This chapter shows you how to write Nix expressions, which instruct Nix
how to build packages. It starts with a simple example (a Nix expression
for GNU Hello), and then moves on to a more in-depth look at the Nix
expression language.

> **Note**
> 
> This chapter is mostly about the Nix expression language. For more
> extensive information on adding packages to the Nix Packages
> collection (such as functions in the standard environment and coding
> conventions), please consult [its
> manual](http://nixos.org/nixpkgs/manual/).