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This is Info file ../../info/lispref.info, produced by Makeinfo-1.63 from the input file lispref.texi. Edition History: GNU Emacs Lisp Reference Manual Second Edition (v2.01), May 1993 GNU Emacs Lisp Reference Manual Further Revised (v2.02), August 1993 Lucid Emacs Lisp Reference Manual (for 19.10) First Edition, March 1994 XEmacs Lisp Programmer's Manual (for 19.12) Second Edition, April 1995 GNU Emacs Lisp Reference Manual v2.4, June 1995 XEmacs Lisp Programmer's Manual (for 19.13) Third Edition, July 1995 Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995 Free Software Foundation, Inc. Copyright (C) 1994, 1995 Sun Microsystems, Inc. Copyright (C) 1995 Amdahl Corporation. Copyright (C) 1995 Ben Wing. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that this permission notice may be stated in a translation approved by the Foundation. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the section entitled "GNU General Public License" is included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the section entitled "GNU General Public License" may be included in a translation approved by the Free Software Foundation instead of in the original English. File: lispref.info, Node: Simple Macro, Next: Expansion, Up: Macros A Simple Example of a Macro =========================== Suppose we would like to define a Lisp construct to increment a variable value, much like the `++' operator in C. We would like to write `(inc x)' and have the effect of `(setq x (1+ x))'. Here's a macro definition that does the job: (defmacro inc (var) (list 'setq var (list '1+ var))) When this is called with `(inc x)', the argument `var' has the value `x'--*not* the *value* of `x'. The body of the macro uses this to construct the expansion, which is `(setq x (1+ x))'. Once the macro definition returns this expansion, Lisp proceeds to evaluate it, thus incrementing `x'. File: lispref.info, Node: Expansion, Next: Compiling Macros, Prev: Simple Macro, Up: Macros Expansion of a Macro Call ========================= A macro call looks just like a function call in that it is a list which starts with the name of the macro. The rest of the elements of the list are the arguments of the macro. Evaluation of the macro call begins like evaluation of a function call except for one crucial difference: the macro arguments are the actual expressions appearing in the macro call. They are not evaluated before they are given to the macro definition. By contrast, the arguments of a function are results of evaluating the elements of the function call list. Having obtained the arguments, Lisp invokes the macro definition just as a function is invoked. The argument variables of the macro are bound to the argument values from the macro call, or to a list of them in the case of a `&rest' argument. And the macro body executes and returns its value just as a function body does. The second crucial difference between macros and functions is that the value returned by the macro body is not the value of the macro call. Instead, it is an alternate expression for computing that value, also known as the "expansion" of the macro. The Lisp interpreter proceeds to evaluate the expansion as soon as it comes back from the macro. Since the expansion is evaluated in the normal manner, it may contain calls to other macros. It may even be a call to the same macro, though this is unusual. You can see the expansion of a given macro call by calling `macroexpand'. - Function: macroexpand FORM &optional ENVIRONMENT This function expands FORM, if it is a macro call. If the result is another macro call, it is expanded in turn, until something which is not a macro call results. That is the value returned by `macroexpand'. If FORM is not a macro call to begin with, it is returned as given. Note that `macroexpand' does not look at the subexpressions of FORM (although some macro definitions may do so). Even if they are macro calls themselves, `macroexpand' does not expand them. The function `macroexpand' does not expand calls to inline functions. Normally there is no need for that, since a call to an inline function is no harder to understand than a call to an ordinary function. If ENVIRONMENT is provided, it specifies an alist of macro definitions that shadow the currently defined macros. Byte compilation uses this feature. (defmacro inc (var) (list 'setq var (list '1+ var))) => inc (macroexpand '(inc r)) => (setq r (1+ r)) (defmacro inc2 (var1 var2) (list 'progn (list 'inc var1) (list 'inc var2))) => inc2 (macroexpand '(inc2 r s)) => (progn (inc r) (inc s)) ; `inc' not expanded here. File: lispref.info, Node: Compiling Macros, Next: Defining Macros, Prev: Expansion, Up: Macros Macros and Byte Compilation =========================== You might ask why we take the trouble to compute an expansion for a macro and then evaluate the expansion. Why not have the macro body produce the desired results directly? The reason has to do with compilation. When a