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| author | Marcus Crestani <marcus@crestani.de> |
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| date | Sat, 13 Dec 2014 14:20:17 +0100 |
| parents | 9fae6227ede5 |
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| rev | line source |
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| 428 | 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. |
| 428 | 4 @c See the file lispref.texi for copying conditions. |
| 5 @setfilename ../../info/eval.info | |
| 6 @node Evaluation, Control Structures, Symbols, Top | |
| 7 @chapter Evaluation | |
| 8 @cindex evaluation | |
| 9 @cindex interpreter | |
| 10 @cindex interpreter | |
| 11 @cindex value of expression | |
| 12 | |
| 13 The @dfn{evaluation} of expressions in XEmacs Lisp is performed by the | |
| 14 @dfn{Lisp interpreter}---a program that receives a Lisp object as input | |
| 15 and computes its @dfn{value as an expression}. How it does this depends | |
| 16 on the data type of the object, according to rules described in this | |
| 17 chapter. The interpreter runs automatically to evaluate portions of | |
| 18 your program, but can also be called explicitly via the Lisp primitive | |
| 19 function @code{eval}. | |
| 20 | |
| 21 @ifinfo | |
| 22 @menu | |
| 23 * Intro Eval:: Evaluation in the scheme of things. | |
| 24 * Eval:: How to invoke the Lisp interpreter explicitly. | |
| 25 * Forms:: How various sorts of objects are evaluated. | |
| 26 * Quoting:: Avoiding evaluation (to put constants in the program). | |
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27 * Multiple values:: Functions may return more than one result. |
| 428 | 28 @end menu |
| 29 | |
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30 @node Intro Eval, Eval, Evaluation, Evaluation |
| 428 | 31 @section Introduction to Evaluation |
| 32 | |
| 33 The Lisp interpreter, or evaluator, is the program that computes | |
| 444 | 34 the value of an expression that is given to it. When a function |
| 428 | 35 written in Lisp is called, the evaluator computes the value of the |
| 36 function by evaluating the expressions in the function body. Thus, | |
| 37 running any Lisp program really means running the Lisp interpreter. | |
| 38 | |
| 39 How the evaluator handles an object depends primarily on the data | |
| 40 type of the object. | |
| 41 @end ifinfo | |
| 42 | |
| 43 @cindex forms | |
| 44 @cindex expression | |
| 45 A Lisp object that is intended for evaluation is called an | |
| 46 @dfn{expression} or a @dfn{form}. The fact that expressions are data | |
| 47 objects and not merely text is one of the fundamental differences | |
| 48 between Lisp-like languages and typical programming languages. Any | |
| 49 object can be evaluated, but in practice only numbers, symbols, lists | |
| 50 and strings are evaluated very often. | |
| 51 | |
| 52 It is very common to read a Lisp expression and then evaluate the | |
| 53 expression, but reading and evaluation are separate activities, and | |
| 54 either can be performed alone. Reading per se does not evaluate | |
| 55 anything; it converts the printed representation of a Lisp object to the | |
| 56 object itself. It is up to the caller of @code{read} whether this | |
| 57 object is a form to be evaluated, or serves some entirely different | |
| 58 purpose. @xref{Input Functions}. | |
| 59 | |
| 60 Do not confuse evaluation with command key interpretation. The | |
| 61 editor command loop translates keyboard input into a command (an | |
| 62 interactively callable function) using the active keymaps, and then | |
| 63 uses @code{call-interactively} to invoke the command. The execution of | |
| 64 the command itself involves evaluation if the command is written in | |
| 65 Lisp, but that is not a part of command key interpretation itself. | |
| 66 @xref{Command Loop}. | |
| 67 | |
| 68 @cindex recursive evaluation | |
| 69 Evaluation is a recursive process. That is, evaluation of a form may | |
| 70 call @code{eval} to evaluate parts of the form. For example, evaluation | |
| 71 of a function call first evaluates each argument of the function call, | |
| 72 and then evaluates each form in the function body. Consider evaluation | |
| 73 of the form @code{(car x)}: the subform @code{x} must first be evaluated | |
| 74 recursively, so that its value can be passed as an argument to the | |
| 75 function @code{car}. | |
| 76 | |
| 77 Evaluation of a function call ultimately calls the function specified | |
| 2492 | 78 in it. @xref{Functions and Commands}. The execution of the function may itself work |
| 428 | 79 by evaluating the function definition; or the function may be a Lisp |
| 80 primitive implemented in C, or it may be a byte-compiled function | |
| 81 (@pxref{Byte Compilation}). | |
| 82 | |
| 83 @cindex environment | |
| 84 The evaluation of forms takes place in a context called the | |
| 85 @dfn{environment}, which consists of the current values and bindings of | |
| 86 all Lisp variables.@footnote{This definition of ``environment'' is | |
| 87 specifically not intended to include all the data that can affect the | |
| 88 result of a program.} Whenever the form refers to a variable without | |
| 89 creating a new binding for it, the value of the binding in the current | |
| 90 environment is used. @xref{Variables}. | |
| 91 | |
