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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
177c0ea7 | 3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1995, 1998 Free Software Foundation, Inc. |
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4 | @c See the file elisp.texi for copying conditions. |
5 | @setfilename ../info/macros | |
f9f59935 | 6 | @node Macros, Customization, Functions, Top |
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7 | @chapter Macros |
8 | @cindex macros | |
9 | ||
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. | |
15 | ||
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 | |
19 | or parts of them. | |
20 | ||
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}. | |
24 | ||
25 | @menu | |
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. | |
33 | @end menu | |
34 | ||
35 | @node Simple Macro | |
36 | @section A Simple Example of a Macro | |
37 | ||
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: | |
42 | ||
43 | @findex inc | |
44 | @example | |
45 | @group | |
46 | (defmacro inc (var) | |
47 | (list 'setq var (list '1+ var))) | |
48 | @end group | |
49 | @end example | |
50 | ||
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51 | When this is called with @code{(inc x)}, the argument @var{var} is the |
52 | symbol @code{x}---@emph{not} the @emph{value} of @code{x}, as it would | |
53 | be in a function. The body of the macro uses this to construct the | |
54 | expansion, which is @code{(setq x (1+ x))}. Once the macro definition | |
55 | returns this expansion, Lisp proceeds to evaluate it, thus incrementing | |
56 | @code{x}. | |
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57 | |
58 | @node Expansion | |
59 | @section Expansion of a Macro Call | |
60 | @cindex expansion of macros | |
61 | @cindex macro call | |
62 | ||
63 | A macro call looks just like a function call in that it is a list which | |
64 | starts with the name of the macro. The rest of the elements of the list | |
65 | are the arguments of the macro. | |
66 | ||
67 | Evaluation of the macro call begins like evaluation of a function call | |
68 | except for one crucial difference: the macro arguments are the actual | |
69 | expressions appearing in the macro call. They are not evaluated before | |
70 | they are given to the macro definition. By contrast, the arguments of a | |
71 | function are results of evaluating the elements of the function call | |
72 | list. | |
73 | ||
74 | Having obtained the arguments, Lisp invokes the macro definition just | |
75 | as a function is invoked. The argument variables of the macro are bound | |
76 | to the argument values from the macro call, or to a list of them in the | |
77 | case of a @code{&rest} argument. And the macro body executes and | |
78 | returns its value just as a function body does. | |
79 | ||
80 | The second crucial difference between macros and functions is that the | |
81 | value returned by the macro body is not the value of the macro call. | |
82 | Instead, it is an alternate expression for computing that value, also | |
83 | known as the @dfn{expansion} of the macro. The Lisp interpreter | |
84 | proceeds to evaluate the expansion as soon as it comes back from the | |
85 | macro. | |
86 | ||
87 | Since the expansion is evaluated in the normal manner, it may contain | |
88 | calls to other macros. It may even be a call to the same macro, though | |
89 | this is unusual. | |
90 | ||
91 | You can see the expansion of a given macro call by calling | |
92 | @code{macroexpand}. | |
93 | ||
94 | @defun macroexpand form &optional environment | |
95 | @cindex macro expansion | |
96 | This function expands @var{form}, if it is a macro call. If the result | |
97 | is another macro call, it is expanded in turn, until something which is | |
98 | not a macro call results. That is the value returned by | |
99 | @code{macroexpand}. If @var{form} is not a macro call to begin with, it | |
100 | is returned as given. | |
101 | ||
102 | Note that @code{macroexpand} does not look at the subexpressions of | |
103 | @var{form} (although some macro definitions may do so). Even if they | |
104 | are macro calls themselves, @code{macroexpand} does not expand them. | |
