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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: Vectors, Prev: Array Functions, Up: Sequences Arrays Vectors Vectors ======= Arrays in Lisp, like arrays in most languages, are blocks of memory whose elements can be accessed in constant time. A "vector" is a general-purpose array; its elements can be any Lisp objects. (The other kind of array provided in Emacs Lisp is the "string", whose elements must be characters.) The main uses of vectors in Emacs are as syntax tables (vectors of integers) and keymaps (vectors of commands). They are also used internally as part of the representation of a byte-compiled function; if you print such a function, you will see a vector in it. The indices of the elements of a vector are numbered starting with zero in Emacs Lisp. Vectors are printed with square brackets surrounding the elements in their order. Thus, a vector containing the symbols `a', `b' and `c' is printed as `[a b c]'. You can write vectors in the same way in Lisp input. A vector, like a string or a number, is considered a constant for evaluation: the result of evaluating it is the same vector. The elements of the vector are not evaluated. *Note Self-Evaluating Forms::. Here are examples of these principles: (setq avector [1 two '(three) "four" [five]]) => [1 two (quote (three)) "four" [five]] (eval avector) => [1 two (quote (three)) "four" [five]] (eq avector (eval avector)) => t Here are some functions that relate to vectors: * Function: vectorp OBJECT This function returns `t' if OBJECT is a vector. (vectorp [a]) => t (vectorp "asdf") => nil * Function: vector &rest OBJECTS This function creates and returns a vector whose elements are the arguments, OBJECTS. (vector 'foo 23 [bar baz] "rats") => [foo 23 [bar baz] "rats"] (vector) => [] * Function: make-vector INTEGER OBJECT This function returns a new vector consisting of INTEGER elements, each initialized to OBJECT. (setq sleepy (make-vector 9 'Z)) => [Z Z Z Z Z Z Z Z Z] * Function: vconcat &rest SEQUENCES This function returns a new vector containing all the elements of the SEQUENCES. The arguments SEQUENCES may be lists, vectors, or strings. If no SEQUENCES are given, an empty vector is returned. The value is a newly constructed vector that is not `eq' to any existing vector. (setq a (vconcat '(A B C) '(D E F))) => [A B C D E F] (eq a (vconcat a)) => nil (vconcat) => [] (vconcat [A B C] "aa" '(foo (6 7))) => [A B C 97 97 foo (6 7)] When an argument is an integer (not a sequence of integers), it is converted to a string of digits making up the decimal printed representation of the integer. This special case exists for compatibility with Mocklisp, and we don't recommend you take advantage of it. If you want to convert an integer in this way, use `format' (*note Formatting Strings::.) or `int-to-string' (*note String Conversion::.). For other concatenation functions, see `mapconcat' in *Note Mapping Functions::, `concat' in *Note Creating Strings::, and `append' in *Note Building Lists::. The `append' function may be used to convert a vector into a list with the same elements (*note Building Lists::.): (setq avector [1 two (quote (three)) "four" [five]]) => [1 two (quote (three)) "four" [five]] (append avector nil) => (1 two (quote (three)) "four" [five]) File: elisp, Node: Symbols, Next: Evaluation, Prev: Sequences Arrays Vectors, Up: Top Symbols ******* A "symbol" is an object with a unique name. This chapter describes symbols, their components, and how they are created and interned. Property lists are also described. The uses of symbols as variables and as function names are described in separate chapters; see *Note Variables::, and *Note Functions::. You may test whether an arbitrary Lisp object is a symbol with `symbolp': * Function: symbolp OBJECT This function returns `t' if OBJECT is a symbol, `nil' otherwise. * Menu: * Symbol Components:: Symbols have names, values, function definitions and property lists. * Definitions:: A definition says how a symbol will be used. * Creating Symbols:: How symbols are kept unique. * Property Lists:: Each symbol has a property list for recording miscellaneous information. File: elisp, Node: Symbol Components, Next: Definitions, Prev: Symbols, Up: Symbols Symbol Components ================= Each symbol has four components (or "cells"), each of which references another object: Print name The "print name cell" holds a string which names the symbol for reading and printing. See `symbol-name' in *Note Creating Symbols::. Value The "value cell" holds the current value of the symbol as a variable. When a symbol is used as a form, the value of the form is the contents of the symbol's value cell. See `symbol-value' in *Note Accessing Variables::. Function The "function cell" holds the function definition of the symbol. When a symbol is used as a function, its function definition is used in its place. This cell is also used by the editor command loop to record keymaps and keyboard macros. Because each symbol has separate value and function cells, variables and function names do not conflict. See `symbol-function' in *Note Function Cells::. Property list The "property list cell" holds the property list of the symbol. See `symbol-plist' in *Note Property Lists::. The print name cell always holds a string, and cannot be changed. The other three cells can be set individually to any specified Lisp object. The print name cell holds the string that is the name of the symbol. Since symbols are represented textually by their names, it is important not to have two symbols with the same