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diff --git a/lispref/streams.texi b/lispref/streams.texi deleted file mode 100644 index af3787f579d..00000000000 --- a/lispref/streams.texi +++ /dev/null @@ -1,713 +0,0 @@ -@c -*-texinfo-*- -@c This is part of the GNU Emacs Lisp Reference Manual. -@c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. -@c See the file elisp.texi for copying conditions. -@setfilename ../info/streams -@node Streams, Minibuffers, Debugging, Top -@comment node-name, next, previous, up -@chapter Reading and Printing Lisp Objects - - @dfn{Printing} and @dfn{reading} are the operations of converting Lisp -objects to textual form and vice versa. They use the printed -representations and read syntax described in @ref{Types of Lisp Object}. - - This chapter describes the Lisp functions for reading and printing. -It also describes @dfn{streams}, which specify where to get the text (if -reading) or where to put it (if printing). - -@menu -* Streams Intro:: Overview of streams, reading and printing. -* Input Streams:: Various data types that can be used as input streams. -* Input Functions:: Functions to read Lisp objects from text. -* Output Streams:: Various data types that can be used as output streams. -* Output Functions:: Functions to print Lisp objects as text. -* Output Variables:: Variables that control what the printing functions do. -@end menu - -@node Streams Intro -@section Introduction to Reading and Printing -@cindex Lisp reader -@cindex printing -@cindex reading - - @dfn{Reading} a Lisp object means parsing a Lisp expression in textual -form and producing a corresponding Lisp object. This is how Lisp -programs get into Lisp from files of Lisp code. We call the text the -@dfn{read syntax} of the object. For example, the text @samp{(a .@: 5)} -is the read syntax for a cons cell whose @sc{car} is @code{a} and whose -@sc{cdr} is the number 5. - - @dfn{Printing} a Lisp object means producing text that represents that -object---converting the object to its printed representation. Printing -the cons cell described above produces the text @samp{(a .@: 5)}. - - Reading and printing are more or less inverse operations: printing the -object that results from reading a given piece of text often produces -the same text, and reading the text that results from printing an object -usually produces a similar-looking object. For example, printing the -symbol @code{foo} produces the text @samp{foo}, and reading that text -returns the symbol @code{foo}. Printing a list whose elements are -@code{a} and @code{b} produces the text @samp{(a b)}, and reading that -text produces a list (but not the same list) with elements are @code{a} -and @code{b}. - - However, these two operations are not precisely inverses. There are -two kinds of exceptions: - -@itemize @bullet -@item -Printing can produce text that cannot be read. For example, buffers, -windows, frames, subprocesses and markers print into text that starts -with @samp{#}; if you try to read this text, you get an error. There is -no way to read those data types. - -@item -One object can have multiple textual representations. For example, -@samp{1} and @samp{01} represent the same integer, and @samp{(a b)} and -@samp{(a .@: (b))} represent the same list. Reading will accept any of -the alternatives, but printing must choose one of them. -@end itemize - -@node Input Streams -@section Input Streams -@cindex stream (for reading) -@cindex input stream - - Most of the Lisp functions for reading text take an @dfn{input stream} -as an argument. The input stream specifies where or how to get the -characters of the text to be read. Here are the possible types of input -stream: - -@table @asis -@item @var{buffer} -@cindex buffer input stream -The input characters are read from @var{buffer}, starting with the -character directly after point. Point advances as characters are read. - -@item @var{marker} -@cindex marker input stream -The input characters are read from the buffer that @var{marker} is in, -starting with the character directly after the marker. The marker -position advances as characters are read. The value of point in the -buffer has no effect when the stream is a marker. - -@item @var{string} -@cindex string input stream -The input characters are taken from @var{string}, starting at the first -character in the string and using as many characters as