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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
fd897522 | 3 | @c Copyright (C) 1998, 1999 Free Software Foundation, Inc. |
b933f645 RS |
4 | @c See the file elisp.texi for copying conditions. |
5 | @setfilename ../info/advising | |
6 | @node Advising Functions, Debugging, Byte Compilation, Top | |
7 | @chapter Advising Emacs Lisp Functions | |
8 | @cindex advising functions | |
9 | ||
10 | The @dfn{advice} feature lets you add to the existing definition of a | |
a9f0a989 | 11 | function, by @dfn{advising the function}. This is a clean method for a |
b933f645 | 12 | library to customize functions defined by other parts of Emacs---cleaner |
a9f0a989 | 13 | than redefining the whole function. |
b933f645 | 14 | |
1911e6e5 RS |
15 | @cindex piece of advice |
16 | Each function can have multiple @dfn{pieces of advice}, separately | |
17 | defined. Each defined piece of advice can be enabled or disabled | |
18 | explicitly. The enabled pieces of advice for any given function | |
19 | actually take effect when you @dfn{activate} advice for that function, or when | |
20 | that function is subsequently defined or redefined. | |
b933f645 | 21 | |
a9f0a989 RS |
22 | @strong{Usage Note:} Advice is useful for altering the behavior of |
23 | existing calls to an existing function. If you want the new behavior | |
24 | for new calls, or for key bindings, it is cleaner to define a new | |
25 | function (or a new command) which uses the existing function. | |
26 | ||
b933f645 | 27 | @menu |
a9f0a989 RS |
28 | * Simple Advice:: A simple example to explain the basics of advice. |
29 | * Defining Advice:: Detailed description of @code{defadvice}. | |
1911e6e5 | 30 | * Around-Advice:: Wrapping advice around a function's definition. |
a9f0a989 RS |
31 | * Computed Advice:: ...is to @code{defadvice} as @code{fset} is to @code{defun}. |
32 | * Activation of Advice:: Advice doesn't do anything until you activate it. | |
33 | * Enabling Advice:: You can enable or disable each piece of advice. | |
34 | * Preactivation:: Preactivation is a way of speeding up the | |
35 | loading of compiled advice. | |
1911e6e5 | 36 | * Argument Access in Advice:: How advice can access the function's arguments. |
a9f0a989 RS |
37 | * Subr Arguments:: Accessing arguments when advising a primitive. |
38 | * Combined Definition:: How advice is implemented. | |
b933f645 RS |
39 | @end menu |
40 | ||
a9f0a989 RS |
41 | @node Simple Advice |
42 | @section A Simple Advice Example | |
43 | ||
44 | The command @code{next-line} moves point down vertically one or more | |
45 | lines; it is the standard binding of @kbd{C-n}. When used on the last | |
46 | line of the buffer, this command inserts a newline to create a line to | |
1636ca09 GM |
47 | move to if @code{next-line-add-newlines} is non-@code{nil} (its default |
48 | is @code{nil}.) | |
a9f0a989 RS |
49 | |
50 | Suppose you wanted to add a similar feature to @code{previous-line}, | |
51 | which would insert a new line at the beginning of the buffer for the | |
52 | command to move to. How could you do this? | |
53 | ||
54 | You could do it by redefining the whole function, but that is not | |
55 | modular. The advice feature provides a cleaner alternative: you can | |
56 | effectively add your code to the existing function definition, without | |
57 | actually changing or even seeing that definition. Here is how to do | |
58 | this: | |
59 | ||
60 | @example | |
61 | (defadvice previous-line (before next-line-at-end (arg)) | |
62 | "Insert an empty line when moving up from the top line." | |
63 | (if (and next-line-add-newlines (= arg 1) | |
64 | (save-excursion (beginning-of-line) (bobp))) | |
65 | (progn | |
66 | (beginning-of-line) | |
67 | (newline)))) | |
68 | @end example | |
69 | ||
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70 | This expression defines a @dfn{piece of advice} for the function |
71 | @code{previous-line}. This piece of advice is named | |
72 | @code{next-line-at-end}, and the symbol @code{before} says that it is | |
73 | @dfn{before-advice} which should run before the regular definition of | |
74 | @code{previous-line}. @code{(arg)} specifies how the advice code can | |
75 | refer to the function's arguments. | |
76 | ||
77 | When this piece of advice runs, it creates an additional line, in the | |
78 | situation where that is appropriate, but does not move point to that | |
79 | line. This is the correct way to write the advice, because the normal | |
80 | definition will run afterward and will move back to the newly inserted | |
81 | line. | |
82 | ||
83 | Defining the advice doesn't immediately change the function | |