macro call appears in a Lisp program being compiled, the Lisp compiler calls the macro definition just as the interpreter would, and receives an expansion. But instead of evaluating this expansion, it compiles the expansion as if it had appeared directly in the program. As a result, the compiled code produces the value and side effects intended for the macro, but executes at full compiled speed. This would not work if the macro body computed the value and side effects itself--they would be computed at compile time, which is not useful. In order for compilation of macro calls to work, the macros must be defined in Lisp when the calls to them are compiled. The compiler has a special feature to help you do this: if a file being compiled contains a `defmacro' form, the macro is defined temporarily for the rest of the compilation of that file. To use this feature, you must define the macro in the same file where it is used and before its first use. Byte-compiling a file executes any `require' calls at top-level in the file. This is in case the file needs the required packages for proper compilation. One way to ensure that necessary macro definitions are available during compilation is to require the files that define them (*note Named Features::.). To avoid loading the macro definition files when someone *runs* the compiled program, write `eval-when-compile' around the `require' calls (*note Eval During Compile::.). File: lispref.info, Node: Defining Macros, Next: Backquote, Prev: Compiling Macros, Up: Macros Defining Macros =============== A Lisp macro is a list whose CAR is `macro'. Its CDR should be a function; expansion of the macro works by applying the function (with `apply') to the list of unevaluated argument-expressions from the macro call. It is possible to use an anonymous Lisp macro just like an anonymous function, but this is never done, because it does not make sense to pass an anonymous macro to functionals such as `mapcar'. In practice, all Lisp macros have names, and they are usually defined with the special form `defmacro'. - Special Form: defmacro NAME ARGUMENT-LIST BODY-FORMS... `defmacro' defines the symbol NAME as a macro that looks like this: (macro lambda ARGUMENT-LIST . BODY-FORMS) This macro object is stored in the function cell of NAME. The value returned by evaluating the `defmacro' form is NAME, but usually we ignore this value. The shape and meaning of ARGUMENT-LIST is the same as in a function, and the keywords `&rest' and `&optional' may be used (*note Argument List::.). Macros may have a documentation string, but any `interactive' declaration is ignored since macros cannot be called interactively. File: lispref.info, Node: Backquote, Next: Problems with Macros, Prev: Defining Macros, Up: Macros Backquote ========= Macros often need to construct large list structures from a mixture of constants and nonconstant parts. To make this easier, use the macro ``' (often called "backquote"). 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) '(a list of (+ 2 3) elements) => (a list of (+ 2 3) elements) The special marker `,' inside of the argument to backquote indicates a value that isn't constant. Backquote evaluates the argument of `,' and puts the value in the list structure: (list 'a 'list 'of (+ 2 3) 'elements) => (a list of 5 elements) `(a list of ,(+ 2 3) elements) => (a list of 5 elements) You can also "splice" an evaluated value into the resulting list, 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) => (use the words foo bar as elements) Before Emacs version 19.29, ``' used a different syntax which required an extra level of parentheses around the entire backquote construct. Likewise, each `,' or `,@' substition required an extra level of parentheses surrounding both the `,' or `,@' and the following expression. The old syntax required whitespace between the ``', `,' or `,@' and the following expression. This syntax is still accepted, but no longer recommended except for compatibility with old Emacs versions. File: lispref.info, Node: Problems with Macros, Prev: Backquote, Up: Macros Common Problems Using Macros ============================ The basic facts of macro expansion have counterintuitive consequences. 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: lispref.info, Node: Argument Evaluation, Next: Surprising Local Vars, Up: Problems with Macros Evaluating Macro Arguments Repeatedly ------------------------------------- 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. For example, (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.) Here's an equivalent definition simplified through use of backquote: (defmacro for (var from init to final do &rest body) "Execute a simple \"for\" loop. For example, (for i from 1 to 10 do (print i))." `(let ((,var ,init)) (while (<= ,var ,final) ,@body (inc ,var)))) Both forms of this definition (with backquote and without) suffer 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: lispref.info, 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 