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<body class="book toc2 toc-left">
<div id="header">
<h1>Wombat&#8217;s Book of Nix</h1>
<div class="details">
<span id="author" class="author">Amy de Buitléir</span><br>
</div>
<div id="toc" class="toc2">
<div id="toctitle">Table of Contents</div>
<ul class="sectlevel1">
<li><a href="#_introduction">1. Introduction</a>
<ul class="sectlevel2">
<li><a href="#_why_nix">1.1. Why Nix?</a></li>
<li><a href="#_why_flakes">1.2. Why <em>flakes</em>?</a></li>
<li><a href="#_prerequisites">1.3. Prerequisites</a></li>
<li><a href="#_see_an_error_or_want_more">1.4. See an error? Or want more?</a></li>
</ul>
</li>
<li><a href="#_the_nix_language">2. The Nix language</a>
<ul class="sectlevel2">
<li><a href="#_introducing_the_nix_language">2.1. Introducing the Nix language</a></li>
<li><a href="#_data_types">2.2. Data types</a>
<ul class="sectlevel3">
<li><a href="#type-string">2.2.1. Strings</a></li>
<li><a href="#type-integer">2.2.2. Integers</a></li>
<li><a href="#type-float">2.2.3. Floating point numbers</a></li>
<li><a href="#type-boolean">2.2.4. Boolean</a></li>
<li><a href="#type-path">2.2.5. Paths</a></li>
<li><a href="#type-list">2.2.6. Lists</a></li>
<li><a href="#type-set">2.2.7. Attribute sets</a></li>
<li><a href="#type-lambda">2.2.8. Functions</a></li>
</ul>
</li>
<li><a href="#_stop_reading_this_chapter">2.3. Stop reading this chapter!</a></li>
<li><a href="#_the_nix_repl">2.4. The Nix REPL</a></li>
<li><a href="#_variables">2.5. Variables</a>
<ul class="sectlevel3">
<li><a href="#_assignment">2.5.1. Assignment</a></li>
<li><a href="#_immutability">2.5.2. Immutability</a></li>
</ul>
</li>
<li><a href="#_numeric_operations">2.6. Numeric operations</a>
<ul class="sectlevel3">
<li><a href="#_arithmetic_operators">2.6.1. Arithmetic operators</a></li>
<li><a href="#_floating_point_calculations">2.6.2. Floating-point calculations</a></li>
</ul>
</li>
<li><a href="#_string_operations">2.7. String operations</a>
<ul class="sectlevel3">
<li><a href="#_string_concatenation">2.7.1. String concatenation</a></li>
<li><a href="#_string_interpolation">2.7.2. String interpolation</a></li>
<li><a href="#_useful_built_in_functions_for_strings">2.7.3. Useful built-in functions for strings</a></li>
</ul>
</li>
<li><a href="#_boolean_operations">2.8. Boolean operations</a></li>
<li><a href="#_path_operations">2.9. Path operations</a>
<ul class="sectlevel3">
<li><a href="#_concatenating_paths">2.9.1. Concatenating paths</a></li>
<li><a href="#_concatenating_a_path_a_string">2.9.2. Concatenating a path + a string</a></li>
<li><a href="#_concatenating_a_string_a_path">2.9.3. Concatenating a string + a path</a></li>
<li><a href="#_useful_built_in_functions_for_paths">2.9.4. Useful built-in functions for paths</a></li>
</ul>
</li>
<li><a href="#_list_operations">2.10. List operations</a>
<ul class="sectlevel3">
<li><a href="#_list_concatenation">2.10.1. List concatenation</a></li>
<li><a href="#_useful_built_in_functions_for_lists">2.10.2. Useful built-in functions for lists</a></li>
</ul>
</li>
<li><a href="#_attribute_set_operations">2.11. Attribute set operations</a>
<ul class="sectlevel3">
<li><a href="#_selection">2.11.1. Selection</a></li>
<li><a href="#_query">2.11.2. Query</a></li>
<li><a href="#_modification">2.11.3. Modification</a></li>
<li><a href="#rec-attrset">2.11.4. Recursive attribute sets</a></li>
<li><a href="#_useful_built_in_functions_for_attribute_sets">2.11.5. Useful built-in functions for attribute sets</a></li>
</ul>
</li>
<li><a href="#functions">2.12. Functions</a>
<ul class="sectlevel3">
<li><a href="#_anonymous_functions">2.12.1. Anonymous functions</a></li>
<li><a href="#_named_functions_and_function_application">2.12.2. Named functions and function application</a></li>
<li><a href="#_multiple_parameters_using_nested_functions">2.12.3. Multiple parameters using nested functions</a></li>
<li><a href="#_multiple_parameters_using_attribute_sets">2.12.4. Multiple parameters using attribute sets</a></li>
<li><a href="#_optional_parameters">2.12.5. Optional parameters</a></li>
<li><a href="#_patterns">2.12.6. @-patterns</a></li>
</ul>
</li>
<li><a href="#_if_expressions">2.13. If expressions</a></li>
<li><a href="#_let_expressions">2.14. Let expressions</a></li>
<li><a href="#_with_expressions">2.15. With expressions</a></li>
</ul>
</li>
<li><a href="#_hello_flake">3. Hello, flake!</a></li>
<li><a href="#_the_hello_flake_repo">4. The hello-flake repo</a></li>
<li><a href="#_flake_structure">5. Flake structure</a>
<ul class="sectlevel2">
<li><a href="#_description">5.1. Description</a></li>
<li><a href="#_inputs">5.2. Inputs</a></li>
<li><a href="#_outputs">5.3. Outputs</a></li>
</ul>
</li>
<li><a href="#_a_generic_flake">6. A generic flake</a></li>
<li><a href="#_another_look_at_hello_flake">7. Another look at hello-flake</a></li>
<li><a href="#_modifying_the_flake">8. Modifying the flake</a>
<ul class="sectlevel2">
<li><a href="#_the_nix_development_shell">8.1. The Nix development shell</a></li>
<li><a href="#_introducing_a_dependency">8.2. Introducing a dependency</a></li>
<li><a href="#_development_workflows">8.3. Development workflows</a></li>
<li><a href="#_this_all_seems_like_a_hassle">8.4. This all seems like a hassle!</a></li>
</ul>
</li>
<li><a href="#_a_new_flake_from_scratch">9. A new flake from scratch</a>
<ul class="sectlevel2">
<li><a href="#_haskell">9.1. Haskell</a>
<ul class="sectlevel3">
<li><a href="#_a_simple_haskell_program">9.1.1. A simple Haskell program</a></li>
<li><a href="#_optional_testing_before_packaging">9.1.2. (Optional) Testing before packaging</a>
<ul class="sectlevel4">
<li><a href="#_some_unsuitable_shells">Some unsuitable shells</a></li>
<li><a href="#_a_suitable_shell_for_a_quick_test">A suitable shell for a quick test</a></li>
</ul>
</li>
<li><a href="#_the_cabal_file">9.1.3. The cabal file</a></li>
<li><a href="#_optional_building_and_running_with_cabal_install">9.1.4. (Optional) Building and running with cabal-install</a></li>
<li><a href="#_the_nix_flake">9.1.5. The Nix flake</a></li>
</ul>
</li>
<li><a href="#_python">9.2. Python</a>
<ul class="sectlevel3">
<li><a href="#_a_simple_python_program">9.2.1. A simple Python program</a></li>
<li><a href="#_a_python_builder">9.2.2. A Python builder</a></li>
<li><a href="#_the_nix_flake_2">9.2.3. The Nix flake</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="#_recipes">10. Recipes</a>
<ul class="sectlevel2">
<li><a href="#_access_to_a_top_level_package_from_the_nixpkgsnixos_repo">10.1. Access to a top-level package from the Nixpkgs/NixOS repo</a>
<ul class="sectlevel3">
<li><a href="#_from_the_command_line">10.1.1. From the command line</a></li>
<li><a href="#_in_shell_nix">10.1.2. In <code>shell.nix</code></a></li>
<li><a href="#_in_a_bash_script">10.1.3. In a Bash script</a></li>
</ul>
</li>
<li><a href="#_access_to_a_package_defined_in_a_remote_git_repo">10.2. Access to a package defined in a remote git repo</a>
<ul class="sectlevel3">
<li><a href="#_in_shell_nix_2">10.2.1. In <code>shell.nix</code></a></li>
