Proposals for how to checkout a branch with HEAD detached

I would like there to be an interactive way to checkout a branch with HEAD detached, as with git checkout --detach. Below are two proposals for how this could be done.

For an example of why one would want to do this, see this SO answer by me.

Thanks

edit: Apologize for the many edits, I accidentally published my issue prematurely.

If the user tries to checkout a branch which is already checked out in another worktree, the user would be prompted something like this: "The branch master is already checkout out in worktree wrktr, do you wish to check it out head detached (y/n)?". There could be a variable that if set, skipped the prompt and always checked out with detached head or didn't check out at all.
An alternative under magit-branch > checkout > "local branch HEAD detached". I am less in favor of this.

PS: If this is of interest I will gladly try implementing it myself. Just tell me which proposal, or how you think it would best be implemented, and reserve it for me for some amount of time and I will see what I can do :-).

You can already check out the commit at point using b b followed by M-n (next-history-element) and then RET.

This is a technique is not my invention, some commands that are part of Emacs also use the initial "next history element" to provide some secondary default like this.

Of course I meant b b...

I don't understand how that allows me to check out a branch HEAD detached.

Screencast.2021-01-13.13.35.16.mp4

Clone the magit repository and show the status buffer for it.
Move to the line that reads 25f432551 * origin/master magit-commit-ask-to-stage: Reduce width of doc-string.
Press b b. This gives you a minibuffer with content Checkout: .
Press M-n. This changes the minibuffer's content to Checkout: 25f432551.
Press RET to checkout 25f432551 directly.

That does not work for me, even with emacs -q I just get origin/diff-parse.

I can't see your screencast, but your point needs to be at the commit hash for the M-n trick to work. Another approach: you can append ^0 (short for ^{commit}) to a ref to dereference the object, getting the associated commit (e.g., b b master^0 will created a detached head at the commit that master points to).

I can't see your screencast, but your point needs to be at the commit hash for the M-n trick to work.

Hmm, I tried it with point at the commit hash. For commits without branches on them, I get the hash, otherwise I get branch name.

Another approach: you can append ^0 (short for ^{commit}) to a ref to dereference the object, getting the associated commit (e.g., b b master^0 will created a detached head at the commit that master points to).

That works and is indeed an interactive way of checking out a branch headlessly. Sorry for imlplying that it was not possible interactively, I concede, thanks for showing me.

Either way, I still think that prompting the user to checkout a branch headlessly, when trying to checkout a branch already checked out in another worktree, would be an improvement compared to just failing with a git error.

I will close this issue now and request this behaviour in a clean new issue.

Thanks.

your point needs to be at the commit hash for the M-n trick to work

I cannot figure out where that is implemented anymore. Please point me to the code that makes that happen. 😊

I am trying to figure out why this works with ivy-mode but apparently not selectrum-mode. I would have expected something like in magit-submodule-read-name-for-path, but cannot see anything like that in this case.

Hm, I should probably go to bed.

Please point me to the code that makes that happen

I'm using ivy, and, taking another look, it looks like the behavior comes from the ivy's wrapper for next-history-element (which explains why @fossegrim reported not seeing the behavior). The commit at point is inserted by the marked line below:

(defun ivy-next-history-element (arg)
  "Forward to `next-history-element' with ARG."
  (interactive "p")
  (if (and (= minibuffer-history-position 0)
           (equal ivy-text ""))
      (progn
        (when minibuffer-default
          (setq ivy--default (car minibuffer-default)))
        (insert ivy--default)  ; <====================================
        (when (and (with-ivy-window (derived-mode-p 'prog-mode))
                   (eq (ivy-state-caller ivy-last) 'swiper)
                   (not (file-exists-p ivy--default))
                   (not (ivy-ffap-url-p ivy--default))
                   (not (ivy-state-dynamic-collection ivy-last))
                   (> (point) (minibuffer-prompt-end)))
          (ivy--insert-symbol-boundaries)))
    (next-history-element arg))
   ...)

And ivy--default is set to the thing at point in ivy--reset-state:

    (setq ivy--default
          (if (region-active-p)
              (buffer-substring (region-beginning) (region-end))
            (ivy-thing-at-point)))

Ha! So we both thought we had implemented this.

We probably should implement this for the default completion mechanism as well. But I am currently staring down the rabbit hole that is completion and am not sure I want to take a leap right now. It's pretty scary.

I'll reopen this issue as a reminder.

As a workaround, with point at the hash in question, you can of course do this:

M-d b b C-y RET

Add additional default when reading branch or commit

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Proposals for how to checkout a branch with HEAD detached #4291

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Proposals for how to checkout a branch with HEAD detached #4291

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olavfosse commented on Jan 13, 2021

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tarsius commented on Jan 13, 2021

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