The caching trick for libsolv here could be a game changer for performance in a lot of Linux distro package managers, where libsolv's completeness comes at a substantial performance cost.

I've never looked too deeply into how conda works because I'm not that into Python, but I guess I should. It seems there are some valuable general lessons here— I wish that more package managers, and in particular, Nix, maintained more version combinations in their repos. Searching through Nixpkgs commits to find compatible versions of things because you have to rely on the implicit versioning of the monorepo sucks.

Adding dependency resolution to Nix as it stands would normally add a big performance cost. Something like this would help with that problem a lot, and Nix already usually runs with a persistent daemon. Maybe a similar technique could fit in well here.

I'm not sure how 'dependency resolution' would work in a system like Nix; since Nix has no notion of "package", "dependency", "version", etc.

> Searching through Nixpkgs commits to find compatible versions of things because you have to rely on the implicit versioning of the monorepo sucks.

I get what you're saying, but I gave up on thinking of Nix/Nixpkgs in terms of "package management". It's much closer to a build system, like GNU Make; rather than, say, APT/Yum.

When it comes to versions of things in Nixpkgs: most things are defined as a function of other things; i.e. the arguments to that function are (roughly) its "dependencies". If we want some different version, we can call those functions with different arguments (or use the various `override` helpers included in their return values). Once we've overridden the things we care about, we can usually take the fixed-point/closure of that package set (whether it's the whole of Nixpkgs, or just e.g. python3Packages, or whatever); that way, our changes get propagated through all of the function calls to ensure everything's referencing our versions. Admittedly, this can be quite tedious!

I'm aware of how Nixpkgs currently works. :)

It's a huge limitation for many reasonable ways that people want to use Nix. It's a shortcut that Nix has gotten away with, not a superior approach. Look at the machinery mach-nix implements with a cached database of PyPi and how it has to scan across different Nixpkgs revisions just to automatically choose compatible versions of deps when it does its code generation for a clear illustration.

There are lots of source-based package management systems which rely on a mix of implicit versioning based on what happens to be exist under an unversioned name in the monorepo and explicit versioning, Gentoo Portage for example.

There's no reason that we couldn't have either

  a. a complement to Nixpkgs which serves as a record of which build recipes have successfully built what versions, and what versions their deps were at

  b. richer metadata in Nixpkgs about versions that can be used to support multiple versions of same-named packages in the repository, and a smarter callPackage that uses those version constraints to choose, when necessary, which versions of dependencies to put into scope when calling a package (yes, I know there is ongoing work to remove callPackage, or at least explicit invocations of it)

Hacking things together with nix-index just to get a little automagical code generation where users can specify package versions rather than Nixpkgs commits is a kludge. It's cumbersome and rightly turns some developers off from using Nix. Nixpkgs should be enriched or complemented in order to obviate that approach, and the Nix ecosystem will need tooling that has an actual dependency resolver to work with it— even if the only maintained combination of versions for distribution is pretty much singular, and implicitly encoded in Nixpkgs just as it is today.

Spack essentially is nix with dependency resolution. spack packages can have conditional (nearly) anything: versions, options, compiler choices, dependencies (including virtual deps). The resolver can make choices about all of those.

See here for more details: https://arxiv.org/abs/2210.08404