105 | ||
106 | The function @code{macroexpand} does not expand calls to inline functions. | |
107 | Normally there is no need for that, since a call to an inline function is | |
108 | no harder to understand than a call to an ordinary function. | |
109 | ||
110 | If @var{environment} is provided, it specifies an alist of macro | |
111 | definitions that shadow the currently defined macros. Byte compilation | |
112 | uses this feature. | |
113 | ||
114 | @smallexample | |
115 | @group | |
116 | (defmacro inc (var) | |
117 | (list 'setq var (list '1+ var))) | |
118 | @result{} inc | |
119 | @end group | |
120 | ||
121 | @group | |
122 | (macroexpand '(inc r)) | |
123 | @result{} (setq r (1+ r)) | |
124 | @end group | |
125 | ||
126 | @group | |
127 | (defmacro inc2 (var1 var2) | |
128 | (list 'progn (list 'inc var1) (list 'inc var2))) | |
129 | @result{} inc2 | |
130 | @end group | |
131 | ||
132 | @group | |
133 | (macroexpand '(inc2 r s)) | |
134 | @result{} (progn (inc r) (inc s)) ; @r{@code{inc} not expanded here.} | |
135 | @end group | |
136 | @end smallexample | |
137 | @end defun | |
138 | ||
139 | @node Compiling Macros | |
140 | @section Macros and Byte Compilation | |
141 | @cindex byte-compiling macros | |
142 | ||
143 | You might ask why we take the trouble to compute an expansion for a | |
144 | macro and then evaluate the expansion. Why not have the macro body | |
145 | produce the desired results directly? The reason has to do with | |
146 | compilation. | |
147 | ||
148 | When a macro call appears in a Lisp program being compiled, the Lisp | |
149 | compiler calls the macro definition just as the interpreter would, and | |
150 | receives an expansion. But instead of evaluating this expansion, it | |
151 | compiles the expansion as if it had appeared directly in the program. | |
152 | As a result, the compiled code produces the value and side effects | |
153 | intended for the macro, but executes at full compiled speed. This would | |
154 | not work if the macro body computed the value and side effects | |
155 | itself---they would be computed at compile time, which is not useful. | |
156 | ||
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157 | In order for compilation of macro calls to work, the macros must |
158 | already be defined in Lisp when the calls to them are compiled. The | |
159 | compiler has a special feature to help you do this: if a file being | |
160 | compiled contains a @code{defmacro} form, the macro is defined | |
161 | temporarily for the rest of the compilation of that file. To make this | |
162 | feature work, you must put the @code{defmacro} in the same file where it | |
163 | is used, and before its first use. | |
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164 | |
165 | Byte-compiling a file executes any @code{require} calls at top-level | |
166 | in the file. This is in case the file needs the required packages for | |
167 | proper compilation. One way to ensure that necessary macro definitions | |
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168 | are available during compilation is to require the files that define |
169 | them (@pxref{Named Features}). To avoid loading the macro definition files | |
170 | when someone @emph{runs} the compiled program, write | |
171 | @code{eval-when-compile} around the @code{require} calls (@pxref{Eval | |
172 | During Compile}). | |
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173 | |
174 | @node Defining Macros | |
175 | @section Defining Macros | |
176 | ||
177 | A Lisp macro is a list whose @sc{car} is @code{macro}. Its @sc{cdr} should | |
178 | be a function; expansion of the macro works by applying the function | |
179 | (with @code{apply}) to the list of unevaluated argument-expressions | |
180 | from the macro call. | |
181 | ||
182 | It is possible to use an anonymous Lisp macro just like an anonymous | |
183 | function, but this is never done, because it does not make sense to pass | |
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184 | an anonymous macro to functionals such as @code{mapcar}. In practice, |
185 | all Lisp macros have names, and they are usually defined with the | |
