Mercurial > hg > xemacs-beta
annotate man/lispref/streams.texi @ 5889:bd644055ef44
Correct a bug in #'check-type, non-setf'able PLACEs
lisp/ChangeLog addition:
2015-04-11 Aidan Kehoe <kehoea@parhasard.net>
* cl-macs.el:
* cl-macs.el (check-type):
Correct the sense of the type test here when PLACE is not
setf'able, something which gave confusing errors with literal
fixnums or, e.g., (+ 30 40).
tests/ChangeLog addition:
2015-04-11 Aidan Kehoe <kehoea@parhasard.net>
* automated/lisp-tests.el:
Check for a bug just fixed in cl-macs.el.
| author | Aidan Kehoe <kehoea@parhasard.net> |
|---|---|
| date | Sat, 11 Apr 2015 18:06:17 +0100 |
| parents | 9fae6227ede5 |
| children |
| rev | line source |
|---|---|
| 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/streams.info | |
| 6 @node Read and Print, Minibuffers, Debugging, Top | |
| 7 @chapter Reading and Printing Lisp Objects | |
| 8 | |
| 9 @dfn{Printing} and @dfn{reading} are the operations of converting Lisp | |
| 10 objects to textual form and vice versa. They use the printed | |
| 11 representations and read syntax described in @ref{Lisp Data Types}. | |
| 12 | |
| 13 This chapter describes the Lisp functions for reading and printing. | |
| 14 It also describes @dfn{streams}, which specify where to get the text (if | |
| 15 reading) or where to put it (if printing). | |
| 16 | |
| 17 @menu | |
| 18 * Streams Intro:: Overview of streams, reading and printing. | |
| 19 * Input Streams:: Various data types that can be used as input streams. | |
| 20 * Input Functions:: Functions to read Lisp objects from text. | |
| 21 * Output Streams:: Various data types that can be used as output streams. | |
| 22 * Output Functions:: Functions to print Lisp objects as text. | |
| 23 * Output Variables:: Variables that control what the printing functions do. | |
| 24 @end menu | |
| 25 | |
|
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26 @node Streams Intro, Input Streams, Read and Print, Read and Print |
| 428 | 27 @section Introduction to Reading and Printing |
| 28 @cindex Lisp reader | |
| 29 @cindex printing | |
| 30 @cindex reading | |
| 31 | |
| 32 @dfn{Reading} a Lisp object means parsing a Lisp expression in textual | |
| 33 form and producing a corresponding Lisp object. This is how Lisp | |
| 34 programs get into Lisp from files of Lisp code. We call the text the | |
| 35 @dfn{read syntax} of the object. For example, the text @samp{(a .@: 5)} | |
| 36 is the read syntax for a cons cell whose @sc{car} is @code{a} and whose | |
| 37 @sc{cdr} is the number 5. | |
| 38 | |
| 39 @dfn{Printing} a Lisp object means producing text that represents that | |
| 40 object---converting the object to its printed representation. Printing | |
| 41 the cons cell described above produces the text @samp{(a .@: 5)}. | |
| 42 | |
| 43 Reading and printing are more or less inverse operations: printing the | |
| 44 object that results from reading a given piece of text often produces | |
| 45 the same text, and reading the text that results from printing an object | |
| 46 usually produces a similar-looking object. For example, printing the | |
| 47 symbol @code{foo} produces the text @samp{foo}, and reading that text | |
| 48 returns the symbol @code{foo}. Printing a list whose elements are | |
| 49 @code{a} and @code{b} produces the text @samp{(a b)}, and reading that | |
| 50 text produces a list (but not the same list) with elements @code{a} | |
| 51 and @code{b}. | |
| 52 | |
| 53 However, these two operations are not precisely inverses. There are | |
| 54 three kinds of exceptions: | |
| 55 | |
| 56 @itemize @bullet | |
| 57 @item | |
| 58 Printing can produce text that cannot be read. For example, buffers, | |
| 59 windows, frames, subprocesses and markers print into text that starts | |
| 60 with @samp{#}; if you try to read this text, you get an error. There is | |
| 61 no way to read those data types. | |
| 62 | |
| 63 @item | |
| 64 One object can have multiple textual representations. For example, | |
| 65 @samp{1} and @samp{01} represent the same integer, and @samp{(a b)} and | |
| 66 @samp{(a .@: (b))} represent the same list. Reading will accept any of | |
