unofficial mirror of emacs-devel@gnu.org 
 help / color / mirror / code / Atom feed
blob bcee5b1078919a1eefa1be5d3427e61dddee11a0 23118 bytes (raw)
name: doc/lispref/macros.texi 	 # note: path name is non-authoritative(*)

  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
 
@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990-1995, 1998, 2001-2019 Free Software Foundation,
@c Inc.
@c See the file elisp.texi for copying conditions.
@node Macros
@chapter Macros
@cindex macros

  @dfn{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 @dfn{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.  @xref{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.
* 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.
@end menu

@node Simple Macro
@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
write @code{(inc x)} and have the effect of @code{(setq x (1+ x))}.
Here's a macro definition that does the job:

@findex inc
@example
@group
(defmacro inc (var)
   (list 'setq var (list '1+ var)))
@end group
@end example

  When this is called with @code{(inc x)}, the argument @var{var} is the
symbol @code{x}---@emph{not} the @emph{value} of @code{x}, as it would
be in a function.  The body of the macro uses this to construct the
expansion, which is @code{(setq x (1+ x))}.  Once the macro definition
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
@cindex 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 @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 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.

  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.

  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}.

@defun macroexpand form &optional environment
@cindex macro expansion
This function expands @var{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
@code{macroexpand}.  If @var{form} is not a macro call to begin with, it
is returned as given.

Note that @code{macroexpand} does not look at the subexpressions of
@var{form} (although some macro definitions may do so).  Even if they
are macro calls themselves, @code{macroexpand} does not expand them.

The function @code{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 @var{environment} is provided, it specifies an alist of macro
definitions that shadow the currently defined macros.  Byte compilation
uses this feature.

@example
@group
(defmacro inc (var)
    (list 'setq var (list '1+ var)))
@end group

@group
(macroexpand '(inc r))
     @result{} (setq r (1+ r))
@end group

@group
(defmacro inc2 (var1 var2)
    (list 'progn (list 'inc var1) (list 'inc var2)))
@end group

@group
(macroexpand '(inc2 r s))
     @result{} (progn (inc r) (inc s))  ; @r{@code{inc} not expanded here.}
@end group
@end example
@end defun


@defun macroexpand-all form &optional environment
@code{macroexpand-all} expands macros like @code{macroexpand}, but
will look for and expand all macros in @var{form}, not just at the
top-level.  If no macros are expanded, the return value is @code{eq}
to @var{form}.

Repeating the example used for @code{macroexpand} above with
@code{macroexpand-all}, we see that @code{macroexpand-all} @emph{does}
expand the embedded calls to @code{inc}:

@example
(macroexpand-all '(inc2 r s))
     @result{} (progn (setq r (1+ r)) (setq s (1+ s)))
@end example

@end defun

@defun macroexpand-1 form &optional environment
This function expands macros like @code{macroexpand}, but it only
performs one step of the expansion: if the result is another macro
call, @code{macroexpand-1} will not expand it.
@end defun

@node Compiling Macros
@section Macros and Byte Compilation
@cindex byte-compiling macros

  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
already 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 @code{defmacro} form, the macro is defined
temporarily for the rest of the compilation of that file.

  Byte-compiling a file also executes any @code{require} calls at
top-level in the file, so you can ensure that necessary macro
definitions are available during compilation by requiring the files
that define them (@pxref{Named Features}).  To avoid loading the macro
definition files when someone @emph{runs} the compiled program, write
@code{eval-when-compile} around the @code{require} calls (@pxref{Eval
During Compile}).

@node Defining Macros
@section Defining Macros
@cindex defining macros
@cindex macro, how to define

  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 almost always
defined with the @code{defmacro} macro.

@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{args} . @var{body})
@end example

(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

  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
@example
@group
(defmacro t-becomes-nil (variable)
  `(if (eq ,variable t)
       (setq ,variable nil)))
@end group

@group
(t-becomes-nil foo)
     @equiv{} (if (eq foo t) (setq foo nil))
@end group
@end example
@end example

@node Problems with Macros
@section Common Problems Using Macros
@cindex macro caveats

  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
* Wrong Time::             Do the work in the expansion, not in the macro.
* Argument Evaluation::    The expansion should evaluate each macro arg once.
* Surprising Local Vars::  Local variable bindings in the expansion
                              require special care.
* Eval During Expansion::  Don't evaluate them; put them in the expansion.
* Repeated Expansion::     Avoid depending on how many times expansion is done.
@end menu

@node Wrong Time
@subsection Wrong Time

  The most common problem in writing macros is doing some of the
real work prematurely---while expanding the macro, rather than in the
expansion itself.  For instance, one real package had this macro
definition:

@example
(defmacro my-set-buffer-multibyte (arg)
  (if (fboundp 'set-buffer-multibyte)
      (set-buffer-multibyte arg)))
@end example

With this erroneous macro definition, the program worked fine when
interpreted but failed when compiled.  This macro definition called
@code{set-buffer-multibyte} during compilation, which was wrong, and
then did nothing when the compiled package was run.  The definition
that the programmer really wanted was this:

@example
(defmacro my-set-buffer-multibyte (arg)
  (if (fboundp 'set-buffer-multibyte)
      `(set-buffer-multibyte ,arg)))
@end example

@noindent
This macro expands, if appropriate, into a call to
@code{set-buffer-multibyte} that will be executed when the compiled
program is actually run.