name. The Lisp reader ensures this: every time it reads a symbol, it looks for an existing symbol with the specified name before it creates a new one. (In GNU Emacs Lisp, this is done with a hashing algorithm that uses an obarray; see *Note Creating Symbols::.) In normal usage, the function cell usually contains a function or macro, as that is what the Lisp interpreter expects to see there (*note Evaluation::.). Keyboard macros (*note Keyboard Macros::.), keymaps (*note Keymaps::.) and autoload objects (*note Autoloading::.) are also sometimes stored in the function cell of symbols. We often refer to "the function `foo'" when we really mean the function stored in the function cell of the symbol `foo'. The distinction will be made only when necessary. Similarly, the property list cell normally holds a correctly formatted property list (*note Property Lists::.), as a number of functions will expect to see a property list there. The function cell or the value cell may be "void", which means that the cell does not reference any object. (This is not the same thing as holding the symbol `void', nor the same as holding the symbol `nil'.) Examining the value of a cell which is void results in an error, such as `Symbol's value as variable is void'. The four functions `symbol-name', `symbol-value', `symbol-plist', and `symbol-function' return the contents of the four cells. Here as an example we show the contents of the four cells of the symbol `buffer-file-name': (symbol-name 'buffer-file-name) => "buffer-file-name" (symbol-value 'buffer-file-name) => "/gnu/elisp/symbols.texi" (symbol-plist 'buffer-file-name) => (variable-documentation 29529) (symbol-function 'buffer-file-name) => #<subr buffer-file-name> Because this symbol is the variable which holds the name of the file being visited in the current buffer, the value cell contents we see are the name of the source file of this chapter of the Emacs Lisp Manual. The property list cell contains the list `(variable-documentation 29529)' which tells the documentation functions where to find documentation about `buffer-file-name' in the `DOC' file. (29529 is the offset from the beginning of the `DOC' file where the documentation for the function begins.) The function cell contains the function for returning the name of the file. Since `buffer-file-name' is a primitive function, its function definition has no read syntax and prints in hash notation (*note Primitive Function Type::.). A function definition written in Lisp will have a lambda expression (or byte-code) in this cell. File: elisp, Node: Definitions, Next: Creating Symbols, Prev: Symbol Components, Up: Symbols Defining Symbols ================ A "definition" in Lisp is a special form that announces your intention to use a certain symbol in a particular way. In Emacs Lisp, you can define a symbol as a variable, or define it as a function (or macro), or both independently. A definition construct typically specifies a value or meaning for the symbol for one kind of use, plus documentation for its meaning when used in this way. Thus, when you define a symbol as a variable, you can supply an initial value for the variable, plus documentation for the variable. `defvar' and `defconst' are definitions that establish a symbol as a global variable. They are documented in detail in *Note Defining Variables::. `defun' defines a symbol as a function, creating a lambda expression and storing it in the function cell of the symbol. This lambda expression thus becomes the function definition of the symbol. (The term "function definition", meaning the contents of the function cell, is derived from the idea that `defun' gives the symbol its definition as a function.) *Note Functions::. `defmacro' defines a symbol as a macro. It creates a macro object and stores it in the function cell of the symbol. Note that a given symbol can be a macro or a function, but not both at once, because both macro and function definitions are kept in the function cell, and that cell can hold only one Lisp object at any given time. *Note Macros::. In GNU Emacs Lisp, a definition is not required in order to use a symbol as a variable or function. Thus, you can make a symbol a global variable with `setq', whether you define it first or not. The real purpose of definitions is to guide programmers and programming tools. They inform programmers who read the code that certain symbols are *intended* to be used as variables, or as functions. In addition, utilities such as `etags' and `make-docfile' can recognize definitions, and add the appropriate information to tag tables and the `emacs/etc/DOC-VERSION' file. *Note Accessing Documentation::. File: elisp, Node: Creating Symbols, Next: Property Lists, Prev: Definitions, Up: Symbols Creating and Interning Symbols ============================== To understand how symbols are created in GNU Emacs Lisp, it is necessary to know how Lisp reads them. It is essential to ensure that every time Lisp reads the same set of characters, it finds the same symbol. Failure to do so would be disastrous. When the Lisp reader encounters a symbol, it reads all the characters of the name. Then it "hashes" those characters to find an index in a table called an "obarray". Hashing is an efficient method of looking something up. For example, instead of searching a telephone book cover to cover when looking up Jan Jones, you start with the J's and go from there. That is a simple version of hashing. Each element of the obarray is a "bucket" which holds all the symbols with a given hash code; to look for a given name, it is sufficient to look