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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1998, 1999, 2001, 2002, | |
114f9c96 | 4 | @c 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 Free Software Foundation, Inc. |
b8d4c8d0 | 5 | @c See the file elisp.texi for copying conditions. |
6336d8c3 | 6 | @setfilename ../../info/streams |
b8d4c8d0 GM |
7 | @node Read and Print, Minibuffers, Debugging, Top |
8 | @comment node-name, next, previous, up | |
9 | @chapter Reading and Printing Lisp Objects | |
10 | ||
11 | @dfn{Printing} and @dfn{reading} are the operations of converting Lisp | |
12 | objects to textual form and vice versa. They use the printed | |
13 | representations and read syntax described in @ref{Lisp Data Types}. | |
14 | ||
15 | This chapter describes the Lisp functions for reading and printing. | |
16 | It also describes @dfn{streams}, which specify where to get the text (if | |
17 | reading) or where to put it (if printing). | |
18 | ||
19 | @menu | |
20 | * Streams Intro:: Overview of streams, reading and printing. | |
21 | * Input Streams:: Various data types that can be used as input streams. | |
22 | * Input Functions:: Functions to read Lisp objects from text. | |
23 | * Output Streams:: Various data types that can be used as output streams. | |
24 | * Output Functions:: Functions to print Lisp objects as text. | |
25 | * Output Variables:: Variables that control what the printing functions do. | |
26 | @end menu | |
27 | ||
28 | @node Streams Intro | |
29 | @section Introduction to Reading and Printing | |
30 | @cindex Lisp reader | |
31 | @cindex printing | |
32 | @cindex reading | |
33 | ||
34 | @dfn{Reading} a Lisp object means parsing a Lisp expression in textual | |
35 | form and producing a corresponding Lisp object. This is how Lisp | |
36 | programs get into Lisp from files of Lisp code. We call the text the | |
37 | @dfn{read syntax} of the object. For example, the text @samp{(a .@: 5)} | |
38 | is the read syntax for a cons cell whose @sc{car} is @code{a} and whose | |
39 | @sc{cdr} is the number 5. | |
40 | ||
41 | @dfn{Printing} a Lisp object means producing text that represents that | |
42 | object---converting the object to its @dfn{printed representation} | |
43 | (@pxref{Printed Representation}). Printing the cons cell described | |
44 | above produces the text @samp{(a .@: 5)}. | |
45 | ||
46 | Reading and printing are more or less inverse operations: printing the | |
47 | object that results from reading a given piece of text often produces | |
48 | the same text, and reading the text that results from printing an object | |
49 | usually produces a similar-looking object. For example, printing the | |
50 | symbol @code{foo} produces the text @samp{foo}, and reading that text | |
51 | returns the symbol @code{foo}. Printing a list whose elements are | |
52 | @code{a} and @code{b} produces the text @samp{(a b)}, and reading that | |
53 | text produces a list (but not the same list) with elements @code{a} | |
54 | and @code{b}. | |
55 | ||
56 | However, these two operations are not precisely inverse to each other. | |
57 | There are three kinds of exceptions: | |
58 | ||
59 | @itemize @bullet | |
60 | @item | |
61 | Printing can produce text that cannot be read. For example, buffers, | |
62 | windows, frames, subprocesses and markers print as text that starts | |
63 | with @samp{#}; if you try to read this text, you get an error. There is | |
64 | no way to read those data types. | |
65 | ||
66 | @item | |
67 | One object can have multiple textual representations. For example, | |
68 | @samp{1} and @samp{01} represent the same integer, and @samp{(a b)} and | |
69 | @samp{(a .@: (b))} represent the same list. Reading will accept any of | |
70 | the alternatives, but printing must choose one of them. | |
71 | ||
72 | @item | |
73 | Comments can appear at certain points in the middle of an object's | |
74 | read sequence without affecting the result of reading it. | |
75 | @end itemize | |
76 | ||
77 | @node Input Streams | |
78 | @section Input Streams | |
79 | @cindex stream (for reading) | |
80 | @cindex input stream | |
81 | ||
82 | Most of the Lisp functions for reading text take an @dfn{input stream} | |
