Playing Hansel and Gretel

• Mark Ring
• Registers
• Bookmarks

Bookmarks are like Registers, with a few special traits:

• They are long-lived (they survive reboots).
• They can re-open closed buffers.
• They have arbitrarily long names.
• They are context aware.

Consider them as specialized, super convenient Registers.

Just like Registers are stored in the variable register-alist, Bookmarks are kept in the equivalent bookmark-alist. Nothing new, here: again, it’s all a combination of the same, basic building blocks.

One command to rule them all

If you read the Emacs Manual on Bookmarks, you will see a bunch of functions to manipulate them. They are always useful to know, but I would suggest you to start with a way simpler approach: just go with Consult.
With Consult, there is a single command to keep in mind: C-x r b (consult-bookmark).

• When you want to bookmark a position, run (consult-bookmark) and give the bookmark a name.
• When you want to jump back to it, run (consult-bookmark) again, and select the bookmark from the list.

No need to remember multiple commands.

Naturally, in perfect consult style, the bookmark list can be filtered as you type, and it provides you with a real-time preview of the file you are going to jump back to. I often find it handy to run (consult-bookmark) only to preview a file: then, I hit C-g to jump back to what I was working.

Oh, and you can also jump to bookmarks using (consult-buffer): just prefix your input with m followed by a space, and you will be filtering bookmarks instead of buffers. Neat!

Saved, by default

When you quit Emacs (do you, really?), Bookmarks are automatically serialized in the file bookmarks, in the directory specified by bookmark-default-file — by default, where your init.el file is.

Try to open that file: it just contains the bookmark-alist variable, serialized and prettified. No magic.

Should you ever desire to save the Bookmarks before quitting Emacs, use M-x bookmark-save RET. Of course, bookmark-save comes together with its alter-ego bookmark-load.

Bookmarks are pluggable

Bookmarks are just items in a variable. This, and the existence of bookmark-save and bookmark-load, could inspire you to have multiple sets of bookmarks in different files, and to load them on demand, maybe when you switch from one project to another, just like direnv does with your .profile.

Indeed, this is what the package Bookmark+ does.
But if you are curious, you could develop this by yourself, on top of the built-in building blocks, with a bunch of Lisp lines. Here’s an idea:

(defun my/switch-project-bookmarks ()
  (setq bookmark-default-file 
        (concat (projectile-project-root) ".bookmarks"))
  (bookmark-load bookmark-default-file t))

(add-hook 'projectile-after-switch-project-hook #'my/switch-project-bookmarks)

This lets you have a .bookmarks file in each project, and load it whenever you enter its folder. It’s a draft, by no means ready for production, but it could give you some inspiration.

Like Dired, but for Bookmarks

There are indeed a few, interesting built-in functions for manipulating bookmark-alist.

One is edit-bookmark.
Run it, and you will enter bmenu, a Dired-like environment: just like Dired lets you create, filter, select, delete, modify and manipulate files, edit-bookmark lets you do the same with Bookmarks.
Here are some keybindings you can use:

Now, only the Almighty Flying Spaghetti Monster knows how much I dislike having to keep all the keybindings in my mind. It’s just beyond my possibilities. Having the memory of a goldfish, what I often do is to run C-h b (describe-bindings): it displays the set of all the active keybindings for the current buffer, each with its documentation, in a beautiful interactive and foldable list, from which I can even jump to the documentation.
After all, Emacs is sold as a self-documenting editor for a reason.

Resurrecting buffers

What if you jump to a bookmark and you have already killed the buffer it targets? In the previous post we sow how Registers deal with this with some less than crystalline machinery. On this regards, bookmarks are way more linear: they always store a file path, not a buffer reference.
Create a bookmark, visit the bookmark-alist variable (M-x describe-variable RET bookmark-alist) and notice the field filename. For example:

(("writing"
 (filename . "/home/arialdo/prg/personal/arialdomartini.github.io/_posts/emacs/2024-03-30-emacs-bookmarks.md")
 (front-context-string . "\n\n# Less common ")
 (rear-context-string . "ookmark and the ")
 (position . 4605)
 (last-modified 26400 56170 174644 0))
 ...

So, it’s always trivial for a bookmark to re-open a buffer by just visiting the file.

Moved files

What if the file was moved?
Ideally, Emacs could detect when the file is being moved or renamed, on the assumption this is done using Emacs itself: if so, Emacs could reflect the change updating the item in bookmark-alist.

That’s not what happens, though. Instead, if you move or rename a file, its bookmarks will be orphaned. When trying to jump to the file, not finding it, Emacs would invoke (bookmark-relocate) This function will ask you the new file location, and will then update the filename field for the affected bookmark.

Modified files

What instead if the file is not moved, but the specific line the bookmark targets changes, maybe because other lines have been added or deleted?
This is where the context awarness of bookmarks comes into play. Read the content of your bookmarks (M-x describe-variable RET bookmark-alist RET). You will see something like:

(("writing"
 (filename . "/home/arialdo/prg/personal/arialdomartini.github.io/_posts/emacs/2024-03-30-emacs-bookmarks.md")
 (front-context-string . "\n\n# Less common ")
 (rear-context-string . "ookmark and the ")
 (position . 4605)
 (last-modified 26400 56170 174644 0))
 ...

Not only does each bookmark contain the exact location in the file, in the field position: it also holds a bit of the surrounding context, in the front-context-string and rear-context-string fields. When jumping to the bookmark, the 3 together are used, in the attempt of compensating the possibly occurred changes.

I wish you a great day and many happy hours of coding!

(Thanks to Protesilaos for coaching me and for the kind review)

References

• Protesilaos Stavrou
• consult
• Emacs Manual - Bookmarks
• direnv
• Bookmark+

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