Laziness

    Nix evaluates expression only when needed.
    This is a great feature when working with packages.

        nix-repl> let a = builtins.div 4 0; b = 6; in b
            6

    Since a is not needed, there's no error about division by zero, because the expression is not in need to be evaluated.
    That's why we can have all the packages defined on demand, yet have access to specific packages very quickly.