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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. | |
4 | @c See the file elisp.texi for copying conditions. | |
5 | @setfilename ../info/control | |
6 | @node Control Structures, Variables, Evaluation, Top | |
7 | @chapter Control Structures | |
8 | @cindex special forms for control structures | |
9 | @cindex control structures | |
10 | ||
11 | A Lisp program consists of expressions or @dfn{forms} (@pxref{Forms}). | |
12 | We control the order of execution of the forms by enclosing them in | |
13 | @dfn{control structures}. Control structures are special forms which | |
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14 | control when, whether, or how many times to execute the forms they |
15 | contain. | |
83ac6b45 | 16 | |
3e099569 | 17 | The simplest order of execution is sequential execution: first form |
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18 | @var{a}, then form @var{b}, and so on. This is what happens when you |
19 | write several forms in succession in the body of a function, or at top | |
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20 | level in a file of Lisp code---the forms are executed in the order |
21 | written. We call this @dfn{textual order}. For example, if a function | |
22 | body consists of two forms @var{a} and @var{b}, evaluation of the | |
23 | function evaluates first @var{a} and then @var{b}, and the function's | |
24 | value is the value of @var{b}. | |
25 | ||
26 | Explicit control structures make possible an order of execution other | |
27 | than sequential. | |
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28 | |
29 | Emacs Lisp provides several kinds of control structure, including | |
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30 | other varieties of sequencing, conditionals, iteration, and (controlled) |
31 | jumps---all discussed below. The built-in control structures are | |
32 | special forms since their subforms are not necessarily evaluated or not | |
33 | evaluated sequentially. You can use macros to define your own control | |
34 | structure constructs (@pxref{Macros}). | |
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35 | |
36 | @menu | |
37 | * Sequencing:: Evaluation in textual order. | |
38 | * Conditionals:: @code{if}, @code{cond}. | |
39 | * Combining Conditions:: @code{and}, @code{or}, @code{not}. | |
40 | * Iteration:: @code{while} loops. | |
41 | * Nonlocal Exits:: Jumping out of a sequence. | |
42 | @end menu | |
43 | ||
44 | @node Sequencing | |
45 | @section Sequencing | |
46 | ||
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47 | Evaluating forms in the order they appear is the most common way |
48 | control passes from one form to another. In some contexts, such as in a | |
49 | function body, this happens automatically. Elsewhere you must use a | |
50 | control structure construct to do this: @code{progn}, the simplest | |
51 | control construct of Lisp. | |
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52 | |
53 | A @code{progn} special form looks like this: | |
54 | ||
55 | @example | |
56 | @group | |
57 | (progn @var{a} @var{b} @var{c} @dots{}) | |
58 | @end group | |
59 | @end example | |
60 | ||
61 | @noindent | |
62 | and it says to execute the forms @var{a}, @var{b}, @var{c} and so on, in | |
63 | that order. These forms are called the body of the @code{progn} form. | |
64 | The value of the last form in the body becomes the value of the entire | |
65 | @code{progn}. | |
66 | ||
67 | @cindex implicit @code{progn} | |
68 | In the early days of Lisp, @code{progn} was the only way to execute | |
69 | two or more forms in succession and use the value of the last of them. | |
70 | But programmers found they often needed to use a @code{progn} in the | |
71 | body of a function, where (at that time) only one form was allowed. So | |
72 | the body of a function was made into an ``implicit @code{progn}'': | |
73 | several forms are allowed just as in the body of an actual @code{progn}. | |
74 | Many other control structures likewise contain an implicit @code{progn}. | |
75 | As a result, @code{progn} is not used as often as it used to be. It is | |
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76 | needed now most often inside an @code{unwind-protect}, @code{and}, |
77 | @code{or}, or in the @var{then}-part of an @code{if}. | |
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78 | |
79 | @defspec progn forms@dots{} | |
80 | This special form evaluates all of the @var{forms}, in textual | |
81 | order, returning the result of the final form. | |
82 | ||
83 | @example | |
84 | @group | |
85 | (progn (print "The first form") | |
86 | (print "The second form") | |
87 | (print "The third form")) | |
88 | @print{} "The first form" | |
89 | @print{} "The second form" | |
90 | @print{} "The third form" | |
91 | @result{} "The third form" | |
92 | @end group | |
93 | @end example | |
94 | @end defspec | |
95 | ||
96 | Two other control constructs likewise evaluate a series of forms but return | |
97 | a different value: | |
98 | ||
99 | @defspec prog1 form1 forms@dots{} | |
100 | This special form evaluates @var{form1} and all of the @var{forms}, in | |
101 | textual order, returning the result of @var{form1}. | |
102 | ||
103 | @example | |
104 | @group | |
105 | (prog1 (print "The first form") | |
106 | (print "The second form") | |
107 | (print "The third form")) | |
108 | @print{} "The first form" | |
109 | @print{} "The second form" | |
110 | @print{} "The third form" | |
111 | @result{} "The first form" | |
112 | @end group | |
113 | @end example | |
114 | ||
115 | Here is a way to remove the first element from a list in the variable | |
116 | @code{x}, then return the value of that former element: | |
117 | ||
118 | @example | |
119 | (prog1 (car x) (setq x (cdr x))) | |
120 | @end example | |
121 | @end defspec | |
122 | ||
123 | @defspec prog2 form1 form2 forms@dots{} | |
124 | This special form evaluates @var{form1}, @var{form2}, and all of the | |
125 | following @var{forms}, in textual order, returning the result of | |
126 | @var{form2}. | |
127 | ||
128 | @example | |
129 | @group | |
130 | (prog2 (print "The first form") | |
131 | (print "The second form") | |
132 | (print "The third form")) | |
133 | @print{} "The first form" | |
134 | @print{} "The second form" | |
135 | @print{} "The third form" | |
136 | @result{} "The second form" | |
137 | @end group | |
138 | @end example | |
139 | @end defspec | |
140 | ||
141 | @node Conditionals | |
142 | @section Conditionals | |
143 | @cindex conditional evaluation | |
