Nix os  vs Parrot GNU/Linux

It has all of hackage in it's package manager (which is confusingly named "nix" as well) due to being based around hashing and allowing you to compile in a sort of virtual machine (really just changing the PATH variable temporarily) it solves many of the versioning problems that you commonly have with cabal. Here's a tutorial (there's many others as well) http://www.cse.chalmers.se/~bernardy/nix.html.
Also I should note that you can use the package manager a la carte on Mac and most any linux distro.

Since NixOS stores all its packages in isolation from each other in /nix/store and because of the declarative configuration model, upgrading NoxOS systems is extremely reliable. Furthermore, it gives you the ability to roll back upgrades.

Every package in your system is generated from a configuration file. This makes it very easy to reproduce that environment. Just copy the config file into a new machine and it's done.

You can use and test snapshots without rebooting. Booting into snapshots or test configurations is possible without risking the system's stability.

As everything is isolated, you can have on the same machine multiple configurations to meet project requirements that would be mutually exclusive on other OSes.

It is compatible to Debian >=testing.

It is always based on the latest testing release so it includes more recent software than a stable based distribution.

It focus the MATE desktop which makes it unique compared to the more general distros.