causes 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', really 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 already 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 never appear anywhere except where put by `for'. Here is a definition of `for' that 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: lispref.info, 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 do 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 'c) (foo a) ==> (setq a t) => t ; but this set `a', not `c'. It makes a difference whether the user's variable is named `a' or `x', because `a' conflicts with the macro argument variable `a'. Another reason not to call `eval' in a macro definition is that it probably won't do what you intend in a compiled program. The byte-compiler runs macro definitions while compiling the program, when the program's own computations (which you might have wished to access with `eval') don't occur and its local variable bindings don't exist. The safe way to work with the run-time value of an expression is to put the expression into the macro expansion, so that its value is computed as part of executing the expansion. File: lispref.info, 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 empty-object () (list 'quote (cons nil nil))) (defun initialize (condition) (let ((object (empty-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 `empty-object' is expanded during compilation, producing a single "constant" `(nil)' that is reused and altered each time `initialize' is called. One way to avoid pathological cases like this is to think of `empty-object' as a funny kind of constant, not as a memory allocation construct. You wouldn't use `setcar' on a constant such as `'(nil)', so naturally you won't use it on `(empty-object)' either. File: lispref.info, 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. XEmacs 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 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. * Named Features:: Loading a library if it isn't already loaded. * Unloading:: How to "unload" a library that was loaded. * Hooks for Loading:: Providing code to be run when particular libraries are loaded. File: lispref.info, Node: How Programs Do Loading, Next: Autoload, Up: Loading How Programs Do Loading ======================= Emacs Lisp has several interfaces for loading. For example, `autoload' creates a placeholder object for a function in a file; trying to call the autoloading function loads the file to get the function's real definition (*note Autoload::.). `require' loads a file if it isn't already loaded (*note Named 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 is FILENAME with `.elc' appended. If such a file exists, it is loaded. If there is no file by that name, then `load' looks for a file named `FILENAME.el'. If that file exists, it is loaded. Finally, if neither of those names is found, `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, you must specify the precise file name you want. If FILENAME is a relative file name, such as `foo' or `baz/foo.bar', `load' searches for the file using the variable `load-path'. It appends FILENAME to each of the directories listed in `load-path', and loads the first file it finds whose name matches. The current default directory is tried only if it is specified in `load-path', where `nil' stands for the default directory. `load' tries all three possible suffixes in the first directory in `load-path', then all three suffixes in the second directory, and so on. If you get a warning that `foo.elc' is older than `foo.el', it means you should consider recompiling `foo.el'. *Note Byte Compilation::. Messages like `Loading foo...' and `Loading foo...done' appear in the echo area during loading unless NOMESSAGE is non-`nil'. Any unhandled errors while loading a file terminate loading. If the load was done for the sake of `autoload', any function definitions made during the loading are undone. If `load' can't find the file to load, then normally it signals the error `file-error' (with `Cannot open load file FILENAME'). But if MISSING-OK is non-`nil', then `load' just returns `nil'. You can use the variable `load-read-function' to specify a function for `load' to use instead of `read' for reading expressions. See below. `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 that exists; otherwise its default value is specified in `emacs/src/paths.h' when XEmacs is built. The syntax of `EMACSLOADPATH' is the same as used for `PATH'; `:' (or `;', according to the operating system) separates directory names, 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/bil/emacs:/usr/lib/emacs/lisp Here is how to set it using `sh': export EMACSLOADPATH EMACSLOADPATH=.:/user/bil/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/bil/emacs" "/usr/local/lisplib" "~/emacs") load-path)) In this example, the path searches the current working directory first, followed then by the `/user/bil/emacs' directory, the `/usr/local/lisplib' directory, and