</ul>
</li>
<li><a href="#_access_to_a_flake_defined_in_a_remote_git_repo">10.3. Access to a flake defined in a remote git repo</a>
<ul class="sectlevel3">
<li><a href="#_in_shell_nix_3">10.3.1. In <code>shell.nix</code></a></li>
</ul>
</li>
<li><a href="#_access_to_a_haskell_library_package_in_the_nixpkgs_repo_without_a_cabal_file">10.4. Access to a Haskell library package in the nixpkgs repo (without a <code>.cabal</code> file)</a>
<ul class="sectlevel3">
<li><a href="#_in_shell_nix_4">10.4.1. In <code>shell.nix</code></a></li>
<li><a href="#_in_a_haskell_script">10.4.2. In a Haskell script</a></li>
</ul>
</li>
<li><a href="#_access_to_a_haskell_package_on_your_local_computer">10.5. Access to a Haskell package on your local computer</a>
<ul class="sectlevel3">
<li><a href="#_in_shell_nix_5">10.5.1. In <code>shell.nix</code></a></li>
</ul>
</li>
<li><a href="#_access_to_a_haskell_package_on_your_local_computer_with_interdependencies">10.6. Access to a Haskell package on your local computer, with interdependencies</a>
<ul class="sectlevel3">
<li><a href="#_in_shell_nix_6">10.6.1. In <code>shell.nix</code></a></li>
</ul>
</li>
<li><a href="#_access_to_a_python_library_package_in_the_nixpkgs_repo_without_using_a_python_builder">10.7. Access to a Python library package in the nixpkgs repo (without using a Python builder)</a>
<ul class="sectlevel3">
<li><a href="#_in_a_python_script">10.7.1. In a Python script</a></li>
</ul>
</li>
<li><a href="#_set_an_environment_variable">10.8. Set an environment variable</a>
<ul class="sectlevel3">
<li><a href="#_in_shell_nix_7">10.8.1. In <code>shell.nix</code></a></li>
</ul>
</li>
</ul>
</li>
</ul>
</div>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="imageblock">
<div class="content">
<img src="images/wombat.svg" alt="wombat reclining against a lambda">
</div>
</div>
<div class="paragraph">
<p>This book is available
<a href="https://mhwombat.codeberg.page/nix-book/">online</a>
and as a downloadable
<a href="https://codeberg.org/mhwombat/nix-book/raw/branch/pages/wombats-book-of-nix.pdf">PDF</a>.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_introduction">1. Introduction</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="_why_nix">1.1. Why Nix?</h3>
<div class="paragraph">
<p>If you’ve opened this PDF, you already have your own motivation for
learning Nix. Here’s how it helps me. As a researcher, I tend to work on
a series of short-term projects, mostly demos and prototypes. For each
one, I typically develop some software using a compiler, often with some
open source libraries. Often I use other tools to analyse data or
generate documentation, for example.</p>
</div>
<div class="paragraph">
<p>Problems would arise when handing off the project to colleagues; they
would report errors when trying to build or run the project. Belatedly I
would realise that my code relies on a library that they need to
install. Or perhaps they had installed the library, but the version
they’re using is incompatible.</p>
</div>
<div class="paragraph">
<p>Using containers helped with the problem. However, I didn’t want to
<em>develop</em> in a container. I did all my development in my nice, familiar,
environment with my custom aliases and shell prompt. and <em>then</em> I
containerised the software. This added step was annoying for me, and if
my colleague wanted to do some additional development, they would
probably extract all of the source code from the container first anyway.
Containers are great, but this isn’t the ideal use case for them.</p>
</div>
<div class="paragraph">
<p>Nix allows me to work in my custom environment, but forces me to specify
any dependencies. It automatically tracks the version of each dependency
so that it can replicate the environment wherever and whenever it’s
needed.</p>
</div>
</div>
<div class="sect2">
<h3 id="_why_flakes">1.2. Why <em>flakes</em>?</h3>
<div class="paragraph">
<p>Flakes are labeled as an experimental feature, so it might seem safer to
avoid them. However, they have been in use for years, and there is
widespread adoption, so the aren’t going away any time soon. Flakes are
easier to understand, and offer more features than the traditional Nix
approach. After weighing the pros and cons, I feel it’s better to learn
and use flakes; and this seems to be the general consensus.</p>
</div>
</div>
<div class="sect2">
<h3 id="_prerequisites">1.3. Prerequisites</h3>
<div class="paragraph">
<p>To follow along with the examples in this book, you will need access to a computer
or (virtual machine) with Nix (or NixOS) installed and <em>flakes enabled</em>.</p>
</div>
<div class="paragraph">
<p>I recommend the installer from
<a href="https://zero-to-nix.com/start/install">zero-to-nix.com</a>. This installer
automatically enables flakes.</p>
</div>
<div class="paragraph">
<p>More documentation (and another installer) available at
<a href="https://nixos.org/">nixos.org</a>.</p>
</div>
<div class="paragraph">
<p>To <em>enable flakes on an existing Nix or NixOS installation</em>,
see the instructions in the <a href="https://nixos.wiki/wiki/Flakes">NixOS wiki</a>.</p>
</div>
<div class="admonitionblock important">
<table>
<tr>
<td class="icon">
<i class="fa icon-important" title="Important"></i>
</td>
<td class="content">
<div class="paragraph">
<p>There are hyphenated and unhyphenated versions of many Nix commands.
For example, <code>nix-shell</code> and <code>nix shell</code> are two different commands.
Don&#8217;t confuse them!</p>
</div>
<div class="paragraph">
<p>Generally speaking, the unhyphenated versions are for working directly
with flakes, while the hyphenated versions are for everything else.</p>
</div>
</td>
</tr>
</table>
</div>
</div>
<div class="sect2">
<h3 id="_see_an_error_or_want_more">1.4. See an error? Or want more?</h3>
<div class="paragraph">
<p>If notice an error, or you&#8217;re interested in an area that isn&#8217;t covered in this book, feel free to open an
<a href="https://codeberg.org/mhwombat/nix-book/issues">issue</a>.</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_the_nix_language">2. The Nix language</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="_introducing_the_nix_language">2.1. Introducing the Nix language</h3>
<div class="paragraph">
<p>Nix and NixOS use a functional programming language called <em>Nix</em>
to specify how to build and install software,
and how to configure system, user, and project-specific environments.
(Yes, “Nix” is the name of both the package manager and the language it uses.)</p>
</div>
<div class="paragraph">
<p>Nix is a <em>functional</em> language.
In a <em>procedural</em> language such as C or Java,
the focus is on writing a series of <em>steps</em> (statements) to achieve a desired result.
By contrast, in a functional language the focus is on <em>defining</em> the desired result.</p>
</div>
<div class="paragraph">
<p>In the case of Nix, the desired result is usually a <em>derivation</em>:
a software package that has been built and is ready for use.
