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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
73b0cd50 | 3 | @c Copyright (C) 1990-1994, 1998-1999, 2001-2011 Free Software Foundation, Inc. |
b8d4c8d0 | 4 | @c See the file elisp.texi for copying conditions. |
6336d8c3 | 5 | @setfilename ../../info/streams |
b8d4c8d0 GM |
6 | @node Read and Print, Minibuffers, Debugging, Top |
7 | @comment node-name, next, previous, up | |
8 | @chapter Reading and Printing Lisp Objects | |
9 | ||
10 | @dfn{Printing} and @dfn{reading} are the operations of converting Lisp | |
11 | objects to textual form and vice versa. They use the printed | |
12 | representations and read syntax described in @ref{Lisp Data Types}. | |
13 | ||
14 | This chapter describes the Lisp functions for reading and printing. | |
15 | It also describes @dfn{streams}, which specify where to get the text (if | |
16 | reading) or where to put it (if printing). | |
17 | ||
18 | @menu | |
19 | * Streams Intro:: Overview of streams, reading and printing. | |
20 | * Input Streams:: Various data types that can be used as input streams. | |
21 | * Input Functions:: Functions to read Lisp objects from text. | |
22 | * Output Streams:: Various data types that can be used as output streams. | |
23 | * Output Functions:: Functions to print Lisp objects as text. | |
24 | * Output Variables:: Variables that control what the printing functions do. | |
25 | @end menu | |
26 | ||
27 | @node Streams Intro | |
28 | @section Introduction to Reading and Printing | |
29 | @cindex Lisp reader | |
30 | @cindex printing | |
31 | @cindex reading | |
32 | ||
33 | @dfn{Reading} a Lisp object means parsing a Lisp expression in textual | |
34 | form and producing a corresponding Lisp object. This is how Lisp | |
35 | programs get into Lisp from files of Lisp code. We call the text the | |
36 | @dfn{read syntax} of the object. For example, the text @samp{(a .@: 5)} | |
37 | is the read syntax for a cons cell whose @sc{car} is @code{a} and whose | |
38 | @sc{cdr} is the number 5. | |
39 | ||
40 | @dfn{Printing} a Lisp object means producing text that represents that | |
41 | object---converting the object to its @dfn{printed representation} | |
42 | (@pxref{Printed Representation}). Printing the cons cell described | |
43 | above produces the text @samp{(a .@: 5)}. | |
44 | ||
45 | Reading and printing are more or less inverse operations: printing the | |
46 | object that results from reading a given piece of text often produces | |
47 | the same text, and reading the text that results from printing an object | |
48 | usually produces a similar-looking object. For example, printing the | |
49 | symbol @code{foo} produces the text @samp{foo}, and reading that text | |
50 | returns the symbol @code{foo}. Printing a list whose elements are | |
51 | @code{a} and @code{b} produces the text @samp{(a b)}, and reading that | |
52 | text produces a list (but not the same list) with elements @code{a} | |
53 | and @code{b}. | |
54 | ||
55 | However, these two operations are not precisely inverse to each other. | |
56 | There are three kinds of exceptions: | |
57 | ||
58 | @itemize @bullet | |
59 | @item | |
60 | Printing can produce text that cannot be read. For example, buffers, | |
61 | windows, frames, subprocesses and markers print as text that starts | |
62 | with @samp{#}; if you try to read this text, you get an error. There is | |
63 | no way to read those data types. | |
64 | ||
65 | @item | |
66 | One object can have multiple textual representations. For example, | |
67 | @samp{1} and @samp{01} represent the same integer, and @samp{(a b)} and | |
68 | @samp{(a .@: (b))} represent the same list. Reading will accept any of | |
69 | the alternatives, but printing must choose one of them. | |
70 | ||
71 | @item | |
72 | Comments can appear at certain points in the middle of an object's | |
73 | read sequence without affecting the result of reading it. | |
74 | @end itemize | |
75 | ||
76 | @node Input Streams | |
77 | @section Input Streams | |
78 | @cindex stream (for reading) | |
79 | @cindex input stream | |
80 | ||
81 | Most of the Lisp functions for reading text take an @dfn{input stream} | |
82 | as an argument. The input stream specifies where or how to get the | |