through all the symbols in the bucket for that name's hash code. If a symbol with the desired name is found, then it is used. If no such symbol is found, then a new symbol is created and added to the obarray bucket. Adding a symbol to an obarray is called "interning" it, and the symbol is then called an "interned symbol". In Emacs Lisp, a symbol may be interned in only one obarray. Common Lisp note: in Common Lisp, a symbol may be interned in several obarrays at once. If a symbol is not in the obarray, then there is no way for Lisp to find it when its name is read. Such a symbol is called an "uninterned symbol" relative to the obarray. An uninterned symbol has all the other characteristics of symbols. It is possible, though uncommon, for two different symbols to have the same name in different obarrays; they are not `eq' or `equal'. In Emacs Lisp, an obarray is represented as a vector. Each element of the vector is a bucket; its value is either an interned symbol whose name hashes to that bucket, or 0 if the bucket is empty. Each interned symbol has an internal link (invisible to the user) to the next symbol in the bucket. Because these links are invisible, there is no way to scan the symbols in an obarray except using `mapatoms' (below). The order of symbols in a bucket is not significant. In an empty obarray, every element is 0, and you can create an obarray with `(make-vector LENGTH 0)'. Prime numbers as lengths tend to result in good hashing; lengths one less than a power of two are also good. Most of the functions below take a name and sometimes an obarray as arguments. A `wrong-type-argument' error is signaled if the name is not a string, or if the obarray is not a vector. * Function: symbol-name SYMBOL This function returns the string that is SYMBOL's name. For example: (symbol-name 'foo) => "foo" Changing the string by substituting characters, etc, will change the name of the symbol, but will fail to update the obarray, so don't do it! * Function: make-symbol NAME This function returns a newly-allocated uninterned symbol whose name is NAME (which must be a string). Its value and function definition are void, and its property list is `nil'. In the example below, the value of `sym' is not `eq' to `foo' because it is a distinct uninterned symbol whose name is also `foo'. (setq sym (make-symbol "foo")) => foo (eq sym 'foo) => nil * Function: intern NAME &optional OBARRAY This function returns the interned symbol whose name is NAME. If there is no such symbol in the obarray, a new one is created, added to the obarray, and returned. If OBARRAY is supplied, it specifies the obarray to use; otherwise, the value of the global variable `obarray' is used. (setq sym (intern "foo")) => foo (eq sym 'foo) => t * Function: intern-soft NAME &optional OBARRAY This function returns the symbol whose name is NAME, or `nil' if a symbol with that name is not found in the obarray. Therefore, you can use `intern-soft' to test whether a symbol with a given name is interned. If OBARRAY is supplied, it specifies the obarray to use; otherwise the value of the global variable `obarray' is used. (intern-soft "frazzle") ; No such symbol exists. => nil (make-symbol "frazzle") ; Create an uninterned one. => frazzle (intern-soft "frazzle") ; That one cannot be found. => nil (setq sym (intern "frazzle")) ; Create an interned one. => frazzle (intern-soft "frazzle") ; That one can be found! => frazzle (eq sym 'frazzle) ; And it is the same one. => t * Variable: obarray This variable is the standard obarray for use by `intern' and `read'. * Function: mapatoms FUNCTION &optional OBARRAY This function applies FUNCTION to every symbol in OBARRAY. It returns `nil'. If OBARRAY is not supplied, it defaults to the value of `obarray', the standard obarray for ordinary symbols. (setq count 0) => 0 (defun count-syms (s) (setq count (1+ count))) => count-syms (mapatoms 'count-syms) => nil count => 1871 See `documentation' in *Note Accessing Documentation::, for another example using `mapatoms'. File: elisp, Node: Property Lists, Prev: Creating Symbols, Up: Symbols Property Lists ============== A "property list" ("plist" for short) is a list of paired elements stored in the property list cell of a symbol. Each of the pairs associates a property name (usually a symbol) with a property or value. Property lists are generally used to record information about a symbol, such as how to compile it, the name of the file where it was defined, or perhaps even the grammatical class of the symbol (representing a word) in a language understanding system. The property names and property values may be any Lisp objects, but the names are usually symbols. They are compared using `eq'. Here is an example of a property list, found on the symbol `progn' when the compiler is loaded: (lisp-indent-hook 0 byte-compile byte-compile-progn) Here `lisp-indent-hook' and `byte-compile' are property names, and the other two elements are the corresponding values. Association lists (*note Association Lists::.) are very similar to property lists. In contrast to association lists, the order of the pairs in the property list is not significant since the property names must be distinct. Property lists are better than association lists when it is necessary to attach information to various Lisp function names or variables. If all the pairs are recorded in one association list, it will be necessary to search that entire list each time a function or variable is to be operated on. By contrast, if the information is recorded in the property lists of the function names or variables themselves, each search