| 67 the alternatives, but printing must choose one of them. | |
| 68 | |
| 69 @item | |
| 70 Comments can appear at certain points in the middle of an object's | |
| 71 read sequence without affecting the result of reading it. | |
| 72 @end itemize | |
| 73 | |
|
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74 @node Input Streams, Input Functions, Streams Intro, Read and Print |
| 428 | 75 @section Input Streams |
| 76 @cindex stream (for reading) | |
| 77 @cindex input stream | |
| 78 | |
| 79 Most of the Lisp functions for reading text take an @dfn{input stream} | |
| 80 as an argument. The input stream specifies where or how to get the | |
| 81 characters of the text to be read. Here are the possible types of input | |
| 82 stream: | |
| 83 | |
| 84 @table @asis | |
| 85 @item @var{buffer} | |
| 86 @cindex buffer input stream | |
| 87 The input characters are read from @var{buffer}, starting with the | |
| 88 character directly after point. Point advances as characters are read. | |
| 89 | |
| 90 @item @var{marker} | |
| 91 @cindex marker input stream | |
| 92 The input characters are read from the buffer that @var{marker} is in, | |
| 93 starting with the character directly after the marker. The marker | |
| 94 position advances as characters are read. The value of point in the | |
| 95 buffer has no effect when the stream is a marker. | |
| 96 | |
| 97 @item @var{string} | |
| 98 @cindex string input stream | |
| 99 The input characters are taken from @var{string}, starting at the first | |
| 100 character in the string and using as many characters as required. | |
| 101 | |
| 102 @item @var{function} | |
| 103 @cindex function input stream | |
| 104 The input characters are generated by @var{function}, one character per | |
| 105 call. Normally @var{function} is called with no arguments, and should | |
| 106 return a character. | |
| 107 | |
| 108 @cindex unreading | |
| 109 Occasionally @var{function} is called with one argument (always a | |
| 110 character). When that happens, @var{function} should save the argument | |
| 111 and arrange to return it on the next call. This is called | |
| 112 @dfn{unreading} the character; it happens when the Lisp reader reads one | |
| 113 character too many and wants to ``put it back where it came from''. | |
| 114 | |
| 115 @item @code{t} | |
| 116 @cindex @code{t} input stream | |
| 117 @code{t} used as a stream means that the input is read from the | |
| 118 minibuffer. In fact, the minibuffer is invoked once and the text | |
| 119 given by the user is made into a string that is then used as the | |
| 120 input stream. | |
| 121 | |
| 122 @item @code{nil} | |
| 123 @cindex @code{nil} input stream | |
| 124 @code{nil} supplied as an input stream means to use the value of | |
| 125 @code{standard-input} instead; that value is the @dfn{default input | |
| 126 stream}, and must be a non-@code{nil} input stream. | |
| 127 | |
| 128 @item @var{symbol} | |
| 129 A symbol as input stream is equivalent to the symbol's function | |
| 130 definition (if any). | |
| 131 @end table | |
| 132 | |
| 133 Here is an example of reading from a stream that is a buffer, showing | |
| 134 where point is located before and after: | |
| 135 | |
| 136 @example | |
| 137 @group | |
| 138 ---------- Buffer: foo ---------- | |
| 139 This@point{} is the contents of foo. | |
| 140 ---------- Buffer: foo ---------- | |
| 141 @end group | |
| 142 | |
| 143 @group | |
| 144 (read (get-buffer "foo")) | |
| 145 @result{} is | |
| 146 @end group | |
| 147 @group | |
| 148 (read (get-buffer "foo")) | |
| 149 @result{} the | |
| 150 @end group | |
| 151 | |
| 152 @group | |
| 153 ---------- Buffer: foo ---------- | |
| 154 This is the@point{} contents of foo. | |
| 155 ---------- Buffer: foo ---------- | |
| 156 @end group | |
| 157 @end example | |
| 158 | |
| 159 @noindent | |
| 160 Note that the first read skips a space. Reading skips any amount of | |
| 161 whitespace preceding the significant text. | |
| 162 | |
| 163 In Emacs 18, reading a symbol discarded the delimiter terminating the | |
| 164 symbol. Thus, point would end up at the beginning of @samp{contents} | |