required. - -@item @var{function} -@cindex function input stream -The input characters are generated by @var{function}, one character per -call. Normally @var{function} is called with no arguments, and should -return a character. - -@cindex unreading -Occasionally @var{function} is called with one argument (always a -character). When that happens, @var{function} should save the argument -and arrange to return it on the next call. This is called -@dfn{unreading} the character; it happens when the Lisp reader reads one -character too many and wants to ``put it back where it came from''. - -@item @code{t} -@cindex @code{t} input stream -@code{t} used as a stream means that the input is read from the -minibuffer. In fact, the minibuffer is invoked once and the text -given by the user is made into a string that is then used as the -input stream. - -@item @code{nil} -@cindex @code{nil} input stream -@code{nil} supplied as an input stream means to use the value of -@code{standard-input} instead; that value is the @dfn{default input -stream}, and must be a non-@code{nil} input stream. - -@item @var{symbol} -A symbol as input stream is equivalent to the symbol's function -definition (if any). -@end table - - Here is an example of reading from a stream which is a buffer, showing -where point is located before and after: - -@example -@group ----------- Buffer: foo ---------- -This@point{} is the contents of foo. ----------- Buffer: foo ---------- -@end group - -@group -(read (get-buffer "foo")) - @result{} is -@end group -@group -(read (get-buffer "foo")) - @result{} the -@end group - -@group ----------- Buffer: foo ---------- -This is the@point{} contents of foo. ----------- Buffer: foo ---------- -@end group -@end example - -@noindent -Note that the first read skips a space at the beginning of the buffer. -Reading skips any amount of whitespace preceding the significant text. - - In Emacs 18, reading a symbol discarded the delimiter terminating the -symbol. Thus, point would end up at the beginning of @samp{contents} -rather than after @samp{the}. The Emacs 19 behavior is superior because -it correctly handles input such as @samp{bar(foo)}, where the delimiter -that ends one object is needed as the beginning of another object. - - Here is an example of reading from a stream that is a marker, -initialized to point at the beginning of the buffer shown. The value -read is the symbol @code{This}. - -@example -@group - ----------- Buffer: foo ---------- -This is the contents of foo. ----------- Buffer: foo ---------- -@end group - -@group -(setq m (set-marker (make-marker) 1 (get-buffer "foo"))) - @result{} #<marker at 1 in foo> -@end group -@group -(read m) - @result{} This -@end group -@group -m - @result{} #<marker at 6 in foo> ;; @r{After the first space.} -@end group -@end example - - Here we read from the contents of a string: - -@example -@group -(read "(When in) the course") - @result{} (When in) -@end group -@end example - - The following example reads from the minibuffer. The -prompt is: @w{@samp{Lisp expression: }}. (That is always the prompt -used when you read from the stream @code{t}.) The user's input is shown -following the prompt. - -@example -@group -(read t) - @result{} 23 ----------- Buffer: Minibuffer ---------- -Lisp expression: @kbd{23 @key{RET}} ----------- Buffer: Minibuffer ---------- -@end group -@end example - - Finally, here is an example of a stream that is a function, named -@code{useless-stream}. Before we use the stream, we initialize the -variable @code{useless-list} to a list of characters. Then each call to -the function @code{useless-stream} obtains the next characters in the list -or unreads a character by adding it to the front of the list. - -@example -@group -(setq useless-list (append "XY()" nil)) - @result{} (88 89 40 41) -@end group - -@group -(defun useless-stream (&optional unread) - (if unread - (setq useless-list (cons unread useless-list)) - (prog1 (car useless-list) - (setq useless-list (cdr useless-list))))) - @result{} useless-stream -@end group -@end example - -@noindent -Now we read using the stream thus constructed: - -@example -@group -(read 'useless-stream) - @result{} XY -@end group - -@group -useless-list - @result{} (41) -@end group -@end example - -@noindent -Note that the close parenthesis remains in the list. The reader has -read it, discovered that it ended the input, and