Flesh out the sketch a bit more and add some constraints.

plugins/incremental.lisp

 
 (in-package :coleslaw-incremental)
 
-;; FIXME: We currently never update the site for config changes.
+;; KLUDGE: We currently never update the site for config changes.
 ;; Examples to consider include changing the theme or domain of the site.
 ;; Both would require full site recompiles. Consequently, it seems reasonable
 ;; to expect that incremental plugin users:
 ;;   A) have done a full build of their site
 ;;   B) have a cl-store dump of the database at ~/.coleslaw.db
 ;;      ^ we should provide a script or plugin just for this
+;;   C) move the original deployment to a location of their choice and
+;;      set it as staging-dir in coleslaw's config prior to enabling incremental builds
+;;   D) to further simplify *my* life, we assume the date of a piece of content will
+;;      never be changed retroactively, only its tags
 
 ;; NOTE: We're gonna be a bit dirty here and monkey patch. The compilation model
 ;; still isn't an "exposed" part of Coleslaw. After some experimentation maybe
 ;; we'll settle on an interface.
 
-(defvar *changed-content* nil
-  "A list of changed content instances to iterate over and write out to disk.")
+(defvar *transients* '(coleslaw::numeric-index coleslaw::feed coleslaw::tag-feed)
+  "A list of document types that should be regenerated on *any* change to the blog.")
 
 (defun coleslaw::load-content ()
   (let ((db-file (rel-path (user-homedir-pathname) ".coleslaw.db")))
     (setf coleslaw::*site* (cl-store:restore db-file))
     (loop for (status path) in (get-updated-files)
        do (update-content status path))
+    (coleslaw::update-content-metadata)
+    (dolist (doc-type *transients*)
+      (discover (find-class doc-type)))
     (cl-store:store coleslaw::*site* db-file)))
 
 (defun update-content (status path)
       ;; This feels way too clever. I wish I could think of a better option.
       (flet ((class-name-p (x class)
                (string-equal x (symbol-name (class-name class)))))
+        ;; If the updated file's extension doesn't match one of our content types,
+        ;; we don't need to mess with it at all. Otherwise, since the class is
+        ;; annoyingly tricky to determine, pass it along.
         (when-let (ctype (find extension ctypes :test #'class-name-p))
           (call-next-method status path :ctype ctype))))))
 
+;; TODO: We should check to see if a *new* tag or month exists
+;; and create an index appropriately. If the last content from a
+;; given month or with a given tag is deleted, just drop the index.
+;; (And also remove it from *all-months* / *all-tags*. Should we store those?)
+;; Additionally, the tag/month lists won't be updated on tag/month index pages.
+
 (defmethod process-change ((status (eql :deleted)) path &key)
-  (let ((obj (find-content-by-path path)))
-    ))
+  (let ((old (find-content-by-path path)))
+    ;; TODO: Remove from any tag and month indexes.
+    (delete-document old)))
 
 (defmethod process-change ((status (eql :modified)) path &key)
-  (let ((obj (find-content-by-path path)))
+  (let ((old (find-content-by-path path))
+        (new (construct ctype (read-content path))))
+    (setf (gethash (page-url old) coleslaw::*site*) new)
+    ;; TODO:
+    ;; Iterate over tags in new, setting old to new in each tag index's content.
+    ;; If there are new tags/date, add it to relevant indices.
+    ;; If tags/date are removed, remove from relevant indices.
     ))
 
 (defmethod process-change ((status (eql :added)) path &key ctype)
-  (let ((obj (construct ctype (read-content path))))
+  (let ((new (construct ctype (read-content path))))
     ))
 
+(defun delete-document (document)
+  "Given a DOCUMENT, delete it from the staging directory and in-memory DB."
+  (let ((url (page-url document)))
+    (delete-file (rel-path (staging-dir *config*) (namestring url)))
+    (remhash (page-url document) coleslaw::*site*)))
+
 (defun coleslaw::compile-blog (staging)
   "lulz. Do it live. DO IT ALL LIVE."
+  (dolist (doc-type *transients*)
+    (publish (find-class doc-type)))
   ;; FIXME: This doesn't cover prev/next links for posts, theme-fn for feeds.
   (mapcar #'write-document *changed-content*))