-This is Info file ../../info/lispref.info, produced by Makeinfo version
-1.68 from the input file lispref.texi.
+This is ../info/lispref.info, produced by makeinfo version 4.0b from
+lispref/lispref.texi.
INFO-DIR-SECTION XEmacs Editor
START-INFO-DIR-ENTRY
Foundation instead of in the original English.
\1f
+File: lispref.info, Node: Intro to Buffer-Local, Next: Creating Buffer-Local, Up: Buffer-Local Variables
+
+Introduction to Buffer-Local Variables
+--------------------------------------
+
+ A buffer-local variable has a buffer-local binding associated with a
+particular buffer. The binding is in effect when that buffer is
+current; otherwise, it is not in effect. If you set the variable while
+a buffer-local binding is in effect, the new value goes in that binding,
+so the global binding is unchanged; this means that the change is
+visible in that buffer alone.
+
+ A variable may have buffer-local bindings in some buffers but not in
+others. The global binding is shared by all the buffers that don't have
+their own bindings. Thus, if you set the variable in a buffer that does
+not have a buffer-local binding for it, the new value is visible in all
+buffers except those with buffer-local bindings. (Here we are assuming
+that there are no `let'-style local bindings to complicate the issue.)
+
+ The most common use of buffer-local bindings is for major modes to
+change variables that control the behavior of commands. For example, C
+mode and Lisp mode both set the variable `paragraph-start' to specify
+that only blank lines separate paragraphs. They do this by making the
+variable buffer-local in the buffer that is being put into C mode or
+Lisp mode, and then setting it to the new value for that mode.
+
+ The usual way to make a buffer-local binding is with
+`make-local-variable', which is what major mode commands use. This
+affects just the current buffer; all other buffers (including those yet
+to be created) continue to share the global value.
+
+ A more powerful operation is to mark the variable as "automatically
+buffer-local" by calling `make-variable-buffer-local'. You can think
+of this as making the variable local in all buffers, even those yet to
+be created. More precisely, the effect is that setting the variable
+automatically makes the variable local to the current buffer if it is
+not already so. All buffers start out by sharing the global value of
+the variable as usual, but any `setq' creates a buffer-local binding
+for the current buffer. The new value is stored in the buffer-local
+binding, leaving the (default) global binding untouched. The global
+value can no longer be changed with `setq'; you need to use
+`setq-default' to do that.
+
+ Local variables in a file you edit are also represented by
+buffer-local bindings for the buffer that holds the file within XEmacs.
+*Note Auto Major Mode::.
+
+\1f
+File: lispref.info, Node: Creating Buffer-Local, Next: Default Value, Prev: Intro to Buffer-Local, Up: Buffer-Local Variables
+
+Creating and Deleting Buffer-Local Bindings
+-------------------------------------------
+
+ - Command: make-local-variable variable
+ This function creates a buffer-local binding in the current buffer
+ for VARIABLE (a symbol). Other buffers are not affected. The
+ value returned is VARIABLE.
+
+ The buffer-local value of VARIABLE starts out as the same value
+ VARIABLE previously had. If VARIABLE was void, it remains void.
+
+ ;; In buffer `b1':
+ (setq foo 5) ; Affects all buffers.
+ => 5
+ (make-local-variable 'foo) ; Now it is local in `b1'.
+ => foo
+ foo ; That did not change
+ => 5 ; the value.
+ (setq foo 6) ; Change the value
+ => 6 ; in `b1'.
+ foo
+ => 6
+
+ ;; In buffer `b2', the value hasn't changed.
+ (save-excursion
+ (set-buffer "b2")
+ foo)
+ => 5
+
+ Making a variable buffer-local within a `let'-binding for that
+ variable does not work. This is because `let' does not distinguish
+ between different kinds of bindings; it knows only which variable
+ the binding was made for.
+
+ *Please note:* do not use `make-local-variable' for a hook
+ variable. Instead, use `make-local-hook'. *Note Hooks::.
+
+ - Command: make-variable-buffer-local variable
+ This function marks VARIABLE (a symbol) automatically
+ buffer-local, so that any subsequent attempt to set it will make it
+ local to the current buffer at the time.
+
+ The value returned is VARIABLE.
+
+ - Function: local-variable-p variable buffer &optional after-set
+ This returns `t' if VARIABLE is buffer-local in buffer BUFFER;
+ else `nil'.
