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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
ba318903 | 3 | @c Copyright (C) 1990-1994, 1998-1999, 2001-2014 Free Software |
ab422c4d | 4 | @c Foundation, Inc. |
b8d4c8d0 | 5 | @c See the file elisp.texi for copying conditions. |
ecc6530d | 6 | @node Read and Print |
b8d4c8d0 GM |
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 | ||
26 | @node Streams Intro | |
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 @dfn{printed representation} | |
41 | (@pxref{Printed Representation}). Printing the cons cell described | |
42 | above produces the text @samp{(a .@: 5)}. | |
43 | ||
44 | Reading and printing are more or less inverse operations: printing the | |
45 | object that results from reading a given piece of text often produces | |
46 | the same text, and reading the text that results from printing an object | |
47 | usually produces a similar-looking object. For example, printing the | |
48 | symbol @code{foo} produces the text @samp{foo}, and reading that text | |
49 | returns the symbol @code{foo}. Printing a list whose elements are | |
50 | @code{a} and @code{b} produces the text @samp{(a b)}, and reading that | |
51 | text produces a list (but not the same list) with elements @code{a} | |
52 | and @code{b}. | |
53 | ||
54 | However, these two operations are not precisely inverse to each other. | |
55 | There are three kinds of exceptions: | |
56 | ||
57 | @itemize @bullet | |
58 | @item | |
59 | Printing can produce text that cannot be read. For example, buffers, | |
60 | windows, frames, subprocesses and markers print as text that starts | |
61 | with @samp{#}; if you try to read this text, you get an error. There is | |
62 | no way to read those data types. | |
63 | ||
64 | @item | |
65 | One object can have multiple textual representations. For example, | |
66 | @samp{1} and @samp{01} represent the same integer, and @samp{(a b)} and | |
67 | @samp{(a .@: (b))} represent the same list. Reading will accept any of | |
68 | the alternatives, but printing must choose one of them. | |
69 | ||
70 | @item | |
71 | Comments can appear at certain points in the middle of an object's | |
72 | read sequence without affecting the result of reading it. | |
73 | @end itemize | |
74 | ||
75 | @node Input Streams | |
76 | @section Input Streams | |
77 | @cindex stream (for reading) | |
78 | @cindex input stream | |
79 | ||
80 | Most of the Lisp functions for reading text take an @dfn{input stream} | |
81 | as an argument. The input stream specifies where or how to get the | |
82 | characters of the text to be read. Here are the possible types of input | |
83 | stream: | |
84 | ||
85 | @table @asis | |
86 | @item @var{buffer} | |
87 | @cindex buffer input stream | |
88 | The input characters are read from @var{buffer}, starting with the | |
89 | character directly after point. Point advances as characters are read. | |
90 | ||
91 | @item @var{marker} | |
92 | @cindex marker input stream | |
93 | The input characters are read from the buffer that @var{marker} is in, | |
94 | starting with the character directly after the marker. The marker | |
95 | position advances as characters are read. The value of point in the | |
96 | buffer has no effect when the stream is a marker. | |
97 | ||
98 | @item @var{string} | |
99 | @cindex string input stream | |
100 | The input characters are taken from @var{string}, starting at the first | |
101 | character in the string and using as many characters as required. | |
102 | ||
103 | @item @var{function} | |
104 | @cindex function input stream | |
105 | The input characters are generated by @var{function}, which must support | |
106 | two kinds of calls: | |
107 | ||
108 | @itemize @bullet | |
109 | @item | |
110 | When it is called with no arguments, it should return the next character. | |
111 | ||
112 | @item | |
113 | When it is called with one argument (always a character), @var{function} | |
114 | should save the argument and arrange to return it on the next call. | |
