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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
fd897522 GM |
3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1998, 1999 |
4 | @c Free Software Foundation, Inc. | |
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5 | @c See the file elisp.texi for copying conditions. |
6 | @setfilename ../info/streams | |
05fd2b65 | 7 | @node Read and Print, Minibuffers, Debugging, Top |
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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 | |
05fd2b65 | 13 | representations and read syntax described in @ref{Lisp Data Types}. |
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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 | |
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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)}. | |
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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 | |
b664e483 | 53 | text produces a list (but not the same list) with elements @code{a} |
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54 | and @code{b}. |
55 | ||
a40d4712 PR |
56 | However, these two operations are not precisely inverse to each other. |
57 | There are three kinds of exceptions: | |
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58 | |
59 | @itemize @bullet | |
60 | @item | |
61 | Printing can produce text that cannot be read. For example, buffers, | |
f9f59935 | 62 | windows, frames, subprocesses and markers print as text that starts |
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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. | |
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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. | |
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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 | |
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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 | |
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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. | |
129 | ||
130 | @item @code{nil} | |
131 | @cindex @code{nil} input stream | |
132 | @code{nil} supplied as an input stream means to use the value of | |
133 | @code{standard-input} instead; that value is the @dfn{default input | |
134 | stream}, and must be a non-@code{nil} input stream. | |
135 | ||
136 | @item @var{symbol} | |
137 | A symbol as input stream is equivalent to the symbol's function | |
138 | definition (if any). | |
139 | @end table | |
140 | ||
b664e483 | 141 | Here is an example of reading from a stream that is a buffer, showing |
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142 | where point is located before and after: |
143 | ||
144 | @example | |
145 | @group | |
146 | ---------- Buffer: foo ---------- | |
147 | This@point{} is the contents of foo. | |
148 | ---------- Buffer: foo ---------- | |
149 | @end group | |
150 | ||
151 | @group | |
152 | (read (get-buffer "foo")) | |
153 | @result{} is | |
154 | @end group | |
155 | @group | |
156 | (read (get-buffer "foo")) | |
157 | @result{} the | |
158 | @end group | |
159 | ||
160 | @group | |
161 | ---------- Buffer: foo ---------- | |
162 | This is the@point{} contents of foo. | |
163 | ---------- Buffer: foo ---------- | |
164 | @end group | |
165 | @end example | |
166 | ||
167 | @noindent | |
b664e483 RS |
168 | Note that the first read skips a space. Reading skips any amount of |
169 | whitespace preceding the significant text. | |
5e8db0c6 | 170 | |
5e8db0c6 | 171 | Here is an example of reading from a stream that is a marker, |
b664e483 | 172 | initially positioned at the beginning of the buffer shown. The value |
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173 | read is the symbol @code{This}. |
174 | ||
175 | @example | |
176 | @group | |
177 | ||
178 | ---------- Buffer: foo ---------- | |
179 | This is the contents of foo. | |
180 | ---------- Buffer: foo ---------- | |
181 | @end group | |
182 | ||