+
+ If optional third arg AFTER-SET is non-`nil', return `t' if SYMBOL
+ would be buffer-local after it is set, regardless of whether it is
+ so presently.
+
+ A `nil' value for BUFFER is _not_ the same as `(current-buffer)',
+ but means "no buffer". Specifically:
+
+ If BUFFER is `nil' and AFTER-SET is `nil', a return value of `t'
+ indicates that the variable is one of the special built-in
+ variables that is always buffer-local. (This includes
+ `buffer-file-name', `buffer-read-only', `buffer-undo-list', and
+ others.)
+
+ If BUFFER is `nil' and AFTER-SET is `t', a return value of `t'
+ indicates that the variable has had `make-variable-buffer-local'
+ applied to it.
+
+ - Function: buffer-local-variables &optional buffer
+ This function returns a list describing the buffer-local variables
+ in buffer BUFFER. It returns an association list (*note
+ Association Lists::) in which each association contains one
+ buffer-local variable and its value. When a buffer-local variable
+ is void in BUFFER, then it appears directly in the resulting list.
+ If BUFFER is omitted, the current buffer is used.
+
+ (make-local-variable 'foobar)
+ (makunbound 'foobar)
+ (make-local-variable 'bind-me)
+ (setq bind-me 69)
+ (setq lcl (buffer-local-variables))
+ ;; First, built-in variables local in all buffers:
+ => ((mark-active . nil)
+ (buffer-undo-list nil)
+ (mode-name . "Fundamental")
+ ...
+ ;; Next, non-built-in local variables.
+ ;; This one is local and void:
+ foobar
+ ;; This one is local and nonvoid:
+ (bind-me . 69))
+
+ Note that storing new values into the CDRs of cons cells in this
+ list does _not_ change the local values of the variables.
+
+ - Command: kill-local-variable variable
+ This function deletes the buffer-local binding (if any) for
+ VARIABLE (a symbol) in the current buffer. As a result, the
+ global (default) binding of VARIABLE becomes visible in this
+ buffer. Usually this results in a change in the value of
+ VARIABLE, since the global value is usually different from the
+ buffer-local value just eliminated.
+
+ If you kill the local binding of a variable that automatically
+ becomes local when set, this makes the global value visible in the
+ current buffer. However, if you set the variable again, that will
+ once again create a local binding for it.
+
+ `kill-local-variable' returns VARIABLE.
+
+ This function is a command because it is sometimes useful to kill
+ one buffer-local variable interactively, just as it is useful to
+ create buffer-local variables interactively.
+
+ - Function: kill-all-local-variables
+ This function eliminates all the buffer-local variable bindings of
+ the current buffer except for variables marked as "permanent". As
+ a result, the buffer will see the default values of most variables.
+
+ This function also resets certain other information pertaining to
+ the buffer: it sets the local keymap to `nil', the syntax table to
+ the value of `standard-syntax-table', and the abbrev table to the
+ value of `fundamental-mode-abbrev-table'.
+
+ Every major mode command begins by calling this function, which
+ has the effect of switching to Fundamental mode and erasing most
+ of the effects of the previous major mode. To ensure that this
+ does its job, the variables that major modes set should not be
+ marked permanent.
+
+ `kill-all-local-variables' returns `nil'.
+
+ A local variable is "permanent" if the variable name (a symbol) has a
+`permanent-local' property that is non-`nil'. Permanent locals are
+appropriate for data pertaining to where the file came from or how to
+save it, rather than with how to edit the contents.
+
+\1f
+File: lispref.info, Node: Default Value, Prev: Creating Buffer-Local, Up: Buffer-Local Variables
+
+The Default Value of a Buffer-Local Variable
+--------------------------------------------
+
+ The global value of a variable with buffer-local bindings is also
+called the "default" value, because it is the value that is in effect
+except when specifically overridden.
+
+ The functions `default-value' and `setq-default' access and change a
+variable's default value regardless of whether the current buffer has a
+buffer-local binding. For example, you could use `setq-default' to
+change the default setting of `paragraph-start' for most buffers; and
+this would work even when you are in a C or Lisp mode buffer that has a
+buffer-local value for this variable.
+
+ The special forms `defvar' and `defconst' also set the default value
+(if they set the variable at all), rather than any local value.
+
+ - Function: default-value symbol
+ This function returns SYMBOL's default value. This is the value
+ that is seen in buffers that do not have their own values for this
+ variable. If SYMBOL is not buffer-local, this is equivalent to
+ `symbol-value' (*note Accessing Variables::).
+
+ - Function: default-boundp symbol
+ The function `default-boundp' tells you whether SYMBOL's default
+ value is nonvoid. If `(default-boundp 'foo)' returns `nil', then
+ `(default-value 'foo)' would get an error.
+
+ `default-boundp' is to `default-value' as `boundp' is to
+ `symbol-value'.
+
+ - Special Form: setq-default symbol value
+ This sets the default value of SYMBOL to VALUE. It does not
+ evaluate SYMBOL, but does evaluate VALUE. The value of the
+ `setq-default' form is VALUE.
+
+ If a SYMBOL is not buffer-local for the current buffer, and is not
+ marked automatically buffer-local, `setq-default' has the same
+ effect as `setq'. If SYMBOL is buffer-local for the current
+ buffer, then this changes the value that other buffers will see
+ (as long as they don't have a buffer-local value), but not the
+ value that the current buffer sees.
+
+ ;; In buffer `foo':
+ (make-local-variable 'local)
+ => local
+ (setq local 'value-in-foo)
+ => value-in-foo
+ (setq-default local 'new-default)
+ => new-default
+ local
+ => value-in-foo
+ (default-value 'local)
+ => new-default
+
+ ;; In (the new) buffer `bar':
+ local
+ => new-default
+ (default-value 'local)
+ => new-default
+ (setq local 'another-default)
+ => another-default
+ (default-value 'local)
+ => another-default
+
+ ;; Back in buffer `foo':
+ local
+ => value-in-foo
+ (default-value 'local)
+ => another-default
+
+ - Function: set-default symbol value
+ This function is like `setq-default', except that SYMBOL is
+ evaluated.
+
+ (set-default (car '(a b c)) 23)
+ => 23
+ (default-value 'a)
+ => 23
+
+\1f
File: lispref.info, Node: Variable Aliases, Prev: Buffer-Local Variables, Up: Variables
Variable Aliases
returned. Any time you change the value of the former variable, the
value of the latter is actually changed. This is useful in cases where
you want to rename a variable but still make old code work (*note
-Obsoleteness::.).
+Obsoleteness::).
- - Function: defvaralias VARIABLE ALIAS
+ - Function: defvaralias variable alias
This function defines VARIABLE as an alias for ALIAS.
Thenceforth, any operations performed on VARIABLE will actually be
performed on ALIAS. Both VARIABLE and ALIAS should be symbols.
variable, a variable that has a buffer-local value in any buffer,
or the symbols `nil' or `t'.
- - Function: variable-alias VARIABLE
+ - Function: variable-alias variable &optional follow-past-lisp-magic
If VARIABLE is aliased to another variable, this function returns
that variable. VARIABLE should be a symbol. If VARIABLE is not
aliased, this function returns `nil'.
- - Function: indirect-variable OBJECT
+ - Function: indirect-variable object &optional follow-past-lisp-magic
This function returns the variable at the end of OBJECT's
variable-alias chain. If OBJECT is a symbol, follow all variable
aliases and return the final (non-aliased) symbol. If OBJECT is
A "command" is an object that `command-execute' can invoke; it is
a possible definition for a key sequence. Some functions are
commands; a function written in Lisp is a command if it contains an
- interactive declaration (*note Defining Commands::.). Such a
+ interactive declaration (*note Defining Commands::). Such a
function can be called from Lisp expressions like other functions;
in this case, the fact that the function is a command makes no
difference.
A "compiled function" is a function that has been compiled by the
byte compiler. *Note Compiled-Function Type::.
- - Function: subrp OBJECT
+ - Function: subrp object
This function returns `t' if OBJECT is a built-in function (i.e.,
a Lisp primitive).
(subrp (symbol-function 'message))
=> t
- - Function: compiled-function-p OBJECT
+ - Function: compiled-function-p object
This function returns `t' if OBJECT is a compiled function. For
example:
It is not often useful to write a lambda expression as the CAR of a
form in this way. You can get the same result, of making local
variables and giving them values, using the special form `let' (*note
-Local Variables::.). And `let' is clearer and easier to use. In
+Local Variables::). And `let' is clearer and easier to use. In
practice, lambda expressions are either stored as the function
definitions of symbols, to produce named functions, or passed as
-arguments to other functions (*note Anonymous Functions::.).