83 | as an argument. The input stream specifies where or how to get the | |
84 | characters of the text to be read. Here are the possible types of input | |
85 | stream: | |
86 | ||
87 | @table @asis | |
88 | @item @var{buffer} | |
89 | @cindex buffer input stream | |
90 | The input characters are read from @var{buffer}, starting with the | |
91 | character directly after point. Point advances as characters are read. | |
92 | ||
93 | @item @var{marker} | |
94 | @cindex marker input stream | |
95 | The input characters are read from the buffer that @var{marker} is in, | |
96 | starting with the character directly after the marker. The marker | |
97 | position advances as characters are read. The value of point in the | |
98 | buffer has no effect when the stream is a marker. | |
99 | ||
100 | @item @var{string} | |
101 | @cindex string input stream | |
102 | The input characters are taken from @var{string}, starting at the first | |
103 | character in the string and using as many characters as required. | |
104 | ||
105 | @item @var{function} | |
106 | @cindex function input stream | |
107 | The input characters are generated by @var{function}, which must support | |
108 | two kinds of calls: | |
109 | ||
110 | @itemize @bullet | |
111 | @item | |
112 | When it is called with no arguments, it should return the next character. | |
113 | ||
114 | @item | |
115 | When it is called with one argument (always a character), @var{function} | |
116 | should save the argument and arrange to return it on the next call. | |
117 | This is called @dfn{unreading} the character; it happens when the Lisp | |
118 | reader reads one character too many and wants to ``put it back where it | |
119 | came from.'' In this case, it makes no difference what value | |
120 | @var{function} returns. | |
121 | @end itemize | |
122 | ||
123 | @item @code{t} | |
124 | @cindex @code{t} input stream | |
125 | @code{t} used as a stream means that the input is read from the | |
126 | minibuffer. In fact, the minibuffer is invoked once and the text | |
127 | given by the user is made into a string that is then used as the | |
128 | input stream. If Emacs is running in batch mode, standard input is used | |
129 | instead of the minibuffer. For example, | |
130 | @example | |
131 | (message "%s" (read t)) | |
132 | @end example | |
133 | will read a Lisp expression from standard input and print the result | |
134 | to standard output. | |
135 | ||
136 | @item @code{nil} | |
137 | @cindex @code{nil} input stream | |
138 | @code{nil} supplied as an input stream means to use the value of | |
139 | @code{standard-input} instead; that value is the @dfn{default input | |
140 | stream}, and must be a non-@code{nil} input stream. | |
141 | ||
142 | @item @var{symbol} | |
143 | A symbol as input stream is equivalent to the symbol's function | |
144 | definition (if any). | |
145 | @end table | |
146 | ||
147 | Here is an example of reading from a stream that is a buffer, showing | |
148 | where point is located before and after: | |
149 | ||
150 | @example | |
151 | @group | |
152 | ---------- Buffer: foo ---------- | |
153 | This@point{} is the contents of foo. | |
154 | ---------- Buffer: foo ---------- | |
155 | @end group | |
156 | ||
157 | @group | |
158 | (read (get-buffer "foo")) | |
159 | @result{} is | |
160 | @end group | |
161 | @group | |
162 | (read (get-buffer "foo")) | |
163 | @result{} the | |
164 | @end group | |
165 | ||
166 | @group | |
167 | ---------- Buffer: foo ---------- | |
168 | This is the@point{} contents of foo. | |
169 | ---------- Buffer: foo ---------- | |
170 | @end group | |
171 | @end example | |
172 | ||
173 | @noindent | |
174 | Note that the first read skips a space. Reading skips any amount of | |
175 | whitespace preceding the significant text. | |
176 | ||
177 | Here is an example of reading from a stream that is a marker, | |
178 | initially positioned at the beginning of the buffer shown. The value | |
179 | read is the symbol @code{This}. | |
180 | ||
181 | @example | |
182 | @group | |
183 | ||
184 | ---------- Buffer: foo ---------- | |
185 | This is the contents of foo. | |
186 | ---------- Buffer: foo ---------- | |
187 | @end group | |