| 165 rather than after @samp{the}. The Emacs 19 behavior is superior because | |
| 166 it correctly handles input such as @samp{bar(foo)}, where the | |
| 167 open-parenthesis that ends one object is needed as the beginning of | |
| 168 another object. | |
| 169 | |
| 170 Here is an example of reading from a stream that is a marker, | |
| 171 initially positioned at the beginning of the buffer shown. The value | |
| 172 read is the symbol @code{This}. | |
| 173 | |
| 174 @example | |
| 175 @group | |
| 176 | |
| 177 ---------- Buffer: foo ---------- | |
| 178 This is the contents of foo. | |
| 179 ---------- Buffer: foo ---------- | |
| 180 @end group | |
| 181 | |
| 182 @group | |
| 183 (setq m (set-marker (make-marker) 1 (get-buffer "foo"))) | |
| 184 @result{} #<marker at 1 in foo> | |
| 185 @end group | |
| 186 @group | |
| 187 (read m) | |
| 188 @result{} This | |
| 189 @end group | |
| 190 @group | |
| 191 m | |
| 192 @result{} #<marker at 5 in foo> ;; @r{Before the first space.} | |
| 193 @end group | |
| 194 @end example | |
| 195 | |
| 196 Here we read from the contents of a string: | |
| 197 | |
| 198 @example | |
| 199 @group | |
| 200 (read "(When in) the course") | |
| 201 @result{} (When in) | |
| 202 @end group | |
| 203 @end example | |
| 204 | |
| 205 The following example reads from the minibuffer. The | |
| 206 prompt is: @w{@samp{Lisp expression: }}. (That is always the prompt | |
| 207 used when you read from the stream @code{t}.) The user's input is shown | |
| 208 following the prompt. | |
| 209 | |
| 210 @example | |
| 211 @group | |
| 212 (read t) | |
| 213 @result{} 23 | |
| 214 ---------- Buffer: Minibuffer ---------- | |
| 215 Lisp expression: @kbd{23 @key{RET}} | |
| 216 ---------- Buffer: Minibuffer ---------- | |
| 217 @end group | |
| 218 @end example | |
| 219 | |
| 220 Finally, here is an example of a stream that is a function, named | |
| 221 @code{useless-stream}. Before we use the stream, we initialize the | |
| 222 variable @code{useless-list} to a list of characters. Then each call to | |
| 223 the function @code{useless-stream} obtains the next character in the list | |
| 224 or unreads a character by adding it to the front of the list. | |
| 225 | |
| 226 @example | |
| 227 @group | |
| 228 (setq useless-list (append "XY()" nil)) | |
| 229 @result{} (88 89 40 41) | |
| 230 @end group | |
| 231 | |
| 232 @group | |
| 233 (defun useless-stream (&optional unread) | |
| 234 (if unread | |
| 235 (setq useless-list (cons unread useless-list)) | |
| 236 (prog1 (car useless-list) | |
| 237 (setq useless-list (cdr useless-list))))) | |
| 238 @result{} useless-stream | |
| 239 @end group | |
| 240 @end example | |
| 241 | |
| 242 @noindent | |
| 243 Now we read using the stream thus constructed: | |
| 244 | |
| 245 @example | |
| 246 @group | |
| 247 (read 'useless-stream) | |
| 248 @result{} XY | |
| 249 @end group | |
| 250 | |
| 251 @group | |
| 252 useless-list | |
| 253 @result{} (40 41) | |
| 254 @end group | |
| 255 @end example | |
| 256 | |
| 257 @noindent | |
| 258 Note that the open and close parentheses remains in the list. The Lisp | |
| 259 reader encountered the open parenthesis, decided that it ended the | |
| 260 input, and unread it. Another attempt to read from the stream at this | |
| 261 point would read @samp{()} and return @code{nil}. | |
| 262 | |
| 263 @ignore @c Not in XEmacs | |
| 264 @defun get-file-char | |
| 265 This function is used internally as an input stream to read from the | |
| 266 input file opened by the function @code{load}. Don't use this function | |
| 267 yourself. | |
| 268 @end defun | |
| 269 @end ignore | |
| 270 | |
|
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271 @node Input Functions, Output Streams, Input Streams, Read and Print |
| 428 | 272 @section Input Functions |
| 273 | |
| 274 This section describes the Lisp functions and variables that pertain | |
| 275 to reading. | |
| 276 | |
| 277 In the functions below, @var{stream} stands for an input stream (see | |
| 278 the previous section). If @var{stream} is @code{nil} or omitted, it | |
| 279 defaults to the value of @code{standard-input}. | |
| 280 | |
| 281 @kindex end-of-file | |