+arguments to other functions (*note Anonymous Functions::).
However, calls to explicit lambda expressions were very useful in the
old days of Lisp, before the special form `let' was invented. At that
source file, but since these spaces come before the starting
double-quote, they are not part of the string. Some people make a
practice of indenting any additional lines of the string so that the
-text lines up in the program source. *This is a mistake.* The
+text lines up in the program source. _This is a mistake._ The
indentation of the following lines is inside the string; what looks
nice in the source code will look ugly when displayed by the help
commands.
However, a symbol can serve as the name of a function. This happens
when you put the function in the symbol's "function cell" (*note Symbol
-Components::.). Then the symbol itself becomes a valid, callable
+Components::). Then the symbol itself becomes a valid, callable
function, equivalent to the list or subr-object that its function cell
refers to. The contents of the function cell are also called the
symbol's "function definition". The procedure of using a symbol's
distinguish.
Even so, keep in mind that a function need not have a unique name.
-While a given function object *usually* appears in the function cell of
+While a given function object _usually_ appears in the function cell of
only one symbol, this is just a matter of convenience. It is easy to
store it in several symbols using `fset'; then each of the symbols is
equally well a name for the same function.
is called "defining a function", and it is done with the `defun'
special form.
- - Special Form: defun NAME ARGUMENT-LIST BODY-FORMS
+ - Special Form: defun name argument-list body-forms
`defun' is the usual way to define new Lisp functions. It defines
the symbol NAME as a function that looks like this:
`defun' stores this lambda expression in the function cell of
NAME. It returns the value NAME, but usually we ignore this value.
- As described previously (*note Lambda Expressions::.),
+ As described previously (*note Lambda Expressions::),
ARGUMENT-LIST is a list of argument names and may include the
keywords `&optional' and `&rest'. Also, the first two forms in
BODY-FORMS may be a documentation string and an interactive
distinguish deliberate redefinition from unintentional
redefinition.
- - Function: define-function NAME DEFINITION
- - Function: defalias NAME DEFINITION
+ - Function: define-function name definition
+ - Function: defalias name definition
These equivalent special forms define the symbol NAME as a
function, with definition DEFINITION (which can be any valid Lisp
function).
specific function name is being defined--especially where that
name appears explicitly in the source file being loaded. This is
because `define-function' and `defalias' record which file defined
- the function, just like `defun'. (*note Unloading::.).
+ the function, just like `defun'. (*note Unloading::).
By contrast, in programs that manipulate function definitions for
other purposes, it is better to use `fset', which does not keep
run time which function to call. To do that, use the functions
`funcall' and `apply'.
- - Function: funcall FUNCTION &rest ARGUMENTS
+ - Function: funcall function &rest arguments
`funcall' calls FUNCTION with ARGUMENTS, and returns whatever
FUNCTION returns.
that you can use any expression to obtain the function to be
called. It also means that `funcall' does not see the expressions
you write for the ARGUMENTS, only their values. These values are
- *not* evaluated a second time in the act of calling FUNCTION;
+ _not_ evaluated a second time in the act of calling FUNCTION;
`funcall' enters the normal procedure for calling a function at the
place where the arguments have already been evaluated.
Compare these example with the examples of `apply'.
- - Function: apply FUNCTION &rest ARGUMENTS
+ - Function: apply function &rest arguments
`apply' calls FUNCTION with ARGUMENTS, just like `funcall' but
with one difference: the last of ARGUMENTS is a list of arguments
to give to FUNCTION, rather than a single argument. We also say
function as the argument. Here are two different kinds of no-op
function:
- - Function: identity ARG
+ - Function: identity arg
This function returns ARG and has no side effects.
- - Function: ignore &rest ARGS
+ - Command: ignore &rest args
This function ignores any arguments and returns `nil'.
\1f
=================
A "mapping function" applies a given function to each element of a
-list or other collection. XEmacs Lisp has three such functions;
-`mapcar' and `mapconcat', which scan a list, are described here. For
-the third mapping function, `mapatoms', see *Note Creating Symbols::.
+list or other collection. XEmacs Lisp has several such functions;
+`mapcar' and `mapconcat', which scan a list, are described here.
+*Note Creating Symbols::, for the function `mapatoms' which maps over
+the symbols in an obarray.