84 | @code{previous-line}. That happens when you @dfn{activate} the advice, | |
85 | like this: | |
86 | ||
87 | @example | |
88 | (ad-activate 'previous-line) | |
89 | @end example | |
90 | ||
91 | @noindent | |
92 | This is what actually begins to use the advice that has been defined so | |
93 | far for the function @code{previous-line}. Henceforth, whenever that | |
94 | function is run, whether invoked by the user with @kbd{C-p} or | |
95 | @kbd{M-x}, or called from Lisp, it runs the advice first, and its | |
96 | regular definition second. | |
97 | ||
98 | This example illustrates before-advice, which is one @dfn{class} of | |
99 | advice: it runs before the function's base definition. There are two | |
100 | other advice classes: @dfn{after-advice}, which runs after the base | |
101 | definition, and @dfn{around-advice}, which lets you specify an | |
102 | expression to wrap around the invocation of the base definition. | |
103 | ||
b933f645 RS |
104 | @node Defining Advice |
105 | @section Defining Advice | |
1911e6e5 RS |
106 | @cindex defining advice |
107 | @cindex advice, defining | |
b933f645 RS |
108 | |
109 | To define a piece of advice, use the macro @code{defadvice}. A call | |
110 | to @code{defadvice} has the following syntax, which is based on the | |
1911e6e5 | 111 | syntax of @code{defun} and @code{defmacro}, but adds more: |
b933f645 RS |
112 | |
113 | @findex defadvice | |
114 | @example | |
115 | (defadvice @var{function} (@var{class} @var{name} | |
116 | @r{[}@var{position}@r{]} @r{[}@var{arglist}@r{]} | |
117 | @var{flags}...) | |
118 | @r{[}@var{documentation-string}@r{]} | |
119 | @r{[}@var{interactive-form}@r{]} | |
120 | @var{body-forms}...) | |
121 | @end example | |
122 | ||
123 | @noindent | |
124 | Here, @var{function} is the name of the function (or macro or special | |
125 | form) to be advised. From now on, we will write just ``function'' when | |
126 | describing the entity being advised, but this always includes macros and | |
127 | special forms. | |
128 | ||
a9f0a989 RS |
129 | @cindex class of advice |
130 | @cindex before-advice | |
131 | @cindex after-advice | |
132 | @cindex around-advice | |
133 | @var{class} specifies the @dfn{class} of the advice---one of @code{before}, | |
b933f645 RS |
134 | @code{after}, or @code{around}. Before-advice runs before the function |
135 | itself; after-advice runs after the function itself; around-advice is | |
136 | wrapped around the execution of the function itself. After-advice and | |
137 | around-advice can override the return value by setting | |
138 | @code{ad-return-value}. | |
139 | ||
1911e6e5 RS |
140 | @defvar ad-return-value |
141 | While advice is executing, after the function's original definition has | |
142 | been executed, this variable holds its return value, which will | |
143 | ultimately be returned to the caller after finishing all the advice. | |
144 | After-advice and around-advice can arrange to return some other value | |
145 | by storing it in this variable. | |
146 | @end defvar | |
b933f645 | 147 | |
a9f0a989 RS |
148 | The argument @var{name} is the name of the advice, a non-@code{nil} |
149 | symbol. The advice name uniquely identifies one piece of advice, within all | |
150 | the pieces of advice in a particular class for a particular | |
151 | @var{function}. The name allows you to refer to the piece of | |
152 | advice---to redefine it, or to enable or disable it. | |
153 | ||
154 | In place of the argument list in an ordinary definition, an advice | |
155 | definition calls for several different pieces of information. | |
156 | ||
b933f645 RS |
157 | The optional @var{position} specifies where, in the current list of |
158 | advice of the specified @var{class}, this new advice should be placed. | |
1911e6e5 RS |
159 | It should be either @code{first}, @code{last} or a number that specifies |
160 | a zero-based position (@code{first} is equivalent to 0). If no position | |
161 | is specified, the default is @code{first}. Position values outside the | |
162 | range of existing positions in this class are mapped to the beginning or | |
163 | the end of the range, whichever is closer. The @var{position} value is | |
164 | ignored when redefining an existing piece of advice. | |
b933f645 RS |
165 | |
166 | The optional @var{arglist} can be used to define the argument list for | |
a9f0a989 RS |
167 | the sake of advice. This becomes the argument list of the combined |
168 | definition that is generated in order to run the advice (@pxref{Combined | |
169 | Definition}). Therefore, the advice expressions can use the argument | |
170 | variables in this list to access argument values. | |
171 | ||
a40d4712 PR |