183 | @group | |
184 | (setq m (set-marker (make-marker) 1 (get-buffer "foo"))) | |
185 | @result{} #<marker at 1 in foo> | |
186 | @end group | |
187 | @group | |
188 | (read m) | |
189 | @result{} This | |
190 | @end group | |
191 | @group | |
192 | m | |
b664e483 | 193 | @result{} #<marker at 5 in foo> ;; @r{Before the first space.} |
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194 | @end group |
195 | @end example | |
196 | ||
197 | Here we read from the contents of a string: | |
198 | ||
199 | @example | |
200 | @group | |
201 | (read "(When in) the course") | |
202 | @result{} (When in) | |
203 | @end group | |
204 | @end example | |
205 | ||
206 | The following example reads from the minibuffer. The | |
207 | prompt is: @w{@samp{Lisp expression: }}. (That is always the prompt | |
208 | used when you read from the stream @code{t}.) The user's input is shown | |
209 | following the prompt. | |
210 | ||
211 | @example | |
212 | @group | |
213 | (read t) | |
214 | @result{} 23 | |
215 | ---------- Buffer: Minibuffer ---------- | |
216 | Lisp expression: @kbd{23 @key{RET}} | |
217 | ---------- Buffer: Minibuffer ---------- | |
218 | @end group | |
219 | @end example | |
220 | ||
221 | Finally, here is an example of a stream that is a function, named | |
222 | @code{useless-stream}. Before we use the stream, we initialize the | |
223 | variable @code{useless-list} to a list of characters. Then each call to | |
b664e483 | 224 | the function @code{useless-stream} obtains the next character in the list |
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225 | or unreads a character by adding it to the front of the list. |
226 | ||
227 | @example | |
228 | @group | |
229 | (setq useless-list (append "XY()" nil)) | |
230 | @result{} (88 89 40 41) | |
231 | @end group | |
232 | ||
233 | @group | |
234 | (defun useless-stream (&optional unread) | |
235 | (if unread | |
236 | (setq useless-list (cons unread useless-list)) | |
237 | (prog1 (car useless-list) | |
238 | (setq useless-list (cdr useless-list))))) | |
239 | @result{} useless-stream | |
240 | @end group | |
241 | @end example | |
242 | ||
243 | @noindent | |
244 | Now we read using the stream thus constructed: | |
245 | ||
246 | @example | |
247 | @group | |
248 | (read 'useless-stream) | |
249 | @result{} XY | |
250 | @end group | |
251 | ||
252 | @group | |
253 | useless-list | |
b664e483 | 254 | @result{} (40 41) |
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255 | @end group |
256 | @end example | |
257 | ||
258 | @noindent | |
a9f0a989 | 259 | Note that the open and close parentheses remain in the list. The Lisp |
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260 | reader encountered the open parenthesis, decided that it ended the |
261 | input, and unread it. Another attempt to read from the stream at this | |
262 | point would read @samp{()} and return @code{nil}. | |
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263 | |
264 | @defun get-file-char | |
265 | This function is used internally as an input stream to read from the | |
266 | input file opened by the function @code{load}. Don't use this function | |
267 | yourself. | |
268 | @end defun | |
269 | ||
270 | @node Input Functions | |
271 | @section Input Functions | |
272 | ||
273 | This section describes the Lisp functions and variables that pertain | |
274 | to reading. | |
275 | ||
276 | In the functions below, @var{stream} stands for an input stream (see | |
277 | the previous section). If @var{stream} is @code{nil} or omitted, it | |
278 | defaults to the value of @code{standard-input}. | |
279 | ||
280 | @kindex end-of-file | |
281 | An @code{end-of-file} error is signaled if reading encounters an | |
b664e483 | 282 | unterminated list, vector, or string. |
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283 | |
284 | @defun read &optional stream | |
285 | This function reads one textual Lisp expression from @var{stream}, | |
286 | returning it as a Lisp object. This is the basic Lisp input function. | |
287 | @end defun | |