the `~/emacs' directory, which are then followed by the standard directories for Lisp code. The command line options `-l' or `-load' specify a Lisp library to load as part of Emacs startup. Since this file might be in the current directory, Emacs 18 temporarily adds the current directory to the front of `load-path' so the file can be found there. Newer Emacs versions also find such files in the current directory, but without altering `load-path'. Dumping Emacs uses a special value of `load-path'. If the value of `load-path' at the end of dumping is unchanged (that is, still the same special value), the dumped Emacs switches to the ordinary `load-path' value when it starts up, as described above. But if `load-path' has any other value at the end of dumping, that value is used for execution of the dumped Emacs also. Therefore, if you want to change `load-path' temporarily for loading a few libraries in `site-init.el' or `site-load.el', you should bind `load-path' locally with `let' around the calls to `load'. - Variable: load-in-progress This variable is non-`nil' if Emacs is in the process of loading a file, and it is `nil' otherwise. - Variable: load-read-function This variable specifies an alternate expression-reading function for `load' and `eval-region' to use instead of `read'. The function should accept one argument, just as `read' does. Normally, the variable's value is `nil', which means those functions should use `read'. To learn how `load' is used to build XEmacs, see *Note Building XEmacs::. File: lispref.info, Node: Autoload, Next: Repeated Loading, Prev: How Programs Do Loading, Up: Loading Autoload ======== The "autoload" facility allows you to make a function or macro known in Lisp, but put off loading the file that defines it. The first call to the function automatically reads the proper file to install the real definition and other associated code, then runs the real definition as if it had been loaded all along. There are two ways to set up an autoloaded function: by calling `autoload', and by writing a special "magic" comment in the source before the real definition. `autoload' is the low-level primitive for autoloading; any Lisp program can call `autoload' at any time. Magic comments do nothing on their own; they serve as a guide for the command `update-file-autoloads', which constructs calls to `autoload' and arranges to execute them when Emacs is built. Magic comments are the most convenient way to make a function autoload, but only for packages installed along with Emacs. - Function: autoload FUNCTION FILENAME &optional DOCSTRING INTERACTIVE TYPE This function defines the function (or macro) named FUNCTION so as to load automatically from FILENAME. The string FILENAME specifies the file to load to get the real definition of FUNCTION. The argument DOCSTRING is the documentation string for the function. Normally, this is the identical to the documentation string in the function definition itself. Specifying the documentation string in the call to `autoload' makes it possible to look at the documentation without loading the function's 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; it's not needed unless the user actually calls FUNCTION, and when that happens, it's time to load the real definition. You can autoload macros and keymaps as well as ordinary functions. Specify TYPE as `macro' if FUNCTION is really a macro. Specify TYPE as `keymap' if FUNCTION is really a keymap. Various parts of Emacs need to know this information without loading the real definition. An autoloaded keymap loads automatically during key lookup when a prefix key's binding is the symbol FUNCTION. Autoloading does not occur for other kinds of access to the keymap. In particular, it does not happen when a Lisp program gets the keymap from the value of a variable and calls `define-key'; not even if the variable name is the same symbol FUNCTION. If FUNCTION already has a non-void function definition that is not an autoload object, `autoload' does nothing and returns `nil'. If the function cell of FUNCTION is void, or is already an autoload object, then it is defined as an autoload object like this: (autoload FILENAME DOCSTRING INTERACTIVE TYPE) For example, (symbol-function 'run-prolog) => (autoload "prolog" 169681 t nil) In this case, `"prolog"' is the name of the file to load, 169681 refers 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 or a keymap. The autoloaded file usually contains 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 its 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 might fail to work properly for the lack of certain subroutines defined later in the file and not loaded successfully. XEmacs as distributed comes with many autoloaded functions. The calls to `autoload' are in the file `loaddefs.el'. There is a convenient way of updating them automatically. If the autoloaded file fails to define the desired Lisp function or macro, then an error is signaled with data `"Autoloading failed to define