string output streams

get-output-stream-string doesn’t just return the string accumulated so far in a string-stream. It also resets the accumulation, so you can use the same stream multiple times for separate results. Here’s a simple string splitter:

(defun split (string &optional (split-character #\Space))
  (let ((result '())
        (stream (make-string-output-stream)))
    (loop for char across string
          if (char= char split-character)
          do (push (get-output-stream-string stream) result)
          do (write-char char stream))
    (push (get-output-stream-string stream) result)
    (nreverse result)))

It could be used like this:

* (split "The quick brown fox.")
("The" "quick" "brown" "fox.")

I learned about this technique from an Erik Naggum post.