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This is Info file elisp, produced by Makeinfo-1.47 from the 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: Related Topics, Prev: Function Cells, Up: Functions Other Topics Related to Functions ================================= Here is a table of several functions that do things related to function calling and function definitions. `apply' *Note Calling Functions::. `autoload' *Note Autoload::. `call-interactively' *Note Interactive Call::. `commandp' *Note Interactive Call::. `documentation' *Note Accessing Documentation::. `eval' *Note Eval::. `funcall' *Note Calling Functions::. `ignore' *Note Calling Functions::. `interactive' *Note Using Interactive::. `interactive-p' *Note Interactive Call::. `mapatoms' *Note Creating Symbols::. `mapcar' *Note Mapping Functions::. `mapconcat' *Note Mapping Functions::. `undefined' *Note Key Lookup::. File: elisp, 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. * 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: elisp, Node: Simple Macro, Next: Expansion, Prev: Macros, 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 will do 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: elisp, 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). If they are macro calls themselves, `macroexpand' will not expand them. If ENVIRONMENT is provided, it specifies an alist of macro definitions that shadow the currently defined macros. This is used by byte compilation. (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: elisp, 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 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. While byte-compiling a file, any `require' calls at top-level are executed. One way to ensure that necessary macro definitions are available during compilation is to require the file that defines them. *Note Features::. File: elisp, 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 mapping functions 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. File: elisp, Node: Backquote, Next: Problems with Macros, Prev: Defining Macros, Up: Macros Backquote ========= It could prove rather awkward to write macros of significant size, simply due to the number of times the function `list' needs to be called. To make writing these forms easier, a macro ``' (often called "backquote") exists. Backquote allows you to quote a list, but selectively evaluate elements of that list. In the simplest case, it is identical to the special form `quote' (*note Quoting::.). For example, these two forms yield identical results: (` (a list of (+ 2 3) elements)) => (a list of (+ 2 3) elements) (quote (a list of (+ 2 3) elements)) => (a list of (+ 2 3) elements) By inserting a special marker, `,', inside of the argument to backquote, it is possible to evaluate desired portions of the argument: (list 'a 'list 'of (+ 2 3) 'elements) => (a list of 5 elements) (` (a list of (, (+ 2 3)) elements)) => (a list of 5 elements) It is also possible to have an evaluated list "spliced" into the resulting list by using the special marker `,@'. The elements of the spliced list become elements at the same level as the other elements of the resulting list. The equivalent code without using ``' is often unreadable. Here are some examples: (setq some-list '(2 3)) => (2 3) (cons 1 (append some-list '(4) some-list)) => (1 2 3 4 2 3) (` (1 (,@ some-list) 4 (,@ some-list))) => (1 2 3 4 2 3) (setq list '(hack foo bar)) => (hack foo bar) (cons 'use (cons 'the (cons 'words (append (cdr list) '(as elements))))) => (use the words foo bar as elements) (` (use the words (,@ (cdr list)) as elements (,@ nil))) => (use the words foo bar as elements) The reason for `(,@ nil)' is to avoid a bug in Emacs version 18. The bug occurs when a call to `,@' is followed only by constant elements. Thus, (` (use the words (,@ (cdr list)) as elements)) would not work, though it really ought to. `(,@ nil)' avoids the problem by being a nonconstant element that does not affect the result. -- Macro: ` LIST This macro returns LIST as `quote' would, except that the list is copied each time this expression is evaluated, and any sublist of the form `(, SUBEXP)' is replaced by the value of SUBEXP. Any sublist of the form `(,@ LISTEXP)' is replaced by evaluating LISTEXP and splicing its elements into the containing list in place of this sublist. (A single sublist can in this way be replaced by any number of new elements in the containing list.) There are certain contexts in which `,' would not be recognized and should not be used: ;; Use of a `,' expression as the CDR of a list. (` (a . (, 1))) ; Not `(a . 