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Use-package behaviour on Emacs

2018-11-26

So, originally, this post was going to be about how to setup emacs for Rust development. But at the end, if you go to Julen Blanchard’s blog he will give you the basic packages that you should be using.

More interestingly, I have learned something new about use-package because of it. And that new thing is that the different keywords that you can pass on the setup of a package are evaluated on an specific order.

Let’s use examples based on my setup of Rust. Originally I added the code as follows:

(use-package rust-mode
  :defer t
  :ensure t)

(use-package cargo
  :defer t
  :ensure t
  :config
  (add-hook rust-mode-hook cargo-minor-mode))

With that setup when I loaded a rust file the cargo-mode was not loaded. I wondered if there was some relation to the fact that cargo package was being defered. So I changed the code to the version below and lo and behold my cargo mode loaded.

(use-package rust-mode
  :defer t
  :ensure t
  :config
  (add-hook rust-mode-hook cargo-minor-mode))

(use-package cargo
  :defer t
  :ensure t)

So if I am using :defer t then the code under :config was not being loaded. rust-mode autoloads based on the extension, which triggers the config which then makes the cargo-mode load. At least at the moment that is what I expect is happening, though I will confirm at another moment.

But that wasn’t right to me, why the setup of my rust-mode package should know anything about cargo-mode? Clearly I was indicating the dependencies incorrectly.

So decided to look into the code of use-package and there it was, a defcustom in which there was a comment indicating that the order is important. Of course, :config is the last item on that order, with:defer just a few lines above. Looking at the list, I discovered that there is keyword :hook. And well, I do wonder what it is for ….

So converted the code to this:

(use-package rust-mode
  :defer t
  :ensure t)

(use-package cargo
  :defer t
  :ensure t
  :hook
  ((rust-mode . cargo-minor-mode)))

And now the cargo-mode loads as I expect whenever a rust file is opened, while keeping the dependency direction correctly


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