172 | The argument list used in advice need not be the same as the argument |
173 | list used in the original function, but must be compatible with it, so | |
174 | that it can handle the ways the function is actually called. If two | |
175 | pieces of advice for a function both specify an argument list, they must | |
176 | specify the same argument list. | |
b933f645 | 177 | |
8241495d | 178 | @xref{Argument Access in Advice}, for more information about argument |
a40d4712 PR |
179 | lists and advice, and a more flexible way for advice to access the |
180 | arguments. | |
8241495d | 181 | |
1911e6e5 RS |
182 | The remaining elements, @var{flags}, are symbols that specify further |
183 | information about how to use this piece of advice. Here are the valid | |
184 | symbols and their meanings: | |
b933f645 RS |
185 | |
186 | @table @code | |
187 | @item activate | |
a9f0a989 RS |
188 | Activate the advice for @var{function} now. Changes in a function's |
189 | advice always take effect the next time you activate advice for the | |
190 | function; this flag says to do so, for @var{function}, immediately after | |
191 | defining this piece of advice. | |
192 | ||
193 | @cindex forward advice | |
194 | This flag has no effect if @var{function} itself is not defined yet (a | |
195 | situation known as @dfn{forward advice}), because it is impossible to | |
196 | activate an undefined function's advice. However, defining | |
197 | @var{function} will automatically activate its advice. | |
b933f645 RS |
198 | |
199 | @item protect | |
200 | Protect this piece of advice against non-local exits and errors in | |
1911e6e5 RS |
201 | preceding code and advice. Protecting advice places it as a cleanup in |
202 | an @code{unwind-protect} form, so that it will execute even if the | |
a9f0a989 | 203 | previous code gets an error or uses @code{throw}. @xref{Cleanups}. |
b933f645 RS |
204 | |
205 | @item compile | |
a9f0a989 RS |
206 | Compile the combined definition that is used to run the advice. This |
207 | flag is ignored unless @code{activate} is also specified. | |
208 | @xref{Combined Definition}. | |
b933f645 RS |
209 | |
210 | @item disable | |
a9f0a989 RS |
211 | Initially disable this piece of advice, so that it will not be used |
212 | unless subsequently explicitly enabled. @xref{Enabling Advice}. | |
b933f645 RS |
213 | |
214 | @item preactivate | |
215 | Activate advice for @var{function} when this @code{defadvice} is | |
216 | compiled or macroexpanded. This generates a compiled advised definition | |
217 | according to the current advice state, which will be used during | |
8241495d | 218 | activation if appropriate. @xref{Preactivation}. |
b933f645 RS |
219 | |
220 | This is useful only if this @code{defadvice} is byte-compiled. | |
221 | @end table | |
222 | ||
223 | The optional @var{documentation-string} serves to document this piece of | |
a9f0a989 RS |
224 | advice. When advice is active for @var{function}, the documentation for |
225 | @var{function} (as returned by @code{documentation}) combines the | |
226 | documentation strings of all the advice for @var{function} with the | |
227 | documentation string of its original function definition. | |
b933f645 RS |
228 | |
229 | The optional @var{interactive-form} form can be supplied to change the | |
230 | interactive behavior of the original function. If more than one piece | |
231 | of advice has an @var{interactive-form}, then the first one (the one | |
232 | with the smallest position) found among all the advice takes precedence. | |
233 | ||
234 | The possibly empty list of @var{body-forms} specifies the body of the | |
235 | advice. The body of an advice can access or change the arguments, the | |
236 | return value, the binding environment, and perform any other kind of | |
237 | side effect. | |
238 | ||
239 | @strong{Warning:} When you advise a macro, keep in mind that macros are | |
240 | expanded when a program is compiled, not when a compiled program is run. | |
241 | All subroutines used by the advice need to be available when the byte | |
242 | compiler expands the macro. | |
243 | ||
41982e72 GM |
244 | @deffn Command ad-unadvise function |
245 | This command deletes the advice from @var{function}. | |
246 | @end deffn | |
247 | ||
248 | @deffn Command ad-unadvise-all | |
249 | This command deletes all pieces of advice from all functions. | |
250 | @end deffn | |
251 | ||
1911e6e5 RS |
252 | @node Around-Advice |
253 | @section Around-Advice | |
254 | ||
255 | Around-advice lets you ``wrap'' a Lisp expression ``around'' the | |
256 | original function definition. You specify where the original function | |
257 | definition should go by means of the special symbol @code{ad-do-it}. | |
