| 1 | @c -*-texinfo-*- |
| 2 | @c This is part of the GNU Emacs Lisp Reference Manual. |
| 3 | @c Copyright (C) 1990-1995, 1998, 2001-2012 Free Software Foundation, Inc. |
| 4 | @c See the file elisp.texi for copying conditions. |
| 5 | @node Macros |
| 6 | @chapter Macros |
| 7 | @cindex macros |
| 8 | |
| 9 | @dfn{Macros} enable you to define new control constructs and other |
| 10 | language features. A macro is defined much like a function, but instead |
| 11 | of telling how to compute a value, it tells how to compute another Lisp |
| 12 | expression which will in turn compute the value. We call this |
| 13 | expression the @dfn{expansion} of the macro. |
| 14 | |
| 15 | Macros can do this because they operate on the unevaluated expressions |
| 16 | for the arguments, not on the argument values as functions do. They can |
| 17 | therefore construct an expansion containing these argument expressions |
| 18 | or parts of them. |
| 19 | |
| 20 | If you are using a macro to do something an ordinary function could |
| 21 | do, just for the sake of speed, consider using an inline function |
| 22 | instead. @xref{Inline Functions}. |
| 23 | |
| 24 | @menu |
| 25 | * Simple Macro:: A basic example. |
| 26 | * Expansion:: How, when and why macros are expanded. |
| 27 | * Compiling Macros:: How macros are expanded by the compiler. |
| 28 | * Defining Macros:: How to write a macro definition. |
| 29 | * Problems with Macros:: Don't evaluate the macro arguments too many times. |
| 30 | Don't hide the user's variables. |
| 31 | * Indenting Macros:: Specifying how to indent macro calls. |
| 32 | @end menu |
| 33 | |
| 34 | @node Simple Macro |
| 35 | @section A Simple Example of a Macro |
| 36 | |
| 37 | Suppose we would like to define a Lisp construct to increment a |
| 38 | variable value, much like the @code{++} operator in C. We would like to |
| 39 | write @code{(inc x)} and have the effect of @code{(setq x (1+ x))}. |
| 40 | Here's a macro definition that does the job: |
| 41 | |
| 42 | @findex inc |
| 43 | @example |
| 44 | @group |
| 45 | (defmacro inc (var) |
| 46 | (list 'setq var (list '1+ var))) |
| 47 | @end group |
| 48 | @end example |
| 49 | |
| 50 | When this is called with @code{(inc x)}, the argument @var{var} is the |
| 51 | symbol @code{x}---@emph{not} the @emph{value} of @code{x}, as it would |
| 52 | be in a function. The body of the macro uses this to construct the |
| 53 | expansion, which is @code{(setq x (1+ x))}. Once the macro definition |
| 54 | returns this expansion, Lisp proceeds to evaluate it, thus incrementing |
| 55 | @code{x}. |
| 56 | |
| 57 | @node Expansion |
| 58 | @section Expansion of a Macro Call |
| 59 | @cindex expansion of macros |
| 60 | @cindex macro call |
| 61 | |
| 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. |
| 65 | |
| 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 |
| 71 | list. |
| 72 | |
| 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. |
| 78 | |
| 79 | The second crucial difference between macros and functions is that |
| 80 | the value returned by the macro body is an alternate Lisp expression, |
| 81 | also known as the @dfn{expansion} of the macro. The Lisp interpreter |
| 82 | proceeds to evaluate the expansion as soon as it comes back from the |
| 83 | macro. |
| 84 | |
| 85 | Since the expansion is evaluated in the normal manner, it may contain |
| 86 | calls to other macros. It may even be a call to the same macro, though |
| 87 | this is unusual. |
| 88 | |
| 89 | You can see the expansion of a given macro call by calling |
| 90 | @code{macroexpand}. |
| 91 | |
| 92 | @defun macroexpand form &optional environment |
| 93 | @cindex macro expansion |
| 94 | This function expands @var{form}, if it is a macro call. If the result |
| 95 | is another macro call, it is expanded in turn, until something which is |
