If I installed an Arch-based distro, is it possible to some how configure pacman to download and install from the Clear Linux’s repos and if it is not available then download from the Arch Linux’s repo instead?
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…this doesn’t work at all. Two different distros. rpm might be used as package. But the requiered dependencies are different and by the reason why clear linux is so fast: almost all of the dependencies are highly optimised.
e.g.: Octave (math program) which depends on openblas (linear algebra and matrix manipulation). Octaves speed depends heavily on openblas.
You download it from ClearLinux Repo - not possible but let us assume for a moment it is. Now you install it on arch. Then you start it it will use openblas of the Arch repo which has no Clear Linux tweaks so you will have almost no benefits. Besides it isnt possible to install.
This is just one example with a rather easy dependency chain. You need the whole environment - btw this is also one of the reasons why CL uses Bundles.
What you can do is having a look at the build configurations of certain pkgs in the CL repo and rebuild them with some of this flags on you arch distro. And in the nextstep also rebuild the touched dependencies. Then you might have some Arch compatible Program which is making use of the CL sauce …but be aware that it might break the program or something else on the system not liking you new libraries.