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| author | Henry Thompson <ht@markup.co.uk> |
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| date | Thu, 16 Dec 2021 18:48:58 +0000 |
| parents | 9fae6227ede5 |
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| rev | line source |
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| 0 | 1 @c -*-texinfo-*- |
| 2 @c This is part of the XEmacs Lisp Reference Manual. | |
| 444 | 3 @c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. |
| 0 | 4 @c See the file lispref.texi for copying conditions. |
| 5 @setfilename ../../info/control.info | |
| 6 @node Control Structures, Variables, Evaluation, Top | |
| 7 @chapter Control Structures | |
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8 @cindex special operators for control structures |
| 0 | 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 | |
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13 @dfn{control structures}. Control structures are special operators which |
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14 control when, whether, or how many times to execute the subforms of |
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15 their containing forms. |
| 0 | 16 |
| 17 The simplest order of execution is sequential execution: first form | |
| 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 | |
| 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. | |
| 28 | |
| 29 XEmacs Lisp provides several kinds of control structure, including | |
| 30 other varieties of sequencing, conditionals, iteration, and (controlled) | |
| 31 jumps---all discussed below. The built-in control structures are | |
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32 special operators since their enclosing forms' subforms are not |
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33 necessarily evaluated or not evaluated sequentially. You can use macros |
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34 to define your own control structure constructs (@pxref{Macros}). |
| 0 | 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 | |
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44 @node Sequencing, Conditionals, Control Structures, Control Structures |
| 0 | 45 @section Sequencing |
| 46 | |
| 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. | |
| 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 | |
| 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}. | |
| 78 | |
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79 @deffn {Special Operator} progn forms@dots{} |
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80 This special operator evaluates all of the @var{forms}, in textual |
| 0 | 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 | |
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94 @end deffn |
| 0 | 95 |
| 96 Two other control constructs likewise evaluate a series of forms but return | |
| 97 a different value: | |
| 98 | |
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99 @deffn {Special Operator} prog1 form1 forms@dots{} |
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100 This special operator evaluates @var{form1} and all of the @var{forms}, in |
| 0 | 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 | |
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121 @end deffn |
| 0 | 122 |
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123 @deffn {Special Operator} prog2 form1 form2 forms@dots{} |
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124 This special operator evaluates @var{form1}, @var{form2}, and all of the |
| 0 | 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 | |
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139 @end deffn |
| 0 | 140 |
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141 @node Conditionals, Combining Conditions, Sequencing, Control Structures |
| 0 | 142 @section Conditionals |
| 143 @cindex conditional evaluation | |
| 144 | |
| 145 Conditional control structures choose among alternatives. XEmacs 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 | |
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149 @deffn {Special Operator} if condition then-form else-forms@dots{} |
| 0 | 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 | |
| 444 | 155 an example of an implicit @code{progn}. @xref{Sequencing}.) |
| 0 | 156 |
| 157 If @var{condition} has the value @code{nil}, and no @var{else-forms} are | |
| 158 given, @code{if} returns @code{nil}. | |
| 159 | |
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160 @code{if} is a special operator because the branch that is not selected is |
| 0 | 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 | |
| 444 | 166 (if nil |
| 167 (print 'true) | |
| 0 | 168 'very-false) |
| 169 @result{} very-false | |
| 170 @end group | |
| 171 @end example | |
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172 @end deffn |
| 0 | 173 |
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174 @deffn {Special Operator} cond clause@dots{} |
| 0 | 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 | |
| 444 | 229 For example, |
| 0 | 230 |
| 231 @example | |
| 232 @group | |
| 233 (cond ((eq a 'hack) 'foo) | |
| 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. | |
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242 @end deffn |
| 0 | 243 |
| 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: | |
| 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 | |
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256 @node Combining Conditions, Iteration, Conditionals, Control Structures |
| 0 | 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 | |
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271 @deffn {Special Operator} and conditions@dots{} |
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272 The @code{and} special operator tests whether all the @var{conditions} are |