function FUNCTION-NAME"'. A magic autoload comment looks like `;;;###autoload', on a line by itself, just before the real definition of the function in its autoloadable source file. The command `M-x update-file-autoloads' writes a corresponding `autoload' call into `loaddefs.el'. Building Emacs loads `loaddefs.el' and thus calls `autoload'. `M-x update-directory-autoloads' is even more powerful; it updates autoloads for all files in the current directory. The same magic comment can copy any kind of form into `loaddefs.el'. If the form following the magic comment is not a function definition, it is copied verbatim. You can also use a magic comment to execute a form at build time *without* executing it when the file itself is loaded. To do this, write the form "on the same line" as the magic comment. Since it is in a comment, it does nothing when you load the source file; but `update-file-autoloads' copies it to `loaddefs.el', where it is executed while building Emacs. The following example shows how `doctor' is prepared for autoloading with a magic comment: ;;;###autoload (defun doctor () "Switch to *doctor* buffer and start giving psychotherapy." (interactive) (switch-to-buffer "*doctor*") (doctor-mode)) Here's what that produces 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 tell `make-docfile' to put the documentation string in the `etc/DOC' file. *Note Building XEmacs::. File: lispref.info, Node: Repeated Loading, Next: Named Features, Prev: Autoload, Up: Loading Repeated Loading ================ You may load one 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 it came from. 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 Lisp library file, keep in mind that the file 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))) To add an element to a list just once, use `add-to-list' (*note Setting Variables::.). Occasionally you will want to test explicitly whether a library has already been loaded. Here's one way to test, in a library, whether it has been loaded before: (defvar foo-was-loaded) (if (not (boundp 'foo-was-loaded)) EXECUTE-FIRST-TIME-ONLY) (setq foo-was-loaded t) If the library uses `provide' to provide a named feature, you can use `featurep' to test whether the library has been loaded. *Note Named Features::. File: lispref.info, Node: Named Features, Next: Unloading, 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. A feature name is a symbol that stands for a collection of functions, variables, etc. The file that defines them should "provide" the feature. Another program that uses them may ensure they are defined by "requiring" the feature. This loads the file of definitions if it hasn't been loaded already. 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'; if it fails to do so, `require' signals an error. Features are normally named after the files that provide them, 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: (defun run-prolog () "Run an inferior Prolog process, input and output via buffer *prolog*." (interactive) (require 'comint) (switch-to-buffer (make-comint "prolog" prolog-program-name)) (inferior-prolog-mode)) The expression `(require 'comint)' loads the file `comint.el' if it has not yet been loaded. This ensures that `make-comint' is defined. The `comint.el' file contains the following top-level expression: (provide 'comint) This adds `comint' to the global `features' list, so that `(require 'comint)' will henceforth know that nothing needs to be done. When `require' is used at top level in a file, it takes effect when you byte-compile that file (*note Byte Compilation::.) as well as when you load it. 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. The compiler ignores the `provide', then processes the `require' by loading the file in question. Loading the file does execute the `provide' call, so the subsequent `require' call does nothing while loading. - Function: provide FEATURE This function announces that FEATURE is now loaded, or being loaded, into the current XEmacs 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) When a file is loaded to satisfy an autoload, and it stops due to an error in the evaluating its 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 XEmacs 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 loading the file fails to provide FEATURE, `require' signals an error, `Required feature FEATURE was not provided'. - Function: featurep FEATURE This function returns `t' if FEATURE has been provided in the current XEmacs 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 XEmacs 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: lispref.info, Node: Unloading, Next: Hooks for Loading, Prev: Named Features, Up: Loading Unloading ========= You can discard the functions and variables loaded by a library to reclaim memory for other Lisp objects. To do this, use the function `unload-feature': - Command: unload-feature FEATURE &optional FORCE This command unloads the library that provided feature FEATURE. It undefines all