@node Argument Evaluation
@subsection Evaluating Macro Arguments Repeatedly

  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 for-loop construct.

@findex for
@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)))))))
@end group

@group
(for i from 1 to 3 do
   (setq square (* i i))
   (princ (format "\n%d %d" i square)))
@expansion{}
@end group
@group
(let ((i 1))
  (while (<= i 3)
    (setq square (* i i))
    (princ (format "\n%d %d" i square))
    (inc i)))
@end group
@group

     @print{}1       1
     @print{}2       4
     @print{}3       9
@result{} nil
@end group
@end example

@noindent
The arguments @code{from}, @code{to}, and @code{do} in this macro are
syntactic sugar; they are entirely ignored.  The idea is that you
will write noise words (such as @code{from}, @code{to}, and @code{do})
in those positions in the macro call.

Here's an equivalent definition simplified through use of backquote:

@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))."
  `(let ((,var ,init))
     (while (<= ,var ,final)
       ,@@body
       (inc ,var))))
@end group
@end example

Both forms of this definition (with backquote and without) suffer from
the defect that @var{final} is evaluated on every iteration.  If
@var{final} is a constant, this is not a problem.  If it is a more
complex form, say @code{(long-complex-calculation x)}, this can slow
down the execution significantly.  If @var{final} has side effects,
executing it more than once is probably incorrect.

@cindex macro argument evaluation
A well-designed macro definition takes steps to avoid this problem by
producing an expansion that evaluates the argument expressions exactly
once unless repeated evaluation is part of the intended purpose of the
macro.  Here is a correct expansion for the @code{for} macro:

@example
@group
(let ((i 1)
      (max 3))
  (while (<= i max)
    (setq square (* i i))
    (princ (format "%d      %d" i square))
    (inc i)))
@end group
@end example

Here is a macro definition that creates this expansion:

@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))."
  `(let ((,var ,init)
         (max ,final))
     (while (<= ,var max)
       ,@@body
       (inc ,var))))
@end group
@end example

  Unfortunately, this fix introduces another problem,
described in the following section.

@node Surprising Local Vars
@subsection Local Variables in Macro Expansions

@ifnottex
  In the previous section, the definition of @code{for} was fixed as
follows to make the expansion evaluate the macro arguments the proper
number of times:

@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))."
@end group
@group
  `(let ((,var ,init)
         (max ,final))
     (while (<= ,var max)
       ,@@body
       (inc ,var))))
@end group
@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:

@example
@group
(let ((max 0))
  (for x from 0 to 10 do
    (let ((this (frob x)))
      (if (< max this)
          (setq max this)))))
@end group
@end example

@noindent
The references to @code{max} inside the body of the @code{for}, which
are supposed to refer to the user's binding of @code{max}, really access
the binding made by @code{for}.

The way to correct this is to use an uninterned symbol instead of
@code{max} (@pxref{Creating Symbols}).  The uninterned symbol can be
bound and referred to just like any other symbol, but since it is
created by @code{for}, we know that it cannot already appear in the
user's program.  Since it is not interned, there is no way the user can
put it into the program later.  It will never appear anywhere except
where put by @code{for}.  Here is a definition of @code{for} that works
this way:

@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))."
  (let ((tempvar (make-symbol "max")))
    `(let ((,var ,init)
           (,tempvar ,final))
       (while (<= ,var ,tempvar)
         ,@@body
         (inc ,var)))))
@end group
@end example

@noindent
This creates an uninterned symbol named @code{max} and puts it in the
expansion instead of the usual interned symbol @code{max} that appears
in expressions ordinarily.

@node Eval During Expansion
@subsection Evaluating Macro Arguments in Expansion

  Another problem can happen if the macro definition itself
evaluates any of the macro argument expressions, such as by calling
@code{eval} (@pxref{Eval}).  If the argument is supposed to refer to the
user's variables, you may have trouble if the user happens to use a
variable with the same name as one of the macro arguments.  Inside the
macro body, the macro argument binding is the most local binding of this
variable, so any references inside the form being evaluated do refer to
it.  Here is an example:

@example
@group
(defmacro foo (a)
  (list 'setq (eval a) t))
@end group
@group
(setq x 'b)
(foo x) @expansion{} (setq b t)
     @result{} t                  ; @r{and @code{b} has been set.}
;; @r{but}
(setq a 'c)
(foo a) @expansion{} (setq a t)
     @result{} t                  ; @r{but this set @code{a}, not @code{c}.}

@end group
@end example

  It makes a difference whether the user's variable is named @code{a} or
@code{x}, because @code{a} conflicts with the macro argument variable
@code{a}.

