`nix-shell` cannot evaluate arguments without a value

[amrsoll]

These are definitely my own skill issues, because I do not understand how you are supposed to use the example shell.nix, as it takes arguments but no values are given to them when I simply run nix-shell

$ nix-shell
error: cannot evaluate a function that has an argument without a value ('mkShell')
       Nix attempted to evaluate a function as a top level expression; in
       this case it must have its arguments supplied either by default
       values, or passed explicitly with '--arg' or '--argstr'. See
       https://nix.dev/manual/nix/stable/language/syntax.html#functions.
       at /home/alex/workspace/bootloader/shell.nix:1:3:
           1| { mkShell
            |   ^
           2| , zephyr

I am used to have shell.nix files that start with this argument

{ pkgs ? import <nixpkgs> {} }: