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1 | @c -*-texinfo-*- |
2 | @c This is part of the GNU Emacs Lisp Reference Manual. | |
3 | @c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. | |
4 | @c See the file elisp.texi for copying conditions. | |
5 | @setfilename ../info/streams | |
05fd2b65 | 6 | @node Read and Print, Minibuffers, Debugging, Top |
5e8db0c6 RS |
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 | |
05fd2b65 | 12 | representations and read syntax described in @ref{Lisp Data Types}. |
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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 printed representation. Printing | |
42 | the cons cell described 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 | |
b664e483 | 51 | text produces a list (but not the same list) with elements @code{a} |
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52 | and @code{b}. |
53 | ||
54 | However, these two operations are not precisely inverses. There are | |
bfe721d1 | 55 | three kinds of exceptions: |
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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 into 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. | |
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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. | |
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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}, one character per | |
106 | call. Normally @var{function} is called with no arguments, and should | |
107 | return a character. | |
108 | ||
109 | @cindex unreading | |
110 | Occasionally @var{function} is called with one argument (always a | |
111 | character). When that happens, @var{function} should save the argument | |
112 | and arrange to return it on the next call. This is called | |
113 | @dfn{unreading} the character; it happens when the Lisp reader reads one | |
114 | character too many and wants to ``put it back where it came from''. | |
115 | ||
116 | @item @code{t} | |
117 | @cindex @code{t} input stream | |
118 | @code{t} used as a stream means that the input is read from the | |
119 | minibuffer. In fact, the minibuffer is invoked once and the text | |
120 | given by the user is made into a string that is then used as the | |
121 | input stream. | |
122 | ||
123 | @item @code{nil} | |
124 | @cindex @code{nil} input stream | |
125 | @code{nil} supplied as an input stream means to use the value of | |
126 | @code{standard-input} instead; that value is the @dfn{default input | |
127 | stream}, and must be a non-@code{nil} input stream. | |
128 | ||
129 | @item @var{symbol} | |
130 | A symbol as input stream is equivalent to the symbol's function | |
131 | definition (if any). | |
132 | @end table | |
133 | ||
b664e483 | 134 | Here is an example of reading from a stream that is a buffer, showing |
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135 | where point is located before and after: |
136 | ||
137 | @example | |
138 | @group | |
139 | ---------- Buffer: foo ---------- | |
140 | This@point{} is the contents of foo. | |
141 | ---------- Buffer: foo ---------- | |
142 | @end group | |
143 | ||
144 | @group | |
145 | (read (get-buffer "foo")) | |
146 | @result{} is | |
147 | @end group | |
148 | @group | |
149 | (read (get-buffer "foo")) | |
150 | @result{} the | |
151 | @end group | |
152 | ||
153 | @group | |
154 | ---------- Buffer: foo ---------- | |
155 | This is the@point{} contents of foo. | |
156 | ---------- Buffer: foo ---------- | |
157 | @end group | |
158 | @end example | |
159 | ||
160 | @noindent | |
b664e483 RS |
161 | Note that the first read skips a space. Reading skips any amount of |
162 | whitespace preceding the significant text. | |
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163 | |
164 | In Emacs 18, reading a symbol discarded the delimiter terminating the | |
165 | symbol. Thus, point would end up at the beginning of @samp{contents} | |
166 | rather than after @samp{the}. The Emacs 19 behavior is superior because | |
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167 | it correctly handles input such as @samp{bar(foo)}, where the |
168 | open-parenthesis that ends one object is needed as the beginning of | |
169 | another object. | |
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170 | |
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) |
5e8db0c6 RS |
255 | @end group |
256 | @end example | |
257 | ||
258 | @noindent | |
b664e483 RS |
259 | Note that the open and close parentheses remains in the list. The Lisp |
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 RS |
296 | If @var{start} is supplied, then reading begins at index @var{start} in |
297 | the string (where the first character is at index 0). If @var{end} is | |
298 | also supplied, then reading stops just before that index, as if the rest | |
299 | 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 |
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356 | characters are inserted. The value of point in the buffer has no effect |
357 | on printing when the stream is a marker. | |
358 | ||
359 | @item @var{function} | |
360 | @cindex function output stream | |
361 | The output characters are passed to @var{function}, which is responsible | |
362 | for storing them away. It is called with a single character as | |
363 | argument, as many times as there are characters to be output, and is | |
364 | free to do anything at all with the characters it receives. | |
365 | ||
366 | @item @code{t} | |
367 | @cindex @code{t} output stream | |
368 | The output characters are displayed in the echo area. | |
369 | ||
370 | @item @code{nil} | |
371 | @cindex @code{nil} output stream | |
372 | @code{nil} specified as an output stream means to the value of | |
373 | @code{standard-output} instead; that value is the @dfn{default output | |
374 | stream}, and must be a non-@code{nil} output stream. | |
375 | ||
376 | @item @var{symbol} | |
377 | A symbol as output stream is equivalent to the symbol's function | |
378 | definition (if any). | |
379 | @end table | |
380 | ||
b664e483 RS |
381 | Many of the valid output streams are also valid as input streams. The |
382 | difference between input and output streams is therefore mostly one of | |
383 | how you use a Lisp object, not a distinction of types of object. | |
384 | ||
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385 | Here is an example of a buffer used as an output stream. Point is |
386 | initially located as shown immediately before the @samp{h} in | |
387 | @samp{the}. At the end, point is located directly before that same | |
388 | @samp{h}. | |
389 | ||
390 | @cindex print example | |
391 | @example | |
392 | @group | |
393 | ---------- Buffer: foo ---------- | |
394 | This is t@point{}he contents of foo. | |
395 | ---------- Buffer: foo ---------- | |
396 | @end group | |
397 | ||
398 | (print "This is the output" (get-buffer "foo")) | |
399 | @result{} "This is the output" | |
400 | ||
401 | @group | |
402 | ---------- Buffer: foo ---------- | |
403 | This is t | |
404 | "This is the output" | |
405 | @point{}he contents of foo. | |
406 | ---------- Buffer: foo ---------- | |
407 | @end group | |
408 | @end example | |
409 | ||
410 | Now we show a use of a marker as an output stream. Initially, the | |
b664e483 RS |
411 | marker is in buffer @code{foo}, between the @samp{t} and the @samp{h} in |
412 | the word @samp{the}. At the end, the marker has advanced over the | |
413 | inserted text so that it remains positioned before the same @samp{h}. | |
414 | Note that the location of point, shown in the usual fashion, has no | |
415 | effect. | |
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416 | |
417 | @example | |
418 | @group | |
419 | ---------- Buffer: foo ---------- | |
420 | "This is the @point{}output" | |
421 | ---------- Buffer: foo ---------- | |
422 | @end group | |
423 | ||
424 | @group | |
425 | m | |
426 | @result{} #<marker at 11 in foo> | |
427 | @end group | |
428 | ||
429 | @group | |
430 | (print "More output for foo." m) | |
431 | @result{} "More output for foo." | |
432 | @end group | |
433 | ||
434 | @group | |
435 | ---------- Buffer: foo ---------- | |
436 | "This is t | |
437 | "More output for foo." | |
438 | he @point{}output" | |
439 | ---------- Buffer: foo ---------- | |
440 | @end group | |
441 | ||
442 | @group | |
443 | m | |
444 | @result{} #<marker at 35 in foo> | |
445 | @end group | |
446 | @end example | |
447 | ||
448 | The following example shows output to the echo area: | |
449 | ||
450 | @example | |
451 | @group | |
452 | (print "Echo Area output" t) | |
453 | @result{} "Echo Area output" | |
454 | ---------- Echo Area ---------- | |
455 | "Echo Area output" | |
456 | ---------- Echo Area ---------- | |
457 | @end group | |
458 | @end example | |
459 | ||
460 | Finally, we show the use of a function as an output stream. The | |
461 | function @code{eat-output} takes each character that it is given and | |
462 | conses it onto the front of the list @code{last-output} (@pxref{Building | |
463 | Lists}). At the end, the list contains all the characters output, but | |
464 | in reverse order. | |
465 | ||
466 | @example | |
467 | @group | |
468 | (setq last-output nil) | |
469 | @result{} nil | |
470 | @end group | |
471 | ||
472 | @group | |
473 | (defun eat-output (c) | |
474 | (setq last-output (cons c last-output))) | |
475 | @result{} eat-output | |
476 | @end group | |