258 | Where this symbol occurs inside the around-advice body, it is replaced | |
259 | with a @code{progn} containing the forms of the surrounded code. Here | |
260 | is an example: | |
261 | ||
262 | @example | |
263 | (defadvice foo (around foo-around) | |
264 | "Ignore case in `foo'." | |
265 | (let ((case-fold-search t)) | |
266 | ad-do-it)) | |
267 | @end example | |
268 | ||
269 | @noindent | |
270 | Its effect is to make sure that case is ignored in | |
271 | searches when the original definition of @code{foo} is run. | |
272 | ||
273 | @defvar ad-do-it | |
274 | This is not really a variable, but it is somewhat used like one | |
275 | in around-advice. It specifies the place to run the function's | |
276 | original definition and other ``earlier'' around-advice. | |
277 | @end defvar | |
278 | ||
279 | If the around-advice does not use @code{ad-do-it}, then it does not run | |
280 | the original function definition. This provides a way to override the | |
281 | original definition completely. (It also overrides lower-positioned | |
282 | pieces of around-advice). | |
283 | ||
bbf6f18c KH |
284 | If the around-advice uses @code{ad-do-it} more than once, the original |
285 | definition is run at each place. In this way, around-advice can execute | |
286 | the original definition (and lower-positioned pieces of around-advice) | |
287 | several times. Another way to do that is by using @code{ad-do-it} | |
288 | inside of a loop. | |
289 | ||
b933f645 RS |
290 | @node Computed Advice |
291 | @section Computed Advice | |
292 | ||
293 | The macro @code{defadvice} resembles @code{defun} in that the code for | |
294 | the advice, and all other information about it, are explicitly stated in | |
295 | the source code. You can also create advice whose details are computed, | |
296 | using the function @code{ad-add-advice}. | |
297 | ||
298 | @defun ad-add-advice function advice class position | |
299 | Calling @code{ad-add-advice} adds @var{advice} as a piece of advice to | |
300 | @var{function} in class @var{class}. The argument @var{advice} has | |
301 | this form: | |
302 | ||
303 | @example | |
304 | (@var{name} @var{protected} @var{enabled} @var{definition}) | |
305 | @end example | |
306 | ||
307 | Here @var{protected} and @var{enabled} are flags, and @var{definition} | |
a9f0a989 RS |
308 | is the expression that says what the advice should do. If @var{enabled} |
309 | is @code{nil}, this piece of advice is initially disabled | |
310 | (@pxref{Enabling Advice}). | |
b933f645 RS |
311 | |
312 | If @var{function} already has one or more pieces of advice in the | |
313 | specified @var{class}, then @var{position} specifies where in the list | |
314 | to put the new piece of advice. The value of @var{position} can either | |
315 | be @code{first}, @code{last}, or a number (counting from 0 at the | |
316 | beginning of the list). Numbers outside the range are mapped to the | |
8241495d RS |
317 | beginning or the end of the range, whichever is closer. The |
318 | @var{position} value is ignored when redefining an existing piece of | |
319 | advice. | |
b933f645 RS |
320 | |
321 | If @var{function} already has a piece of @var{advice} with the same | |
322 | name, then the position argument is ignored and the old advice is | |
323 | replaced with the new one. | |
324 | @end defun | |
325 | ||
326 | @node Activation of Advice | |
327 | @section Activation of Advice | |
328 | @cindex activating advice | |
1911e6e5 | 329 | @cindex advice, activating |
b933f645 RS |
330 | |
331 | By default, advice does not take effect when you define it---only when | |
332 | you @dfn{activate} advice for the function that was advised. You can | |
333 | request the activation of advice for a function when you define the | |
334 | advice, by specifying the @code{activate} flag in the @code{defadvice}. | |
335 | But normally you activate the advice for a function by calling the | |
336 | function @code{ad-activate} or one of the other activation commands | |
337 | listed below. | |
338 | ||
339 | Separating the activation of advice from the act of defining it permits | |
340 | you to add several pieces of advice to one function efficiently, without | |
341 | redefining the function over and over as each advice is added. More | |
342 | importantly, it permits defining advice for a function before that | |
343 | function is actually defined. | |
344 | ||
a9f0a989 RS |
345 | When a function's advice is first activated, the function's original |
346 | definition is saved, and all enabled pieces of advice for that function | |
347 | are combined with the original definition to make a new definition. | |