| 0 | 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 | |
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323 @end deffn |
| 0 | 324 |
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325 @deffn {Special Operator} or conditions@dots{} |
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326 The @code{or} special operator tests whether at least one of the |
| 0 | 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} | |
| 444 | 363 (if @var{arg2} @var{arg2} |
| 0 | 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. | |
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372 @end deffn |
| 0 | 373 |
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374 @node Iteration, Nonlocal Exits, Combining Conditions, Control Structures |
| 0 | 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 | |
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382 in XEmacs Lisp with the special operator @code{while}: |
| 0 | 383 |
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384 @deffn {Special Operator} while condition forms@dots{} |
| 0 | 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 | |
| 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 deffn |
| 0 | 431 |
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432 @node Nonlocal Exits, , Iteration, Control Structures |
| 0 | 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 XEmacs 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 | |
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449 @node Catch and Throw, Examples of Catch, Nonlocal Exits, Nonlocal Exits |
| 0 | 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{} | |
| 464 (throw 'foo t) | |
| 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 operator when it exits that form |
| 0 | 492 (@pxref{Cleanups}). |
| 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} | |
| 500 that throw back to the editor command loop (@pxref{Recursive Editing}). | |
| 501 | |
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502 @deffn {Special Operator} catch tag body@dots{} |
| 0 | 503 @cindex tag on run time stack |
| 504 @code{catch} establishes a return point for the @code{throw} function. The | |
| 505 return point is distinguished from other such return points by @var{tag}, | |
| 506 which may be any Lisp object. The argument @var{tag} is evaluated normally | |
| 507 before the return point is established. | |
| 508 | |
| 509 With the return point in effect, @code{catch} evaluates the forms of the | |
| 510 @var{body} in textual order. If the forms execute normally, without | |
| 511 error or nonlocal exit, the value of the last body form is returned from | |
| 512 the @code{catch}. | |
| 513 | |
| 514 If a @code{throw} is done within @var{body} specifying the same value | |
| 515 @var{tag}, the @code{catch} exits immediately; the value it returns is | |
| 516 whatever was specified as the second argument of @code{throw}. | |
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517 @end deffn |
| 0 | 518 |
| 519 @defun throw tag value | |
| 520 The purpose of @code{throw} is to return from a return point previously | |
| 521 established with @code{catch}. The argument @var{tag} is used to choose | |
| 522 among the various existing return points; it must be @code{eq} to the value | |
| 523 specified in the @code{catch}. If multiple return points match @var{tag}, | |
| 524 the innermost one is used. | |
| 525 | |
| 526 The argument @var{value} is used as the value to return from that | |
| 527 @code{catch}. | |
| 528 | |
| 529 @kindex no-catch | |
| 530 If no return point is in effect with tag @var{tag}, then a @code{no-catch} | |
| 531 error is signaled with data @code{(@var{tag} @var{value})}. | |
| 532 @end defun | |
| 533 | |
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534 @node Examples of Catch, Errors, Catch and Throw, Nonlocal Exits |
| 0 | 535 @subsection Examples of @code{catch} and @code{throw} |
| 536 | |
| 537 One way to use @code{catch} and @code{throw} is to exit from a doubly | |
| 538 nested loop. (In most languages, this would be done with a ``go to''.) | |
| 539 Here we compute @code{(foo @var{i} @var{j})} for @var{i} and @var{j} | |
| 540 varying from 0 to 9: | |
| 541 | |
| 542 @example | |
| 543 @group | |
| 544 (defun search-foo () | |
| 545 (catch 'loop | |
| 546 (let ((i 0)) | |
| 547 (while (< i 10) | |
| 548 (let ((j 0)) | |
| 549 (while (< j 10) | |
| 550 (if (foo i j) | |
| 551 (throw 'loop (list i j))) | |
| 552 (setq j (1+ j)))) | |
| 553 (setq i (1+ i)))))) | |
| 554 @end group | |
| 555 @end example | |
| 556 | |
| 557 @noindent | |
| 558 If @code{foo} ever returns non-@code{nil}, we stop immediately and return a | |
| 559 list of @var{i} and @var{j}. If @code{foo} always returns @code{nil}, the | |
| 560 @code{catch} returns normally, and the value is @code{nil}, since that | |
| 561 is the result of the @code{while}. | |
| 562 | |
| 563 Here are two tricky examples, slightly different, showing two | |
| 564 return points at once. First, two return points with the same tag, | |
| 565 @code{hack}: | |
| 566 | |
| 567 @example | |
| 568 @group | |
| 569 (defun catch2 (tag) | |
| 570 (catch tag | |
| 571 (throw 'hack 'yes))) | |
| 572 @result{} catch2 | |
| 573 @end group | |
| 574 | |
| 575 @group | |
| 444 | 576 (catch 'hack |
| 0 | 577 (print (catch2 'hack)) |
| 578 'no) | |
| 579 @print{} yes | |
| 580 @result{} no | |
| 581 @end group | |
| 582 @end example | |
| 583 | |
| 584 @noindent | |
| 585 Since both return points have tags that match the @code{throw}, it goes to | |
| 586 the inner one, the one established in @code{catch2}. Therefore, | |
| 587 @code{catch2} returns normally with value @code{yes}, and this value is | |
| 588 printed. Finally the second body form in the outer @code{catch}, which is | |
| 589 @code{'no}, is evaluated and returned from the outer @code{catch}. | |
| 590 | |
| 591 Now let's change the argument given to @code{catch2}: | |
| 592 | |
