@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
-@c Copyright (C) 1990-1995, 1998, 2001-2011 Free Software Foundation, Inc.
+@c Copyright (C) 1990-1995, 1998, 2001-2015 Free Software Foundation,
+@c Inc.
@c See the file elisp.texi for copying conditions.
-@setfilename ../../info/macros
-@node Macros, Customization, Functions, Top
+@node Macros
@chapter Macros
@cindex macros
* 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.
* Indenting Macros:: Specifying how to indent macro calls.
@section A Simple Example of a Macro
Suppose we would like to define a Lisp construct to increment a
-variable value, much like the @code{++} operator in C. We would like to
+variable value, much like the @code{++} operator in C@. We would like to
write @code{(inc x)} and have the effect of @code{(setq x (1+ x))}.
Here's a macro definition that does the job:
returns this expansion, Lisp proceeds to evaluate it, thus incrementing
@code{x}.
+@defun macrop object
+This predicate tests whether its argument is a macro, and returns
+@code{t} if so, @code{nil} otherwise.
+@end defun
+
@node Expansion
@section Expansion of a Macro Call
@cindex expansion of macros
case of a @code{&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 @dfn{expansion} of the macro. The Lisp interpreter
+ The second crucial difference between macros and functions is that
+the value returned by the macro body is an alternate Lisp expression,
+also known as the @dfn{expansion} of the macro. The Lisp interpreter
proceeds to evaluate the expansion as soon as it comes back from the
macro.
calls to other macros. It may even be a call to the same macro, though
this is unusual.
+ Note that Emacs tries to expand macros when loading an uncompiled
+Lisp file. This is not always possible, but if it is, it speeds up
+subsequent execution. @xref{How Programs Do Loading}.
+
You can see the expansion of a given macro call by calling
@code{macroexpand}.
definitions that shadow the currently defined macros. Byte compilation
uses this feature.
-@smallexample
+@example
@group
(defmacro inc (var)
(list 'setq var (list '1+ var)))
- @result{} inc
@end group
@group
@group
(defmacro inc2 (var1 var2)
(list 'progn (list 'inc var1) (list 'inc var2)))
- @result{} inc2
@end group
@group
(macroexpand '(inc2 r s))
@result{} (progn (inc r) (inc s)) ; @r{@code{inc} not expanded here.}
@end group
-@end smallexample
+@end example
@end defun
@code{macroexpand-all}, we see that @code{macroexpand-all} @emph{does}
expand the embedded calls to @code{inc}:
-@smallexample
+@example
(macroexpand-all '(inc2 r s))
@result{} (progn (setq r (1+ r)) (setq s (1+ s)))
-@end smallexample
+@end example
@end defun
@node Defining Macros
@section Defining Macros
+@cindex defining macros
+@cindex macro, how to define
- A Lisp macro is a list whose @sc{car} is @code{macro}. Its @sc{cdr} should
-be a function; expansion of the macro works by applying the function
-(with @code{apply}) to the list of unevaluated argument-expressions
-from the macro call.
+ A Lisp macro object is a list whose @sc{car} is @code{macro}, and
+whose @sc{cdr} is a function. Expansion of the macro works
+by applying the function (with @code{apply}) to the list of
+@emph{unevaluated} arguments 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 @code{mapcar}. In practice,
-all Lisp macros have names, and they are usually defined with the
-special form @code{defmacro}.
+function, but this is never done, because it does not make sense to
+pass an anonymous macro to functionals such as @code{mapcar}. In
+practice, all Lisp macros have names, and they are almost always
+defined with the @code{defmacro} macro.
-@defspec defmacro name argument-list body-forms@dots{}
-@code{defmacro} defines the symbol @var{name} as a macro that looks
-like this:
+@defmac defmacro name args [doc] [declare] body@dots{}
+@code{defmacro} defines the symbol @var{name} (which should not be
+quoted) as a macro that looks like this:
@example
-(macro lambda @var{argument-list} . @var{body-forms})
+(macro lambda @var{args} . @var{body})
@end example
-(Note that the @sc{cdr} of this list is a function---a lambda expression.)
-This macro object is stored in the function cell of @var{name}. The
-value returned by evaluating the @code{defmacro} form is @var{name}, but
-usually we ignore this value.
-
-The shape and meaning of @var{argument-list} is the same as in a
-function, and the keywords @code{&rest} and @code{&optional} may be used
-(@pxref{Argument List}). Macros may have a documentation string, but
-any @code{interactive} declaration is ignored since macros cannot be
-called interactively.