unread it. Another -attempt to read from the stream at this point would get an error due to -the unmatched close parenthesis. - -@defun get-file-char -This function is used internally as an input stream to read from the -input file opened by the function @code{load}. Don't use this function -yourself. -@end defun - -@node Input Functions -@section Input Functions - - This section describes the Lisp functions and variables that pertain -to reading. - - In the functions below, @var{stream} stands for an input stream (see -the previous section). If @var{stream} is @code{nil} or omitted, it -defaults to the value of @code{standard-input}. - -@kindex end-of-file - An @code{end-of-file} error is signaled if reading encounters an -unterminated list, vector or string. - -@defun read &optional stream -This function reads one textual Lisp expression from @var{stream}, -returning it as a Lisp object. This is the basic Lisp input function. -@end defun - -@defun read-from-string string &optional start end -@cindex string to object -This function reads the first textual Lisp expression from the text in -@var{string}. It returns a cons cell whose @sc{car} is that expression, -and whose @sc{cdr} is an integer giving the position of the next -remaining character in the string (i.e., the first one not read). - -If @var{start} is supplied, then reading begins at index @var{start} in the -string (where the first character is at index 0). If @var{end} is also -supplied, then reading stops at that index as if the rest of the string -were not there. - -For example: - -@example -@group -(read-from-string "(setq x 55) (setq y 5)") - @result{} ((setq x 55) . 11) -@end group -@group -(read-from-string "\"A short string\"") - @result{} ("A short string" . 16) -@end group - -@group -;; @r{Read starting at the first character.} -(read-from-string "(list 112)" 0) - @result{} ((list 112) . 10) -@end group -@group -;; @r{Read starting at the second character.} -(read-from-string "(list 112)" 1) - @result{} (list . 6) -@end group -@group -;; @r{Read starting at the seventh character,} -;; @r{and stopping at the ninth.} -(read-from-string "(list 112)" 6 8) - @result{} (11 . 8) -@end group -@end example -@end defun - -@defvar standard-input -This variable holds the default input stream---the stream that -@code{read} uses when the @var{stream} argument is @code{nil}. -@end defvar - -@node Output Streams -@section Output Streams -@cindex stream (for printing) -@cindex output stream - - An output stream specifies what to do with the characters produced -by printing. Most print functions accept an output stream as an -optional argument. Here are the possible types of output stream: - -@table @asis -@item @var{buffer} -@cindex buffer output stream -The output characters are inserted into @var{buffer} at point. -Point advances as characters are inserted. - -@item @var{marker} -@cindex marker output stream -The output characters are inserted into the buffer that @var{marker} -points into, at the marker position. The position advances as -characters are inserted. The value of point in the buffer has no effect -on printing when the stream is a marker. - -@item @var{function} -@cindex function output stream -The output characters are passed to @var{function}, which is responsible -for storing them away. It is called with a single character as -argument, as many times as there are characters to be output, and is -free to do anything at all with the characters it receives. - -@item @code{t} -@cindex @code{t} output stream -The output characters are displayed in the echo area. - -@item @code{nil} -@cindex @code{nil} output stream -@code{nil} specified as an output stream means to the value of -@code{standard-output} instead; that value is the @dfn{default output -stream}, and must be a non-@code{nil} output stream. - -@item @var{symbol} -A symbol as output stream is equivalent to the symbol's function -definition (if any). -@end table - - Here is an example of a buffer used as an output stream. Point is -initially located as shown immediately before the @samp{h} in -@samp{the}. At the end, point is located directly before that same -@samp{h}. - -@cindex print example -@example -@group ----------- Buffer: foo ---------- -This is t@point{}he contents of foo. ----------- Buffer: foo ---------- -@end group - -(print "This is the output" (get-buffer "foo")) - @result{} "This