will scan only the length of one property list, which is usually short. For this reason, the documentation for a variable is recorded in a property named `variable-documentation'. The byte compiler likewise uses properties to record those functions needing special treatment. However, association lists have their own advantages. Depending on your application, it may be faster to add an association to the front of an association list than to update a property. All properties for a symbol are stored in the same property list, so there is a possibility of a conflict between different uses of a property name. (For this reason, it is a good idea to use property names that are probably unique, such as by including the name of the library in the property name.) An association list may be used like a stack where associations are pushed on the front of the list and later discarded; this is not possible with a property list. * Function: symbol-plist SYMBOL This function returns the property list of SYMBOL. * Function: setplist SYMBOL PLIST This function sets SYMBOL's property list to PLIST. Normally, PLIST should be a well-formed property list, but this is not enforced. (setplist 'foo '(a 1 b (2 3) c nil)) => (a 1 b (2 3) c nil) (symbol-plist 'foo) => (a 1 b (2 3) c nil) For symbols in special obarrays, which are not used for ordinary purposes, it may make sense to use the property list cell in a nonstandard fashion; in fact, the abbrev mechanism does so (*note Abbrevs::.). * Function: get SYMBOL PROPERTY This function finds the value of the property named PROPERTY in SYMBOL's property list. If there is no such property, `nil' is returned. Thus, there is no distinction between a value of `nil' and the absence of the property. The name PROPERTY is compared with the existing property names using `eq', so any object is a legitimate property. See `put' for an example. * Function: put SYMBOL PROPERTY VALUE This function puts VALUE onto SYMBOL's property list under the property name PROPERTY, replacing any previous value. (put 'fly 'verb 'transitive) =>'transitive (put 'fly 'noun '(a buzzing little bug)) => (a buzzing little bug) (get 'fly 'verb) => transitive (symbol-plist 'fly) => (verb transitive noun (a buzzing little bug)) File: elisp, Node: Evaluation, Next: Control Structures, Prev: Symbols, Up: Top Evaluation ********** The "evaluation" of expressions in Emacs Lisp is performed by the "Lisp interpreter"--a program that receives a Lisp object as input and computes its "value as an expression". The value is computed in a fashion that depends on the data type of the object, following rules described in this chapter. The interpreter runs automatically to evaluate portions of your program, but can also be called explicitly via the Lisp primitive function `eval'. * Menu: * Intro Eval:: Evaluation in the scheme of things. * Eval:: How to invoke the Lisp interpreter explicitly. * Forms:: How various sorts of objects are evaluated. * Quoting:: Avoiding evaluation (to put constants in the program). File: elisp, Node: Intro Eval, Next: Eval, Prev: Evaluation, Up: Evaluation Introduction to Evaluation ========================== The Lisp interpreter, or evaluator, is the program which computes the value of an expression which is given to it. When a function written in Lisp is called, the evaluator computes the value of the function by evaluating the expressions in the function body. Thus, running any Lisp program really means running the Lisp interpreter. How the evaluator handles an object depends primarily on the data type of the object. A Lisp object which is intended for evaluation is called an "expression" or a "form". The fact that expressions are data objects and not merely text is one of the fundamental differences between Lisp-like languages and typical programming languages. Any object can be evaluated, but in practice only numbers, symbols, lists and strings are evaluated very often. It is very common to read a Lisp expression and then evaluate the expression, but reading and evaluation are separate activities, and either can be performed alone. Reading per se does not evaluate anything; it converts the printed representation of a Lisp object to the object itself. It is up to the caller of `read' whether this object is a form to be evaluated, or serves some entirely different purpose. *Note Input Functions::. Do not confuse evaluation with command key interpretation. The editor command loop translates keyboard input into a command (an interactively callable function) using the current keymaps, and then uses `call-interactively' to invoke the command. The execution of the command itself involves evaluation if the command is written in Lisp, but that is not a part of command key interpretation itself. *Note Command Loop::. Evaluation is a recursive process. That is, evaluation of a form may cause `eval' to be called again in order to evaluate parts of the form. For example, evaluation of a function call first evaluates each argument of the function call, and then evaluates each form in the function body. Consider evaluation of the form `(car x)': the subform `x' must first be evaluated recursively, so that its value can be passed as an argument to the function `car'. The evaluation of forms takes place in a context called the "environment", which consists of the current values and bindings of all Lisp variables. Whenever the form refers to a variable without creating a new binding for it, the value of the current binding is used. *Note Variables::. Evaluation of a form may create new environments for recursive evaluation by