Cloning, narrowing, and Emacs

I don't write much Elisp anymore — for the most part I have my Emacs doing what I need, and I'd rather use the standard features that I do not have to maintain myself.

But lately I've started using two standard features more than before:

So I've found myself doing these steps often: first clone the buffer, then narrow the clone. Then usually I'll display both buffers in adjacent windows, moving back-and-forth, but knowing that a stray keystroke will not send one or the other buffer scrolling off to a weird position.

The one irritation is that the built-in clone-indirect-buffer modifies the setup of buffers and windows in the frame — and I do not like it when something messes with my mise en place! So here is a quick interactive Elisp function for cloning the buffer and narrowing it to the active region.

(defun clone-and-narrow (region-start region-end)
  "Clone the current buffer, and narrow the clone to the current (live) region"

  ;; Provide the start and end of the current region as the two
  ;; arguments
  (interactive "r")

    ;; Make sure there is a well-defined and live region specified by
    ;; the arguments
    ((and (use-region-p) region-start region-end)

     ;; Deactivate the mark
     (deactivate-mark t)

      (let ((new-buffer))

       ;; Return a new buffer viewing the same file as the current
       ;; buffer, and use that buffer as current for subsequent
       ;; operations.
       (setf new-buffer (clone-indirect-buffer nil nil))
       (set-buffer new-buffer)

       ;; Narrow the buffer to the given region.
       (narrow-to-region region-start region-end)

       ;; Display the new clone within the current window.
       (switch-to-buffer new-buffer)))

    ;; Show a message if there is no live region, but no beepy drama
    (t (message "Void region"))))
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