83 | characters of the text to be read. Here are the possible types of input | |
84 | stream: | |
85 | ||
86 | @table @asis | |
87 | @item @var{buffer} | |
88 | @cindex buffer input stream | |
89 | The input characters are read from @var{buffer}, starting with the | |
90 | character directly after point. Point advances as characters are read. | |
91 | ||
92 | @item @var{marker} | |
93 | @cindex marker input stream | |
94 | The input characters are read from the buffer that @var{marker} is in, | |
95 | starting with the character directly after the marker. The marker | |
96 | position advances as characters are read. The value of point in the | |
97 | buffer has no effect when the stream is a marker. | |
98 | ||
99 | @item @var{string} | |
100 | @cindex string input stream | |
101 | The input characters are taken from @var{string}, starting at the first | |
102 | character in the string and using as many characters as required. | |
103 | ||
104 | @item @var{function} | |
105 | @cindex function input stream | |
106 | The input characters are generated by @var{function}, which must support | |
107 | two kinds of calls: | |
108 | ||
109 | @itemize @bullet | |
110 | @item | |
111 | When it is called with no arguments, it should return the next character. | |
112 | ||
113 | @item | |
114 | When it is called with one argument (always a character), @var{function} | |
115 | should save the argument and arrange to return it on the next call. | |
116 | This is called @dfn{unreading} the character; it happens when the Lisp | |
117 | reader reads one character too many and wants to ``put it back where it | |
118 | came from.'' In this case, it makes no difference what value | |
119 | @var{function} returns. | |
120 | @end itemize | |
121 | ||
122 | @item @code{t} | |
123 | @cindex @code{t} input stream | |
124 | @code{t} used as a stream means that the input is read from the | |
125 | minibuffer. In fact, the minibuffer is invoked once and the text | |
126 | given by the user is made into a string that is then used as the | |
127 | input stream. If Emacs is running in batch mode, standard input is used | |
128 | instead of the minibuffer. For example, | |
129 | @example | |
130 | (message "%s" (read t)) | |
131 | @end example | |
132 | will read a Lisp expression from standard input and print the result | |
133 | to standard output. | |
134 | ||
135 | @item @code{nil} | |
136 | @cindex @code{nil} input stream | |
137 | @code{nil} supplied as an input stream means to use the value of | |
138 | @code{standard-input} instead; that value is the @dfn{default input | |
139 | stream}, and must be a non-@code{nil} input stream. | |
140 | ||
141 | @item @var{symbol} | |
142 | A symbol as input stream is equivalent to the symbol's function | |
143 | definition (if any). | |
144 | @end table | |
145 | ||
146 | Here is an example of reading from a stream that is a buffer, showing | |
147 | where point is located before and after: | |
148 | ||
149 | @example | |
150 | @group | |
151 | ---------- Buffer: foo ---------- | |
152 | This@point{} is the contents of foo. | |
153 | ---------- Buffer: foo ---------- | |
154 | @end group | |
155 | ||
156 | @group | |
157 | (read (get-buffer "foo")) | |
158 | @result{} is | |
159 | @end group | |
160 | @group | |
161 | (read (get-buffer "foo")) | |
162 | @result{} the | |
163 | @end group | |
164 | ||
165 | @group | |
166 | ---------- Buffer: foo ---------- | |
167 | This is the@point{} contents of foo. | |
168 | ---------- Buffer: foo ---------- | |
169 | @end group | |
170 | @end example | |
171 | ||
172 | @noindent | |
173 | Note that the first read skips a space. Reading skips any amount of | |
174 | whitespace preceding the significant text. | |
175 | ||
176 | Here is an example of reading from a stream that is a marker, | |
177 | initially positioned at the beginning of the buffer shown. The value | |
178 | read is the symbol @code{This}. | |
179 | ||
180 | @example | |
181 | @group | |
182 | ||
183 | ---------- Buffer: foo ---------- | |
184 | This is the contents of foo. | |
185 | ---------- Buffer: foo ---------- | |
186 | @end group | |
187 | ||
188 | @group | |
189 | (setq m (set-marker (make-marker) 1 (get-buffer "foo"))) | |