| 593 @example | |
| 594 @group | |
| 595 (defun catch2 (tag) | |
| 596 (catch tag | |
| 597 (throw 'hack 'yes))) | |
| 598 @result{} catch2 | |
| 599 @end group | |
| 600 | |
| 601 @group | |
| 602 (catch 'hack | |
| 603 (print (catch2 'quux)) | |
| 604 'no) | |
| 605 @result{} yes | |
| 606 @end group | |
| 607 @end example | |
| 608 | |
| 609 @noindent | |
| 610 We still have two return points, but this time only the outer one has | |
| 611 the tag @code{hack}; the inner one has the tag @code{quux} instead. | |
| 612 Therefore, @code{throw} makes the outer @code{catch} return the value | |
| 613 @code{yes}. The function @code{print} is never called, and the | |
| 614 body-form @code{'no} is never evaluated. | |
| 615 | |
| 2297 | 616 In most cases the formal tag for a catch is a quoted symbol or a |
| 617 variable whose value is a symbol. Both styles are demonstrated above. | |
| 618 In definitions of derived control structures, an anonymous tag may be | |
| 619 desired. A gensym could be used, but since catch tags are compared | |
| 620 using @code{eq}, any Lisp object can be used. An occasionally | |
| 621 encountered idiom is to bind a local variable to @code{(cons nil nil)}, | |
| 622 and use the variable as the formal tag. | |
| 623 | |
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624 @node Errors, Cleanups, Examples of Catch, Nonlocal Exits |
| 0 | 625 @subsection Errors |
| 626 @cindex errors | |
| 627 | |
| 628 When XEmacs 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, XEmacs'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 | |
| 638 structures, or created temporary buffers that should be deleted before | |
| 639 the program is finished. In such cases, you would use | |
| 640 @code{unwind-protect} to establish @dfn{cleanup expressions} to be | |
| 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. | |
| 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 XEmacs does when you report an error. | |
| 653 * Handling Errors:: How you can trap errors and continue execution. | |
| 654 * Error Symbols:: How errors are classified for trapping them. | |
| 655 @end menu | |
| 656 | |
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657 @node Signaling Errors, Processing of Errors, Errors, Errors |
| 0 | 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 | |
| 398 | 665 @code{error}, @code{signal}, and others. |
| 0 | 666 |
| 444 | 667 Quitting, which happens when the user types @kbd{C-g}, is not |
| 0 | 668 considered an error, but it is handled almost like an error. |
| 669 @xref{Quitting}. | |
| 670 | |
| 444 | 671 XEmacs has a rich hierarchy of error symbols predefined via @code{deferror}. |
| 672 | |
| 673 @example | |
| 674 error | |
| 675 syntax-error | |
| 676 invalid-read-syntax | |
| 677 list-formation-error | |
| 678 malformed-list | |
| 679 malformed-property-list | |
| 680 circular-list | |
| 681 circular-property-list | |
| 682 | |
| 683 invalid-argument | |
| 684 wrong-type-argument | |
| 685 args-out-of-range | |
| 686 wrong-number-of-arguments | |
| 687 invalid-function | |
| 688 no-catch | |
| 689 | |
| 690 invalid-state | |
| 691 void-function | |
| 692 cyclic-function-indirection | |
| 693 void-variable | |
| 694 cyclic-variable-indirection | |
| 695 | |
| 696 invalid-operation | |
| 697 invalid-change | |
| 698 setting-constant | |
| 699 editing-error | |
| 700 beginning-of-buffer | |
| 701 end-of-buffer | |
| 702 buffer-read-only | |
| 703 io-error | |
| 704 end-of-file | |
| 705 arith-error | |
| 706 range-error | |
| 707 domain-error | |
| 708 singularity-error | |
| 709 overflow-error | |
| 710 underflow-error | |
| 711 @end example | |
| 712 | |
| 713 The five most common errors you will probably use or base your new | |
| 714 errors off of are @code{syntax-error}, @code{invalid-argument}, | |
| 715 @code{invalid-state}, @code{invalid-operation}, and | |
| 716 @code{invalid-change}. Note the semantic differences: | |
| 717 | |
| 718 @itemize @bullet | |
| 719 @item | |
| 720 @code{syntax-error} is for errors in complex structures: parsed strings, | |
| 721 lists, and the like. | |
| 722 | |
| 723 @item | |
| 724 @code{invalid-argument} is for errors in a simple value. Typically, the | |
| 725 entire value, not just one part of it, is wrong. | |
| 726 | |
| 727 @item | |
| 728 @code{invalid-state} means that some settings have been changed in such | |
| 729 a way that their current state is unallowable. More and more, code is | |
| 730 being written more carefully, and catches the error when the settings | |
| 731 are being changed, rather than afterwards. This leads us to the next | |
| 732 error: | |
| 733 | |
| 734 @item | |
| 735 @code{invalid-change} means that an attempt is being made to change some | |
| 736 settings into an invalid state. @code{invalid-change} is a type of | |
| 737 @code{invalid-operation}. | |
| 738 | |
| 739 @item | |
| 740 @code{invalid-operation} refers to all cases where code is trying to do | |
| 741 something that's disallowed. This includes file errors, buffer errors | |
| 742 (e.g. running off the end of a buffer), @code{invalid-change} as just | |
| 743 mentioned, and arithmetic errors. | |
| 744 @end itemize | |
| 745 | |
| 746 @defun error datum &rest args | |
| 747 This function signals a non-continuable error. | |
| 748 | |
| 749 @var{datum} should normally be an error symbol, i.e. a symbol defined | |
| 750 using @code{define-error}. @var{args} will be made into a list, and | |
| 751 @var{datum} and @var{args} passed as the two arguments to @code{signal}, | |
| 752 the most basic error handling function. | |
| 0 | 753 |
| 398 | 754 This error is not continuable: you cannot continue execution after the |
| 444 | 755 error using the debugger @kbd{r} command. See also @code{cerror}. |
| 756 | |
| 757 The correct semantics of @var{args} varies from error to error, but for | |
| 758 most errors that need to be generated in Lisp code, the first argument | |