- - Function: mapcar FUNCTION SEQUENCE
+ Mapping functions should never modify the sequence being mapped over.
+The results are unpredictable.
+
+ - Function: mapcar function sequence
`mapcar' applies FUNCTION to each element of SEQUENCE in turn, and
returns a list of the results.
- The argument SEQUENCE may be a list, a vector, or a string. The
- result is always a list. The length of the result is the same as
- the length of SEQUENCE.
+ The argument SEQUENCE can be any kind of sequence; that is, a
+ list, a vector, a bit vector, or a string. The result is always a
+ list. The length of the result is the same as the length of
+ SEQUENCE.
For example:
=> (2 3 4)
(mapcar 'char-to-string "abc")
=> ("a" "b" "c")
-
+
;; Call each function in `my-hooks'.
(mapcar 'funcall my-hooks)
-
+
(defun mapcar* (f &rest args)
"Apply FUNCTION to successive cars of all ARGS.
Return the list of results."
(apply 'mapcar* f
;; Recurse for rest of elements.
(mapcar 'cdr args)))))
-
+
(mapcar* 'cons '(a b c) '(1 2 3 4))
=> ((a . 1) (b . 2) (c . 3))
- - Function: mapconcat FUNCTION SEQUENCE SEPARATOR
+ - Function: mapconcat function sequence separator
`mapconcat' applies FUNCTION to each element of SEQUENCE: the
results, which must be strings, are concatenated. Between each
pair of result strings, `mapconcat' inserts the string SEPARATOR.
punctuation.
The argument FUNCTION must be a function that can take one
- argument and return a string.
+ argument and return a string. The argument SEQUENCE can be any
+ kind of sequence; that is, a list, a vector, a bit vector, or a
+ string.
(mapconcat 'symbol-name
'(The cat in the hat)
" ")
=> "The cat in the hat"
-
+
(mapconcat (function (lambda (x) (format "%c" (1+ x))))
"HAL-8000"
"")
=> (lambda (x) (+ 12 x))
This computes a list that looks like `(lambda (x) (+ 12 x))' and makes
-it the value (*not* the function definition!) of `silly'.
+it the value (_not_ the function definition!) of `silly'.
Here is how we might call this function:
(funcall silly 1)
=> 13
-(It does *not* work to write `(silly 1)', because this function is not
-the *function definition* of `silly'. We have not given `silly' any
+(It does _not_ work to write `(silly 1)', because this function is not
+the _function definition_ of `silly'. We have not given `silly' any
function definition, just a value as a variable.)
Most of the time, anonymous functions are constants that appear in
In such cases, we usually use the special form `function' instead of
simple quotation to quote the anonymous function.
- - Special Form: function FUNCTION-OBJECT
+ - Special Form: function function-object
This special form returns FUNCTION-OBJECT without evaluating it.
In this, it is equivalent to `quote'. However, it serves as a
note to the XEmacs Lisp compiler that FUNCTION-OBJECT is intended
See also the function `indirect-function' in *Note Function
Indirection::.
- - Function: symbol-function SYMBOL
+ - Function: symbol-function symbol
This returns the object in the function cell of SYMBOL. If the
symbol's function cell is void, a `void-function' error is
signaled.
`fboundp'. After you have given a symbol a function definition, you
can make it void once more using `fmakunbound'.
- - Function: fboundp SYMBOL
- This function returns `t' if the symbol has an object in its
- function cell, `nil' otherwise. It does not check that the object
- is a legitimate function.
+ - Function: fboundp symbol
+ This function returns `t' if SYMBOL has an object in its function
+ cell, `nil' otherwise. It does not check that the object is a
+ legitimate function.
- - Function: fmakunbound SYMBOL
+ - Function: fmakunbound symbol
This function makes SYMBOL's function cell void, so that a
subsequent attempt to access this cell will cause a `void-function'
error. (See also `makunbound', in *Note Local Variables::.)
(foo 1)
error--> Symbol's function definition is void: foo
- - Function: fset SYMBOL OBJECT
+ - Function: fset symbol object
This function stores OBJECT in the function cell of SYMBOL. The
result is OBJECT. Normally OBJECT should be a function or the
name of a function, but this is not checked.
* Giving a symbol a function definition that is not a list and
therefore cannot be made with `defun'. For example, you can
- use `fset' to give a symbol `s1' a function definition which
- is another symbol `s2'; then `s1' serves as an alias for
- whatever definition `s2' presently has.