190 | @result{} #<marker at 1 in foo> | |
191 | @end group | |
192 | @group | |
193 | (read m) | |
194 | @result{} This | |
195 | @end group | |
196 | @group | |
197 | m | |
198 | @result{} #<marker at 5 in foo> ;; @r{Before the first space.} | |
199 | @end group | |
200 | @end example | |
201 | ||
202 | Here we read from the contents of a string: | |
203 | ||
204 | @example | |
205 | @group | |
206 | (read "(When in) the course") | |
207 | @result{} (When in) | |
208 | @end group | |
209 | @end example | |
210 | ||
211 | The following example reads from the minibuffer. The | |
212 | prompt is: @w{@samp{Lisp expression: }}. (That is always the prompt | |
213 | used when you read from the stream @code{t}.) The user's input is shown | |
214 | following the prompt. | |
215 | ||
216 | @example | |
217 | @group | |
218 | (read t) | |
219 | @result{} 23 | |
220 | ---------- Buffer: Minibuffer ---------- | |
221 | Lisp expression: @kbd{23 @key{RET}} | |
222 | ---------- Buffer: Minibuffer ---------- | |
223 | @end group | |
224 | @end example | |
225 | ||
226 | Finally, here is an example of a stream that is a function, named | |
227 | @code{useless-stream}. Before we use the stream, we initialize the | |
228 | variable @code{useless-list} to a list of characters. Then each call to | |
229 | the function @code{useless-stream} obtains the next character in the list | |
230 | or unreads a character by adding it to the front of the list. | |
231 | ||
232 | @example | |
233 | @group | |
234 | (setq useless-list (append "XY()" nil)) | |
235 | @result{} (88 89 40 41) | |
236 | @end group | |
237 | ||
238 | @group | |
239 | (defun useless-stream (&optional unread) | |
240 | (if unread | |
241 | (setq useless-list (cons unread useless-list)) | |
242 | (prog1 (car useless-list) | |
243 | (setq useless-list (cdr useless-list))))) | |
244 | @result{} useless-stream | |
245 | @end group | |
246 | @end example | |
247 | ||
248 | @noindent | |
249 | Now we read using the stream thus constructed: | |
250 | ||
251 | @example | |
252 | @group | |
253 | (read 'useless-stream) | |
254 | @result{} XY | |
255 | @end group | |
256 | ||
257 | @group | |
258 | useless-list | |
259 | @result{} (40 41) | |
260 | @end group | |
261 | @end example | |
262 | ||
263 | @noindent | |
264 | Note that the open and close parentheses remain in the list. The Lisp | |
265 | reader encountered the open parenthesis, decided that it ended the | |
266 | input, and unread it. Another attempt to read from the stream at this | |
267 | point would read @samp{()} and return @code{nil}. | |
268 | ||
269 | @defun get-file-char | |
270 | This function is used internally as an input stream to read from the | |
271 | input file opened by the function @code{load}. Don't use this function | |
272 | yourself. | |
273 | @end defun | |
274 | ||
275 | @node Input Functions | |
276 | @section Input Functions | |
277 | ||
278 | This section describes the Lisp functions and variables that pertain | |
279 | to reading. | |
280 | ||
281 | In the functions below, @var{stream} stands for an input stream (see | |
282 | the previous section). If @var{stream} is @code{nil} or omitted, it | |
283 | defaults to the value of @code{standard-input}. | |
284 | ||
285 | @kindex end-of-file | |
286 | An @code{end-of-file} error is signaled if reading encounters an | |
287 | unterminated list, vector, or string. | |
288 | ||
289 | @defun read &optional stream | |
290 | This function reads one textual Lisp expression from @var{stream}, | |
291 | returning it as a Lisp object. This is the basic Lisp input function. | |
292 | @end defun | |
293 | ||
294 | @defun read-from-string string &optional start end | |
295 | @cindex string to object | |
296 | This function reads the first textual Lisp expression from the text in | |
297 | @var{string}. It returns a cons cell whose @sc{car} is that expression, | |
298 | and whose @sc{cdr} is an integer giving the position of the next | |
299 | remaining character in the string (i.e., the first one not read). | |
300 | ||
301 | If @var{start} is supplied, then reading begins at index @var{start} in | |
302 | the string (where the first character is at index 0). If you specify | |