186 | special form @code{defmacro}. | |
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187 | |
188 | @defspec defmacro name argument-list body-forms@dots{} | |
189 | @code{defmacro} defines the symbol @var{name} as a macro that looks | |
190 | like this: | |
191 | ||
192 | @example | |
193 | (macro lambda @var{argument-list} . @var{body-forms}) | |
194 | @end example | |
195 | ||
969fe9b5 | 196 | (Note that the @sc{cdr} of this list is a function---a lambda expression.) |
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197 | This macro object is stored in the function cell of @var{name}. The |
198 | value returned by evaluating the @code{defmacro} form is @var{name}, but | |
199 | usually we ignore this value. | |
200 | ||
201 | The shape and meaning of @var{argument-list} is the same as in a | |
202 | function, and the keywords @code{&rest} and @code{&optional} may be used | |
203 | (@pxref{Argument List}). Macros may have a documentation string, but | |
204 | any @code{interactive} declaration is ignored since macros cannot be | |
205 | called interactively. | |
206 | @end defspec | |
207 | ||
208 | @node Backquote | |
209 | @section Backquote | |
210 | @cindex backquote (list substitution) | |
211 | @cindex ` (list substitution) | |
22697dac | 212 | @findex ` |
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213 | |
214 | Macros often need to construct large list structures from a mixture of | |
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215 | constants and nonconstant parts. To make this easier, use the @samp{`} |
216 | syntax (usually called @dfn{backquote}). | |
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217 | |
218 | Backquote allows you to quote a list, but selectively evaluate | |
219 | elements of that list. In the simplest case, it is identical to the | |
220 | special form @code{quote} (@pxref{Quoting}). For example, these | |
221 | two forms yield identical results: | |
222 | ||
223 | @example | |
224 | @group | |
22697dac | 225 | `(a list of (+ 2 3) elements) |
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226 | @result{} (a list of (+ 2 3) elements) |
227 | @end group | |
228 | @group | |
22697dac | 229 | '(a list of (+ 2 3) elements) |
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230 | @result{} (a list of (+ 2 3) elements) |
231 | @end group | |
232 | @end example | |
233 | ||
f57ddf67 | 234 | @findex , @r{(with Backquote)} |
bfe721d1 | 235 | The special marker @samp{,} inside of the argument to backquote |
73804d4b | 236 | indicates a value that isn't constant. Backquote evaluates the |
bfe721d1 | 237 | argument of @samp{,} and puts the value in the list structure: |
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238 | |
239 | @example | |
240 | @group | |
241 | (list 'a 'list 'of (+ 2 3) 'elements) | |
242 | @result{} (a list of 5 elements) | |
243 | @end group | |
244 | @group | |
22697dac | 245 | `(a list of ,(+ 2 3) elements) |
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246 | @result{} (a list of 5 elements) |
247 | @end group | |
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248 | @end example |
249 | ||
250 | Substitution with @samp{,} is allowed at deeper levels of the list | |
251 | structure also. For example: | |
252 | ||
253 | @example | |
254 | @group | |
255 | (defmacro t-becomes-nil (variable) | |
256 | `(if (eq ,variable t) | |
257 | (setq ,variable nil))) | |
258 | @end group | |
259 | ||
260 | @group | |
261 | (t-becomes-nil foo) | |
262 | @equiv{} (if (eq foo t) (setq foo nil)) | |
263 | @end group | |
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264 | @end example |
265 | ||
f57ddf67 | 266 | @findex ,@@ @r{(with Backquote)} |
73804d4b | 267 | @cindex splicing (with backquote) |
f9f59935 | 268 | You can also @dfn{splice} an evaluated value into the resulting list, |
bfe721d1 | 269 | using the special marker @samp{,@@}. The elements of the spliced list |
73804d4b | 270 | become elements at the same level as the other elements of the resulting |
bfe721d1 | 271 | list. The equivalent code without using @samp{`} is often unreadable. |
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272 | Here are some examples: |
273 | ||
274 | @example | |