| 282 An @code{end-of-file} error is signaled if reading encounters an | |
| 283 unterminated list, vector, or string. | |
| 284 | |
| 285 @defun read &optional stream | |
| 286 This function reads one textual Lisp expression from @var{stream}, | |
| 287 returning it as a Lisp object. This is the basic Lisp input function. | |
| 288 @end defun | |
| 289 | |
| 290 @defun read-from-string string &optional start end | |
| 291 @cindex string to object | |
| 292 This function reads the first textual Lisp expression from the text in | |
| 293 @var{string}. It returns a cons cell whose @sc{car} is that expression, | |
| 294 and whose @sc{cdr} is an integer giving the position of the next | |
| 295 remaining character in the string (i.e., the first one not read). | |
| 296 | |
| 297 If @var{start} is supplied, then reading begins at index @var{start} in | |
| 298 the string (where the first character is at index 0). If @var{end} is | |
| 299 also supplied, then reading stops just before that index, as if the rest | |
| 300 of the string were not there. | |
| 301 | |
| 302 For example: | |
| 303 | |
| 304 @example | |
| 305 @group | |
| 306 (read-from-string "(setq x 55) (setq y 5)") | |
| 307 @result{} ((setq x 55) . 11) | |
| 308 @end group | |
| 309 @group | |
| 310 (read-from-string "\"A short string\"") | |
| 311 @result{} ("A short string" . 16) | |
| 312 @end group | |
| 313 | |
| 314 @group | |
| 315 ;; @r{Read starting at the first character.} | |
| 316 (read-from-string "(list 112)" 0) | |
| 317 @result{} ((list 112) . 10) | |
| 318 @end group | |
| 319 @group | |
| 320 ;; @r{Read starting at the second character.} | |
| 321 (read-from-string "(list 112)" 1) | |
| 322 @result{} (list . 5) | |
| 323 @end group | |
| 324 @group | |
| 325 ;; @r{Read starting at the seventh character,} | |
| 326 ;; @r{and stopping at the ninth.} | |
| 327 (read-from-string "(list 112)" 6 8) | |
| 328 @result{} (11 . 8) | |
| 329 @end group | |
| 330 @end example | |
| 331 @end defun | |
| 332 | |
| 333 @defvar standard-input | |
| 334 This variable holds the default input stream---the stream that | |
| 335 @code{read} uses when the @var{stream} argument is @code{nil}. | |
| 336 @end defvar | |
| 337 | |
|
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338 @node Output Streams, Output Functions, Input Functions, Read and Print |
| 428 | 339 @section Output Streams |
| 340 @cindex stream (for printing) | |
| 341 @cindex output stream | |
| 342 | |
| 343 An output stream specifies what to do with the characters produced | |
| 344 by printing. Most print functions accept an output stream as an | |
| 345 optional argument. Here are the possible types of output stream: | |
| 346 | |
| 347 @table @asis | |
| 348 @item @var{buffer} | |
| 349 @cindex buffer output stream | |
| 350 The output characters are inserted into @var{buffer} at point. | |
| 351 Point advances as characters are inserted. | |
| 352 | |
| 353 @item @var{marker} | |
| 354 @cindex marker output stream | |
| 355 The output characters are inserted into the buffer that @var{marker} | |
| 356 points into, at the marker position. The marker position advances as | |
| 357 characters are inserted. The value of point in the buffer has no effect | |
| 358 on printing when the stream is a marker. | |
| 359 | |
| 360 @item @var{function} | |
| 361 @cindex function output stream | |
| 362 The output characters are passed to @var{function}, which is responsible | |
| 363 for storing them away. It is called with a single character as | |
| 364 argument, as many times as there are characters to be output, and is | |
| 365 free to do anything at all with the characters it receives. | |
| 366 | |
| 367 @item @code{t} | |
| 368 @cindex @code{t} output stream | |
| 369 The output characters are displayed in the echo area. | |
| 370 | |
| 371 @item @code{nil} | |
| 372 @cindex @code{nil} output stream | |
| 373 @code{nil} specified as an output stream means to the value of | |
| 374 @code{standard-output} instead; that value is the @dfn{default output | |
| 375 stream}, and must be a non-@code{nil} output stream. | |
| 376 | |
| 377 @item @var{symbol} | |
| 378 A symbol as output stream is equivalent to the symbol's function | |