477 | ||
478 | @group | |
479 | (print "This is the output" 'eat-output) | |
480 | @result{} "This is the output" | |
481 | @end group | |
482 | ||
483 | @group | |
484 | last-output | |
485 | @result{} (10 34 116 117 112 116 117 111 32 101 104 | |
486 | 116 32 115 105 32 115 105 104 84 34 10) | |
487 | @end group | |
488 | @end example | |
489 | ||
490 | @noindent | |
491 | Now we can put the output in the proper order by reversing the list: | |
492 | ||
493 | @example | |
494 | @group | |
495 | (concat (nreverse last-output)) | |
496 | @result{} " | |
497 | \"This is the output\" | |
498 | " | |
499 | @end group | |
500 | @end example | |
501 | ||
b664e483 RS |
502 | @noindent |
503 | Calling @code{concat} converts the list to a string so you can see its | |
504 | contents more clearly. | |
505 | ||
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506 | @node Output Functions |
507 | @section Output Functions | |
508 | ||
509 | This section describes the Lisp functions for printing Lisp objects. | |
510 | ||
511 | @cindex @samp{"} in printing | |
512 | @cindex @samp{\} in printing | |
513 | @cindex quoting characters in printing | |
514 | @cindex escape characters in printing | |
515 | Some of the Emacs printing functions add quoting characters to the | |
516 | output when necessary so that it can be read properly. The quoting | |
517 | characters used are @samp{"} and @samp{\}; they distinguish strings from | |
518 | symbols, and prevent punctuation characters in strings and symbols from | |
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519 | being taken as delimiters when reading. @xref{Printed Representation}, |
520 | for full details. You specify quoting or no quoting by the choice of | |
521 | printing function. | |
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522 | |
523 | If the text is to be read back into Lisp, then it is best to print | |
524 | with quoting characters to avoid ambiguity. Likewise, if the purpose is | |
525 | to describe a Lisp object clearly for a Lisp programmer. However, if | |
526 | the purpose of the output is to look nice for humans, then it is better | |
527 | to print without quoting. | |
528 | ||
529 | Printing a self-referent Lisp object requires an infinite amount of | |
530 | text. In certain cases, trying to produce this text leads to a stack | |
531 | overflow. Emacs detects such recursion and prints @samp{#@var{level}} | |
532 | instead of recursively printing an object already being printed. For | |
533 | example, here @samp{#0} indicates a recursive reference to the object at | |
534 | level 0 of the current print operation: | |
535 | ||
536 | @example | |
537 | (setq foo (list nil)) | |
538 | @result{} (nil) | |
539 | (setcar foo foo) | |
540 | @result{} (#0) | |
541 | @end example | |
542 | ||
543 | In the functions below, @var{stream} stands for an output stream. | |
544 | (See the previous section for a description of output streams.) If | |
545 | @var{stream} is @code{nil} or omitted, it defaults to the value of | |
546 | @code{standard-output}. | |
547 | ||
548 | @defun print object &optional stream | |
549 | @cindex Lisp printer | |
550 | The @code{print} function is a convenient way of printing. It outputs | |
551 | the printed representation of @var{object} to @var{stream}, printing in | |
552 | addition one newline before @var{object} and another after it. Quoting | |
553 | characters are used. @code{print} returns @var{object}. For example: | |
554 | ||
555 | @example | |
556 | @group | |
557 | (progn (print 'The\ cat\ in) | |
558 | (print "the hat") | |
559 | (print " came back")) | |
560 | @print{} | |
561 | @print{} The\ cat\ in | |
562 | @print{} | |
563 | @print{} "the hat" | |
564 | @print{} | |
565 | @print{} " came back" | |
566 | @print{} | |
567 | @result{} " came back" | |
568 | @end group | |
569 | @end example | |
570 | @end defun | |
571 | ||
572 | @defun prin1 object &optional stream | |
573 | This function outputs the printed representation of @var{object} to | |
b664e483 RS |
574 | @var{stream}. It does not print newlines to separate output as |
575 | @code{print} does, but it does use quoting characters just like | |
576 | @code{print}. It returns @var{object}. | |
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577 | |
578 | @example | |
579 | @group | |
580 | (progn (prin1 'The\ cat\ in) | |
581 | (prin1 "the hat") | |
582 | (prin1 " came back")) | |
583 | @print{} The\ cat\ in"the hat"" came back" | |
584 | @result{} " came back" | |
585 | @end group | |
586 | @end example | |
587 | @end defun | |
588 | ||
589 | @defun princ object &optional stream | |
590 | This function outputs the printed representation of @var{object} to | |
591 | @var{stream}. It returns @var{object}. | |