The Nix language has been designed for that purpose,
and thus has some features you don&#8217;t typically find in general-purpose languages.</p>
</div>
</div>
<div class="sect2">
<h3 id="_data_types">2.2. Data types</h3>
<div class="sect3">
<h4 id="type-string">2.2.1. Strings</h4>
<div class="paragraph">
<p>Strings are enclosed by double quotes (<code>"</code>), or <em>two</em> single quotes (<code>'</code>).</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">"Hello, world!"</pre>
</div>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">''This string contains "double quotes"''</pre>
</div>
</div>
<div class="paragraph">
<p>They can span multiple lines.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">''Old pond
A frog jumps in
The sound of water
-- Basho''</pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="type-integer">2.2.2. Integers</h4>
<div class="literalblock">
<div class="content">
<pre class="nowrap">7
256</pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="type-float">2.2.3. Floating point numbers</h4>
<div class="literalblock">
<div class="content">
<pre class="nowrap">3.14
6.022e23</pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="type-boolean">2.2.4. Boolean</h4>
<div class="paragraph">
<p>The Boolean values in Nix are <code>true</code> and <code>false</code>.</p>
</div>
</div>
<div class="sect3">
<h4 id="type-path">2.2.5. Paths</h4>
<div class="paragraph">
<p>File paths are play an important role in building software, so Nix has a special data type for them.
Paths may be absolute (e.g. <code>/bin/sh</code>) or relative (e.g. <code>./data/file1.csv</code>).
Note that paths are not enclosed in quotation marks; they are not strings!</p>
</div>
<div class="paragraph">
<p>Enclosing a path in angle brackets, e.g. &lt;nixpkgs&gt; causes the directories
listed in the environment variable NIX_PATH to be searched for the given
file or directory name.
These are called <em>lookup paths</em>.</p>
</div>
</div>
<div class="sect3">
<h4 id="type-list">2.2.6. Lists</h4>
<div class="paragraph">
<p>List elements are enclosed in square brackets and separated by spaces (not commas).
The elements need not be of the same type.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">[ "apple" 123 ./build.sh false ]</pre>
</div>
</div>
<div class="paragraph">
<p>Lists can be empty.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">[]</pre>
</div>
</div>
<div class="paragraph">
<p>List elements can be of any type, and can even be lists themselves.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">[ [ 1 2 ] [ 3 4 ] ]</pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="type-set">2.2.7. Attribute sets</h4>
<div class="paragraph">
<p>Attribute sets associate keys with values.
They are enclosed in curly brackets, and the associations are terminated by semi-colons.
Note that the final semi-colon before the closing bracket is required.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">{ name = "Professor Paws"; age = 10; species = "cat"; }</pre>
</div>
</div>
<div class="paragraph">
<p>The keys of an attribute set must be strings.
When the key is not a valid identifier, it must be enclosed in quotation marks.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">{ abc = true; "123" = false; }</pre>
</div>
</div>
<div class="paragraph">
<p>Attribute sets can be empty.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">{}</pre>
</div>
</div>
<div class="paragraph">
<p>Values of attribute sets can be of any type, and can even be attribute sets themselves.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">{ name = { first = "Professor"; last = "Paws"; }; age = 10; species = "cat"; }</pre>
</div>
</div>
<div class="paragraph">
<p>In <a href="#rec-attrset">Section 2.11.4, &#8220;Recursive attribute sets&#8221;</a> you will be introduced to a special type of attribute set.</p>
</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<i class="fa icon-note" title="Note"></i>
</td>
<td class="content">
<div class="paragraph">
<p>In some Nix documentation, and in many articles about Nix,
attribute sets are simply called "sets".</p>
</div>
</td>
</tr>
</table>
</div>
</div>
<div class="sect3">
<h4 id="type-lambda">2.2.8. Functions</h4>
<div class="paragraph">
<p>We&#8217;ll learn how to write functions later in this chapter.
For now, note that functions are "first-class values",
meaning that they can be treated like any other data type.
For example, a function can be assigned to a variable, appear as an element in a list,
or be associated with a key in an attribute set.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">[ "apple" 123 ./build.sh false (x: x*x) ]
{ name = "Professor Paws"; age = 10; species = "cat"; formula = (x: x*2); }</pre>
</div>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_stop_reading_this_chapter">2.3. Stop reading this chapter!</h3>
<div class="paragraph">
<p>When I first began using Nix, it seemed logical to start by learning the Nix language.
However, after following an in-depth tutorial,
I found that I didn&#8217;t know how to do anything useful with the language!
It wasn&#8217;t until later that I understood what I was missing:
a guide to the most useful library functions.</p>
</div>
<div class="paragraph">
<p>When working with Nix or NixOS,
it&#8217;s very rare that you&#8217;ll want to write something from scratch.
Instead, you&#8217;ll use one of the many library functions
that make things easier and shield you from the underlying complexity.
Many of these functions are language-specific,
and the documentation for them may be inadequate.
Often the easiest (or only) way to learn to use them
is to find an example that does something similar to what you want,
and then modify the function parameters to suit your needs.</p>
</div>
<div class="paragraph">
<p>At this point you&#8217;ve learned enough of the Nix language
to do the majority of common Nix tasks.
So when I say "Stop reading this chapter!", I&#8217;m only half-joking.
Instead I suggest that you <em>skim</em> the rest of this chapter,
paying special attention to anything marked with <span class="icon"><i class="fa fa-exclamation-circle"></i></span>.
Then move on to the following chapters
where you will learn how to develop and package software using Nix.
Afterward, come back to this chapter and read it in more detail.</p>
</div>
<div class="paragraph">
<p>While writing this book, I anticipated that readers would want to skip around,
alternating between pure learning and learning-by-doing.
I&#8217;ve tried to structure the book to support that;
sometimes repeating information from earlier chapters that you might have skipped.</p>
</div>
</div>
<div class="sect2">
<h3 id="_the_nix_repl">2.4. The Nix REPL</h3>
<div class="paragraph">
<p>The Nix REPL <sup class="footnote">[<a id="_footnoteref_1" class="footnote" href="#_footnotedef_1" title="View footnote.">1</a>]</sup>
is an interactive environment for evaluating and debugging Nix code.
It&#8217;s also a good place to begin learning Nix.
Enter it using the command <code>nix repl</code>.
Within the REPL, type <code>:?</code> to see a list of available commands.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>$# echo &quot;$ nix repl&quot;
Welcome to Nix 2.18.1. Type :? for help.
nix-repl&gt; :?
The following commands are available:
&lt;expr&gt; Evaluate and print expression
&lt;x&gt; = &lt;expr&gt; Bind expression to variable
:a, :add &lt;expr&gt; Add attributes from resulting set to scope
:b &lt;expr&gt; Build a derivation
:bl &lt;expr&gt; Build a derivation, creating GC roots in the
working directory
:e, :edit &lt;expr&gt; Open package or function in $EDITOR
:i &lt;expr&gt; Build derivation, then install result into
current profile
:l, :load &lt;path&gt; Load Nix expression and add it to scope
:lf, :load-flake &lt;ref&gt; Load Nix flake and add it to scope
:p, :print &lt;expr&gt; Evaluate and print expression recursively
:q, :quit Exit nix-repl
:r, :reload Reload all files
:sh &lt;expr&gt; Build dependencies of derivation, then start
nix-shell
:t &lt;expr&gt; Describe result of evaluation
:u &lt;expr&gt; Build derivation, then start nix-shell
:doc &lt;expr&gt; Show documentation of a builtin function
:log &lt;expr&gt; Show logs for a derivation
:te, :trace-enable [bool] Enable, disable or toggle showing traces for
errors
:?, :help Brings up this help menu</code></pre>
</div>
</div>
<div class="paragraph">
<p>A command that is useful to beginners is <code>:t</code>, which tells you the type of an expression.</p>
</div>
<div class="paragraph">
<p>Note that the command to exit the REPL is <code>:q</code> (or <code>:quit</code> if you prefer).</p>
</div>
</div>
<div class="sect2">
<h3 id="_variables">2.5. Variables</h3>
<div class="sect3">
<h4 id="_assignment">2.5.1. Assignment</h4>
<div class="paragraph">
<p>You can declare variables in Nix and assign values to them.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; a = 7
nix-repl&gt; b = 3
nix-repl&gt; a - b
4</code></pre>
</div>
</div>
<div class="admonitionblock important">
<table>
<tr>
<td class="icon">
<i class="fa icon-important" title="Important"></i>
</td>
<td class="content">
<div class="paragraph">
<p>The spaces before and after operators aren&#8217;t always required.