| 96 | not a macro call results. That is the value returned by |
| 97 | @code{macroexpand}. If @var{form} is not a macro call to begin with, it |
| 98 | is returned as given. |
| 99 | |
| 100 | Note that @code{macroexpand} does not look at the subexpressions of |
| 101 | @var{form} (although some macro definitions may do so). Even if they |
| 102 | are macro calls themselves, @code{macroexpand} does not expand them. |
| 103 | |
| 104 | The function @code{macroexpand} does not expand calls to inline functions. |
| 105 | Normally there is no need for that, since a call to an inline function is |
| 106 | no harder to understand than a call to an ordinary function. |
| 107 | |
| 108 | If @var{environment} is provided, it specifies an alist of macro |
| 109 | definitions that shadow the currently defined macros. Byte compilation |
| 110 | uses this feature. |
| 111 | |
| 112 | @example |
| 113 | @group |
| 114 | (defmacro inc (var) |
| 115 | (list 'setq var (list '1+ var))) |
| 116 | @end group |
| 117 | |
| 118 | @group |
| 119 | (macroexpand '(inc r)) |
| 120 | @result{} (setq r (1+ r)) |
| 121 | @end group |
| 122 | |
| 123 | @group |
| 124 | (defmacro inc2 (var1 var2) |
| 125 | (list 'progn (list 'inc var1) (list 'inc var2))) |
| 126 | @end group |
| 127 | |
| 128 | @group |
| 129 | (macroexpand '(inc2 r s)) |
| 130 | @result{} (progn (inc r) (inc s)) ; @r{@code{inc} not expanded here.} |
| 131 | @end group |
| 132 | @end example |
| 133 | @end defun |
| 134 | |
| 135 | |
| 136 | @defun macroexpand-all form &optional environment |
| 137 | @code{macroexpand-all} expands macros like @code{macroexpand}, but |
| 138 | will look for and expand all macros in @var{form}, not just at the |
| 139 | top-level. If no macros are expanded, the return value is @code{eq} |
| 140 | to @var{form}. |
| 141 | |
| 142 | Repeating the example used for @code{macroexpand} above with |
| 143 | @code{macroexpand-all}, we see that @code{macroexpand-all} @emph{does} |
| 144 | expand the embedded calls to @code{inc}: |
| 145 | |
| 146 | @example |
| 147 | (macroexpand-all '(inc2 r s)) |
| 148 | @result{} (progn (setq r (1+ r)) (setq s (1+ s))) |
| 149 | @end example |
| 150 | |
| 151 | @end defun |
| 152 | |
| 153 | @node Compiling Macros |
| 154 | @section Macros and Byte Compilation |
| 155 | @cindex byte-compiling macros |
| 156 | |
| 157 | You might ask why we take the trouble to compute an expansion for a |
| 158 | macro and then evaluate the expansion. Why not have the macro body |
| 159 | produce the desired results directly? The reason has to do with |
| 160 | compilation. |
| 161 | |
| 162 | When a macro call appears in a Lisp program being compiled, the Lisp |
| 163 | compiler calls the macro definition just as the interpreter would, and |
| 164 | receives an expansion. But instead of evaluating this expansion, it |
| 165 | compiles the expansion as if it had appeared directly in the program. |
| 166 | As a result, the compiled code produces the value and side effects |
| 167 | intended for the macro, but executes at full compiled speed. This would |
| 168 | not work if the macro body computed the value and side effects |
| 169 | itself---they would be computed at compile time, which is not useful. |
| 170 | |
| 171 | In order for compilation of macro calls to work, the macros must |
| 172 | already be defined in Lisp when the calls to them are compiled. The |
| 173 | compiler has a special feature to help you do this: if a file being |
| 174 | compiled contains a @code{defmacro} form, the macro is defined |
| 175 | temporarily for the rest of the compilation of that file. |
| 176 | |
| 177 | Byte-compiling a file also executes any @code{require} calls at |
| 178 | top-level in the file, so you can ensure that necessary macro |
| 179 | definitions are available during compilation by requiring the files |