1)' => (a \, 1) ;; Use of `,' in a vector. (` [a (, 1) c]) ; Not `[a 1 c]' error--> Wrong type argument ;; Use of a `,' as the entire argument of ``'. (` (, 2)) ; Not 2 => (\, 2) Common Lisp note: in Common Lisp, `,' and `,@' are implemented as reader macros, so they do not require parentheses. Emacs Lisp implements them as functions because reader macros are not supported (to save space). File: elisp, Node: Problems with Macros, Prev: Backquote, Up: Macros Common Problems Using Macros ============================ The basic facts of macro expansion have all been described above, but there consequences are often counterintuitive. This section describes some important consequences that can lead to trouble, and rules to follow to avoid trouble. * Menu: * Argument Evaluation:: The expansion should evaluate each macro arg once. * Surprising Local Vars:: Local variable bindings in the expansion require special care. * Eval During Expansion:: Don't evaluate them; put them in the expansion. * Repeated Expansion:: Avoid depending on how many times expansion is done. File: elisp, Node: Argument Evaluation, Next: Surprising Local Vars, Prev: Problems with Macros, Up: Problems with Macros Evaluating Macro Arguments Too Many Times ----------------------------------------- When defining a macro you must pay attention to the number of times the arguments will be evaluated when the expansion is executed. The following macro (used to facilitate iteration) illustrates the problem. This macro allows us to write a simple "for" loop such as one might find in Pascal. (defmacro for (var from init to final do &rest body) "Execute a simple \"for\" loop, e.g., (for i from 1 to 10 do (print i))." (list 'let (list (list var init)) (cons 'while (cons (list '<= var final) (append body (list (list 'inc var))))))) => for (for i from 1 to 3 do (setq square (* i i)) (princ (format "\n%d %d" i square))) ==> (let ((i 1)) (while (<= i 3) (setq square (* i i)) (princ (format "%d %d" i square)) (inc i))) -|1 1 -|2 4 -|3 9 => nil (The arguments `from', `to', and `do' in this macro are "syntactic sugar"; they are entirely ignored. The idea is that you will write noise words (such as `from', `to', and `do') in those positions in the macro call.) This macro suffers from the defect that FINAL is evaluated on every iteration. If FINAL is a constant, this is not a problem. If it is a more complex form, say `(long-complex-calculation x)', this can slow down the execution significantly. If FINAL has side effects, executing it more than once is probably incorrect. A well-designed macro definition takes steps to avoid this problem by producing an expansion that evaluates the argument expressions exactly once unless repeated evaluation is part of the intended purpose of the macro. Here is a correct expansion for the `for' macro: (let ((i 1) (max 3)) (while (<= i max) (setq square (* i i)) (princ (format "%d %d" i square)) (inc i))) Here is a macro definition that creates this expansion: (defmacro for (var from init to final do &rest body) "Execute a simple for loop: (for i from 1 to 10 do (print i))." (` (let (((, var) (, init)) (max (, final))) (while (<= (, var) max) (,@ body) (inc (, var)))))) Unfortunately, this introduces another problem. Proceed to the following node. File: elisp, Node: Surprising Local Vars, Next: Eval During Expansion, Prev: Argument Evaluation, Up: Problems with Macros Local Variables in Macro Expansions ----------------------------------- In the previous section, the definition of `for' was fixed as follows to make the expansion evaluate the macro arguments the proper number of times: (defmacro for (var from init to final do &rest body) "Execute a simple for loop: (for i from 1 to 10 do (print i))." (` (let (((, var) (, init)) (max (, final))) (while (<= (, var) max) (,@ body) (inc (, var)))))) The new definition of `for' has a new problem: it introduces a local variable named `max' which the user does not expect. This will cause trouble in examples such as the following: (let ((max 0)) (for x from 0 to 10 do (let ((this (frob x))) (if (< max this) (setq max this))))) The references to `max' inside the body of the `for', which are supposed to refer to the user's binding of `max', will instead access the binding made by `for'. The way to correct this is to use an uninterned symbol instead of `max' (*note Creating Symbols::.). The uninterned symbol can be bound and referred to just like any other symbol, but since it is created by `for', we know that it cannot appear in the user's program. Since it is not interned, there is no way the user can put it into the program later. It will not appear anywhere except where put by `for'. Here is a definition of `for' which works this way: (defmacro for (var from init to final do &rest body) "Execute a simple for loop: (for i from 1 to 10 do (print i))." (let ((tempvar (make-symbol "max"))) (` (let (((, var) (, init)) ((, tempvar) (, final))) (while (<= (, var) (, tempvar)) (,@ body) (inc (, var))))))) This creates an uninterned symbol named `max' and puts it in the expansion instead of the usual interned symbol `max' that appears in expressions ordinarily. File: elisp, Node: Eval During Expansion, Next: Repeated Expansion, Prev: Surprising Local Vars, Up: Problems with Macros Evaluating Macro Arguments in Expansion --------------------------------------- Another problem can happen if you evaluate any of the macro argument expressions during the computation of the expansion, such as by calling `eval' (*note Eval::.). If the argument is supposed to refer to the user's variables, you may have trouble if the user happens to use a variable with the same name as one of the macro arguments. Inside the macro body, the macro argument binding is the most local binding of this variable, so any references inside the form being evaluated will refer to it. Here is an example: (defmacro foo (a) (list 'setq (eval a) t)) => foo (setq x 'b) (foo x) ==> (setq b t) => t ; and `b' has been set. ;; but (setq a 'b) (foo a) ==> (setq 'b t) ; invalid! error--> Symbol's value is void: b It makes a difference whether the user types `a' or `x', because `a' conflicts with the macro argument variable `a'. In general it is best to avoid calling `eval' in a macro definition at all. File: elisp, Node: Repeated Expansion, Prev: Eval During Expansion, Up: Problems with Macros How Many Times is the Macro Expanded? ------------------------------------- Occasionally problems result from the fact that a macro call is expanded each time it is evaluated in an interpreted function, but is expanded only once (during compilation) for a compiled function. If the macro definition has side effects, they will work differently depending on how many times the macro is expanded. In particular, constructing objects is a kind of side effect. If the macro is called once, then the objects are constructed only once. In other words, the same structure of objects is used each time the macro call is executed. In interpreted operation, the macro is reexpanded each time, producing a fresh collection of objects each time. Usually this does not matter--the objects have the same contents whether they are shared or not. But if the surrounding program does side effects on the objects, it makes a difference whether they are shared. Here is an example: (defmacro new-object () (list 'quote (cons nil nil))) (defun initialize (condition) (let ((object (new-object))) (if condition (setcar object condition)) object)) If `initialize' is interpreted, a new list `(nil)' is constructed each time `initialize' is called. Thus, no side-effect survives between calls. If `initialize' is compiled, then the macro `new-object' is expanded during compilation, producing a single "constant" `(nil)' that is reused and altered each time `initialize' is called. File: elisp, Node: Loading, Next: Byte Compilation, Prev: Macros, Up: Top Loading ******* Loading a file of Lisp code means bringing its contents into the Lisp environment in the form of Lisp objects. Emacs finds and opens the file, reads the text, evaluates each form, and then closes the file. The load functions evaluate all the expressions in a file just as the `eval-current-buffer' function evaluates all the expressions in a buffer. The difference is that the load functions read and evaluate the text in the file as found on disk, not the text in an Emacs buffer. The loaded file must contain Lisp expressions, either as source code or, optionally, as byte-compiled code. Each form in the file is called a "top-level form". There is no special format for the forms in a loadable file; any form in a file may equally well be typed directly into a buffer and evaluated there. (Indeed, most code is tested this way.) Most often, the forms are function definitions and variable definitions. A file containing Lisp code is often called a "library". Thus, the "Rmail library" is a file containing code for Rmail mode. Similarly, a "Lisp library directory" is a directory of files containing Lisp code. * Menu: * How Programs Do Loading:: The `load' function and others. * Autoload:: Setting up a function to autoload. * Repeated Loading:: Precautions about loading a file twice. * Features:: Loading a library if it isn't already loaded. File: elisp, Node: How Programs Do Loading, Next: Autoload, Prev: Loading, Up: Loading How Programs Do Loading ======================= There are several interface functions for loading. For example, the `autoload' function creates a Lisp object that loads a file when it is evaluated (*note Autoload::.). `require' also causes files to be loaded (*note Features::.). Ultimately, all these facilities call the `load' function to do the work. -- Function: load FILENAME &optional MISSING-OK NOMESSAGE NOSUFFIX This function