Writing a Formulae for KMonad which depends on a Cask during runtime

[niklaas]

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HOMEBREW_VERSION: 4.2.1-30-gf3c687e
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Core cask tap last commit: 5 hours ago
Core cask tap JSON: 30 Dec 16:22 UTC
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Uncommitted files:
  ?? Formula/k/kmonad.rb

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  ?? Casks/k/karabiner-driverkit.rb

Description of issue

Hi! I'm working on a Formulae for KMonad. They have pretty detailed installation instructions and I am able to build it locally on Mac OS. However, I am wondering about how to write the Formulae given that KMonad depends on a Cask during runtime.

KMonad depends on "system extensions". When building, you can decide between two different system extensions: either kext or dext. kext is for older Mac OS versions; dext is for newer versions. To support a wider range of systems that are actively in use, I decided to go for the latter.

There are two options to obtain dext (feel free to skip this part):

1. Install pqrs-org/Karabiner-Elements since it comes with dext.

   There is a Cask for Karabiner Elements. I have learned it's not possible to specify a Cask as a dependency of a Formula. Anyway, depending on Karabiner Element has the downside that the version the Formulae builds potentially not to be compatible with the one that comes with Karabiner Elements. (The currently active pull request [Mac] Upgrade Karabiner-DriverKit-VirtualHIDDevice to v3.1.0 kmonad/kmonad#780 is an example of KMonad catching up with the version that comes shipped with Karabiner Elements.)

2. Install the PKG for dext that comes with KMonad as part of a git submodule

   As outlined in the installation instructions, it's pretty straightforward to install the version of dext that is bundled with KMonad in form of a git submodule if you just go for the pre-compiled binary version. You just have to install a PKG file. That said, I suspect this shouldn't be done as part of the Formulae because Formulaes are not meant for PKGs. There is indeed a way to compile the PKG as part of the Formulae but that would require signing the extension, which I would like to avoid because it sounds like a rabbit hole.

While dext comes with karabiner-elements, given that the version shipped with kmonad and karabiner-elements can be out of sync, I suggest that I create a separate Cask responsible for installing the version that comes with kmonad as a submodule. Thus, I'm suggesting the following set-up:

Create a kmonad Formulae that's responsible for building KMonad, and create a kmonad-driverkit Cask that installs the PKG that comes with KMonad as a submodule. In the caveats section of the kmonad Formulae, highlight that the kmonad-driverkit Cask is required and the karabiner-elements Cask conflicts with the kmonad-driverkit Cask.

[p-linnane]

Formulae cannot depend on casks. Formulae in homebrew-core must be both open source and built from source. If something needs system extensions it can only be added as a cask, since those must be signed.

[aryairani]

Is it true that formulae in a tap outside of homebrew-core still cannot depend on casks? (see https://github.com/orgs/Homebrew/discussions/5275#discussioncomment-9066648)