144 | ||
145 | Conditional control structures choose among alternatives. Emacs Lisp | |
146 | has two conditional forms: @code{if}, which is much the same as in other | |
147 | languages, and @code{cond}, which is a generalized case statement. | |
148 | ||
149 | @defspec if condition then-form else-forms@dots{} | |
150 | @code{if} chooses between the @var{then-form} and the @var{else-forms} | |
151 | based on the value of @var{condition}. If the evaluated @var{condition} is | |
152 | non-@code{nil}, @var{then-form} is evaluated and the result returned. | |
153 | Otherwise, the @var{else-forms} are evaluated in textual order, and the | |
154 | value of the last one is returned. (The @var{else} part of @code{if} is | |
155 | an example of an implicit @code{progn}. @xref{Sequencing}.) | |
156 | ||
157 | If @var{condition} has the value @code{nil}, and no @var{else-forms} are | |
158 | given, @code{if} returns @code{nil}. | |
159 | ||
3e099569 | 160 | @code{if} is a special form because the branch that is not selected is |
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161 | never evaluated---it is ignored. Thus, in the example below, |
162 | @code{true} is not printed because @code{print} is never called. | |
163 | ||
164 | @example | |
165 | @group | |
166 | (if nil | |
167 | (print 'true) | |
168 | 'very-false) | |
169 | @result{} very-false | |
170 | @end group | |
171 | @end example | |
172 | @end defspec | |
173 | ||
174 | @defspec cond clause@dots{} | |
175 | @code{cond} chooses among an arbitrary number of alternatives. Each | |
176 | @var{clause} in the @code{cond} must be a list. The @sc{car} of this | |
177 | list is the @var{condition}; the remaining elements, if any, the | |
178 | @var{body-forms}. Thus, a clause looks like this: | |
179 | ||
180 | @example | |
181 | (@var{condition} @var{body-forms}@dots{}) | |
182 | @end example | |
183 | ||
184 | @code{cond} tries the clauses in textual order, by evaluating the | |
185 | @var{condition} of each clause. If the value of @var{condition} is | |
186 | non-@code{nil}, the clause ``succeeds''; then @code{cond} evaluates its | |
187 | @var{body-forms}, and the value of the last of @var{body-forms} becomes | |
188 | the value of the @code{cond}. The remaining clauses are ignored. | |
189 | ||
190 | If the value of @var{condition} is @code{nil}, the clause ``fails'', so | |
191 | the @code{cond} moves on to the following clause, trying its | |
192 | @var{condition}. | |
193 | ||
194 | If every @var{condition} evaluates to @code{nil}, so that every clause | |
195 | fails, @code{cond} returns @code{nil}. | |
196 | ||
197 | A clause may also look like this: | |
198 | ||
199 | @example | |
200 | (@var{condition}) | |
201 | @end example | |
202 | ||
203 | @noindent | |
204 | Then, if @var{condition} is non-@code{nil} when tested, the value of | |
205 | @var{condition} becomes the value of the @code{cond} form. | |
206 | ||
207 | The following example has four clauses, which test for the cases where | |
208 | the value of @code{x} is a number, string, buffer and symbol, | |
209 | respectively: | |
210 | ||
211 | @example | |
212 | @group | |
213 | (cond ((numberp x) x) | |
214 | ((stringp x) x) | |
215 | ((bufferp x) | |
216 | (setq temporary-hack x) ; @r{multiple body-forms} | |
217 | (buffer-name x)) ; @r{in one clause} | |
218 | ((symbolp x) (symbol-value x))) | |
219 | @end group | |
220 | @end example | |
221 | ||
222 | Often we want to execute the last clause whenever none of the previous | |
223 | clauses was successful. To do this, we use @code{t} as the | |
224 | @var{condition} of the last clause, like this: @code{(t | |
225 | @var{body-forms})}. The form @code{t} evaluates to @code{t}, which is | |
226 | never @code{nil}, so this clause never fails, provided the @code{cond} | |
227 | gets to it at all. | |
228 | ||
229 | For example, | |
230 | ||
231 | @example | |
232 | @group | |
3e099569 | 233 | (cond ((eq a 'hack) 'foo) |
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234 | (t "default")) |
235 | @result{} "default" | |
236 | @end group | |
237 | @end example | |
238 | ||
239 | @noindent | |
240 | This expression is a @code{cond} which returns @code{foo} if the value | |
241 | of @code{a} is 1, and returns the string @code{"default"} otherwise. | |
242 | @end defspec | |
243 | ||
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244 | Any conditional construct can be expressed with @code{cond} or with |
245 | @code{if}. Therefore, the choice between them is a matter of style. | |
246 | For example: | |
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247 | |
248 | @example | |
249 | @group | |
250 | (if @var{a} @var{b} @var{c}) | |
251 | @equiv{} | |
252 | (cond (@var{a} @var{b}) (t @var{c})) | |
253 | @end group | |
254 | @end example | |
255 | ||
256 | @node Combining Conditions | |
257 | @section Constructs for Combining Conditions | |
258 | ||
259 | This section describes three constructs that are often used together | |
260 | with @code{if} and @code{cond} to express complicated conditions. The | |
261 | constructs @code{and} and @code{or} can also be used individually as | |
262 | kinds of multiple conditional constructs. | |
263 | ||
264 | @defun not condition | |
265 | This function tests for the falsehood of @var{condition}. It returns | |
266 | @code{t} if @var{condition} is @code{nil}, and @code{nil} otherwise. | |
267 | The function @code{not} is identical to @code{null}, and we recommend | |
268 | using the name @code{null} if you are testing for an empty list. | |
269 | @end defun | |
270 | ||
271 | @defspec and conditions@dots{} | |
272 | The @code{and} special form tests whether all the @var{conditions} are | |
273 | true. It works by evaluating the @var{conditions} one by one in the | |
274 | order written. | |
275 | ||
276 | If any of the @var{conditions} evaluates to @code{nil}, then the result | |
277 | of the @code{and} must be @code{nil} regardless of the remaining | |
278 | @var{conditions}; so @code{and} returns right away, ignoring the | |
279 | remaining @var{conditions}. | |
280 | ||
281 | If all the @var{conditions} turn out non-@code{nil}, then the value of | |
282 | the last of them becomes the value of the @code{and} form. | |
283 | ||
284 | Here is an example. The first condition returns the integer 1, which is | |
285 | not @code{nil}. Similarly, the second condition returns the integer 2, | |
286 | which is not @code{nil}. The third condition is @code{nil}, so the | |
287 | remaining condition is never evaluated. | |
288 | ||
289 | @example | |
290 | @group | |
