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* lisp/emacs-lisp/cursor-sensor.el: Add motivation

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Stefan Monnier 2018-12-09 20:56:35 -05:00
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lisp/emacs-lisp/cursor-sensor.el
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@ -38,6 +38,27 @@
;; called just before redisplay happens, according to the movement of ;; called just before redisplay happens, according to the movement of
;; the cursor since the last redisplay. ;; the cursor since the last redisplay.
;;;; Motivation
;; The old properties were very problematic in practice because they
;; operate at a much lower level and hence affect all motion
;; *functions* like goto-char, forward-char, ... hence breaking
;; invariants like:
;;
;; (forward-char N) == (progn (forward-char N1) (forward-char (- N N1)))
;; (point) == (progn (forward-char N) (forward-char -N) (point))
;; (+ N (point)) == (progn (forward-char N) (point))
;;
;; The problems would usually show up due to interaction between
;; unrelated code working in the same buffer, where one code used those
;; properties and the other (unknowingly) assumed those aren't used.
;; In practice a *lot* of code assumes there's no such funny business.
;;
;; Worse: all(?) packages using those properties don't actually want those
;; properties to affect motion at such a low-level, they only want to
;; affect the overall effect of commands, but not the effect of every
;; single point-motion that a given command happened to use internally.
;;; Code: ;;; Code:
;;;###autoload ;;;###autoload