| 92 @cindex side effect | |
| 93 Evaluation of a form may create new environments for recursive | |
| 94 evaluation by binding variables (@pxref{Local Variables}). These | |
| 95 environments are temporary and vanish by the time evaluation of the form | |
| 96 is complete. The form may also make changes that persist; these changes | |
| 97 are called @dfn{side effects}. An example of a form that produces side | |
| 98 effects is @code{(setq foo 1)}. | |
| 99 | |
| 100 The details of what evaluation means for each kind of form are | |
| 101 described below (@pxref{Forms}). | |
| 102 | |
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103 @node Eval, Forms, Intro Eval, Evaluation |
| 428 | 104 @section Eval |
| 105 @c ??? Perhaps this should be the last section in the chapter. | |
| 106 | |
| 107 Most often, forms are evaluated automatically, by virtue of their | |
| 108 occurrence in a program being run. On rare occasions, you may need to | |
| 109 write code that evaluates a form that is computed at run time, such as | |
| 110 after reading a form from text being edited or getting one from a | |
| 111 property list. On these occasions, use the @code{eval} function. | |
| 112 | |
| 113 @strong{Please note:} it is generally cleaner and more flexible to call | |
| 114 functions that are stored in data structures, rather than to evaluate | |
| 115 expressions stored in data structures. Using functions provides the | |
| 116 ability to pass information to them as arguments. | |
| 117 | |
| 118 The functions and variables described in this section evaluate forms, | |
| 119 specify limits to the evaluation process, or record recently returned | |
| 120 values. Loading a file also does evaluation (@pxref{Loading}). | |
| 121 | |
| 122 @defun eval form | |
| 123 This is the basic function for performing evaluation. It evaluates | |
| 124 @var{form} in the current environment and returns the result. How the | |
| 125 evaluation proceeds depends on the type of the object (@pxref{Forms}). | |
| 126 | |
| 127 Since @code{eval} is a function, the argument expression that appears | |
| 128 in a call to @code{eval} is evaluated twice: once as preparation before | |
| 129 @code{eval} is called, and again by the @code{eval} function itself. | |
| 130 Here is an example: | |
| 131 | |
| 132 @example | |
| 133 @group | |
| 134 (setq foo 'bar) | |
| 135 @result{} bar | |
| 136 @end group | |
| 137 @group | |
| 138 (setq bar 'baz) | |
| 139 @result{} baz | |
| 140 ;; @r{@code{eval} receives argument @code{bar}, which is the value of @code{foo}} | |
| 141 (eval foo) | |
| 142 @result{} baz | |
| 143 (eval 'foo) | |
| 144 @result{} bar | |
| 145 @end group | |
| 146 @end example | |
| 147 | |
| 148 The number of currently active calls to @code{eval} is limited to | |
| 149 @code{max-lisp-eval-depth} (see below). | |
| 150 @end defun | |
| 151 | |
| 152 @deffn Command eval-region start end &optional stream | |
| 153 This function evaluates the forms in the current buffer in the region | |
| 154 defined by the positions @var{start} and @var{end}. It reads forms from | |
| 155 the region and calls @code{eval} on them until the end of the region is | |
| 156 reached, or until an error is signaled and not handled. | |
| 157 | |
| 158 If @var{stream} is supplied, @code{standard-output} is bound to it | |
| 159 during the evaluation. | |
| 160 | |
| 161 You can use the variable @code{load-read-function} to specify a function | |
| 162 for @code{eval-region} to use instead of @code{read} for reading | |
| 163 expressions. @xref{How Programs Do Loading}. | |
| 164 | |
| 165 @code{eval-region} always returns @code{nil}. | |
| 166 @end deffn | |
| 167 | |
| 168 @cindex evaluation of buffer contents | |
| 169 @deffn Command eval-buffer buffer &optional stream | |
| 170 This is like @code{eval-region} except that it operates on the whole | |
| 171 contents of @var{buffer}. | |
| 172 @end deffn | |
| 173 | |
| 174 @defvar max-lisp-eval-depth | |
| 175 This variable defines the maximum depth allowed in calls to @code{eval}, | |
| 176 @code{apply}, and @code{funcall} before an error is signaled (with error | |
| 177 message @code{"Lisp nesting exceeds max-lisp-eval-depth"}). This counts | |
| 178 internal uses of those functions, such as for calling the functions | |
| 179 mentioned in Lisp expressions, and recursive evaluation of function call | |
| 180 arguments and function body forms. | |
| 181 | |
| 182 This limit, with the associated error when it is exceeded, is one way | |
| 183 that Lisp avoids infinite recursion on an ill-defined function. | |
| 184 @cindex Lisp nesting error | |
| 185 | |
| 458 | 186 The default value of this variable is 1000. If you set it to a value |
| 428 | 187 less than 100, Lisp will reset it to 100 if the given value is reached. |
| 188 | |
| 189 @code{max-specpdl-size} provides another limit on nesting. | |
| 190 @xref{Local Variables}. | |
| 191 @end defvar | |
| 192 | |
| 193 @defvar values | |
| 194 The value of this variable is a list of the values returned by all the | |
| 195 expressions that were read from buffers (including the minibuffer), | |
| 196 evaluated, and printed. The elements are ordered most recent first. | |
| 197 | |
| 198 @example | |
| 199 @group | |
| 200 (setq x 1) | |
| 201 @result{} 1 | |
| 202 @end group | |
| 203 @group | |