| 759 should be a string describing the *context* of the error (i.e. the exact | |
| 760 operation being performed and what went wrong), and the remaining | |
| 761 arguments or \"frobs\" (most often, there is one) specify the offending | |
| 762 object(s) and/or provide additional details such as the exact error when | |
| 763 a file error occurred, e.g.: | |
| 764 | |
| 765 @itemize @bullet | |
| 766 @item | |
| 767 the buffer in which an editing error occurred. | |
| 768 @item | |
| 769 an invalid value that was encountered. (In such cases, the string | |
| 770 should describe the purpose or \"semantics\" of the value [e.g. if the | |
| 771 value is an argument to a function, the name of the argument; if the value | |
| 772 is the value corresponding to a keyword, the name of the keyword; if the | |
| 773 value is supposed to be a list length, say this and say what the purpose | |
| 774 of the list is; etc.] as well as specifying why the value is invalid, if | |
| 775 that's not self-evident.) | |
| 776 @item | |
| 777 the file in which an error occurred. (In such cases, there should be a | |
| 778 second frob, probably a string, specifying the exact error that occurred. | |
| 779 This does not occur in the string that precedes the first frob, because | |
| 780 that frob describes the exact operation that was happening. | |
| 781 @end itemize | |
| 782 | |
| 783 For historical compatibility, DATUM can also be a string. In this case, | |
| 784 @var{datum} and @var{args} are passed together as the arguments to | |
| 785 @code{format}, and then an error is signalled using the error symbol | |
| 786 @code{error} and formatted string. Although this usage of @code{error} | |
| 787 is very common, it is deprecated because it totally defeats the purpose | |
| 788 of having structured errors. There is now a rich set of defined errors | |
| 789 to use. | |
| 790 | |
| 791 See also @code{cerror}, @code{signal}, and @code{signal-error}." | |
| 398 | 792 |
| 0 | 793 These examples show typical uses of @code{error}: |
| 794 | |
| 795 @example | |
| 796 @group | |
| 444 | 797 (error 'syntax-error |
| 798 "Dialog descriptor must supply at least one button" | |
| 799 descriptor) | |
| 800 @end group | |
| 801 | |
| 802 @group | |
| 803 (error "You have committed an error. | |
| 0 | 804 Try something else.") |
| 444 | 805 @error{} You have committed an error. |
| 0 | 806 Try something else. |
| 807 @end group | |
| 808 | |
| 809 @group | |
| 810 (error "You have committed %d errors." 10) | |
| 444 | 811 @error{} You have committed 10 errors. |
| 0 | 812 @end group |
| 813 @end example | |
| 814 | |
| 815 If you want to use your own string as an error message verbatim, don't | |
| 816 just write @code{(error @var{string})}. If @var{string} contains | |
| 817 @samp{%}, it will be interpreted as a format specifier, with undesirable | |
| 818 results. Instead, use @code{(error "%s" @var{string})}. | |
| 819 @end defun | |
| 820 | |
| 444 | 821 @defun cerror datum &rest args |
| 398 | 822 This function behaves like @code{error}, except that the error it |
| 823 signals is continuable. That means that debugger commands @kbd{c} and | |
| 824 @kbd{r} can resume execution. | |
| 825 @end defun | |
| 826 | |
| 0 | 827 @defun signal error-symbol data |
| 398 | 828 This function signals a continuable error named by @var{error-symbol}. |
| 829 The argument @var{data} is a list of additional Lisp objects relevant to | |
| 830 the circumstances of the error. | |
| 0 | 831 |
| 832 The argument @var{error-symbol} must be an @dfn{error symbol}---a symbol | |
| 833 bearing a property @code{error-conditions} whose value is a list of | |
| 834 condition names. This is how XEmacs Lisp classifies different sorts of | |
| 835 errors. | |
| 836 | |
| 837 The number and significance of the objects in @var{data} depends on | |
| 398 | 838 @var{error-symbol}. For example, with a @code{wrong-type-argument} |
| 839 error, there are two objects in the list: a predicate that describes the | |
| 840 type that was expected, and the object that failed to fit that type. | |
| 0 | 841 @xref{Error Symbols}, for a description of error symbols. |
| 842 | |
| 843 Both @var{error-symbol} and @var{data} are available to any error | |
| 844 handlers that handle the error: @code{condition-case} binds a local | |
| 845 variable to a list of the form @code{(@var{error-symbol} .@: | |
| 846 @var{data})} (@pxref{Handling Errors}). If the error is not handled, | |
| 847 these two values are used in printing the error message. | |
| 848 | |
| 398 | 849 The function @code{signal} can return, if the debugger is invoked and |
| 850 the user invokes the ``return from signal'' option. If you want the | |
| 851 error not to be continuable, use @code{signal-error} instead. Note that | |
| 852 in FSF Emacs @code{signal} never returns. | |
| 0 | 853 |
| 854 @smallexample | |
| 855 @group | |
| 856 (signal 'wrong-number-of-arguments '(x y)) | |
| 857 @error{} Wrong number of arguments: x, y | |
| 858 @end group | |
| 859 | |
| 860 @group | |
| 398 | 861 (signal 'no-such-error '("My unknown error condition")) |
| 862 @error{} Peculiar error (no-such-error "My unknown error condition") | |
| 0 | 863 @end group |
| 864 @end smallexample | |
| 865 @end defun | |
| 866 | |
| 398 | 867 @defun signal-error error-symbol data |
| 868 This function behaves like @code{signal}, except that the error it | |
| 869 signals is not continuable. | |
| 870 @end defun | |
| 871 | |
| 872 @defmac check-argument-type predicate argument | |
| 873 This macro checks that @var{argument} satisfies @var{predicate}. If | |
| 874 that is not the case, it signals a continuable | |
| 875 @code{wrong-type-argument} error until the returned value satisfies | |
| 876 @var{predicate}, and assigns the returned value to @var{argument}. In | |
| 877 other words, execution of the program will not continue until | |
| 878 @var{predicate} is met. | |
