function to CARs. (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 returns 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: elisp, Node: Anonymous Functions, Next: Function Cells, Prev: Mapping Functions, Up: Functions Anonymous Functions =================== In Lisp, a function is a list that starts with `lambda' (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 Emacs Lisp compiler that FUNCTION-OBJECT is intended to be used only as a function, and therefore can safely be compiled. *Note Quoting::, for comparison. 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 (arg) (+ arg 5)) 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.