303 | @var{end}, then reading is forced to stop just before that index, as if | |
304 | the rest of the string were not there. | |
305 | ||
306 | For example: | |
307 | ||
308 | @example | |
309 | @group | |
310 | (read-from-string "(setq x 55) (setq y 5)") | |
311 | @result{} ((setq x 55) . 11) | |
312 | @end group | |
313 | @group | |
314 | (read-from-string "\"A short string\"") | |
315 | @result{} ("A short string" . 16) | |
316 | @end group | |
317 | ||
318 | @group | |
319 | ;; @r{Read starting at the first character.} | |
320 | (read-from-string "(list 112)" 0) | |
321 | @result{} ((list 112) . 10) | |
322 | @end group | |
323 | @group | |
324 | ;; @r{Read starting at the second character.} | |
325 | (read-from-string "(list 112)" 1) | |
326 | @result{} (list . 5) | |
327 | @end group | |
328 | @group | |
329 | ;; @r{Read starting at the seventh character,} | |
330 | ;; @r{and stopping at the ninth.} | |
331 | (read-from-string "(list 112)" 6 8) | |
332 | @result{} (11 . 8) | |
333 | @end group | |
334 | @end example | |
335 | @end defun | |
336 | ||
337 | @defvar standard-input | |
338 | This variable holds the default input stream---the stream that | |
339 | @code{read} uses when the @var{stream} argument is @code{nil}. | |
340 | The default is @code{t}, meaning use the minibuffer. | |
341 | @end defvar | |
342 | ||
dd449674 CY |
343 | @defvar read-circle |
344 | If non-@code{nil}, this variable enables the reading of circular and | |
345 | shared structures. @xref{Circular Objects}. Its default value is | |
346 | @code{t}. | |
347 | @end defvar | |
348 | ||
b8d4c8d0 GM |
349 | @node Output Streams |
350 | @section Output Streams | |
351 | @cindex stream (for printing) | |
352 | @cindex output stream | |
353 | ||
354 | An output stream specifies what to do with the characters produced | |
355 | by printing. Most print functions accept an output stream as an | |
356 | optional argument. Here are the possible types of output stream: | |
357 | ||
358 | @table @asis | |
359 | @item @var{buffer} | |
360 | @cindex buffer output stream | |
361 | The output characters are inserted into @var{buffer} at point. | |
362 | Point advances as characters are inserted. | |
363 | ||
364 | @item @var{marker} | |
365 | @cindex marker output stream | |
366 | The output characters are inserted into the buffer that @var{marker} | |
367 | points into, at the marker position. The marker position advances as | |
368 | characters are inserted. The value of point in the buffer has no effect | |
369 | on printing when the stream is a marker, and this kind of printing | |
370 | does not move point (except that if the marker points at or before the | |
371 | position of point, point advances with the surrounding text, as | |
372 | usual). | |
373 | ||
374 | @item @var{function} | |
375 | @cindex function output stream | |
376 | The output characters are passed to @var{function}, which is responsible | |
377 | for storing them away. It is called with a single character as | |
378 | argument, as many times as there are characters to be output, and | |
379 | is responsible for storing the characters wherever you want to put them. | |
380 | ||
381 | @item @code{t} | |
382 | @cindex @code{t} output stream | |
383 | The output characters are displayed in the echo area. | |
384 | ||
385 | @item @code{nil} | |
386 | @cindex @code{nil} output stream | |
387 | @code{nil} specified as an output stream means to use the value of | |
388 | @code{standard-output} instead; that value is the @dfn{default output | |
389 | stream}, and must not be @code{nil}. | |
390 | ||
391 | @item @var{symbol} | |
392 | A symbol as output stream is equivalent to the symbol's function | |
393 | definition (if any). | |
394 | @end table | |
395 | ||
396 | Many of the valid output streams are also valid as input streams. The | |
397 | difference between input and output streams is therefore more a matter | |
398 | of how you use a Lisp object, than of different types of object. | |
399 | ||
400 | Here is an example of a buffer used as an output stream. Point is | |
401 | initially located as shown immediately before the @samp{h} in | |
402 | @samp{the}. At the end, point is located directly before that same | |
403 | @samp{h}. | |
404 | ||
405 | @cindex print example | |