| 180 | that define them (@pxref{Named Features}). To avoid loading the macro |
| 181 | definition files when someone @emph{runs} the compiled program, write |
| 182 | @code{eval-when-compile} around the @code{require} calls (@pxref{Eval |
| 183 | During Compile}). |
| 184 | |
| 185 | @node Defining Macros |
| 186 | @section Defining Macros |
| 187 | |
| 188 | A Lisp macro object is a list whose @sc{car} is @code{macro}, and |
| 189 | whose @sc{cdr} is a lambda expression. Expansion of the macro works |
| 190 | by applying the lambda expression (with @code{apply}) to the list of |
| 191 | @emph{unevaluated} arguments from the macro call. |
| 192 | |
| 193 | It is possible to use an anonymous Lisp macro just like an anonymous |
| 194 | function, but this is never done, because it does not make sense to |
| 195 | pass an anonymous macro to functionals such as @code{mapcar}. In |
| 196 | practice, all Lisp macros have names, and they are almost always |
| 197 | defined with the @code{defmacro} macro. |
| 198 | |
| 199 | @defmac defmacro name args [doc] [declare] body@dots{} |
| 200 | @code{defmacro} defines the symbol @var{name} (which should not be |
| 201 | quoted) as a macro that looks like this: |
| 202 | |
| 203 | @example |
| 204 | (macro lambda @var{args} . @var{body}) |
| 205 | @end example |
| 206 | |
| 207 | (Note that the @sc{cdr} of this list is a lambda expression.) This |
| 208 | macro object is stored in the function cell of @var{name}. The |
| 209 | meaning of @var{args} is the same as in a function, and the keywords |
| 210 | @code{&rest} and @code{&optional} may be used (@pxref{Argument List}). |
| 211 | Neither @var{name} nor @var{args} should be quoted. The return value |
| 212 | of @code{defmacro} is undefined. |
| 213 | |
| 214 | @var{doc}, if present, should be a string specifying the macro's |
| 215 | documentation string. @var{declare}, if present, should be a |
| 216 | @code{declare} form specifying metadata for the macro (@pxref{Declare |
| 217 | Form}). Note that macros cannot have interactive declarations, since |
| 218 | they cannot be called interactively. |
| 219 | @end defmac |
| 220 | |
| 221 | Macros often need to construct large list structures from a mixture |
| 222 | of constants and nonconstant parts. To make this easier, use the |
| 223 | @samp{`} syntax (@pxref{Backquote}). For example: |
| 224 | |
| 225 | @example |
| 226 | @example |
| 227 | @group |
| 228 | (defmacro t-becomes-nil (variable) |
| 229 | `(if (eq ,variable t) |
| 230 | (setq ,variable nil))) |
| 231 | @end group |
| 232 | |
| 233 | @group |
| 234 | (t-becomes-nil foo) |
| 235 | @equiv{} (if (eq foo t) (setq foo nil)) |
| 236 | @end group |
| 237 | @end example |
| 238 | @end example |
| 239 | |
| 240 | The body of a macro definition can include a @code{declare} form, |
| 241 | which specifies additional properties about the macro. @xref{Declare |
| 242 | Form}. |
| 243 | |
| 244 | @node Problems with Macros |
| 245 | @section Common Problems Using Macros |
| 246 | |
| 247 | Macro expansion can have counterintuitive consequences. This |
| 248 | section describes some important consequences that can lead to |
| 249 | trouble, and rules to follow to avoid trouble. |
| 250 | |
| 251 | @menu |
| 252 | * Wrong Time:: Do the work in the expansion, not in the macro. |
| 253 | * Argument Evaluation:: The expansion should evaluate each macro arg once. |
| 254 | * Surprising Local Vars:: Local variable bindings in the expansion |
| 255 | require special care. |
| 256 | * Eval During Expansion:: Don't evaluate them; put them in the expansion. |
| 257 | * Repeated Expansion:: Avoid depending on how many times expansion is done. |
| 258 | @end menu |
| 259 | |
| 260 | @node Wrong Time |
| 261 | @subsection Wrong Time |
| 262 | |
| 263 | The most common problem in writing macros is doing some of the |