is the output" - -@group ----------- Buffer: foo ---------- -This is t -"This is the output" -@point{}he contents of foo. ----------- Buffer: foo ---------- -@end group -@end example - - Now we show a use of a marker as an output stream. Initially, the -marker points in buffer @code{foo}, between the @samp{t} and the -@samp{h} in the word @samp{the}. At the end, the marker has been -advanced over the inserted text so that it still points before the same -@samp{h}. Note that the location of point, shown in the usual fashion, -has no effect. - -@example -@group ----------- Buffer: foo ---------- -"This is the @point{}output" ----------- Buffer: foo ---------- -@end group - -@group -m - @result{} #<marker at 11 in foo> -@end group - -@group -(print "More output for foo." m) - @result{} "More output for foo." -@end group - -@group ----------- Buffer: foo ---------- -"This is t -"More output for foo." -he @point{}output" ----------- Buffer: foo ---------- -@end group - -@group -m - @result{} #<marker at 35 in foo> -@end group -@end example - - The following example shows output to the echo area: - -@example -@group -(print "Echo Area output" t) - @result{} "Echo Area output" ----------- Echo Area ---------- -"Echo Area output" ----------- Echo Area ---------- -@end group -@end example - - Finally, we show the use of a function as an output stream. The -function @code{eat-output} takes each character that it is given and -conses it onto the front of the list @code{last-output} (@pxref{Building -Lists}). At the end, the list contains all the characters output, but -in reverse order. - -@example -@group -(setq last-output nil) - @result{} nil -@end group - -@group -(defun eat-output (c) - (setq last-output (cons c last-output))) - @result{} eat-output -@end group - -@group -(print "This is the output" 'eat-output) - @result{} "This is the output" -@end group - -@group -last-output - @result{} (10 34 116 117 112 116 117 111 32 101 104 - 116 32 115 105 32 115 105 104 84 34 10) -@end group -@end example - -@noindent -Now we can put the output in the proper order by reversing the list: - -@example -@group -(concat (nreverse last-output)) - @result{} " -\"This is the output\" -" -@end group -@end example - -@node Output Functions -@section Output Functions - - This section describes the Lisp functions for printing Lisp objects. - -@cindex @samp{"} in printing -@cindex @samp{\} in printing -@cindex quoting characters in printing -@cindex escape characters in printing - Some of the Emacs printing functions add quoting characters to the -output when necessary so that it can be read properly. The quoting -characters used are @samp{"} and @samp{\}; they distinguish strings from -symbols, and prevent punctuation characters in strings and symbols from -being taken as delimiters. @xref{Printed Representation}, for full -details. You specify quoting or no quoting by the choice of printing -function. - - If the text is to be read back into Lisp, then it is best to print -with quoting characters to avoid ambiguity. Likewise, if the purpose is -to describe a Lisp object clearly for a Lisp programmer. However, if -the purpose of the output is to look nice for humans, then it is better -to print without quoting. - - Printing a self-referent Lisp object requires an infinite amount of -text. In certain cases, trying to produce this text leads to a stack -overflow. Emacs detects such recursion and prints @samp{#@var{level}} -instead of recursively printing an object already being printed. For -example, here @samp{#0} indicates a recursive reference to the object at -level 0 of the current print operation: - -@example -(setq foo (list nil)) - @result{} (nil) -(setcar foo foo) - @result{} (#0) -@end example - - In the functions below, @var{stream} stands for an output stream. -(See the previous section for a description of output streams.) If -@var{stream} is @code{nil} or omitted, it defaults to the value of -@code{standard-output}. - -@defun print object &optional stream -@cindex Lisp printer -The @code{print} function is a convenient way of printing. It outputs -the printed representation of @var{object} to @var{stream}, printing in -addition one newline before @var{object} and another after it. Quoting -characters are used. @code{print} returns @var{object}. For example: - -@example -@group -(progn (print 'The\ cat\ in) - (print "the