115 | This is called @dfn{unreading} the character; it happens when the Lisp | |
116 | reader reads one character too many and wants to ``put it back where it | |
16152b76 | 117 | came from''. In this case, it makes no difference what value |
b8d4c8d0 GM |
118 | @var{function} returns. |
119 | @end itemize | |
120 | ||
121 | @item @code{t} | |
122 | @cindex @code{t} input stream | |
123 | @code{t} used as a stream means that the input is read from the | |
124 | minibuffer. In fact, the minibuffer is invoked once and the text | |
125 | given by the user is made into a string that is then used as the | |
126 | input stream. If Emacs is running in batch mode, standard input is used | |
127 | instead of the minibuffer. For example, | |
128 | @example | |
129 | (message "%s" (read t)) | |
130 | @end example | |
131 | will read a Lisp expression from standard input and print the result | |
132 | to standard output. | |
133 | ||
134 | @item @code{nil} | |
135 | @cindex @code{nil} input stream | |
136 | @code{nil} supplied as an input stream means to use the value of | |
137 | @code{standard-input} instead; that value is the @dfn{default input | |
138 | stream}, and must be a non-@code{nil} input stream. | |
139 | ||
140 | @item @var{symbol} | |
141 | A symbol as input stream is equivalent to the symbol's function | |
142 | definition (if any). | |
143 | @end table | |
144 | ||
145 | Here is an example of reading from a stream that is a buffer, showing | |
146 | where point is located before and after: | |
147 | ||
148 | @example | |
149 | @group | |
150 | ---------- Buffer: foo ---------- | |
151 | This@point{} is the contents of foo. | |
152 | ---------- Buffer: foo ---------- | |
153 | @end group | |
154 | ||
155 | @group | |
156 | (read (get-buffer "foo")) | |
157 | @result{} is | |
158 | @end group | |
159 | @group | |
160 | (read (get-buffer "foo")) | |
161 | @result{} the | |
162 | @end group | |
163 | ||
164 | @group | |
165 | ---------- Buffer: foo ---------- | |
166 | This is the@point{} contents of foo. | |
167 | ---------- Buffer: foo ---------- | |
168 | @end group | |
169 | @end example | |
170 | ||
171 | @noindent | |
172 | Note that the first read skips a space. Reading skips any amount of | |
173 | whitespace preceding the significant text. | |
174 | ||
175 | Here is an example of reading from a stream that is a marker, | |
176 | initially positioned at the beginning of the buffer shown. The value | |
177 | read is the symbol @code{This}. | |
178 | ||
179 | @example | |
180 | @group | |
181 | ||
182 | ---------- Buffer: foo ---------- | |
183 | This is the contents of foo. | |
184 | ---------- Buffer: foo ---------- | |
185 | @end group | |
186 | ||
187 | @group | |
188 | (setq m (set-marker (make-marker) 1 (get-buffer "foo"))) | |
189 | @result{} #<marker at 1 in foo> | |
190 | @end group | |
191 | @group | |
192 | (read m) | |
193 | @result{} This | |
194 | @end group | |
195 | @group | |
196 | m | |
197 | @result{} #<marker at 5 in foo> ;; @r{Before the first space.} | |
198 | @end group | |
199 | @end example | |
200 | ||
201 | Here we read from the contents of a string: | |
202 | ||
203 | @example | |
204 | @group | |
205 | (read "(When in) the course") | |
206 | @result{} (When in) | |
207 | @end group | |
208 | @end example | |
209 | ||
210 | The following example reads from the minibuffer. The | |
211 | prompt is: @w{@samp{Lisp expression: }}. (That is always the prompt | |
212 | used when you read from the stream @code{t}.) The user's input is shown | |
213 | following the prompt. | |
214 | ||
215 | @example | |
216 | @group | |
217 | (read t) | |
218 | @result{} 23 | |
219 | ---------- Buffer: Minibuffer ---------- | |
220 | Lisp expression: @kbd{23 @key{RET}} | |
221 | ---------- Buffer: Minibuffer ---------- | |
222 | @end group | |
223 | @end example | |
224 | ||
225 | Finally, here is an example of a stream that is a function, named | |
226 | @code{useless-stream}. Before we use the stream, we initialize the | |