| 264 | real work prematurely---while expanding the macro, rather than in the |
| 265 | expansion itself. For instance, one real package had this macro |
| 266 | definition: |
| 267 | |
| 268 | @example |
| 269 | (defmacro my-set-buffer-multibyte (arg) |
| 270 | (if (fboundp 'set-buffer-multibyte) |
| 271 | (set-buffer-multibyte arg))) |
| 272 | @end example |
| 273 | |
| 274 | With this erroneous macro definition, the program worked fine when |
| 275 | interpreted but failed when compiled. This macro definition called |
| 276 | @code{set-buffer-multibyte} during compilation, which was wrong, and |
| 277 | then did nothing when the compiled package was run. The definition |
| 278 | that the programmer really wanted was this: |
| 279 | |
| 280 | @example |
| 281 | (defmacro my-set-buffer-multibyte (arg) |
| 282 | (if (fboundp 'set-buffer-multibyte) |
| 283 | `(set-buffer-multibyte ,arg))) |
| 284 | @end example |
| 285 | |
| 286 | @noindent |
| 287 | This macro expands, if appropriate, into a call to |
| 288 | @code{set-buffer-multibyte} that will be executed when the compiled |
| 289 | program is actually run. |
| 290 | |
| 291 | @node Argument Evaluation |
| 292 | @subsection Evaluating Macro Arguments Repeatedly |
| 293 | |
| 294 | When defining a macro you must pay attention to the number of times |
| 295 | the arguments will be evaluated when the expansion is executed. The |
| 296 | following macro (used to facilitate iteration) illustrates the |
| 297 | problem. This macro allows us to write a ``for'' loop construct. |
| 298 | |
| 299 | @findex for |
| 300 | @example |
| 301 | @group |
| 302 | (defmacro for (var from init to final do &rest body) |
| 303 | "Execute a simple \"for\" loop. |
| 304 | For example, (for i from 1 to 10 do (print i))." |
| 305 | (list 'let (list (list var init)) |
| 306 | (cons 'while |
| 307 | (cons (list '<= var final) |
| 308 | (append body (list (list 'inc var))))))) |
| 309 | @end group |
| 310 | |
| 311 | @group |
| 312 | (for i from 1 to 3 do |
| 313 | (setq square (* i i)) |
| 314 | (princ (format "\n%d %d" i square))) |
| 315 | @expansion{} |
| 316 | @end group |
| 317 | @group |
| 318 | (let ((i 1)) |
| 319 | (while (<= i 3) |
| 320 | (setq square (* i i)) |
| 321 | (princ (format "\n%d %d" i square)) |
| 322 | (inc i))) |
| 323 | @end group |
| 324 | @group |
| 325 | |
| 326 | @print{}1 1 |
| 327 | @print{}2 4 |
| 328 | @print{}3 9 |
| 329 | @result{} nil |
| 330 | @end group |
| 331 | @end example |
| 332 | |
| 333 | @noindent |
| 334 | The arguments @code{from}, @code{to}, and @code{do} in this macro are |
| 335 | ``syntactic sugar''; they are entirely ignored. The idea is that you |
| 336 | will write noise words (such as @code{from}, @code{to}, and @code{do}) |
| 337 | in those positions in the macro call. |
| 338 | |
| 339 | Here's an equivalent definition simplified through use of backquote: |
| 340 | |
| 341 | @example |
| 342 | @group |
| 343 | (defmacro for (var from init to final do &rest body) |
| 344 | "Execute a simple \"for\" loop. |
| 345 | For example, (for i from 1 to 10 do (print i))." |
| 346 | `(let ((,var ,init)) |
| 347 | (while (<= ,var ,final) |
| 348 | ,@@body |
| 349 | (inc ,var)))) |
| 350 | @end group |
| 351 | @end example |
| 352 | |
| 353 | Both forms of this definition (with backquote and without) suffer from |
| 354 | the defect that @var{final} is evaluated on every iteration. If |
| 355 | @var{final} is a constant, this is not a problem. If it is a more |
| 356 | complex form, say @code{(long-complex-calculation x)}, this can slow |
| 357 | down the execution significantly. If @var{final} has side effects, |
| 358 | executing it more than once is probably incorrect. |
| 359 | |
| 360 | @cindex macro argument evaluation |
| 361 | A well-designed macro definition takes steps to avoid this problem by |
