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 \'(\"         


        

Answer 1

Assuming a list of strings and a single character delimiter, the following should work efficiently for frequent invocation on short lists:

(defun join (list &optional (delimiter #\&))
  (with-output-to-string (stream)
    (join-to-stream stream list delimiter)))

(defun join-to-stream (stream list &optional (delimiter #\&))
  (destructuring-bind (&optional first &rest rest) list
    (when first
      (write-string first stream)
      (when rest
        (write-char delimiter stream)
        (join-to-stream stream rest delimiter)))))