Using Emacs - 31 - elfeed part 3 - macros

In part 2 I talked about how I used Hyrdas to quickly navigate through elfeed tags. It was a nice step up but the fact that I still had to manually edit my configuration code for every new tag to update the hydra was a problem.

Basically, I had to somehow or other, take a list of all the active tags and with it build a defhydra command that will then make my Hydra.

Fortunately, emacs, being a lisp, has macros. I'm not talking about keyboard macros which I talked about in episode 15 but rather Lisp style macros. Macros let you transform code and then execute the transformed code.

The example I give in the video:

(defmacro infix (a op b)
  `(,op ,a ,b))

(infix 3 + 8) ; evaluates to 11

This transforms the 3+8 into (+ 3 8) and then evaluates it to be 11.

We can use this idea with our Hydra.

We can use the call elfeed-db-get-all-tags to get a list of all the tags in our database. I decided that if I had an uppercase letter in the tag, I'd use the lowercase version of that letter as my "hotkey" and if it didn't, I'd just use the first letter.

So, given a tag list of:

(active blogs cs eDucation emacs local misc sports star tech unread webcomics)

I'd want a "hotkey" of b for logs and d for eDucation.

The routine z/hasCap tests to see if a tag has a capital letter in it and z/get-hydra-option-key returns the final "hotkey:"

(defun z/hasCap (s) ""
       (let ((case-fold-search nil))
       (string-match-p "[[:upper:]]" s)

(defun z/get-hydra-option-key (s)
  "returns single upper case letter (converted to lower) or first"
  (let ( (loc (z/hasCap s)))
    (if loc
	(downcase (substring s loc (+ loc 1)))
      (substring s 0 1)

mz/make-elfeed-cats takes a list of tags and returns a list of items where each item is in the form expected by the hydra definition:

("t" (elfeed-search-set-filter "@6-months-ago +tagname") "tagname")
(defun mz/make-elfeed-cats (tags)
  "Returns a list of lists. Each one is line for the hydra configuratio in the form
     (c function hint)"
  (mapcar (lambda (tag)
	    (let* (
		   (tagstring (symbol-name tag))
		   (c (z/get-hydra-option-key tagstring))
	      (list c (append '(elfeed-search-set-filter) (list (format "@6-months-ago +%s" tagstring) ))tagstring  )))

Finally, here's our macro:

(defmacro mz/make-elfeed-hydra ()
  `(defhydra mz/hydra-elfeed ()
     ,@(mz/make-elfeed-cats (elfeed-db-get-all-tags))
     ("*" (elfeed-search-set-filter "@6-months-ago +star") "Starred")
     ("M" elfeed-toggle-star "Mark")
     ("A" (elfeed-search-set-filter "@6-months-ago") "All")
     ("T" (elfeed-search-set-filter "@1-day-ago") "Today")
     ("Q" bjm/elfeed-save-db-and-bury "Quit Elfeed" :color blue)
     ("q" nil "quit" :color blue)

The line that starts with *,@* calls the routine that builds lines of code for all the tags in the database and the macro leaves me with the defhydra I need.

I then redefine the hydra every time I need it, just in case tags changed:

(defun mz/make-and-run-elfeed-hydra ()

and bind mz/make-and-run-elfeed-hydra to j and J in my elfeed keymap (this code goes in the bind section of my use-package elfeed section):

("j" . mz/make-and-run-elfeed-hydra)
("J" . mz/make-and-run-elfeed-hydra)

As long as I remember to name my tags in a way that they don't conflict with one another I can quickly navigate all around elfeed.

Macros FTW!!!!

Here's the video:



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