275 | @group | |
276 | (setq some-list '(2 3)) | |
277 | @result{} (2 3) | |
278 | @end group | |
279 | @group | |
280 | (cons 1 (append some-list '(4) some-list)) | |
281 | @result{} (1 2 3 4 2 3) | |
282 | @end group | |
283 | @group | |
22697dac | 284 | `(1 ,@@some-list 4 ,@@some-list) |
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285 | @result{} (1 2 3 4 2 3) |
286 | @end group | |
287 | ||
288 | @group | |
289 | (setq list '(hack foo bar)) | |
290 | @result{} (hack foo bar) | |
291 | @end group | |
292 | @group | |
293 | (cons 'use | |
294 | (cons 'the | |
295 | (cons 'words (append (cdr list) '(as elements))))) | |
296 | @result{} (use the words foo bar as elements) | |
297 | @end group | |
298 | @group | |
22697dac | 299 | `(use the words ,@@(cdr list) as elements) |
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300 | @result{} (use the words foo bar as elements) |
301 | @end group | |
302 | @end example | |
303 | ||
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304 | In old Emacs versions, before version 19.29, @samp{`} used a different |
305 | syntax which required an extra level of parentheses around the entire | |
306 | backquote construct. Likewise, each @samp{,} or @samp{,@@} substitution | |
307 | required an extra level of parentheses surrounding both the @samp{,} or | |
308 | @samp{,@@} and the following expression. The old syntax required | |
309 | whitespace between the @samp{`}, @samp{,} or @samp{,@@} and the | |
310 | following expression. | |
22697dac | 311 | |
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312 | This syntax is still accepted, for compatibility with old Emacs |
313 | versions, but we recommend not using it in new programs. | |
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314 | |
315 | @node Problems with Macros | |
316 | @section Common Problems Using Macros | |
317 | ||
318 | The basic facts of macro expansion have counterintuitive consequences. | |
319 | This section describes some important consequences that can lead to | |
320 | trouble, and rules to follow to avoid trouble. | |
321 | ||
322 | @menu | |
0bae67ad | 323 | * Wrong Time:: Do the work in the expansion, not in the macro. |
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324 | * Argument Evaluation:: The expansion should evaluate each macro arg once. |
325 | * Surprising Local Vars:: Local variable bindings in the expansion | |
326 | require special care. | |
327 | * Eval During Expansion:: Don't evaluate them; put them in the expansion. | |
328 | * Repeated Expansion:: Avoid depending on how many times expansion is done. | |
329 | @end menu | |
330 | ||
0bae67ad RS |
331 | @node Wrong Time |
332 | @subsection Wrong Time | |
333 | ||
334 | The most common problem in writing macros is doing too some of the | |
335 | real work prematurely---while expanding the macro, rather than in the | |
336 | expansion itself. For instance, one real package had this nmacro | |
337 | definition: | |
338 | ||
339 | @example | |
340 | (defmacro my-set-buffer-multibyte (arg) | |
341 | (if (fboundp 'set-buffer-multibyte) | |
342 | (set-buffer-multibyte arg))) | |
343 | @end example | |
344 | ||
345 | With this erroneous macro definition, the program worked fine when | |
346 | interpreted but failed when compiled. This macro definition called | |
347 | @code{set-buffer-multibyte} during compilation, which was wrong, and | |
348 | then did nothing when the compiled package was run. The definition | |
349 | that the programmer really wanted was this: | |
350 | ||
351 | @example | |
352 | (defmacro my-set-buffer-multibyte (arg) | |
353 | (if (fboundp 'set-buffer-multibyte) | |
354 | `(set-buffer-multibyte ,arg))) | |
355 | @end example | |
356 | ||
357 | @noindent | |
358 | This macro expands, if appropriate, into a call to | |
359 | @code{set-buffer-multibyte} that will be executed when the compiled | |
360 | program is actually run. | |
361 | ||
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362 | @node Argument Evaluation |
363 | @subsection Evaluating Macro Arguments Repeatedly | |
364 | ||
365 | When defining a macro you must pay attention to the number of times | |
