Yep! I'm not a nix expert but my understanding is that this is typically done with overlays. So you can, say, swap out openmpi for mpich, but mpich requires a

Yep! I'm not a nix expert but my understanding is that this is typically done with overlays. So you can, say, swap out openmpi for mpich, but mpich requires a different version of hwloc that conflicts with the one defined in some other branch of your derivation, you're out of luck. In Spack that is handled through dependency resolution. There have been discussions of how to deal with this in Nix, e.g.: https://discourse.nixos.org/t/concept-use-any-package-version/6515 https://discourse.nixos.org/t/concept-use-any-package-versio.... Similarly, for packages like HDF5 that tend to be depended on with options that affect its API (+parallel, ~parallel, etc.), I do not think there are automatic ways to get nix to ensure that your request for hdf5+parallel is consistent with other packages in the graph. Possibly worth a mention: for uarch flags like the ones shown here: https://nixos.wiki/wiki/Build_flags https://nixos.wiki/wiki/Build_flags, the user is responsible for setting them specifically for the requested compiler. We've abstracted that with `archspec` (https://github.com/archspec/archspec https://github.com/archspec/archspec) so that you can just ask for a target and the correct compiler flags are injected. e.g., `target=cascadelake` or `target=zen3`. So if you switch compiler, you do not have to look up these details. See https://tgamblin.github.io/pubs/archspec-canopie-hpc-2020.pdf https://tgamblin.github.io/pubs/archspec-canopie-hpc-2020.pd... for more on how it works. The salient parts of archspec aren't in a python library but in a JSON file (https://github.com/archspec/archspec-json/blob/master/cpu/microarchitectures.json https://github.com/archspec/archspec-json/blob/master/cpu/mi...) so it's something that could probably be incorporated into Nix.