227 | variable @code{useless-list} to a list of characters. Then each call to | |
228 | the function @code{useless-stream} obtains the next character in the list | |
229 | or unreads a character by adding it to the front of the list. | |
230 | ||
231 | @example | |
232 | @group | |
233 | (setq useless-list (append "XY()" nil)) | |
234 | @result{} (88 89 40 41) | |
235 | @end group | |
236 | ||
237 | @group | |
238 | (defun useless-stream (&optional unread) | |
239 | (if unread | |
240 | (setq useless-list (cons unread useless-list)) | |
241 | (prog1 (car useless-list) | |
242 | (setq useless-list (cdr useless-list))))) | |
243 | @result{} useless-stream | |
244 | @end group | |
245 | @end example | |
246 | ||
247 | @noindent | |
248 | Now we read using the stream thus constructed: | |
249 | ||
250 | @example | |
251 | @group | |
252 | (read 'useless-stream) | |
253 | @result{} XY | |
254 | @end group | |
255 | ||
256 | @group | |
257 | useless-list | |
258 | @result{} (40 41) | |
259 | @end group | |
260 | @end example | |
261 | ||
262 | @noindent | |
263 | Note that the open and close parentheses remain in the list. The Lisp | |
264 | reader encountered the open parenthesis, decided that it ended the | |
265 | input, and unread it. Another attempt to read from the stream at this | |
266 | point would read @samp{()} and return @code{nil}. | |
267 | ||
b8d4c8d0 GM |
268 | @node Input Functions |
269 | @section Input Functions | |
270 | ||
271 | This section describes the Lisp functions and variables that pertain | |
272 | to reading. | |
273 | ||
274 | In the functions below, @var{stream} stands for an input stream (see | |
275 | the previous section). If @var{stream} is @code{nil} or omitted, it | |
276 | defaults to the value of @code{standard-input}. | |
277 | ||
278 | @kindex end-of-file | |
279 | An @code{end-of-file} error is signaled if reading encounters an | |
280 | unterminated list, vector, or string. | |
281 | ||
282 | @defun read &optional stream | |
283 | This function reads one textual Lisp expression from @var{stream}, | |
284 | returning it as a Lisp object. This is the basic Lisp input function. | |
285 | @end defun | |
286 | ||
287 | @defun read-from-string string &optional start end | |
288 | @cindex string to object | |
289 | This function reads the first textual Lisp expression from the text in | |
290 | @var{string}. It returns a cons cell whose @sc{car} is that expression, | |
291 | and whose @sc{cdr} is an integer giving the position of the next | |
292 | remaining character in the string (i.e., the first one not read). | |
293 | ||
294 | If @var{start} is supplied, then reading begins at index @var{start} in | |
295 | the string (where the first character is at index 0). If you specify | |
296 | @var{end}, then reading is forced to stop just before that index, as if | |
297 | the rest of the string were not there. | |
298 | ||
299 | For example: | |
300 | ||
301 | @example | |
302 | @group | |
303 | (read-from-string "(setq x 55) (setq y 5)") | |
304 | @result{} ((setq x 55) . 11) | |
305 | @end group | |
306 | @group | |
307 | (read-from-string "\"A short string\"") | |
308 | @result{} ("A short string" . 16) | |
309 | @end group | |
310 | ||
311 | @group | |
312 | ;; @r{Read starting at the first character.} | |
313 | (read-from-string "(list 112)" 0) | |
314 | @result{} ((list 112) . 10) | |
315 | @end group | |
316 | @group | |
317 | ;; @r{Read starting at the second character.} | |
318 | (read-from-string "(list 112)" 1) | |
319 | @result{} (list . 5) | |
320 | @end group | |
321 | @group | |
322 | ;; @r{Read starting at the seventh character,} | |
323 | ;; @r{and stopping at the ninth.} | |
324 | (read-from-string "(list 112)" 6 8) | |
325 | @result{} (11 . 8) | |
326 | @end group | |
327 | @end example | |
328 | @end defun | |
329 | ||
330 | @defvar standard-input | |
331 | This variable holds the default input stream---the stream that | |
332 | @code{read} uses when the @var{stream} argument is @code{nil}. | |