406 | @example | |
407 | @group | |
408 | ---------- Buffer: foo ---------- | |
409 | This is t@point{}he contents of foo. | |
410 | ---------- Buffer: foo ---------- | |
411 | @end group | |
412 | ||
413 | (print "This is the output" (get-buffer "foo")) | |
414 | @result{} "This is the output" | |
415 | ||
416 | @group | |
417 | ---------- Buffer: foo ---------- | |
418 | This is t | |
419 | "This is the output" | |
420 | @point{}he contents of foo. | |
421 | ---------- Buffer: foo ---------- | |
422 | @end group | |
423 | @end example | |
424 | ||
425 | Now we show a use of a marker as an output stream. Initially, the | |
426 | marker is in buffer @code{foo}, between the @samp{t} and the @samp{h} in | |
427 | the word @samp{the}. At the end, the marker has advanced over the | |
428 | inserted text so that it remains positioned before the same @samp{h}. | |
429 | Note that the location of point, shown in the usual fashion, has no | |
430 | effect. | |
431 | ||
432 | @example | |
433 | @group | |
434 | ---------- Buffer: foo ---------- | |
435 | This is the @point{}output | |
436 | ---------- Buffer: foo ---------- | |
437 | @end group | |
438 | ||
439 | @group | |
440 | (setq m (copy-marker 10)) | |
441 | @result{} #<marker at 10 in foo> | |
442 | @end group | |
443 | ||
444 | @group | |
445 | (print "More output for foo." m) | |
446 | @result{} "More output for foo." | |
447 | @end group | |
448 | ||
449 | @group | |
450 | ---------- Buffer: foo ---------- | |
451 | This is t | |
452 | "More output for foo." | |
453 | he @point{}output | |
454 | ---------- Buffer: foo ---------- | |
455 | @end group | |
456 | ||
457 | @group | |
458 | m | |
459 | @result{} #<marker at 34 in foo> | |
460 | @end group | |
461 | @end example | |
462 | ||
463 | The following example shows output to the echo area: | |
464 | ||
465 | @example | |
466 | @group | |
467 | (print "Echo Area output" t) | |
468 | @result{} "Echo Area output" | |
469 | ---------- Echo Area ---------- | |
470 | "Echo Area output" | |
471 | ---------- Echo Area ---------- | |
472 | @end group | |
473 | @end example | |
474 | ||
475 | Finally, we show the use of a function as an output stream. The | |
476 | function @code{eat-output} takes each character that it is given and | |
477 | conses it onto the front of the list @code{last-output} (@pxref{Building | |
478 | Lists}). At the end, the list contains all the characters output, but | |
479 | in reverse order. | |
480 | ||
481 | @example | |
482 | @group | |
483 | (setq last-output nil) | |
484 | @result{} nil | |
485 | @end group | |
486 | ||
487 | @group | |
488 | (defun eat-output (c) | |
489 | (setq last-output (cons c last-output))) | |
490 | @result{} eat-output | |
491 | @end group | |
492 | ||
493 | @group | |
494 | (print "This is the output" 'eat-output) | |
495 | @result{} "This is the output" | |
496 | @end group | |
497 | ||
498 | @group | |
499 | last-output | |
500 | @result{} (10 34 116 117 112 116 117 111 32 101 104 | |
501 | 116 32 115 105 32 115 105 104 84 34 10) | |
502 | @end group | |
503 | @end example | |
504 | ||
505 | @noindent | |
506 | Now we can put the output in the proper order by reversing the list: | |
507 | ||
508 | @example | |
509 | @group | |
510 | (concat (nreverse last-output)) | |
511 | @result{} " | |
512 | \"This is the output\" | |
513 | " | |
514 | @end group | |
515 | @end example | |
516 | ||
517 | @noindent | |
518 | Calling @code{concat} converts the list to a string so you can see its | |
519 | contents more clearly. | |
520 | ||
521 | @node Output Functions | |
522 | @section Output Functions | |
523 | ||
524 | This section describes the Lisp functions for printing Lisp | |
525 | objects---converting objects into their printed representation. | |
526 | ||
527 | @cindex @samp{"} in printing | |
528 | @cindex @samp{\} in printing | |
529 | @cindex quoting characters in printing | |
530 | @cindex escape characters in printing | |
531 | Some of the Emacs printing functions add quoting characters to the | |
532 | output when necessary so that it can be read properly. The quoting | |
533 | characters used are @samp{"} and @samp{\}; they distinguish strings from | |
534 | symbols, and prevent punctuation characters in strings and symbols from | |