However, you can get unexpected results when you omit them, as shown in the following example.
Nix allows hyphens (<code>-</code>) in variable names,
so <code>a-b</code> is interpreted as the name of a variable rather than a subtraction operation.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; a-b
error: undefined variable &#39;a-b&#39;
at «string»:1:1:
1| a-b
| ^</code></pre>
</div>
</div>
</td>
</tr>
</table>
</div>
</div>
<div class="sect3">
<h4 id="_immutability">2.5.2. Immutability</h4>
<div class="paragraph">
<p>In Nix, values are <em>immutable</em>;
once you assign a value to a variable, you cannot change it.
You can, however, create a new variable with the same name, but in a different scope.
Don&#8217;t worry if you don&#8217;t completely understand the previous sentence;
we will see some examples in <a href="#_functions">[_functions]</a>, <a href="#_let_expressions">Section 2.14, &#8220;Let expressions&#8221;</a>, and <a href="#_with_expressions">Section 2.15, &#8220;With expressions&#8221;</a>.</p>
</div>
<div class="admonitionblock important">
<table>
<tr>
<td class="icon">
<i class="fa icon-important" title="Important"></i>
</td>
<td class="content">
<div class="paragraph">
<p>In the Nix REPL, it may seem like the values of variables can be changed,
in <em>apparent</em> contradiction to the previous paragraph.
In truth, the REPL works some behind-the-scenes "magic",
effectively creating a new nested scope with each assignment.
This makes it much easier to experiment in the REPL.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; x = 1
nix-repl&gt; x
1
nix-repl&gt; x = x + 1 # creates a new variable called &quot;x&quot;; the original is no longer in scope
nix-repl&gt; x
2</code></pre>
</div>
</div>
</td>
</tr>
</table>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_numeric_operations">2.6. Numeric operations</h3>
<div class="sect3">
<h4 id="_arithmetic_operators">2.6.1. Arithmetic operators</h4>
<div class="paragraph">
<p>The usual arithmetic operators are provided.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; 1 + 2 # addition
3
nix-repl&gt; 5 - 3 # subtraction
2
nix-repl&gt; 3 * 4 # multiplication
12
nix-repl&gt; 6 / 2 # division
3
nix-repl&gt; -1 # negation
-1</code></pre>
</div>
</div>
<div class="admonitionblock important">
<table>
<tr>
<td class="icon">
<i class="fa icon-important" title="Important"></i>
</td>
<td class="content">
<div class="paragraph">
<p>As mentioned in <a href="#_variables">Section 2.5, &#8220;Variables&#8221;</a>,
you can get unexpected results when you omit spaces around operators.
Consider the following example.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; 6/2
/home/amy/codeberg/nix-book/6/2</code></pre>
</div>
</div>
<div class="paragraph">
<p>What happened?
Let&#8217;s use the <code>:t</code> command to find out the type of the expression.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; :t 6/2
a path</code></pre>
</div>
</div>
<div class="paragraph">
<p>If an expression can be interpreted as a path, Nix will do so.
This is useful, because paths are <em>far</em> more commonly used in Nix expressions that arithmetic operators.
In this case, Nix interpreted <code>6/2</code> as a relative path from the current directory,
which in the above example was <code>/home/amy/codeberg/nix-book</code>.</p>
</div>
<div class="paragraph">
<p>Adding a space after the <code>/</code> operator produces the expected result.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; 6/ 2
3</code></pre>
</div>
</div>
<div class="paragraph">
<p>To avoid surprises and improve readability, I prefer to use spaces before and after all operators.</p>
</div>
</td>
</tr>
</table>
</div>
</div>
<div class="sect3">
<h4 id="_floating_point_calculations">2.6.2. Floating-point calculations</h4>
<div class="paragraph">
<p>Numbers without a decimal point are assumed to be integers.
To ensure that a number is interpreted as a floating-point value, add a decimal point.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; :t 5
an integer
nix-repl&gt; :t 5.0
a float
nix-repl&gt; :t 5.
a float</code></pre>
</div>
</div>
<div class="paragraph">
<p>In the example below, the first expression results in integer division (rounding down),
while the second produces a floating-point result.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; 5 / 3
1
nix-repl&gt; 5.0 / 3
1.66667</code></pre>
</div>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_string_operations">2.7. String operations</h3>
<div class="sect3">
<h4 id="_string_concatenation">2.7.1. String concatenation</h4>
<div class="paragraph">
<p>String concatenation uses the <code>+</code> operator.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; &quot;Hello, &quot; + &quot;world!&quot;
&quot;Hello, world!&quot;</code></pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="_string_interpolation">2.7.2. String interpolation</h4>
<div class="paragraph">
<p>You can use the <code>${<em>variable</em>}</code> syntax to insert the value of a variable within a string.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; name = &quot;Wombat&quot;
nix-repl&gt; &quot;Hi, I&#39;m ${name}.&quot;
&quot;Hi, I&#39;m Wombat.&quot;</code></pre>
</div>
</div>
<div class="admonitionblock important">
<table>
<tr>
<td class="icon">
<i class="fa icon-important" title="Important"></i>
</td>
<td class="content">
<div class="paragraph">
<p>You cannot mix numbers and strings.
Earlier we set <code>a = 7</code>, so the following expression fails.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; &quot;My favourite number is ${a}.&quot;
error:
… while evaluating a path segment
at «string»:1:25:
1| &quot;My favourite number is ${a}.&quot;
| ^
error: cannot coerce an integer to a string</code></pre>
</div>
</div>
<div class="paragraph">
<p>Nix does provide functions for converting between types; we&#8217;ll see these in <a href="#_built_in_functions">[_built_in_functions]</a>.</p>
</div>
</td>
</tr>
</table>
</div>
</div>
<div class="sect3">
<h4 id="_useful_built_in_functions_for_strings">2.7.3. Useful built-in functions for strings</h4>
<div class="paragraph">
<p>Nix provides some built-in functions for working with strings;
a few examples are shown below.
For more information on these and other built-in functions, see the Nix Manual
(<a href="https://nixos.org/manual/nix/stable/language/builtins" class="bare">https://nixos.org/manual/nix/stable/language/builtins</a>).</p>
</div>
<div class="paragraph">
<p>How long is this string?</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; builtins.stringLength &quot;supercalifragilisticexpialidocious&quot;
34</code></pre>
</div>
</div>
<div class="paragraph">
<p>Given a startiing position and a length, extract a substring.
The first character of a string has index <code>0</code>.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; builtins.substring 3 6 &quot;hayneedlestack&quot;
&quot;needle&quot;</code></pre>
</div>
</div>
<div class="paragraph">
<p>Convert an expression to a string.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; builtins.toString 7
&quot;7&quot;
nix-repl&gt; builtins.toString (3*4 + 1)
&quot;13&quot;</code></pre>
</div>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_boolean_operations">2.8. Boolean operations</h3>
<div class="paragraph">
<p>The usual boolean operators are available.
Recall that earlier we set <code>a = 7</code> and <code>b = 3</code>.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; a == 7 # equality test
true
nix-repl&gt; b != 3 # inequality
false
nix-repl&gt; a &gt; 12 # greater than
false
nix-repl&gt; b &gt;= 2 # greater than or equal
true
nix-repl&gt; a &lt; b # less than
false
nix-repl&gt; b &lt;= a # less than or equal
true
nix-repl&gt; !true # logical negation
false
nix-repl&gt; (3 * a == 21) &amp;&amp; (a &gt; b) # logical AND
true
nix-repl&gt; (b &gt; a) || (b &gt; 10) # logical OR
false</code></pre>
</div>
</div>
<div class="paragraph">
<p>One operator that might be unfamiliar to you is <em>logical implication</em>, which uses the symbol <code>&#8594;</code>.