| 204 (list 'A (1+ 2) auto-save-default) | |
| 205 @result{} (A 3 t) | |
| 206 @end group | |
| 207 @group | |
| 208 values | |
| 209 @result{} ((A 3 t) 1 @dots{}) | |
| 210 @end group | |
| 211 @end example | |
| 212 | |
| 213 This variable is useful for referring back to values of forms recently | |
| 214 evaluated. It is generally a bad idea to print the value of | |
| 215 @code{values} itself, since this may be very long. Instead, examine | |
| 216 particular elements, like this: | |
| 217 | |
| 218 @example | |
| 219 @group | |
| 220 ;; @r{Refer to the most recent evaluation result.} | |
| 221 (nth 0 values) | |
| 222 @result{} (A 3 t) | |
| 223 @end group | |
| 224 @group | |
| 225 ;; @r{That put a new element on,} | |
| 226 ;; @r{so all elements move back one.} | |
| 227 (nth 1 values) | |
| 228 @result{} (A 3 t) | |
| 229 @end group | |
| 230 @group | |
| 231 ;; @r{This gets the element that was next-to-most-recent} | |
| 232 ;; @r{before this example.} | |
| 233 (nth 3 values) | |
| 234 @result{} 1 | |
| 235 @end group | |
| 236 @end example | |
| 237 @end defvar | |
| 238 | |
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239 @node Forms, Quoting, Eval, Evaluation |
| 428 | 240 @section Kinds of Forms |
| 241 | |
| 242 A Lisp object that is intended to be evaluated is called a @dfn{form}. | |
| 243 How XEmacs evaluates a form depends on its data type. XEmacs has three | |
| 244 different kinds of form that are evaluated differently: symbols, lists, | |
| 245 and ``all other types''. This section describes all three kinds, | |
| 246 starting with ``all other types'' which are self-evaluating forms. | |
| 247 | |
| 248 @menu | |
| 249 * Self-Evaluating Forms:: Forms that evaluate to themselves. | |
| 250 * Symbol Forms:: Symbols evaluate as variables. | |
| 251 * Classifying Lists:: How to distinguish various sorts of list forms. | |
| 252 * Function Indirection:: When a symbol appears as the car of a list, | |
| 253 we find the real function via the symbol. | |
| 254 * Function Forms:: Forms that call functions. | |
| 255 * Macro Forms:: Forms that call macros. | |
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256 * Special Operators:: ``Special operators'' are idiosyncratic primitives, |
| 428 | 257 most of them extremely important. |
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258 Also known as special forms. |
| 428 | 259 * Autoloading:: Functions set up to load files |
| 260 containing their real definitions. | |
| 261 @end menu | |
| 262 | |
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263 @node Self-Evaluating Forms, Symbol Forms, Forms, Forms |
| 428 | 264 @subsection Self-Evaluating Forms |
| 265 @cindex vector evaluation | |
| 266 @cindex literal evaluation | |
| 267 @cindex self-evaluating form | |
| 268 | |
| 269 A @dfn{self-evaluating form} is any form that is not a list or symbol. | |
| 270 Self-evaluating forms evaluate to themselves: the result of evaluation | |
| 271 is the same object that was evaluated. Thus, the number 25 evaluates to | |
| 272 25, and the string @code{"foo"} evaluates to the string @code{"foo"}. | |
| 273 Likewise, evaluation of a vector does not cause evaluation of the | |
| 274 elements of the vector---it returns the same vector with its contents | |
| 275 unchanged. | |
| 276 | |
| 277 @example | |
| 278 @group | |
| 279 '123 ; @r{An object, shown without evaluation.} | |
| 280 @result{} 123 | |
| 281 @end group | |
| 282 @group | |
| 283 123 ; @r{Evaluated as usual---result is the same.} | |
| 284 @result{} 123 | |
| 285 @end group | |
| 286 @group | |
| 287 (eval '123) ; @r{Evaluated ``by hand''---result is the same.} | |
| 288 @result{} 123 | |
| 289 @end group | |
| 290 @group | |
| 291 (eval (eval '123)) ; @r{Evaluating twice changes nothing.} | |
| 292 @result{} 123 | |
| 293 @end group | |
| 294 @end example | |
| 295 | |
| 296 It is common to write numbers, characters, strings, and even vectors | |
| 297 in Lisp code, taking advantage of the fact that they self-evaluate. | |
| 298 However, it is quite unusual to do this for types that lack a read | |
| 299 syntax, because there's no way to write them textually. It is possible | |
| 300 to construct Lisp expressions containing these types by means of a Lisp | |
| 301 program. Here is an example: | |
| 302 | |
| 303 @example | |
| 304 @group | |
| 305 ;; @r{Build an expression containing a buffer object.} | |
| 306 (setq buffer (list 'print (current-buffer))) | |
| 307 @result{} (print #<buffer eval.texi>) | |
| 308 @end group | |
| 309 @group | |
| 310 ;; @r{Evaluate it.} | |
| 311 (eval buffer) | |
| 312 @print{} #<buffer eval.texi> | |
| 313 @result{} #<buffer eval.texi> | |
| 314 @end group | |
| 315 @end example | |
| 316 | |
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317 @node Symbol Forms, Classifying Lists, Self-Evaluating Forms, Forms |
| 428 | 318 @subsection Symbol Forms |
| 319 @cindex symbol evaluation | |
| 320 | |
| 321 When a symbol is evaluated, it is treated as a variable. The result | |
| 322 is the variable's value, if it has one. If it has none (if its value | |
| 323 cell is void), an error is signaled. For more information on the use of | |
| 324 variables, see @ref{Variables}. | |
| 325 | |
| 326 In the following example, we set the value of a symbol with | |
| 327 @code{setq}. Then we evaluate the symbol, and get back the value that | |