333 | The default is @code{t}, meaning use the minibuffer. | |
334 | @end defvar | |
335 | ||
dd449674 CY |
336 | @defvar read-circle |
337 | If non-@code{nil}, this variable enables the reading of circular and | |
338 | shared structures. @xref{Circular Objects}. Its default value is | |
339 | @code{t}. | |
340 | @end defvar | |
341 | ||
b8d4c8d0 GM |
342 | @node Output Streams |
343 | @section Output Streams | |
344 | @cindex stream (for printing) | |
345 | @cindex output stream | |
346 | ||
347 | An output stream specifies what to do with the characters produced | |
348 | by printing. Most print functions accept an output stream as an | |
349 | optional argument. Here are the possible types of output stream: | |
350 | ||
351 | @table @asis | |
352 | @item @var{buffer} | |
353 | @cindex buffer output stream | |
354 | The output characters are inserted into @var{buffer} at point. | |
355 | Point advances as characters are inserted. | |
356 | ||
357 | @item @var{marker} | |
358 | @cindex marker output stream | |
359 | The output characters are inserted into the buffer that @var{marker} | |
360 | points into, at the marker position. The marker position advances as | |
361 | characters are inserted. The value of point in the buffer has no effect | |
362 | on printing when the stream is a marker, and this kind of printing | |
363 | does not move point (except that if the marker points at or before the | |
364 | position of point, point advances with the surrounding text, as | |
365 | usual). | |
366 | ||
367 | @item @var{function} | |
368 | @cindex function output stream | |
369 | The output characters are passed to @var{function}, which is responsible | |
370 | for storing them away. It is called with a single character as | |
371 | argument, as many times as there are characters to be output, and | |
372 | is responsible for storing the characters wherever you want to put them. | |
373 | ||
374 | @item @code{t} | |
375 | @cindex @code{t} output stream | |
376 | The output characters are displayed in the echo area. | |
377 | ||
378 | @item @code{nil} | |
379 | @cindex @code{nil} output stream | |
380 | @code{nil} specified as an output stream means to use the value of | |
381 | @code{standard-output} instead; that value is the @dfn{default output | |
382 | stream}, and must not be @code{nil}. | |
383 | ||
384 | @item @var{symbol} | |
385 | A symbol as output stream is equivalent to the symbol's function | |
386 | definition (if any). | |
387 | @end table | |
388 | ||
389 | Many of the valid output streams are also valid as input streams. The | |
390 | difference between input and output streams is therefore more a matter | |
391 | of how you use a Lisp object, than of different types of object. | |
392 | ||
393 | Here is an example of a buffer used as an output stream. Point is | |
394 | initially located as shown immediately before the @samp{h} in | |
395 | @samp{the}. At the end, point is located directly before that same | |
396 | @samp{h}. | |
397 | ||
398 | @cindex print example | |
399 | @example | |
400 | @group | |
401 | ---------- Buffer: foo ---------- | |
402 | This is t@point{}he contents of foo. | |
403 | ---------- Buffer: foo ---------- | |
404 | @end group | |
405 | ||
406 | (print "This is the output" (get-buffer "foo")) | |
407 | @result{} "This is the output" | |
408 | ||
409 | @group | |
410 | ---------- Buffer: foo ---------- | |
411 | This is t | |
412 | "This is the output" | |
413 | @point{}he contents of foo. | |
414 | ---------- Buffer: foo ---------- | |
415 | @end group | |
416 | @end example | |
417 | ||
418 | Now we show a use of a marker as an output stream. Initially, the | |
419 | marker is in buffer @code{foo}, between the @samp{t} and the @samp{h} in | |
420 | the word @samp{the}. At the end, the marker has advanced over the | |
421 | inserted text so that it remains positioned before the same @samp{h}. | |
422 | Note that the location of point, shown in the usual fashion, has no | |