The expression <code>u &#8594; v</code> is equivalent to <code>!u || v</code>.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; u = false
nix-repl&gt; v = true
nix-repl&gt; u -&gt; v
true
nix-repl&gt; v -&gt; u
false</code></pre>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_path_operations">2.9. Path operations</h3>
<div class="paragraph">
<p>Any expression that contains a forward slash (<code>/</code>) <em>not</em> followed by a space
is interpreted as a path.
To refer to a file or directory relative to the current directory, prefix it with <code>./</code>.
You can specify the current directory as <code>./.</code></p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; ./file.txt
/home/amy/codeberg/nix-book/file.txt
nix-repl&gt; ./.
/home/amy/codeberg/nix-book</code></pre>
</div>
</div>
<div class="sect3">
<h4 id="_concatenating_paths">2.9.1. Concatenating paths</h4>
<div class="paragraph">
<p>Paths can be concatenated to produce a new path.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; /home/wombat + /bin/sh
/home/wombat/bin/sh
nix-repl&gt; :t /home/wombat + /bin/sh
a path</code></pre>
</div>
</div>
<div class="admonitionblock important">
<table>
<tr>
<td class="icon">
<i class="fa icon-important" title="Important"></i>
</td>
<td class="content">
<div class="paragraph">
<p>Relative paths are made absolute when they are parsed, which occurs before concatenation.
This is why the result in the example below is not <code>/home/wombat/file.txt</code>.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; /home/wombat + ./file.txt
/home/wombat/home/amy/codeberg/nix-book/file.txt</code></pre>
</div>
</div>
</td>
</tr>
</table>
</div>
</div>
<div class="sect3">
<h4 id="_concatenating_a_path_a_string">2.9.2. Concatenating a path + a string</h4>
<div class="paragraph">
<p>A path can be concatenated with a string to produce a new path.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; /home/wombat + &quot;/file.txt&quot;
/home/wombat/file.txt
nix-repl&gt; :t /home/wombat + &quot;/file.txt&quot;
a path</code></pre>
</div>
</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<i class="fa icon-note" title="Note"></i>
</td>
<td class="content">
<div class="paragraph">
<p>The Nix reference manual says that the string must not "have a string context" that refers to a store path.
String contexts are beyond the scope of this book;
for more information see <a href="https://nixos.org/manual/nix/stable/language/operators#path-concatenation" class="bare">https://nixos.org/manual/nix/stable/language/operators#path-concatenation</a>.</p>
</div>
</td>
</tr>
</table>
</div>
</div>
<div class="sect3">
<h4 id="_concatenating_a_string_a_path">2.9.3. Concatenating a string + a path</h4>
<div class="paragraph">
<p>Strings can be concatenated with paths, but with a side-effect that may surprise you:
if the path exists, the file is copied to the Nix store!
The result is a string, not a path.</p>
</div>
<div class="paragraph">
<p>In the example below, the file <code>file.txt</code> is copied to <code>/nix/store/gp8ba25gpwvbqizqfr58jr014gmv1hd8-file.txt</code>
(not, as you might expect, to <code>/home/wombat/nix/store/gp8ba25gpwvbqizqfr58jr014gmv1hd8-file.txt</code>).</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; &quot;/home/wombat&quot; + ./file.txt
&quot;/home/wombat/nix/store/gp8ba25gpwvbqizqfr58jr014gmv1hd8-file.txt&quot;</code></pre>
</div>
</div>
<div class="paragraph">
<p>The path must exist.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; &quot;/home/wombat&quot; + ./no-such-file.txt
error (ignored): error: end of string reached
error: getting status of &#39;/home/amy/codeberg/nix-book/no-such-file.txt&#39;: No such file or directory</code></pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="_useful_built_in_functions_for_paths">2.9.4. Useful built-in functions for paths</h4>
<div class="paragraph">
<p>Nix provides some built-in functions for working with paths;
a few examples are shown below.
For more information on these and other built-in functions, see the Nix Manual
(<a href="https://nixos.org/manual/nix/stable/language/builtins" class="bare">https://nixos.org/manual/nix/stable/language/builtins</a>).</p>
</div>
<div class="paragraph">
<p>Does the path exist?</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; builtins.pathExists ./index.html
true
nix-repl&gt; builtins.pathExists /no/such/path
false</code></pre>
</div>
</div>
<div class="paragraph">
<p>Get a list of the files in a directory, along with the type of each file.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; builtins.readDir ./.
{ &quot;.envrc&quot; = &quot;regular&quot;; &quot;.git&quot; = &quot;directory&quot;; &quot;.gitignore&quot; = &quot;regular&quot;; Makefile = &quot;regular&quot;; images = &quot;directory&quot;; &quot;index.html&quot; = &quot;regular&quot;; &quot;shell.nix&quot; = &quot;regular&quot;; source = &quot;directory&quot;; themes = &quot;directory&quot;; &quot;wombats-book-of-nix.pdf&quot; = &quot;regular&quot;; }</code></pre>
</div>
</div>
<div class="paragraph">
<p>Read the contents of a file into a string.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; builtins.readFile ./.envrc
&quot;use nix\n&quot;</code></pre>
</div>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code></code></pre>
</div>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_list_operations">2.10. List operations</h3>
<div class="sect3">
<h4 id="_list_concatenation">2.10.1. List concatenation</h4>
<div class="paragraph">
<p>Lists can be concatenated using the <code>++</code> operator.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; [ 1 2 3 ] ++ [ &quot;apple&quot; &quot;banana&quot; ]
[ 1 2 3 &quot;apple&quot; &quot;banana&quot; ]</code></pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="_useful_built_in_functions_for_lists">2.10.2. Useful built-in functions for lists</h4>
<div class="paragraph">
<p>Nix provides some built-in functions for working with lists;
a few examples are shown below.
For more information on these and other built-in functions, see the Nix Manual
(<a href="https://nixos.org/manual/nix/stable/language/builtins" class="bare">https://nixos.org/manual/nix/stable/language/builtins</a>).</p>
</div>
<div class="paragraph">
<p>Testing if an element appears in a list.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; fruit = [ &quot;apple&quot; &quot;banana&quot; &quot;canteloupe&quot; ]
nix-repl&gt; builtins.elem &quot;apple&quot; fruit
true
nix-repl&gt; builtins.elem &quot;broccoli&quot; fruit
false</code></pre>
</div>
</div>
<div class="paragraph">
<p>Selecting an item from a list by index.
The first element in a list has index <code>0</code>.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; builtins.elemAt fruit 0
&quot;apple&quot;
nix-repl&gt; builtins.elemAt fruit 2
&quot;canteloupe&quot;</code></pre>
</div>
</div>
<div class="paragraph">
<p>Determining the number of elements in a list.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; builtins.length fruit
3</code></pre>
</div>
</div>
<div class="paragraph">
<p>Accessing the first element of a list.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; builtins.head fruit
&quot;apple&quot;</code></pre>
</div>
</div>
<div class="paragraph">
<p>Dropping the first element of a list.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; builtins.tail fruit
[ &quot;banana&quot; &quot;canteloupe&quot; ]</code></pre>
</div>
</div>
<div class="paragraph">
<p>Functions are useful for working with lists.
