From Newsgroup: comp.lang.lisp

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
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