366 | the arguments will be evaluated when the expansion is executed. The | |
367 | following macro (used to facilitate iteration) illustrates the problem. | |
368 | This macro allows us to write a simple ``for'' loop such as one might | |
369 | find in Pascal. | |
370 | ||
371 | @findex for | |
372 | @smallexample | |
373 | @group | |
374 | (defmacro for (var from init to final do &rest body) | |
375 | "Execute a simple \"for\" loop. | |
376 | For example, (for i from 1 to 10 do (print i))." | |
377 | (list 'let (list (list var init)) | |
378 | (cons 'while (cons (list '<= var final) | |
379 | (append body (list (list 'inc var))))))) | |
380 | @end group | |
381 | @result{} for | |
382 | ||
383 | @group | |
384 | (for i from 1 to 3 do | |
385 | (setq square (* i i)) | |
386 | (princ (format "\n%d %d" i square))) | |
387 | @expansion{} | |
388 | @end group | |
389 | @group | |
390 | (let ((i 1)) | |
391 | (while (<= i 3) | |
392 | (setq square (* i i)) | |
393 | (princ (format "%d %d" i square)) | |
394 | (inc i))) | |
395 | @end group | |
396 | @group | |
397 | ||
398 | @print{}1 1 | |
399 | @print{}2 4 | |
400 | @print{}3 9 | |
401 | @result{} nil | |
402 | @end group | |
403 | @end smallexample | |
404 | ||
405 | @noindent | |
969fe9b5 | 406 | The arguments @code{from}, @code{to}, and @code{do} in this macro are |
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407 | ``syntactic sugar''; they are entirely ignored. The idea is that you |
408 | will write noise words (such as @code{from}, @code{to}, and @code{do}) | |
969fe9b5 | 409 | in those positions in the macro call. |
73804d4b | 410 | |
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411 | Here's an equivalent definition simplified through use of backquote: |
412 | ||
413 | @smallexample | |
414 | @group | |
415 | (defmacro for (var from init to final do &rest body) | |
416 | "Execute a simple \"for\" loop. | |
417 | For example, (for i from 1 to 10 do (print i))." | |
bfe721d1 KH |
418 | `(let ((,var ,init)) |
419 | (while (<= ,var ,final) | |
420 | ,@@body | |
421 | (inc ,var)))) | |
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422 | @end group |
423 | @end smallexample | |
424 | ||
425 | Both forms of this definition (with backquote and without) suffer from | |
426 | the defect that @var{final} is evaluated on every iteration. If | |
427 | @var{final} is a constant, this is not a problem. If it is a more | |
428 | complex form, say @code{(long-complex-calculation x)}, this can slow | |
429 | down the execution significantly. If @var{final} has side effects, | |
430 | executing it more than once is probably incorrect. | |
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431 | |
432 | @cindex macro argument evaluation | |
433 | A well-designed macro definition takes steps to avoid this problem by | |
434 | producing an expansion that evaluates the argument expressions exactly | |
435 | once unless repeated evaluation is part of the intended purpose of the | |
436 | macro. Here is a correct expansion for the @code{for} macro: | |
437 | ||
438 | @smallexample | |
439 | @group | |
440 | (let ((i 1) | |
441 | (max 3)) | |
442 | (while (<= i max) | |
443 | (setq square (* i i)) | |
444 | (princ (format "%d %d" i square)) | |
445 | (inc i))) | |
446 | @end group | |
447 | @end smallexample | |
448 | ||
177c0ea7 | 449 | Here is a macro definition that creates this expansion: |
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450 | |
451 | @smallexample | |
452 | @group | |
453 | (defmacro for (var from init to final do &rest body) | |
454 | "Execute a simple for loop: (for i from 1 to 10 do (print i))." | |
bfe721d1 KH |
455 | `(let ((,var ,init) |
456 | (max ,final)) | |
457 | (while (<= ,var max) | |
458 | ,@@body | |
459 | (inc ,var)))) | |
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460 | @end group |
461 | @end smallexample | |
462 | ||
969fe9b5 RS |
463 | Unfortunately, this fix introduces another problem, |
464 | described in the following section. | |
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465 | |
466 | @node Surprising Local Vars | |
467 | @subsection Local Variables in Macro Expansions | |
468 | ||
37680279 | 469 | @ifnottex |
