The Nix expression language is a pure, lazy, functional language. Purity means that operations in the language don't have side-effects (for instance, there is no variable assignment). Laziness means that arguments to functions are evaluated only when they are needed. Functional means that functions are normal values that can be passed around and manipulated in interesting ways. The language is not a full-featured, general purpose language. Its main job is to describe packages, compositions of packages, and the variability within packages. This section presents the various features of the language.