Functions will be introduced in <a href="#functions">Section 2.12, &#8220;Functions&#8221;</a>,
but the following examples should be somewhat self-explanatory.</p>
</div>
<div class="paragraph">
<p>Using a function to filter (select elements from) a list.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; numbers = [ 1 3 6 8 9 15 25 ]
nix-repl&gt; isBig = n: n &gt; 10 # is the number &quot;big&quot; (greater than 10)?
nix-repl&gt; builtins.filter isBig numbers # get just the &quot;big&quot; numbers
[ 15 25 ]</code></pre>
</div>
</div>
<div class="paragraph">
<p>Applying a function to all the elements in a list.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; double = n: 2*n # multiply by two
nix-repl&gt; builtins.map double numbers # double each element in the list
[ 2 6 12 16 18 30 50 ]</code></pre>
</div>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_attribute_set_operations">2.11. Attribute set operations</h3>
<div class="sect3">
<h4 id="_selection">2.11.1. Selection</h4>
<div class="paragraph">
<p>The <code>.</code> operator selects an attribute from a set.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; animal = { name = { first = &quot;Professor&quot;; last = &quot;Paws&quot;; }; age = 10; species = &quot;cat&quot;; }
nix-repl&gt; animal . age
10
nix-repl&gt; animal . name . first
&quot;Professor&quot;</code></pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="_query">2.11.2. Query</h4>
<div class="paragraph">
<p>We can use the <code>?</code> operator to find out if a set has a particular attribute.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; animal ? species
true
nix-repl&gt; animal ? bicycle
false</code></pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="_modification">2.11.3. Modification</h4>
<div class="paragraph">
<p>We can use the <code>//</code> operator to modify an attribute set.
Recall that Nix values are immutable, so the result is a new value (the original is not modified).</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; animal // { species = &quot;tiger&quot;; }
{ age = 10; name = { ... }; species = &quot;tiger&quot;; }</code></pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="rec-attrset">2.11.4. Recursive attribute sets</h4>
<div class="paragraph">
<p>An ordinary attribute set cannot refer to its own elements.
To do this, you need a <em>recursive</em> attribute set.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; { x = 3; y = 4*x; }
error: undefined variable &#39;x&#39;
at «string»:1:16:
1| { x = 3; y = 4*x; }
| ^
nix-repl&gt; rec { x = 3; y = 4*x; }
{ x = 3; y = 12; }</code></pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="_useful_built_in_functions_for_attribute_sets">2.11.5. Useful built-in functions for attribute sets</h4>
<div class="paragraph">
<p>Nix provides some built-in functions for working with attribute sets;
a few examples are shown below.
For more information on these and other built-in functions, see the Nix Manual
(<a href="https://nixos.org/manual/nix/stable/language/builtins" class="bare">https://nixos.org/manual/nix/stable/language/builtins</a>).</p>
</div>
<div class="paragraph">
<p>Get an alphabetical list of the keys.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; builtins.attrNames animal
[ &quot;age&quot; &quot;name&quot; &quot;species&quot; ]</code></pre>
</div>
</div>
<div class="paragraph">
<p>Get the values associated with each key, in alphabetical order by the key names.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; builtins.attrValues animal
[ 10 &quot;Professor Paws&quot; &quot;cat&quot; ]</code></pre>
</div>
</div>
<div class="paragraph">
<p>What value is associated with a key?</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; builtins.getAttr &quot;age&quot; animal
10</code></pre>
</div>
</div>
<div class="paragraph">
<p>Does the set have a value for a key?</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; builtins.hasAttr &quot;name&quot; animal
true
nix-repl&gt; builtins.hasAttr &quot;car&quot; animal
false</code></pre>
</div>
</div>
<div class="paragraph">
<p>Remove one or more keys and associated values from a set.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; builtins.removeAttrs animal [ &quot;age&quot; &quot;species&quot; ]
{ name = &quot;Professor Paws&quot;; }</code></pre>
</div>
</div>
</div>
</div>
<div class="sect2">
<h3 id="functions">2.12. Functions</h3>
<div class="sect3">
<h4 id="_anonymous_functions">2.12.1. Anonymous functions</h4>
<div class="paragraph">
<p>Functions are defined using the syntax <code><em>parameter</em>: <em>expression</em></code>,
where the <em>expression</em> typically involves the <em>parameter</em>.
Consider the following example.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; x: x + 1
«lambda @ «string»:1:1»</code></pre>
</div>
</div>
<div class="paragraph">
<p>We created a function that adds <code>1</code> to its input.
However, it doesn&#8217;t have a name, so we can&#8217;t use it directly.
Anonymous functions do have their uses, as we shall see shortly.</p>
</div>
<div class="paragraph">
<p>Note that the message printed by the NixREPL when we created the function uses the term <em>lambda</em>.
This derives from a branch of mathematics called <em>lambda calculus</em>.
Lambda calculus was the inspiration for most functional languages such as Nix.
Functional programmers often call anonymous functions "lambdas".</p>
</div>
<div class="paragraph">
<p>The Nix REPL confirms that the expression <code>x: x + 1</code> defines a function.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; :t x: x + 1
a function</code></pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="_named_functions_and_function_application">2.12.2. Named functions and function application</h4>
<div class="paragraph">
<p>How can we use a function?
Recall from <a href="#type-lambda">Section 2.2.8, &#8220;Functions&#8221;</a> that functions can be treated like any other data type.
In particular, we can assign it to a variable.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; f = x: x + 1
nix-repl&gt; f
«lambda @ «string»:1:2»</code></pre>
</div>
</div>
<div class="paragraph">
<p>Procedural languages such as C or Java often use parenthesis to apply a function to a value, e.g. <code>f(5)</code>.
Nix, like lambda calculus and most functional languages, does not require parenthesis for function application.
This reduces visual clutter when chaining a series of functions.</p>
</div>
<div class="paragraph">
<p>Now that our function has a name, we can use it.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; f 5
6</code></pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="_multiple_parameters_using_nested_functions">2.12.3. Multiple parameters using nested functions</h4>
<div class="paragraph">
<p>Functions in Nix always have a single parameter.
To define a calculation that requires more than one parameter,
we create functions that return functions!</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; add = a: (b: a+b)</code></pre>
</div>
</div>
<div class="paragraph">
<p>We have created a function called <code>add</code>.
When applied to a parameter <code>a</code>, it returns a new function that adds <code>a</code> to its input.
Note that the expression <code>(b: a+b)</code> is an anonymous function.
We never call it directly, so it doesn&#8217;t need a name.
Anonymous functions are useful after all!</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; add 3 # Returns a function that adds 3 to any input
«lambda @ «string»:1:6»</code></pre>
</div>
</div>
<div class="paragraph">
<p>Now let&#8217;s apply <code>add 3</code> to the value <code>5</code>.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; (add 3) 5
8</code></pre>
</div>
</div>
<div class="paragraph">
<p>In fact, the parentheses aren&#8217;t needed.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; add 3 5
8</code></pre>
</div>
</div>
<div class="paragraph">
<p>In case that wasn&#8217;t clear, let&#8217;s repeat those steps, but in more detail.
The function <code>add</code> takes a single parameter <code>a</code>,
and returns a new function that takes a single parameter <code>b</code>, and returns the value <code>a + b</code>.
Let&#8217;s apply <code>add</code> to the value <code>3</code>, and give the resulting new function a name, <code>g</code>.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; g = add 3</code></pre>
</div>
</div>
<div class="paragraph">
<p>The function <code>g</code> takes a single parameter and adds <code>3</code> to it.
The Nix REPL confirms that <code>g</code> is indeed a function.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; :t g
a function</code></pre>
</div>
</div>
<div class="paragraph">
<p>Now we can apply <code>g</code> to a number to get a new number.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; g 5
8</code></pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="_multiple_parameters_using_attribute_sets">2.12.4. Multiple parameters using attribute sets</h4>
<div class="paragraph">
<p>I said earlier that a function in Nix always has a single parameter.
However, that parameter need not be a simple value; it could be a list or an attribute set.
To specify an attribute set as a parameter, we use a <em>set pattern</em>,
which has the form</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">{ _name1_, _name2_, ... }</pre>
</div>
</div>
<div class="paragraph">
<p>Note that while the key-value associations in attribute sets are separated by semi-colons,
the key names in the attribute <em>set pattern</em> are separated by commas.
