Flexible macro to output to a stream (default returns a string)

with-output-to-stream.lisp

(defmacro with-output-to-stream ((var &optional stream &key (element-type 'character)) &body body)
  (let ((gstream (gensym "STREAM"))
        (gresult (gensym "RESULT")))
    `(let* ((,gstream ,stream)
            (,var (case ,gstream
                    ((t) *standard-output*)
                    ((nil) (make-string-output-stream :element-type ',element-type))
                    (otherwise ,gstream)))
            (,gresult (multiple-value-list (progn ,@body))))
       (if ,gstream
           (values-list ,gresult)
           (get-output-stream-string ,var)))))