| 328 @code{setq} stored. | |
| 329 | |
| 330 @example | |
| 331 @group | |
| 332 (setq a 123) | |
| 333 @result{} 123 | |
| 334 @end group | |
| 335 @group | |
| 336 (eval 'a) | |
| 337 @result{} 123 | |
| 338 @end group | |
| 339 @group | |
| 340 a | |
| 341 @result{} 123 | |
| 342 @end group | |
| 343 @end example | |
| 344 | |
| 345 The symbols @code{nil} and @code{t} are treated specially, so that the | |
| 346 value of @code{nil} is always @code{nil}, and the value of @code{t} is | |
| 347 always @code{t}; you cannot set or bind them to any other values. Thus, | |
| 348 these two symbols act like self-evaluating forms, even though | |
| 349 @code{eval} treats them like any other symbol. | |
| 350 | |
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351 @node Classifying Lists, Function Indirection, Symbol Forms, Forms |
| 428 | 352 @subsection Classification of List Forms |
| 353 @cindex list form evaluation | |
| 354 | |
| 355 A form that is a nonempty list is either a function call, a macro | |
| 356 call, or a special form, according to its first element. These three | |
| 357 kinds of forms are evaluated in different ways, described below. The | |
| 358 remaining list elements constitute the @dfn{arguments} for the function, | |
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359 macro, or special operator. |
| 428 | 360 |
| 361 The first step in evaluating a nonempty list is to examine its first | |
| 362 element. This element alone determines what kind of form the list is | |
| 363 and how the rest of the list is to be processed. The first element is | |
| 364 @emph{not} evaluated, as it would be in some Lisp dialects such as | |
| 365 Scheme. | |
| 366 | |
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367 @node Function Indirection, Function Forms, Classifying Lists, Forms |
| 428 | 368 @subsection Symbol Function Indirection |
| 369 @cindex symbol function indirection | |
| 370 @cindex indirection | |
| 371 @cindex void function | |
| 372 | |
| 373 If the first element of the list is a symbol then evaluation examines | |
| 374 the symbol's function cell, and uses its contents instead of the | |
| 375 original symbol. If the contents are another symbol, this process, | |
| 376 called @dfn{symbol function indirection}, is repeated until it obtains a | |
| 377 non-symbol. @xref{Function Names}, for more information about using a | |
| 378 symbol as a name for a function stored in the function cell of the | |
| 379 symbol. | |
| 380 | |
| 381 One possible consequence of this process is an infinite loop, in the | |
| 382 event that a symbol's function cell refers to the same symbol. Or a | |
| 383 symbol may have a void function cell, in which case the subroutine | |
| 384 @code{symbol-function} signals a @code{void-function} error. But if | |
| 385 neither of these things happens, we eventually obtain a non-symbol, | |
| 386 which ought to be a function or other suitable object. | |
| 387 | |
| 388 @kindex invalid-function | |
| 389 @cindex invalid function | |
| 390 More precisely, we should now have a Lisp function (a lambda | |
| 391 expression), a byte-code function, a primitive function, a Lisp macro, a | |
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392 special operator, or an autoload object. Each of these types is a case |
| 428 | 393 described in one of the following sections. If the object is not one of |
| 394 these types, the error @code{invalid-function} is signaled. | |
| 395 | |
| 396 The following example illustrates the symbol indirection process. We | |
| 397 use @code{fset} to set the function cell of a symbol and | |
| 398 @code{symbol-function} to get the function cell contents | |
| 399 (@pxref{Function Cells}). Specifically, we store the symbol @code{car} | |
| 400 into the function cell of @code{first}, and the symbol @code{first} into | |
| 401 the function cell of @code{erste}. | |
| 402 | |
| 403 @smallexample | |
| 404 @group | |
| 405 ;; @r{Build this function cell linkage:} | |
| 406 ;; ------------- ----- ------- ------- | |
| 407 ;; | #<subr car> | <-- | car | <-- | first | <-- | erste | | |
| 408 ;; ------------- ----- ------- ------- | |
| 409 @end group | |
| 410 @end smallexample | |
| 411 | |
| 412 @smallexample | |
| 413 @group | |
| 414 (symbol-function 'car) | |
| 415 @result{} #<subr car> | |
| 416 @end group | |
| 417 @group | |
| 418 (fset 'first 'car) | |
| 419 @result{} car | |
| 420 @end group | |
| 421 @group | |
| 422 (fset 'erste 'first) | |
| 423 @result{} first | |
| 424 @end group | |
| 425 @group | |
| 426 (erste '(1 2 3)) ; @r{Call the function referenced by @code{erste}.} | |
| 427 @result{} 1 | |
| 428 @end group | |
| 429 @end smallexample | |
| 430 | |
| 431 By contrast, the following example calls a function without any symbol | |
| 432 function indirection, because the first element is an anonymous Lisp | |
| 433 function, not a symbol. | |
| 434 | |
| 435 @smallexample | |
| 436 @group | |
| 437 ((lambda (arg) (erste arg)) | |
| 444 | 438 '(1 2 3)) |
| 428 | 439 @result{} 1 |
| 440 @end group | |
| 441 @end smallexample | |
| 442 | |
| 443 @noindent | |
| 444 Executing the function itself evaluates its body; this does involve | |
| 445 symbol function indirection when calling @code{erste}. | |
| 446 | |
| 447 The built-in function @code{indirect-function} provides an easy way to | |