291 | (and (print 1) (print 2) nil (print 3)) | |
292 | @print{} 1 | |
293 | @print{} 2 | |
294 | @result{} nil | |
295 | @end group | |
296 | @end example | |
297 | ||
298 | Here is a more realistic example of using @code{and}: | |
299 | ||
300 | @example | |
301 | @group | |
302 | (if (and (consp foo) (eq (car foo) 'x)) | |
303 | (message "foo is a list starting with x")) | |
304 | @end group | |
305 | @end example | |
306 | ||
307 | @noindent | |
308 | Note that @code{(car foo)} is not executed if @code{(consp foo)} returns | |
309 | @code{nil}, thus avoiding an error. | |
310 | ||
311 | @code{and} can be expressed in terms of either @code{if} or @code{cond}. | |
312 | For example: | |
313 | ||
314 | @example | |
315 | @group | |
316 | (and @var{arg1} @var{arg2} @var{arg3}) | |
317 | @equiv{} | |
318 | (if @var{arg1} (if @var{arg2} @var{arg3})) | |
319 | @equiv{} | |
320 | (cond (@var{arg1} (cond (@var{arg2} @var{arg3})))) | |
321 | @end group | |
322 | @end example | |
323 | @end defspec | |
324 | ||
325 | @defspec or conditions@dots{} | |
326 | The @code{or} special form tests whether at least one of the | |
327 | @var{conditions} is true. It works by evaluating all the | |
328 | @var{conditions} one by one in the order written. | |
329 | ||
330 | If any of the @var{conditions} evaluates to a non-@code{nil} value, then | |
331 | the result of the @code{or} must be non-@code{nil}; so @code{or} returns | |
332 | right away, ignoring the remaining @var{conditions}. The value it | |
333 | returns is the non-@code{nil} value of the condition just evaluated. | |
334 | ||
335 | If all the @var{conditions} turn out @code{nil}, then the @code{or} | |
336 | expression returns @code{nil}. | |
337 | ||
338 | For example, this expression tests whether @code{x} is either 0 or | |
339 | @code{nil}: | |
340 | ||
341 | @example | |
342 | (or (eq x nil) (eq x 0)) | |
343 | @end example | |
344 | ||
345 | Like the @code{and} construct, @code{or} can be written in terms of | |
346 | @code{cond}. For example: | |
347 | ||
348 | @example | |
349 | @group | |
350 | (or @var{arg1} @var{arg2} @var{arg3}) | |
351 | @equiv{} | |
352 | (cond (@var{arg1}) | |
353 | (@var{arg2}) | |
354 | (@var{arg3})) | |
355 | @end group | |
356 | @end example | |
357 | ||
358 | You could almost write @code{or} in terms of @code{if}, but not quite: | |
359 | ||
360 | @example | |
361 | @group | |
362 | (if @var{arg1} @var{arg1} | |
363 | (if @var{arg2} @var{arg2} | |
364 | @var{arg3})) | |
365 | @end group | |
366 | @end example | |
367 | ||
368 | @noindent | |
369 | This is not completely equivalent because it can evaluate @var{arg1} or | |
370 | @var{arg2} twice. By contrast, @code{(or @var{arg1} @var{arg2} | |
371 | @var{arg3})} never evaluates any argument more than once. | |
372 | @end defspec | |
373 | ||
374 | @node Iteration | |
375 | @section Iteration | |
376 | @cindex iteration | |
377 | @cindex recursion | |
378 | ||
379 | Iteration means executing part of a program repetitively. For | |
380 | example, you might want to repeat some computation once for each element | |
381 | of a list, or once for each integer from 0 to @var{n}. You can do this | |
382 | in Emacs Lisp with the special form @code{while}: | |
383 | ||
384 | @defspec while condition forms@dots{} | |
385 | @code{while} first evaluates @var{condition}. If the result is | |
386 | non-@code{nil}, it evaluates @var{forms} in textual order. Then it | |
387 | reevaluates @var{condition}, and if the result is non-@code{nil}, it | |
388 | evaluates @var{forms} again. This process repeats until @var{condition} | |
389 | evaluates to @code{nil}. | |
390 | ||
391 | There is no limit on the number of iterations that may occur. The loop | |
392 | will continue until either @var{condition} evaluates to @code{nil} or | |
393 | until an error or @code{throw} jumps out of it (@pxref{Nonlocal Exits}). | |
394 | ||
395 | The value of a @code{while} form is always @code{nil}. | |
396 | ||
397 | @example | |
398 | @group | |
399 | (setq num 0) | |
400 | @result{} 0 | |
401 | @end group | |
402 | @group | |
403 | (while (< num 4) | |
404 | (princ (format "Iteration %d." num)) | |
405 | (setq num (1+ num))) | |
406 | @print{} Iteration 0. | |
407 | @print{} Iteration 1. | |
408 | @print{} Iteration 2. | |
409 | @print{} Iteration 3. | |
410 | @result{} nil | |
411 | @end group | |
412 | @end example | |
413 | ||
414 | If you would like to execute something on each iteration before the | |
415 | end-test, put it together with the end-test in a @code{progn} as the | |
416 | first argument of @code{while}, as shown here: | |
417 | ||
418 | @example | |
419 | @group | |
420 | (while (progn | |
421 | (forward-line 1) | |
422 | (not (looking-at "^$")))) | |
423 | @end group | |
424 | @end example | |
425 | ||
426 | @noindent | |
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427 | This moves forward one line and continues moving by lines until it |
428 | reaches an empty. It is unusual in that the @code{while} has no body, | |
429 | just the end test (which also does the real work of moving point). | |
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430 | @end defspec |
431 | ||
432 | @node Nonlocal Exits | |
433 | @section Nonlocal Exits | |
434 | @cindex nonlocal exits | |
435 | ||
436 | A @dfn{nonlocal exit} is a transfer of control from one point in a | |
437 | program to another remote point. Nonlocal exits can occur in Emacs Lisp | |
438 | as a result of errors; you can also use them under explicit control. | |
439 | Nonlocal exits unbind all variable bindings made by the constructs being | |
440 | exited. | |
441 | ||
442 | @menu | |
443 | * Catch and Throw:: Nonlocal exits for the program's own purposes. | |
444 | * Examples of Catch:: Showing how such nonlocal exits can be written. | |
445 | * Errors:: How errors are signaled and handled. | |
446 | * Cleanups:: Arranging to run a cleanup form if an error happens. | |
447 | @end menu | |
448 | ||
449 | @node Catch and Throw | |
450 | @subsection Explicit Nonlocal Exits: @code{catch} and @code{throw} | |
451 | ||
452 | Most control constructs affect only the flow of control within the | |
453 | construct itself. The function @code{throw} is the exception to this | |
454 | rule of normal program execution: it performs a nonlocal exit on | |
455 | request. (There are other exceptions, but they are for error handling | |
456 | only.) @code{throw} is used inside a @code{catch}, and jumps back to | |
457 | that @code{catch}. For example: | |
458 | ||
459 | @example | |
460 | @group | |
461 | (catch 'foo | |
462 | (progn | |
463 | @dots{} | |