Here&#8217;s an example.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; greet = { first, last }: &quot;Hello ${first} ${last}! May I call you ${first}?&quot;
nix-repl&gt; greet { first=&quot;Amy&quot;; last=&quot;de Buitléir&quot;; }
&quot;Hello Amy de Buitléir! May I call you Amy?&quot;</code></pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="_optional_parameters">2.12.5. Optional parameters</h4>
<div class="paragraph">
<p>We can make some values in an attribute set optional by providing default values,
using the syntax <code><em>name</em> ? <em>value</em></code>.
This is illustrated below.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; greet = { first, last ? &quot;whatever-your-lastname-is&quot;, topic ? &quot;Nix&quot; }: &quot;Hello ${first} ${last}! May I call you ${first}? Are you enjoying learning ${topic}?&quot;
nix-repl&gt; greet { first=&quot;Amy&quot;; }
&quot;Hello Amy whatever-your-lastname-is! May I call you Amy? Are you enjoying learning Nix?&quot;
nix-repl&gt; greet { first=&quot;Amy&quot;; topic=&quot;Mathematics&quot;;}
&quot;Hello Amy whatever-your-lastname-is! May I call you Amy? Are you enjoying learning Mathematics?&quot;</code></pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="_patterns">2.12.6. @-patterns</h4>
<div class="paragraph">
<p>TODO</p>
</div>
<div class="paragraph">
<p>TODO</p>
</div>
<div class="paragraph">
<p>TODO</p>
</div>
<div class="paragraph">
<p>TODO</p>
</div>
<div class="paragraph">
<p>TODO</p>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_if_expressions">2.13. If expressions</h3>
<div class="paragraph">
<p>The conditional construct in Nix is an <em>expression</em>, not a <em>statement</em>.
Since expressions must have values in all cases, you must specify both the <code>then</code> and the <code>else</code> branch.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; a = 7
nix-repl&gt; b = 3
nix-repl&gt; if a &gt; b then &quot;yes&quot; else &quot;no&quot;
&quot;yes&quot;</code></pre>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_let_expressions">2.14. Let expressions</h3>
<div class="paragraph">
<p>A <code>let</code> expression defines a value with a local scope.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; let x = 3; in x*x
9
nix-repl&gt; let x = 3; y = 2; in x*x + y
11</code></pre>
</div>
</div>
<div class="paragraph">
<p>You can also nest <code>let</code> expressions.
The previous expression is equivalent to the following.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; let x = 3; in let y = 2; in x*x + y
11</code></pre>
</div>
</div>
<div class="admonitionblock important">
<table>
<tr>
<td class="icon">
<i class="fa icon-important" title="Important"></i>
</td>
<td class="content">
<div class="paragraph">
<p>A variable defined inside a <code>let</code> expression will "shadow" an outer variable with the same name.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; x = 100
nix-repl&gt; let x = 3; in x*x
9
nix-repl&gt; let x = 3; in let x = 7; in x+1
8</code></pre>
</div>
</div>
</td>
</tr>
</table>
</div>
<div class="paragraph">
<p>A variable in a let expression can refer to another variable in the expression.
This is similar to how recursive attribute sets work.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; let x = 3; y = x + 1; in x*y
12</code></pre>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_with_expressions">2.15. With expressions</h3>
<div class="paragraph">
<p>A <code>with</code> expression is somewhat similar to a <code>let</code> expression,
but it brings all of the associations in an attribute set into scope.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; point = { x1 = 3; x2 = 2; }
nix-repl&gt; with point; x1*x1 + x2
11</code></pre>
</div>
</div>
<div class="admonitionblock important">
<table>
<tr>
<td class="icon">
<i class="fa icon-important" title="Important"></i>
</td>
<td class="content">
<div class="paragraph">
<p>Unlike a <code>let</code> expression, a variable defined inside a <code>with</code> expression will <em>not</em>
"shadow" an outer variable with the same name.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; name = &quot;Amy&quot;
nix-repl&gt; animal = { name = &quot;Professor Paws&quot;; age = 10; species = &quot;cat&quot;; }
nix-repl&gt; with animal; &quot;Hello, &quot; + name
&quot;Hello, Amy&quot;</code></pre>
</div>
</div>
<div class="paragraph">
<p>However, you can refer to the variable in the inner scope
using the attribute selection operator (<code>.</code>).</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code><span></span>nix-repl&gt; with animal; &quot;Hello, &quot; + animal.name
&quot;Hello, Professor Paws&quot;</code></pre>
</div>
</div>
</td>
</tr>
</table>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_hello_flake">3. Hello, flake!</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Before learning to write Nix flakes, let’s learn how to use them. I’ve
created a simple example of a flake in this git
<a href="https://codeberg.org/mhwombat/hello-flake">repository</a>. To run this
flake, you don’t need to install anything; simply run the command below.
The first time you use a flake, Nix has to fetch and build it, which
may take time. Subsequent invocations should be instantaneous.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ nix run "git+https://codeberg.org/mhwombat/hello-flake"
Hello from your flake!</pre>
</div>
</div>
<div class="paragraph">
<p>That’s a lot to type every time we want to use this package. Instead, we
can enter a shell with the package available to us, using the
<code>nix shell</code> command.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ nix shell "git+https://codeberg.org/mhwombat/hello-flake"</pre>
</div>
</div>
<div class="paragraph">
<p>In this shell, the command is on our <code>$PATH</code>, so we can execute the
command by name.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ hello-flake
Hello from your flake!</pre>
</div>
</div>
<div class="paragraph">
<p>Nix didn’t <em>install</em> the package; it merely built and placed it in a
directory called the &#8220;Nix store&#8221;. Thus we can have multiple versions
of a package without worrying about conflicts. We can find out the
location of the executable, if we’re curious.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ which hello-flake
/nix/store/qskl8ajlgnl654fhgsmv74yv8x9r3kzg-hello-flake/bin/hello-flake</pre>
</div>
</div>
<div class="paragraph">
<p>Once we exit that shell, the <code>hello-flake</code> command is no longer
directly available.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ exit
$ hello-flake
sh: line 3: hello-flake: command not found</pre>
</div>
</div>
<div class="paragraph">
<p>However, we can still run the command using the store path we found
earlier. That’s not particularly convenient, but it does demonstrate
that the package remains in the store for future use.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">/nix/store/0xbn2hi6h1m5h4kc02vwffs2cydrbc0r-hello-flake/bin/hello-flake</pre>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_the_hello_flake_repo">4. The hello-flake repo</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Let’s clone the repository and see how the flake is defined.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ git clone https://codeberg.org/mhwombat/hello-flake
Cloning into 'hello-flake'...
$ cd hello-flake
$ ls
flake.lock
flake.nix
hello-flake
LICENSE
README.md</pre>
</div>
</div>
<div class="paragraph">
<p>This is a simple repo with just a few files. Like most git repos, it
includes <code>LICENSE</code>, which contains the software license, and <code>README.md</code>
which provides information about the repo.</p>
</div>
<div class="paragraph">
<p>The <code>hello-flake</code> file is the executable we ran earlier.
This particular executable is just a shell script, so we can view it.
