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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
ba318903 | 3 | @c Copyright (C) 1998-1999, 2001-2014 Free Software Foundation, Inc. |
b8d4c8d0 | 4 | @c See the file elisp.texi for copying conditions. |
ecc6530d | 5 | @node Advising Functions |
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6 | @chapter Advising Emacs Lisp Functions |
7 | @cindex advising functions | |
8 | ||
b8d4c8d0 | 9 | @cindex piece of advice |
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10 | The @dfn{advice} feature lets you add to the existing definition of |
11 | a function, by @dfn{advising the function}. A function can have | |
12 | multiple @dfn{pieces of advice}, each of which can be separately | |
13 | defined, and separately enabled or disabled (@pxref{Activation of | |
14 | Advice}). Each piece of advice can alter almost anything about the | |
15 | function, including its argument list, what the function does when it | |
16 | runs, and the value it returns. | |
17 | ||
18 | Advice can be useful for altering the behavior of an existing | |
19 | function without having to redefine the whole function. However, it | |
20 | can be a source of bugs, since existing callers to the function may | |
21 | assume the old behavior, and work incorrectly when the behavior is | |
22 | changed by advice. Advice can also cause confusion in debugging, if | |
23 | the person doing the debugging does not notice or remember that the | |
24 | function has been modified by advice. | |
25 | ||
26 | For these reasons, advice should be reserved for the cases where you | |
27 | cannot modify a function's behavior in any other way. If it is | |
28 | possible to do the same thing via a hook, that is preferable | |
29 | (@pxref{Hooks}). If you simply want to change what a particular key | |
30 | does, it may be better to write a new command, and remap the old | |
31 | command's key bindings to the new one (@pxref{Remapping Commands}). | |
32 | In particular, Emacs's own source files should not put advice on | |
33 | functions in Emacs. (There are currently a few exceptions to this | |
34 | convention, but we aim to correct them.) | |
35 | ||
36 | Macros can also be advised, in much the same way as functions. | |
37 | However, special forms (@pxref{Special Forms}) cannot be advised. | |
38 | ||
39 | It is possible to advise a primitive (@pxref{What Is a Function}), | |
40 | but one should typically @emph{not} do so, for two reasons. Firstly, | |
41 | some primitives are used by the advice mechanism, and advising them | |
42 | could cause an infinite recursion. Secondly, many primitives are | |
43 | called directly from C, and such calls ignore advice; hence, one ends | |
44 | up in a confusing situation where some calls (occurring from Lisp | |
45 | code) obey the advice and other calls (from C code) do not. | |
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46 | |
47 | @menu | |
48 | * Simple Advice:: A simple example to explain the basics of advice. | |
49 | * Defining Advice:: Detailed description of @code{defadvice}. | |
50 | * Around-Advice:: Wrapping advice around a function's definition. | |
51 | * Computed Advice:: ...is to @code{defadvice} as @code{fset} is to @code{defun}. | |
52 | * Activation of Advice:: Advice doesn't do anything until you activate it. | |
53 | * Enabling Advice:: You can enable or disable each piece of advice. | |
54 | * Preactivation:: Preactivation is a way of speeding up the | |
55 | loading of compiled advice. | |
56 | * Argument Access in Advice:: How advice can access the function's arguments. | |
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57 | * Combined Definition:: How advice is implemented. |
58 | @end menu | |
59 | ||
60 | @node Simple Advice | |
61 | @section A Simple Advice Example | |
62 | ||
63 | The command @code{next-line} moves point down vertically one or more | |
64 | lines; it is the standard binding of @kbd{C-n}. When used on the last | |
65 | line of the buffer, this command inserts a newline to create a line to | |
66 | move to if @code{next-line-add-newlines} is non-@code{nil} (its default | |
67 | is @code{nil}.) | |
68 | ||
69 | Suppose you wanted to add a similar feature to @code{previous-line}, | |
70 | which would insert a new line at the beginning of the buffer for the | |
71 | command to move to (when @code{next-line-add-newlines} is | |
72 | non-@code{nil}). How could you do this? | |
73 | ||
74 | You could do it by redefining the whole function, but that is not | |
75 | modular. The advice feature provides a cleaner alternative: you can | |
76 | effectively add your code to the existing function definition, without | |
77 | actually changing or even seeing that definition. Here is how to do | |
