First pass at WRITE-DOCUMENT, document exported protocol helpers.

docs/hacking.md

 All current Content Types and Indexes implement the protocol faithfully.
 It consists of 2 "class" methods, 2 instance methods, and an invariant.
 
+There are also 4 helper functions provided that should prove useful in
+implementing new content types.
 
 **Class Methods**:
 
   the site's git repo with the lowercased class-name as a file extension,
   i.e. (".post" for POST files).
 
+**Protocol Helpers**:
+
+- `add-document`: Add the document to *coleslaw*'s in-memory
+  database. It will error if the `page-url` of the document is not
+  unique. Such a hash collision represents content on the site being
+  shadowed/overwritten. This should be used in your `discover` method.
+- `write-document`: Write the document out to disk as HTML. It takes
+  an optional template name and render-args to pass to the template.
+  This should be used in your `publish` method.
+- `find-all`: Return a list of all documents of the requested class.
+  This is often used in the `publish` method to iterate over documents
+  of a given type.
+- `purge-all`: Remove all instances of the requested class from the DB.
+  This is primarily used at the REPL or for debugging but it is also
+  used in a `:before` method on `discover` to keep it idempotent.
+
 ### Current Content Types & Indexes
 
 There are 5 INDEX subclasses at present: TAG-INDEX, MONTH-INDEX,

src/documents.lisp

 
 (defgeneric render (document &key &allow-other-keys)
   (:documentation "Render the given DOCUMENT to HTML."))
+
+(defun write-document (document &optional theme-fn &rest render-args)
+  "Write the given DOCUMENT to disk as HTML. If THEME-FN is present,
+use it as the template passing any RENDER-ARGS."
+  (let ((html (if render-args
+                  (apply #'render-page document theme-fn render-args)
+                  (render-page document nil))))
+    (write-file (page-path obj) html)))

src/packages.lisp

            #:discover
            #:publish
            #:page-url
-           #:render))
+           #:render
+           #:write-document))