Yep S1 still exists. Nix does not do destructive updating. When you install or update a package, you'll always be able to revert to the previous state. The best explanation of that process can be found in the original paper:

Nix: A Safe and Policy-Free System for Software Deployment - http://www.st.ewi.tudelft.nl/~dolstra/pubs/nspfssd-lisa2004-...

I really hope nix will become standard practice, it has some significant advantages over existing package managers.

Purely functional does not mean "has no state at all". This meme needs a stake stuck through it, though beats me how to do it. Functional languages exist and do real work.

In this case it means that packages are handled as functional programming languages handle data values. New ones are created from old ones, and unreferenced values are cleaned up via garbage collection.

It doesn't seem unreasonable to think that "purely functional" means "consisting of pure functions". Why not just say "functional" if that's what is meant?

The aim of Nix is to make function from dependencies to resulting packages a pure function. Nix seems to have achieved it with respect to set of packages installed on a system; the most problematic thing now is that some packages manage to detect CPU version at build time... Fortunately, most of them allow to disable this feature.

Installing the "same" package doesn't overwrite previous versions -- all packages are persistent -- so dependencies continue to work.

It is the mutation of packages, silently breaking API and ABI compatibility, that is the bane of distro management. Nix is a great attempt to address this.

That was solved decades ago with static linking. The bane of distro management is keeping everything as up to date as possible without breaking things.

And the bane of static linking is that a vulnerability in say libc will require the re-compilation & upgrade of a gazillion packages.

Exactly. And is the result different from keeping old versions of libc around and linking all programs you haven't updated against it? From the description, that's what this system does.

If dependencies never change in place, you're emulating static linking with a dynamic linker.

With ubiquitous high bandwidth and huge disk drives, that doesn't seem to be as much of a problem as it used to be.

Yes, but you are still at the mercy of whoever is statically building (and packages) your binaries. Bad for binary linux distributions, worse for proprietary software.

Nix model is that you work with expressions that allow to build packages from source. Binary packages are provided in format "if you performed build described by hash X you would get the contents of this archive".

For proprietary software, you have to consider the binary as-distributed "source" and build a version with proper dynamic linker an library path. Some third party binary wouldn't run as-is on NixOS, but there is a package that sets correct linking settings post-factum.

I think of it as git-for-system. All the data lies in the store/repository. Nix offers you to pick different isolated branches as needed.