| 879 | |
| 880 @var{argument} is not evaluated, and should be a symbol. | |
| 881 @var{predicate} is evaluated, and should name a function. | |
| 882 | |
| 883 As shown in the following example, @code{check-argument-type} is useful | |
| 884 in low-level code that attempts to ensure the sanity of its data before | |
| 885 proceeding. | |
| 886 | |
| 887 @example | |
| 888 @group | |
| 889 (defun cache-object-internal (object wlist) | |
| 890 ;; @r{Before doing anything, make sure that @var{wlist} is indeed} | |
| 891 ;; @r{a weak list, which is what we expect.} | |
| 892 (check-argument-type 'weak-list-p wlist) | |
| 893 @dots{}) | |
| 894 @end group | |
| 895 @end example | |
| 896 @end defmac | |
| 0 | 897 |
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898 @node Processing of Errors, Handling Errors, Signaling Errors, Errors |
| 0 | 899 @subsubsection How XEmacs Processes Errors |
| 900 | |
| 901 When an error is signaled, @code{signal} searches for an active | |
| 902 @dfn{handler} for the error. A handler is a sequence of Lisp | |
| 903 expressions designated to be executed if an error happens in part of the | |
| 904 Lisp program. If the error has an applicable handler, the handler is | |
| 905 executed, and control resumes following the handler. The handler | |
| 906 executes in the environment of the @code{condition-case} that | |
| 907 established it; all functions called within that @code{condition-case} | |
| 908 have already been exited, and the handler cannot return to them. | |
| 909 | |
| 910 If there is no applicable handler for the error, the current command is | |
| 911 terminated and control returns to the editor command loop, because the | |
| 912 command loop has an implicit handler for all kinds of errors. The | |
| 913 command loop's handler uses the error symbol and associated data to | |
| 914 print an error message. | |
| 915 | |
| 398 | 916 Errors in command loop are processed using the @code{command-error} |
| 917 function, which takes care of some necessary cleanup, and prints a | |
| 918 formatted error message to the echo area. The functions that do the | |
| 919 formatting are explained below. | |
| 920 | |
| 921 @defun display-error error-object stream | |
| 922 This function displays @var{error-object} on @var{stream}. | |
| 923 @var{error-object} is a cons of error type, a symbol, and error | |
| 924 arguments, a list. If the error type symbol of one of its error | |
| 901 | 925 condition superclasses has a @code{display-error} property, that |
| 398 | 926 function is invoked for printing the actual error message. Otherwise, |
| 927 the error is printed as @samp{Error: arg1, arg2, ...}. | |
| 928 @end defun | |
| 929 | |
| 930 @defun error-message-string error-object | |
| 931 This function converts @var{error-object} to an error message string, | |
| 932 and returns it. The message is equivalent to the one that would be | |
| 933 printed by @code{display-error}, except that it is conveniently returned | |
| 934 in string form. | |
| 935 @end defun | |
| 936 | |
| 0 | 937 @cindex @code{debug-on-error} use |
| 938 An error that has no explicit handler may call the Lisp debugger. The | |
| 939 debugger is enabled if the variable @code{debug-on-error} (@pxref{Error | |
| 940 Debugging}) is non-@code{nil}. Unlike error handlers, the debugger runs | |
| 941 in the environment of the error, so that you can examine values of | |
| 942 variables precisely as they were at the time of the error. | |
| 943 | |
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944 @node Handling Errors, Error Symbols, Processing of Errors, Errors |
| 0 | 945 @subsubsection Writing Code to Handle Errors |
| 946 @cindex error handler | |
| 947 @cindex handling errors | |
| 948 | |
| 949 The usual effect of signaling an error is to terminate the command | |
| 950 that is running and return immediately to the XEmacs editor command loop. | |
| 951 You can arrange to trap errors occurring in a part of your program by | |
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952 establishing an error handler, with the special operator |
| 0 | 953 @code{condition-case}. A simple example looks like this: |
| 954 | |
| 955 @example | |
| 956 @group | |
| 957 (condition-case nil | |
| 958 (delete-file filename) | |
| 959 (error nil)) | |
| 960 @end group | |
| 961 @end example | |
| 962 | |
| 963 @noindent | |
| 964 This deletes the file named @var{filename}, catching any error and | |
| 965 returning @code{nil} if an error occurs. | |
| 966 | |
| 967 The second argument of @code{condition-case} is called the | |
| 968 @dfn{protected form}. (In the example above, the protected form is a | |
| 969 call to @code{delete-file}.) The error handlers go into effect when | |
| 970 this form begins execution and are deactivated when this form returns. | |
| 971 They remain in effect for all the intervening time. In particular, they | |
| 972 are in effect during the execution of functions called by this form, in | |
| 973 their subroutines, and so on. This is a good thing, since, strictly | |
| 974 speaking, errors can be signaled only by Lisp primitives (including | |
| 975 @code{signal} and @code{error}) called by the protected form, not by the | |
| 976 protected form itself. | |
| 977 | |
| 978 The arguments after the protected form are handlers. Each handler | |
| 979 lists one or more @dfn{condition names} (which are symbols) to specify | |
| 980 which errors it will handle. The error symbol specified when an error | |
| 981 is signaled also defines a list of condition names. A handler applies | |
| 982 to an error if they have any condition names in common. In the example | |
| 983 above, there is one handler, and it specifies one condition name, | |
| 984 @code{error}, which covers all errors. | |
| 985 | |
| 986 The search for an applicable handler checks all the established handlers | |