| 448 perform symbol function indirection explicitly. | |
| 449 | |
| 444 | 450 @defun indirect-function object |
| 451 This function returns the meaning of @var{object} as a function. If | |
| 452 @var{object} is a symbol, then it finds @var{object}'s function | |
| 453 definition and starts over with that value. If @var{object} is not a | |
| 454 symbol, then it returns @var{object} itself. | |
| 428 | 455 |
| 456 Here is how you could define @code{indirect-function} in Lisp: | |
| 457 | |
| 458 @smallexample | |
| 459 (defun indirect-function (function) | |
| 460 (if (symbolp function) | |
| 461 (indirect-function (symbol-function function)) | |
| 462 function)) | |
| 463 @end smallexample | |
| 464 @end defun | |
| 465 | |
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466 @node Function Forms, Macro Forms, Function Indirection, Forms |
| 428 | 467 @subsection Evaluation of Function Forms |
| 468 @cindex function form evaluation | |
| 469 @cindex function call | |
| 470 | |
| 471 If the first element of a list being evaluated is a Lisp function | |
| 472 object, byte-code object or primitive function object, then that list is | |
| 473 a @dfn{function call}. For example, here is a call to the function | |
| 474 @code{+}: | |
| 475 | |
| 476 @example | |
| 477 (+ 1 x) | |
| 478 @end example | |
| 479 | |
| 480 The first step in evaluating a function call is to evaluate the | |
| 481 remaining elements of the list from left to right. The results are the | |
| 482 actual argument values, one value for each list element. The next step | |
| 483 is to call the function with this list of arguments, effectively using | |
| 484 the function @code{apply} (@pxref{Calling Functions}). If the function | |
| 485 is written in Lisp, the arguments are used to bind the argument | |
| 486 variables of the function (@pxref{Lambda Expressions}); then the forms | |
| 487 in the function body are evaluated in order, and the value of the last | |
| 488 body form becomes the value of the function call. | |
| 489 | |
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490 @node Macro Forms, Special Operators, Function Forms, Forms |
| 428 | 491 @subsection Lisp Macro Evaluation |
| 492 @cindex macro call evaluation | |
| 493 | |
| 494 If the first element of a list being evaluated is a macro object, then | |
| 495 the list is a @dfn{macro call}. When a macro call is evaluated, the | |
| 496 elements of the rest of the list are @emph{not} initially evaluated. | |
| 497 Instead, these elements themselves are used as the arguments of the | |
| 498 macro. The macro definition computes a replacement form, called the | |
| 499 @dfn{expansion} of the macro, to be evaluated in place of the original | |
| 500 form. The expansion may be any sort of form: a self-evaluating | |
| 501 constant, a symbol, or a list. If the expansion is itself a macro call, | |
| 502 this process of expansion repeats until some other sort of form results. | |
| 503 | |
| 504 Ordinary evaluation of a macro call finishes by evaluating the | |
| 505 expansion. However, the macro expansion is not necessarily evaluated | |
| 506 right away, or at all, because other programs also expand macro calls, | |
| 507 and they may or may not evaluate the expansions. | |
| 508 | |
| 509 Normally, the argument expressions are not evaluated as part of | |
| 510 computing the macro expansion, but instead appear as part of the | |
| 511 expansion, so they are computed when the expansion is computed. | |
| 512 | |
| 513 For example, given a macro defined as follows: | |
| 514 | |
| 515 @example | |
| 516 @group | |
| 517 (defmacro cadr (x) | |
| 518 (list 'car (list 'cdr x))) | |
| 519 @end group | |
| 520 @end example | |
| 521 | |
| 522 @noindent | |
| 523 an expression such as @code{(cadr (assq 'handler list))} is a macro | |
| 524 call, and its expansion is: | |
| 525 | |
| 526 @example | |
| 527 (car (cdr (assq 'handler list))) | |
| 528 @end example | |
| 529 | |
| 530 @noindent | |
| 531 Note that the argument @code{(assq 'handler list)} appears in the | |
| 532 expansion. | |
| 533 | |
| 534 @xref{Macros}, for a complete description of XEmacs Lisp macros. | |
| 535 | |
|
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536 @node Special Operators, Autoloading, Macro Forms, Forms |
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537 @subsection Special Operators |
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538 @cindex special operator evaluation |
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539 @cindex special form |
| 428 | 540 |
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541 A @dfn{special operator} (historically, and less logically, a |
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542 @dfn{special form}) is a primitive function specially marked so that |
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543 its arguments are not all evaluated. Most special operators define control |
| 428 | 544 structures or perform variable bindings---things which functions cannot |
| 545 do. | |
| 546 | |
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547 Each special operator has its own rules for which arguments are evaluated |
| 428 | 548 and which are used without evaluation. Whether a particular argument is |
