2 @c This is part of the XEmacs Lisp Reference Manual.
3 @c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc.
4 @c See the file lispref.texi for copying conditions.
5 @setfilename ../../info/macros.info
6 @node Macros, Loading, Functions, Top
10 @dfn{Macros} enable you to define new control constructs and other
11 language features. A macro is defined much like a function, but instead
12 of telling how to compute a value, it tells how to compute another Lisp
13 expression which will in turn compute the value. We call this
14 expression the @dfn{expansion} of the macro.
16 Macros can do this because they operate on the unevaluated expressions
17 for the arguments, not on the argument values as functions do. They can
18 therefore construct an expansion containing these argument expressions
21 If you are using a macro to do something an ordinary function could
22 do, just for the sake of speed, consider using an inline function
23 instead. @xref{Inline Functions}.
26 * Simple Macro:: A basic example.
27 * Expansion:: How, when and why macros are expanded.
28 * Compiling Macros:: How macros are expanded by the compiler.
29 * Defining Macros:: How to write a macro definition.
30 * Backquote:: Easier construction of list structure.
31 * Problems with Macros:: Don't evaluate the macro arguments too many times.
32 Don't hide the user's variables.
36 @section A Simple Example of a Macro
38 Suppose we would like to define a Lisp construct to increment a
39 variable value, much like the @code{++} operator in C. We would like to
40 write @code{(inc x)} and have the effect of @code{(setq x (1+ x))}.
41 Here's a macro definition that does the job:
47 (list 'setq var (list '1+ var)))
51 When this is called with @code{(inc x)}, the argument @code{var} has
52 the value @code{x}---@emph{not} the @emph{value} of @code{x}. The body
53 of the macro uses this to construct the expansion, which is @code{(setq
54 x (1+ x))}. Once the macro definition returns this expansion, Lisp
55 proceeds to evaluate it, thus incrementing @code{x}.
58 @section Expansion of a Macro Call
59 @cindex expansion of macros
62 A macro call looks just like a function call in that it is a list which
63 starts with the name of the macro. The rest of the elements of the list
64 are the arguments of the macro.
66 Evaluation of the macro call begins like evaluation of a function call
67 except for one crucial difference: the macro arguments are the actual
68 expressions appearing in the macro call. They are not evaluated before
69 they are given to the macro definition. By contrast, the arguments of a
70 function are results of evaluating the elements of the function call
73 Having obtained the arguments, Lisp invokes the macro definition just
74 as a function is invoked. The argument variables of the macro are bound
75 to the argument values from the macro call, or to a list of them in the
76 case of a @code{&rest} argument. And the macro body executes and
77 returns its value just as a function body does.
79 The second crucial difference between macros and functions is that the
80 value returned by the macro body is not the value of the macro call.
81 Instead, it is an alternate expression for computing that value, also
82 known as the @dfn{expansion} of the macro. The Lisp interpreter
83 proceeds to evaluate the expansion as soon as it comes back from the
86 Since the expansion is evaluated in the normal manner, it may contain
87 calls to other macros. It may even be a call to the same macro, though
90 You can see the expansion of a given macro call by calling
93 @defun macroexpand form &optional environment
94 @cindex macro expansion
95 This function expands @var{form}, if it is a macro call. If the result
96 is another macro call, it is expanded in turn, until something which is
97 not a macro call results. That is the value returned by
98 @code{macroexpand}. If @var{form} is not a macro call to begin with, it
101 Note that @code{macroexpand} does not look at the subexpressions of
102 @var{form} (although some macro definitions may do so). Even if they
103 are macro calls themselves, @code{macroexpand} does not expand them.
105 The function @code{macroexpand} does not expand calls to inline functions.
106 Normally there is no need for that, since a call to an inline function is
107 no harder to understand than a call to an ordinary function.