| 987 starting with the most recently established one. Thus, if two nested | |
| 988 @code{condition-case} forms offer to handle the same error, the inner of | |
| 989 the two will actually handle it. | |
| 990 | |
| 991 When an error is handled, control returns to the handler. Before this | |
| 992 happens, XEmacs unbinds all variable bindings made by binding constructs | |
| 993 that are being exited and executes the cleanups of all | |
| 994 @code{unwind-protect} forms that are exited. Once control arrives at | |
| 995 the handler, the body of the handler is executed. | |
| 996 | |
| 997 After execution of the handler body, execution continues by returning | |
| 998 from the @code{condition-case} form. Because the protected form is | |
| 999 exited completely before execution of the handler, the handler cannot | |
| 1000 resume execution at the point of the error, nor can it examine variable | |
| 1001 bindings that were made within the protected form. All it can do is | |
| 1002 clean up and proceed. | |
| 1003 | |
| 1004 @code{condition-case} is often used to trap errors that are | |
| 1005 predictable, such as failure to open a file in a call to | |
| 1006 @code{insert-file-contents}. It is also used to trap errors that are | |
| 1007 totally unpredictable, such as when the program evaluates an expression | |
| 1008 read from the user. | |
| 1009 | |
| 398 | 1010 @cindex @code{debug-on-signal} use |
| 1011 Even when an error is handled, the debugger may still be called if the | |
| 1012 variable @code{debug-on-signal} (@pxref{Error Debugging}) is | |
| 1013 non-@code{nil}. Note that this may yield unpredictable results with | |
| 1014 code that traps expected errors as normal part of its operation. Do not | |
| 1015 set @code{debug-on-signal} unless you know what you are doing. | |
| 1016 | |
| 0 | 1017 Error signaling and handling have some resemblance to @code{throw} and |
| 1018 @code{catch}, but they are entirely separate facilities. An error | |
| 1019 cannot be caught by a @code{catch}, and a @code{throw} cannot be handled | |
| 1020 by an error handler (though using @code{throw} when there is no suitable | |
| 1021 @code{catch} signals an error that can be handled). | |
| 1022 | |
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1023 @deffn {Special Operator} condition-case var protected-form handlers@dots{} |
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1024 This special operator establishes the error handlers @var{handlers} around |
| 0 | 1025 the execution of @var{protected-form}. If @var{protected-form} executes |
| 1026 without error, the value it returns becomes the value of the | |
| 1027 @code{condition-case} form; in this case, the @code{condition-case} has | |
| 1028 no effect. The @code{condition-case} form makes a difference when an | |
| 1029 error occurs during @var{protected-form}. | |
| 1030 | |
| 1031 Each of the @var{handlers} is a list of the form @code{(@var{conditions} | |
| 1032 @var{body}@dots{})}. Here @var{conditions} is an error condition name | |
| 1033 to be handled, or a list of condition names; @var{body} is one or more | |
| 1034 Lisp expressions to be executed when this handler handles an error. | |
| 1035 Here are examples of handlers: | |
| 1036 | |
| 1037 @smallexample | |
| 1038 @group | |
| 1039 (error nil) | |
| 1040 | |
| 1041 (arith-error (message "Division by zero")) | |
| 1042 | |
| 1043 ((arith-error file-error) | |
| 1044 (message | |
| 1045 "Either division by zero or failure to open a file")) | |
| 1046 @end group | |
| 1047 @end smallexample | |
| 1048 | |
| 1049 Each error that occurs has an @dfn{error symbol} that describes what | |
| 1050 kind of error it is. The @code{error-conditions} property of this | |
| 1051 symbol is a list of condition names (@pxref{Error Symbols}). Emacs | |
| 1052 searches all the active @code{condition-case} forms for a handler that | |
| 1053 specifies one or more of these condition names; the innermost matching | |
| 1054 @code{condition-case} handles the error. Within this | |
| 1055 @code{condition-case}, the first applicable handler handles the error. | |
| 1056 | |
| 1057 After executing the body of the handler, the @code{condition-case} | |
| 1058 returns normally, using the value of the last form in the handler body | |
| 1059 as the overall value. | |
| 1060 | |
| 1061 The argument @var{var} is a variable. @code{condition-case} does not | |
| 1062 bind this variable when executing the @var{protected-form}, only when it | |
| 1063 handles an error. At that time, it binds @var{var} locally to a list of | |
| 1064 the form @code{(@var{error-symbol} . @var{data})}, giving the | |
| 1065 particulars of the error. The handler can refer to this list to decide | |
| 1066 what to do. For example, if the error is for failure opening a file, | |
| 1067 the file name is the second element of @var{data}---the third element of | |
| 1068 @var{var}. | |
| 1069 | |
| 1070 If @var{var} is @code{nil}, that means no variable is bound. Then the | |
| 1071 error symbol and associated data are not available to the handler. | |
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1072 @end deffn |
| 0 | 1073 |
| 1074 @cindex @code{arith-error} example | |
| 1075 Here is an example of using @code{condition-case} to handle the error | |
| 1076 that results from dividing by zero. The handler prints out a warning | |
| 1077 message and returns a very large number. | |
| 1078 | |
| 1079 @smallexample | |
| 1080 @group | |
| 1081 (defun safe-divide (dividend divisor) | |
| 444 | 1082 (condition-case err |
| 0 | 1083 ;; @r{Protected form.} |
| 444 | 1084 (/ dividend divisor) |
| 0 | 1085 ;; @r{The handler.} |
| 1086 (arith-error ; @r{Condition.} | |
| 1087 (princ (format "Arithmetic error: %s" err)) | |
| 1088 1000000))) | |
| 1089 @result{} safe-divide | |
| 1090 @end group | |
| 1091 | |
| 1092 @group | |
| 1093 (safe-divide 5 0) | |
| 1094 @print{} Arithmetic error: (arith-error) | |