78 | this: | |
79 | ||
80 | @example | |
81 | (defadvice previous-line (before next-line-at-end | |
82 | (&optional arg try-vscroll)) | |
83 | "Insert an empty line when moving up from the top line." | |
84 | (if (and next-line-add-newlines (= arg 1) | |
85 | (save-excursion (beginning-of-line) (bobp))) | |
86 | (progn | |
87 | (beginning-of-line) | |
88 | (newline)))) | |
89 | @end example | |
90 | ||
91 | This expression defines a @dfn{piece of advice} for the function | |
92 | @code{previous-line}. This piece of advice is named | |
93 | @code{next-line-at-end}, and the symbol @code{before} says that it is | |
94 | @dfn{before-advice} which should run before the regular definition of | |
95 | @code{previous-line}. @code{(&optional arg try-vscroll)} specifies | |
96 | how the advice code can refer to the function's arguments. | |
97 | ||
98 | When this piece of advice runs, it creates an additional line, in the | |
99 | situation where that is appropriate, but does not move point to that | |
100 | line. This is the correct way to write the advice, because the normal | |
101 | definition will run afterward and will move back to the newly inserted | |
102 | line. | |
103 | ||
104 | Defining the advice doesn't immediately change the function | |
105 | @code{previous-line}. That happens when you @dfn{activate} the advice, | |
106 | like this: | |
107 | ||
108 | @example | |
109 | (ad-activate 'previous-line) | |
110 | @end example | |
111 | ||
112 | @noindent | |
113 | This is what actually begins to use the advice that has been defined so | |
114 | far for the function @code{previous-line}. Henceforth, whenever that | |
115 | function is run, whether invoked by the user with @kbd{C-p} or | |
116 | @kbd{M-x}, or called from Lisp, it runs the advice first, and its | |
117 | regular definition second. | |
118 | ||
119 | This example illustrates before-advice, which is one @dfn{class} of | |
120 | advice: it runs before the function's base definition. There are two | |
121 | other advice classes: @dfn{after-advice}, which runs after the base | |
122 | definition, and @dfn{around-advice}, which lets you specify an | |
123 | expression to wrap around the invocation of the base definition. | |
124 | ||
125 | @node Defining Advice | |
126 | @section Defining Advice | |
127 | @cindex defining advice | |
128 | @cindex advice, defining | |
129 | ||
130 | To define a piece of advice, use the macro @code{defadvice}. A call | |
131 | to @code{defadvice} has the following syntax, which is based on the | |
132 | syntax of @code{defun} and @code{defmacro}, but adds more: | |
133 | ||
134 | @findex defadvice | |
135 | @example | |
136 | (defadvice @var{function} (@var{class} @var{name} | |
137 | @r{[}@var{position}@r{]} @r{[}@var{arglist}@r{]} | |
138 | @var{flags}...) | |
139 | @r{[}@var{documentation-string}@r{]} | |
140 | @r{[}@var{interactive-form}@r{]} | |
141 | @var{body-forms}...) | |
142 | @end example | |
143 | ||
144 | @noindent | |
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145 | Here, @var{function} is the name of the function (or macro) to be |
146 | advised. From now on, we will write just ``function'' when describing | |
147 | the entity being advised, but this always includes macros. | |
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148 | |
149 | In place of the argument list in an ordinary definition, an advice | |
150 | definition calls for several different pieces of information. | |
151 | ||
152 | @cindex class of advice | |
153 | @cindex before-advice | |
154 | @cindex after-advice | |
155 | @cindex around-advice | |
156 | @var{class} specifies the @dfn{class} of the advice---one of @code{before}, | |
157 | @code{after}, or @code{around}. Before-advice runs before the function | |
158 | itself; after-advice runs after the function itself; around-advice is | |
159 | wrapped around the execution of the function itself. After-advice and | |
160 | around-advice can override the return value by setting | |
161 | @code{ad-return-value}. | |
162 | ||
163 | @defvar ad-return-value | |
164 | While advice is executing, after the function's original definition has | |
165 | been executed, this variable holds its return value, which will | |
166 | ultimately be returned to the caller after finishing all the advice. | |
167 | After-advice and around-advice can arrange to return some other value | |
168 | by storing it in this variable. | |
169 | @end defvar | |
170 | ||
171 | The argument @var{name} is the name of the advice, a non-@code{nil} | |
172 | symbol. The advice name uniquely identifies one piece of advice, within all | |
173 | the pieces of advice in a particular class for a particular | |
174 | @var{function}. The name allows you to refer to the piece of | |
