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Output Functions

This section describes the Lisp functions for printing Lisp objects.

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 `\' and `"'; they are used to distinguish strings from symbols, and to prevent punctuation characters in strings and symbols from being taken as delimiters. See section Printed Representation and Read Syntax, 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 `#level' instead of recursively printing an object already being printed. For example, here `#0' indicates a recursive reference to the object at level 0 of the current print operation:

(setq foo (list nil))
     => (nil)
(setcar foo foo)
     => (#0)

In the functions below, stream stands for an output stream. (See the previous section for a description of output streams.) If stream is nil or omitted, it defaults to the value of standard-output.

Function: print object &optional stream

The print is a convenient way of printing. It outputs the printed representation of object to stream, printing in addition one newline before object and another after it. Quoting characters are used. print returns object. For example:

(progn (print 'The\ cat\ in)
       (print "the hat")
       (print " came back"))
     -| 
     -| The\ cat\ in
     -| 
     -| "the hat"
     -| 
     -| " came back"
     -| 
     => " came back"

Function: prin1 object &optional stream

This function outputs the printed representation of object to stream. It does not print any spaces or newlines to separate output as print does, but it does use quoting characters just like print. It returns object.

(progn (prin1 'The\ cat\ in) 
       (prin1 "the hat") 
       (prin1 " came back"))
     -| The\ cat\ in"the hat"" came back"
     => " came back"

Function: princ object &optional stream

This function outputs the printed representation of object to stream. It returns object.

This function is intended to produce output that is readable by people, not by read, so quoting characters are not used and double-quotes are not printed around the contents of strings. It does not add any spacing between calls.

(progn
  (princ 'The\ cat)
  (princ " in the \"hat\""))
     -| The cat in the "hat"
     => " in the \"hat\""

Function: terpri &optional stream

This function outputs a newline to stream. The name stands for "terminate print".

Function: write-char character &optional stream

This function outputs character to stream. It returns character.

Function: prin1-to-string object &optional noescape

This function returns a string containing the text that prin1 would have printed for the same argument.

(prin1-to-string 'foo)
     => "foo"
(prin1-to-string (mark-marker))
     => "#<marker at 2773 in strings.texi>"

If noescape is non-nil, that inhibits use of quoting characters in the output. (This argument is supported in Emacs versions 19 and later.)

(prin1-to-string "foo")
     => "\"foo\""
(prin1-to-string "foo" t)
     => "foo"

See format, in section Conversion of Characters and Strings, for other ways to obtain the printed representation of a Lisp object as a string.

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