535 | being taken as delimiters when reading. @xref{Printed Representation}, | |
536 | for full details. You specify quoting or no quoting by the choice of | |
537 | printing function. | |
538 | ||
539 | If the text is to be read back into Lisp, then you should print with | |
540 | quoting characters to avoid ambiguity. Likewise, if the purpose is to | |
541 | describe a Lisp object clearly for a Lisp programmer. However, if the | |
542 | purpose of the output is to look nice for humans, then it is usually | |
543 | better to print without quoting. | |
544 | ||
545 | Lisp objects can refer to themselves. Printing a self-referential | |
546 | object in the normal way would require an infinite amount of text, and | |
547 | the attempt could cause infinite recursion. Emacs detects such | |
548 | recursion and prints @samp{#@var{level}} instead of recursively printing | |
549 | an object already being printed. For example, here @samp{#0} indicates | |
550 | a recursive reference to the object at level 0 of the current print | |
551 | operation: | |
552 | ||
553 | @example | |
554 | (setq foo (list nil)) | |
555 | @result{} (nil) | |
556 | (setcar foo foo) | |
557 | @result{} (#0) | |
558 | @end example | |
559 | ||
560 | In the functions below, @var{stream} stands for an output stream. | |
561 | (See the previous section for a description of output streams.) If | |
562 | @var{stream} is @code{nil} or omitted, it defaults to the value of | |
563 | @code{standard-output}. | |
564 | ||
565 | @defun print object &optional stream | |
566 | @cindex Lisp printer | |
567 | The @code{print} function is a convenient way of printing. It outputs | |
568 | the printed representation of @var{object} to @var{stream}, printing in | |
569 | addition one newline before @var{object} and another after it. Quoting | |
570 | characters are used. @code{print} returns @var{object}. For example: | |
571 | ||
572 | @example | |
573 | @group | |
574 | (progn (print 'The\ cat\ in) | |
575 | (print "the hat") | |
576 | (print " came back")) | |
577 | @print{} | |
578 | @print{} The\ cat\ in | |
579 | @print{} | |
580 | @print{} "the hat" | |
581 | @print{} | |
582 | @print{} " came back" | |
583 | @result{} " came back" | |
584 | @end group | |
585 | @end example | |
586 | @end defun | |
587 | ||
588 | @defun prin1 object &optional stream | |
589 | This function outputs the printed representation of @var{object} to | |
590 | @var{stream}. It does not print newlines to separate output as | |
591 | @code{print} does, but it does use quoting characters just like | |
592 | @code{print}. It returns @var{object}. | |
593 | ||
594 | @example | |
595 | @group | |
596 | (progn (prin1 'The\ cat\ in) | |
597 | (prin1 "the hat") | |
598 | (prin1 " came back")) | |
599 | @print{} The\ cat\ in"the hat"" came back" | |
600 | @result{} " came back" | |
601 | @end group | |
602 | @end example | |
603 | @end defun | |
604 | ||
605 | @defun princ object &optional stream | |
606 | This function outputs the printed representation of @var{object} to | |
607 | @var{stream}. It returns @var{object}. | |
608 | ||
609 | This function is intended to produce output that is readable by people, | |
610 | not by @code{read}, so it doesn't insert quoting characters and doesn't | |
611 | put double-quotes around the contents of strings. It does not add any | |
612 | spacing between calls. | |
613 | ||
614 | @example | |
615 | @group | |
616 | (progn | |
617 | (princ 'The\ cat) | |
618 | (princ " in the \"hat\"")) | |
619 | @print{} The cat in the "hat" | |
620 | @result{} " in the \"hat\"" | |
621 | @end group | |
622 | @end example | |
623 | @end defun | |
624 | ||
625 | @defun terpri &optional stream | |
626 | @cindex newline in print | |
627 | This function outputs a newline to @var{stream}. The name stands | |
628 | for ``terminate print.'' | |
629 | @end defun | |
630 | ||
631 | @defun write-char character &optional stream | |
632 | This function outputs @var{character} to @var{stream}. It returns | |
633 | @var{character}. | |
634 | @end defun | |
635 | ||
636 | @defun prin1-to-string object &optional noescape | |
637 | @cindex object to string | |
638 | This function returns a string containing the text that @code{prin1} | |