288 | ||
289 | @defun read-from-string string &optional start end | |
290 | @cindex string to object | |
291 | This function reads the first textual Lisp expression from the text in | |
292 | @var{string}. It returns a cons cell whose @sc{car} is that expression, | |
293 | and whose @sc{cdr} is an integer giving the position of the next | |
294 | remaining character in the string (i.e., the first one not read). | |
295 | ||
b664e483 | 296 | If @var{start} is supplied, then reading begins at index @var{start} in |
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297 | the string (where the first character is at index 0). If you specify |
298 | @var{end}, then reading is forced to stop just before that index, as if | |
299 | the rest of the string were not there. | |
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300 | |
301 | For example: | |
302 | ||
303 | @example | |
304 | @group | |
305 | (read-from-string "(setq x 55) (setq y 5)") | |
306 | @result{} ((setq x 55) . 11) | |
307 | @end group | |
308 | @group | |
309 | (read-from-string "\"A short string\"") | |
310 | @result{} ("A short string" . 16) | |
311 | @end group | |
312 | ||
313 | @group | |
314 | ;; @r{Read starting at the first character.} | |
315 | (read-from-string "(list 112)" 0) | |
316 | @result{} ((list 112) . 10) | |
317 | @end group | |
318 | @group | |
319 | ;; @r{Read starting at the second character.} | |
320 | (read-from-string "(list 112)" 1) | |
b664e483 | 321 | @result{} (list . 5) |
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322 | @end group |
323 | @group | |
324 | ;; @r{Read starting at the seventh character,} | |
325 | ;; @r{and stopping at the ninth.} | |
326 | (read-from-string "(list 112)" 6 8) | |
327 | @result{} (11 . 8) | |
328 | @end group | |
329 | @end example | |
330 | @end defun | |
331 | ||
332 | @defvar standard-input | |
333 | This variable holds the default input stream---the stream that | |
334 | @code{read} uses when the @var{stream} argument is @code{nil}. | |
335 | @end defvar | |
336 | ||
337 | @node Output Streams | |
338 | @section Output Streams | |
339 | @cindex stream (for printing) | |
340 | @cindex output stream | |
341 | ||
342 | An output stream specifies what to do with the characters produced | |
343 | by printing. Most print functions accept an output stream as an | |
344 | optional argument. Here are the possible types of output stream: | |
345 | ||
346 | @table @asis | |
347 | @item @var{buffer} | |
348 | @cindex buffer output stream | |
349 | The output characters are inserted into @var{buffer} at point. | |
350 | Point advances as characters are inserted. | |
351 | ||
352 | @item @var{marker} | |
353 | @cindex marker output stream | |
354 | The output characters are inserted into the buffer that @var{marker} | |
b664e483 | 355 | points into, at the marker position. The marker position advances as |
5e8db0c6 | 356 | characters are inserted. The value of point in the buffer has no effect |
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357 | on printing when the stream is a marker, and this kind of printing |
358 | does not move point. | |
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359 | |
360 | @item @var{function} | |
361 | @cindex function output stream | |
362 | The output characters are passed to @var{function}, which is responsible | |
363 | for storing them away. It is called with a single character as | |
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364 | argument, as many times as there are characters to be output, and |
365 | is responsible for storing the characters wherever you want to put them. | |
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366 | |
367 | @item @code{t} | |
368 | @cindex @code{t} output stream | |
369 | The output characters are displayed in the echo area. | |
370 | ||
371 | @item @code{nil} | |
372 | @cindex @code{nil} output stream | |
a9f0a989 | 373 | @code{nil} specified as an output stream means to use the value of |