348 | (Pieces of advice that are currently disabled are not used; see | |
349 | @ref{Enabling Advice}.) This definition is installed, and optionally | |
350 | byte-compiled as well, depending on conditions described below. | |
b933f645 RS |
351 | |
352 | In all of the commands to activate advice, if @var{compile} is @code{t}, | |
353 | the command also compiles the combined definition which implements the | |
354 | advice. | |
355 | ||
356 | @deffn Command ad-activate function &optional compile | |
8241495d | 357 | This command activates all the advice defined for @var{function}. |
b933f645 RS |
358 | @end deffn |
359 | ||
a9f0a989 RS |
360 | To activate advice for a function whose advice is already active is not |
361 | a no-op. It is a useful operation which puts into effect any changes in | |
1911e6e5 RS |
362 | that function's advice since the previous activation of advice for that |
363 | function. | |
b933f645 RS |
364 | |
365 | @deffn Command ad-deactivate function | |
366 | This command deactivates the advice for @var{function}. | |
1911e6e5 RS |
367 | @cindex deactivating advice |
368 | @cindex advice, deactivating | |
b933f645 RS |
369 | @end deffn |
370 | ||
41982e72 GM |
371 | @deffn Command ad-update function &optional compile |
372 | This command activates the advice for @var{function} | |
373 | if its advice is already activated. This is useful | |
374 | if you change the advice. | |
375 | @end deffn | |
376 | ||
b933f645 RS |
377 | @deffn Command ad-activate-all &optional compile |
378 | This command activates the advice for all functions. | |
379 | @end deffn | |
380 | ||
381 | @deffn Command ad-deactivate-all | |
382 | This command deactivates the advice for all functions. | |
383 | @end deffn | |
384 | ||
41982e72 GM |
385 | @deffn Command ad-update-all &optional compile |
386 | This command activates the advice for all functions | |
387 | whose advice is already activated. This is useful | |
388 | if you change the advice of some functions. | |
389 | @end deffn | |
390 | ||
b933f645 RS |
391 | @deffn Command ad-activate-regexp regexp &optional compile |
392 | This command activates all pieces of advice whose names match | |
393 | @var{regexp}. More precisely, it activates all advice for any function | |
394 | which has at least one piece of advice that matches @var{regexp}. | |
395 | @end deffn | |
396 | ||
397 | @deffn Command ad-deactivate-regexp regexp | |
1911e6e5 | 398 | This command deactivates all pieces of advice whose names match |
b933f645 RS |
399 | @var{regexp}. More precisely, it deactivates all advice for any |
400 | function which has at least one piece of advice that matches | |
401 | @var{regexp}. | |
402 | @end deffn | |
403 | ||
404 | @deffn Command ad-update-regexp regexp &optional compile | |
405 | This command activates pieces of advice whose names match @var{regexp}, | |
a9f0a989 | 406 | but only those for functions whose advice is already activated. |
1911e6e5 | 407 | @cindex reactivating advice |
b933f645 | 408 | |
a9f0a989 RS |
409 | Reactivating a function's advice is useful for putting into effect all |
410 | the changes that have been made in its advice (including enabling and | |
411 | disabling specific pieces of advice; @pxref{Enabling Advice}) since the | |
412 | last time it was activated. | |
b933f645 RS |
413 | @end deffn |
414 | ||
415 | @deffn Command ad-start-advice | |
a9f0a989 | 416 | Turn on automatic advice activation when a function is defined or |
41982e72 GM |
417 | redefined. If you turn on this mode, then advice takes effect |
418 | immediately when defined. | |
a9f0a989 RS |
419 | @end deffn |
420 | ||
421 | @deffn Command ad-stop-advice | |
b933f645 RS |
422 | Turn off automatic advice activation when a function is defined or |
423 | redefined. | |
424 | @end deffn | |
425 | ||
426 | @defopt ad-default-compilation-action | |
427 | This variable controls whether to compile the combined definition | |
428 | that results from activating advice for a function. | |
41982e72 | 429 | |
080a57ba | 430 | A value of @code{always} specifies to compile unconditionally. |
41982e72 GM |
431 | A value of @code{nil} specifies never compile the advice. |
432 | ||
433 | A value of @code{maybe} specifies to compile if the byte-compiler is | |
434 | already loaded. A value of @code{like-original} specifies to compile | |
080a57ba | 435 | the advice if the original definition of the advised function is |
41982e72 GM |
436 | compiled or a built-in function. |
437 | ||
438 | This variable takes effect only if the @var{compile} argument of | |
439 | @code{ad-activate} (or any of the above functions) was supplied as | |
440 | @code{nil}. If that argument is non-@code{nil}, that means | |