  Another problem with calling @code{eval} in a macro definition is that
it probably won't do what you intend in a compiled program.  The
byte compiler runs macro definitions while compiling the program, when
the program's own computations (which you might have wished to access
with @code{eval}) don't occur and its local variable bindings don't
exist.

  To avoid these problems, @strong{don't evaluate an argument expression
while computing the macro expansion}.  Instead, substitute the
expression into the macro expansion, so that its value will be computed
as part of executing the expansion.  This is how the other examples in
this chapter work.

@node Repeated Expansion
@subsection How Many Times is the Macro Expanded?
@cindex side effect

  Occasionally problems result from the fact that a macro call is
expanded each time it is evaluated in an interpreted function, but is
expanded only once (during compilation) for a compiled function.  If the
macro definition has side effects, they will work differently depending
on how many times the macro is expanded.

  Therefore, you should avoid side effects in computation of the
macro expansion, unless you really know what you are doing.

  One special kind of side effect can't be avoided: constructing Lisp
objects.  Almost all macro expansions include constructed lists; that is
the whole point of most macros.  This is usually safe; there is just one
case where you must be careful: when the object you construct is part of a
quoted constant in the macro expansion.

  If the macro is expanded just once, in compilation, then the object is
constructed just once, during compilation.  But in interpreted
execution, the macro is expanded each time the macro call runs, and this
means a new object is constructed each time.

  In most clean Lisp code, this difference won't matter.  It can matter
only if you perform side-effects on the objects constructed by the macro
definition.  Thus, to avoid trouble, @strong{avoid side effects on
objects constructed by macro definitions}.  Here is an example of how
such side effects can get you into trouble:

@lisp
@group
(defmacro empty-object ()
  (list 'quote (cons nil nil)))
@end group

@group
(defun initialize (condition)
  (let ((object (empty-object)))
    (if condition
        (setcar object condition))
    object))
@end group
@end lisp

@noindent
If @code{initialize} is interpreted, a new list @code{(nil)} is
constructed each time @code{initialize} is called.  Thus, no side effect
survives between calls.  If @code{initialize} is compiled, then the
macro @code{empty-object} is expanded during compilation, producing a
single constant @code{(nil)} that is reused and altered each time
@code{initialize} is called.

One way to avoid pathological cases like this is to think of
@code{empty-object} as a funny kind of constant, not as a memory
allocation construct.  You wouldn't use @code{setcar} on a constant such
as @code{'(nil)}, so naturally you won't use it on @code{(empty-object)}
either.

@node Indenting Macros
@section Indenting Macros

  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}))
@end example

@noindent
@cindex @code{lisp-indent-function} property
This results in the @code{lisp-indent-function} property being set on
the macro name.

@noindent
Here are the possibilities for @var{indent-spec}:

@table @asis
@item @code{nil}
This is the same as no property---use the standard indentation pattern.
@item @code{defun}
Handle this function like a @samp{def} construct: treat the second
line as the start of a @dfn{body}.
@item an integer, @var{number}
The first @var{number} arguments of the function are
@dfn{distinguished} arguments; the rest are considered the body
of the expression.  A line in the expression is indented according to
whether the first argument on it is distinguished or not.  If the
argument is part of the body, the line is indented @code{lisp-body-indent}
more columns than the open-parenthesis starting the containing
expression.  If the argument is distinguished and is either the first
or second argument, it is indented @emph{twice} that many extra columns.
If the argument is distinguished and not the first or second argument,
the line uses the standard pattern.
@item a symbol, @var{symbol}
@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.
@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
difference between returning a number and returning a list is that a
number says that all following lines at the same nesting level should
be indented just like this one; a list says that following lines might
call for different indentations.  This makes a difference when the
indentation is being computed by @kbd{C-M-q}; if the value is a
number, @kbd{C-M-q} need not recalculate indentation for the following
lines until the end of the list.
@end table

debug log:

solving bcee5b1078 ...
found bcee5b1078 in https://yhetil.org/emacs-devel/87tvew7bxj.fsf@tcd.ie/
found a422ba9750 in https://git.savannah.gnu.org/cgit/emacs.git
preparing index
index prepared:
100644 a422ba9750d2c899a8950aa754d9f5fdb3958aa1	doc/lispref/macros.texi

applying [1/1] https://yhetil.org/emacs-devel/87tvew7bxj.fsf@tcd.ie/
diff --git a/doc/lispref/macros.texi b/doc/lispref/macros.texi
index a422ba9750..bcee5b1078 100644

Checking patch doc/lispref/macros.texi...
Applied patch doc/lispref/macros.texi cleanly.

index at:
100644 bcee5b1078919a1eefa1be5d3427e61dddee11a0	doc/lispref/macros.texi

(*) Git path names are given by the tree(s) the blob belongs to.
    Blobs themselves have no identifier aside from the hash of its contents.^

Code repositories for project(s) associated with this public inbox

	https://git.savannah.gnu.org/cgit/emacs.git

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).