finds and opens a file of Lisp code, evaluates all the forms in it, and closes the file. To find the file, `load' first looks for a file named `FILENAME.elc', that is, for a file whose name has `.elc' appended. If such a file exists, it is loaded. But if there is no file by that name, then `load' looks for a file whose name has `.el' appended. If that file exists, it is loaded. Finally, if there is no file by either name, `load' looks for a file named FILENAME with nothing appended, and loads it if it exists. (The `load' function is not clever about looking at FILENAME. In the perverse case of a file named `foo.el.el', evaluation of `(load "foo.el")' will indeed find it.) If the optional argument NOSUFFIX is non-`nil', then the suffixes `.elc' and `.el' are not tried. In this case, the file name must be specified precisely. If FILENAME is a relative file name, such as `foo.bar' or `baz/foo.bar', Emacs searches for the file using the variable `load-path'. Emacs does this by appending FILENAME to each of the directories listed in `load-path', and loading the first file it finds whose name matches. The current default directory is tried only if it is specified in `load-path', where it is represented as `nil'. All three possible suffixes are tried in the first directory in `load-path', then all three in the second directory in `load-path', etc. Messages like `Loading foo...' and `Loading foo...done' are printed in the echo area while loading unless NOMESSAGE is non-`nil'. Any errors that are encountered while loading a file cause `load' to abort. If the load was done for the sake of `autoload', certain kinds of top-level forms, those which define functions, are undone. The error `file-error' is signaled (with `Cannot open load file FILENAME') if no file is found. No error is signaled if MISSING-OK is non-`nil'--then `load' just returns `nil'. `load' returns `t' if the file loads successfully. -- User Option: load-path The value of this variable is a list of directories to search when loading files with `load'. Each element is a string (which must be a directory name) or `nil' (which stands for the current working directory). The value of `load-path' is initialized from the environment variable `EMACSLOADPATH', if it exists; otherwise it is set to the default specified in `emacs/src/paths.h' when Emacs is built. The syntax of `EMACSLOADPATH' is the same as that of `PATH'; fields are separated by `:', and `.' is used for the current default directory. Here is an example of how to set your `EMACSLOADPATH' variable from a `csh' `.login' file: setenv EMACSLOADPATH .:/user/liberte/emacs:/usr/local/lib/emacs/lisp Here is how to set it using `sh': export EMACSLOADPATH EMACSLOADPATH=.:/user/liberte/emacs:/usr/local/lib/emacs/lisp Here is an example of code you can place in a `.emacs' file to add several directories to the front of your default `load-path': (setq load-path (append (list nil "/user/liberte/emacs" "/usr/local/lisplib") load-path)) In this example, the path searches the current working directory first, followed by `/user/liberte/emacs' and `/usr/local/lisplib', which are then followed by the standard directories for Lisp code. When Emacs 18 is processing command options `-l' or `-load' which specify Lisp libraries to be loaded, it temporarily adds the current directory to the front of `load-path' so that files in the current directory can be specified easily. Emacs version 19 will also find such files in the current directory but without altering `load-path'. -- Variable: load-in-progress This variable is non-`nil' if Emacs is in the process of loading a file, and it is `nil' otherwise. This is how `defun' and `provide' determine whether a load is in progress, so that their effect can be undone if the load fails. To learn how `load' is used to build Emacs, see *Note Building Emacs::. File: elisp, Node: Autoload, Next: Repeated Loading, Prev: How Programs Do Loading, Up: Loading Autoload ======== The "autoload" facility allows you to make a function or macro available but put off loading its actual definition. An attempt to call a symbol whose definition is an autoload object automatically reads the file to install the real definition and its other associated code, and then calls the real definition. To prepare a function or macro for autoloading, you must call `autoload', specifying the function name and the name of the file to be loaded. This is usually done when Emacs is first built, by files such as `emacs/lisp/loaddefs.el'. The following example shows how `doctor' is prepared for autoloading in `loaddefs.el': (autoload 'doctor "doctor" "\ Switch to *doctor* buffer and start giving psychotherapy." t) The backslash and newline immediately following the double-quote are a convention used only in the preloaded Lisp files such