175 | advice---to redefine it, or to enable or disable it. | |
176 | ||
177 | The optional @var{position} specifies where, in the current list of | |
178 | advice of the specified @var{class}, this new advice should be placed. | |
179 | It should be either @code{first}, @code{last} or a number that specifies | |
180 | a zero-based position (@code{first} is equivalent to 0). If no position | |
181 | is specified, the default is @code{first}. Position values outside the | |
182 | range of existing positions in this class are mapped to the beginning or | |
183 | the end of the range, whichever is closer. The @var{position} value is | |
184 | ignored when redefining an existing piece of advice. | |
185 | ||
186 | The optional @var{arglist} can be used to define the argument list for | |
187 | the sake of advice. This becomes the argument list of the combined | |
188 | definition that is generated in order to run the advice (@pxref{Combined | |
189 | Definition}). Therefore, the advice expressions can use the argument | |
190 | variables in this list to access argument values. | |
191 | ||
192 | The argument list used in advice need not be the same as the argument | |
193 | list used in the original function, but must be compatible with it, so | |
194 | that it can handle the ways the function is actually called. If two | |
195 | pieces of advice for a function both specify an argument list, they must | |
196 | specify the same argument list. | |
197 | ||
198 | @xref{Argument Access in Advice}, for more information about argument | |
199 | lists and advice, and a more flexible way for advice to access the | |
200 | arguments. | |
201 | ||
202 | The remaining elements, @var{flags}, are symbols that specify further | |
203 | information about how to use this piece of advice. Here are the valid | |
204 | symbols and their meanings: | |
205 | ||
206 | @table @code | |
207 | @item activate | |
208 | Activate the advice for @var{function} now. Changes in a function's | |
209 | advice always take effect the next time you activate advice for the | |
210 | function; this flag says to do so, for @var{function}, immediately after | |
211 | defining this piece of advice. | |
212 | ||
213 | @cindex forward advice | |
214 | This flag has no immediate effect if @var{function} itself is not defined yet (a | |
215 | situation known as @dfn{forward advice}), because it is impossible to | |
216 | activate an undefined function's advice. However, defining | |
217 | @var{function} will automatically activate its advice. | |
218 | ||
219 | @item protect | |
220 | Protect this piece of advice against non-local exits and errors in | |
221 | preceding code and advice. Protecting advice places it as a cleanup in | |
222 | an @code{unwind-protect} form, so that it will execute even if the | |
223 | previous code gets an error or uses @code{throw}. @xref{Cleanups}. | |
224 | ||
225 | @item compile | |
226 | Compile the combined definition that is used to run the advice. This | |
227 | flag is ignored unless @code{activate} is also specified. | |
228 | @xref{Combined Definition}. | |
229 | ||
230 | @item disable | |
231 | Initially disable this piece of advice, so that it will not be used | |
232 | unless subsequently explicitly enabled. @xref{Enabling Advice}. | |
233 | ||
234 | @item preactivate | |
235 | Activate advice for @var{function} when this @code{defadvice} is | |
236 | compiled or macroexpanded. This generates a compiled advised definition | |
237 | according to the current advice state, which will be used during | |
238 | activation if appropriate. @xref{Preactivation}. | |
239 | ||
240 | This is useful only if this @code{defadvice} is byte-compiled. | |
241 | @end table | |
242 | ||
243 | The optional @var{documentation-string} serves to document this piece of | |
244 | advice. When advice is active for @var{function}, the documentation for | |
245 | @var{function} (as returned by @code{documentation}) combines the | |
246 | documentation strings of all the advice for @var{function} with the | |
247 | documentation string of its original function definition. | |
248 | ||
249 | The optional @var{interactive-form} form can be supplied to change the | |
250 | interactive behavior of the original function. If more than one piece | |
251 | of advice has an @var{interactive-form}, then the first one (the one | |
252 | with the smallest position) found among all the advice takes precedence. | |
253 | ||
254 | The possibly empty list of @var{body-forms} specifies the body of the | |
255 | advice. The body of an advice can access or change the arguments, the | |
256 | return value, the binding environment, and perform any other kind of | |
257 | side effect. | |
258 | ||
