Give Emacs Psychic Completion Powers with prescient.el

• M-x
• describe-variable / describe-function
• find-file
• switch-to-buffer
• company-mode

It saves time to make the list of recommendations more tailored to your usage!

Prescient is a sorting and filtering extension for Ivy, company-mode, and Selectrum. It provides:

• Sorting candidates based on how frequently you select them
• Configurable filtering rules to quickly find what you’re looking for

Since we’ve been using Ivy in the configuration for this channel, let’s try Prescient with it.

First, try M-x without Prescient turned on, then install ivy-prescient (make sure to load it after Counsel!)

(use-package ivy-prescient
  :after counsel
  :config
  (ivy-prescient-mode 1))

Now run M-x again and take a look at the results. Select a command, then run M-x again, see that it is suggested first!

Prescient is remembering the selections you’ve made for M-x (or counsel-M-x in this case) and it sorts them first based on some configurable rules:

• prescient-history-length: How many chosen candidates will be remembered
• prescient-frequency-decay: A multiplier applied to each frequent candidate each selection (default: 0.997)
• prescient-frequency-threshold: Threshold used for “forgetting” a command that isn’t used frequently anymore (default: 0.5)

NOTE: These variables apply to all usages of Prescient, not just for Ivy!

• counsel-find-file
• counsel-describe-variable
• counsel-switch-buffer - Doesn’t work!

• Commands to exclude: ivy-prescient-sort-commands

By default it contains ivy-switch-buffer which counsel-switch-buffer uses internally.

You might have noticed M-x is now sorting all commands by shortest-first. If this is distracting to you, it can be disabled!

(setq prescient-sort-length-enable nil)

Set prescient-filter-method:

;; This is the default value!
(setq prescient-filter-method '(literal regexp fuzzy))

• literal: the subquery must be a substring of the candidate. Supports char folding.
• initialism: the subquery must match a substring of the initials of the candidate.
• prefix: words (substrings of only word characters) match the beginning of words found in the candidate, in order, separated by the same non-word characters that separate words in the query. This is similar to the completion style partial or what you get from Ivy by default
• anchored: words are separated by capital letters or symbols, with capital letters being the start of a new word. This is similar to prefix, but allows for less typing.
• fuzzy: the characters of the subquery must match some subset of those of the candidate, in the correct order but not necessarily contiguous.
• regexp: the subquery is interpreted directly as a regular expression. (Try ^p\w+d)

If you are too used to Ivy’s filtering styles, you can use those while still keeping Prescient’s sorting:

(setq ivy-prescient-enable-filtering nil)

NOTE: Make sure you toggle ivy-prescient-mode after setting this for it to take effect!

ivy-prescient changes how results are highlighted when you start typing. To emulate the old highlighting you can use ivy-prescient-retain-classic-highlighting:

(setq ivy-prescient-retain-classic-highlighting t)

Prescient also has an extension for Company to provide candidate sorting based on frequency:

(use-package company-prescient
  :after company
  :config
  (company-prescient-mode 1))

NOTE: Prescient filtering is not applied to company-mode! It would require a custom completion backend.

Remembering your history is great, but it’s even better when it is remembered each time you start Emacs:

;; Remember candidate frequencies across sessions
(prescient-persist-mode 1)

ivy-prescient needs a new maintainer: https://github.com/raxod502/prescient.el/issues/65

If you like using Prescient with Ivy, consider helping out!

We will talk about Selectrum in another video; we’ll hook it up to Prescient then!