nixpkgs/doc/functions/debug.xml

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 <title>Debugging Nix Expressions</title>

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

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