423 | effect. | |
424 | ||
425 | @example | |
426 | @group | |
427 | ---------- Buffer: foo ---------- | |
428 | This is the @point{}output | |
429 | ---------- Buffer: foo ---------- | |
430 | @end group | |
431 | ||
432 | @group | |
433 | (setq m (copy-marker 10)) | |
434 | @result{} #<marker at 10 in foo> | |
435 | @end group | |
436 | ||
437 | @group | |
438 | (print "More output for foo." m) | |
439 | @result{} "More output for foo." | |
440 | @end group | |
441 | ||
442 | @group | |
443 | ---------- Buffer: foo ---------- | |
444 | This is t | |
445 | "More output for foo." | |
446 | he @point{}output | |
447 | ---------- Buffer: foo ---------- | |
448 | @end group | |
449 | ||
450 | @group | |
451 | m | |
452 | @result{} #<marker at 34 in foo> | |
453 | @end group | |
454 | @end example | |
455 | ||
456 | The following example shows output to the echo area: | |
457 | ||
458 | @example | |
459 | @group | |
460 | (print "Echo Area output" t) | |
461 | @result{} "Echo Area output" | |
462 | ---------- Echo Area ---------- | |
463 | "Echo Area output" | |
464 | ---------- Echo Area ---------- | |
465 | @end group | |
466 | @end example | |
467 | ||
468 | Finally, we show the use of a function as an output stream. The | |
469 | function @code{eat-output} takes each character that it is given and | |
470 | conses it onto the front of the list @code{last-output} (@pxref{Building | |
471 | Lists}). At the end, the list contains all the characters output, but | |
472 | in reverse order. | |
473 | ||
474 | @example | |
475 | @group | |
476 | (setq last-output nil) | |
477 | @result{} nil | |
478 | @end group | |
479 | ||
480 | @group | |
481 | (defun eat-output (c) | |
482 | (setq last-output (cons c last-output))) | |
483 | @result{} eat-output | |
484 | @end group | |
485 | ||
486 | @group | |
487 | (print "This is the output" 'eat-output) | |
488 | @result{} "This is the output" | |
489 | @end group | |
490 | ||
491 | @group | |
492 | last-output | |
493 | @result{} (10 34 116 117 112 116 117 111 32 101 104 | |
494 | 116 32 115 105 32 115 105 104 84 34 10) | |
495 | @end group | |
496 | @end example | |
497 | ||
498 | @noindent | |
499 | Now we can put the output in the proper order by reversing the list: | |
500 | ||
501 | @example | |
502 | @group | |
503 | (concat (nreverse last-output)) | |
504 | @result{} " | |
505 | \"This is the output\" | |
506 | " | |
507 | @end group | |
508 | @end example | |
509 | ||
510 | @noindent | |
511 | Calling @code{concat} converts the list to a string so you can see its | |
512 | contents more clearly. | |
513 | ||
514 | @node Output Functions | |
515 | @section Output Functions | |
516 | ||
517 | This section describes the Lisp functions for printing Lisp | |
518 | objects---converting objects into their printed representation. | |
519 | ||
520 | @cindex @samp{"} in printing | |
521 | @cindex @samp{\} in printing | |
522 | @cindex quoting characters in printing | |
523 | @cindex escape characters in printing | |
524 | Some of the Emacs printing functions add quoting characters to the | |
525 | output when necessary so that it can be read properly. The quoting | |
526 | characters used are @samp{"} and @samp{\}; they distinguish strings from | |
527 | symbols, and prevent punctuation characters in strings and symbols from | |
528 | being taken as delimiters when reading. @xref{Printed Representation}, | |
529 | for full details. You specify quoting or no quoting by the choice of | |
530 | printing function. | |
531 | ||
532 | If the text is to be read back into Lisp, then you should print with | |
533 | quoting characters to avoid ambiguity. Likewise, if the purpose is to | |
534 | describe a Lisp object clearly for a Lisp programmer. However, if the | |
535 | purpose of the output is to look nice for humans, then it is usually | |
536 | better to print without quoting. | |
537 | ||
538 | Lisp objects can refer to themselves. Printing a self-referential | |
539 | object in the normal way would require an infinite amount of text, and | |