binding variables (*note Local Variables::.). These environments are temporary and will be gone by the time evaluation of the form is complete. The form may also make changes that persist; these changes are called "side-effects". An example of a form that produces side-effects is `(setq foo 1)'. Finally, evaluation of one particular function call, `byte-code', invokes the "byte-code interpreter" on its arguments. Although the byte-code interpreter is not the same as the Lisp interpreter, it uses the same environment as the Lisp interpreter, and may on occasion invoke the Lisp interpreter. (*Note Byte Compilation::.) The details of what evaluation means for each kind of form are described below (*note Forms::.). File: elisp, Node: Eval, Next: Forms, Prev: Intro Eval, Up: Evaluation Eval ==== Most often, forms are evaluated automatically, by virtue of their occurrence in a program being run. On rare occasions, you may need to write code that evaluates a form that is computed at run time, such as when the form is read from text being edited or found on a property list. On these occasions, use the `eval' function. The functions and variables described in this section evaluate forms, specify limits to the evaluation process, or record recently returned values. Evaluation is also performed by `load' (*note Loading::.). * Function: eval FORM This is the basic function for performing evaluation. It evaluates FORM in the current environment and returns the result. How the evaluation proceeds depends on the type of the object (*note Forms::.). Since `eval' is a function, the argument expression that appears in a call to `eval' is evaluated twice: once as preparation before `eval' is called, and again by the `eval' function itself. Here is an example: (setq foo 'bar) => bar (setq bar 'baz) => baz ;; `eval' is called on the form `bar', which is the value of `foo' (eval foo) => baz The number of currently active calls to `eval' is limited to `max-lisp-eval-depth'. * Command: eval-current-buffer &optional STREAM This function evaluates the forms in the current buffer. It reads forms from the buffer and calls `eval' on them until the end of the buffer is reached, or until an error is signaled and not handled. If STREAM is supplied, the variable `standard-output' is bound to STREAM during the evaluation (*note Output Functions::.). `eval-current-buffer' always returns `nil'. * Command: eval-region START END &optional STREAM This function evaluates the forms in the current buffer in the region defined by the positions START and END. It reads forms from the region and calls `eval' on them until the end of the region is reached, or until an error is signaled and not handled. If STREAM is supplied, `standard-output' is bound to it for the duration of the command. `eval-region' always returns `nil'. * Variable: max-lisp-eval-depth This variable defines the maximum depth allowed in calls to `eval', `apply', and `funcall' before an error is signaled (with error message `"Lisp nesting exceeds max-lisp-eval-depth"'). `eval' is called recursively to evaluate the arguments of Lisp function calls and to evaluate bodies of functions. This limit, with the associated error when it is exceeded, is one way that Lisp avoids infinite recursion on an ill-defined function. The default value of this variable is 200. If you set it to a value less than 100, Lisp will reset it to 100 if the given value is reached. * Variable: values The value of this variable is a list of values returned by all expressions which were read from buffers (including the minibuffer), evaluated, and printed. The elements are in order, most recent first. (setq x 1) => 1 (list 'A (1+ 2) auto-save-default) => (A 3 t) values => ((A 3 t) 1 ...) This variable is useful for referring back to values of forms recently evaluated. It is generally a bad idea to print the value of `values' itself, since this may be very long. Instead, examine particular elements, like this: ;; Refer to the most recent evaluation result. (nth 0 values) => (A 3 t) ;; That put a new element on, so all elements move back one. (nth 1 values) => (A 3 t) ;; This gets the element that was next-to-last before this example. (nth 3 values) => 1 File: elisp, Node: Forms, Next: Quoting, Prev: Eval, Up: Evaluation Kinds of Forms ============== A Lisp object that is intended to be evaluated is called a "form". How Emacs evaluates a form depends on its data type. Emacs has three different kinds of form that are evaluated differently: symbols, lists, and "all other types". All three kinds are described in this section, starting with "all other types" which are self-evaluating forms. * Menu: * Self-Evaluating Forms:: Forms that evaluate to themselves. * Symbol Forms:: Symbols evaluate as variables. * Classifying Lists:: How to distinguish various sorts of list forms. * Function Forms:: Forms that call functions. * Macro Forms:: Forms that call macros. * Special Forms:: "Special forms" are idiosyncratic primitives, most of them extremely important. * Autoloading:: Functions set up to load files containing their real definitions. File: elisp, Node: Self-Evaluating Forms, Next: Symbol Forms, Prev: Forms, Up: Forms Self-Evaluating Forms --------------------- A "self-evaluating form" is any form that is not a list or symbol. Self-evaluating forms evaluate to themselves: the result of evaluation is the same object that was evaluated. Thus, the number 25 evaluates to 25, and the string `"foo"' evaluates to the string `"foo"'. Likewise, evaluation of a vector does not cause evaluation of the elements of the vector--it returns the