| 549 evaluated may depend on the results of evaluating other arguments. | |
| 550 | |
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551 Here is a list, in alphabetical order, of all of the special operators in |
| 428 | 552 XEmacs Lisp with a reference to where each is described. |
| 553 | |
| 554 @table @code | |
| 555 @item and | |
| 556 @pxref{Combining Conditions} | |
| 557 | |
| 558 @item catch | |
| 559 @pxref{Catch and Throw} | |
| 560 | |
| 561 @item cond | |
| 562 @pxref{Conditionals} | |
| 563 | |
| 564 @item condition-case | |
| 565 @pxref{Handling Errors} | |
| 566 | |
| 567 @item defconst | |
| 568 @pxref{Defining Variables} | |
| 569 | |
| 570 @item defmacro | |
| 571 @pxref{Defining Macros} | |
| 572 | |
| 573 @item defun | |
| 574 @pxref{Defining Functions} | |
| 575 | |
| 576 @item defvar | |
| 577 @pxref{Defining Variables} | |
| 578 | |
| 579 @item function | |
| 580 @pxref{Anonymous Functions} | |
| 581 | |
| 582 @item if | |
| 583 @pxref{Conditionals} | |
| 584 | |
| 585 @item interactive | |
| 586 @pxref{Interactive Call} | |
| 587 | |
| 588 @item let | |
| 589 @itemx let* | |
| 590 @pxref{Local Variables} | |
| 591 | |
| 592 @item or | |
| 593 @pxref{Combining Conditions} | |
| 594 | |
| 595 @item prog1 | |
| 596 @itemx prog2 | |
| 597 @itemx progn | |
| 598 @pxref{Sequencing} | |
| 599 | |
| 600 @item quote | |
| 601 @pxref{Quoting} | |
| 602 | |
| 603 @item save-current-buffer | |
| 604 @pxref{Excursions} | |
| 605 | |
| 606 @item save-excursion | |
| 607 @pxref{Excursions} | |
| 608 | |
| 609 @item save-restriction | |
| 610 @pxref{Narrowing} | |
| 611 | |
| 612 @item save-selected-window | |
| 613 @pxref{Excursions} | |
| 614 | |
| 615 @item save-window-excursion | |
| 616 @pxref{Window Configurations} | |
| 617 | |
| 618 @item setq | |
| 619 @pxref{Setting Variables} | |
| 620 | |
| 621 @item setq-default | |
| 622 @pxref{Creating Buffer-Local} | |
| 623 | |
| 624 @item unwind-protect | |
| 625 @pxref{Nonlocal Exits} | |
| 626 | |
| 627 @item while | |
| 628 @pxref{Iteration} | |
| 629 | |
| 630 @item with-output-to-temp-buffer | |
| 631 @pxref{Temporary Displays} | |
| 632 @end table | |
| 633 | |
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634 @cindex CL note---special operators compared |
| 428 | 635 @quotation |
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636 @b{Common Lisp note:} here are some comparisons of special operators in |
| 428 | 637 XEmacs Lisp and Common Lisp. @code{setq}, @code{if}, and |
|
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638 @code{catch} are special operators in both XEmacs Lisp and Common Lisp. |
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639 @code{defun} is a special operator in XEmacs Lisp, but a macro in Common |
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640 Lisp. @code{save-excursion} is a special operator in XEmacs Lisp, but |
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641 doesn't exist in Common Lisp. @code{throw} is a special operator in |
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642 both Common Lisp and XEmacs Lisp (because it must be able to throw |
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643 multiple values).@refill |
| 428 | 644 @end quotation |
| 645 | |
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646 @node Autoloading, , Special Operators, Forms |
| 428 | 647 @subsection Autoloading |
| 648 | |
| 649 The @dfn{autoload} feature allows you to call a function or macro | |
| 650 whose function definition has not yet been loaded into XEmacs. It | |
| 651 specifies which file contains the definition. When an autoload object | |
| 652 appears as a symbol's function definition, calling that symbol as a | |
| 653 function automatically loads the specified file; then it calls the real | |
| 654 definition loaded from that file. @xref{Autoload}. | |
| 655 | |
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656 @node Quoting, Multiple values, Forms, Evaluation |
| 428 | 657 @section Quoting |
| 658 @cindex quoting | |
| 659 | |
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660 The special operator @code{quote} returns its single argument, as written, |
| 428 | 661 without evaluating it. This provides a way to include constant symbols |
| 662 and lists, which are not self-evaluating objects, in a program. (It is | |
| 663 not necessary to quote self-evaluating objects such as numbers, strings, | |
| 664 and vectors.) | |
| 665 | |
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666 @deffn {Special Operator} quote object |
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667 This special operator returns @var{object}, without evaluating it. |
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668 @end deffn |
| 428 | 669 |
| 670 @cindex @samp{'} for quoting | |
| 671 @cindex quoting using apostrophe | |
| 672 @cindex apostrophe for quoting | |
| 673 Because @code{quote} is used so often in programs, Lisp provides a | |
| 674 convenient read syntax for it. An apostrophe character (@samp{'}) | |
| 675 followed by a Lisp object (in read syntax) expands to a list whose first | |
| 676 element is @code{quote}, and whose second element is the object. Thus, | |
| 677 the read syntax @code{'x} is an abbreviation for @code{(quote x)}. | |
| 678 | |
| 679 Here are some examples of expressions that use @code{quote}: | |
| 680 | |