109 If @var{environment} is provided, it specifies an alist of macro
110 definitions that shadow the currently defined macros. Byte compilation
116 (list 'setq var (list '1+ var)))
121 (macroexpand '(inc r))
122 @result{} (setq r (1+ r))
126 (defmacro inc2 (var1 var2)
127 (list 'progn (list 'inc var1) (list 'inc var2)))
132 (macroexpand '(inc2 r s))
133 @result{} (progn (inc r) (inc s)) ; @r{@code{inc} not expanded here.}
138 @node Compiling Macros
139 @section Macros and Byte Compilation
140 @cindex byte-compiling macros
142 You might ask why we take the trouble to compute an expansion for a
143 macro and then evaluate the expansion. Why not have the macro body
144 produce the desired results directly? The reason has to do with
147 When a macro call appears in a Lisp program being compiled, the Lisp
148 compiler calls the macro definition just as the interpreter would, and
149 receives an expansion. But instead of evaluating this expansion, it
150 compiles the expansion as if it had appeared directly in the program.
151 As a result, the compiled code produces the value and side effects
152 intended for the macro, but executes at full compiled speed. This would
153 not work if the macro body computed the value and side effects
154 itself---they would be computed at compile time, which is not useful.
156 In order for compilation of macro calls to work, the macros must be
157 defined in Lisp when the calls to them are compiled. The compiler has a
158 special feature to help you do this: if a file being compiled contains a
159 @code{defmacro} form, the macro is defined temporarily for the rest of
160 the compilation of that file. To use this feature, you must define the
161 macro in the same file where it is used and before its first use.
163 Byte-compiling a file executes any @code{require} calls at top-level
164 in the file. This is in case the file needs the required packages for
165 proper compilation. One way to ensure that necessary macro definitions
166 are available during compilation is to require the files that define
167 them (@pxref{Named Features}). To avoid loading the macro definition files
168 when someone @emph{runs} the compiled program, write
169 @code{eval-when-compile} around the @code{require} calls (@pxref{Eval
172 @node Defining Macros
173 @section Defining Macros
175 A Lisp macro is a list whose @sc{car} is @code{macro}. Its @sc{cdr} should
176 be a function; expansion of the macro works by applying the function
177 (with @code{apply}) to the list of unevaluated argument-expressions
180 It is possible to use an anonymous Lisp macro just like an anonymous
181 function, but this is never done, because it does not make sense to pass
182 an anonymous macro to functionals such as @code{mapcar}. In practice,
183 all Lisp macros have names, and they are usually defined with the
184 special form @code{defmacro}.
186 @defspec defmacro name argument-list body-forms@dots{}
187 @code{defmacro} defines the symbol @var{name} as a macro that looks
191 (macro lambda @var{argument-list} . @var{body-forms})
194 This macro object is stored in the function cell of @var{name}. The
195 value returned by evaluating the @code{defmacro} form is @var{name}, but
196 usually we ignore this value.
198 The shape and meaning of @var{argument-list} is the same as in a
199 function, and the keywords @code{&rest} and @code{&optional} may be used
200 (@pxref{Argument List}). Macros may have a documentation string, but
201 any @code{interactive} declaration is ignored since macros cannot be
202 called interactively.
207 @cindex backquote (list substitution)
208 @cindex ` (list substitution)
211 Macros often need to construct large list structures from a mixture of
212 constants and nonconstant parts. To make this easier, use the macro
213 @samp{`} (often called @dfn{backquote}).
215 Backquote allows you to quote a list, but selectively evaluate
216 elements of that list. In the simplest case, it is identical to the
217 special form @code{quote} (@pxref{Quoting}). For example, these
218 two forms yield identical results:
222 `(a list of (+ 2 3) elements)
223 @result{} (a list of (+ 2 3) elements)
226 '(a list of (+ 2 3) elements)
227 @result{} (a list of (+ 2 3) elements)
231 @findex , @r{(with Backquote)}
232 The special marker @samp{,} inside of the argument to backquote
233 indicates a value that isn't constant. Backquote evaluates the
234 argument of @samp{,} and puts the value in the list structure:
238 (list 'a 'list 'of (+ 2 3) 'elements)
239 @result{} (a list of 5 elements)
242 `(a list of ,(+ 2 3) elements)
243 @result{} (a list of 5 elements)
247 @findex ,@@ @r{(with Backquote)}
248 @cindex splicing (with backquote)
249 You can also @dfn{splice} an evaluated value into the resulting list,
250 using the special marker @samp{,@@}. The elements of the spliced list
251 become elements at the same level as the other elements of the resulting
252 list. The equivalent code without using @samp{`} is often unreadable.