3e099569 | 464 | (throw 'foo t) |
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465 | @dots{})) |
466 | @end group | |
467 | @end example | |
468 | ||
469 | @noindent | |
470 | The @code{throw} transfers control straight back to the corresponding | |
471 | @code{catch}, which returns immediately. The code following the | |
472 | @code{throw} is not executed. The second argument of @code{throw} is used | |
473 | as the return value of the @code{catch}. | |
474 | ||
475 | The @code{throw} and the @code{catch} are matched through the first | |
476 | argument: @code{throw} searches for a @code{catch} whose first argument | |
477 | is @code{eq} to the one specified. Thus, in the above example, the | |
478 | @code{throw} specifies @code{foo}, and the @code{catch} specifies the | |
479 | same symbol, so that @code{catch} is applicable. If there is more than | |
480 | one applicable @code{catch}, the innermost one takes precedence. | |
481 | ||
482 | Executing @code{throw} exits all Lisp constructs up to the matching | |
483 | @code{catch}, including function calls. When binding constructs such as | |
484 | @code{let} or function calls are exited in this way, the bindings are | |
485 | unbound, just as they are when these constructs exit normally | |
486 | (@pxref{Local Variables}). Likewise, @code{throw} restores the buffer | |
487 | and position saved by @code{save-excursion} (@pxref{Excursions}), and | |
488 | the narrowing status saved by @code{save-restriction} and the window | |
489 | selection saved by @code{save-window-excursion} (@pxref{Window | |
490 | Configurations}). It also runs any cleanups established with the | |
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491 | @code{unwind-protect} special form when it exits that form |
492 | (@pxref{Cleanups}). | |
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493 | |
494 | The @code{throw} need not appear lexically within the @code{catch} | |
495 | that it jumps to. It can equally well be called from another function | |
496 | called within the @code{catch}. As long as the @code{throw} takes place | |
497 | chronologically after entry to the @code{catch}, and chronologically | |
498 | before exit from it, it has access to that @code{catch}. This is why | |
499 | @code{throw} can be used in commands such as @code{exit-recursive-edit} | |
3e099569 | 500 | that throw back to the editor command loop (@pxref{Recursive Editing}). |
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501 | |
502 | @cindex CL note---only @code{throw} in Emacs | |
503 | @quotation | |
3e099569 | 504 | @b{Common Lisp note:} Most other versions of Lisp, including Common Lisp, |
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505 | have several ways of transferring control nonsequentially: @code{return}, |
506 | @code{return-from}, and @code{go}, for example. Emacs Lisp has only | |
507 | @code{throw}. | |
508 | @end quotation | |
509 | ||
510 | @defspec catch tag body@dots{} | |
511 | @cindex tag on run time stack | |
512 | @code{catch} establishes a return point for the @code{throw} function. The | |
513 | return point is distinguished from other such return points by @var{tag}, | |
514 | which may be any Lisp object. The argument @var{tag} is evaluated normally | |
515 | before the return point is established. | |
516 | ||
517 | With the return point in effect, @code{catch} evaluates the forms of the | |
518 | @var{body} in textual order. If the forms execute normally, without | |
519 | error or nonlocal exit, the value of the last body form is returned from | |
520 | the @code{catch}. | |
521 | ||
522 | If a @code{throw} is done within @var{body} specifying the same value | |
523 | @var{tag}, the @code{catch} exits immediately; the value it returns is | |
524 | whatever was specified as the second argument of @code{throw}. | |
525 | @end defspec | |
526 | ||
527 | @defun throw tag value | |
528 | The purpose of @code{throw} is to return from a return point previously | |
529 | established with @code{catch}. The argument @var{tag} is used to choose | |
530 | among the various existing return points; it must be @code{eq} to the value | |
531 | specified in the @code{catch}. If multiple return points match @var{tag}, | |
532 | the innermost one is used. | |
533 | ||
534 | The argument @var{value} is used as the value to return from that | |
535 | @code{catch}. | |
536 | ||
537 | @kindex no-catch | |
538 | If no return point is in effect with tag @var{tag}, then a @code{no-catch} | |
539 | error is signaled with data @code{(@var{tag} @var{value})}. | |
540 | @end defun | |
541 | ||
542 | @node Examples of Catch | |
543 | @subsection Examples of @code{catch} and @code{throw} | |
544 | ||
545 | One way to use @code{catch} and @code{throw} is to exit from a doubly | |
546 | nested loop. (In most languages, this would be done with a ``go to''.) | |
547 | Here we compute @code{(foo @var{i} @var{j})} for @var{i} and @var{j} | |
548 | varying from 0 to 9: | |
549 | ||
550 | @example | |
551 | @group | |
552 | (defun search-foo () | |
553 | (catch 'loop | |
554 | (let ((i 0)) | |
555 | (while (< i 10) | |
556 | (let ((j 0)) | |
557 | (while (< j 10) | |
558 | (if (foo i j) | |
559 | (throw 'loop (list i j))) | |
560 | (setq j (1+ j)))) | |
561 | (setq i (1+ i)))))) | |
562 | @end group | |
563 | @end example | |
564 | ||
565 | @noindent | |
566 | If @code{foo} ever returns non-@code{nil}, we stop immediately and return a | |
567 | list of @var{i} and @var{j}. If @code{foo} always returns @code{nil}, the | |
568 | @code{catch} returns normally, and the value is @code{nil}, since that | |
569 | is the result of the @code{while}. | |
570 | ||
571 | Here are two tricky examples, slightly different, showing two | |
572 | return points at once. First, two return points with the same tag, | |
573 | @code{hack}: | |
574 | ||
575 | @example | |
576 | @group | |
577 | (defun catch2 (tag) | |
578 | (catch tag | |
579 | (throw 'hack 'yes))) | |
580 | @result{} catch2 | |
581 | @end group | |
582 | ||
583 | @group | |
584 | (catch 'hack | |
585 | (print (catch2 'hack)) | |
586 | 'no) | |
587 | @print{} yes | |
588 | @result{} no | |
589 | @end group | |
590 | @end example | |
591 | ||
592 | @noindent | |
593 | Since both return points have tags that match the @code{throw}, it goes to | |
594 | the inner one, the one established in @code{catch2}. Therefore, | |
595 | @code{catch2} returns normally with value @code{yes}, and this value is | |
596 | printed. Finally the second body form in the outer @code{catch}, which is | |
597 | @code{'no}, is evaluated and returned from the outer @code{catch}. | |
