GNU ELPA - cursor-undo

Cursor-undo allows you to undo cursor movement commands using the Emacs standard `undo' command.

For frequent cursor movements such as up/down/left/right, it combines the movements of the same direction into a single undo entry. This prevents the undo command from reversing each individual character movement separately. For example, if you move the cursor 20 characters to the right and then 10 lines up, the first undo will go down 10 lines back, and the next undo move back 20 characters left. On the other hand, for search commands that often jump across multiple pages, each search command has its own undo entry, allowing you to undo them one at a time rather than as a combined operation.

This cursor-undo functionality has existed in my local Emacs init file for over 11+ years, since version 0 on 2013-06-26. It was originally intended to support my ELPA package BriefMode only, but I later found it would be more useful if implemented in a more generalized way. For years I have hoped for an official implementation of this feature, which is commonly seen among various editors. Considering my implementation using advice functions a bit inelegant so I have always hesitated to release it till recently.

Until there is official support for the cursor undo feature, this package serves most common daily needs. The core design is to align with Emacs's native `undo' function by recording cursor positions and screen-relative position undo entries in the `buffer-undo-list' in accordance with its documentation.

As this package primarily consists of advice functions to wrap cursor movement commands, each cursor movement command needs to be manually wrapped with `def-cursor-undo'. For interactive functions that heavily invoke advised cursor movement commands, you may even need to advise them with `disable-cursor-tracking' to prevent generating numerous distinct cursor undo entries from a single command. For user convenience, I have prepared ready `def-cursor-undo' advice sets for standard Emacs cursor movement commands, Brief Editor mode, Viper mode, and EVIL mode.

This package is released as a standard ELPA package. Just go to Emacs main menu -> "Options" -> "Manage Emacs Packages" and install it. Once this package is installed, you only need to enable cursor-undo mode by adding the following line into your Emacs init file .emacs or init.el:

(cursor-undo 1)

That's all. You can now move your cursor around and use stand Emacs `undo' (C-_, C-/ or C-x u) to move it back. Even in read-only buffers you can still undo cursor movements. This is convenient especially when browsing a huge file.

Notice that the original `undo' cannot be performed in read-only buffers. Here, I also advised the undo operation in read-only buffers as long as the pending undo list is still a cursor movement.

This also enables a editing trick: If you are just editing a big file and moving the cursor to browse other parts but forgot where you were, you can undo cursor movements to go back to your last edited position by long-holding <undo> until the last editing command. However, by doing so you risk missing your last edited operation as it might flash by so quickly that you don't even notice but keep undoing other cursor commands you don't want to undo.

In this case, you can switch the buffer to read-only mode (by setting `buffer-read-only' to t), then long press <undo> until the undo command warns you that you're trying to edit a read-only buffer. At this point, you're exactly at the latest editing position you are looking for. You can then safely set `buffer-read-only' flag back to NIL and continue your editing.

If you choose to use the default Emacs `undo' system, you should be able to use `evil-undo' to undo cursor movements. If your choice is tree-undo or another undo system, you might need to use Emacs default `undo' (C-_, C-/ or C-x u …) to undo cursor movements.

The default `viper-undo' is advised to allow cursor-undo. If you find the advised function not working properly, consider comment out the following source code `(define-advice viper-undo …' to restore the original `viper-undo' function and use Emacs default `undo' (C-_, C-/ or C-x u …) to undo cursor movements.

Why do I use advice functions instead of the more generalized pre/post command hooks? I bet you wouldn't like all the cursor movements in your debugging session being recorded as undo entries; when you start `undo'ing you will find the cursor replaying your previous debugging session in reverse order. Similar effects happens in `semantic' mode and many others. If it record every movement you will soon find it difficult to use in many situations. Therefore, only the chosen editing key commands are advised with `def-cursor-undo'. If you find something missing, just advise it with your own `def-cursor-undo'.

Luke Lee