540 | the attempt could cause infinite recursion. Emacs detects such | |
541 | recursion and prints @samp{#@var{level}} instead of recursively printing | |
542 | an object already being printed. For example, here @samp{#0} indicates | |
543 | a recursive reference to the object at level 0 of the current print | |
544 | operation: | |
545 | ||
546 | @example | |
547 | (setq foo (list nil)) | |
548 | @result{} (nil) | |
549 | (setcar foo foo) | |
550 | @result{} (#0) | |
551 | @end example | |
552 | ||
553 | In the functions below, @var{stream} stands for an output stream. | |
554 | (See the previous section for a description of output streams.) If | |
555 | @var{stream} is @code{nil} or omitted, it defaults to the value of | |
556 | @code{standard-output}. | |
557 | ||
558 | @defun print object &optional stream | |
559 | @cindex Lisp printer | |
560 | The @code{print} function is a convenient way of printing. It outputs | |
561 | the printed representation of @var{object} to @var{stream}, printing in | |
562 | addition one newline before @var{object} and another after it. Quoting | |
563 | characters are used. @code{print} returns @var{object}. For example: | |
564 | ||
565 | @example | |
566 | @group | |
567 | (progn (print 'The\ cat\ in) | |
568 | (print "the hat") | |
569 | (print " came back")) | |
570 | @print{} | |
571 | @print{} The\ cat\ in | |
572 | @print{} | |
573 | @print{} "the hat" | |
574 | @print{} | |
575 | @print{} " came back" | |
576 | @result{} " came back" | |
577 | @end group | |
578 | @end example | |
579 | @end defun | |
580 | ||
581 | @defun prin1 object &optional stream | |
582 | This function outputs the printed representation of @var{object} to | |
583 | @var{stream}. It does not print newlines to separate output as | |
584 | @code{print} does, but it does use quoting characters just like | |
585 | @code{print}. It returns @var{object}. | |
586 | ||
587 | @example | |
588 | @group | |
589 | (progn (prin1 'The\ cat\ in) | |
590 | (prin1 "the hat") | |
591 | (prin1 " came back")) | |
592 | @print{} The\ cat\ in"the hat"" came back" | |
593 | @result{} " came back" | |
594 | @end group | |
595 | @end example | |
596 | @end defun | |
597 | ||
598 | @defun princ object &optional stream | |
599 | This function outputs the printed representation of @var{object} to | |
600 | @var{stream}. It returns @var{object}. | |
601 | ||
602 | This function is intended to produce output that is readable by people, | |
603 | not by @code{read}, so it doesn't insert quoting characters and doesn't | |
604 | put double-quotes around the contents of strings. It does not add any | |
605 | spacing between calls. | |
606 | ||
607 | @example | |
608 | @group | |
609 | (progn | |
610 | (princ 'The\ cat) | |
611 | (princ " in the \"hat\"")) | |
612 | @print{} The cat in the "hat" | |
613 | @result{} " in the \"hat\"" | |
614 | @end group | |
615 | @end example | |
616 | @end defun | |
617 | ||
618 | @defun terpri &optional stream | |
619 | @cindex newline in print | |
620 | This function outputs a newline to @var{stream}. The name stands | |
16152b76 | 621 | for ``terminate print''. |
b8d4c8d0 GM |
622 | @end defun |
623 | ||
624 | @defun write-char character &optional stream | |
625 | This function outputs @var{character} to @var{stream}. It returns | |
626 | @var{character}. | |
627 | @end defun | |
628 | ||
629 | @defun prin1-to-string object &optional noescape | |
630 | @cindex object to string | |
631 | This function returns a string containing the text that @code{prin1} | |
632 | would have printed for the same argument. | |
633 | ||
634 | @example | |
635 | @group | |
636 | (prin1-to-string 'foo) | |
637 | @result{} "foo" | |
638 | @end group | |
639 | @group | |
640 | (prin1-to-string (mark-marker)) | |
641 | @result{} "#<marker at 2773 in strings.texi>" | |
642 | @end group | |
643 | @end example | |
644 | ||
645 | If @var{noescape} is non-@code{nil}, that inhibits use of quoting | |