253 Here are some examples:
257 (setq some-list '(2 3))
261 (cons 1 (append some-list '(4) some-list))
262 @result{} (1 2 3 4 2 3)
265 `(1 ,@@some-list 4 ,@@some-list)
266 @result{} (1 2 3 4 2 3)
270 (setq list '(hack foo bar))
271 @result{} (hack foo bar)
276 (cons 'words (append (cdr list) '(as elements)))))
277 @result{} (use the words foo bar as elements)
280 `(use the words ,@@(cdr list) as elements)
281 @result{} (use the words foo bar as elements)
286 In older versions of Emacs (before XEmacs 19.12 or FSF Emacs version
287 19.29), @samp{`} used a different syntax which required an extra level
288 of parentheses around the entire backquote construct. Likewise, each
289 @samp{,} or @samp{,@@} substitution required an extra level of
290 parentheses surrounding both the @samp{,} or @samp{,@@} and the
291 following expression. The old syntax required whitespace between the
292 @samp{`}, @samp{,} or @samp{,@@} and the following expression.
294 This syntax is still accepted, but no longer recommended except for
295 compatibility with old Emacs versions.
298 @node Problems with Macros
299 @section Common Problems Using Macros
301 The basic facts of macro expansion have counterintuitive consequences.
302 This section describes some important consequences that can lead to
303 trouble, and rules to follow to avoid trouble.
306 * Argument Evaluation:: The expansion should evaluate each macro arg once.
307 * Surprising Local Vars:: Local variable bindings in the expansion
308 require special care.
309 * Eval During Expansion:: Don't evaluate them; put them in the expansion.
310 * Repeated Expansion:: Avoid depending on how many times expansion is done.
313 @node Argument Evaluation
314 @subsection Evaluating Macro Arguments Repeatedly
316 When defining a macro you must pay attention to the number of times
317 the arguments will be evaluated when the expansion is executed. The
318 following macro (used to facilitate iteration) illustrates the problem.
319 This macro allows us to write a simple ``for'' loop such as one might
325 (defmacro for (var from init to final do &rest body)
326 "Execute a simple \"for\" loop.
327 For example, (for i from 1 to 10 do (print i))."
328 (list 'let (list (list var init))
329 (cons 'while (cons (list '<= var final)
330 (append body (list (list 'inc var)))))))
335 (for i from 1 to 3 do
336 (setq square (* i i))
337 (princ (format "\n%d %d" i square)))
343 (setq square (* i i))
344 (princ (format "%d %d" i square))
357 (The arguments @code{from}, @code{to}, and @code{do} in this macro are
358 ``syntactic sugar''; they are entirely ignored. The idea is that you
359 will write noise words (such as @code{from}, @code{to}, and @code{do})
360 in those positions in the macro call.)
362 Here's an equivalent definition simplified through use of backquote:
366 (defmacro for (var from init to final do &rest body)
367 "Execute a simple \"for\" loop.
368 For example, (for i from 1 to 10 do (print i))."
370 (while (<= ,var ,final)
376 Both forms of this definition (with backquote and without) suffer from
377 the defect that @var{final} is evaluated on every iteration. If
378 @var{final} is a constant, this is not a problem. If it is a more
379 complex form, say @code{(long-complex-calculation x)}, this can slow
380 down the execution significantly. If @var{final} has side effects,
381 executing it more than once is probably incorrect.
383 @cindex macro argument evaluation
384 A well-designed macro definition takes steps to avoid this problem by
385 producing an expansion that evaluates the argument expressions exactly
386 once unless repeated evaluation is part of the intended purpose of the
387 macro. Here is a correct expansion for the @code{for} macro:
394 (setq square (* i i))
395 (princ (format "%d %d" i square))
400 Here is a macro definition that creates this expansion:
404 (defmacro for (var from init to final do &rest body)
405 "Execute a simple for loop: (for i from 1 to 10 do (print i))."
414 Unfortunately, this introduces another problem.
416 Proceed to the following node.
419 @node Surprising Local Vars
420 @subsection Local Variables in Macro Expansions
423 In the previous section, the definition of @code{for} was fixed as
424 follows to make the expansion evaluate the macro arguments the proper
429 (defmacro for (var from init to final do &rest body)
430 "Execute a simple for loop: (for i from 1 to 10 do (print i))."
