The Nix Language
The Nix language is a lazy lambda calculus, a minimal programming language that was designed expressly for configuring Nix packages (see below). Since it is a lazy lambda calculus, it can’t really do side effects; for the most part, all it can do is evaluate expressions to a value. However, since it was designed for configuring packages, it has a few built-in features for the limited side effects required for that purpose. We list it first because Nix the language is pervasive throughout the other uses of the name Nix: it is the language the package configurations are written in; it is the language your NixOS configuration is written in and you may use it on the command line, especially with NixOS and NixOps, where commands may be Nix expressions. There is a REPL available for the Nix language. There are tutorialsLearn the Nix language: Learn Nix in Y Minutes and Nix By Example are good starting points.
to help you get comfortable with the language itself, particularly if you are unaccustomed to lazy lambda calculi.