same vector with its contents unchanged. '123 ; An object, shown without evaluation. => 123 123 ; Evaluated as usual---result is the same. => 123 (eval '123) ; Evaluated ``by hand''---result is the same. => 123 (eval (eval '123)) ; Evaluating twice changes nothing. => 123 It is common to write numbers, characters, strings, and even vectors in Lisp code, taking advantage of the fact that they self-evaluate. However, it is quite unusual to do this for types that lack a read syntax, because it is inconvenient and not very useful; however, it is possible to put them inside Lisp programs when they are constructed from subexpressions rather than read. Here is an example: ;; Build such an expression. (setq buffer (list 'print (current-buffer))) => (print #<buffer eval.texi>) ;; Evaluate it. (eval buffer) -| #<buffer eval.texi> => #<buffer eval.texi> File: elisp, Node: Symbol Forms, Next: Classifying Lists, Prev: Self-Evaluating Forms, Up: Forms Symbol Forms ------------ When a symbol is evaluated, it is treated as a variable. The result is the variable's value, if it has one. If it has none (if its value cell is void), an error is signaled. For more information on the use of variables, see *Note Variables::. In the following example, the value of a symbol is set with `setq'. When the symbol is later evaluated, that value is returned. (setq a 123) => 123 (eval 'a) => 123 a => 123 The symbols `nil' and `t' are treated specially, so that the value of `nil' is always `nil', and the value of `t' is always `t'. Thus, these two symbols act like self-evaluating forms, even though `eval' treats them like any other symbol. File: elisp, Node: Classifying Lists, Next: Function Forms, Prev: Symbol Forms, Up: Forms Classification of List Forms ---------------------------- A form that is a nonempty list is either a function call, a macro call, or a special form, according to its first element. These three kinds of forms are evaluated in different ways, described below. The rest of the list consists of "arguments" for the function, macro or special form. The first step in evaluating a nonempty list is to examine its first element. This element alone determines what kind of form the list is and how the rest of the list is to be processed. The first element is *not* evaluated, as it would be in some Lisp dialects including Scheme. If the first element of the list is a symbol, as it most commonly is, then the symbol's function cell is examined, and its contents are used instead of the original symbol. If the contents are another symbol, this process, called "symbol function indirection", is repeated until a non-symbol is obtained. One possible consequence of this process is an infinite loop, in the event that a symbol's function cell refers to the same symbol. Or a symbol may have a void function cell, causing a `void-function' error. But if neither of these things happens, we eventually obtain a non-symbol, which ought to be a function or other suitable object. More precisely, we should now have a Lisp function (a lambda expression), a primitive function, a Lisp macro, a special form, or an autoload object. Each of these types is a case described in one of the following sections. If the object is not one of these types, the error `invalid-function' is signaled. The following example illustrates the symbol indirection process. We use `fset' to set the function cell of a symbol and `symbol-function' to get the function cell contents (*note Function Cells::.). Specifically, we store the symbol `car' into the function cell of `first', and the symbol `first' into the function cell of `erste'. ;; Build this function cell linkage: ;; ------------- ----- ------- ------- ;; | #<subr car> | <-- | car | <-- | first | <-- | erste | ;; ------------- ----- ------- ------- (symbol-function 'car) => #<subr car> (fset 'first 'car) => car (fset 'erste 'first) => first (erste '(1 2 3)) ; Call the function referenced by `erste'. => 1 By contrast, the following example calls a function without any symbol function indirection, because the first element is an anonymous Lisp function, not a symbol. ((lambda (arg) (erste arg)) '(1 2 3)) => 1 After that function is called, its body is evaluated; this does involve symbol function indirection when calling `erste'. File: elisp, Node: Function Forms, Next: Macro Forms, Prev: Classifying Lists, Up: Forms Evaluation of Function Forms ---------------------------- If the first element of a list being evaluated is a Lisp function object or primitive function object, then that list is a "function call". For example, here is a call to the function `+': (+ 1 x) When a function call is evaluated, the first step is to evaluate the remaining elements of the list in the order they appear. The results are the actual argument values, one argument from each element. Then the function is called with this list of arguments, effectively using the function `apply' (*note Calling Functions::.). If the function is written in Lisp, the arguments are used to bind the argument variables of the function (*note Lambda Expressions::.); then the forms in the function body are evaluated in order, and the result of the last one is used as the value of the function call. File: elisp, Node: Macro Forms, Next: Special Forms, Prev: Function Forms, Up: Forms Lisp Macro Evaluation --------------------- If the first element of a list being evaluated is a macro object, then the list is a "macro call". When a macro call is evaluated, the elements of the rest of the list are *not* initially evaluated. Instead, these elements themselves are used as the arguments