Guix is a GNU project, and doesn't allow proprietary packages in their collection at all.
My reasoning was very simple. Nixos makes it easy to install packages on a per user basis. This would mean there is a lot of redundancy if another user also needs the same package. A snapshot/dedup filesystem will easily solve the problem.
Nix doesn't store duplicates; a hash is calculated, based on the inputs (source code, compilers, libraries, etc.) and if an output with that hash already exists, it will be used. If not, the configured binary caches will be queried, to see if a pre-built binary can be downloaded. If not, the inputs are fetched (following the same process) and the build is performed.
Is there any other implementation than posix in the wild these days?
Yes, I know it is fixed. But who knows what one runs into!
And this: http://blog.moertel.com/posts/2005-08-22-how-to-change-symli...
* I share the concern of the author on symlinks farm. It is scary! I would like it to be dealt with in the filesystem layer (Plan 9 had a snapshot based filesystem - fossil - years ago). Symlinks have all sorts of weird semantics on different Unix machines.
* Another of my gripe with nixos is that it makes Unix, a single user machine! Sure, packages need not be installed in a user-local way. I may be ignorant of other possibilities here.
* More care for licenses. I still use Debian because they really care for licenses. Last I looked, nixos was in no way close to Debian in terms of documenting the various copyrights and licenses of files pertaining to a package.
Otherwise, Nixos is a great idea and a huge step forward.