188 | ||
189 | @group | |
190 | (setq m (set-marker (make-marker) 1 (get-buffer "foo"))) | |
191 | @result{} #<marker at 1 in foo> | |
192 | @end group | |
193 | @group | |
194 | (read m) | |
195 | @result{} This | |
196 | @end group | |
197 | @group | |
198 | m | |
199 | @result{} #<marker at 5 in foo> ;; @r{Before the first space.} | |
200 | @end group | |
201 | @end example | |
202 | ||
203 | Here we read from the contents of a string: | |
204 | ||
205 | @example | |
206 | @group | |
207 | (read "(When in) the course") | |
208 | @result{} (When in) | |
209 | @end group | |
210 | @end example | |
211 | ||
212 | The following example reads from the minibuffer. The | |
213 | prompt is: @w{@samp{Lisp expression: }}. (That is always the prompt | |
214 | used when you read from the stream @code{t}.) The user's input is shown | |
215 | following the prompt. | |
216 | ||
217 | @example | |
218 | @group | |
219 | (read t) | |
220 | @result{} 23 | |
221 | ---------- Buffer: Minibuffer ---------- | |
222 | Lisp expression: @kbd{23 @key{RET}} | |
223 | ---------- Buffer: Minibuffer ---------- | |
224 | @end group | |
225 | @end example | |
226 | ||
227 | Finally, here is an example of a stream that is a function, named | |
228 | @code{useless-stream}. Before we use the stream, we initialize the | |
229 | variable @code{useless-list} to a list of characters. Then each call to | |
230 | the function @code{useless-stream} obtains the next character in the list | |
231 | or unreads a character by adding it to the front of the list. | |
232 | ||
233 | @example | |
234 | @group | |
235 | (setq useless-list (append "XY()" nil)) | |
236 | @result{} (88 89 40 41) | |
237 | @end group | |
238 | ||
239 | @group | |
240 | (defun useless-stream (&optional unread) | |
241 | (if unread | |
242 | (setq useless-list (cons unread useless-list)) | |
243 | (prog1 (car useless-list) | |
244 | (setq useless-list (cdr useless-list))))) | |
245 | @result{} useless-stream | |
246 | @end group | |
247 | @end example | |
248 | ||
249 | @noindent | |
250 | Now we read using the stream thus constructed: | |
251 | ||
252 | @example | |
253 | @group | |
254 | (read 'useless-stream) | |
255 | @result{} XY | |
256 | @end group | |
257 | ||
258 | @group | |
259 | useless-list | |
260 | @result{} (40 41) | |
261 | @end group | |
262 | @end example | |
263 | ||
264 | @noindent | |
265 | Note that the open and close parentheses remain in the list. The Lisp | |
266 | reader encountered the open parenthesis, decided that it ended the | |
267 | input, and unread it. Another attempt to read from the stream at this | |
268 | point would read @samp{()} and return @code{nil}. | |
269 | ||
270 | @defun get-file-char | |
271 | This function is used internally as an input stream to read from the | |
272 | input file opened by the function @code{load}. Don't use this function | |
273 | yourself. | |
274 | @end defun | |
275 | ||
276 | @node Input Functions | |
277 | @section Input Functions | |
278 | ||
279 | This section describes the Lisp functions and variables that pertain | |
280 | to reading. | |
281 | ||
282 | In the functions below, @var{stream} stands for an input stream (see | |
283 | the previous section). If @var{stream} is @code{nil} or omitted, it | |
284 | defaults to the value of @code{standard-input}. | |
285 | ||
286 | @kindex end-of-file | |
287 | An @code{end-of-file} error is signaled if reading encounters an | |
288 | unterminated list, vector, or string. | |
289 | ||
290 | @defun read &optional stream | |
291 | This function reads one textual Lisp expression from @var{stream}, | |
292 | returning it as a Lisp object. This is the basic Lisp input function. | |
293 | @end defun | |
294 | ||
295 | @defun read-from-string string &optional start end | |
296 | @cindex string to object | |
297 | This function reads the first textual Lisp expression from the text in | |
298 | @var{string}. It returns a cons cell whose @sc{car} is that expression, | |
299 | and whose @sc{cdr} is an integer giving the position of the next | |
300 | remaining character in the string (i.e., the first one not read). | |
301 | ||
302 | If @var{start} is supplied, then reading begins at index @var{start} in | |
303 | the string (where the first character is at index 0). If you specify | |
304 | @var{end}, then reading is forced to stop just before that index, as if | |
305 | the rest of the string were not there. | |
306 | ||
307 | For example: | |
308 | ||
309 | @example | |
310 | @group | |
311 | (read-from-string "(setq x 55) (setq y 5)") | |
312 | @result{} ((setq x 55) . 11) | |
313 | @end group | |
314 | @group | |
315 | (read-from-string "\"A short string\"") | |
316 | @result{} ("A short string" . 16) | |
317 | @end group | |
318 | ||
319 | @group | |
320 | ;; @r{Read starting at the first character.} | |
321 | (read-from-string "(list 112)" 0) | |
322 | @result{} ((list 112) . 10) | |
323 | @end group | |
324 | @group | |
325 | ;; @r{Read starting at the second character.} | |
326 | (read-from-string "(list 112)" 1) | |
327 | @result{} (list . 5) | |
328 | @end group | |
329 | @group | |
330 | ;; @r{Read starting at the seventh character,} | |
331 | ;; @r{and stopping at the ninth.} | |
332 | (read-from-string "(list 112)" 6 8) | |
333 | @result{} (11 . 8) | |
334 | @end group | |
335 | @end example | |
336 | @end defun | |
337 | ||
338 | @defvar standard-input | |
339 | This variable holds the default input stream---the stream that | |
340 | @code{read} uses when the @var{stream} argument is @code{nil}. | |
341 | The default is @code{t}, meaning use the minibuffer. | |
342 | @end defvar | |
343 | ||
dd449674 CY |
344 | @defvar read-circle |
345 | If non-@code{nil}, this variable enables the reading of circular and | |
346 | shared structures. @xref{Circular Objects}. Its default value is | |
347 | @code{t}. | |
348 | @end defvar | |
349 | ||
b8d4c8d0 GM |
350 | @node Output Streams |
351 | @section Output Streams | |
352 | @cindex stream (for printing) | |
353 | @cindex output stream | |
354 | ||
355 | An output stream specifies what to do with the characters produced | |
356 | by printing. Most print functions accept an output stream as an | |
357 | optional argument. Here are the possible types of output stream: | |
358 | ||
359 | @table @asis | |
360 | @item @var{buffer} | |
361 | @cindex buffer output stream | |
362 | The output characters are inserted into @var{buffer} at point. | |
363 | Point advances as characters are inserted. | |
364 | ||
365 | @item @var{marker} | |
366 | @cindex marker output stream | |
367 | The output characters are inserted into the buffer that @var{marker} | |
368 | points into, at the marker position. The marker position advances as | |
369 | characters are inserted. The value of point in the buffer has no effect | |
370 | on printing when the stream is a marker, and this kind of printing | |
371 | does not move point (except that if the marker points at or before the | |
372 | position of point, point advances with the surrounding text, as | |
373 | usual). | |
374 | ||
375 | @item @var{function} | |
376 | @cindex function output stream | |
377 | The output characters are passed to @var{function}, which is responsible | |
378 | for storing them away. It is called with a single character as | |
379 | argument, as many times as there are characters to be output, and | |
380 | is responsible for storing the characters wherever you want to put them. | |
381 | ||
382 | @item @code{t} | |
383 | @cindex @code{t} output stream | |
384 | The output characters are displayed in the echo area. | |
385 | ||
386 | @item @code{nil} | |
387 | @cindex @code{nil} output stream | |
388 | @code{nil} specified as an output stream means to use the value of | |
389 | @code{standard-output} instead; that value is the @dfn{default output | |
390 | stream}, and must not be @code{nil}. | |
391 | ||
392 | @item @var{symbol} | |
393 | A symbol as output stream is equivalent to the symbol's function | |
394 | definition (if any). | |
395 | @end table | |
396 | ||
397 | Many of the valid output streams are also valid as input streams. The | |
398 | difference between input and output streams is therefore more a matter | |
399 | of how you use a Lisp object, than of different types of object. | |
400 | ||
401 | Here is an example of a buffer used as an output stream. Point is | |
402 | initially located as shown immediately before the @samp{h} in | |
403 | @samp{the}. At the end, point is located directly before that same | |
404 | @samp{h}. | |
405 | ||
406 | @cindex print example | |
407 | @example | |
408 | @group | |
409 | ---------- Buffer: foo ---------- | |
410 | This is t@point{}he contents of foo. | |
411 | ---------- Buffer: foo ---------- | |
412 | @end group | |
413 | ||
414 | (print "This is the output" (get-buffer "foo")) | |
415 | @result{} "This is the output" | |
416 | ||
417 | @group | |
418 | ---------- Buffer: foo ---------- | |
419 | This is t | |
420 | "This is the output" | |
421 | @point{}he contents of foo. | |
422 | ---------- Buffer: foo ---------- | |
423 | @end group | |
424 | @end example | |
425 | ||
426 | Now we show a use of a marker as an output stream. Initially, the | |
427 | marker is in buffer @code{foo}, between the @samp{t} and the @samp{h} in | |
428 | the word @samp{the}. At the end, the marker has advanced over the | |
429 | inserted text so that it remains positioned before the same @samp{h}. | |
430 | Note that the location of point, shown in the usual fashion, has no | |
431 | effect. | |
432 | ||
433 | @example | |
434 | @group | |
435 | ---------- Buffer: foo ---------- | |
436 | This is the @point{}output | |
437 | ---------- Buffer: foo ---------- | |
438 | @end group | |
439 | ||
440 | @group | |
441 | (setq m (copy-marker 10)) | |
442 | @result{} #<marker at 10 in foo> | |
443 | @end group | |
444 | ||
445 | @group | |
446 | (print "More output for foo." m) | |
447 | @result{} "More output for foo." | |
448 | @end group | |
449 | ||
450 | @group | |
451 | ---------- Buffer: foo ---------- | |
452 | This is t | |
453 | "More output for foo." | |
454 | he @point{}output | |
455 | ---------- Buffer: foo ---------- | |
456 | @end group | |
457 | ||
458 | @group | |
459 | m | |
460 | @result{} #<marker at 34 in foo> | |
461 | @end group | |
462 | @end example | |
463 | ||
464 | The following example shows output to the echo area: | |
465 | ||
466 | @example | |
467 | @group | |
468 | (print "Echo Area output" t) | |
469 | @result{} "Echo Area output" | |
470 | ---------- Echo Area ---------- | |
471 | "Echo Area output" | |
472 | ---------- Echo Area ---------- | |
473 | @end group | |
474 | @end example | |
475 | ||
476 | Finally, we show the use of a function as an output stream. The | |
477 | function @code{eat-output} takes each character that it is given and | |
478 | conses it onto the front of the list @code{last-output} (@pxref{Building | |
479 | Lists}). At the end, the list contains all the characters output, but | |
480 | in reverse order. | |
481 | ||
482 | @example | |
483 | @group | |
484 | (setq last-output nil) | |
485 | @result{} nil | |
486 | @end group | |
487 | ||
488 | @group | |
489 | (defun eat-output (c) | |
490 | (setq last-output (cons c last-output))) | |
491 | @result{} eat-output | |
492 | @end group | |
493 | ||
494 | @group | |
495 | (print "This is the output" 'eat-output) | |
496 | @result{} "This is the output" | |
497 | @end group | |
498 | ||
499 | @group | |
500 | last-output | |
501 | @result{} (10 34 116 117 112 116 117 111 32 101 104 | |
502 | 116 32 115 105 32 115 105 104 84 34 10) | |
503 | @end group | |
504 | @end example | |
505 | ||
506 | @noindent | |
507 | Now we can put the output in the proper order by reversing the list: | |
508 | ||
509 | @example | |
510 | @group | |
511 | (concat (nreverse last-output)) | |
512 | @result{} " | |
513 | \"This is the output\" | |
514 | " | |
515 | @end group | |
516 | @end example | |
517 | ||
518 | @noindent | |
519 | Calling @code{concat} converts the list to a string so you can see its | |
520 | contents more clearly. | |
521 | ||
522 | @node Output Functions | |
523 | @section Output Functions | |
524 | ||
525 | This section describes the Lisp functions for printing Lisp | |
526 | objects---converting objects into their printed representation. | |
527 | ||
528 | @cindex @samp{"} in printing | |
529 | @cindex @samp{\} in printing | |
530 | @cindex quoting characters in printing | |
531 | @cindex escape characters in printing | |
532 | Some of the Emacs printing functions add quoting characters to the | |
533 | output when necessary so that it can be read properly. The quoting | |