| 1095 @result{} 1000000 | |
| 1096 @end group | |
| 1097 @end smallexample | |
| 1098 | |
| 1099 @noindent | |
| 398 | 1100 The handler specifies condition name @code{arith-error} so that it will |
| 1101 handle only division-by-zero errors. Other kinds of errors will not be | |
| 1102 handled, at least not by this @code{condition-case}. Thus, | |
| 0 | 1103 |
| 1104 @smallexample | |
| 1105 @group | |
| 1106 (safe-divide nil 3) | |
| 1107 @error{} Wrong type argument: integer-or-marker-p, nil | |
| 1108 @end group | |
| 1109 @end smallexample | |
| 1110 | |
| 1111 Here is a @code{condition-case} that catches all kinds of errors, | |
| 1112 including those signaled with @code{error}: | |
| 1113 | |
| 1114 @smallexample | |
| 1115 @group | |
| 1116 (setq baz 34) | |
| 1117 @result{} 34 | |
| 1118 @end group | |
| 1119 | |
| 1120 @group | |
| 1121 (condition-case err | |
| 1122 (if (eq baz 35) | |
| 1123 t | |
| 1124 ;; @r{This is a call to the function @code{error}.} | |
| 1125 (error "Rats! The variable %s was %s, not 35" 'baz baz)) | |
| 1126 ;; @r{This is the handler; it is not a form.} | |
| 444 | 1127 (error (princ (format "The error was: %s" err)) |
| 0 | 1128 2)) |
| 1129 @print{} The error was: (error "Rats! The variable baz was 34, not 35") | |
| 1130 @result{} 2 | |
| 1131 @end group | |
| 1132 @end smallexample | |
| 1133 | |
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1134 @node Error Symbols, , Handling Errors, Errors |
| 0 | 1135 @subsubsection Error Symbols and Condition Names |
| 1136 @cindex error symbol | |
| 1137 @cindex error name | |
| 1138 @cindex condition name | |
| 1139 @cindex user-defined error | |
| 1140 @kindex error-conditions | |
| 1141 | |
| 1142 When you signal an error, you specify an @dfn{error symbol} to specify | |
| 1143 the kind of error you have in mind. Each error has one and only one | |
| 1144 error symbol to categorize it. This is the finest classification of | |
| 1145 errors defined by the XEmacs Lisp language. | |
| 1146 | |
| 1147 These narrow classifications are grouped into a hierarchy of wider | |
| 1148 classes called @dfn{error conditions}, identified by @dfn{condition | |
| 1149 names}. The narrowest such classes belong to the error symbols | |
| 1150 themselves: each error symbol is also a condition name. There are also | |
| 1151 condition names for more extensive classes, up to the condition name | |
| 1152 @code{error} which takes in all kinds of errors. Thus, each error has | |
| 1153 one or more condition names: @code{error}, the error symbol if that | |
| 1154 is distinct from @code{error}, and perhaps some intermediate | |
| 1155 classifications. | |
| 1156 | |
| 398 | 1157 In other words, each error condition @dfn{inherits} from another error |
| 1158 condition, with @code{error} sitting at the top of the inheritance | |
| 1159 hierarchy. | |
| 1160 | |
| 1161 @defun define-error error-symbol error-message &optional inherits-from | |
| 1162 This function defines a new error, denoted by @var{error-symbol}. | |
| 1163 @var{error-message} is an informative message explaining the error, and | |
| 1164 will be printed out when an unhandled error occurs. @var{error-symbol} | |
| 1165 is a sub-error of @var{inherits-from} (which defaults to @code{error}). | |
| 0 | 1166 |
| 398 | 1167 @code{define-error} internally works by putting on @var{error-symbol} |
| 1168 an @code{error-message} property whose value is @var{error-message}, and | |
| 1169 an @code{error-conditions} property that is a list of @var{error-symbol} | |
| 1170 followed by each of its super-errors, up to and including @code{error}. | |
| 1171 You will sometimes see code that sets this up directly rather than | |
| 1172 calling @code{define-error}, but you should @emph{not} do this yourself, | |
| 1173 unless you wish to maintain compatibility with FSF Emacs, which does not | |
| 1174 provide @code{define-error}. | |
| 1175 @end defun | |
| 0 | 1176 |
| 398 | 1177 Here is how we define a new error symbol, @code{new-error}, that |
| 1178 belongs to a range of errors called @code{my-own-errors}: | |
| 0 | 1179 |
| 1180 @example | |
| 1181 @group | |
| 398 | 1182 (define-error 'my-own-errors "A whole range of errors" 'error) |
| 1183 (define-error 'new-error "A new error" 'my-own-errors) | |
| 0 | 1184 @end group |
| 1185 @end example | |
| 1186 | |
| 1187 @noindent | |
| 398 | 1188 @code{new-error} has three condition names: @code{new-error}, the |
| 1189 narrowest classification; @code{my-own-errors}, which we imagine is a | |
| 1190 wider classification; and @code{error}, which is the widest of all. | |
| 1191 | |
| 1192 Note that it is not legal to try to define an error unless its | |
| 1193 super-error is also defined. For instance, attempting to define | |
| 1194 @code{new-error} before @code{my-own-errors} are defined will signal an | |
| 1195 error. | |
| 0 | 1196 |
| 1197 The error string should start with a capital letter but it should | |
| 1198 not end with a period. This is for consistency with the rest of Emacs. | |
| 398 | 1199 |
| 0 | 1200 Naturally, XEmacs will never signal @code{new-error} on its own; only |
| 1201 an explicit call to @code{signal} (@pxref{Signaling Errors}) in your | |
| 1202 code can do this: | |
| 1203 | |
| 1204 @example | |
| 1205 @group | |
| 1206 (signal 'new-error '(x y)) | |
| 1207 @error{} A new error: x, y | |
| 1208 @end group | |
| 1209 @end example | |
| 1210 | |
| 1211 This error can be handled through any of the three condition names. | |
| 1212 This example handles @code{new-error} and any other errors in the class | |
| 1213 @code{my-own-errors}: | |
| 1214 | |
| 1215 @example | |
| 1216 @group | |
| 1217 (condition-case foo | |
| 1218 (bar nil t) | |
| 1219 (my-own-errors nil)) | |
| 1220 @end group | |
| 1221 @end example | |
| 1222 | |
| 1223 The significant way that errors are classified is by their condition | |