442 The new definition of @code{for} has a new problem: it introduces a
443 local variable named @code{max} which the user does not expect. This
444 causes trouble in examples such as the following:
449 (for x from 0 to 10 do
450 (let ((this (frob x)))
457 The references to @code{max} inside the body of the @code{for}, which
458 are supposed to refer to the user's binding of @code{max}, really access
459 the binding made by @code{for}.
461 The way to correct this is to use an uninterned symbol instead of
462 @code{max} (@pxref{Creating Symbols}). The uninterned symbol can be
463 bound and referred to just like any other symbol, but since it is
464 created by @code{for}, we know that it cannot already appear in the
465 user's program. Since it is not interned, there is no way the user can
466 put it into the program later. It will never appear anywhere except
467 where put by @code{for}. Here is a definition of @code{for} that works
472 (defmacro for (var from init to final do &rest body)
473 "Execute a simple for loop: (for i from 1 to 10 do (print i))."
474 (let ((tempvar (make-symbol "max")))
477 (while (<= ,var ,tempvar)
484 This creates an uninterned symbol named @code{max} and puts it in the
485 expansion instead of the usual interned symbol @code{max} that appears
486 in expressions ordinarily.
488 @node Eval During Expansion
489 @subsection Evaluating Macro Arguments in Expansion
491 Another problem can happen if you evaluate any of the macro argument
492 expressions during the computation of the expansion, such as by calling
493 @code{eval} (@pxref{Eval}). If the argument is supposed to refer to the
494 user's variables, you may have trouble if the user happens to use a
495 variable with the same name as one of the macro arguments. Inside the
496 macro body, the macro argument binding is the most local binding of this
497 variable, so any references inside the form being evaluated do refer
498 to it. Here is an example:
503 (list 'setq (eval a) t))
508 (foo x) @expansion{} (setq b t)
509 @result{} t ; @r{and @code{b} has been set.}
512 (foo a) @expansion{} (setq a t)
513 @result{} t ; @r{but this set @code{a}, not @code{c}.}
518 It makes a difference whether the user's variable is named @code{a} or
519 @code{x}, because @code{a} conflicts with the macro argument variable
522 Another reason not to call @code{eval} in a macro definition is that
523 it probably won't do what you intend in a compiled program. The
524 byte-compiler runs macro definitions while compiling the program, when
525 the program's own computations (which you might have wished to access
526 with @code{eval}) don't occur and its local variable bindings don't
529 The safe way to work with the run-time value of an expression is to
530 put the expression into the macro expansion, so that its value is
531 computed as part of executing the expansion.
533 @node Repeated Expansion
534 @subsection How Many Times is the Macro Expanded?
536 Occasionally problems result from the fact that a macro call is
537 expanded each time it is evaluated in an interpreted function, but is
538 expanded only once (during compilation) for a compiled function. If the
539 macro definition has side effects, they will work differently depending
540 on how many times the macro is expanded.
542 In particular, constructing objects is a kind of side effect. If the
543 macro is called once, then the objects are constructed only once. In
544 other words, the same structure of objects is used each time the macro
545 call is executed. In interpreted operation, the macro is reexpanded
546 each time, producing a fresh collection of objects each time. Usually
547 this does not matter---the objects have the same contents whether they
548 are shared or not. But if the surrounding program does side effects
549 on the objects, it makes a difference whether they are shared. Here is
554 (defmacro empty-object ()
555 (list 'quote (cons nil nil)))
559 (defun initialize (condition)
560 (let ((object (empty-object)))
562 (setcar object condition))
568 If @code{initialize} is interpreted, a new list @code{(nil)} is
569 constructed each time @code{initialize} is called. Thus, no side effect
570 survives between calls. If @code{initialize} is compiled, then the
571 macro @code{empty-object} is expanded during compilation, producing a
572 single ``constant'' @code{(nil)} that is reused and altered each time
573 @code{initialize} is called.
575 One way to avoid pathological cases like this is to think of
576 @code{empty-object} as a funny kind of constant, not as a memory
577 allocation construct. You wouldn't use @code{setcar} on a constant such
578 as @code{'(nil)}, so naturally you won't use it on @code{(empty-object)}