5e8db0c6 | 374 | @code{standard-output} instead; that value is the @dfn{default output |
a9f0a989 | 375 | stream}, and must not be @code{nil}. |
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376 | |
377 | @item @var{symbol} | |
378 | A symbol as output stream is equivalent to the symbol's function | |
379 | definition (if any). | |
380 | @end table | |
381 | ||
b664e483 | 382 | Many of the valid output streams are also valid as input streams. The |
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383 | difference between input and output streams is therefore more a matter |
384 | of how you use a Lisp object, than of different types of object. | |
b664e483 | 385 | |
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386 | Here is an example of a buffer used as an output stream. Point is |
387 | initially located as shown immediately before the @samp{h} in | |
388 | @samp{the}. At the end, point is located directly before that same | |
389 | @samp{h}. | |
390 | ||
391 | @cindex print example | |
392 | @example | |
393 | @group | |
394 | ---------- Buffer: foo ---------- | |
395 | This is t@point{}he contents of foo. | |
396 | ---------- Buffer: foo ---------- | |
397 | @end group | |
398 | ||
399 | (print "This is the output" (get-buffer "foo")) | |
400 | @result{} "This is the output" | |
401 | ||
402 | @group | |
403 | ---------- Buffer: foo ---------- | |
404 | This is t | |
405 | "This is the output" | |
406 | @point{}he contents of foo. | |
407 | ---------- Buffer: foo ---------- | |
408 | @end group | |
409 | @end example | |
410 | ||
411 | Now we show a use of a marker as an output stream. Initially, the | |
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412 | marker is in buffer @code{foo}, between the @samp{t} and the @samp{h} in |
413 | the word @samp{the}. At the end, the marker has advanced over the | |
414 | inserted text so that it remains positioned before the same @samp{h}. | |
415 | Note that the location of point, shown in the usual fashion, has no | |
416 | effect. | |
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417 | |
418 | @example | |
419 | @group | |
420 | ---------- Buffer: foo ---------- | |
a9f0a989 | 421 | This is the @point{}output |
5e8db0c6 RS |
422 | ---------- Buffer: foo ---------- |
423 | @end group | |
424 | ||
425 | @group | |
1911e6e5 RS |
426 | (setq m (copy-marker 10)) |
427 | @result{} #<marker at 10 in foo> | |
5e8db0c6 RS |
428 | @end group |
429 | ||
430 | @group | |
431 | (print "More output for foo." m) | |
432 | @result{} "More output for foo." | |
433 | @end group | |
434 | ||
435 | @group | |
436 | ---------- Buffer: foo ---------- | |
a9f0a989 | 437 | This is t |
5e8db0c6 | 438 | "More output for foo." |
a9f0a989 | 439 | he @point{}output |
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440 | ---------- Buffer: foo ---------- |
441 | @end group | |
442 | ||
443 | @group | |
444 | m | |
1911e6e5 | 445 | @result{} #<marker at 34 in foo> |
5e8db0c6 RS |
446 | @end group |
447 | @end example | |
448 | ||
449 | The following example shows output to the echo area: | |
450 | ||
451 | @example | |
452 | @group | |
453 | (print "Echo Area output" t) | |
454 | @result{} "Echo Area output" | |
455 | ---------- Echo Area ---------- | |
456 | "Echo Area output" | |
457 | ---------- Echo Area ---------- | |
458 | @end group | |
459 | @end example | |
460 | ||
461 | Finally, we show the use of a function as an output stream. The | |
462 | function @code{eat-output} takes each character that it is given and | |
463 | conses it onto the front of the list @code{last-output} (@pxref{Building | |
464 | Lists}). At the end, the list contains all the characters output, but | |
465 | in reverse order. | |
466 | ||
467 | @example | |
468 | @group | |
469 | (setq last-output nil) | |
470 | @result{} nil | |
471 | @end group | |
472 | ||
473 | @group | |
474 | (defun eat-output (c) | |
475 | (setq last-output (cons c last-output))) | |
476 | @result{} eat-output | |
477 | @end group | |
478 | ||
479 | @group | |
480 | (print "This is the output" 'eat-output) | |