| 1224 names---the names used to match errors with handlers. An error symbol | |
| 1225 serves only as a convenient way to specify the intended error message | |
| 1226 and list of condition names. It would be cumbersome to give | |
| 1227 @code{signal} a list of condition names rather than one error symbol. | |
| 1228 | |
| 1229 By contrast, using only error symbols without condition names would | |
| 1230 seriously decrease the power of @code{condition-case}. Condition names | |
| 1231 make it possible to categorize errors at various levels of generality | |
| 1232 when you write an error handler. Using error symbols alone would | |
| 1233 eliminate all but the narrowest level of classification. | |
| 1234 | |
| 444 | 1235 |
| 398 | 1236 |
| 0 | 1237 @xref{Standard Errors}, for a list of all the standard error symbols |
| 1238 and their conditions. | |
| 1239 | |
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1240 @node Cleanups, , Errors, Nonlocal Exits |
| 0 | 1241 @subsection Cleaning Up from Nonlocal Exits |
| 1242 | |
| 1243 The @code{unwind-protect} construct is essential whenever you | |
| 1244 temporarily put a data structure in an inconsistent state; it permits | |
| 1245 you to ensure the data are consistent in the event of an error or throw. | |
| 1246 | |
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1247 @deffn {Special Operator} unwind-protect body cleanup-forms@dots{} |
| 0 | 1248 @cindex cleanup forms |
| 1249 @cindex protected forms | |
| 1250 @cindex error cleanup | |
| 1251 @cindex unwinding | |
| 1252 @code{unwind-protect} executes the @var{body} with a guarantee that the | |
| 1253 @var{cleanup-forms} will be evaluated if control leaves @var{body}, no | |
| 1254 matter how that happens. The @var{body} may complete normally, or | |
| 1255 execute a @code{throw} out of the @code{unwind-protect}, or cause an | |
| 1256 error; in all cases, the @var{cleanup-forms} will be evaluated. | |
| 1257 | |
| 1258 If the @var{body} forms finish normally, @code{unwind-protect} returns | |
| 1259 the value of the last @var{body} form, after it evaluates the | |
| 1260 @var{cleanup-forms}. If the @var{body} forms do not finish, | |
| 1261 @code{unwind-protect} does not return any value in the normal sense. | |
| 1262 | |
| 1263 Only the @var{body} is actually protected by the @code{unwind-protect}. | |
| 1264 If any of the @var{cleanup-forms} themselves exits nonlocally (e.g., via | |
| 1265 a @code{throw} or an error), @code{unwind-protect} is @emph{not} | |
| 1266 guaranteed to evaluate the rest of them. If the failure of one of the | |
| 1267 @var{cleanup-forms} has the potential to cause trouble, then protect it | |
| 1268 with another @code{unwind-protect} around that form. | |
| 1269 | |
| 1270 The number of currently active @code{unwind-protect} forms counts, | |
| 1271 together with the number of local variable bindings, against the limit | |
| 1272 @code{max-specpdl-size} (@pxref{Local Variables}). | |
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1273 @end deffn |
| 0 | 1274 |
| 1275 For example, here we make an invisible buffer for temporary use, and | |
| 1276 make sure to kill it before finishing: | |
| 1277 | |
| 1278 @smallexample | |
| 1279 @group | |
| 1280 (save-excursion | |
| 1281 (let ((buffer (get-buffer-create " *temp*"))) | |
| 1282 (set-buffer buffer) | |
| 1283 (unwind-protect | |
| 1284 @var{body} | |
| 1285 (kill-buffer buffer)))) | |
| 1286 @end group | |
| 1287 @end smallexample | |
| 1288 | |
| 1289 @noindent | |
| 1290 You might think that we could just as well write @code{(kill-buffer | |
| 1291 (current-buffer))} and dispense with the variable @code{buffer}. | |
| 1292 However, the way shown above is safer, if @var{body} happens to get an | |
| 1293 error after switching to a different buffer! (Alternatively, you could | |
| 1294 write another @code{save-excursion} around the body, to ensure that the | |
| 1295 temporary buffer becomes current in time to kill it.) | |
| 1296 | |
| 1297 @findex ftp-login | |
| 1298 Here is an actual example taken from the file @file{ftp.el}. It | |
| 1299 creates a process (@pxref{Processes}) to try to establish a connection | |
| 1300 to a remote machine. As the function @code{ftp-login} is highly | |
| 1301 susceptible to numerous problems that the writer of the function cannot | |
| 1302 anticipate, it is protected with a form that guarantees deletion of the | |
| 1303 process in the event of failure. Otherwise, XEmacs might fill up with | |
| 1304 useless subprocesses. | |
| 1305 | |
| 1306 @smallexample | |
| 1307 @group | |
| 1308 (let ((win nil)) | |
| 1309 (unwind-protect | |
| 1310 (progn | |
| 1311 (setq process (ftp-setup-buffer host file)) | |
| 1312 (if (setq win (ftp-login process host user password)) | |
| 1313 (message "Logged in") | |
| 1314 (error "Ftp login failed"))) | |
| 1315 (or win (and process (delete-process process))))) | |
| 1316 @end group | |
| 1317 @end smallexample | |
| 1318 | |
| 1319 This example actually has a small bug: if the user types @kbd{C-g} to | |
| 1320 quit, and the quit happens immediately after the function | |
| 1321 @code{ftp-setup-buffer} returns but before the variable @code{process} is | |
| 1322 set, the process will not be killed. There is no easy way to fix this bug, | |
| 1323 but at least it is very unlikely. | |
| 1324 | |
| 1325 Here is another example which uses @code{unwind-protect} to make sure | |
| 1326 to kill a temporary buffer. In this example, the value returned by | |
| 1327 @code{unwind-protect} is used. | |
| 1328 | |
| 1329 @smallexample | |
| 1330 (defun shell-command-string (cmd) | |
| 1331 "Return the output of the shell command CMD, as a string." | |
| 1332 (save-excursion | |
| 1333 (set-buffer (generate-new-buffer " OS*cmd")) | |
| 1334 (shell-command cmd t) | |
| 1335 (unwind-protect | |
| 1336 (buffer-string) | |
| 1337 (kill-buffer (current-buffer))))) | |
| 1338 @end smallexample |