639 | would have printed for the same argument. | |
640 | ||
641 | @example | |
642 | @group | |
643 | (prin1-to-string 'foo) | |
644 | @result{} "foo" | |
645 | @end group | |
646 | @group | |
647 | (prin1-to-string (mark-marker)) | |
648 | @result{} "#<marker at 2773 in strings.texi>" | |
649 | @end group | |
650 | @end example | |
651 | ||
652 | If @var{noescape} is non-@code{nil}, that inhibits use of quoting | |
653 | characters in the output. (This argument is supported in Emacs versions | |
654 | 19 and later.) | |
655 | ||
656 | @example | |
657 | @group | |
658 | (prin1-to-string "foo") | |
659 | @result{} "\"foo\"" | |
660 | @end group | |
661 | @group | |
662 | (prin1-to-string "foo" t) | |
663 | @result{} "foo" | |
664 | @end group | |
665 | @end example | |
666 | ||
667 | See @code{format}, in @ref{Formatting Strings}, for other ways to obtain | |
668 | the printed representation of a Lisp object as a string. | |
669 | @end defun | |
670 | ||
671 | @defmac with-output-to-string body@dots{} | |
672 | This macro executes the @var{body} forms with @code{standard-output} set | |
673 | up to feed output into a string. Then it returns that string. | |
674 | ||
675 | For example, if the current buffer name is @samp{foo}, | |
676 | ||
677 | @example | |
678 | (with-output-to-string | |
679 | (princ "The buffer is ") | |
680 | (princ (buffer-name))) | |
681 | @end example | |
682 | ||
683 | @noindent | |
684 | returns @code{"The buffer is foo"}. | |
685 | @end defmac | |
686 | ||
36cb87a1 LMI |
687 | @defun pp object &optional stream |
688 | This function outputs @var{object} to @var{stream}, just like | |
689 | @code{prin1}, but does it in a more ``pretty'' way. That is, it'll | |
690 | indent and fill the object to make it more readable for humans. | |
691 | @end defun | |
692 | ||
b8d4c8d0 GM |
693 | @node Output Variables |
694 | @section Variables Affecting Output | |
695 | @cindex output-controlling variables | |
696 | ||
697 | @defvar standard-output | |
698 | The value of this variable is the default output stream---the stream | |
699 | that print functions use when the @var{stream} argument is @code{nil}. | |
700 | The default is @code{t}, meaning display in the echo area. | |
701 | @end defvar | |
702 | ||
703 | @defvar print-quoted | |
704 | If this is non-@code{nil}, that means to print quoted forms using | |
705 | abbreviated reader syntax. @code{(quote foo)} prints as @code{'foo}, | |
706 | @code{(function foo)} as @code{#'foo}, and backquoted forms print | |
707 | using modern backquote syntax. | |
708 | @end defvar | |
709 | ||
710 | @defvar print-escape-newlines | |
711 | @cindex @samp{\n} in print | |
712 | @cindex escape characters | |
713 | If this variable is non-@code{nil}, then newline characters in strings | |
714 | are printed as @samp{\n} and formfeeds are printed as @samp{\f}. | |
715 | Normally these characters are printed as actual newlines and formfeeds. | |
716 | ||
717 | This variable affects the print functions @code{prin1} and @code{print} | |
718 | that print with quoting. It does not affect @code{princ}. Here is an | |
719 | example using @code{prin1}: | |
720 | ||
721 | @example | |
722 | @group | |
723 | (prin1 "a\nb") | |
724 | @print{} "a | |
725 | @print{} b" | |
726 | @result{} "a | |
727 | b" | |
728 | @end group | |
729 | ||
730 | @group | |
731 | (let ((print-escape-newlines t)) | |
732 | (prin1 "a\nb")) | |
733 | @print{} "a\nb" | |
734 | @result{} "a | |
735 | b" | |
736 | @end group | |
737 | @end example | |
738 | ||
739 | @noindent | |
740 | In the second expression, the local binding of | |
741 | @code{print-escape-newlines} is in effect during the call to | |
742 | @code{prin1}, but not during the printing of the result. | |
743 | @end defvar | |
744 | ||
745 | @defvar print-escape-nonascii | |
746 | If this variable is non-@code{nil}, then unibyte non-@acronym{ASCII} | |
747 | characters in strings are unconditionally printed as backslash sequences | |
748 | by the print functions @code{prin1} and @code{print} that print with | |
749 | quoting. | |
750 | ||
751 | Those functions also use backslash sequences for unibyte non-@acronym{ASCII} | |