598 | ||
599 | Now let's change the argument given to @code{catch2}: | |
600 | ||
601 | @example | |
602 | @group | |
603 | (defun catch2 (tag) | |
604 | (catch tag | |
605 | (throw 'hack 'yes))) | |
606 | @result{} catch2 | |
607 | @end group | |
608 | ||
609 | @group | |
610 | (catch 'hack | |
611 | (print (catch2 'quux)) | |
612 | 'no) | |
613 | @result{} yes | |
614 | @end group | |
615 | @end example | |
616 | ||
617 | @noindent | |
3e099569 RS |
618 | We still have two return points, but this time only the outer one has |
619 | the tag @code{hack}; the inner one has the tag @code{quux} instead. | |
620 | Therefore, @code{throw} makes the outer @code{catch} return the value | |
621 | @code{yes}. The function @code{print} is never called, and the | |
622 | body-form @code{'no} is never evaluated. | |
83ac6b45 RS |
623 | |
624 | @node Errors | |
625 | @subsection Errors | |
626 | @cindex errors | |
627 | ||
628 | When Emacs Lisp attempts to evaluate a form that, for some reason, | |
629 | cannot be evaluated, it @dfn{signals} an @dfn{error}. | |
630 | ||
631 | When an error is signaled, Emacs's default reaction is to print an | |
632 | error message and terminate execution of the current command. This is | |
633 | the right thing to do in most cases, such as if you type @kbd{C-f} at | |
634 | the end of the buffer. | |
635 | ||
636 | In complicated programs, simple termination may not be what you want. | |
637 | For example, the program may have made temporary changes in data | |
3e099569 | 638 | structures, or created temporary buffers that should be deleted before |
83ac6b45 RS |
639 | the program is finished. In such cases, you would use |
640 | @code{unwind-protect} to establish @dfn{cleanup expressions} to be | |
3e099569 RS |
641 | evaluated in case of error. (@xref{Cleanups}.) Occasionally, you may |
642 | wish the program to continue execution despite an error in a subroutine. | |
643 | In these cases, you would use @code{condition-case} to establish | |
644 | @dfn{error handlers} to recover control in case of error. | |
83ac6b45 RS |
645 | |
646 | Resist the temptation to use error handling to transfer control from | |
647 | one part of the program to another; use @code{catch} and @code{throw} | |
648 | instead. @xref{Catch and Throw}. | |
649 | ||
650 | @menu | |
651 | * Signaling Errors:: How to report an error. | |
652 | * Processing of Errors:: What Emacs does when you report an error. | |
653 | * Handling Errors:: How you can trap errors and continue execution. | |
3e099569 | 654 | * Error Symbols:: How errors are classified for trapping them. |
83ac6b45 RS |
655 | @end menu |
656 | ||
657 | @node Signaling Errors | |
658 | @subsubsection How to Signal an Error | |
659 | @cindex signaling errors | |
660 | ||
661 | Most errors are signaled ``automatically'' within Lisp primitives | |
662 | which you call for other purposes, such as if you try to take the | |
663 | @sc{car} of an integer or move forward a character at the end of the | |
664 | buffer; you can also signal errors explicitly with the functions | |
665 | @code{error} and @code{signal}. | |
666 | ||
667 | Quitting, which happens when the user types @kbd{C-g}, is not | |
668 | considered an error, but it is handled almost like an error. | |
669 | @xref{Quitting}. | |
670 | ||
671 | @defun error format-string &rest args | |
672 | This function signals an error with an error message constructed by | |
673 | applying @code{format} (@pxref{String Conversion}) to | |
674 | @var{format-string} and @var{args}. | |
675 | ||
676 | These examples show typical uses of @code{error}: | |
677 | ||
678 | @example | |
679 | @group | |
680 | (error "You have committed an error. | |
681 | Try something else.") | |
682 | @error{} You have committed an error. | |
683 | Try something else. | |
684 | @end group | |
685 | ||
686 | @group | |
687 | (error "You have committed %d errors." 10) | |
688 | @error{} You have committed 10 errors. | |
689 | @end group | |
690 | @end example | |
691 | ||
692 | @code{error} works by calling @code{signal} with two arguments: the | |
693 | error symbol @code{error}, and a list containing the string returned by | |
694 | @code{format}. | |
695 | ||
696 | If you want to use your own string as an error message verbatim, don't | |
697 | just write @code{(error @var{string})}. If @var{string} contains | |
698 | @samp{%}, it will be interpreted as a format specifier, with undesirable | |
699 | results. Instead, use @code{(error "%s" @var{string})}. | |
700 | @end defun | |
701 | ||
702 | @defun signal error-symbol data | |
703 | This function signals an error named by @var{error-symbol}. The | |
704 | argument @var{data} is a list of additional Lisp objects relevant to the | |
705 | circumstances of the error. | |
706 | ||
707 | The argument @var{error-symbol} must be an @dfn{error symbol}---a symbol | |
708 | bearing a property @code{error-conditions} whose value is a list of | |
709 | condition names. This is how Emacs Lisp classifies different sorts of | |
710 | errors. | |
711 | ||
712 | The number and significance of the objects in @var{data} depends on | |
713 | @var{error-symbol}. For example, with a @code{wrong-type-arg} error, | |
3e099569 RS |
714 | there are two objects in the list: a predicate that describes the type |
715 | that was expected, and the object that failed to fit that type. | |
716 | @xref{Error Symbols}, for a description of error symbols. | |
83ac6b45 RS |
717 | |
718 | Both @var{error-symbol} and @var{data} are available to any error | |
3e099569 | 719 | handlers that handle the error: @code{condition-case} binds a local |
83ac6b45 RS |
720 | variable to a list of the form @code{(@var{error-symbol} .@: |
721 | @var{data})} (@pxref{Handling Errors}). If the error is not handled, | |
722 | these two values are used in printing the error message. | |
723 | ||
724 | The function @code{signal} never returns (though in older Emacs versions | |
725 | it could sometimes return). | |
726 | ||
727 | @smallexample | |
728 | @group | |
729 | (signal 'wrong-number-of-arguments '(x y)) | |
730 | @error{} Wrong number of arguments: x, y | |
731 | @end group | |
732 | ||
733 | @group | |
734 | (signal 'no-such-error '("My unknown error condition.")) | |
735 | @error{} peculiar error: "My unknown error condition." | |
736 | @end group | |
737 | @end smallexample | |
738 | @end defun | |
739 | ||
740 | @cindex CL note---no continuable errors | |
741 | @quotation | |
742 | @b{Common Lisp note:} Emacs Lisp has nothing like the Common Lisp | |
743 | concept of continuable errors. | |
744 | @end quotation | |