441 | to compile the advice regardless. | |
b933f645 RS |
442 | @end defopt |
443 | ||
1911e6e5 RS |
444 | If the advised definition was constructed during ``preactivation'' |
445 | (@pxref{Preactivation}), then that definition must already be compiled, | |
446 | because it was constructed during byte-compilation of the file that | |
447 | contained the @code{defadvice} with the @code{preactivate} flag. | |
b933f645 RS |
448 | |
449 | @node Enabling Advice | |
450 | @section Enabling and Disabling Advice | |
1911e6e5 RS |
451 | @cindex enabling advice |
452 | @cindex advice, enabling and disabling | |
453 | @cindex disabling advice | |
b933f645 RS |
454 | |
455 | Each piece of advice has a flag that says whether it is enabled or | |
1911e6e5 RS |
456 | not. By enabling or disabling a piece of advice, you can turn it on |
457 | and off without having to undefine and redefine it. For example, here is | |
b933f645 RS |
458 | how to disable a particular piece of advice named @code{my-advice} for |
459 | the function @code{foo}: | |
460 | ||
461 | @example | |
462 | (ad-disable-advice 'foo 'before 'my-advice) | |
463 | @end example | |
464 | ||
1911e6e5 | 465 | This function by itself only changes the enable flag for a piece of |
a9f0a989 | 466 | advice. To make the change take effect in the advised definition, you |
b933f645 RS |
467 | must activate the advice for @code{foo} again: |
468 | ||
469 | @example | |
470 | (ad-activate 'foo) | |
471 | @end example | |
472 | ||
473 | @deffn Command ad-disable-advice function class name | |
474 | This command disables the piece of advice named @var{name} in class | |
475 | @var{class} on @var{function}. | |
476 | @end deffn | |
477 | ||
478 | @deffn Command ad-enable-advice function class name | |
479 | This command enables the piece of advice named @var{name} in class | |
480 | @var{class} on @var{function}. | |
481 | @end deffn | |
482 | ||
a9f0a989 RS |
483 | You can also disable many pieces of advice at once, for various |
484 | functions, using a regular expression. As always, the changes take real | |
485 | effect only when you next reactivate advice for the functions in | |
486 | question. | |
b933f645 RS |
487 | |
488 | @deffn Command ad-disable-regexp regexp | |
489 | This command disables all pieces of advice whose names match | |
490 | @var{regexp}, in all classes, on all functions. | |
491 | @end deffn | |
492 | ||
493 | @deffn Command ad-enable-regexp regexp | |
494 | This command enables all pieces of advice whose names match | |
495 | @var{regexp}, in all classes, on all functions. | |
496 | @end deffn | |
497 | ||
498 | @node Preactivation | |
499 | @section Preactivation | |
1911e6e5 RS |
500 | @cindex preactivating advice |
501 | @cindex advice, preactivating | |
b933f645 RS |
502 | |
503 | Constructing a combined definition to execute advice is moderately | |
504 | expensive. When a library advises many functions, this can make loading | |
505 | the library slow. In that case, you can use @dfn{preactivation} to | |
506 | construct suitable combined definitions in advance. | |
507 | ||
508 | To use preactivation, specify the @code{preactivate} flag when you | |
509 | define the advice with @code{defadvice}. This @code{defadvice} call | |
510 | creates a combined definition which embodies this piece of advice | |
511 | (whether enabled or not) plus any other currently enabled advice for the | |
512 | same function, and the function's own definition. If the | |
513 | @code{defadvice} is compiled, that compiles the combined definition | |
514 | also. | |
515 | ||
a9f0a989 RS |
516 | When the function's advice is subsequently activated, if the enabled |
517 | advice for the function matches what was used to make this combined | |
518 | definition, then the existing combined definition is used, thus avoiding | |
519 | the need to construct one. Thus, preactivation never causes wrong | |
b933f645 | 520 | results---but it may fail to do any good, if the enabled advice at the |
a9f0a989 | 521 | time of activation doesn't match what was used for preactivation. |
b933f645 RS |
522 | |
523 | Here are some symptoms that can indicate that a preactivation did not | |
524 | work properly, because of a mismatch. | |
525 | ||
526 | @itemize @bullet | |
527 | @item | |
528 | Activation of the advised | |
529 | function takes longer than usual. | |
530 | @item | |
531 | The byte-compiler gets | |
532 | loaded while an advised function gets activated. | |
533 | @item | |
534 | @code{byte-compile} is included in the value of @code{features} even | |
535 | though you did not ever explicitly use the byte-compiler. | |
536 | @end itemize | |
537 | ||
538 | Compiled preactivated advice works properly even if the function itself | |
