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19.4 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:

buffer
The output characters are inserted into buffer at point. Point advances as characters are inserted.
marker
The output characters are inserted into the buffer that 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).
function
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 responsible for storing the characters wherever you want to put them.
t
The output characters are displayed in the echo area.
nil
nil specified as an output stream means to use the value of standard-output instead; that value is the default output stream, and must not be nil.
symbol
A symbol as output stream is equivalent to the symbol's function definition (if any).

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 ‘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 is in buffer foo, between the ‘t’ and the ‘h’ in the word ‘the’. At the end, the marker has advanced over the inserted text so that it remains positioned 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 ----------
     
     (setq m (copy-marker 10))
          ⇒ #<marker at 10 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 ----------
     
     m
          ⇒ #<marker at 34 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 the use of a function as an output stream. The function eat-output takes each character that it is given and conses it onto the front of the list last-output (see 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"
     
     last-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\"
     "

Calling concat converts the list to a string so you can see its contents more clearly.