These are development shells using nix package manager.
To run them install Nix from NixOS and then in your shell run the following:
$ nix-shell ./erlang-shell.nixSee the page Declarative and reproducible developer environments for a more detailed explanation of creating a shell environment using Nix.
And, the nice thing about these shells is that they can be cascaded. So, for instance if you need a shell with Python and OCaml then launch one dev-shell. From this new dev-shell launch the other dev-shell. Below is an example:
$ nix-shell ./python-shell.nix
[nix-shell ~] $ nix-shell ./ocaml-shell.nix
[nix-shell ~] $ ocaml --version
[nix-shell ~] $ python --version