534 | characters used are @samp{"} and @samp{\}; they distinguish strings from | |
535 | symbols, and prevent punctuation characters in strings and symbols from | |
536 | being taken as delimiters when reading. @xref{Printed Representation}, | |
537 | for full details. You specify quoting or no quoting by the choice of | |
538 | printing function. | |
539 | ||
540 | If the text is to be read back into Lisp, then you should print with | |
541 | quoting characters to avoid ambiguity. Likewise, if the purpose is to | |
542 | describe a Lisp object clearly for a Lisp programmer. However, if the | |
543 | purpose of the output is to look nice for humans, then it is usually | |
544 | better to print without quoting. | |
545 | ||
546 | Lisp objects can refer to themselves. Printing a self-referential | |
547 | object in the normal way would require an infinite amount of text, and | |
548 | the attempt could cause infinite recursion. Emacs detects such | |
549 | recursion and prints @samp{#@var{level}} instead of recursively printing | |
550 | an object already being printed. For example, here @samp{#0} indicates | |
551 | a recursive reference to the object at level 0 of the current print | |
552 | operation: | |
553 | ||
554 | @example | |
555 | (setq foo (list nil)) | |
556 | @result{} (nil) | |
557 | (setcar foo foo) | |
558 | @result{} (#0) | |
559 | @end example | |
560 | ||
561 | In the functions below, @var{stream} stands for an output stream. | |
562 | (See the previous section for a description of output streams.) If | |
563 | @var{stream} is @code{nil} or omitted, it defaults to the value of | |
564 | @code{standard-output}. | |
565 | ||
566 | @defun print object &optional stream | |
567 | @cindex Lisp printer | |
568 | The @code{print} function is a convenient way of printing. It outputs | |
569 | the printed representation of @var{object} to @var{stream}, printing in | |
570 | addition one newline before @var{object} and another after it. Quoting | |
571 | characters are used. @code{print} returns @var{object}. For example: | |
572 | ||
573 | @example | |
574 | @group | |
575 | (progn (print 'The\ cat\ in) | |
576 | (print "the hat") | |
577 | (print " came back")) | |
578 | @print{} | |
579 | @print{} The\ cat\ in | |
580 | @print{} | |
581 | @print{} "the hat" | |
582 | @print{} | |
583 | @print{} " came back" | |
584 | @result{} " came back" | |
585 | @end group | |
586 | @end example | |
587 | @end defun | |
588 | ||
589 | @defun prin1 object &optional stream | |
590 | This function outputs the printed representation of @var{object} to | |
591 | @var{stream}. It does not print newlines to separate output as | |
592 | @code{print} does, but it does use quoting characters just like | |
593 | @code{print}. It returns @var{object}. | |
594 | ||
595 | @example | |
596 | @group | |
597 | (progn (prin1 'The\ cat\ in) | |
598 | (prin1 "the hat") | |
599 | (prin1 " came back")) | |
600 | @print{} The\ cat\ in"the hat"" came back" | |
601 | @result{} " came back" | |
602 | @end group | |
603 | @end example | |
604 | @end defun | |
605 | ||
606 | @defun princ object &optional stream | |
607 | This function outputs the printed representation of @var{object} to | |
608 | @var{stream}. It returns @var{object}. | |
609 | ||
610 | This function is intended to produce output that is readable by people, | |
611 | not by @code{read}, so it doesn't insert quoting characters and doesn't | |
612 | put double-quotes around the contents of strings. It does not add any | |
613 | spacing between calls. | |
614 | ||
615 | @example | |
616 | @group | |
617 | (progn | |
618 | (princ 'The\ cat) | |
619 | (princ " in the \"hat\"")) | |
620 | @print{} The cat in the "hat" | |
621 | @result{} " in the \"hat\"" | |
622 | @end group | |
623 | @end example | |
624 | @end defun | |
625 | ||
626 | @defun terpri &optional stream | |
627 | @cindex newline in print | |
628 | This function outputs a newline to @var{stream}. The name stands | |
629 | for ``terminate print.'' | |
630 | @end defun | |
631 | ||
632 | @defun write-char character &optional stream | |
633 | This function outputs @var{character} to @var{stream}. It returns | |
634 | @var{character}. | |