592 | ||
593 | This function is intended to produce output that is readable by people, | |
594 | not by @code{read}, so it doesn't insert quoting characters and doesn't | |
595 | put double-quotes around the contents of strings. It does not add any | |
596 | spacing between calls. | |
597 | ||
598 | @example | |
599 | @group | |
600 | (progn | |
601 | (princ 'The\ cat) | |
602 | (princ " in the \"hat\"")) | |
603 | @print{} The cat in the "hat" | |
604 | @result{} " in the \"hat\"" | |
605 | @end group | |
606 | @end example | |
607 | @end defun | |
608 | ||
609 | @defun terpri &optional stream | |
610 | @cindex newline in print | |
611 | This function outputs a newline to @var{stream}. The name stands | |
612 | for ``terminate print''. | |
613 | @end defun | |
614 | ||
615 | @defun write-char character &optional stream | |
616 | This function outputs @var{character} to @var{stream}. It returns | |
617 | @var{character}. | |
618 | @end defun | |
619 | ||
620 | @defun prin1-to-string object &optional noescape | |
621 | @cindex object to string | |
622 | This function returns a string containing the text that @code{prin1} | |
623 | would have printed for the same argument. | |
624 | ||
625 | @example | |
626 | @group | |
627 | (prin1-to-string 'foo) | |
628 | @result{} "foo" | |
629 | @end group | |
630 | @group | |
631 | (prin1-to-string (mark-marker)) | |
632 | @result{} "#<marker at 2773 in strings.texi>" | |
633 | @end group | |
634 | @end example | |
635 | ||
636 | If @var{noescape} is non-@code{nil}, that inhibits use of quoting | |
637 | characters in the output. (This argument is supported in Emacs versions | |
638 | 19 and later.) | |
639 | ||
640 | @example | |
641 | @group | |
642 | (prin1-to-string "foo") | |
643 | @result{} "\"foo\"" | |
644 | @end group | |
645 | @group | |
646 | (prin1-to-string "foo" t) | |
647 | @result{} "foo" | |
648 | @end group | |
649 | @end example | |
650 | ||
651 | See @code{format}, in @ref{String Conversion}, for other ways to obtain | |
652 | the printed representation of a Lisp object as a string. | |
653 | @end defun | |
654 | ||
655 | @node Output Variables | |
656 | @section Variables Affecting Output | |
657 | ||
658 | @defvar standard-output | |
659 | The value of this variable is the default output stream---the stream | |
660 | that print functions use when the @var{stream} argument is @code{nil}. | |
661 | @end defvar | |
662 | ||
663 | @defvar print-escape-newlines | |
664 | @cindex @samp{\n} in print | |
665 | @cindex escape characters | |
666 | If this variable is non-@code{nil}, then newline characters in strings | |
667 | are printed as @samp{\n} and formfeeds are printed as @samp{\f}. | |
668 | Normally these characters are printed as actual newlines and formfeeds. | |
669 | ||
670 | This variable affects the print functions @code{prin1} and @code{print}, | |
671 | as well as everything that uses them. It does not affect @code{princ}. | |
672 | Here is an example using @code{prin1}: | |
673 | ||
674 | @example | |
675 | @group | |
676 | (prin1 "a\nb") | |
677 | @print{} "a | |
678 | @print{} b" | |
679 | @result{} "a | |
680 | b" | |
681 | @end group | |
682 | ||
683 | @group | |
684 | (let ((print-escape-newlines t)) | |
685 | (prin1 "a\nb")) | |
686 | @print{} "a\nb" | |
687 | @result{} "a | |
688 | b" | |
689 | @end group | |
690 | @end example | |
691 | ||
692 | @noindent | |
693 | In the second expression, the local binding of | |
694 | @code{print-escape-newlines} is in effect during the call to | |
695 | @code{prin1}, but not during the printing of the result. | |
696 | @end defvar | |
697 | ||
698 | @defvar print-length | |
699 | @cindex printing limits | |
700 | The value of this variable is the maximum number of elements of a list | |
701 | that will be printed. If a list being printed has more than this many | |
b664e483 | 702 | elements, it is abbreviated with an ellipsis. |
5e8db0c6 RS |
703 | |
704 | If the value is @code{nil} (the default), then there is no limit. | |
705 | ||
706 | @example | |
707 | @group | |
708 | (setq print-length 2) | |
709 | @result{} 2 | |
710 | @end group | |
711 | @group | |
712 | (print '(1 2 3 4 5)) | |
713 | @print{} (1 2 ...) | |
714 | @result{} (1 2 ...) | |
715 | @end group | |
716 | @end example | |
717 | @end defvar | |
718 | ||
719 | @defvar print-level | |
720 | The value of this variable is the maximum depth of nesting of | |
b664e483 | 721 | parentheses and brackets when printed. Any list or vector at a depth |
5e8db0c6 RS |
722 | exceeding this limit is abbreviated with an ellipsis. A value of |
723 | @code{nil} (which is the default) means no limit. | |
724 | ||
725 | This variable exists in version 19 and later versions. | |
726 | @end defvar |