common lisp - What's the canonical way to join strings in a list?

Question

I want to convert ("USERID=XYZ" "USERPWD=123") to "USERID=XYZ&USERPWD=123". I tried

(apply #'concatenate 'string '("USERID=XYZ" "USERPWD=123"))

which will return ""USERID=XYZUSERPWD=123".

But i do not know how to insert '&'? The following function works but seems a bit complicated.

(defun join (list &optional (delim "&"))
    (with-output-to-string (s)
        (when list
            (format s "~A" (first list))
            (dolist (element (rest list))
               (format s "~A~A" delim element)))))

Answer 1

Use FORMAT.

~{ and ~} denote iteration, ~A denotes aesthetic printing, and ~^ (aka Tilde Circumflex in the docs) denotes printing the , only when something follows it.

* (format nil "~{~A~^, ~}" '( 1 2 3 4 ))

"1, 2, 3, 4"
*