-@end defspec
-
- The body of the macro definition can include a @code{declare} form,
-which can specify how @key{TAB} should indent macro calls, and how to
-step through them for Edebug.
-
-@defmac declare @var{specs}@dots{}
-@anchor{Definition of declare}
-A @code{declare} form is used in a macro definition to specify various
-additional information about it. Two kinds of specification are
-currently supported:
-
-@table @code
-@item (debug @var{edebug-form-spec})
-Specify how to step through macro calls for Edebug.
-@xref{Instrumenting Macro Calls}.
-
-@item (indent @var{indent-spec})
-Specify how to indent calls to this macro. @xref{Indenting Macros},
-for more details.
-@end table
-
-A @code{declare} form only has its special effect in the body of a
-@code{defmacro} form if it immediately follows the documentation
-string, if present, or the argument list otherwise. (Strictly
-speaking, @emph{several} @code{declare} forms can follow the
-documentation string or argument list, but since a @code{declare} form
-can have several @var{specs}, they can always be combined into a
-single form.) When used at other places in a @code{defmacro} form, or
-outside a @code{defmacro} form, @code{declare} just returns @code{nil}
-without evaluating any @var{specs}.
+(Note that the @sc{cdr} of this list is a lambda expression.) This
+macro object is stored in the function cell of @var{name}. The
+meaning of @var{args} is the same as in a function, and the keywords
+@code{&rest} and @code{&optional} may be used (@pxref{Argument List}).
+Neither @var{name} nor @var{args} should be quoted. The return value
+of @code{defmacro} is undefined.
+
+@var{doc}, if present, should be a string specifying the macro's
+documentation string. @var{declare}, if present, should be a
+@code{declare} form specifying metadata for the macro (@pxref{Declare
+Form}). Note that macros cannot have interactive declarations, since
+they cannot be called interactively.
@end defmac
- No macro absolutely needs a @code{declare} form, because that form
-has no effect on how the macro expands, on what the macro means in the
-program. It only affects secondary features: indentation and Edebug.
-
-@node Backquote
-@section Backquote
-@cindex backquote (list substitution)
-@cindex ` (list substitution)
-@findex `
-
- Macros often need to construct large list structures from a mixture of
-constants and nonconstant parts. To make this easier, use the @samp{`}
-syntax (usually called @dfn{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 @code{quote} (@pxref{Quoting}). For example, these
-two forms yield identical results:
-
-@example
-@group
-`(a list of (+ 2 3) elements)
- @result{} (a list of (+ 2 3) elements)
-@end group
-@group
-'(a list of (+ 2 3) elements)
- @result{} (a list of (+ 2 3) elements)
-@end group
-@end example
-
-@findex , @r{(with backquote)}
-The special marker @samp{,} inside of the argument to backquote
-indicates a value that isn't constant. Backquote evaluates the
-argument of @samp{,} and puts the value in the list structure:
+ Macros often need to construct large list structures from a mixture
+of constants and nonconstant parts. To make this easier, use the
+@samp{`} syntax (@pxref{Backquote}). For example:
@example
-@group
-(list 'a 'list 'of (+ 2 3) 'elements)
- @result{} (a list of 5 elements)
-@end group
-@group
-`(a list of ,(+ 2 3) elements)
- @result{} (a list of 5 elements)
-@end group
-@end example
-
- Substitution with @samp{,} is allowed at deeper levels of the list
-structure also. For example:
-
@example
@group
(defmacro t-becomes-nil (variable)
@equiv{} (if (eq foo t) (setq foo nil))
@end group
@end example
-
-@findex ,@@ @r{(with backquote)}
-@cindex splicing (with backquote)
- You can also @dfn{splice} an evaluated value into the resulting list,
-using the special marker @samp{,@@}. 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 @samp{`} is often unreadable.
-Here are some examples:
-
-@example
-@group
-(setq some-list '(2 3))
- @result{} (2 3)
-@end group
-@group
-(cons 1 (append some-list '(4) some-list))
- @result{} (1 2 3 4 2 3)
-@end group
-@group
-`(1 ,@@some-list 4 ,@@some-list)
- @result{} (1 2 3 4 2 3)
-@end group
-
-@group
-(setq list '(hack foo bar))
- @result{} (hack foo bar)
-@end group
-@group
-(cons 'use
- (cons 'the
- (cons 'words (append (cdr list) '(as elements)))))
- @result{} (use the words foo bar as elements)
-@end group
-@group
-`(use the words ,@@(cdr list) as elements)
- @result{} (use the words foo bar as elements)
-@end group
@end example
+ The body of a macro definition can include a @code{declare} form,
+which specifies additional properties about the macro. @xref{Declare
+Form}.