of the macro. The macro definition computes a replacement form, called the "expansion" of the macro, which is evaluated in place of the original form. The expansion may be any sort of form: a self-evaluating constant, a symbol or a list. If the expansion is itself a macro call, this process of expansion repeats until some other sort of form results. Normally, the argument expressions are not evaluated as part of computing the macro expansion, but instead appear as part of the expansion, so they are evaluated when the expansion is evaluated. For example, given a macro defined as follows: (defmacro cadr (x) (list 'car (list 'cdr x))) an expression such as `(cadr (assq 'handler list))' is a macro call, and its expansion is: (car (cdr (assq 'handler list))) Note that the argument `(assq 'handler list)' appears in the expansion. *Note Macros::, for a complete description of Emacs Lisp macros. File: elisp, Node: Special Forms, Next: Autoloading, Prev: Macro Forms, Up: Forms Special Forms ------------- A "special form" is a primitive function specially marked so that its arguments are not all evaluated. Special forms define control structures or perform variable bindings--things which functions cannot do. Each special form has its own rules for which arguments are evaluated and which are used without evaluation. Whether a particular argument is evaluated may depend on the results of evaluating other arguments. Here is a list, in alphabetical order, of all of the special forms in Emacs Lisp with a reference to where each is described. `and' *note Combining Conditions::. `catch' *note Catch and Throw::. `cond' *note Conditionals::. `condition-case' *note Errors::. `defconst' *note Defining Variables::. `defmacro' *note Defining Macros::. `defun' *note Defining Functions::. `defvar' *note Defining Variables::. `function' *note Anonymous Functions::. `if' *note Conditionals::. `interactive' *note Interactive Call::. `let' *note Local Variables::. `let*' *note Local Variables::. `or' *note Combining Conditions::. `prog1' *note Sequencing::. `prog2' *note Sequencing::. `progn' *note Sequencing::. `quote' *note Quoting::. `save-excursion' *note Excursions::. `save-restriction' *note Narrowing::. `save-window-excursion' *note Window Configurations::. `setq' *note Setting Variables::. `setq-default' *note Creating Buffer-Local::. `unwind-protect' *note Nonlocal Exits::. `while' *note Iteration::. `with-output-to-temp-buffer' *note Temporary Displays::. Common Lisp note: here are some comparisons of special forms in GNU Emacs Lisp and Common Lisp. `setq', `if', and `catch' are special forms in both Emacs Lisp and Common Lisp. `defun' is a special form in Emacs Lisp, but a macro in Common Lisp. `save-excursion' is a special form in Emacs Lisp, but doesn't exist in Common Lisp. `throw' is a special form in Common Lisp (because it must be able to throw multiple values), but it is a function in Emacs Lisp (which doesn't have multiple values). File: elisp, Node: Autoloading, Prev: Special Forms, Up: Forms Autoloading ----------- The "autoload" feature allows you to call a function or macro whose function definition has not yet been loaded into Emacs. When an autoload object appears as a symbol's function definition and that symbol is used as a function, Emacs will automatically install the real definition (plus other associated code) and then call that definition. (*Note Autoload::.) File: elisp, Node: Quoting, Prev: Forms, Up: Evaluation Quoting ======= The special form `quote' returns its single argument "unchanged". * Special Form: quote OBJECT This special form returns OBJECT, without evaluating it. This allows symbols and lists, which would normally be evaluated, to be included literally in a program. (It is not necessary to quote numbers, strings, and vectors since they are self-evaluating.) Use `function' instead of `quote' when quoting lambda expressions (*note Anonymous Functions::.). Because `quote' is used so often in programs, a convenient read syntax is defined for it. An apostrophe character (`'') followed by a Lisp object (in read syntax) expands to a list whose first element is `quote', and whose second element is the object. Thus, the read syntax `'x' is an abbreviation for `(quote x)'. Here are some examples of expressions that use `quote': (quote (+ 1 2)) => (+ 1 2) (quote foo) => foo 'foo => foo ''foo => (quote foo) '(quote foo) => (quote foo) ['foo] => [(quote foo)] File: elisp, Node: Control Structures, Next: Variables, Prev: Evaluation, Up: Top Control Structures ****************** A Lisp program consists of expressions or "forms" (*note Forms::.). We control the order of execution of the forms by enclosing them in "control structures". Control structures are special forms which control when, whether, or how many times to execute the forms they contain. The simplest control structure is sequential execution: first form A, then form B, and so on. This is what happens when you write several forms in succession in the body of a function, or at top level in a file of Lisp code--the forms are executed in the order they are written. We call this "textual order". For example, if a function body consists of two forms A and B, evaluation of the function evaluates first A and then B, and the function's value is the value of B. Naturally, Emacs Lisp has many kinds of control structures, including other varieties of sequencing, function calls, conditionals, iteration, and (controlled) jumps. The built-in control structures are special forms since their subforms are not necessarily evaluated. You can use macros to define