Re: what's the best way to do this?

Pascal Bourguignon wrote:

  (format t "~{~S~^:~}" list)

 

I suspect this is the best LOOP that can be constructed:
>
(loop for item in list
      for first = t then nil
      if first do (format t "~S" item)
      else do (format t ":~S" item))

 
If you don't want to use the simple format, you should rather avoid it
in the loop.
 
(when list
  (loop initially (princ (car list))
        for item in (cdr list)
        do (princ ":") (princ item) ))

Gauche Scheme

(define seq '(a b c d))

(when (pair? seq)
  (display (pop! seq))
  (for-each (cut  format #t ":~a" <>) seq))

  ===>
a:b:c:d