+
@node Problems with Macros
@section Common Problems Using Macros
+@cindex macro caveats
- The basic facts of macro expansion have counterintuitive consequences.
-This section describes some important consequences that can lead to
+ Macro expansion can have counterintuitive consequences. This
+section describes some important consequences that can lead to
trouble, and rules to follow to avoid trouble.
@menu
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.
+following macro (used to facilitate iteration) illustrates the
+problem. This macro allows us to write a ``for'' loop construct.
@findex for
-@smallexample
+@example
@group
(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)))))))
+ (cons 'while
+ (cons (list '<= var final)
+ (append body (list (list 'inc var)))))))
@end group
-@result{} for
@group
(for i from 1 to 3 do
@print{}3 9
@result{} nil
@end group
-@end smallexample
+@end example
@noindent
The arguments @code{from}, @code{to}, and @code{do} in this macro are
Here's an equivalent definition simplified through use of backquote:
-@smallexample
+@example
@group
(defmacro for (var from init to final do &rest body)
"Execute a simple \"for\" loop.
,@@body
(inc ,var))))
@end group
-@end smallexample
+@end example
Both forms of this definition (with backquote and without) suffer from
the defect that @var{final} is evaluated on every iteration. If
once unless repeated evaluation is part of the intended purpose of the
macro. Here is a correct expansion for the @code{for} macro:
-@smallexample
+@example
@group
(let ((i 1)
(max 3))
(princ (format "%d %d" i square))
(inc i)))
@end group
-@end smallexample
+@end example
Here is a macro definition that creates this expansion:
-@smallexample
+@example
@group
(defmacro for (var from init to final do &rest body)
"Execute a simple for loop: (for i from 1 to 10 do (print i))."
,@@body
(inc ,var))))
@end group
-@end smallexample
+@end example
Unfortunately, this fix introduces another problem,
described in the following section.
follows to make the expansion evaluate the macro arguments the proper
number of times:
-@smallexample
+@example
@group
(defmacro for (var from init to final do &rest body)
"Execute a simple for loop: (for i from 1 to 10 do (print i))."
,@@body
(inc ,var))))
@end group
-@end smallexample
+@end example
@end ifnottex
The new definition of @code{for} has a new problem: it introduces a
local variable named @code{max} which the user does not expect. This
causes trouble in examples such as the following:
-@smallexample
+@example
@group
(let ((max 0))
(for x from 0 to 10 do
(if (< max this)
(setq max this)))))
@end group
-@end smallexample
+@end example
@noindent
The references to @code{max} inside the body of the @code{for}, which
where put by @code{for}. Here is a definition of @code{for} that works
this way:
-@smallexample
+@example
@group
(defmacro for (var from init to final do &rest body)
"Execute a simple for loop: (for i from 1 to 10 do (print i))."
,@@body
(inc ,var)))))
@end group
-@end smallexample
+@end example
@noindent
This creates an uninterned symbol named @code{max} and puts it in the
@group
(defmacro foo (a)
(list 'setq (eval a) t))
- @result{} foo
@end group
@group
(setq x 'b)
@node Indenting Macros
@section Indenting Macros
- You can use the @code{declare} form in the macro definition to
-specify how to @key{TAB} should indent calls to the macro. You
-write it like this:
+ Within a macro definition, you can use the @code{declare} form
+(@pxref{Defining Macros}) to specify how @key{TAB} should indent
+calls to the macro. An indentation specification is written like this:
@example
(declare (indent @var{indent-spec}))
@var{symbol} should be a function name; that function is called to
calculate the indentation of a line within this expression. The
function receives two arguments:
+
@table @asis
+@item @var{pos}
+The position at which the line being indented begins.
@item @var{state}
The value returned by @code{parse-partial-sexp} (a Lisp primitive for
indentation and nesting computation) when it parses up to the
beginning of this line.
-@item @var{pos}
-The position at which the line being indented begins.
@end table
+
@noindent
It should return either a number, which is the number of columns of
indentation for that line, or a list whose car is such a number. The