259 | @strong{Warning:} When you advise a macro, keep in mind that macros are | |
260 | expanded when a program is compiled, not when a compiled program is run. | |
261 | All subroutines used by the advice need to be available when the byte | |
262 | compiler expands the macro. | |
263 | ||
264 | @deffn Command ad-unadvise function | |
25dec365 | 265 | This command deletes all pieces of advice from @var{function}. |
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266 | @end deffn |
267 | ||
268 | @deffn Command ad-unadvise-all | |
269 | This command deletes all pieces of advice from all functions. | |
270 | @end deffn | |
271 | ||
272 | @node Around-Advice | |
273 | @section Around-Advice | |
274 | ||
275 | Around-advice lets you ``wrap'' a Lisp expression ``around'' the | |
276 | original function definition. You specify where the original function | |
277 | definition should go by means of the special symbol @code{ad-do-it}. | |
278 | Where this symbol occurs inside the around-advice body, it is replaced | |
279 | with a @code{progn} containing the forms of the surrounded code. Here | |
280 | is an example: | |
281 | ||
282 | @example | |
283 | (defadvice foo (around foo-around) | |
284 | "Ignore case in `foo'." | |
285 | (let ((case-fold-search t)) | |
286 | ad-do-it)) | |
287 | @end example | |
288 | ||
289 | @noindent | |
290 | Its effect is to make sure that case is ignored in | |
291 | searches when the original definition of @code{foo} is run. | |
292 | ||
293 | @defvar ad-do-it | |
294 | This is not really a variable, rather a place-holder that looks like a | |
295 | variable. You use it in around-advice to specify the place to run the | |
296 | function's original definition and other ``earlier'' around-advice. | |
297 | @end defvar | |
298 | ||
299 | If the around-advice does not use @code{ad-do-it}, then it does not run | |
300 | the original function definition. This provides a way to override the | |
301 | original definition completely. (It also overrides lower-positioned | |
302 | pieces of around-advice). | |
303 | ||
304 | If the around-advice uses @code{ad-do-it} more than once, the original | |
305 | definition is run at each place. In this way, around-advice can execute | |
306 | the original definition (and lower-positioned pieces of around-advice) | |
307 | several times. Another way to do that is by using @code{ad-do-it} | |
308 | inside of a loop. | |
309 | ||
310 | @node Computed Advice | |
311 | @section Computed Advice | |
312 | ||
313 | The macro @code{defadvice} resembles @code{defun} in that the code for | |
314 | the advice, and all other information about it, are explicitly stated in | |
315 | the source code. You can also create advice whose details are computed, | |
316 | using the function @code{ad-add-advice}. | |
317 | ||
318 | @defun ad-add-advice function advice class position | |
319 | Calling @code{ad-add-advice} adds @var{advice} as a piece of advice to | |
794f204b | 320 | @var{function} in class @var{class}. The argument @var{advice} has |
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321 | this form: |
322 | ||
323 | @example | |
324 | (@var{name} @var{protected} @var{enabled} @var{definition}) | |
325 | @end example | |
326 | ||
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327 | @noindent |
328 | Here, @var{protected} and @var{enabled} are flags; if @var{protected} | |
329 | is non-@code{nil}, the advice is protected against non-local exits | |
330 | (@pxref{Defining Advice}), and if @var{enabled} is @code{nil} the | |
331 | advice is initially disabled (@pxref{Enabling Advice}). | |
332 | @var{definition} should have the form | |
333 | ||
334 | @example | |
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336 | @end example |
337 | ||
338 | @noindent | |
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339 | where @var{lambda} is a lambda expression; this lambda expression is |
340 | called in order to perform the advice. @xref{Lambda Expressions}. | |
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341 | |
342 | If the @var{function} argument to @code{ad-add-advice} already has one | |
343 | or more pieces of advice in the specified @var{class}, then | |
344 | @var{position} specifies where in the list to put the new piece of | |
345 | advice. The value of @var{position} can either be @code{first}, | |
346 | @code{last}, or a number (counting from 0 at the beginning of the | |
347 | list). Numbers outside the range are mapped to the beginning or the | |
348 | end of the range, whichever is closer. The @var{position} value is | |
349 | ignored when redefining an existing piece of advice. | |
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350 | |
351 | If @var{function} already has a piece of @var{advice} with the same | |
