join.lisp

; a join function using FORMAT

(defun join-aux (stream &rest args)
  (declare (ignore args)
           (special delim))
  (princ delim stream))

(defun join (list delim)
  (declare (special delim))
  (format nil "~{~a~^~/JOIN-AUX/~:*~}" list))

; I haven't seen a possibility to pass the delimiter directly, so that
; some format directive can make use of it. We have to work around it.
;
; The trick:
; * a global function JOIN-AUX takes a stream and ignores the rest of its
;   args and prints the delimiter instead
; * the delimiter is accessed via a special variable using dynamic binding
; * in the format string, the function JOIN-AUX gets called with the next argument
;   and then we back off using ~:* this one argument, so that it remains unprocessed
;   for the iteration.