481 | @result{} "This is the output" | |
482 | @end group | |
483 | ||
484 | @group | |
485 | last-output | |
486 | @result{} (10 34 116 117 112 116 117 111 32 101 104 | |
487 | 116 32 115 105 32 115 105 104 84 34 10) | |
488 | @end group | |
489 | @end example | |
490 | ||
491 | @noindent | |
492 | Now we can put the output in the proper order by reversing the list: | |
493 | ||
494 | @example | |
495 | @group | |
496 | (concat (nreverse last-output)) | |
497 | @result{} " | |
498 | \"This is the output\" | |
499 | " | |
500 | @end group | |
501 | @end example | |
502 | ||
b664e483 RS |
503 | @noindent |
504 | Calling @code{concat} converts the list to a string so you can see its | |
505 | contents more clearly. | |
506 | ||
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507 | @node Output Functions |
508 | @section Output Functions | |
509 | ||
f9f59935 RS |
510 | This section describes the Lisp functions for printing Lisp |
511 | objects---converting objects into their printed representation. | |
5e8db0c6 RS |
512 | |
513 | @cindex @samp{"} in printing | |
514 | @cindex @samp{\} in printing | |
515 | @cindex quoting characters in printing | |
516 | @cindex escape characters in printing | |
517 | Some of the Emacs printing functions add quoting characters to the | |
518 | output when necessary so that it can be read properly. The quoting | |
519 | characters used are @samp{"} and @samp{\}; they distinguish strings from | |
520 | symbols, and prevent punctuation characters in strings and symbols from | |
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521 | being taken as delimiters when reading. @xref{Printed Representation}, |
522 | for full details. You specify quoting or no quoting by the choice of | |
523 | printing function. | |
5e8db0c6 | 524 | |
969fe9b5 RS |
525 | If the text is to be read back into Lisp, then you should print with |
526 | quoting characters to avoid ambiguity. Likewise, if the purpose is to | |
527 | describe a Lisp object clearly for a Lisp programmer. However, if the | |
528 | purpose of the output is to look nice for humans, then it is usually | |
529 | better to print without quoting. | |
5e8db0c6 | 530 | |
a9f0a989 RS |
531 | Lisp objects can refer to themselves. Printing a self-referential |
532 | object in the normal way would require an infinite amount of text, and | |
533 | the attempt could cause infinite recursion. Emacs detects such | |
534 | recursion and prints @samp{#@var{level}} instead of recursively printing | |
535 | an object already being printed. For example, here @samp{#0} indicates | |
536 | a recursive reference to the object at level 0 of the current print | |
537 | operation: | |
5e8db0c6 RS |
538 | |
539 | @example | |
540 | (setq foo (list nil)) | |
541 | @result{} (nil) | |
542 | (setcar foo foo) | |
543 | @result{} (#0) | |
544 | @end example | |
545 | ||
546 | In the functions below, @var{stream} stands for an output stream. | |
547 | (See the previous section for a description of output streams.) If | |
548 | @var{stream} is @code{nil} or omitted, it defaults to the value of | |
549 | @code{standard-output}. | |
550 | ||
551 | @defun print object &optional stream | |
552 | @cindex Lisp printer | |
553 | The @code{print} function is a convenient way of printing. It outputs | |
554 | the printed representation of @var{object} to @var{stream}, printing in | |
555 | addition one newline before @var{object} and another after it. Quoting | |
556 | characters are used. @code{print} returns @var{object}. For example: | |
557 | ||
558 | @example | |
559 | @group | |
560 | (progn (print 'The\ cat\ in) | |
561 | (print "the hat") | |
562 | (print " came back")) | |
563 | @print{} | |
564 | @print{} The\ cat\ in | |
565 | @print{} | |
566 | @print{} "the hat" | |
567 | @print{} | |
568 | @print{} " came back" | |
569 | @print{} | |
570 | @result{} " came back" | |
571 | @end group | |
572 | @end example | |
573 | @end defun | |
574 | ||
575 | @defun prin1 object &optional stream | |