352 | name, then the position argument is ignored and the old advice is | |
353 | replaced with the new one. | |
354 | @end defun | |
355 | ||
356 | @node Activation of Advice | |
357 | @section Activation of Advice | |
358 | @cindex activating advice | |
359 | @cindex advice, activating | |
360 | ||
361 | By default, advice does not take effect when you define it---only when | |
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362 | you @dfn{activate} advice for the function. However, the advice will |
363 | be activated automatically if you define or redefine the function | |
364 | later. You can request the activation of advice for a function when | |
365 | you define the advice, by specifying the @code{activate} flag in the | |
366 | @code{defadvice}; or you can activate the advice separately by calling | |
367 | the function @code{ad-activate} or one of the other activation | |
368 | commands listed below. | |
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369 | |
370 | Separating the activation of advice from the act of defining it permits | |
371 | you to add several pieces of advice to one function efficiently, without | |
372 | redefining the function over and over as each advice is added. More | |
373 | importantly, it permits defining advice for a function before that | |
374 | function is actually defined. | |
375 | ||
376 | When a function's advice is first activated, the function's original | |
377 | definition is saved, and all enabled pieces of advice for that function | |
378 | are combined with the original definition to make a new definition. | |
379 | (Pieces of advice that are currently disabled are not used; see | |
380 | @ref{Enabling Advice}.) This definition is installed, and optionally | |
381 | byte-compiled as well, depending on conditions described below. | |
382 | ||
383 | In all of the commands to activate advice, if @var{compile} is | |
384 | @code{t} (or anything but @code{nil} or a negative number), the | |
385 | command also compiles the combined definition which implements the | |
386 | advice. If it is @code{nil} or a negative number, what happens | |
387 | depends on @code{ad-default-compilation-action} as described below. | |
388 | ||
389 | @deffn Command ad-activate function &optional compile | |
390 | This command activates all the advice defined for @var{function}. | |
391 | @end deffn | |
392 | ||
393 | Activating advice does nothing if @var{function}'s advice is already | |
394 | active. But if there is new advice, added since the previous time you | |
395 | activated advice for @var{function}, it activates the new advice. | |
396 | ||
397 | @deffn Command ad-deactivate function | |
398 | This command deactivates the advice for @var{function}. | |
399 | @cindex deactivating advice | |
400 | @c @cindex advice, deactivating "advice, activating" is just above | |
401 | @end deffn | |
402 | ||
403 | @deffn Command ad-update function &optional compile | |
404 | This command activates the advice for @var{function} | |
405 | if its advice is already activated. This is useful | |
406 | if you change the advice. | |
407 | @end deffn | |
408 | ||
409 | @deffn Command ad-activate-all &optional compile | |
410 | This command activates the advice for all functions. | |
411 | @end deffn | |
412 | ||
413 | @deffn Command ad-deactivate-all | |
414 | This command deactivates the advice for all functions. | |
415 | @end deffn | |
416 | ||
417 | @deffn Command ad-update-all &optional compile | |
418 | This command activates the advice for all functions | |
419 | whose advice is already activated. This is useful | |
420 | if you change the advice of some functions. | |
421 | @end deffn | |
422 | ||
423 | @deffn Command ad-activate-regexp regexp &optional compile | |
424 | This command activates all pieces of advice whose names match | |
425 | @var{regexp}. More precisely, it activates all advice for any function | |
426 | which has at least one piece of advice that matches @var{regexp}. | |
427 | @end deffn | |
428 | ||
429 | @deffn Command ad-deactivate-regexp regexp | |
430 | This command deactivates all pieces of advice whose names match | |
431 | @var{regexp}. More precisely, it deactivates all advice for any | |
432 | function which has at least one piece of advice that matches | |
433 | @var{regexp}. | |
434 | @end deffn | |
435 | ||
436 | @deffn Command ad-update-regexp regexp &optional compile | |
437 | This command activates pieces of advice whose names match @var{regexp}, | |
438 | but only those for functions whose advice is already activated. | |
439 | @cindex reactivating advice | |
440 | ||
441 | Reactivating a function's advice is useful for putting into effect all | |
442 | the changes that have been made in its advice (including enabling and | |
