Loading ‘~/.emacs’: unescaped character literals `?[', `?]' detected, `?\[', `?\]' expected!

(defun transform-square-brackets-to-round-ones(string-to-transform)
                  "Transforms [ into ( and ] into ), other chars left unchanged."
                  (concat 
                   (mapcar #'(lambda (c) (if (equal c ?[) ?\( (if (equal c ?]) ?\) c))) string-to-transform)))