hat") - (print " came back")) - @print{} - @print{} The\ cat\ in - @print{} - @print{} "the hat" - @print{} - @print{} " came back" - @print{} - @result{} " came back" -@end group -@end example -@end defun - -@defun prin1 object &optional stream -This function outputs the printed representation of @var{object} to -@var{stream}. It does not print any spaces or newlines to separate -output as @code{print} does, but it does use quoting characters just -like @code{print}. It returns @var{object}. - -@example -@group -(progn (prin1 'The\ cat\ in) - (prin1 "the hat") - (prin1 " came back")) - @print{} The\ cat\ in"the hat"" came back" - @result{} " came back" -@end group -@end example -@end defun - -@defun princ object &optional stream -This function outputs the printed representation of @var{object} to -@var{stream}. It returns @var{object}. - -This function is intended to produce output that is readable by people, -not by @code{read}, so it doesn't insert quoting characters and doesn't -put double-quotes around the contents of strings. It does not add any -spacing between calls. - -@example -@group -(progn - (princ 'The\ cat) - (princ " in the \"hat\"")) - @print{} The cat in the "hat" - @result{} " in the \"hat\"" -@end group -@end example -@end defun - -@defun terpri &optional stream -@cindex newline in print -This function outputs a newline to @var{stream}. The name stands -for ``terminate print''. -@end defun - -@defun write-char character &optional stream -This function outputs @var{character} to @var{stream}. It returns -@var{character}. -@end defun - -@defun prin1-to-string object &optional noescape -@cindex object to string -This function returns a string containing the text that @code{prin1} -would have printed for the same argument. - -@example -@group -(prin1-to-string 'foo) - @result{} "foo" -@end group -@group -(prin1-to-string (mark-marker)) - @result{} "#<marker at 2773 in strings.texi>" -@end group -@end example - -If @var{noescape} is non-@code{nil}, that inhibits use of quoting -characters in the output. (This argument is supported in Emacs versions -19 and later.) - -@example -@group -(prin1-to-string "foo") - @result{} "\"foo\"" -@end group -@group -(prin1-to-string "foo" t) - @result{} "foo" -@end group -@end example - -See @code{format}, in @ref{String Conversion}, for other ways to obtain -the printed representation of a Lisp object as a string. -@end defun - -@node Output Variables -@section Variables Affecting Output - -@defvar standard-output -The value of this variable is the default output stream---the stream -that print functions use when the @var{stream} argument is @code{nil}. -@end defvar - -@defvar print-escape-newlines -@cindex @samp{\n} in print -@cindex escape characters -If this variable is non-@code{nil}, then newline characters in strings -are printed as @samp{\n} and formfeeds are printed as @samp{\f}. -Normally these characters are printed as actual newlines and formfeeds. - -This variable affects the print functions @code{prin1} and @code{print}, -as well as everything that uses them. It does not affect @code{princ}. -Here is an example using @code{prin1}: - -@example -@group -(prin1 "a\nb") - @print{} "a - @print{} b" - @result{} "a -b" -@end group - -@group -(let ((print-escape-newlines t)) - (prin1 "a\nb")) - @print{} "a\nb" - @result{} "a -b" -@end group -@end example - -@noindent -In the second expression, the local binding of -@code{print-escape-newlines} is in effect during the call to -@code{prin1}, but not during the printing of the result. -@end defvar - -@defvar print-length -@cindex printing limits -The value of this variable is the maximum number of elements of a list -that will be printed. If a list being printed has more than this many -elements, then it is abbreviated with an ellipsis. - -If the value is @code{nil} (the default), then there is no limit. - -@example -@group -(setq print-length 2) - @result{} 2 -@end group -@group -(print '(1 2 3 4 5)) - @print{} (1 2 ...) - @result{} (1 2 ...) -@end group -@end example -@end defvar - -@defvar print-level -The value of this variable is the maximum depth of nesting of -parentheses that will be printed. Any list or vector at a depth -exceeding this limit is abbreviated with an ellipsis. A value of -@code{nil} (which is the default) means no limit. - -This variable exists in version 19 and later versions. -@end defvar |