745 | ||
746 | @node Processing of Errors | |
747 | @subsubsection How Emacs Processes Errors | |
748 | ||
749 | When an error is signaled, @code{signal} searches for an active | |
750 | @dfn{handler} for the error. A handler is a sequence of Lisp | |
751 | expressions designated to be executed if an error happens in part of the | |
752 | Lisp program. If the error has an applicable handler, the handler is | |
753 | executed, and control resumes following the handler. The handler | |
3e099569 | 754 | executes in the environment of the @code{condition-case} that |
83ac6b45 RS |
755 | established it; all functions called within that @code{condition-case} |
756 | have already been exited, and the handler cannot return to them. | |
757 | ||
758 | If there is no applicable handler for the error, the current command is | |
759 | terminated and control returns to the editor command loop, because the | |
760 | command loop has an implicit handler for all kinds of errors. The | |
761 | command loop's handler uses the error symbol and associated data to | |
762 | print an error message. | |
763 | ||
764 | @cindex @code{debug-on-error} use | |
765 | An error that has no explicit handler may call the Lisp debugger. The | |
766 | debugger is enabled if the variable @code{debug-on-error} (@pxref{Error | |
767 | Debugging}) is non-@code{nil}. Unlike error handlers, the debugger runs | |
768 | in the environment of the error, so that you can examine values of | |
769 | variables precisely as they were at the time of the error. | |
770 | ||
771 | @node Handling Errors | |
772 | @subsubsection Writing Code to Handle Errors | |
773 | @cindex error handler | |
774 | @cindex handling errors | |
775 | ||
776 | The usual effect of signaling an error is to terminate the command | |
777 | that is running and return immediately to the Emacs editor command loop. | |
778 | You can arrange to trap errors occurring in a part of your program by | |
779 | establishing an error handler, with the special form | |
780 | @code{condition-case}. A simple example looks like this: | |
781 | ||
782 | @example | |
783 | @group | |
784 | (condition-case nil | |
785 | (delete-file filename) | |
786 | (error nil)) | |
787 | @end group | |
788 | @end example | |
789 | ||
790 | @noindent | |
791 | This deletes the file named @var{filename}, catching any error and | |
792 | returning @code{nil} if an error occurs. | |
793 | ||
794 | The second argument of @code{condition-case} is called the | |
795 | @dfn{protected form}. (In the example above, the protected form is a | |
796 | call to @code{delete-file}.) The error handlers go into effect when | |
797 | this form begins execution and are deactivated when this form returns. | |
798 | They remain in effect for all the intervening time. In particular, they | |
3e099569 RS |
799 | are in effect during the execution of functions called by this form, in |
800 | their subroutines, and so on. This is a good thing, since, strictly | |
83ac6b45 RS |
801 | speaking, errors can be signaled only by Lisp primitives (including |
802 | @code{signal} and @code{error}) called by the protected form, not by the | |
803 | protected form itself. | |
804 | ||
805 | The arguments after the protected form are handlers. Each handler | |
806 | lists one or more @dfn{condition names} (which are symbols) to specify | |
807 | which errors it will handle. The error symbol specified when an error | |
808 | is signaled also defines a list of condition names. A handler applies | |
809 | to an error if they have any condition names in common. In the example | |
810 | above, there is one handler, and it specifies one condition name, | |
811 | @code{error}, which covers all errors. | |
812 | ||
813 | The search for an applicable handler checks all the established handlers | |
814 | starting with the most recently established one. Thus, if two nested | |
815 | @code{condition-case} forms offer to handle the same error, the inner of | |
816 | the two will actually handle it. | |
817 | ||
818 | When an error is handled, control returns to the handler. Before this | |
819 | happens, Emacs unbinds all variable bindings made by binding constructs | |
820 | that are being exited and executes the cleanups of all | |
821 | @code{unwind-protect} forms that are exited. Once control arrives at | |
822 | the handler, the body of the handler is executed. | |
823 | ||
824 | After execution of the handler body, execution continues by returning | |
825 | from the @code{condition-case} form. Because the protected form is | |
826 | exited completely before execution of the handler, the handler cannot | |
827 | resume execution at the point of the error, nor can it examine variable | |
828 | bindings that were made within the protected form. All it can do is | |
829 | clean up and proceed. | |
830 | ||
831 | @code{condition-case} is often used to trap errors that are | |
832 | predictable, such as failure to open a file in a call to | |
833 | @code{insert-file-contents}. It is also used to trap errors that are | |
834 | totally unpredictable, such as when the program evaluates an expression | |
835 | read from the user. | |
836 | ||
837 | Error signaling and handling have some resemblance to @code{throw} and | |
838 | @code{catch}, but they are entirely separate facilities. An error | |
839 | cannot be caught by a @code{catch}, and a @code{throw} cannot be handled | |
840 | by an error handler (though using @code{throw} when there is no suitable | |
3e099569 | 841 | @code{catch} signals an error that can be handled). |
83ac6b45 RS |
842 | |
843 | @defspec condition-case var protected-form handlers@dots{} | |
844 | This special form establishes the error handlers @var{handlers} around | |
845 | the execution of @var{protected-form}. If @var{protected-form} executes | |
846 | without error, the value it returns becomes the value of the | |
847 | @code{condition-case} form; in this case, the @code{condition-case} has | |
848 | no effect. The @code{condition-case} form makes a difference when an | |
849 | error occurs during @var{protected-form}. | |
850 | ||
851 | Each of the @var{handlers} is a list of the form @code{(@var{conditions} | |
852 | @var{body}@dots{})}. Here @var{conditions} is an error condition name | |
853 | to be handled, or a list of condition names; @var{body} is one or more | |
854 | Lisp expressions to be executed when this handler handles an error. | |
855 | Here are examples of handlers: | |
856 | ||
857 | @smallexample | |
858 | @group | |
859 | (error nil) | |
860 | ||
861 | (arith-error (message "Division by zero")) | |
862 | ||