635 | @end defun | |
636 | ||
637 | @defun prin1-to-string object &optional noescape | |
638 | @cindex object to string | |
639 | This function returns a string containing the text that @code{prin1} | |
640 | would have printed for the same argument. | |
641 | ||
642 | @example | |
643 | @group | |
644 | (prin1-to-string 'foo) | |
645 | @result{} "foo" | |
646 | @end group | |
647 | @group | |
648 | (prin1-to-string (mark-marker)) | |
649 | @result{} "#<marker at 2773 in strings.texi>" | |
650 | @end group | |
651 | @end example | |
652 | ||
653 | If @var{noescape} is non-@code{nil}, that inhibits use of quoting | |
654 | characters in the output. (This argument is supported in Emacs versions | |
655 | 19 and later.) | |
656 | ||
657 | @example | |
658 | @group | |
659 | (prin1-to-string "foo") | |
660 | @result{} "\"foo\"" | |
661 | @end group | |
662 | @group | |
663 | (prin1-to-string "foo" t) | |
664 | @result{} "foo" | |
665 | @end group | |
666 | @end example | |
667 | ||
668 | See @code{format}, in @ref{Formatting Strings}, for other ways to obtain | |
669 | the printed representation of a Lisp object as a string. | |
670 | @end defun | |
671 | ||
672 | @defmac with-output-to-string body@dots{} | |
673 | This macro executes the @var{body} forms with @code{standard-output} set | |
674 | up to feed output into a string. Then it returns that string. | |
675 | ||
676 | For example, if the current buffer name is @samp{foo}, | |
677 | ||
678 | @example | |
679 | (with-output-to-string | |
680 | (princ "The buffer is ") | |
681 | (princ (buffer-name))) | |
682 | @end example | |
683 | ||
684 | @noindent | |
685 | returns @code{"The buffer is foo"}. | |
686 | @end defmac | |
687 | ||
688 | @node Output Variables | |
689 | @section Variables Affecting Output | |
690 | @cindex output-controlling variables | |
691 | ||
692 | @defvar standard-output | |
693 | The value of this variable is the default output stream---the stream | |
694 | that print functions use when the @var{stream} argument is @code{nil}. | |
695 | The default is @code{t}, meaning display in the echo area. | |
696 | @end defvar | |
697 | ||
698 | @defvar print-quoted | |
699 | If this is non-@code{nil}, that means to print quoted forms using | |
700 | abbreviated reader syntax. @code{(quote foo)} prints as @code{'foo}, | |
701 | @code{(function foo)} as @code{#'foo}, and backquoted forms print | |
702 | using modern backquote syntax. | |
703 | @end defvar | |
704 | ||
705 | @defvar print-escape-newlines | |
706 | @cindex @samp{\n} in print | |
707 | @cindex escape characters | |
708 | If this variable is non-@code{nil}, then newline characters in strings | |
709 | are printed as @samp{\n} and formfeeds are printed as @samp{\f}. | |
710 | Normally these characters are printed as actual newlines and formfeeds. | |
711 | ||
712 | This variable affects the print functions @code{prin1} and @code{print} | |
713 | that print with quoting. It does not affect @code{princ}. Here is an | |
714 | example using @code{prin1}: | |
715 | ||
716 | @example | |
717 | @group | |
718 | (prin1 "a\nb") | |
719 | @print{} "a | |
720 | @print{} b" | |
721 | @result{} "a | |
722 | b" | |
723 | @end group | |
724 | ||
725 | @group | |
726 | (let ((print-escape-newlines t)) | |
727 | (prin1 "a\nb")) | |
728 | @print{} "a\nb" | |
729 | @result{} "a | |
730 | b" | |
731 | @end group | |
732 | @end example | |
733 | ||
734 | @noindent | |
735 | In the second expression, the local binding of | |
736 | @code{print-escape-newlines} is in effect during the call to | |
737 | @code{prin1}, but not during the printing of the result. | |
738 | @end defvar | |
739 | ||
740 | @defvar print-escape-nonascii | |
741 | If this variable is non-@code{nil}, then unibyte non-@acronym{ASCII} | |
742 | characters in strings are unconditionally printed as backslash sequences | |
743 | by the print functions @code{prin1} and @code{print} that print with | |
744 | quoting. | |
745 | ||
746 | Those functions also use backslash sequences for unibyte non-@acronym{ASCII} | |
747 | characters, regardless of the value of this variable, when the output | |
748 | stream is a multibyte buffer or a marker pointing into one. | |
749 | @end defvar | |