443 | disabling specific pieces of advice; @pxref{Enabling Advice}) since the | |
444 | last time it was activated. | |
445 | @end deffn | |
446 | ||
447 | @deffn Command ad-start-advice | |
448 | Turn on automatic advice activation when a function is defined or | |
449 | redefined. This is the default mode. | |
450 | @end deffn | |
451 | ||
452 | @deffn Command ad-stop-advice | |
453 | Turn off automatic advice activation when a function is defined or | |
454 | redefined. | |
455 | @end deffn | |
456 | ||
457 | @defopt ad-default-compilation-action | |
458 | This variable controls whether to compile the combined definition | |
459 | that results from activating advice for a function. | |
460 | ||
461 | A value of @code{always} specifies to compile unconditionally. | |
462 | A value of @code{never} specifies never compile the advice. | |
463 | ||
b1baed0b | 464 | A value of @code{maybe} specifies to compile if the byte compiler is |
b8d4c8d0 GM |
465 | already loaded. A value of @code{like-original} specifies to compile |
466 | the advice if the original definition of the advised function is | |
467 | compiled or a built-in function. | |
468 | ||
469 | This variable takes effect only if the @var{compile} argument of | |
470 | @code{ad-activate} (or any of the above functions) did not force | |
471 | compilation. | |
472 | @end defopt | |
473 | ||
474 | If the advised definition was constructed during ``preactivation'' | |
475 | (@pxref{Preactivation}), then that definition must already be compiled, | |
476 | because it was constructed during byte-compilation of the file that | |
477 | contained the @code{defadvice} with the @code{preactivate} flag. | |
478 | ||
479 | @node Enabling Advice | |
480 | @section Enabling and Disabling Advice | |
481 | @cindex enabling advice | |
482 | @cindex advice, enabling and disabling | |
483 | @cindex disabling advice | |
484 | ||
485 | Each piece of advice has a flag that says whether it is enabled or | |
486 | not. By enabling or disabling a piece of advice, you can turn it on | |
487 | and off without having to undefine and redefine it. For example, here is | |
488 | how to disable a particular piece of advice named @code{my-advice} for | |
489 | the function @code{foo}: | |
490 | ||
491 | @example | |
492 | (ad-disable-advice 'foo 'before 'my-advice) | |
493 | @end example | |
494 | ||
495 | This function by itself only changes the enable flag for a piece of | |
496 | advice. To make the change take effect in the advised definition, you | |
497 | must activate the advice for @code{foo} again: | |
498 | ||
499 | @example | |
500 | (ad-activate 'foo) | |
501 | @end example | |
502 | ||
503 | @deffn Command ad-disable-advice function class name | |
504 | This command disables the piece of advice named @var{name} in class | |
505 | @var{class} on @var{function}. | |
506 | @end deffn | |
507 | ||
508 | @deffn Command ad-enable-advice function class name | |
509 | This command enables the piece of advice named @var{name} in class | |
510 | @var{class} on @var{function}. | |
511 | @end deffn | |
512 | ||
513 | You can also disable many pieces of advice at once, for various | |
514 | functions, using a regular expression. As always, the changes take real | |
515 | effect only when you next reactivate advice for the functions in | |
516 | question. | |
517 | ||
518 | @deffn Command ad-disable-regexp regexp | |
519 | This command disables all pieces of advice whose names match | |
520 | @var{regexp}, in all classes, on all functions. | |
521 | @end deffn | |
522 | ||
523 | @deffn Command ad-enable-regexp regexp | |
524 | This command enables all pieces of advice whose names match | |
525 | @var{regexp}, in all classes, on all functions. | |
526 | @end deffn | |
527 | ||
528 | @node Preactivation | |
529 | @section Preactivation | |
530 | @cindex preactivating advice | |
531 | @cindex advice, preactivating | |
532 | ||
533 | Constructing a combined definition to execute advice is moderately | |
534 | expensive. When a library advises many functions, this can make loading | |
535 | the library slow. In that case, you can use @dfn{preactivation} to | |
536 | construct suitable combined definitions in advance. | |
537 | ||
538 | To use preactivation, specify the @code{preactivate} flag when you | |
539 | define the advice with @code{defadvice}. This @code{defadvice} call | |
540 | creates a combined definition which embodies this piece of advice | |
541 | (whether enabled or not) plus any other currently enabled advice for the | |
542 | same function, and the function's own definition. If the | |
543 | @code{defadvice} is compiled, that compiles the combined definition | |
544 | also. | |
545 | ||
