(lispref.info)Output Streams

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Output Streams

   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:

     The output characters are inserted into BUFFER at point.  Point
     advances as characters are inserted.

     The output characters are inserted into the buffer that MARKER is
     in at the marker position.  The position advances as characters are
     inserted.  The value of point in the buffer has no effect when the
     stream is a marker.

     The output characters are passed to 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.

     The output characters are displayed in the echo area.

     `nil' specified as an output stream means that the value of
     `standard-output' should be used as the output stream; that value
     is the "default output stream", and must be a non-`nil' output

     A symbol as output stream is equivalent to the symbol's function
     definition (if any).

   Here is an example of a buffer used as an output stream.  Point is
initially located as shown immediately before the `h' in `the'.  At the
end, point is located directly before that same `h'.

     ---------- Buffer: foo ----------
     This is t-!-he contents of foo.
     ---------- Buffer: foo ----------
     (print "This is the output" (get-buffer "foo"))
          => "This is the output"
     ---------- Buffer: foo ----------
     This is t
     "This is the output"
     -!-he contents of foo.
     ---------- Buffer: foo ----------

   Now we show a use of a marker as an output stream.  Initially, the
marker points in buffer `foo', between the `t' and the `h' in the word
`the'.  At the end, the marker has been advanced over the inserted text
so that it still points before the same `h'.  Note that the location of
point, shown in the usual fashion, has no effect.

     ---------- Buffer: foo ----------
     "This is the -!-output"
     ---------- Buffer: foo ----------
          => #<marker at 11 in foo>
     (print "More output for foo." m)
          => "More output for foo."
     ---------- Buffer: foo ----------
     "This is t
     "More output for foo."
     he -!-output"
     ---------- Buffer: foo ----------
          => #<marker at 35 in foo>

   The following example shows output to the echo area:

     (print "Echo Area output" t)
          => "Echo Area output"
     ---------- Echo Area ----------
     "Echo Area output"
     ---------- Echo Area ----------

   Finally, we show an output stream which is a function.  The function
`eat-output' takes each character that it is given and conses it onto
the front of the list `last-output' (Note: Building Lists.).  At the
end, the list contains all the characters output, but in reverse order.

     (setq last-output nil)
          => nil
     (defun eat-output (c)
       (setq last-output (cons c last-output)))
          => eat-output
     (print "This is the output" 'eat-output)
          => "This is the output"
          => (10 34 116 117 112 116 117 111 32 101 104
         116 32 115 105 32 115 105 104 84 34 10)

Now we can put the output in the proper order by reversing the list:

     (concat (nreverse last-output))
          => "
     \"This is the output\"

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