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I once had emacs trim trailing whitespace upon leaving a line, if that line had been modified.

Note: I already have trim at save, it is not what I am asking for here. I have the package ws-butler, but it seems to only act on save.

It was working with my previous config in emacs 26, but I cannot see a package other than ws-butler there, I suspect it might be some built-in I fail to configure?

What could possibly have been doing that trimming?

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    Maybe ws-trim.el(search for ws-trim on the page)? The canonical distribution was hosted on the lysator FTP server. I found a copy at github.com/windley/emacs/blob/master/ws-trim.el however.
    – NickD
    Oct 17, 2022 at 13:30
  • did you have a hook calling delete-trailing-whitespace?
    – nega
    Oct 17, 2022 at 13:57
  • @NickD: you got it, thanks!! I even had a copy of ws-trim.el in my old config repo, although the setting up of it was gone from the init.el.
    – Gauthier
    Oct 18, 2022 at 10:50
  • @NickD If you want to make an answer of your comment, I'd gladly give you the internet points.
    – Gauthier
    Oct 18, 2022 at 10:50
  • Glad that the ws-trim mention helped, but I'd rather you provided an answer that describes how you use it to answer your question. Just mentioning ws-trim was easy but I would have to do significant amounts of research to provide a good answer and I won't have the time to do that any time soon - if ever !-)
    – NickD
    Oct 18, 2022 at 12:53

1 Answer 1

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Thanks to @NickD for finding the package I was missing: ws-trim.

It is mentioned here, which is where I found it in the first place, long ago.

Download the source, put it somewhere in emacs load-path (I have (add-to-list 'load-path "~/.emacs.d/includes") in my init).

The following gives me the minor mode with the behavior I am looking for:

;; ws-trim trims trailing whitespace at navigating from line.
(require 'ws-trim)
(setq ws-trim-level 1) ; 1 -> only modified lines are trimmed.
(setq ws-trim-method-hook '(ws-trim-trailing))
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    One nit: it is preferable to use add-hook to modify a hook: for one, it checks for duplicates, thereby avoiding problems down the line; also, if it has some functions on it already, the setq will get rid of them - that might be a big problem. It is not much of a problem here (presumably, nobody else - some library or package - would want to add anything to it), but it is a good habit to get into. Just replace the last setq with (add-hook 'ws-trim-method-hook #'ws-trim-trailing).
    – NickD
    Oct 19, 2022 at 1:24
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    @NickD The lib does this: ;;;###autoload (defvar ws-trim-method-hook '(ws-trim-leading ws-trim-trailing)). Isn't add-hook going to just add to the list? I don't want to trim leading, so I thought the hook needed replacing?
    – Gauthier
    Oct 19, 2022 at 7:45
  • You are right that you need to replace it. It's unfortunate that the library does that initialization (IMO). I would rather have it give you an empty hook that users can populate as they want. In this case, I would add a (defvar ws-trim-method-hook nil) before ws-trim.el is loaded to enforce my preference and then use add-hook in my regular way, but it is a minor nit.
    – NickD
    Oct 19, 2022 at 14:07

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