Factor purge-all calls in discover to a before method.

src/documents.lisp

   (:method (doc-type)
     (let* ((class-name (class-name doc-type))
            (file-type (string-downcase (symbol-name class-name))))
-      (purge-all class-name)
       (do-files (file (repo *config*) file-type)
         (let ((obj (construct class-name (read-content file))))
           (add-document obj))))))
 
+(defmethod discover :before (doc-type)
+  (purge-all (class-name doc-type)))
+
 ;; Instance Methods
 
 (defgeneric page-url (document)

src/indexes.lisp

   (format nil "tag/~a" (index-slug object)))
 
 (defmethod discover ((doc-type (eql (find-class 'tag-index))))
-  (purge-all (class-name doc-type))
   (let ((content (by-date (find-all 'post))))
     (dolist (tag (all-tags))
       (add-document (index-by-tag tag content)))))
   (format nil "date/~a" (index-slug object)))
 
 (defmethod discover ((doc-type (eql (find-class 'month-index))))
-  (purge-all (class-name doc-type))
   (let ((content (by-date (find-all 'post))))
     (dolist (month (all-months))
       (add-document (index-by-month month content)))))
   (format nil "~d" (index-slug object)))
 
 (defmethod discover ((doc-type (eql (find-class 'numeric-index))))
-  (purge-all (class-name doc-type))
   (let ((content (by-date (find-all 'post))))
     (dotimes (i (ceiling (length content) 10))
       (add-document (index-by-n i content)))))