863 | ((arith-error file-error) | |
864 | (message | |
865 | "Either division by zero or failure to open a file")) | |
866 | @end group | |
867 | @end smallexample | |
868 | ||
3e099569 | 869 | Each error that occurs has an @dfn{error symbol} that describes what |
83ac6b45 | 870 | kind of error it is. The @code{error-conditions} property of this |
3e099569 RS |
871 | symbol is a list of condition names (@pxref{Error Symbols}). Emacs |
872 | searches all the active @code{condition-case} forms for a handler that | |
83ac6b45 RS |
873 | specifies one or more of these condition names; the innermost matching |
874 | @code{condition-case} handles the error. Within this | |
875 | @code{condition-case}, the first applicable handler handles the error. | |
876 | ||
877 | After executing the body of the handler, the @code{condition-case} | |
878 | returns normally, using the value of the last form in the handler body | |
879 | as the overall value. | |
880 | ||
881 | The argument @var{var} is a variable. @code{condition-case} does not | |
882 | bind this variable when executing the @var{protected-form}, only when it | |
883 | handles an error. At that time, it binds @var{var} locally to a list of | |
884 | the form @code{(@var{error-symbol} . @var{data})}, giving the | |
885 | particulars of the error. The handler can refer to this list to decide | |
886 | what to do. For example, if the error is for failure opening a file, | |
887 | the file name is the second element of @var{data}---the third element of | |
888 | @var{var}. | |
889 | ||
890 | If @var{var} is @code{nil}, that means no variable is bound. Then the | |
891 | error symbol and associated data are not available to the handler. | |
892 | @end defspec | |
893 | ||
894 | @cindex @code{arith-error} example | |
895 | Here is an example of using @code{condition-case} to handle the error | |
896 | that results from dividing by zero. The handler prints out a warning | |
897 | message and returns a very large number. | |
898 | ||
899 | @smallexample | |
900 | @group | |
901 | (defun safe-divide (dividend divisor) | |
902 | (condition-case err | |
903 | ;; @r{Protected form.} | |
904 | (/ dividend divisor) | |
905 | ;; @r{The handler.} | |
906 | (arith-error ; @r{Condition.} | |
907 | (princ (format "Arithmetic error: %s" err)) | |
908 | 1000000))) | |
909 | @result{} safe-divide | |
910 | @end group | |
911 | ||
912 | @group | |
913 | (safe-divide 5 0) | |
914 | @print{} Arithmetic error: (arith-error) | |
915 | @result{} 1000000 | |
916 | @end group | |
917 | @end smallexample | |
918 | ||
919 | @noindent | |
920 | The handler specifies condition name @code{arith-error} so that it will handle only division-by-zero errors. Other kinds of errors will not be handled, at least not by this @code{condition-case}. Thus, | |
921 | ||
922 | @smallexample | |
923 | @group | |
924 | (safe-divide nil 3) | |
925 | @error{} Wrong type argument: integer-or-marker-p, nil | |
926 | @end group | |
927 | @end smallexample | |
928 | ||
929 | Here is a @code{condition-case} that catches all kinds of errors, | |
930 | including those signaled with @code{error}: | |
931 | ||
932 | @smallexample | |
933 | @group | |
934 | (setq baz 34) | |
935 | @result{} 34 | |
936 | @end group | |
937 | ||
938 | @group | |
939 | (condition-case err | |
940 | (if (eq baz 35) | |
941 | t | |
942 | ;; @r{This is a call to the function @code{error}.} | |
943 | (error "Rats! The variable %s was %s, not 35." 'baz baz)) | |
944 | ;; @r{This is the handler; it is not a form.} | |
945 | (error (princ (format "The error was: %s" err)) | |
946 | 2)) | |
947 | @print{} The error was: (error "Rats! The variable baz was 34, not 35.") | |
948 | @result{} 2 | |
949 | @end group | |
950 | @end smallexample | |
951 | ||
3e099569 | 952 | @node Error Symbols |
83ac6b45 RS |
953 | @subsubsection Error Symbols and Condition Names |
954 | @cindex error symbol | |
955 | @cindex error name | |
956 | @cindex condition name | |
957 | @cindex user-defined error | |
958 | @kindex error-conditions | |
959 | ||
960 | When you signal an error, you specify an @dfn{error symbol} to specify | |
961 | the kind of error you have in mind. Each error has one and only one | |
962 | error symbol to categorize it. This is the finest classification of | |
963 | errors defined by the Emacs Lisp language. | |
964 | ||
965 | These narrow classifications are grouped into a hierarchy of wider | |
966 | classes called @dfn{error conditions}, identified by @dfn{condition | |
967 | names}. The narrowest such classes belong to the error symbols | |
968 | themselves: each error symbol is also a condition name. There are also | |
969 | condition names for more extensive classes, up to the condition name | |
970 | @code{error} which takes in all kinds of errors. Thus, each error has | |
971 | one or more condition names: @code{error}, the error symbol if that | |
972 | is distinct from @code{error}, and perhaps some intermediate | |
973 | classifications. | |
974 | ||
975 | In order for a symbol to be an error symbol, it must have an | |
976 | @code{error-conditions} property which gives a list of condition names. | |
3e099569 | 977 | This list defines the conditions that this kind of error belongs to. |
83ac6b45 RS |
978 | (The error symbol itself, and the symbol @code{error}, should always be |
979 | members of this list.) Thus, the hierarchy of condition names is | |
980 | defined by the @code{error-conditions} properties of the error symbols. | |
981 | ||
982 | In addition to the @code{error-conditions} list, the error symbol | |
983 | should have an @code{error-message} property whose value is a string to | |
984 | be printed when that error is signaled but not handled. If the | |
985 | @code{error-message} property exists, but is not a string, the error | |
986 | message @samp{peculiar error} is used. | |
987 | @cindex peculiar error | |
988 | ||
989 | Here is how we define a new error symbol, @code{new-error}: | |
990 | ||
991 | @example | |
992 | @group | |
993 | (put 'new-error | |
994 | 'error-conditions | |
995 | '(error my-own-errors new-error)) | |
996 | @result{} (error my-own-errors new-error) | |
997 | @end group | |
998 | @group | |
999 | (put 'new-error 'error-message "A new error") | |
1000 | @result{} "A new error" | |
1001 | @end group | |
1002 | @end example | |
1003 | ||
1004 | @noindent | |
1005 | This error has three condition names: @code{new-error}, the narrowest | |
1006 | classification; @code{my-own-errors}, which we imagine is a wider | |
1007 | classification; and @code{error}, which is the widest of all. | |
1008 | ||
