@c -*-texinfo-*- @c This is part of the GNU Emacs Lisp Reference Manual. @c Copyright (C) 1990, 1991, 1992, 1993, 1994, 1998, 1999, 2001, 2002, @c 2003, 2004, 2005, 2006, 2007 Free Software Foundation, Inc. @c See the file elisp.texi for copying conditions. @setfilename ../info/streams @node Read and Print, 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{Lisp Data Types}. 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 @dfn{printed representation} (@pxref{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 @code{a} and @code{b}. However, these two operations are not precisely inverse to each other. There are three kinds of exceptions: @itemize @bullet @item Printing can produce text that cannot be read. For example, buffers, windows, frames, subprocesses and markers print as 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. @item Comments can appear at certain points in the middle of an object's read sequence without affecting the result of reading it. @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}, which must support two kinds of calls: @itemize @bullet @item When it is called with no arguments, it should return the next character. @item When it is called with one argument (always a character), @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.'' In this case, it makes no difference what value @var{function} returns. @end itemize @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. If Emacs is running in batch mode, standard input is used instead of the minibuffer. For example, @example (message "%s" (read t)) @end example will read a Lisp expression from standard input and print the result to standard output. @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 that 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. Reading skips any amount of whitespace preceding the significant text. Here is an example of reading from a stream that is a marker, initially positioned 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{} # @end group @group (read m) @result{} This @end group @group m @result{} # ;; @r{Before 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 character 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{} (40 41) @end group @end example @noindent Note that the open and close parentheses remain in the list. The Lisp reader encountered the open parenthesis, decided that it ended the input, and unread it. Another attempt to read from the stream at this point would read @samp{()} and return @code{nil}. @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 you specify @var{end}, then reading is forced to stop just before 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 . 5) @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}. The default is @code{t}, meaning use the minibuffer. @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 marker position advances as characters are inserted. The value of point in the buffer has no effect on printing when the stream is a marker, and this kind of printing does not move point (except that if the marker points at or before the position of point, point advances with the surrounding text, as usual). @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 responsible for storing the characters wherever you want to put them. @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 use the value of @code{standard-output} instead; that value is the @dfn{default output stream}, and must not be @code{nil}. @item @var{symbol} A symbol as output stream is equivalent to the symbol's function definition (if any). @end table Many of the valid output streams are also valid as input streams. The difference between input and output streams is therefore more a matter of how you use a Lisp object, than of different types of object. 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 is in buffer @code{foo}, between the @samp{t} and the @samp{h} in the word @samp{the}. At the end, the marker has advanced over the inserted text so that it remains positioned 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 (setq m (copy-marker 10)) @result{} # @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{} # @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 @noindent Calling @code{concat} converts the list to a string so you can see its contents more clearly. @node Output Functions @section Output Functions This section describes the Lisp functions for printing Lisp objects---converting objects into their printed representation. @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 when reading. @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 you should 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 usually better to print without quoting. Lisp objects can refer to themselves. Printing a self-referential object in the normal way would require an infinite amount of text, and the attempt could cause infinite recursion. 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" @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 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{} "#" @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{Formatting Strings}, for other ways to obtain the printed representation of a Lisp object as a string. @end defun @defmac with-output-to-string body@dots{} This macro executes the @var{body} forms with @code{standard-output} set up to feed output into a string. Then it returns that string. For example, if the current buffer name is @samp{foo}, @example (with-output-to-string (princ "The buffer is ") (princ (buffer-name))) @end example @noindent returns @code{"The buffer is foo"}. @end defmac @node Output Variables @section Variables Affecting Output @cindex output-controlling variables @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}. The default is @code{t}, meaning display in the echo area. @end defvar @defvar print-quoted If this is non-@code{nil}, that means to print quoted forms using abbreviated reader syntax. @code{(quote foo)} prints as @code{'foo}, @code{(function foo)} as @code{#'foo}, and backquoted forms print using modern backquote syntax. @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} that print with quoting. 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-escape-nonascii If this variable is non-@code{nil}, then unibyte non-@acronym{ASCII} characters in strings are unconditionally printed as backslash sequences by the print functions @code{prin1} and @code{print} that print with quoting. Those functions also use backslash sequences for unibyte non-@acronym{ASCII} characters, regardless of the value of this variable, when the output stream is a multibyte buffer or a marker pointing into one. @end defvar @defvar print-escape-multibyte If this variable is non-@code{nil}, then multibyte non-@acronym{ASCII} characters in strings are unconditionally printed as backslash sequences by the print functions @code{prin1} and @code{print} that print with quoting. Those functions also use backslash sequences for multibyte non-@acronym{ASCII} characters, regardless of the value of this variable, when the output stream is a unibyte buffer or a marker pointing into one. @end defvar @defvar print-length @cindex printing limits The value of this variable is the maximum number of elements to print in any list, vector or bool-vector. If an object being printed has more than this many elements, 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 and brackets when 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. @end defvar @defopt eval-expression-print-length @defoptx eval-expression-print-level These are the values for @code{print-length} and @code{print-level} used by @code{eval-expression}, and thus, indirectly, by many interactive evaluation commands (@pxref{Lisp Eval,, Evaluating Emacs-Lisp Expressions, emacs, The GNU Emacs Manual}). @end defopt These variables are used for detecting and reporting circular and shared structure: @defvar print-circle If non-@code{nil}, this variable enables detection of circular and shared structure in printing. @end defvar @defvar print-gensym If non-@code{nil}, this variable enables detection of uninterned symbols (@pxref{Creating Symbols}) in printing. When this is enabled, uninterned symbols print with the prefix @samp{#:}, which tells the Lisp reader to produce an uninterned symbol. @end defvar @defvar print-continuous-numbering If non-@code{nil}, that means number continuously across print calls. This affects the numbers printed for @samp{#@var{n}=} labels and @samp{#@var{m}#} references. Don't set this variable with @code{setq}; you should only bind it temporarily to @code{t} with @code{let}. When you do that, you should also bind @code{print-number-table} to @code{nil}. @end defvar @defvar print-number-table This variable holds a vector used internally by printing to implement the @code{print-circle} feature. You should not use it except to bind it to @code{nil} when you bind @code{print-continuous-numbering}. @end defvar @defvar float-output-format This variable specifies how to print floating point numbers. Its default value is @code{nil}, meaning use the shortest output that represents the number without losing information. To control output format more precisely, you can put a string in this variable. The string should hold a @samp{%}-specification to be used in the C function @code{sprintf}. For further restrictions on what you can use, see the variable's documentation string. @end defvar @ignore arch-tag: 07636b8c-c4e3-4735-9e06-2e864320b434 @end ignore