as `loaddefs.el'; they cause the documentation string to be put in the `etc/DOC' file. (*Note Building Emacs::.) In any other source file, you would write just this: (autoload 'doctor "doctor" "Switch to *doctor* buffer and start giving psychotherapy." t) Calling `autoload' creates an autoload object containing the name of the file and some other information, and makes this the definition of the specified symbol. When you later try to call that symbol as a function or macro, the file is loaded; the loading should redefine that symbol with its proper definition. After the file completes loading, the function or macro is called as if it had been there originally. If, at the end of loading the file, the desired Lisp function or macro has not been defined, then the error `error' is signaled (with data `"Autoloading failed to define function FUNCTION-NAME"'). The autoloaded file may, of course, contain other definitions and may require or provide one or more features. If the file is not completely loaded (due to an error in the evaluation of the contents) any function definitions or `provide' calls that occurred during the load are undone. This is to ensure that the next attempt to call any function autoloading from this file will try again to load the file. If not for this, then some of the functions in the file might appear defined, but they may fail to work properly for the lack of certain subroutines defined later in the file and not loaded successfully. -- Function: autoload SYMBOL FILENAME &optional DOCSTRING INTERACTIVE MACRO This function defines the function (or macro) named SYMBOL so as to load automatically from FILENAME. The string FILENAME is a file name which will be passed to `load' when the function is called. The argument DOCSTRING is the documentation string for the function. Normally, this is the same string that is in the function definition itself. This makes it possible to look at the documentation without loading the real definition. If INTERACTIVE is non-`nil', then the function can be called interactively. This lets completion in `M-x' work without loading the function's real definition. The complete interactive specification need not be given here. If MACRO is non-`nil', then the function is really a macro. If SYMBOL already has a non-`nil' function definition that is not an autoload object, `autoload' does nothing and returns `nil'. If the function cell of SYMBOL is void, or is already an autoload object, then it is set to an autoload object that looks like this: (autoload FILENAME DOCSTRING INTERACTIVE MACRO) For example, (symbol-function 'run-prolog) => (autoload "prolog" 169681 t nil) In this case, `"prolog"' is the name of the file to load, 169681 is the reference to the documentation string in the `emacs/etc/DOC' file (*note Documentation Basics::.), `t' means the function is interactive, and `nil' that it is not a macro. File: elisp, Node: Repeated Loading, Next: Features, Prev: Autoload, Up: Loading Repeated Loading ================ You may load a file more than once in an Emacs session. For example, after you have rewritten and reinstalled a function definition by editing it in a buffer, you may wish to return to the original version; you can do this by reloading the file in which it is located. When you load or reload files, bear in mind that the `load' and `load-library' functions automatically load a byte-compiled file rather than a non-compiled file of similar name. If you rewrite a file that you intend to save and reinstall, remember to byte-compile it if necessary; otherwise you may find yourself inadvertently reloading the older, byte-compiled file instead of your newer, non-compiled file! When writing the forms in a library, keep in mind that the library might be loaded more than once. For example, the choice of `defvar' vs. `defconst' for defining a variable depends on whether it is desirable to reinitialize the variable if the library is reloaded: `defconst' does so, and `defvar' does not. (*Note Defining Variables::.) The simplest way to add an element to an alist is like this: (setq minor-mode-alist (cons '(leif-mode " Leif") minor-mode-alist)) But this would add multiple elements if the library is reloaded. To avoid the problem, write this: (or (assq 'leif-mode minor-mode-alist) (setq minor-mode-alist (cons '(leif-mode " Leif") minor-mode-alist))) Occasionally you will want to test explicitly whether a library has already been loaded; you can do so as follows: (if (not (boundp 'foo-was-loaded)) EXECUTE-FIRST-TIME-ONLY) (setq foo-was-loaded t) File: elisp, Node: Features, Prev: Repeated Loading, Up: Loading Features ======== `provide' and `require' are an alternative to `autoload' for loading files automatically. They work in terms of named "features". Autoloading is triggered by