1009 | Naturally, Emacs will never signal @code{new-error} on its own; only | |
3e099569 RS |
1010 | an explicit call to @code{signal} (@pxref{Signaling Errors}) in your |
1011 | code can do this: | |
83ac6b45 RS |
1012 | |
1013 | @example | |
1014 | @group | |
1015 | (signal 'new-error '(x y)) | |
1016 | @error{} A new error: x, y | |
1017 | @end group | |
1018 | @end example | |
1019 | ||
1020 | This error can be handled through any of the three condition names. | |
1021 | This example handles @code{new-error} and any other errors in the class | |
1022 | @code{my-own-errors}: | |
1023 | ||
1024 | @example | |
1025 | @group | |
1026 | (condition-case foo | |
1027 | (bar nil t) | |
1028 | (my-own-errors nil)) | |
1029 | @end group | |
1030 | @end example | |
1031 | ||
1032 | The significant way that errors are classified is by their condition | |
1033 | names---the names used to match errors with handlers. An error symbol | |
1034 | serves only as a convenient way to specify the intended error message | |
1035 | and list of condition names. It would be cumbersome to give | |
1036 | @code{signal} a list of condition names rather than one error symbol. | |
1037 | ||
1038 | By contrast, using only error symbols without condition names would | |
1039 | seriously decrease the power of @code{condition-case}. Condition names | |
1040 | make it possible to categorize errors at various levels of generality | |
1041 | when you write an error handler. Using error symbols alone would | |
1042 | eliminate all but the narrowest level of classification. | |
1043 | ||
1044 | @xref{Standard Errors}, for a list of all the standard error symbols | |
1045 | and their conditions. | |
1046 | ||
1047 | @node Cleanups | |
1048 | @subsection Cleaning Up from Nonlocal Exits | |
1049 | ||
1050 | The @code{unwind-protect} construct is essential whenever you | |
1051 | temporarily put a data structure in an inconsistent state; it permits | |
1052 | you to ensure the data are consistent in the event of an error or throw. | |
1053 | ||
1054 | @defspec unwind-protect body cleanup-forms@dots{} | |
1055 | @cindex cleanup forms | |
1056 | @cindex protected forms | |
1057 | @cindex error cleanup | |
1058 | @cindex unwinding | |
1059 | @code{unwind-protect} executes the @var{body} with a guarantee that the | |
1060 | @var{cleanup-forms} will be evaluated if control leaves @var{body}, no | |
1061 | matter how that happens. The @var{body} may complete normally, or | |
1062 | execute a @code{throw} out of the @code{unwind-protect}, or cause an | |
1063 | error; in all cases, the @var{cleanup-forms} will be evaluated. | |
1064 | ||
1065 | If the @var{body} forms finish normally, @code{unwind-protect} returns | |
1066 | the value of the last @var{body} form, after it evaluates the | |
1067 | @var{cleanup-forms}. If the @var{body} forms do not finish, | |
1068 | @code{unwind-protect} does not return any value in the normal sense. | |
1069 | ||
1070 | Only the @var{body} is actually protected by the @code{unwind-protect}. | |
1071 | If any of the @var{cleanup-forms} themselves exits nonlocally (e.g., via | |
1072 | a @code{throw} or an error), @code{unwind-protect} is @emph{not} | |
1073 | guaranteed to evaluate the rest of them. If the failure of one of the | |
1074 | @var{cleanup-forms} has the potential to cause trouble, then protect it | |
1075 | with another @code{unwind-protect} around that form. | |
1076 | ||
1077 | The number of currently active @code{unwind-protect} forms counts, | |
1078 | together with the number of local variable bindings, against the limit | |
1079 | @code{max-specpdl-size} (@pxref{Local Variables}). | |
1080 | @end defspec | |
1081 | ||
1082 | For example, here we make an invisible buffer for temporary use, and | |
1083 | make sure to kill it before finishing: | |
1084 | ||
1085 | @smallexample | |
1086 | @group | |
1087 | (save-excursion | |
1088 | (let ((buffer (get-buffer-create " *temp*"))) | |
1089 | (set-buffer buffer) | |
1090 | (unwind-protect | |
1091 | @var{body} | |
1092 | (kill-buffer buffer)))) | |
1093 | @end group | |
1094 | @end smallexample | |
1095 | ||
1096 | @noindent | |
1097 | You might think that we could just as well write @code{(kill-buffer | |
1098 | (current-buffer))} and dispense with the variable @code{buffer}. | |
1099 | However, the way shown above is safer, if @var{body} happens to get an | |
1100 | error after switching to a different buffer! (Alternatively, you could | |
1101 | write another @code{save-excursion} around the body, to ensure that the | |
1102 | temporary buffer becomes current in time to kill it.) | |
1103 | ||
1104 | @findex ftp-login | |
3e099569 RS |
1105 | Here is an actual example taken from the file @file{ftp.el}. It |
1106 | creates a process (@pxref{Processes}) to try to establish a connection | |
1107 | to a remote machine. As the function @code{ftp-login} is highly | |
1108 | susceptible to numerous problems that the writer of the function cannot | |
1109 | anticipate, it is protected with a form that guarantees deletion of the | |
1110 | process in the event of failure. Otherwise, Emacs might fill up with | |
1111 | useless subprocesses. | |
83ac6b45 RS |
1112 | |
1113 | @smallexample | |
1114 | @group | |
1115 | (let ((win nil)) | |
1116 | (unwind-protect | |
1117 | (progn | |
1118 | (setq process (ftp-setup-buffer host file)) | |
1119 | (if (setq win (ftp-login process host user password)) | |
1120 | (message "Logged in") | |
1121 | (error "Ftp login failed"))) | |
1122 | (or win (and process (delete-process process))))) | |
1123 | @end group | |
1124 | @end smallexample | |
1125 | ||
1126 | This example actually has a small bug: if the user types @kbd{C-g} to | |
1127 | quit, and the quit happens immediately after the function | |
1128 | @code{ftp-setup-buffer} returns but before the variable @code{process} is | |
1129 | set, the process will not be killed. There is no easy way to fix this bug, | |
1130 | but at least it is very unlikely. | |
1131 | ||
1132 | Here is another example which uses @code{unwind-protect} to make sure | |
1133 | to kill a temporary buffer. In this example, the value returned by | |
1134 | @code{unwind-protect} is used. | |
1135 | ||
ec221d13 | 1136 | @smallexample |
83ac6b45 RS |
1137 | (defun shell-command-string (cmd) |
1138 | "Return the output of the shell command CMD, as a string." | |
1139 | (save-excursion | |
1140 | (set-buffer (generate-new-buffer " OS*cmd")) | |
1141 | (shell-command cmd t) | |
1142 | (unwind-protect | |
1143 | (buffer-string) | |
1144 | (kill-buffer (current-buffer))))) | |
ec221d13 | 1145 | @end smallexample |