646 | characters in the output. (This argument is supported in Emacs versions | |
647 | 19 and later.) | |
648 | ||
649 | @example | |
650 | @group | |
651 | (prin1-to-string "foo") | |
652 | @result{} "\"foo\"" | |
653 | @end group | |
654 | @group | |
655 | (prin1-to-string "foo" t) | |
656 | @result{} "foo" | |
657 | @end group | |
658 | @end example | |
659 | ||
660 | See @code{format}, in @ref{Formatting Strings}, for other ways to obtain | |
661 | the printed representation of a Lisp object as a string. | |
662 | @end defun | |
663 | ||
664 | @defmac with-output-to-string body@dots{} | |
665 | This macro executes the @var{body} forms with @code{standard-output} set | |
666 | up to feed output into a string. Then it returns that string. | |
667 | ||
668 | For example, if the current buffer name is @samp{foo}, | |
669 | ||
670 | @example | |
671 | (with-output-to-string | |
672 | (princ "The buffer is ") | |
673 | (princ (buffer-name))) | |
674 | @end example | |
675 | ||
676 | @noindent | |
677 | returns @code{"The buffer is foo"}. | |
678 | @end defmac | |
679 | ||
36cb87a1 LMI |
680 | @defun pp object &optional stream |
681 | This function outputs @var{object} to @var{stream}, just like | |
682 | @code{prin1}, but does it in a more ``pretty'' way. That is, it'll | |
683 | indent and fill the object to make it more readable for humans. | |
684 | @end defun | |
685 | ||
b8d4c8d0 GM |
686 | @node Output Variables |
687 | @section Variables Affecting Output | |
688 | @cindex output-controlling variables | |
689 | ||
690 | @defvar standard-output | |
691 | The value of this variable is the default output stream---the stream | |
692 | that print functions use when the @var{stream} argument is @code{nil}. | |
693 | The default is @code{t}, meaning display in the echo area. | |
694 | @end defvar | |
695 | ||
696 | @defvar print-quoted | |
697 | If this is non-@code{nil}, that means to print quoted forms using | |
1df7defd | 698 | abbreviated reader syntax, e.g., @code{(quote foo)} prints as |
34106abe | 699 | @code{'foo}, and @code{(function foo)} as @code{#'foo}. |
b8d4c8d0 GM |
700 | @end defvar |
701 | ||
702 | @defvar print-escape-newlines | |
703 | @cindex @samp{\n} in print | |
704 | @cindex escape characters | |
705 | If this variable is non-@code{nil}, then newline characters in strings | |
706 | are printed as @samp{\n} and formfeeds are printed as @samp{\f}. | |
707 | Normally these characters are printed as actual newlines and formfeeds. | |
708 | ||
709 | This variable affects the print functions @code{prin1} and @code{print} | |
710 | that print with quoting. It does not affect @code{princ}. Here is an | |
711 | example using @code{prin1}: | |
712 | ||
713 | @example | |
714 | @group | |
715 | (prin1 "a\nb") | |
716 | @print{} "a | |
717 | @print{} b" | |
718 | @result{} "a | |
719 | b" | |
720 | @end group | |
721 | ||
722 | @group | |
723 | (let ((print-escape-newlines t)) | |
724 | (prin1 "a\nb")) | |
725 | @print{} "a\nb" | |
726 | @result{} "a | |
727 | b" | |
728 | @end group | |
729 | @end example | |
730 | ||
731 | @noindent | |
732 | In the second expression, the local binding of | |
733 | @code{print-escape-newlines} is in effect during the call to | |
734 | @code{prin1}, but not during the printing of the result. | |
735 | @end defvar | |
736 | ||
737 | @defvar print-escape-nonascii | |
738 | If this variable is non-@code{nil}, then unibyte non-@acronym{ASCII} | |
739 | characters in strings are unconditionally printed as backslash sequences | |
740 | by the print functions @code{prin1} and @code{print} that print with | |
741 | quoting. | |
742 | ||
743 | Those functions also use backslash sequences for unibyte non-@acronym{ASCII} | |
744 | characters, regardless of the value of this variable, when the output | |
745 | stream is a multibyte buffer or a marker pointing into one. | |
746 | @end defvar | |
747 | ||
748 | @defvar print-escape-multibyte | |
749 | If this variable is non-@code{nil}, then multibyte non-@acronym{ASCII} | |