calling a specific function, but a feature is loaded the first time another program asks for it by name. The use of named features simplifies the task of determining whether required definitions have been defined. A feature name is a symbol that stands for a collection of functions, variables, etc. A program that needs the collection may ensure that they are defined by "requiring" the feature. If the file that contains the feature has not yet been loaded, then it will be loaded (or an error will be signaled if it cannot be loaded). The file thus loaded must "provide" the required feature or an error will be signaled. To require the presence of a feature, call `require' with the feature name as argument. `require' looks in the global variable `features' to see whether the desired feature has been provided already. If not, it loads the feature from the appropriate file. This file should call `provide' at the top-level to add the feature to `features'. Features are normally named after the files they are provided in so that `require' need not be given the file name. For example, in `emacs/lisp/prolog.el', the definition for `run-prolog' includes the following code: (interactive) (require 'shell) (switch-to-buffer (make-shell "prolog" "prolog")) (inferior-prolog-mode)) The expression `(require 'shell)' loads the file `shell.el' if it has not yet been loaded. This ensures that `make-shell' is defined. The `shell.el' file contains the following top-level expression: (provide 'shell) This adds `shell' to the global `features' list when the `shell' file is loaded, so that `(require 'shell)' will henceforth know that nothing needs to be done. When `require' is used at top-level in a file, it takes effect if you byte-compile that file (*note Byte Compilation::.). This is in case the required package contains macros that the byte compiler must know about. Although top-level calls to `require' are evaluated during byte compilation, `provide' calls are not. Therefore, you can ensure that a file of definitions is loaded before it is byte-compiled by including a `provide' followed by a `require' for the same feature, as in the following example. (provide 'my-feature) ; Ignored by byte compiler, evaluated by `load'. (require 'my-feature) ; Evaluated by byte compiler. -- Function: provide FEATURE This function announces that FEATURE is now loaded, or being loaded, into the current Emacs session. This means that the facilities associated with FEATURE are or will be available for other Lisp programs. The direct effect of calling `provide' is to add FEATURE to the front of the list `features' if it is not already in the list. The argument FEATURE must be a symbol. `provide' returns FEATURE. features => (bar bish) (provide 'foo) => foo features => (foo bar bish) During autoloading, if the file is not completely loaded (due to an error in the evaluation of the contents) any function definitions or `provide' calls that occurred during the load are undone. *Note Autoload::. -- Function: require FEATURE &optional FILENAME This function checks whether FEATURE is present in the current Emacs session (using `(featurep FEATURE)'; see below). If it is not, then `require' loads FILENAME with `load'. If FILENAME is not supplied, then the name of the symbol FEATURE is used as the file name to load. If FEATURE is not provided after the file has been loaded, Emacs will signal the error `error' (with data `Required feature FEATURE was not provided'). -- Function: featurep FEATURE This function returns `t' if FEATURE has been provided in the current Emacs session (i.e., FEATURE is a member of `features'.) -- Variable: features The value of this variable is a list of symbols that are the features loaded in the current Emacs session. Each symbol was put in this list with a call to `provide'. The order of the elements in the `features' list is not significant. File: elisp, Node: Byte Compilation, Next: Debugging, Prev: Loading, Up: Top Byte Compilation **************** GNU Emacs Lisp has a "compiler" that translates functions written in Lisp into a special representation called "byte-code" that can be executed more efficiently. The compiler replaces Lisp function definitions with byte-code. When a byte-code function is called, its definition is evaluated by the "byte-code interpreter". Because the byte-compiled code is evaluated by the byte-code interpreter, instead of being executed directly by the machine's hardware (as true compiled code is), byte-code is completely transportable from machine to machine without recompilation. It is not, however, as fast as true compiled code. *Note Compilation Errors::, for how to investigate errors occurring in byte compilation. * Menu: * Compilation Functions:: Byte compilation functions. * Disassembly:: Disassembling byte-code; how