576 | This function outputs the printed representation of @var{object} to | |
b664e483 RS |
577 | @var{stream}. It does not print newlines to separate output as |
578 | @code{print} does, but it does use quoting characters just like | |
579 | @code{print}. It returns @var{object}. | |
5e8db0c6 RS |
580 | |
581 | @example | |
582 | @group | |
583 | (progn (prin1 'The\ cat\ in) | |
584 | (prin1 "the hat") | |
585 | (prin1 " came back")) | |
586 | @print{} The\ cat\ in"the hat"" came back" | |
587 | @result{} " came back" | |
588 | @end group | |
589 | @end example | |
590 | @end defun | |
591 | ||
592 | @defun princ object &optional stream | |
593 | This function outputs the printed representation of @var{object} to | |
594 | @var{stream}. It returns @var{object}. | |
595 | ||
596 | This function is intended to produce output that is readable by people, | |
597 | not by @code{read}, so it doesn't insert quoting characters and doesn't | |
598 | put double-quotes around the contents of strings. It does not add any | |
599 | spacing between calls. | |
600 | ||
601 | @example | |
602 | @group | |
603 | (progn | |
604 | (princ 'The\ cat) | |
605 | (princ " in the \"hat\"")) | |
606 | @print{} The cat in the "hat" | |
607 | @result{} " in the \"hat\"" | |
608 | @end group | |
609 | @end example | |
610 | @end defun | |
611 | ||
612 | @defun terpri &optional stream | |
613 | @cindex newline in print | |
614 | This function outputs a newline to @var{stream}. The name stands | |
615 | for ``terminate print''. | |
616 | @end defun | |
617 | ||
618 | @defun write-char character &optional stream | |
619 | This function outputs @var{character} to @var{stream}. It returns | |
620 | @var{character}. | |
621 | @end defun | |
622 | ||
623 | @defun prin1-to-string object &optional noescape | |
624 | @cindex object to string | |
625 | This function returns a string containing the text that @code{prin1} | |
626 | would have printed for the same argument. | |
627 | ||
628 | @example | |
629 | @group | |
630 | (prin1-to-string 'foo) | |
631 | @result{} "foo" | |
632 | @end group | |
633 | @group | |
634 | (prin1-to-string (mark-marker)) | |
635 | @result{} "#<marker at 2773 in strings.texi>" | |
636 | @end group | |
637 | @end example | |
638 | ||
639 | If @var{noescape} is non-@code{nil}, that inhibits use of quoting | |
640 | characters in the output. (This argument is supported in Emacs versions | |
641 | 19 and later.) | |
642 | ||
643 | @example | |
644 | @group | |
645 | (prin1-to-string "foo") | |
646 | @result{} "\"foo\"" | |
647 | @end group | |
648 | @group | |
649 | (prin1-to-string "foo" t) | |
650 | @result{} "foo" | |
651 | @end group | |
652 | @end example | |
653 | ||
654 | See @code{format}, in @ref{String Conversion}, for other ways to obtain | |
655 | the printed representation of a Lisp object as a string. | |
656 | @end defun | |
657 | ||
f9f59935 | 658 | @defmac with-output-to-string body... |
a9f0a989 RS |
659 | This macro executes the @var{body} forms with @code{standard-output} set |
660 | up to feed output into a string. Then it returns that string. | |
f9f59935 RS |
661 | |
662 | For example, if the current buffer name is @samp{foo}, | |
663 | ||
664 | @example | |
665 | (with-output-to-string | |
666 | (princ "The buffer is ") | |
667 | (princ (buffer-name))) | |
668 | @end example | |
669 | ||
670 | @noindent | |
671 | returns @code{"The buffer is foo"}. | |
672 | @end defmac | |
673 | ||
5e8db0c6 RS |
674 | @node Output Variables |
675 | @section Variables Affecting Output | |
676 | ||
677 | @defvar standard-output | |
678 | The value of this variable is the default output stream---the stream | |
679 | that print functions use when the @var{stream} argument is @code{nil}. | |
680 | @end defvar | |
681 | ||
682 | @defvar print-escape-newlines | |
683 | @cindex @samp{\n} in print | |
684 | @cindex escape characters | |
685 | If this variable is non-@code{nil}, then newline characters in strings | |
686 | are printed as @samp{\n} and formfeeds are printed as @samp{\f}. | |