750 | characters in strings are unconditionally printed as backslash sequences | |
751 | by the print functions @code{prin1} and @code{print} that print with | |
752 | quoting. | |
753 | ||
754 | Those functions also use backslash sequences for multibyte | |
755 | non-@acronym{ASCII} characters, regardless of the value of this variable, | |
756 | when the output stream is a unibyte buffer or a marker pointing into | |
757 | one. | |
758 | @end defvar | |
759 | ||
760 | @defvar print-length | |
761 | @cindex printing limits | |
762 | The value of this variable is the maximum number of elements to print in | |
763 | any list, vector or bool-vector. If an object being printed has more | |
764 | than this many elements, it is abbreviated with an ellipsis. | |
765 | ||
766 | If the value is @code{nil} (the default), then there is no limit. | |
767 | ||
768 | @example | |
769 | @group | |
770 | (setq print-length 2) | |
771 | @result{} 2 | |
772 | @end group | |
773 | @group | |
774 | (print '(1 2 3 4 5)) | |
775 | @print{} (1 2 ...) | |
776 | @result{} (1 2 ...) | |
777 | @end group | |
778 | @end example | |
779 | @end defvar | |
780 | ||
781 | @defvar print-level | |
782 | The value of this variable is the maximum depth of nesting of | |
783 | parentheses and brackets when printed. Any list or vector at a depth | |
784 | exceeding this limit is abbreviated with an ellipsis. A value of | |
785 | @code{nil} (which is the default) means no limit. | |
786 | @end defvar | |
787 | ||
788 | @defopt eval-expression-print-length | |
789 | @defoptx eval-expression-print-level | |
790 | These are the values for @code{print-length} and @code{print-level} | |
791 | used by @code{eval-expression}, and thus, indirectly, by many | |
792 | interactive evaluation commands (@pxref{Lisp Eval,, Evaluating | |
793 | Emacs-Lisp Expressions, emacs, The GNU Emacs Manual}). | |
794 | @end defopt | |
795 | ||
796 | These variables are used for detecting and reporting circular | |
797 | and shared structure: | |
798 | ||
799 | @defvar print-circle | |
dd449674 | 800 | If non-@code{nil}, this variable enables detection of circular and |
544c5fc9 | 801 | shared structure in printing. @xref{Circular Objects}. |
b8d4c8d0 GM |
802 | @end defvar |
803 | ||
804 | @defvar print-gensym | |
805 | If non-@code{nil}, this variable enables detection of uninterned symbols | |
806 | (@pxref{Creating Symbols}) in printing. When this is enabled, | |
807 | uninterned symbols print with the prefix @samp{#:}, which tells the Lisp | |
808 | reader to produce an uninterned symbol. | |
809 | @end defvar | |
810 | ||
811 | @defvar print-continuous-numbering | |
812 | If non-@code{nil}, that means number continuously across print calls. | |
813 | This affects the numbers printed for @samp{#@var{n}=} labels and | |
814 | @samp{#@var{m}#} references. | |
b8d4c8d0 GM |
815 | Don't set this variable with @code{setq}; you should only bind it |
816 | temporarily to @code{t} with @code{let}. When you do that, you should | |
817 | also bind @code{print-number-table} to @code{nil}. | |
818 | @end defvar | |
819 | ||
820 | @defvar print-number-table | |
821 | This variable holds a vector used internally by printing to implement | |
822 | the @code{print-circle} feature. You should not use it except | |
823 | to bind it to @code{nil} when you bind @code{print-continuous-numbering}. | |
824 | @end defvar | |
825 | ||
826 | @defvar float-output-format | |
161c2a25 GM |
827 | This variable specifies how to print floating point numbers. The |
828 | default is @code{nil}, meaning use the shortest output | |
b8d4c8d0 GM |
829 | that represents the number without losing information. |
830 | ||
831 | To control output format more precisely, you can put a string in this | |
832 | variable. The string should hold a @samp{%}-specification to be used | |
833 | in the C function @code{sprintf}. For further restrictions on what | |
834 | you can use, see the variable's documentation string. | |
835 | @end defvar |