| 379 definition (if any). | |
| 380 @end table | |
| 381 | |
| 382 Many of the valid output streams are also valid as input streams. The | |
| 383 difference between input and output streams is therefore mostly one of | |
| 384 how you use a Lisp object, not a distinction of types of object. | |
| 385 | |
| 386 Here is an example of a buffer used as an output stream. Point is | |
| 387 initially located as shown immediately before the @samp{h} in | |
| 388 @samp{the}. At the end, point is located directly before that same | |
| 389 @samp{h}. | |
| 390 | |
| 391 @cindex print example | |
| 392 @example | |
| 393 @group | |
| 394 ---------- Buffer: foo ---------- | |
| 395 This is t@point{}he contents of foo. | |
| 396 ---------- Buffer: foo ---------- | |
| 397 @end group | |
| 398 | |
| 399 (print "This is the output" (get-buffer "foo")) | |
| 400 @result{} "This is the output" | |
| 401 | |
| 402 @group | |
| 403 ---------- Buffer: foo ---------- | |
| 404 This is t | |
| 405 "This is the output" | |
| 406 @point{}he contents of foo. | |
| 407 ---------- Buffer: foo ---------- | |
| 408 @end group | |
| 409 @end example | |
| 410 | |
| 411 Now we show a use of a marker as an output stream. Initially, the | |
| 412 marker is in buffer @code{foo}, between the @samp{t} and the @samp{h} in | |
| 413 the word @samp{the}. At the end, the marker has advanced over the | |
| 414 inserted text so that it remains positioned before the same @samp{h}. | |
| 415 Note that the location of point, shown in the usual fashion, has no | |
| 416 effect. | |
| 417 | |
| 418 @example | |
| 419 @group | |
| 420 ---------- Buffer: foo ---------- | |
| 421 "This is the @point{}output" | |
| 422 ---------- Buffer: foo ---------- | |
| 423 @end group | |
| 424 | |
| 425 @group | |
| 426 m | |
| 427 @result{} #<marker at 11 in foo> | |
| 428 @end group | |
| 429 | |
| 430 @group | |
| 431 (print "More output for foo." m) | |
| 432 @result{} "More output for foo." | |
| 433 @end group | |
| 434 | |
| 435 @group | |
| 436 ---------- Buffer: foo ---------- | |
| 437 "This is t | |
| 438 "More output for foo." | |
| 439 he @point{}output" | |
| 440 ---------- Buffer: foo ---------- | |
| 441 @end group | |
| 442 | |
| 443 @group | |
| 444 m | |
| 445 @result{} #<marker at 35 in foo> | |
| 446 @end group | |
| 447 @end example | |
| 448 | |
| 449 The following example shows output to the echo area: | |
| 450 | |
| 451 @example | |
| 452 @group | |
| 453 (print "Echo Area output" t) | |
| 454 @result{} "Echo Area output" | |
| 455 ---------- Echo Area ---------- | |
| 456 "Echo Area output" | |
| 457 ---------- Echo Area ---------- | |
| 458 @end group | |
| 459 @end example | |
| 460 | |
| 461 Finally, we show the use of a function as an output stream. The | |
| 462 function @code{eat-output} takes each character that it is given and | |
| 463 conses it onto the front of the list @code{last-output} (@pxref{Building | |
| 464 Lists}). At the end, the list contains all the characters output, but | |
| 465 in reverse order. | |
| 466 | |
| 467 @example | |
| 468 @group | |
| 469 (setq last-output nil) | |
| 470 @result{} nil | |
| 471 @end group | |
| 472 | |
| 473 @group | |
| 474 (defun eat-output (c) | |
| 475 (setq last-output (cons c last-output))) | |
| 476 @result{} eat-output | |
| 477 @end group | |
| 478 | |
| 479 @group | |
| 480 (print "This is the output" 'eat-output) | |
| 481 @result{} "This is the output" | |
| 482 @end group | |
| 483 | |
| 484 @group | |
| 485 last-output | |
| 486 @result{} (?\n ?\" ?t ?u ?p ?t ?u ?o ?\ ?e ?h ?t | |
| 487 ?\ ?s ?i ?\ ?s ?i ?h ?T ?\" ?\n) | |
| 488 @end group | |
| 489 @end example | |
| 490 | |
| 491 @noindent | |
| 492 Now we can put the output in the proper order by reversing the list: | |
| 493 | |
| 494 @example | |
| 495 @group | |
| 496 (concat (nreverse last-output)) | |
| 497 @result{} " | |
| 498 \"This is the output\" | |
| 499 " | |
| 500 @end group | |
| 501 @end example | |
| 502 | |
| 503 @noindent | |
| 504 Calling @code{concat} converts the list to a string so you can see its | |
| 505 contents more clearly. | |
| 506 | |
|