687 | Normally these characters are printed as actual newlines and formfeeds. | |
688 | ||
969fe9b5 RS |
689 | This variable affects the print functions @code{prin1} and @code{print} |
690 | that print with quoting. It does not affect @code{princ}. Here is an | |
691 | example using @code{prin1}: | |
5e8db0c6 RS |
692 | |
693 | @example | |
694 | @group | |
695 | (prin1 "a\nb") | |
696 | @print{} "a | |
697 | @print{} b" | |
698 | @result{} "a | |
699 | b" | |
700 | @end group | |
701 | ||
702 | @group | |
703 | (let ((print-escape-newlines t)) | |
704 | (prin1 "a\nb")) | |
705 | @print{} "a\nb" | |
706 | @result{} "a | |
707 | b" | |
708 | @end group | |
709 | @end example | |
710 | ||
711 | @noindent | |
712 | In the second expression, the local binding of | |
713 | @code{print-escape-newlines} is in effect during the call to | |
714 | @code{prin1}, but not during the printing of the result. | |
715 | @end defvar | |
716 | ||
1911e6e5 | 717 | @defvar print-escape-nonascii |
8241495d | 718 | If this variable is non-@code{nil}, then unibyte non-@sc{ascii} |
1911e6e5 RS |
719 | characters in strings are unconditionally printed as backslash sequences |
720 | by the print functions @code{prin1} and @code{print} that print with | |
721 | quoting. | |
5074194e | 722 | |
8241495d | 723 | Those functions also use backslash sequences for unibyte non-@sc{ascii} |
5074194e RS |
724 | characters, regardless of the value of this variable, when the output |
725 | stream is a multibyte buffer or a marker pointing into one. | |
726 | @end defvar | |
727 | ||
5074194e | 728 | @defvar print-escape-multibyte |
8241495d | 729 | If this variable is non-@code{nil}, then multibyte non-@sc{ascii} |
5074194e RS |
730 | characters in strings are unconditionally printed as backslash sequences |
731 | by the print functions @code{prin1} and @code{print} that print with | |
732 | quoting. | |
733 | ||
734 | Those functions also use backslash sequences for multibyte | |
8241495d | 735 | non-@sc{ascii} characters, regardless of the value of this variable, |
5074194e RS |
736 | when the output stream is a unibyte buffer or a marker pointing into |
737 | one. | |
1911e6e5 RS |
738 | @end defvar |
739 | ||
5e8db0c6 RS |
740 | @defvar print-length |
741 | @cindex printing limits | |
f9f59935 RS |
742 | The value of this variable is the maximum number of elements to print in |
743 | any list, vector or bool-vector. If an object being printed has more | |
744 | than this many elements, it is abbreviated with an ellipsis. | |
5e8db0c6 RS |
745 | |
746 | If the value is @code{nil} (the default), then there is no limit. | |
747 | ||
748 | @example | |
749 | @group | |
750 | (setq print-length 2) | |
751 | @result{} 2 | |
752 | @end group | |
753 | @group | |
754 | (print '(1 2 3 4 5)) | |
755 | @print{} (1 2 ...) | |
756 | @result{} (1 2 ...) | |
757 | @end group | |
758 | @end example | |
759 | @end defvar | |
760 | ||
761 | @defvar print-level | |
762 | The value of this variable is the maximum depth of nesting of | |
b664e483 | 763 | parentheses and brackets when printed. Any list or vector at a depth |
5e8db0c6 RS |
764 | exceeding this limit is abbreviated with an ellipsis. A value of |
765 | @code{nil} (which is the default) means no limit. | |
5e8db0c6 | 766 | @end defvar |
8241495d RS |
767 | |
768 | These variables are used for detecting and reporting circular | |
769 | and shared structure---but they are only defined in Emacs 21. | |
770 | ||
771 | @tindex print-circle | |
772 | @defvar print-circle | |
773 | If non-@code{nil}, this variable enables detection of circular | |
774 | and shared structure in printing. | |
775 | @end defvar | |
776 | ||
777 | @tindex print-gensym | |
778 | @defvar print-gensym | |
779 | If non-@code{nil}, this variable enables detection of uninterned symbols | |
780 | (@pxref{Creating Symbols}) in printing. When this is enabled, | |
781 | uninterned symbols print with the prefix @samp{#:}, which tells the Lisp | |
782 | reader to produce an uninterned symbol. | |
783 | @end defvar |