750 | ||
751 | @defvar print-escape-multibyte | |
752 | If this variable is non-@code{nil}, then multibyte non-@acronym{ASCII} | |
753 | characters in strings are unconditionally printed as backslash sequences | |
754 | by the print functions @code{prin1} and @code{print} that print with | |
755 | quoting. | |
756 | ||
757 | Those functions also use backslash sequences for multibyte | |
758 | non-@acronym{ASCII} characters, regardless of the value of this variable, | |
759 | when the output stream is a unibyte buffer or a marker pointing into | |
760 | one. | |
761 | @end defvar | |
762 | ||
763 | @defvar print-length | |
764 | @cindex printing limits | |
765 | The value of this variable is the maximum number of elements to print in | |
766 | any list, vector or bool-vector. If an object being printed has more | |
767 | than this many elements, it is abbreviated with an ellipsis. | |
768 | ||
769 | If the value is @code{nil} (the default), then there is no limit. | |
770 | ||
771 | @example | |
772 | @group | |
773 | (setq print-length 2) | |
774 | @result{} 2 | |
775 | @end group | |
776 | @group | |
777 | (print '(1 2 3 4 5)) | |
778 | @print{} (1 2 ...) | |
779 | @result{} (1 2 ...) | |
780 | @end group | |
781 | @end example | |
782 | @end defvar | |
783 | ||
784 | @defvar print-level | |
785 | The value of this variable is the maximum depth of nesting of | |
786 | parentheses and brackets when printed. Any list or vector at a depth | |
787 | exceeding this limit is abbreviated with an ellipsis. A value of | |
788 | @code{nil} (which is the default) means no limit. | |
789 | @end defvar | |
790 | ||
791 | @defopt eval-expression-print-length | |
792 | @defoptx eval-expression-print-level | |
793 | These are the values for @code{print-length} and @code{print-level} | |
794 | used by @code{eval-expression}, and thus, indirectly, by many | |
795 | interactive evaluation commands (@pxref{Lisp Eval,, Evaluating | |
796 | Emacs-Lisp Expressions, emacs, The GNU Emacs Manual}). | |
797 | @end defopt | |
798 | ||
799 | These variables are used for detecting and reporting circular | |
800 | and shared structure: | |
801 | ||
802 | @defvar print-circle | |
dd449674 | 803 | If non-@code{nil}, this variable enables detection of circular and |
544c5fc9 | 804 | shared structure in printing. @xref{Circular Objects}. |
b8d4c8d0 GM |
805 | @end defvar |
806 | ||
807 | @defvar print-gensym | |
808 | If non-@code{nil}, this variable enables detection of uninterned symbols | |
809 | (@pxref{Creating Symbols}) in printing. When this is enabled, | |
810 | uninterned symbols print with the prefix @samp{#:}, which tells the Lisp | |
811 | reader to produce an uninterned symbol. | |
812 | @end defvar | |
813 | ||
814 | @defvar print-continuous-numbering | |
815 | If non-@code{nil}, that means number continuously across print calls. | |
816 | This affects the numbers printed for @samp{#@var{n}=} labels and | |
817 | @samp{#@var{m}#} references. | |
818 | ||
819 | Don't set this variable with @code{setq}; you should only bind it | |
820 | temporarily to @code{t} with @code{let}. When you do that, you should | |
821 | also bind @code{print-number-table} to @code{nil}. | |
822 | @end defvar | |
823 | ||
824 | @defvar print-number-table | |
825 | This variable holds a vector used internally by printing to implement | |
826 | the @code{print-circle} feature. You should not use it except | |
827 | to bind it to @code{nil} when you bind @code{print-continuous-numbering}. | |
828 | @end defvar | |
829 | ||
830 | @defvar float-output-format | |
831 | This variable specifies how to print floating point numbers. Its | |
832 | default value is @code{nil}, meaning use the shortest output | |
833 | that represents the number without losing information. | |
834 | ||
835 | To control output format more precisely, you can put a string in this | |
836 | variable. The string should hold a @samp{%}-specification to be used | |
837 | in the C function @code{sprintf}. For further restrictions on what | |
838 | you can use, see the variable's documentation string. | |
839 | @end defvar | |
840 | ||
841 | @ignore | |
842 | arch-tag: 07636b8c-c4e3-4735-9e06-2e864320b434 | |
843 | @end ignore |