functions, macros, and variables defined in that library with `defconst', `defvar', `defun', `defmacro', `defsubst' and `defalias'. It then restores any autoloads formerly associated with those symbols. (Loading saves these in the `autoload' property of the symbol.) Ordinarily, `unload-feature' refuses to unload a library on which other loaded libraries depend. (A library A depends on library B if A contains a `require' for B.) If the optional argument FORCE is non-`nil', dependencies are ignored and you can unload any library. The `unload-feature' function is written in Lisp; its actions are based on the variable `load-history'. - Variable: load-history This variable's value is an alist connecting library names with the names of functions and variables they define, the features they provide, and the features they require. Each element is a list and describes one library. The CAR of the list is the name of the library, as a string. The rest of the list is composed of these kinds of objects: * Symbols that were defined by this library. * Lists of the form `(require . FEATURE)' indicating features that were required. * Lists of the form `(provide . FEATURE)' indicating features that were provided. The value of `load-history' may have one element whose CAR is `nil'. This element describes definitions made with `eval-buffer' on a buffer that is not visiting a file. The command `eval-region' updates `load-history', but does so by adding the symbols defined to the element for the file being visited, rather than replacing that element. File: lispref.info, Node: Hooks for Loading, Prev: Unloading, Up: Loading Hooks for Loading ================= You can ask for code to be executed if and when a particular library is loaded, by calling `eval-after-load'. - Function: eval-after-load LIBRARY FORM This function arranges to evaluate FORM at the end of loading the library LIBRARY, if and when LIBRARY is loaded. If LIBRARY is already loaded, it evaluates FORM right away. The library name LIBRARY must exactly match the argument of `load'. To get the proper results when an installed library is found by searching `load-path', you should not include any directory names in LIBRARY. An error in FORM does not undo the load, but does prevent execution of the rest of FORM. In general, well-designed Lisp programs should not use this feature. The clean and modular ways to interact with a Lisp library are (1) examine and set the library's variables (those which are meant for outside use), and (2) call the library's functions. If you wish to do (1), you can do it immediately--there is no need to wait for when the library is loaded. To do (2), you must load the library (preferably with `require'). But it is ok to use `eval-after-load' in your personal customizations if you don't feel they must meet the design standards of programs to be released. - Variable: after-load-alist An alist of expressions to evaluate if and when particular libraries are loaded. Each element looks like this: (FILENAME FORMS...) The function `load' checks `after-load-alist' in order to implement `eval-after-load'. File: lispref.info, Node: Byte Compilation, Next: Debugging, Prev: Loading, Up: Top Byte Compilation **************** XEmacs 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. In general, any version of Emacs can run byte-compiled code produced by recent earlier versions of Emacs, but the reverse is not true. In particular, if you compile a program with Emacs 19.29, the compiled code does not run in earlier versions. Files compiled in versions before 19.29 may not work in 19.29 if they contain character constants with modifier bits, because the bits were renumbered in Emacs 19.29. *Note Compilation Errors::, for how to investigate errors occurring in byte compilation. * Menu: * Speed of Byte-Code:: An example of speedup from byte compilation. * Compilation Functions:: Byte compilation functions. * Docs and Compilation:: Dynamic loading of documentation strings. * Dynamic Loading:: Dynamic loading of individual functions. * Eval During Compile:: Code to be evaluated when you compile. * Byte-Code Objects:: The data type used for byte-compiled functions. * Disassembly:: Disassembling byte-code; how to read byte-code. File: lispref.info, Node: Speed of Byte-Code, Next: Compilation Functions, Up: Byte Compilation Performance of Byte-Compiled Code ================================= A byte-compiled function is not as efficient as a primitive function written in C, but runs much faster than the version written in Lisp. Here is an example: (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) => ("Fri Mar 18 17:25:57 1994" "Fri Mar 18 17:26:28 1994") ; 31 seconds (byte-compile 'silly-loop) => [Compiled code not shown] (silly-loop 100000) => ("Fri Mar 18 17:26:52 1994" "Fri Mar 18 17:26:58 1994") ; 6 seconds In this example, the interpreted code required 31 seconds to run, whereas the byte-compiled code required 6 seconds. These results are representative, but actual results will vary greatly.