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470 | In the previous section, the definition of @code{for} was fixed as |
471 | follows to make the expansion evaluate the macro arguments the proper | |
472 | number of times: | |
473 | ||
474 | @smallexample | |
475 | @group | |
476 | (defmacro for (var from init to final do &rest body) | |
477 | "Execute a simple for loop: (for i from 1 to 10 do (print i))." | |
478 | @end group | |
479 | @group | |
bfe721d1 KH |
480 | `(let ((,var ,init) |
481 | (max ,final)) | |
482 | (while (<= ,var max) | |
483 | ,@@body | |
484 | (inc ,var)))) | |
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485 | @end group |
486 | @end smallexample | |
37680279 | 487 | @end ifnottex |
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488 | |
489 | The new definition of @code{for} has a new problem: it introduces a | |
490 | local variable named @code{max} which the user does not expect. This | |
491 | causes trouble in examples such as the following: | |
492 | ||
ec221d13 | 493 | @smallexample |
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494 | @group |
495 | (let ((max 0)) | |
496 | (for x from 0 to 10 do | |
497 | (let ((this (frob x))) | |
498 | (if (< max this) | |
499 | (setq max this))))) | |
500 | @end group | |
ec221d13 | 501 | @end smallexample |
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502 | |
503 | @noindent | |
504 | The references to @code{max} inside the body of the @code{for}, which | |
505 | are supposed to refer to the user's binding of @code{max}, really access | |
506 | the binding made by @code{for}. | |
507 | ||
508 | The way to correct this is to use an uninterned symbol instead of | |
509 | @code{max} (@pxref{Creating Symbols}). The uninterned symbol can be | |
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510 | bound and referred to just like any other symbol, but since it is |
511 | created by @code{for}, we know that it cannot already appear in the | |
512 | user's program. Since it is not interned, there is no way the user can | |
513 | put it into the program later. It will never appear anywhere except | |
514 | where put by @code{for}. Here is a definition of @code{for} that works | |
515 | this way: | |
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516 | |
517 | @smallexample | |
518 | @group | |
519 | (defmacro for (var from init to final do &rest body) | |
520 | "Execute a simple for loop: (for i from 1 to 10 do (print i))." | |
521 | (let ((tempvar (make-symbol "max"))) | |
bfe721d1 KH |
522 | `(let ((,var ,init) |
523 | (,tempvar ,final)) | |
524 | (while (<= ,var ,tempvar) | |
525 | ,@@body | |
526 | (inc ,var))))) | |
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527 | @end group |
528 | @end smallexample | |
529 | ||
530 | @noindent | |
531 | This creates an uninterned symbol named @code{max} and puts it in the | |
532 | expansion instead of the usual interned symbol @code{max} that appears | |
533 | in expressions ordinarily. | |
534 | ||
535 | @node Eval During Expansion | |
536 | @subsection Evaluating Macro Arguments in Expansion | |
537 | ||
a9f0a989 | 538 | Another problem can happen if the macro definition itself |
f9f59935 | 539 | evaluates any of the macro argument expressions, such as by calling |
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540 | @code{eval} (@pxref{Eval}). If the argument is supposed to refer to the |
541 | user's variables, you may have trouble if the user happens to use a | |
542 | variable with the same name as one of the macro arguments. Inside the | |
543 | macro body, the macro argument binding is the most local binding of this | |
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544 | variable, so any references inside the form being evaluated do refer to |
545 | it. Here is an example: | |
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546 | |
547 | @example | |
548 | @group | |
549 | (defmacro foo (a) | |
550 | (list 'setq (eval a) t)) | |
551 | @result{} foo | |
552 | @end group | |
553 | @group | |
554 | (setq x 'b) | |
555 | (foo x) @expansion{} (setq b t) | |
556 | @result{} t ; @r{and @code{b} has been set.} | |
557 | ;; @r{but} | |
558 | (setq a 'c) | |
559 | (foo a) @expansion{} (setq a t) | |