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507 @node Output Functions, Output Variables, Output Streams, Read and Print |
| 428 | 508 @section Output Functions |
| 509 | |
| 510 This section describes the Lisp functions for printing Lisp objects. | |
| 511 | |
| 512 @cindex @samp{"} in printing | |
| 513 @cindex @samp{\} in printing | |
| 514 @cindex quoting characters in printing | |
| 515 @cindex escape characters in printing | |
| 516 Some of the XEmacs printing functions add quoting characters to the | |
| 517 output when necessary so that it can be read properly. The quoting | |
| 518 characters used are @samp{"} and @samp{\}; they distinguish strings from | |
| 519 symbols, and prevent punctuation characters in strings and symbols from | |
| 520 being taken as delimiters when reading. @xref{Printed Representation}, | |
| 521 for full details. You specify quoting or no quoting by the choice of | |
| 522 printing function. | |
| 523 | |
| 524 If the text is to be read back into Lisp, then it is best to print | |
| 525 with quoting characters to avoid ambiguity. Likewise, if the purpose is | |
| 526 to describe a Lisp object clearly for a Lisp programmer. However, if | |
| 527 the purpose of the output is to look nice for humans, then it is better | |
| 528 to print without quoting. | |
| 529 | |
| 530 Printing a self-referent Lisp object requires an infinite amount of | |
| 531 text. In certain cases, trying to produce this text leads to a stack | |
| 532 overflow. XEmacs detects such recursion and prints @samp{#@var{level}} | |
| 533 instead of recursively printing an object already being printed. For | |
| 534 example, here @samp{#0} indicates a recursive reference to the object at | |
| 535 level 0 of the current print operation: | |
| 536 | |
| 537 @example | |
| 538 (setq foo (list nil)) | |
| 539 @result{} (nil) | |
| 540 (setcar foo foo) | |
| 541 @result{} (#0) | |
| 542 @end example | |
| 543 | |
| 544 In the functions below, @var{stream} stands for an output stream. | |
| 545 (See the previous section for a description of output streams.) If | |
| 546 @var{stream} is @code{nil} or omitted, it defaults to the value of | |
| 547 @code{standard-output}. | |
| 548 | |
| 549 @defun print object &optional stream | |
| 550 @cindex Lisp printer | |
| 551 The @code{print} function is a convenient way of printing. It outputs | |
| 552 the printed representation of @var{object} to @var{stream}, printing in | |
| 553 addition one newline before @var{object} and another after it. Quoting | |
| 554 characters are used. @code{print} returns @var{object}. For example: | |
| 555 | |
| 556 @example | |
| 557 @group | |
| 558 (progn (print 'The\ cat\ in) | |
| 559 (print "the hat") | |
| 560 (print " came back")) | |
| 444 | 561 @print{} |
| 428 | 562 @print{} The\ cat\ in |
| 444 | 563 @print{} |
| 428 | 564 @print{} "the hat" |
| 444 | 565 @print{} |
| 428 | 566 @print{} " came back" |
| 444 | 567 @print{} |
| 428 | 568 @result{} " came back" |
| 569 @end group | |
| 570 @end example | |
| 571 @end defun | |
| 572 | |
| 573 @defun prin1 object &optional stream | |
| 574 This function outputs the printed representation of @var{object} to | |
| 575 @var{stream}. It does not print newlines to separate output as | |
| 576 @code{print} does, but it does use quoting characters just like | |
| 577 @code{print}. It returns @var{object}. | |
| 578 | |
| 579 @example | |
| 580 @group | |
| 444 | 581 (progn (prin1 'The\ cat\ in) |
| 582 (prin1 "the hat") | |
| 428 | 583 (prin1 " came back")) |
| 584 @print{} The\ cat\ in"the hat"" came back" | |
| 585 @result{} " came back" | |
| 586 @end group | |
| 587 @end example | |
| 588 @end defun | |
| 589 | |
| 590 @defun princ object &optional stream | |
| 591 This function outputs the printed representation of @var{object} to | |
| 592 @var{stream}. It returns @var{object}. | |
| 593 | |
| 594 This function is intended to produce output that is readable by people, | |
| 595 not by @code{read}, so it doesn't insert quoting characters and doesn't | |
| 596 put double-quotes around the contents of strings. It does not add any | |
| 597 spacing between calls. | |
| 598 | |
| 599 @example | |
| 600 @group | |
| 601 (progn | |