546 | When the function's advice is subsequently activated, if the enabled | |
547 | advice for the function matches what was used to make this combined | |
548 | definition, then the existing combined definition is used, thus avoiding | |
549 | the need to construct one. Thus, preactivation never causes wrong | |
550 | results---but it may fail to do any good, if the enabled advice at the | |
551 | time of activation doesn't match what was used for preactivation. | |
552 | ||
553 | Here are some symptoms that can indicate that a preactivation did not | |
554 | work properly, because of a mismatch. | |
555 | ||
556 | @itemize @bullet | |
557 | @item | |
558 | Activation of the advised | |
559 | function takes longer than usual. | |
560 | @item | |
b1baed0b | 561 | The byte compiler gets |
b8d4c8d0 GM |
562 | loaded while an advised function gets activated. |
563 | @item | |
564 | @code{byte-compile} is included in the value of @code{features} even | |
b1baed0b | 565 | though you did not ever explicitly use the byte compiler. |
b8d4c8d0 GM |
566 | @end itemize |
567 | ||
568 | Compiled preactivated advice works properly even if the function itself | |
569 | is not defined until later; however, the function needs to be defined | |
570 | when you @emph{compile} the preactivated advice. | |
571 | ||
572 | There is no elegant way to find out why preactivated advice is not being | |
573 | used. What you can do is to trace the function | |
574 | @code{ad-cache-id-verification-code} (with the function | |
575 | @code{trace-function-background}) before the advised function's advice | |
576 | is activated. After activation, check the value returned by | |
577 | @code{ad-cache-id-verification-code} for that function: @code{verified} | |
578 | means that the preactivated advice was used, while other values give | |
579 | some information about why they were considered inappropriate. | |
580 | ||
581 | @strong{Warning:} There is one known case that can make preactivation | |
582 | fail, in that a preconstructed combined definition is used even though | |
583 | it fails to match the current state of advice. This can happen when two | |
584 | packages define different pieces of advice with the same name, in the | |
585 | same class, for the same function. But you should avoid that anyway. | |
586 | ||
587 | @node Argument Access in Advice | |
588 | @section Argument Access in Advice | |
589 | ||
590 | The simplest way to access the arguments of an advised function in the | |
591 | body of a piece of advice is to use the same names that the function | |
592 | definition uses. To do this, you need to know the names of the argument | |
593 | variables of the original function. | |
594 | ||
595 | While this simple method is sufficient in many cases, it has a | |
596 | disadvantage: it is not robust, because it hard-codes the argument names | |
597 | into the advice. If the definition of the original function changes, | |
598 | the advice might break. | |
599 | ||
600 | Another method is to specify an argument list in the advice itself. | |
601 | This avoids the need to know the original function definition's argument | |
602 | names, but it has a limitation: all the advice on any particular | |
603 | function must use the same argument list, because the argument list | |
604 | actually used for all the advice comes from the first piece of advice | |
605 | for that function. | |
606 | ||
607 | A more robust method is to use macros that are translated into the | |
608 | proper access forms at activation time, i.e., when constructing the | |
4b1ed1bb CY |
609 | advised definition. Access macros access actual arguments by their |
610 | (zero-based) position, regardless of how these actual arguments get | |
611 | distributed onto the argument variables of a function. This is robust | |
612 | because in Emacs Lisp the meaning of an argument is strictly | |
613 | determined by its position in the argument list. | |
b8d4c8d0 GM |
614 | |
615 | @defmac ad-get-arg position | |
616 | This returns the actual argument that was supplied at @var{position}. | |
617 | @end defmac | |
618 | ||
619 | @defmac ad-get-args position | |
620 | This returns the list of actual arguments supplied starting at | |
621 | @var{position}. | |
622 | @end defmac | |
623 | ||
624 | @defmac ad-set-arg position value | |
625 | This sets the value of the actual argument at @var{position} to | |
626 | @var{value} | |
627 | @end defmac | |
628 | ||
629 | @defmac ad-set-args position value-list | |
630 | This sets the list of actual arguments starting at @var{position} to | |
631 | @var{value-list}. | |
632 | @end defmac | |
633 | ||
634 | Now an example. Suppose the function @code{foo} is defined as | |
635 | ||
636 | @example | |