539 | is not defined until later; however, the function needs to be defined | |
540 | when you @emph{compile} the preactivated advice. | |
541 | ||
542 | There is no elegant way to find out why preactivated advice is not being | |
543 | used. What you can do is to trace the function | |
544 | @code{ad-cache-id-verification-code} (with the function | |
a9f0a989 RS |
545 | @code{trace-function-background}) before the advised function's advice |
546 | is activated. After activation, check the value returned by | |
b933f645 RS |
547 | @code{ad-cache-id-verification-code} for that function: @code{verified} |
548 | means that the preactivated advice was used, while other values give | |
549 | some information about why they were considered inappropriate. | |
550 | ||
551 | @strong{Warning:} There is one known case that can make preactivation | |
552 | fail, in that a preconstructed combined definition is used even though | |
553 | it fails to match the current state of advice. This can happen when two | |
554 | packages define different pieces of advice with the same name, in the | |
555 | same class, for the same function. But you should avoid that anyway. | |
556 | ||
557 | @node Argument Access in Advice | |
558 | @section Argument Access in Advice | |
559 | ||
560 | The simplest way to access the arguments of an advised function in the | |
561 | body of a piece of advice is to use the same names that the function | |
562 | definition uses. To do this, you need to know the names of the argument | |
563 | variables of the original function. | |
564 | ||
565 | While this simple method is sufficient in many cases, it has a | |
566 | disadvantage: it is not robust, because it hard-codes the argument names | |
567 | into the advice. If the definition of the original function changes, | |
568 | the advice might break. | |
569 | ||
a9f0a989 RS |
570 | Another method is to specify an argument list in the advice itself. |
571 | This avoids the need to know the original function definition's argument | |
572 | names, but it has a limitation: all the advice on any particular | |
573 | function must use the same argument list, because the argument list | |
574 | actually used for all the advice comes from the first piece of advice | |
575 | for that function. | |
576 | ||
b933f645 RS |
577 | A more robust method is to use macros that are translated into the |
578 | proper access forms at activation time, i.e., when constructing the | |
579 | advised definition. Access macros access actual arguments by position | |
1911e6e5 | 580 | regardless of how these actual arguments get distributed onto the |
b933f645 RS |
581 | argument variables of a function. This is robust because in Emacs Lisp |
582 | the meaning of an argument is strictly determined by its position in the | |
583 | argument list. | |
584 | ||
585 | @defmac ad-get-arg position | |
586 | This returns the actual argument that was supplied at @var{position}. | |
587 | @end defmac | |
588 | ||
589 | @defmac ad-get-args position | |
590 | This returns the list of actual arguments supplied starting at | |
591 | @var{position}. | |
592 | @end defmac | |
593 | ||
594 | @defmac ad-set-arg position value | |
595 | This sets the value of the actual argument at @var{position} to | |
596 | @var{value} | |
597 | @end defmac | |
598 | ||
599 | @defmac ad-set-args position value-list | |
600 | This sets the list of actual arguments starting at @var{position} to | |
601 | @var{value-list}. | |
602 | @end defmac | |
603 | ||
604 | Now an example. Suppose the function @code{foo} is defined as | |
605 | ||
606 | @example | |
607 | (defun foo (x y &optional z &rest r) ...) | |
608 | @end example | |
609 | ||
610 | @noindent | |
611 | and is then called with | |
612 | ||
613 | @example | |
614 | (foo 0 1 2 3 4 5 6) | |
615 | @end example | |
616 | ||
617 | @noindent | |
618 | which means that @var{x} is 0, @var{y} is 1, @var{z} is 2 and @var{r} is | |
619 | @code{(3 4 5 6)} within the body of @code{foo}. Here is what | |
620 | @code{ad-get-arg} and @code{ad-get-args} return in this case: | |
621 | ||
622 | @example | |
623 | (ad-get-arg 0) @result{} 0 | |
624 | (ad-get-arg 1) @result{} 1 | |
625 | (ad-get-arg 2) @result{} 2 | |
626 | (ad-get-arg 3) @result{} 3 | |
627 | (ad-get-args 2) @result{} (2 3 4 5 6) | |
628 | (ad-get-args 4) @result{} (4 5 6) | |
629 | @end example | |
630 | ||
631 | Setting arguments also makes sense in this example: | |
632 | ||
633 | @example | |
634 | (ad-set-arg 5 "five") | |
635 | @end example | |
636 | ||
637 | @noindent | |
638 | has the effect of changing the sixth argument to @code{"five"}. If this | |
