There are a lot of already written Nix package definitions covering the most
commonly used applications in Linux and macOS. However, for some reason or
another, from time to time, you may wish to install a package with a slightly
customized definition.
The example below will install yarn without explicitely depending on node, i.e., it will use whatever node version you have available on the environment.
To install simply run nix-env -f yarn.nix -i.
More generally, when extending a package.nix, you can follow the template
below and run with nix-env -f your_file.nix -i