to read byte-code. File: elisp, Node: Compilation Functions, Next: Disassembly, Prev: Byte Compilation, Up: Byte Compilation The Compilation Functions ========================= An individual function or macro definition may be byte-compiled with the `byte-compile' function. A whole file may be byte-compiled with `byte-compile-file' and several files may be byte-compiled with `byte-recompile-directory' or `batch-byte-compile'. Only `defun' and `defmacro' forms in a file are byte-compiled; other top-level forms are not altered by byte compilation. Be careful when byte-compiling code that uses macros. Macro calls are expanded when they are compiled, so the macros must already be defined for proper compilation. For more details, see *Note Compiling Macros::. While byte-compiling a file, any `require' calls at top-level are executed. One way to ensure that necessary macro definitions are available during compilation is to require the file that defines them. *Note Features::. A byte-compiled function is not as efficient as a primitive function written in C, but will run much faster than the version written in Lisp. For a rough comparison, consider the example below: (defun silly-loop (n) "Return time before and after N iterations of a loop." (let ((t1 (current-time-string))) (while (> (setq n (1- n)) 0)) (list t1 (current-time-string)))) => silly-loop (silly-loop 100000) => ("Thu Jan 12 20:18:38 1989" "Thu Jan 12 20:19:29 1989") ; 51 seconds (byte-compile 'silly-loop) => [Compiled code not shown] (silly-loop 100000) => ("Thu Jan 12 20:21:04 1989" "Thu Jan 12 20:21:17 1989") ; 13 seconds In this example, the interpreted code required 51 seconds to run, whereas the byte-compiled code required 13 seconds. These results are representative, but actual results will vary greatly. -- Function: byte-compile SYMBOL This function byte-compiles the function definition of SYMBOL, replacing the previous definition with the compiled one. The function definition of SYMBOL must be the actual code for the function; i.e., the compiler will not follow indirection to another symbol. `byte-compile' does not compile macros. `byte-compile' returns the new, compiled definition of SYMBOL. (defun factorial (integer) "Compute factorial of INTEGER." (if (= 1 integer) 1 (* integer (factorial (1- integer))))) => factorial (byte-compile 'factorial) => (lambda (integer) "Compute factorial of INTEGER." (byte-code "\301^HU\203 ^@\301\202^Q^@\302^H\303^HS!\"\207" [integer 1 * factorial] 4)) The string that is the first argument of `byte-code' is the actual byte-code. Each character in it is an instruction. The vector contains all the constants, variable names and function names used by the function, except for certain primitives that are coded as special instructions. The `byte-compile' function is not autoloaded as are `byte-compile-file' and `byte-recompile-directory'. -- Command: byte-compile-file FILENAME This function compiles a file of Lisp code named FILENAME into a file of byte-code. The output file's name is made by appending `c' to the end of FILENAME. Compilation works by reading the input file one form at a time. If it is a definition of a function or macro, the compiled function or macro definition is written out. Other forms are copied out unchanged. All comments are discarded when the input file is read. This command returns `t'. When called interactively, it prompts for the file name. % ls -l push* -rw-r--r-- 1 lewis 791 Oct 5 20:31 push.el (byte-compile-file "~/emacs/push.el") => t % ls -l push* -rw-r--r-- 1 lewis 791 Oct 5 20:31 push.el -rw-rw-rw- 1 lewis 638 Oct 8 20:25 push.elc -- Command: byte-recompile-directory DIRECTORY FLAG This function recompiles every `.el' file in DIRECTORY that needs recompilation. A file needs recompilation if a `.elc' file exists but is older than the `.el' file. If a `.el' file exists, but there is no corresponding `.elc' file, then FLAG is examined. If it is `nil', the file is ignored. If it is non-`nil', the user is asked whether the file should be compiled. The returned value of this command is unpredictable. -- Function: batch-byte-compile This function runs `byte-compile-file' on the files remaining on the command line. This function must be used only in a batch execution of Emacs, as it kills Emacs on completion. Each file will be processed, even if an error occurs while compiling a previous file. (The file with the error will not, of course, produce any compiled code.) % emacs -batch -f batch-byte-compile *.el -- Function: byte-code CODE-STRING DATA-VECTOR MAX-STACK This is the function that actually interprets byte-code. A byte-compiled function is actually defined with a body that calls `byte-code'. Don't call this function yourself. Only the byte compiler knows how to generate valid calls to this function.