| 362 | producing an expansion that evaluates the argument expressions exactly |
| 363 | once unless repeated evaluation is part of the intended purpose of the |
| 364 | macro. Here is a correct expansion for the @code{for} macro: |
| 365 | |
| 366 | @example |
| 367 | @group |
| 368 | (let ((i 1) |
| 369 | (max 3)) |
| 370 | (while (<= i max) |
| 371 | (setq square (* i i)) |
| 372 | (princ (format "%d %d" i square)) |
| 373 | (inc i))) |
| 374 | @end group |
| 375 | @end example |
| 376 | |
| 377 | Here is a macro definition that creates this expansion: |
| 378 | |
| 379 | @example |
| 380 | @group |
| 381 | (defmacro for (var from init to final do &rest body) |
| 382 | "Execute a simple for loop: (for i from 1 to 10 do (print i))." |
| 383 | `(let ((,var ,init) |
| 384 | (max ,final)) |
| 385 | (while (<= ,var max) |
| 386 | ,@@body |
| 387 | (inc ,var)))) |
| 388 | @end group |
| 389 | @end example |
| 390 | |
| 391 | Unfortunately, this fix introduces another problem, |
| 392 | described in the following section. |
| 393 | |
| 394 | @node Surprising Local Vars |
| 395 | @subsection Local Variables in Macro Expansions |
| 396 | |
| 397 | @ifnottex |
| 398 | In the previous section, the definition of @code{for} was fixed as |
| 399 | follows to make the expansion evaluate the macro arguments the proper |
| 400 | number of times: |
| 401 | |
| 402 | @example |
| 403 | @group |
| 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))." |
| 406 | @end group |
| 407 | @group |
| 408 | `(let ((,var ,init) |
| 409 | (max ,final)) |
| 410 | (while (<= ,var max) |
| 411 | ,@@body |
| 412 | (inc ,var)))) |
| 413 | @end group |
| 414 | @end example |
| 415 | @end ifnottex |
| 416 | |
| 417 | The new definition of @code{for} has a new problem: it introduces a |
| 418 | local variable named @code{max} which the user does not expect. This |
| 419 | causes trouble in examples such as the following: |
| 420 | |
| 421 | @example |
| 422 | @group |
| 423 | (let ((max 0)) |
| 424 | (for x from 0 to 10 do |
| 425 | (let ((this (frob x))) |
| 426 | (if (< max this) |
| 427 | (setq max this))))) |
| 428 | @end group |
| 429 | @end example |
| 430 | |
| 431 | @noindent |
| 432 | The references to @code{max} inside the body of the @code{for}, which |
| 433 | are supposed to refer to the user's binding of @code{max}, really access |
| 434 | the binding made by @code{for}. |
| 435 | |
| 436 | The way to correct this is to use an uninterned symbol instead of |
| 437 | @code{max} (@pxref{Creating Symbols}). The uninterned symbol can be |
| 438 | bound and referred to just like any other symbol, but since it is |
| 439 | created by @code{for}, we know that it cannot already appear in the |
| 440 | user's program. Since it is not interned, there is no way the user can |
| 441 | put it into the program later. It will never appear anywhere except |
| 442 | where put by @code{for}. Here is a definition of @code{for} that works |
| 443 | this way: |
| 444 | |
| 445 | @example |
| 446 | @group |
| 447 | (defmacro for (var from init to final do &rest body) |
| 448 | "Execute a simple for loop: (for i from 1 to 10 do (print i))." |
| 449 | (let ((tempvar (make-symbol "max"))) |
| 450 | `(let ((,var ,init) |
| 451 | (,tempvar ,final)) |
| 452 | (while (<= ,var ,tempvar) |
| 453 | ,@@body |
| 454 | (inc ,var))))) |
| 455 | @end group |
| 456 | @end example |
| 457 | |
| 458 | @noindent |
| 459 | This creates an uninterned symbol named @code{max} and puts it in the |
| 460 | expansion instead of the usual interned symbol @code{max} that appears |
| 461 | in expressions ordinarily. |
| 462 | |
| 463 | @node Eval During Expansion |
| 464 | @subsection Evaluating Macro Arguments in Expansion |
| 465 | |
| 466 | Another problem can happen if the macro definition itself |
