nixpkgs/doc/functions/debug.xml

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<section xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xmlns:xi="http://www.w3.org/2001/XInclude"
xml:id="sec-debug">
<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>