Adding :target option for the TOC keyword in Org Mode

Now that A- can be counted on to happily play with a babysitter for several hours once a week, I’ve decided to alternate consulting and personal projects. Two weeks ago, I used my personal time to make a script that renewed my library books automatically. This week, I set aside time to look at Org Mode. DC had asked me to update the patch I made to allow people to specify a target for the table of contents, and I was curious about whether I could hack something together.

Patch for adding :target to TOC keyword

Here’s a sample file that shows what I mean:

#+OPTIONS: toc:nil
* Not this section
** Heading X
** Heading Y
* Target
  :PROPERTIES:
  :CUSTOM_ID: TargetSection
  :END:
** Heading A
** Heading B
* Another section
#+TOC: headlines 1 :target "Target"

Here’s the core of how to make it work for HTML exports:

(defun org-html-keyword (keyword _contents info)
  "Transcode a KEYWORD element from Org to HTML.
CONTENTS is nil.  INFO is a plist holding contextual information."
  (let ((key (org-element-property :key keyword))
  (value (org-element-property :value keyword)))
    (cond
     ((string= key "HTML") value)
     ((string= key "TOC")
      (let ((case-fold-search t))
  (cond
   ((string-match "\\<headlines\\>" value)
    (let ((depth (and (string-match "\\<[0-9]+\\>" value)
          (string-to-number (match-string 0 value))))
    (scope
     (cond
      ;; link
      ((string-match ":target +\"\\([^\"]+\\)\"" value)
       (let ((link (with-temp-buffer
         (save-excursion
           (insert (org-make-link-string (match-string 1 value))))
         (org-element-link-parser))))
         (pcase (org-element-property :type link)
           ((or "custom-id" "id") (org-export-resolve-id-link link info))
           ("fuzzy" (org-export-resolve-fuzzy-link link info))
           (_ nil))))
      ;; local
      ((string-match-p "\\<local\\>" value) keyword))))
      (org-html-toc depth info scope)))
   ((string= "listings" value) (org-html-list-of-listings info))
   ((string= "tables" value) (org-html-list-of-tables info))))))))

It was a lot of fun Doing the Right Thing(s): writing documentation, adding tests, and making it work for more than just HTML export. I found out where to make the changes by using grep to search for TOC in the Org Mode source code. All the heavy lifting was already done by org-export-collect-headlines, so it was just a matter of passing the right scope. It took me a while to figure out that I needed to pass an Org link element. An easy way of making that element work for both fuzzy and ID-specific links was to insert the target text into a temporary buffer (remembering to use org-make-link-string) and then calling org-element-link-parser.

I tried figuring out how to make it work with a link to another file, but I didn’t get very far, so I figured I’d just wrap things up nicely there.

I wasn’t sure if my original post made it through because I sent it through Gmane and Cc:d DC, who got it with an empty To:, so I ended up submitting it twice. I just realized I forgot to add test-ox-ascii.el. I don’t want to spam the list, so I’ll send that along with other changes if people have feedback.

But look! Open source contributions! I’m so excited. I wonder what I’ll get to do in two weeks from now. =)