Every time I export the blog from Org-mode to HTML, I need to go through new entries and add the YAML header that Jekyll (this blog's site generator) uses. I'm tired of doing that, so it's time to automate it.

First we add the symbol for our not-yet-defined function to the property list that defines this Org project. A property list is a flat list of the form (:param1 value1 ... :paramN valueN). 

;; List contains (("fasciism" ...)) before proper plist begins.
;; cdar = (cdr (car ...)), removing first element of list.
(plist-put (cdar org-publish-project-alist)
           :completion-function
           'mak::fasciism-project-export-completion)

This method for accessing org-publish-project-alist depends on the blog being the first project. While a terrible way to do things, it'll work since Emacs loads these blog entries in sequence so that later additions to my Org projects won't cause this to fail. 

(defun mak::fasciism-project-export-completion (&rest project)
  "Ensure the YAML header exists on blog entries."
  ;; In Org v8, we check a dynamically-scoped variable.
  ;; In Org v9, we get a parameter passed to us.
  (unless project
    (setq project project-plist))
  (let* ((dir (plist-get project :publishing-directory))
         (files (directory-files dir t "\\.html$")))
    ;; For each HTML file.
    (dolist (file files)
      ;; Create a temporary buffer tied to the HTML file.
      (with-temp-file file
       (insert-file-contents file)
         ;; Check for YAML header, inserting if absent.
         (unless (equal (buffer-substring 1 5) "---\n")
           (insert "---\n"
                   "layout: post\n"
                   "---\n"
                   "\n"))))))

While most of the functions used above will be recognizable to your average programmer, I want to take a moment to discuss let*. We've used let before when we needed to create temporary variables, so why do I use let* here? It's because let* allows me to create a variable that references the value assigned to a previous variable in the same assignment list. Specifically, we define dir, and then we use dir in the definition of files. We could have made the definition of files longer, but I prefer this method of breaking it up into smaller assignments that accrete.