NixOS Linux

Generally you don't specify a specific version as a dependency unless it's necessary, but when 'building' the system, specific versions are used. If things break during an upgrade you can roll back to the previous version. Keeping those older builds around will mean keeping older versions of the packages used in those builds (libcurl, for example). Periodically, you can purge older generations (i.e. builds) and collect garbage, which will clear out packages that aren't used anymore. It would be very strange to end up in a situation where you had dozens of instances of libcurl _being used simultaneously_ (though I guess that would be possible if lots of packages had very special requirements, and ended up building libcurl with different flags or something).

There may be circumstances where you have a _specific_ version dependency, and you can specify that too. That would mean having multiple copies of the same package, even in a single generation. Obviously this usually means major versions, so you don't usually see _dozens_ of them. Having specific dependencies on minor versions would be a major code smell IMHO.

Anecdotally, my NixOS systems (both desktops and servers) can grow up to, say, ~50 gb of system files in a few months, at which point I'll clear out older generations and GC, which gets them down closer to a typical Linux distro size. Once or twice, I've seen a system balloon up over 100G when I was doing a lot of experimenting with packages.

Oh, and incidentally, most of the different versions are only visible to the packages that depend on them. Your 'default' environment is one such package, and only sees one of each of it's dependencies. It's not like you have dozens of instances of packages crowding /lib or anything (since /lib doesn't exist in the first place). You have to go looking in the Nix package store to find the actual files.

Sometimes it shines ex:

I messed up my x11 config as well as drivers and just booted into the previous generation like it was no big deal

I needed to install a version of electrum that was only available on a specific git commit. I just specified the commit and booted into a shell with it

Sometimes it sucks:

Learning to package arrive 3rd party software can sometimes be really complicated and it might be the only way forward.

Lots of steam doesn't work out of the box compared to Ubuntu.

Then anouther half day to get comfortable with home-manager to configure my user settings

If you're willing, its worth the effort

Let me give an example: let's say you want to use xmonad on top of Plasma. NixOS doesn't have any simple way to configure that. You have the mutually exclusive options for one or the other, basically.

I know Nix is powerful enough to do this, if I wanted to go offroad and create custom packages/derivations, but that's the point where I am no longer happy with the utility/work tradeoff. It's what ultimately sent me packing back over to arch.

When I came back to arch, I was left hankering for NixOS's declarative package management, if that could be somehow reconciled with the baseline arch package system. This led me to using the aconfmgr utility, which checks in packages and system configuration files into a home-based filetree.

Once you've got it set up, aconfmgr can regenerate your current configuration from scratch, declaratively! I was able to check the aconfmgr tree into my dotfiles repo, so now it tracks my home state AND my system state. Unlike Nix, this still uses file-based configuration, but it's all versioned and fully reproducible, so I no longer particularly care about that caveat.