| 602 (princ 'The\ cat) | |
| 603 (princ " in the \"hat\"")) | |
| 604 @print{} The cat in the "hat" | |
| 605 @result{} " in the \"hat\"" | |
| 606 @end group | |
| 607 @end example | |
| 608 @end defun | |
| 609 | |
| 610 @defun terpri &optional stream | |
| 611 @cindex newline in print | |
| 612 This function outputs a newline to @var{stream}. The name stands | |
| 613 for ``terminate print''. | |
| 614 @end defun | |
| 615 | |
| 616 @defun write-char character &optional stream | |
| 617 This function outputs @var{character} to @var{stream}. It returns | |
| 618 @var{character}. | |
| 619 @end defun | |
| 620 | |
| 621 @defun prin1-to-string object &optional noescape | |
| 622 @cindex object to string | |
| 623 This function returns a string containing the text that @code{prin1} | |
| 624 would have printed for the same argument. | |
| 625 | |
| 626 @example | |
| 627 @group | |
| 628 (prin1-to-string 'foo) | |
| 629 @result{} "foo" | |
| 630 @end group | |
| 631 @group | |
| 632 (prin1-to-string (mark-marker)) | |
| 633 @result{} "#<marker at 2773 in strings.texi>" | |
| 634 @end group | |
| 635 @end example | |
| 636 | |
| 637 If @var{noescape} is non-@code{nil}, that inhibits use of quoting | |
| 638 characters in the output. (This argument is supported in Emacs versions | |
| 639 19 and later.) | |
| 640 | |
| 641 @example | |
| 642 @group | |
| 643 (prin1-to-string "foo") | |
| 644 @result{} "\"foo\"" | |
| 645 @end group | |
| 646 @group | |
| 647 (prin1-to-string "foo" t) | |
| 648 @result{} "foo" | |
| 649 @end group | |
| 650 @end example | |
| 651 | |
| 652 See @code{format}, in @ref{String Conversion}, for other ways to obtain | |
| 653 the printed representation of a Lisp object as a string. | |
| 654 @end defun | |
| 655 | |
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Silence texinfo 5.2 warnings, primarily by adding next, prev, and up
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|
656 @node Output Variables, , Output Functions, Read and Print |
| 428 | 657 @section Variables Affecting Output |
| 658 | |
| 659 @defvar standard-output | |
| 660 The value of this variable is the default output stream---the stream | |
| 661 that print functions use when the @var{stream} argument is @code{nil}. | |
| 662 @end defvar | |
| 663 | |
| 664 @defvar print-escape-newlines | |
| 665 @cindex @samp{\n} in print | |
| 666 @cindex escape characters | |
| 667 If this variable is non-@code{nil}, then newline characters in strings | |
| 668 are printed as @samp{\n} and formfeeds are printed as @samp{\f}. | |
| 669 Normally these characters are printed as actual newlines and formfeeds. | |
| 670 | |
| 671 This variable affects the print functions @code{prin1} and @code{print}, | |
| 672 as well as everything that uses them. It does not affect @code{princ}. | |
| 673 Here is an example using @code{prin1}: | |
| 674 | |
| 675 @example | |
| 676 @group | |
| 677 (prin1 "a\nb") | |
| 678 @print{} "a | |
| 679 @print{} b" | |
| 680 @result{} "a | |
| 681 b" | |
| 682 @end group | |
| 683 | |
| 684 @group | |
| 685 (let ((print-escape-newlines t)) | |
| 686 (prin1 "a\nb")) | |
| 687 @print{} "a\nb" | |
| 688 @result{} "a | |
| 689 b" | |
| 690 @end group | |
| 691 @end example | |
| 692 | |
| 693 @noindent | |
| 694 In the second expression, the local binding of | |
| 695 @code{print-escape-newlines} is in effect during the call to | |
| 696 @code{prin1}, but not during the printing of the result. | |
| 697 @end defvar | |
| 698 | |
| 699 @defvar print-readably | |
| 700 @cindex printing readably | |
| 701 If non-@code{nil}, then all objects will be printed in a readable form. | |
| 702 If an object has no readable representation, then an error is signalled. | |
| 703 When @code{print-readably} is true, compiled-function objects will be | |
| 704 written in @samp{#[...]} form instead of in @samp{#<compiled-function | |
| 705 [...]>} form, and two-element lists of the form @samp{(quote object)} | |
| 706 will be written as the equivalent @samp{'object}. Do not @emph{set} | |
| 707 this variable; bind it instead. | |
| 708 @end defvar | |
| 709 | |
| 710 @defvar print-length | |
| 711 @cindex printing limits | |