| 681 @example | |
| 682 @group | |
| 683 (quote (+ 1 2)) | |
| 684 @result{} (+ 1 2) | |
| 685 @end group | |
| 686 @group | |
| 687 (quote foo) | |
| 688 @result{} foo | |
| 689 @end group | |
| 690 @group | |
| 691 'foo | |
| 692 @result{} foo | |
| 693 @end group | |
| 694 @group | |
| 695 ''foo | |
| 696 @result{} (quote foo) | |
| 697 @end group | |
| 698 @group | |
| 699 '(quote foo) | |
| 700 @result{} (quote foo) | |
| 701 @end group | |
| 702 @group | |
| 703 ['foo] | |
| 704 @result{} [(quote foo)] | |
| 705 @end group | |
| 706 @end example | |
| 707 | |
| 708 Other quoting constructs include @code{function} (@pxref{Anonymous | |
| 709 Functions}), which causes an anonymous lambda expression written in Lisp | |
| 710 to be compiled, and @samp{`} (@pxref{Backquote}), which is used to quote | |
| 711 only part of a list, while computing and substituting other parts. | |
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712 |
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713 @node Multiple values, , Quoting, Evaluation |
|
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714 @section Multiple values |
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715 @cindex multiple values |
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716 |
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717 @noindent |
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718 Under XEmacs, expressions can return zero or more results, using the |
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719 @code{values} and @code{values-list} functions. Results other than the |
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720 first are typically discarded, but special operators are provided to |
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721 access them. |
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722 |
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723 @defun values arguments@dots{} |
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724 This function returns @var{arguments} as multiple values. Callers will |
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725 always receive the first element of @var{arguments}, but must use |
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726 various special operators, described below, to access other elements of |
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727 @var{arguments}. |
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728 |
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729 The idiom @code{(values (function-call argument))}, with one |
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730 argument, is the normal mechanism to avoid passing multiple values to |
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731 the calling form where that is not desired. |
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732 |
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733 XEmacs implements the Common Lisp specification when it comes to the |
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734 exact details of when to discard and when to preserve multiple values; |
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735 see Common Lisp the Language or the Common Lisp hyperspec for more |
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736 details. The most important thing to keep in mind is when multiple |
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737 values are passed as an argument to a function, all but the first are |
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738 discarded. |
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739 @end defun |
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740 |
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741 @defun values-list argument |
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742 This function returns the elements of the lst @var{argument} as multiple |
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743 values. |
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744 @end defun |
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745 |
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746 @defmac multiple-value-bind (var@dots{}) values-form forms@dots{} |
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747 This macro evaluates @var{values-form}, which may return |
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748 multiple values. It then binds the @var{var}s to these respective values, |
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749 as if by @code{let}, and then executes the body @var{forms}. |
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750 If there are more @var{var}s than values, the extra @var{var}s |
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751 are bound to @code{nil}. If there are fewer @var{var}s than |
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752 values, the excess values are ignored. |
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753 @end defmac |
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754 |
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755 @defmac multiple-value-setq (var@dots{}) form |
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756 This macro evaluates @var{form}, which may return multiple |
