My GNU Emacs MH mail folder completion in MH-E

May 19, 2024

When I wrote about understanding the orderless package, I mentioned that orderless doesn't work well with hierarchical completions such as file names, which are completed one component at a time. I also said this mattered to me because MH-E completed the names of mail folders in this part by part manner, but I didn't feel like rewriting MH-E's folder completion system to fix it. Well, you can probably guess what happened next.

In the GNU Emacs way, I didn't so much rewrite MH-E's mail folder completion as add a second folder completion system along side it, and then rebound some keys to use my system. Writing my system was possible because it turned out MH-E had already done most of the work for me, by being able to collect a complete list of all folder names (which it used to support its use of the GNU Emacs Speedbar).

To put the summary up front, I was pleasantly surprised by how easy it was to add my own completion stuff and make use of it within my MH-E environment. At the same time, reverse engineering some of MH-E's internal data structures was a bit annoying and it definitely feels like a bit of a hack (although one that's unlikely to bite me; MH-E is not exactly undergoing rapid and dramatic evolution these days, so those data structures are unlikely to change).

There are many sophisticated way to do minibuffer completion in GNU Emacs, but if your purpose is to work well with orderless, the simplest approach is to generate a list of all of your completion candidates up front and then provide this list to completing-read. This results in code that looks like this:

(defvar cks/mh-folder-history '() "History of MH folder targets.")
(defun cks/mh-get-folder (msg)
  (let ((cks/completion-category 'mh-e-folder-full))
    (completing-read msg (cks/mh-all-folders) nil t "+" cks/mh-folder-history)))

Here I've made the decision that this completion interface should require that I select an existing MH mail folder, to avoid problems. If I want to create a new mail folder I fall back to the standard MH-E functions, with their less convenient completion but greater freedom. I've also decided to give this completion a history, so I can easily re-use my recent folder destinations.

(The cks/completion-category stuff is for forcing the minibuffer completion category so that I can customize how vertico presents it, including listing those recent folder destinations first.)

This 'get MH folder' function is then used in a straightforward way:

(defun mh-refile-msg-full (range folder)
  (interactive (list (mh-interactive-range "Refile")
                     (intern (cks/mh-get-folder "Refile to folder? "))))
  (mh-refile-msg range folder))

This defers all of the hard work to the underlying MH-E command for refiling messages. This is one of the great neat tricks in GNU Emacs with the (interactive ...) form; when you make a function a command with (interactive ...), it's natural to find up with it callable from other ELisp code with the arguments you'd normally be prompted for interactively. So I can reuse the mh-refile-msg command non-interactively, sticking my own interactive frontend on it.

All of the hard work happens in cks/mh-all-folders. Naturally, MH-E maintains its own data structures in a way that it finds convenient, so its 'mh-sub-folders-cache' hash table is not structured as a list of all MH folder names but instead has hash entries storing all of the immediate child folders of a parent plus some information on each (at the root, the 'parent' is nil). So we start with a function to transform various combination of a hash key and a hash value into a MH folder name:

(defun cks/mh-hash-folder-name (key elem)
  (cond
   ((and key elem) (concat key "/" (car elem)))
   (key key)
   (elem (concat "+" (car elem)))))

And then we go over mh-sub-folders-cache using our mapping function with:

(cl-loop for key being the hash-keys of mh-sub-folders-cache
  using (hash-values v)
  collect (cks/mh-hash-folder-name key nil)
  append (cl-loop for sub in v
		  collect (cks/mh-hash-folder-name key sub)))))

After getting this list we need to sort it alphabetically, and also remove duplicate entries just in case (and also a surplus nil entry), using the following:

(sort (remq nil (seq-uniq flist)) 'string-lessp)

(Here, 'flist' is the let variable I have stuck the cl-loop result into. My actual code then removes some folder names I don't want to be there cluttering up the completion list for various reasons.)

There are some additional complications because MH-E will invalidate bits of its sub-folders cache every so often, so we may need to force the entire cache to be rebuilt from scratch (which requires some hackery, but turns out to be very fast these days). I'm not putting those relatively terrible hacks down here (also, the whole thing is somewhat long).

(If I was a clever person I would split this into two functions, one of which generated the full MH mail folder list and the second of which filtered out the stuff I don't want in it. Then I could publish the first function for people's convenience, assuming that anyone was interested. However, my ELisp often evolves organically as I realize what I want.)

Written on 19 May 2024.
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Last modified: Sun May 19 23:38:41 2024
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