How do I filter kill-ring contents?

Question

Is it possible to prevent kill-ring from storing whitespaces/empty lines? Right now after I do a couple of changes and then go to browse-kill-ring I often see something like this:

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   merchant_uuid: "some_uuid"
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   it "creates a webhook" do
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Is there a way of forcing kill-ring to only append meaningful content?

Answer 1

Here's one approach for dealing with blank (i.e. only whitespace) kills. Rather than filtering them out altogether, this will allow at most one blank entry in the kill ring. Each new kill will check the head of the kill-ring and replace it if it is blank.

(defun my/replace-blank-kill (args)
  (let ((string (car args))
        (replace (cdr args))
        (last (car-safe kill-ring)))
    (when (and last (string-blank-p last))
      (setq replace t))
    (list string replace)))

(advice-add 'kill-new :filter-args #'my/replace-blank-kill)

Note that I'm using the Emacs 24.4+ advice mechanism here to filter the arguments that eventually get passed to kill-new.

The effect is that you can still kill and yank a bunch of whitespace, but if you kill some whitespace and then kill something else, the whitespace entry will be discarded.

Answer 2

One problem is that any command may modify the kill-ring so the only hook powerful enough to do this is the post-command-hook. This is inefficient, but AFAIK the only way to do this with rigor. If that sounds fine to you, you can do this:

(defun kill-ring-clean-whitespace-only ()
  (when (string-match "^[[:space:]]*$" (car kill-ring))
    (setq kill-ring (cdr kill-ring))))

(add-hook 'post-command-hook #'kill-ring-clean-whitespace-only)

For efficiency's sake, I assume that all commands append to the kill-ring, this is probably safe, but note that a function may insert text anywhere they wish into the kill-ring. This is rare, though, and I'm sure you don't want to iterate over the entire kill ring on every command.