637 | (defun foo (x y &optional z &rest r) ...) | |
638 | @end example | |
639 | ||
640 | @noindent | |
641 | and is then called with | |
642 | ||
643 | @example | |
644 | (foo 0 1 2 3 4 5 6) | |
645 | @end example | |
646 | ||
647 | @noindent | |
648 | which means that @var{x} is 0, @var{y} is 1, @var{z} is 2 and @var{r} is | |
649 | @code{(3 4 5 6)} within the body of @code{foo}. Here is what | |
650 | @code{ad-get-arg} and @code{ad-get-args} return in this case: | |
651 | ||
652 | @example | |
653 | (ad-get-arg 0) @result{} 0 | |
654 | (ad-get-arg 1) @result{} 1 | |
655 | (ad-get-arg 2) @result{} 2 | |
656 | (ad-get-arg 3) @result{} 3 | |
657 | (ad-get-args 2) @result{} (2 3 4 5 6) | |
658 | (ad-get-args 4) @result{} (4 5 6) | |
659 | @end example | |
660 | ||
661 | Setting arguments also makes sense in this example: | |
662 | ||
663 | @example | |
664 | (ad-set-arg 5 "five") | |
665 | @end example | |
666 | ||
667 | @noindent | |
668 | has the effect of changing the sixth argument to @code{"five"}. If this | |
669 | happens in advice executed before the body of @code{foo} is run, then | |
670 | @var{r} will be @code{(3 4 "five" 6)} within that body. | |
671 | ||
672 | Here is an example of setting a tail of the argument list: | |
673 | ||
674 | @example | |
675 | (ad-set-args 0 '(5 4 3 2 1 0)) | |
676 | @end example | |
677 | ||
678 | @noindent | |
679 | If this happens in advice executed before the body of @code{foo} is run, | |
680 | then within that body, @var{x} will be 5, @var{y} will be 4, @var{z} | |
681 | will be 3, and @var{r} will be @code{(2 1 0)} inside the body of | |
682 | @code{foo}. | |
683 | ||
684 | These argument constructs are not really implemented as Lisp macros. | |
685 | Instead they are implemented specially by the advice mechanism. | |
686 | ||
b8d4c8d0 GM |
687 | @node Combined Definition |
688 | @section The Combined Definition | |
689 | ||
690 | Suppose that a function has @var{n} pieces of before-advice | |
691 | (numbered from 0 through @var{n}@minus{}1), @var{m} pieces of | |
692 | around-advice and @var{k} pieces of after-advice. Assuming no piece | |
693 | of advice is protected, the combined definition produced to implement | |
694 | the advice for a function looks like this: | |
695 | ||
696 | @example | |
697 | (lambda @var{arglist} | |
698 | @r{[} @r{[}@var{advised-docstring}@r{]} @r{[}(interactive ...)@r{]} @r{]} | |
699 | (let (ad-return-value) | |
700 | @r{before-0-body-form}... | |
701 | .... | |
702 | @r{before-@var{n}@minus{}1-body-form}... | |
703 | @r{around-0-body-form}... | |
704 | @r{around-1-body-form}... | |
705 | .... | |
706 | @r{around-@var{m}@minus{}1-body-form}... | |
707 | (setq ad-return-value | |
708 | @r{apply original definition to @var{arglist}}) | |
709 | @r{end-of-around-@var{m}@minus{}1-body-form}... | |
710 | .... | |
711 | @r{end-of-around-1-body-form}... | |
712 | @r{end-of-around-0-body-form}... | |
713 | @r{after-0-body-form}... | |
714 | .... | |
715 | @r{after-@var{k}@minus{}1-body-form}... | |
716 | ad-return-value)) | |
717 | @end example | |
718 | ||
719 | Macros are redefined as macros, which means adding @code{macro} to | |
720 | the beginning of the combined definition. | |
721 | ||
722 | The interactive form is present if the original function or some piece | |
723 | of advice specifies one. When an interactive primitive function is | |
724 | advised, advice uses a special method: it calls the primitive with | |
725 | @code{call-interactively} so that it will read its own arguments. | |
726 | In this case, the advice cannot access the arguments. | |
727 | ||
728 | The body forms of the various advice in each class are assembled | |
729 | according to their specified order. The forms of around-advice @var{l} | |
730 | are included in one of the forms of around-advice @var{l} @minus{} 1. | |
731 | ||
732 | The innermost part of the around advice onion is | |
733 | ||
734 | @display | |
735 | apply original definition to @var{arglist} | |
736 | @end display | |
737 | ||
738 | @noindent | |
739 | whose form depends on the type of the original function. The variable | |
740 | @code{ad-return-value} is set to whatever this returns. The variable is | |
741 | visible to all pieces of advice, which can access and modify it before | |
742 | it is actually returned from the advised function. | |
743 | ||
744 | The semantic structure of advised functions that contain protected | |
745 | pieces of advice is the same. The only difference is that | |
746 | @code{unwind-protect} forms ensure that the protected advice gets | |
747 | executed even if some previous piece of advice had an error or a | |
748 | non-local exit. If any around-advice is protected, then the whole | |
749 | around-advice onion is protected as a result. |