| 712 The value of this variable is the maximum number of elements of a list | |
| 713 that will be printed. If a list being printed has more than this many | |
| 714 elements, it is abbreviated with an ellipsis. | |
| 715 | |
| 716 If the value is @code{nil} (the default), then there is no limit. | |
| 717 | |
| 718 @example | |
| 719 @group | |
| 720 (setq print-length 2) | |
| 721 @result{} 2 | |
| 722 @end group | |
| 723 @group | |
| 724 (print '(1 2 3 4 5)) | |
| 725 @print{} (1 2 ...) | |
| 726 @result{} (1 2 ...) | |
| 727 @end group | |
| 728 @end example | |
| 729 @end defvar | |
| 730 | |
| 731 @defvar print-level | |
| 732 The value of this variable is the maximum depth of nesting of | |
| 733 parentheses and brackets when printed. Any list or vector at a depth | |
| 734 exceeding this limit is abbreviated with an ellipsis. A value of | |
| 735 @code{nil} (which is the default) means no limit. | |
| 736 | |
| 737 This variable exists in version 19 and later versions. | |
| 738 @end defvar | |
| 739 | |
| 740 @defvar print-string-length | |
| 741 @cindex string length, maximum when printing | |
| 742 The value of this variable is the maximum number of characters of a string | |
| 743 that will be printed. If a string being printed has more than this many | |
| 744 characters, it is abbreviated with an ellipsis. | |
| 745 @end defvar | |
| 746 | |
| 747 @defvar print-gensym | |
| 748 @cindex printing uninterned symbols | |
| 749 @cindex uninterned symbols, printing | |
| 750 If non-@code{nil}, then uninterned symbols will be printed specially. | |
| 751 Uninterned symbols are those which are not present in @code{obarray}, | |
| 752 that is, those which were made with @code{make-symbol} or by calling | |
| 753 @code{intern} with a second argument. | |
| 754 | |
| 755 When @code{print-gensym} is true, such symbols will be preceded by | |
| 756 @samp{#:}, which causes the reader to create a new symbol instead of | |
| 757 interning and returning an existing one. Beware: The @samp{#:} syntax | |
| 758 creates a new symbol each time it is seen, so if you print an object | |
| 759 which contains two pointers to the same uninterned symbol, @code{read} | |
| 760 will not duplicate that structure. | |
| 761 | |
| 762 Also, since XEmacs has no real notion of packages, there is no way for | |
| 763 the printer to distinguish between symbols interned in no obarray, and | |
| 764 symbols interned in an alternate obarray. | |
| 765 @end defvar | |
| 766 | |
| 767 @defvar float-output-format | |
| 768 @cindex printing floating-point numbers | |
| 769 @cindex floating-point numbers, printing | |
| 770 This variable holds the format descriptor string that Lisp uses to print | |
| 771 floats. This is a @samp{%}-spec like those accepted by @code{printf} in | |
| 772 C, but with some restrictions. It must start with the two characters | |
| 773 @samp{%.}. After that comes an integer precision specification, and | |
| 774 then a letter which controls the format. The letters allowed are | |
| 775 @samp{e}, @samp{f} and @samp{g}. | |
| 776 | |
| 777 @itemize @bullet | |
| 778 @item | |
| 779 Use @samp{e} for exponential notation | |
| 780 @samp{@var{dig}.@var{digits}e@var{expt}}. | |
| 781 @item | |
| 782 Use @samp{f} for decimal point notation @samp{DIGITS.DIGITS}. | |
| 783 @item | |
| 784 Use @samp{g} to choose the shorter of those two formats for the number | |
| 785 at hand. | |
| 786 @end itemize | |
| 787 | |
| 788 The precision in any of these cases is the number of digits following | |
| 789 the decimal point. With @samp{f}, a precision of 0 means to omit the | |
| 790 decimal point. 0 is not allowed with @samp{f} or @samp{g}. | |
| 791 | |
| 444 | 792 A value of @code{nil} means to use @samp{%.16g}. |
| 428 | 793 |
| 794 Regardless of the value of @code{float-output-format}, a floating point | |
| 795 number will never be printed in such a way that it is ambiguous with an | |
| 796 integer; that is, a floating-point number will always be printed with a | |
| 797 decimal point and/or an exponent, even if the digits following the | |
| 798 decimal point are all zero. This is to preserve read-equivalence. | |
| 799 @end defvar |