752 | characters, regardless of the value of this variable, when the output | |
753 | stream is a multibyte buffer or a marker pointing into one. | |
754 | @end defvar | |
755 | ||
756 | @defvar print-escape-multibyte | |
757 | If this variable is non-@code{nil}, then multibyte non-@acronym{ASCII} | |
758 | characters in strings are unconditionally printed as backslash sequences | |
759 | by the print functions @code{prin1} and @code{print} that print with | |
760 | quoting. | |
761 | ||
762 | Those functions also use backslash sequences for multibyte | |
763 | non-@acronym{ASCII} characters, regardless of the value of this variable, | |
764 | when the output stream is a unibyte buffer or a marker pointing into | |
765 | one. | |
766 | @end defvar | |
767 | ||
768 | @defvar print-length | |
769 | @cindex printing limits | |
770 | The value of this variable is the maximum number of elements to print in | |
771 | any list, vector or bool-vector. If an object being printed has more | |
772 | than this many elements, it is abbreviated with an ellipsis. | |
773 | ||
774 | If the value is @code{nil} (the default), then there is no limit. | |
775 | ||
776 | @example | |
777 | @group | |
778 | (setq print-length 2) | |
779 | @result{} 2 | |
780 | @end group | |
781 | @group | |
782 | (print '(1 2 3 4 5)) | |
783 | @print{} (1 2 ...) | |
784 | @result{} (1 2 ...) | |
785 | @end group | |
786 | @end example | |
787 | @end defvar | |
788 | ||
789 | @defvar print-level | |
790 | The value of this variable is the maximum depth of nesting of | |
791 | parentheses and brackets when printed. Any list or vector at a depth | |
792 | exceeding this limit is abbreviated with an ellipsis. A value of | |
793 | @code{nil} (which is the default) means no limit. | |
794 | @end defvar | |
795 | ||
796 | @defopt eval-expression-print-length | |
797 | @defoptx eval-expression-print-level | |
798 | These are the values for @code{print-length} and @code{print-level} | |
799 | used by @code{eval-expression}, and thus, indirectly, by many | |
800 | interactive evaluation commands (@pxref{Lisp Eval,, Evaluating | |
801 | Emacs-Lisp Expressions, emacs, The GNU Emacs Manual}). | |
802 | @end defopt | |
803 | ||
804 | These variables are used for detecting and reporting circular | |
805 | and shared structure: | |
806 | ||
807 | @defvar print-circle | |
dd449674 | 808 | If non-@code{nil}, this variable enables detection of circular and |
544c5fc9 | 809 | shared structure in printing. @xref{Circular Objects}. |
b8d4c8d0 GM |
810 | @end defvar |
811 | ||
812 | @defvar print-gensym | |
813 | If non-@code{nil}, this variable enables detection of uninterned symbols | |
814 | (@pxref{Creating Symbols}) in printing. When this is enabled, | |
815 | uninterned symbols print with the prefix @samp{#:}, which tells the Lisp | |
816 | reader to produce an uninterned symbol. | |
817 | @end defvar | |
818 | ||
819 | @defvar print-continuous-numbering | |
820 | If non-@code{nil}, that means number continuously across print calls. | |
821 | This affects the numbers printed for @samp{#@var{n}=} labels and | |
822 | @samp{#@var{m}#} references. | |
823 | ||
824 | Don't set this variable with @code{setq}; you should only bind it | |
825 | temporarily to @code{t} with @code{let}. When you do that, you should | |
826 | also bind @code{print-number-table} to @code{nil}. | |
827 | @end defvar | |
828 | ||
829 | @defvar print-number-table | |
830 | This variable holds a vector used internally by printing to implement | |
831 | the @code{print-circle} feature. You should not use it except | |
832 | to bind it to @code{nil} when you bind @code{print-continuous-numbering}. | |
833 | @end defvar | |
834 | ||
835 | @defvar float-output-format | |
836 | This variable specifies how to print floating point numbers. Its | |
837 | default value is @code{nil}, meaning use the shortest output | |
838 | that represents the number without losing information. | |
839 | ||
840 | To control output format more precisely, you can put a string in this | |
841 | variable. The string should hold a @samp{%}-specification to be used | |
842 | in the C function @code{sprintf}. For further restrictions on what | |
843 | you can use, see the variable's documentation string. | |
844 | @end defvar |