+ use `fset' to give a symbol SYMBOL1 a function definition
+ which is another symbol SYMBOL2; then SYMBOL1 serves as an
+ alias for whatever definition SYMBOL2 presently has.
* In constructs for defining or altering functions. If `defun'
were not a primitive, it could be written in Lisp (as a
But it is unmodular and unclean, in any case, for a Lisp file to
redefine a function defined elsewhere.
-\1f
-File: lispref.info, Node: Inline Functions, Next: Related Topics, Prev: Function Cells, Up: Functions
-
-Inline Functions
-================
-
- You can define an "inline function" by using `defsubst' instead of
-`defun'. An inline function works just like an ordinary function
-except for one thing: when you compile a call to the function, the
-function's definition is open-coded into the caller.
-
- Making a function inline makes explicit calls run faster. But it
-also has disadvantages. For one thing, it reduces flexibility; if you
-change the definition of the function, calls already inlined still use
-the old definition until you recompile them. Since the flexibility of
-redefining functions is an important feature of XEmacs, you should not
-make a function inline unless its speed is really crucial.
-
- Another disadvantage is that making a large function inline can
-increase the size of compiled code both in files and in memory. Since
-the speed advantage of inline functions is greatest for small
-functions, you generally should not make large functions inline.
-
- It's possible to define a macro to expand into the same code that an
-inline function would execute. But the macro would have a limitation:
-you can use it only explicitly--a macro cannot be called with `apply',
-`mapcar' and so on. Also, it takes some work to convert an ordinary
-function into a macro. (*Note Macros::.) To convert it into an inline
-function is very easy; simply replace `defun' with `defsubst'. Since
-each argument of an inline function is evaluated exactly once, you
-needn't worry about how many times the body uses the arguments, as you
-do for macros. (*Note Argument Evaluation::.)
-
- Inline functions can be used and open-coded later on in the same
-file, following the definition, just like macros.
-
-\1f
-File: lispref.info, Node: Related Topics, Prev: Inline Functions, Up: Functions
-
-Other Topics Related to Functions
-=================================
-
- Here is a table of several functions that do things related to
-function calling and function definitions. They are documented
-elsewhere, but we provide cross references here.
-
-`apply'
- See *Note Calling Functions::.
-
-`autoload'
- See *Note Autoload::.
-
-`call-interactively'
- See *Note Interactive Call::.
-
-`commandp'
- See *Note Interactive Call::.
-
-`documentation'
- See *Note Accessing Documentation::.
-
-`eval'
- See *Note Eval::.
-
-`funcall'
- See *Note Calling Functions::.
-
-`ignore'
- See *Note Calling Functions::.
-
-`indirect-function'
- See *Note Function Indirection::.
-
-`interactive'
- See *Note Using Interactive::.
-
-`interactive-p'
- See *Note Interactive Call::.
-
-`mapatoms'
- See *Note Creating Symbols::.
-
-`mapcar'
- See *Note Mapping Functions::.
-
-`mapconcat'
- See *Note Mapping Functions::.
-
-`undefined'
- See *Note Key Lookup::.
-
-\1f
-File: lispref.info, Node: Macros, Next: Loading, Prev: Functions, Up: Top
-
-Macros
-******
-
- "Macros" enable you to define new control constructs and other
-language features. A macro is defined much like a function, but instead
-of telling how to compute a value, it tells how to compute another Lisp
-expression which will in turn compute the value. We call this
-expression the "expansion" of the macro.
-
- Macros can do this because they operate on the unevaluated
-expressions for the arguments, not on the argument values as functions
-do. They can therefore construct an expansion containing these
-argument expressions or parts of them.
-
- If you are using a macro to do something an ordinary function could
-do, just for the sake of speed, consider using an inline function
-instead. *Note Inline Functions::.
-
-* Menu:
-
-* Simple Macro:: A basic example.
-* Expansion:: How, when and why macros are expanded.
-* Compiling Macros:: How macros are expanded by the compiler.
-* Defining Macros:: How to write a macro definition.
-* Backquote:: Easier construction of list structure.
-* Problems with Macros:: Don't evaluate the macro arguments too many times.
- Don't hide the user's variables.
-
-\1f
-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'.
-
-\1f
-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.
-
-\1f
-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::.).
-
-\1f
-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.
-
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