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757 values. It then sets the @var{var}s to these respective values, as if by |
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758 @code{setq}. Extra @var{var}s or values are treated the same as |
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759 in @code{multiple-value-bind}. |
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760 @end defmac |
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761 |
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762 @deffn {Special Operator} multiple-value-call function forms@dots{} |
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763 This special operator evaluates function, discarding any multiple |
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764 values. It then evaluates @var{forms}, preserving any multiple values, |
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765 and calls @var{function} as a function with the results. Conceptually, this |
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766 function is a version of @code{apply'}that by-passes the multiple values |
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767 infrastructure, treating multiple values as intercalated lists. |
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768 @end deffn |
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769 |
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770 @defmac multiple-value-list form |
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771 This macro evaluates @var{form} and returns a list of the |
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772 multiple values given by it. |
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773 @end defmac |
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774 |
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775 @deffn {Special Operator} multiple-value-prog1 first body@dots{} |
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776 This special operator evaluates the form @var{first}, then the |
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777 forms @var{body}. It returns the value given by @var{first}, preserving |
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778 any multiple values. This is identical to @code{prog1}, except that |
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779 @code{prog1} always discards multiple values. |
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780 @end deffn |
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781 |
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782 @defmac nth-value n form |
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783 This macro evaluates @var{form} and returns the @var{n}th |
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784 value it gave. @var{n} must be an integer of value zero or more. |
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785 If @var{form} gave insufficient multiple values, @code{nth-value} |
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786 returns @code{nil}. |
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787 @end defmac |
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788 |
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789 @defvar multiple-values-limit |
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790 This constant describes the exclusive upper bound on the number of |
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791 multiple values that @code{values} accepts and that |
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792 @code{multiple-value-bind}, etc. will consume. |
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793 @end defvar |
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794 |
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795 To take full advantage of multiple values, Emacs Lisp code must have |
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796 been compiled by XEmacs 21.5 or later, which is not yet true of the |
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797 XEmacs packages. Matched @code{values} and @code{multiple-value-bind} |
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798 calls will work in code included in the XEmacs packages when run on |
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799 21.5, though the following incantation may be necessary at the start of |
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800 your file, until appropriate code is included in XEmacs 21.4: |
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801 |
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802 @example |
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803 (eval-when-compile (when (eq 'list (symbol-function 'values)) |
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804 (define-compiler-macro values (&rest args) |
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805 (cons 'list args)) |
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806 (define-compiler-macro values-list (list) list))) |
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807 @end example |
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808 |
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809 Such code cannot, unfortunately, rely on XEmacs to discard multiple |
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810 values where that is appropriate. |