Don't configure a package if not installed?

[MetroWind]

What's the best way to not configure a package if it's not installed?

Suppose I have this in my .emacs.el:

(use-package web-mode
  :mode "\\.html?$"
  :config
  ;; some setqs...
  )

If I don't have web-mode installed, the :config part will never trigger, but looks like the :mode setting is still done. So if I try to open a HTML file, it will complain about cannot loading web-mode. Ideally, it would be best that if web-mode is not installed the whole use-package call is disabled. I suppose I can put a :if (package-installed-p 'web-mode) there, but that only works if web-mode is installed through elpa (I'm not sure, is that true?). It could also be slower than necessary (again I'm not sure. Is it slow?). Is there a better way?

[waymondo]

The :if (package-installed-p 'web-mode) method you've described seems reasonable, and will simply look for the presence of the package regardless of the package archive it was installed with.

Do you have a specific reason you don't want to just use :ensure t in this case?

[noctuid]

Use-package uses locate-library for use-package-check-before-init. It would be great if there was an equivalent variable that worked the same way as :if (i.e. it put all non-installation configuration inside a (when (locate-library (symbol-name package)) ...)). Right now I'm using a wrapper around use-package that does this, but it would be nicer if this was builtin.

Edit: This isn't really necessary if you don't have debug-on-error non-nil during init (because of the condition-case-unless-debug), but I set it to t for my init in order to debug non-recoverable errors in my init without having to rerun Emacs. I guess an alternative would be to have a variable to use condition-case instead and never error?

[skangas]

I think you should use :if (locate-library "web-mode"), or :if (or (locate-library "web-mode") (package-installed-p 'web-mode)). Perhaps we should add some notes to that effect to the documentation.

[skangas]

I've now added such an example to the use-package manual: https://git.savannah.gnu.org/cgit/emacs.git/commit/?h=emacs-29&id=a17a6036dd4549514c7cecc7cdd93a571bf57ece

This change will be a part of the use-package shipped with Emacs 29. I'm therefore closing this issue.

[monnier]

I don't really understand why people want to use locate-library here (seems costly to me), but FWIW I think the better test here is :if (fboundp 'web-mode).