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Discovering Useful Functions in Emacs

·4 mins


In one of my first articles about getting set up with packages.el, I left a somewhat expensive line hanging out in the open:

;; init.el
;; --snip--
;; --snip--

This function will call out to any repositories we've configured to search packages from and update our local registry of what's available. That can take a few seconds, and is a bit annoying if you're constantly restarting Emacs to work on your init.el like I am right now. So it would be nice if this function instead looked something like this:

(when <packages-aren't-installed>

We just need a little code to check when <packages-aren't-installed> is true. We already discovered that the variable package-selected-packages, managed by custom.el, is a list of packages that we've chosen to install. We also know that the function package-installed-p checkes whether a particular package is installed or not. In my init.el, I have a block like so:

;; init.el
;; --snip--
  '(package-selected-packages '(markdown-mode marginalia)))
;; --snip--

So it seems like we have the ingredients we need, save for a way to apply the package-installed-p over the whole list package-selected-packages, and reduce down to a single true or false value, depending on if any of the expected packages aren't installed. Altogether, that means we have a very simple map-reduce problem, and I am certain there's a function in Emacs ready for us to use - we just need to find it.

Vertical FIDO Mode

This hidden superpower of Emacs 28.1+, along with the marginalia package described below, are the main tools in my arsenal for surfacing everything from Emacs' many hidden, labyrinthine depths.

;; Set 'flex' to be the main completion style.  This enables a "fuzzy"
;; style of searching for things using the minibuffer
(setq completion-styles '(flex basic partial-completion emacs22))
;; Turn on FIDO (Fake IDO) mode
;; Have TAB complete using the first option and continue, instead of
;; popping up the *Completions* buffer
(define-key icomplete-minibuffer-map [remap minibuffer-complete] 'icomplete-force-complete)
;; Sometimes I have to customize this icomplete-compute-delay variable
;; to 0.0 to avoid delay before the M-x minibuffer pops up
(setq icomplete-compute-delay 0.0)
;; Set the display to be a vertical list of items, instead of a horizontal one

This sets up the minibuffer to show a live update of completion candidates as we type, in an easy-to-read vertical format



While not a part of base Emacs, this little package complements the vertical FIDO display by adding information, usually a docstring, next to each item. After performing a M-x package-refresh-contents and M-x package-install RET marginalia, we get this:


Search Process

The modifications we just made also affect commands that search through variables and functions, like C-h f (describe-function). What we're after right now is some kind of function that acts on a list, and tells us if any of the items are nil after applying the package-installed-p function to them. My gut tells me that a word like "all", "every", or "none" might appear in such a function. After trying a few of these, examining the descriptions, and quickly trying again, a query for the word "any" brought up a promising function called cl-notany


It's close; this one will yield t when every element in the sequence is nil, we want one for when any element is nil. Hitting RET on cl-notany above brings up its help text

cl-notany is an autoloaded compiled Lisp function in ‘cl-extra.el’.

(cl-notany PREDICATE SEQ...)

Return true if PREDICATE is false of every element of SEQ or SEQs.


There's a link on `cl-extra` in this buffer, and my bet is that the function we're looking for would be defined near cl-notany. Following that link takes us to a block of code with the defun for cl-notany. Lo and behold, there's a cl-notevery right below it with a description that matches exactly what we need:

;; cl-extra.el
(defun cl-notany (cl-pred cl-seq &rest cl-rest)
  "Return true if PREDICATE is false of every element of SEQ or SEQs.
\n(fn PREDICATE SEQ...)"
  (not (apply #'cl-some cl-pred cl-seq cl-rest)))

(defun cl-notevery (cl-pred cl-seq &rest cl-rest)
  "Return true if PREDICATE is false of some element of SEQ or SEQs.
\n(fn PREDICATE SEQ...)"
  (not (apply #'cl-every cl-pred cl-seq cl-rest)))

Writing the when block

With cl-notevery in hand, the last step is to apply it in our init.el for managing package refresh and installation.

(when (cl-notevery 'package-installed-p package-selected-packages)
  (add-to-list 'package-archives '("melpa" . "") t)