560 | @result{} t ; @r{but this set @code{a}, not @code{c}.} | |
561 | ||
562 | @end group | |
563 | @end example | |
564 | ||
565 | It makes a difference whether the user's variable is named @code{a} or | |
566 | @code{x}, because @code{a} conflicts with the macro argument variable | |
567 | @code{a}. | |
568 | ||
969fe9b5 | 569 | Another problem with calling @code{eval} in a macro definition is that |
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570 | it probably won't do what you intend in a compiled program. The |
571 | byte-compiler runs macro definitions while compiling the program, when | |
572 | the program's own computations (which you might have wished to access | |
573 | with @code{eval}) don't occur and its local variable bindings don't | |
574 | exist. | |
575 | ||
969fe9b5 | 576 | To avoid these problems, @strong{don't evaluate an argument expression |
a9f0a989 | 577 | while computing the macro expansion}. Instead, substitute the |
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578 | expression into the macro expansion, so that its value will be computed |
579 | as part of executing the expansion. This is how the other examples in | |
580 | this chapter work. | |
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581 | |
582 | @node Repeated Expansion | |
583 | @subsection How Many Times is the Macro Expanded? | |
584 | ||
585 | Occasionally problems result from the fact that a macro call is | |
586 | expanded each time it is evaluated in an interpreted function, but is | |
587 | expanded only once (during compilation) for a compiled function. If the | |
588 | macro definition has side effects, they will work differently depending | |
589 | on how many times the macro is expanded. | |
590 | ||
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591 | Therefore, you should avoid side effects in computation of the |
592 | macro expansion, unless you really know what you are doing. | |
593 | ||
594 | One special kind of side effect can't be avoided: constructing Lisp | |
595 | objects. Almost all macro expansions include constructed lists; that is | |
596 | the whole point of most macros. This is usually safe; there is just one | |
597 | case where you must be careful: when the object you construct is part of a | |
598 | quoted constant in the macro expansion. | |
599 | ||
600 | If the macro is expanded just once, in compilation, then the object is | |
601 | constructed just once, during compilation. But in interpreted | |
602 | execution, the macro is expanded each time the macro call runs, and this | |
603 | means a new object is constructed each time. | |
604 | ||
605 | In most clean Lisp code, this difference won't matter. It can matter | |
606 | only if you perform side-effects on the objects constructed by the macro | |
607 | definition. Thus, to avoid trouble, @strong{avoid side effects on | |
608 | objects constructed by macro definitions}. Here is an example of how | |
609 | such side effects can get you into trouble: | |
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610 | |
611 | @lisp | |
612 | @group | |
613 | (defmacro empty-object () | |
614 | (list 'quote (cons nil nil))) | |
615 | @end group | |
616 | ||
617 | @group | |
618 | (defun initialize (condition) | |
619 | (let ((object (empty-object))) | |
620 | (if condition | |
621 | (setcar object condition)) | |
622 | object)) | |
623 | @end group | |
624 | @end lisp | |
625 | ||
626 | @noindent | |
627 | If @code{initialize} is interpreted, a new list @code{(nil)} is | |
628 | constructed each time @code{initialize} is called. Thus, no side effect | |
629 | survives between calls. If @code{initialize} is compiled, then the | |
630 | macro @code{empty-object} is expanded during compilation, producing a | |
631 | single ``constant'' @code{(nil)} that is reused and altered each time | |
632 | @code{initialize} is called. | |
633 | ||
634 | One way to avoid pathological cases like this is to think of | |
635 | @code{empty-object} as a funny kind of constant, not as a memory | |
636 | allocation construct. You wouldn't use @code{setcar} on a constant such | |
637 | as @code{'(nil)}, so naturally you won't use it on @code{(empty-object)} | |
638 | either. |