| 467 | evaluates any of the macro argument expressions, such as by calling |
| 468 | @code{eval} (@pxref{Eval}). If the argument is supposed to refer to the |
| 469 | user's variables, you may have trouble if the user happens to use a |
| 470 | variable with the same name as one of the macro arguments. Inside the |
| 471 | macro body, the macro argument binding is the most local binding of this |
| 472 | variable, so any references inside the form being evaluated do refer to |
| 473 | it. Here is an example: |
| 474 | |
| 475 | @example |
| 476 | @group |
| 477 | (defmacro foo (a) |
| 478 | (list 'setq (eval a) t)) |
| 479 | @end group |
| 480 | @group |
| 481 | (setq x 'b) |
| 482 | (foo x) @expansion{} (setq b t) |
| 483 | @result{} t ; @r{and @code{b} has been set.} |
| 484 | ;; @r{but} |
| 485 | (setq a 'c) |
| 486 | (foo a) @expansion{} (setq a t) |
| 487 | @result{} t ; @r{but this set @code{a}, not @code{c}.} |
| 488 | |
| 489 | @end group |
| 490 | @end example |
| 491 | |
| 492 | It makes a difference whether the user's variable is named @code{a} or |
| 493 | @code{x}, because @code{a} conflicts with the macro argument variable |
| 494 | @code{a}. |
| 495 | |
| 496 | Another problem with calling @code{eval} in a macro definition is that |
| 497 | it probably won't do what you intend in a compiled program. The |
| 498 | byte compiler runs macro definitions while compiling the program, when |
| 499 | the program's own computations (which you might have wished to access |
| 500 | with @code{eval}) don't occur and its local variable bindings don't |
| 501 | exist. |
| 502 | |
| 503 | To avoid these problems, @strong{don't evaluate an argument expression |
| 504 | while computing the macro expansion}. Instead, substitute the |
| 505 | expression into the macro expansion, so that its value will be computed |
| 506 | as part of executing the expansion. This is how the other examples in |
| 507 | this chapter work. |
| 508 | |
| 509 | @node Repeated Expansion |
| 510 | @subsection How Many Times is the Macro Expanded? |
| 511 | |
| 512 | Occasionally problems result from the fact that a macro call is |
| 513 | expanded each time it is evaluated in an interpreted function, but is |
| 514 | expanded only once (during compilation) for a compiled function. If the |
| 515 | macro definition has side effects, they will work differently depending |
| 516 | on how many times the macro is expanded. |
| 517 | |
| 518 | Therefore, you should avoid side effects in computation of the |
| 519 | macro expansion, unless you really know what you are doing. |
| 520 | |
| 521 | One special kind of side effect can't be avoided: constructing Lisp |
| 522 | objects. Almost all macro expansions include constructed lists; that is |
| 523 | the whole point of most macros. This is usually safe; there is just one |
| 524 | case where you must be careful: when the object you construct is part of a |
| 525 | quoted constant in the macro expansion. |
| 526 | |
| 527 | If the macro is expanded just once, in compilation, then the object is |
| 528 | constructed just once, during compilation. But in interpreted |
| 529 | execution, the macro is expanded each time the macro call runs, and this |
| 530 | means a new object is constructed each time. |
| 531 | |
| 532 | In most clean Lisp code, this difference won't matter. It can matter |
| 533 | only if you perform side-effects on the objects constructed by the macro |
| 534 | definition. Thus, to avoid trouble, @strong{avoid side effects on |
| 535 | objects constructed by macro definitions}. Here is an example of how |
| 536 | such side effects can get you into trouble: |
| 537 | |
| 538 | @lisp |
| 539 | @group |
| 540 | (defmacro empty-object () |
| 541 | (list 'quote (cons nil nil))) |
| 542 | @end group |
| 543 | |
| 544 | @group |
| 545 | (defun initialize (condition) |
