nixpkgs/doc/functions/debug.section.md

Debugging Nix Expressions

Nix is a unityped, dynamic language, this means any value can potentially appear anywhere. Since it is also non-strict, evaluation order and what is ultimately evaluated might surprise you. Therefore, it is important to be able to debug Nix expressions.

In the lib/debug.nix file you will find a number of functions that help (pretty-)printing values while evaluation is running. You can even specify how deep these values should be printed recursively, and transform them on the fly. Please consult the docstrings in lib/debug.nix for usage information.