your own control structure constructs (*note Macros::.). * Menu: * Sequencing:: Evaluation in textual order. * Conditionals:: `if', `cond'. * Combining Conditions:: `and', `or', `not'. * Iteration:: `while' loops. * Nonlocal Exits:: Jumping out of a sequence. File: elisp, Node: Sequencing, Next: Conditionals, Prev: Control Structures, Up: Control Structures Sequencing ========== Evaluating forms in the order they are written is the most common control structure. Sometimes this happens automatically, such as in a function body. Elsewhere you must use a control structure construct to do this: `progn', the simplest control construct of Lisp. A `progn' special form looks like this: (progn A B C ...) and it says to execute the forms A, B, C and so on, in that order. These forms are called the body of the `progn' form. The value of the last form in the body becomes the value of the entire `progn'. When Lisp was young, `progn' was the only way to execute two or more forms in succession and use the value of the last of them. But programmers found they often needed to use a `progn' in the body of a function, where (at that time) only one form was allowed. So the body of a function was made into an "implicit `progn'": several forms are allowed just as in the body of an actual `progn'. Many other control structures likewise contain an implicit `progn'. As a result, `progn' is not used as often as it used to be. It is needed now most often inside of an `unwind-protect', `and', or `or'. * Special Form: progn FORMS... This special form evaluates all of the FORMS, in textual order, returning the result of the final form. (progn (print "The first form") (print "The second form") (print "The third form")) -| "The first form" -| "The second form" -| "The third form" => "The third form" Two other control constructs likewise evaluate a series of forms but return a different value: * Special Form: prog1 FORM1 FORMS... This special form evaluates FORM1 and all of the FORMS, in textual order, returning the result of FORM1. (prog1 (print "The first form") (print "The second form") (print "The third form")) -| "The first form" -| "The second form" -| "The third form" => "The first form" Here is a way to remove the first element from a list in the variable `x', then return the value of that former element: (prog1 (car x) (setq x (cdr x))) * Special Form: prog2 FORM1 FORM2 FORMS... This special form evaluates FORM1, FORM2, and all of the following FORMS, in textual order, returning the result of FORM2. (prog2 (print "The first form") (print "The second form") (print "The third form")) -| "The first form" -| "The second form" -| "The third form" => "The second form" File: elisp, Node: Conditionals, Next: Combining Conditions, Prev: Sequencing, Up: Control Structures Conditionals ============ Conditional control structures choose among alternatives. Emacs Lisp has two conditional forms: `if', which is much the same as in other languages, and `cond', which is a generalized case statement. * Special Form: if CONDITION THEN-FORM ELSE-FORMS... `if' chooses between the THEN-FORM and the ELSE-FORMS based on the value of CONDITION. If the evaluated CONDITION is non-`nil', THEN-FORM is evaluated and the result returned. Otherwise, the ELSE-FORMS are evaluated in textual order, and the value of the last one is returned. (The ELSE part of `if' is an example of an implicit `progn'. *Note Sequencing::.) If CONDITION has the value `nil', and no ELSE-FORMS are given, `if' returns `nil'. `if' is a special form because the branch which is not selected is never evaluated--it is ignored. Thus, in the example below, `true' is not printed because `print' is never called. (if nil (print 'true) 'very-false) => very-false * Special Form: cond CLAUSE... `cond' chooses among an arbitrary number of alternatives. Each CLAUSE in the `cond' must be a list. The CAR of this list is the CONDITION; the remaining elements, if any, the BODY-FORMS. Thus, a clause looks like this: (CONDITION BODY-FORMS...) `cond' tries the clauses in textual order, by evaluating the CONDITION of each clause. If the value of CONDITION is non-`nil', the BODY-FORMS are evaluated, and the value of the last of BODY-FORMS becomes the value of the `cond'. The remaining clauses are ignored. If the value of CONDITION is `nil', the clause "fails", so the `cond' moves on to the following clause, trying its CONDITION. If every CONDITION evaluates to `nil', so that every clause fails, `cond' returns `nil'. A clause may also look like this: (CONDITION) Then, if CONDITION is non-`nil' when tested, the value of CONDITION becomes the value of the `cond' form. The following example has four clauses, which test for the cases where the value of `x' is a number, string, buffer and symbol, respectively: (cond ((numberp x) x) ((stringp x) x) ((bufferp x) (setq temporary-hack x) ; multiple body-forms (buffer-name x)) ; in one clause ((symbolp x) (symbol-value x))) Often we want the last clause to be executed whenever none of the previous clauses was successful. To do this, we use `t' as the CONDITION of the last clause, like this: `(t BODY-FORMS)'. The form `t' evaluates to `t', which is never `nil', so this clause never fails, provided the `cond' gets to it at all. For example, (cond ((eq a 1) 'foo) (t "default")) => "default" This expression is a `cond' which returns `foo' if the value of `a' is 1, and returns the string `"default"' otherwise. Both `cond' and `if' can usually be written in terms of the other. Therefore, the choice between them is a matter of taste and style. For example: (if A B C) == (cond (A B) (t C))