[O] [RFC] Add new export backend for Jekyll

[Łukasz Stelmach]

Jekyll[1] is a static site genrator.  Its most notable deployment are
Github Pages.  It accepts several input formats including HTML, that
is why ox-jekyll is derived from ox-html.  Input files for Jekyll must
be HTML files without any site-specific elements (e.g. menus, headers,
sidebars), because Jekyll will add them.  The HTML code, however, must
be prepended with a piece of YAML code which specifies:

+ page title,
+ page category,
+ layout template to use for the page,
+ publication date.

The main task of ox-jekyll is to read Org '#+KEYWORDS' and output them as
Jekyll's front matter.  At this point also source code blocks are exported
inside special tags instead of html <pre></pre>.

[1] http://jekyllrb.com/

Signed-off-by: Łukasz Stelmach <address@hidden>
---

This is kind of work-in-progress version. "Kind of" because it satisfies
my needs and I would like to ask what others think or need.

How to test this module?

Please comment.

 lisp/ox-jekyll.el | 139 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 139 insertions(+)
 create mode 100644 lisp/ox-jekyll.el

diff --git a/lisp/ox-jekyll.el b/lisp/ox-jekyll.el
new file mode 100644
index 000000000..273792ba9
--- /dev/null
+++ b/lisp/ox-jekyll.el
@@ -0,0 +1,139 @@
+;;; ox-jekyll.el --- Export org files to Jekyll static site generator
+
+;; Copyright (C) 2011-2015 Free Software Foundation, Inc.
+
+;; Author: Łukasz Stelmach <stlman at poczta dot fm>
+;; Keywords: 
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; This library implements a backend to export Org files as HTML
+;; for further processing by Jekyll static site generator.
+;; See Org manual for more information.
+
+;;; Code:
+
+;;; Dependencies
+
+(require 'ox)
+
+;;; Define Back-End
+
+(org-export-define-derived-backend 'jekyll 'html
+  :menu-entry
+  '(?h 1
+       ((?J "As HTML buffer (Jekyll)" org-jekyll-export-as-html)
+        (?j "As HTML file (Jekyll)" org-jekyll-export-to-html)))
+
+  :translate-alist
+  '((template . org-jekyll-html-template)
+    (src-block . org-jekyll-html-src-block))
+
+  :options-alist
+  '((:jekyll-layout "LAYOUT" nil "post" t)
+    (:jekyll-categories "CATEGORIES" nil nil t))
+  ;; TODO: tags, as soon as I learn how they differ from categories
+  ;; https://github.com/jekyll/jekyll/issues/6853
+  )
+
+(defun org-jekyll-export-as-html
+    "TODO: Doc"
+  (&optional async subtreep visible-only body-only ext-plist)
+  (interactive)
+  (org-export-to-buffer 'jekyll "*Org JEKYLL Export*"
+    async subtreep visible-only body-only ext-plist (lambda () (html-mode))))
+
+(defun org-jekyll-export-to-html
+    "TODO: Doc"
+  (&optional async subtreep visible-only body-only ext-plist)
+  (interactive)
+  (let ((file (org-export-output-file-name ".html" subtreep)))
+    (org-export-to-file 'beamer file
+      async subtreep visible-only body-only ext-plist)))
+
+(defun org-jekyll-html-src-block (src-block contents info)
+  "Transcode a SRC-BLOCK element from Org to a highlight block for Jekyll.
+CONTENTS holds the contents of the item.  INFO is a plist holding
+contextual information."
+  (message "org-jekyll-html-src-block")
+  (if (org-export-read-attribute :attr_html src-block :textarea)
+      (org-html--textarea-block src-block)
+    (let ((lang (org-element-property :language src-block))
+          (caption (org-export-get-caption src-block))
+          (code (org-html-format-code src-block info))
+          (label (let ((lbl (org-element-property :name src-block)))
+                   (if (not lbl) ""
+                     (format " id=\"%s\""
+                             (org-export-solidify-link-text lbl))))))
+      (if (not lang) (format "<pre class=\"example\"%s>\n%s</pre>" label code)
+        (format
+         "<div class=\"org-src-container\">\n%s%s\n</div>"
+         (if (not caption) ""
+           (format "<label class=\"org-src-name\">%s</label>"
+                   (org-export-data caption info)))
+         (format "\n{%% highlight %s %%}\n%s{%% endhighlight %%}" lang 
code))))))
+
+(defun org-jekyll-html-template (contents info)
+  "Return complete document for Jekyll, i.e. the HTML contents
+without the <head> and <body> tags but prepended with a front
+matter for Jekyll."
+  (let ((title (org-export-data (plist-get info :title) info))
+        (date (org-export-data (plist-get info :date) info))
+       (categories (org-export-data  (plist-get info :jekyll-categories) info))
+        (now (format-time-string "%FT%T%z" (current-time)))
+        last_mod)
+    (setq last_mod (if (and (stringp date) (/= 0 (length date)))
+                       now
+                     nil))
+    (setq date (if (and (stringp date) (/= 0 (length date)))
+                   date
+                 (with-current-buffer (plist-get info :input-buffer)
+                   (save-excursion
+                     (goto-char (point-min))
+                     (insert "#+DATE: " now "\n")))
+                 now))
+    (concat
+     (format
+     "---
+title: %s
+date: %s
+layout: %s
+%s%s---\n"
+     title date (plist-get info :jekyll-layout)
+     (if (< 0 (length categories))
+        (concat "categories:\n  - "
+                        (mapconcat 'identity
+                                   (split-string categories "," t "\\s-+")
+                                   "\n  - ")         
+                        "\n")
+       "")
+     (when last_mod (concat "last_modified_at: " last_mod "\n")))
+     contents)))
+
+(defun org-html-publish-to-jekyll-html (plist filename pub-dir)
+  "Publish an org file to HTML for postprocessing in Jekyll.
+
+FILENAME is the filename of the Org file to be published.  PLIST
+is the property list for the given project.  PUB-DIR is the
+publishing directory.
+
+Return output file name."
+  (org-publish-org-to 'jekyll filename
+                     (concat "." (or (plist-get plist :html-extension)
+                                     org-html-extension "html"))
+                     plist pub-dir))
-- 
2.11.0