| 546 | (let ((object (empty-object))) |
| 547 | (if condition |
| 548 | (setcar object condition)) |
| 549 | object)) |
| 550 | @end group |
| 551 | @end lisp |
| 552 | |
| 553 | @noindent |
| 554 | If @code{initialize} is interpreted, a new list @code{(nil)} is |
| 555 | constructed each time @code{initialize} is called. Thus, no side effect |
| 556 | survives between calls. If @code{initialize} is compiled, then the |
| 557 | macro @code{empty-object} is expanded during compilation, producing a |
| 558 | single ``constant'' @code{(nil)} that is reused and altered each time |
| 559 | @code{initialize} is called. |
| 560 | |
| 561 | One way to avoid pathological cases like this is to think of |
| 562 | @code{empty-object} as a funny kind of constant, not as a memory |
| 563 | allocation construct. You wouldn't use @code{setcar} on a constant such |
| 564 | as @code{'(nil)}, so naturally you won't use it on @code{(empty-object)} |
| 565 | either. |
| 566 | |
| 567 | @node Indenting Macros |
| 568 | @section Indenting Macros |
| 569 | |
| 570 | Within a macro definition, you can use the @code{declare} form |
| 571 | (@pxref{Defining Macros}) to specify how @key{TAB} should indent |
| 572 | calls to the macro. An indentation specification is written like this: |
| 573 | |
| 574 | @example |
| 575 | (declare (indent @var{indent-spec})) |
| 576 | @end example |
| 577 | |
| 578 | @noindent |
| 579 | Here are the possibilities for @var{indent-spec}: |
| 580 | |
| 581 | @table @asis |
| 582 | @item @code{nil} |
| 583 | This is the same as no property---use the standard indentation pattern. |
| 584 | @item @code{defun} |
| 585 | Handle this function like a @samp{def} construct: treat the second |
| 586 | line as the start of a @dfn{body}. |
| 587 | @item an integer, @var{number} |
| 588 | The first @var{number} arguments of the function are |
| 589 | @dfn{distinguished} arguments; the rest are considered the body |
| 590 | of the expression. A line in the expression is indented according to |
| 591 | whether the first argument on it is distinguished or not. If the |
| 592 | argument is part of the body, the line is indented @code{lisp-body-indent} |
| 593 | more columns than the open-parenthesis starting the containing |
| 594 | expression. If the argument is distinguished and is either the first |
| 595 | or second argument, it is indented @emph{twice} that many extra columns. |
| 596 | If the argument is distinguished and not the first or second argument, |
| 597 | the line uses the standard pattern. |
| 598 | @item a symbol, @var{symbol} |
| 599 | @var{symbol} should be a function name; that function is called to |
| 600 | calculate the indentation of a line within this expression. The |
| 601 | function receives two arguments: |
| 602 | |
| 603 | @table @asis |
| 604 | @item @var{state} |
| 605 | The value returned by @code{parse-partial-sexp} (a Lisp primitive for |
| 606 | indentation and nesting computation) when it parses up to the |
| 607 | beginning of this line. |
| 608 | @item @var{pos} |
| 609 | The position at which the line being indented begins. |
| 610 | @end table |
| 611 | |
| 612 | @noindent |
| 613 | It should return either a number, which is the number of columns of |
| 614 | indentation for that line, or a list whose car is such a number. The |
| 615 | difference between returning a number and returning a list is that a |
| 616 | number says that all following lines at the same nesting level should |
| 617 | be indented just like this one; a list says that following lines might |
| 618 | call for different indentations. This makes a difference when the |
| 619 | indentation is being computed by @kbd{C-M-q}; if the value is a |
| 620 | number, @kbd{C-M-q} need not recalculate indentation for the following |
| 621 | lines until the end of the list. |
| 622 | @end table |