639 | happens in advice executed before the body of @code{foo} is run, then | |
640 | @var{r} will be @code{(3 4 "five" 6)} within that body. | |
641 | ||
642 | Here is an example of setting a tail of the argument list: | |
643 | ||
644 | @example | |
645 | (ad-set-args 0 '(5 4 3 2 1 0)) | |
646 | @end example | |
647 | ||
648 | @noindent | |
649 | If this happens in advice executed before the body of @code{foo} is run, | |
650 | then within that body, @var{x} will be 5, @var{y} will be 4, @var{z} | |
651 | will be 3, and @var{r} will be @code{(2 1 0)} inside the body of | |
652 | @code{foo}. | |
653 | ||
654 | These argument constructs are not really implemented as Lisp macros. | |
655 | Instead they are implemented specially by the advice mechanism. | |
656 | ||
a9f0a989 RS |
657 | @node Subr Arguments |
658 | @section Definition of Subr Argument Lists | |
b933f645 RS |
659 | |
660 | When the advice facility constructs the combined definition, it needs | |
661 | to know the argument list of the original function. This is not always | |
662 | possible for primitive functions. When advice cannot determine the | |
663 | argument list, it uses @code{(&rest ad-subr-args)}, which always works | |
664 | but is inefficient because it constructs a list of the argument values. | |
665 | You can use @code{ad-define-subr-args} to declare the proper argument | |
666 | names for a primitive function: | |
667 | ||
668 | @defun ad-define-subr-args function arglist | |
669 | This function specifies that @var{arglist} should be used as the | |
670 | argument list for function @var{function}. | |
671 | @end defun | |
672 | ||
673 | For example, | |
674 | ||
675 | @example | |
676 | (ad-define-subr-args 'fset '(sym newdef)) | |
677 | @end example | |
678 | ||
679 | @noindent | |
680 | specifies the argument list for the function @code{fset}. | |
681 | ||
682 | @node Combined Definition | |
683 | @section The Combined Definition | |
684 | ||
685 | Suppose that a function has @var{n} pieces of before-advice, @var{m} | |
686 | pieces of around-advice and @var{k} pieces of after-advice. Assuming no | |
687 | piece of advice is protected, the combined definition produced to | |
688 | implement the advice for a function looks like this: | |
689 | ||
690 | @example | |
691 | (lambda @var{arglist} | |
692 | @r{[} @r{[}@var{advised-docstring}@r{]} @r{[}(interactive ...)@r{]} @r{]} | |
693 | (let (ad-return-value) | |
694 | @r{before-0-body-form}... | |
695 | .... | |
696 | @r{before-@var{n}-1-body-form}... | |
697 | @r{around-0-body-form}... | |
698 | @r{around-1-body-form}... | |
699 | .... | |
700 | @r{around-@var{m}-1-body-form}... | |
701 | (setq ad-return-value | |
702 | @r{apply original definition to @var{arglist}}) | |
703 | @r{other-around-@var{m}-1-body-form}... | |
704 | .... | |
705 | @r{other-around-1-body-form}... | |
706 | @r{other-around-0-body-form}... | |
707 | @r{after-0-body-form}... | |
708 | .... | |
709 | @r{after-@var{k}-1-body-form}... | |
710 | ad-return-value)) | |
711 | @end example | |
712 | ||
713 | Macros are redefined as macros, which means adding @code{macro} to | |
714 | the beginning of the combined definition. | |
715 | ||
716 | The interactive form is present if the original function or some piece | |
717 | of advice specifies one. When an interactive primitive function is | |
718 | advised, a special method is used: to call the primitive with | |
719 | @code{call-interactively} so that it will read its own arguments. | |
720 | In this case, the advice cannot access the arguments. | |
721 | ||
722 | The body forms of the various advice in each class are assembled | |
723 | according to their specified order. The forms of around-advice @var{l} | |
724 | are included in one of the forms of around-advice @var{l} @minus{} 1. | |
725 | ||
726 | The innermost part of the around advice onion is | |
727 | ||
728 | @display | |
729 | apply original definition to @var{arglist} | |
730 | @end display | |
731 | ||
732 | @noindent | |
733 | whose form depends on the type of the original function. The variable | |
734 | @code{ad-return-value} is set to whatever this returns. The variable is | |
735 | visible to all pieces of advice, which can access and modify it before | |
736 | it is actually returned from the advised function. | |
737 | ||
738 | The semantic structure of advised functions that contain protected | |
739 | pieces of advice is the same. The only difference is that | |
740 | @code{unwind-protect} forms ensure that the protected advice gets | |
741 | executed even if some previous piece of advice had an error or a | |
742 | non-local exit. If any around-advice is protected, then the whole | |
743 | around-advice onion is protected as a result. |