One of the most surprising aspects of the Nix language is that it is lazy , especially if you have never used a lazy language before. This laziness is what makes much of Nixpkgs possible, and its complexity. One of the simplest ways to observe the laziness is by understanding that only the attributes you access are evaluated. $ nix eval --expr 'let pkgs = { hello = "hi"; broken = throw "never forced"; }; in pkgs.hello' "hi" The more whackier version of this is you can have endless recursion in an attribute set. Nixpkgs is filled with these bottomless attribute sets: $ nix eval -f '<nixpkgs>' 'pkgs.hello' --raw /nix/store/18bbdvag5v2f3d4y37pdbkzvh7s71cw4-hello-2.12.2 $ nix eval -f '<nixpkgs>' 'pkgs.pkgs.pkgs.hello' --raw /nix/store/18bbdvag5v2f3d4y37pdbkzvh7s71cw4-hello-2.12.2 $ nix eval -f '<nixpkgs>' 'pkgs.python3Packages.pkgs.hello' --raw /nix/store/18bbdvag5v2f3d4y37pdbkzvh7s71cw4-hello-2.12.2 The same store path every time. pkgs contains itself, and so does every package set inside it.…