It’s an extremly simple script with just two lines.</p>
</div>
<div class="listingblock">
<div class="title">hello-flake</div>
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="bash"><div class="lineno"><table class="linenotable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span>
<span class="normal">2</span>
<span class="normal">3</span></pre></div></td><td class="code"><div><pre><span></span><span class="tok-ch">#!/usr/bin/env sh</span>
<span class="tok-nb">echo</span><span class="tok-w"> </span><span class="tok-s2">&quot;Hello from your flake!&quot;</span>
</pre></div></td></tr></table></div></code></pre>
</div>
</div>
<div class="paragraph">
<p>Now that we have a copy of the repo, we can execute this script
directly.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ ./hello-flake
Hello from your flake!</pre>
</div>
</div>
<div class="paragraph">
<p>Not terribly exciting, I know. But starting with such a simple package
makes it easier to focus on the flake system without getting bogged down
in the details. We’ll make this script a little more interesting later.</p>
</div>
<div class="paragraph">
<p>Let’s look at another file. The file that defines how to package a flake
is always called <code>flake.nix</code>.</p>
</div>
<div class="listingblock">
<div class="title">flake.nix</div>
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="nix"><div class="lineno"><table class="linenotable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal"> 1</span>
<span class="normal"> 2</span>
<span class="normal"> 3</span>
<span class="normal"> 4</span>
<span class="normal"> 5</span>
<span class="normal"> 6</span>
<span class="normal"> 7</span>
<span class="normal"> 8</span>
<span class="normal"> 9</span>
<span class="normal">10</span>
<span class="normal">11</span>
<span class="normal">12</span>
<span class="normal">13</span>
<span class="normal">14</span>
<span class="normal">15</span>
<span class="normal">16</span>
<span class="normal">17</span>
<span class="normal">18</span>
<span class="normal">19</span>
<span class="normal">20</span>
<span class="normal">21</span>
<span class="normal">22</span>
<span class="normal">23</span>
<span class="normal">24</span>
<span class="normal">25</span>
<span class="normal">26</span>
<span class="normal">27</span>
<span class="normal">28</span>
<span class="normal">29</span>
<span class="normal">30</span>
<span class="normal">31</span>
<span class="normal">32</span>
<span class="normal">33</span>
<span class="normal">34</span>
<span class="normal">35</span>
<span class="normal">36</span>
<span class="normal">37</span>
<span class="normal">38</span>
<span class="normal">39</span>
<span class="normal">40</span>
<span class="normal">41</span>
<span class="normal">42</span>
<span class="normal">43</span>
<span class="normal">44</span></pre></div></td><td class="code"><div><pre><span></span><span class="tok-p">{</span>
<span class="tok-c1"># See https://github.com/mhwombat/nix-for-numbskulls/blob/main/flakes.md</span>
<span class="tok-c1"># for a brief overview of what each section in a flake should or can contain.</span>
<span class="tok-ss">description =</span> <span class="tok-s2">&quot;a very simple and friendly flake&quot;</span><span class="tok-p">;</span>
<span class="tok-ss">inputs =</span> <span class="tok-p">{</span>
nixpkgs<span class="tok-o">.</span><span class="tok-ss">url =</span> <span class="tok-s2">&quot;github:NixOS/nixpkgs&quot;</span><span class="tok-p">;</span>
flake-utils<span class="tok-o">.</span><span class="tok-ss">url =</span> <span class="tok-s2">&quot;github:numtide/flake-utils&quot;</span><span class="tok-p">;</span>
<span class="tok-p">};</span>
<span class="tok-ss">outputs =</span> <span class="tok-p">{</span> self<span class="tok-p">,</span> nixpkgs<span class="tok-p">,</span> flake-utils <span class="tok-p">}:</span>
flake-utils<span class="tok-o">.</span>lib<span class="tok-o">.</span>eachDefaultSystem <span class="tok-p">(</span>system<span class="tok-p">:</span>
<span class="tok-k">let</span>
<span class="tok-ss">pkgs =</span> <span class="tok-nb">import</span> nixpkgs <span class="tok-p">{</span> <span class="tok-k">inherit</span> system<span class="tok-p">;</span> <span class="tok-p">};</span>
<span class="tok-k">in</span>
<span class="tok-p">{</span>
<span class="tok-ss">packages =</span> <span class="tok-k">rec</span> <span class="tok-p">{</span>
<span class="tok-ss">hello =</span> pkgs<span class="tok-o">.</span>stdenv<span class="tok-o">.</span>mkDerivation <span class="tok-k">rec</span> <span class="tok-p">{</span>
<span class="tok-ss">name =</span> <span class="tok-s2">&quot;hello-flake&quot;</span><span class="tok-p">;</span>
<span class="tok-ss">src =</span> <span class="tok-o">.</span><span class="tok-l">/.</span><span class="tok-p">;</span>
<span class="tok-ss">unpackPhase =</span> <span class="tok-s2">&quot;true&quot;</span><span class="tok-p">;</span>
<span class="tok-ss">buildPhase =</span> <span class="tok-s2">&quot;:&quot;</span><span class="tok-p">;</span>
<span class="tok-ss">installPhase =</span>
<span class="tok-s1">&#39;&#39;</span>
<span class="tok-s1"> mkdir -p $out/bin</span>
<span class="tok-s1"> cp $src/hello-flake $out/bin/hello-flake</span>
<span class="tok-s1"> chmod +x $out/bin/hello-flake</span>
<span class="tok-s1"> &#39;&#39;</span><span class="tok-p">;</span>
<span class="tok-p">};</span>
<span class="tok-ss">default =</span> hello<span class="tok-p">;</span>
<span class="tok-p">};</span>
<span class="tok-ss">apps =</span> <span class="tok-k">rec</span> <span class="tok-p">{</span>
<span class="tok-ss">hello =</span> flake-utils<span class="tok-o">.</span>lib<span class="tok-o">.</span>mkApp <span class="tok-p">{</span> <span class="tok-ss">drv =</span> self<span class="tok-o">.</span>packages<span class="tok-o">.</span><span class="tok-err">$</span><span class="tok-p">{</span>system<span class="tok-p">}</span><span class="tok-o">.</span>hello<span class="tok-p">;</span> <span class="tok-p">};</span>
<span class="tok-ss">default =</span> hello<span class="tok-p">;</span>
<span class="tok-p">};</span>
<span class="tok-p">}</span>
<span class="tok-p">);</span>
<span class="tok-p">}</span>
</pre></div></td></tr></table></div></code></pre>
</div>
</div>
<div class="paragraph">
<p>If this is your first time seeing a flake definition, it probably looks
intimidating.
Flakes are written in the Nix language, introduced in an earlier chapter.
However, you don&#8217;t really need to know Nix to follow this example.
For now, I’d like to focus on the inputs section.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="nix"><span></span><span class="tok-ss">inputs =</span> <span class="tok-p">{</span>
nixpkgs<span class="tok-o">.</span><span class="tok-ss">url =</span> <span class="tok-s2">&quot;github:NixOS/nixpkgs&quot;</span><span class="tok-p">;</span>
flake-utils<span class="tok-o">.</span><span class="tok-ss">url =</span> <span class="tok-s2">&quot;github:numtide/flake-utils&quot;</span><span class="tok-p">;</span>
<span class="tok-p">};</span></code></pre>
</div>
</div>
<div class="paragraph">
<p>There are just two entries, one for <code>nixpkgs</code> and one for <code>flake-utils</code>.
The first one, <code>nixpkgs</code> refers to the collection of standard software
packages that can be installed with the Nix package manager. The second,
<code>flake-utils</code>, is a collection of utilities that simplify writing
flakes. The important thing to note is that the <code>hello-flake</code> package
<em>depends</em> on <code>nixpkgs</code> and <code>flake-utils</code>.</p>
</div>
<div class="paragraph">
<p>Finally, let’s look at <code>flake.lock</code>, or rather, just part of it.</p>
</div>
<div class="listingblock">
<div class="title">flake.lock</div>
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="linenums"><span></span>{
&quot;nodes&quot;: {
&quot;flake-utils&quot;: {
&quot;inputs&quot;: {
&quot;systems&quot;: &quot;systems&quot;
},
&quot;locked&quot;: {
&quot;lastModified&quot;: 1681202837,
&quot;narHash&quot;: &quot;sha256-H+Rh19JDwRtpVPAWp64F+rlEtxUWBAQW28eAi3SRSzg=&quot;,
&quot;owner&quot;: &quot;numtide&quot;,
&quot;repo&quot;: &quot;flake-utils&quot;,
&quot;rev&quot;: &quot;cfacdce06f30d2b68473a46042957675eebb3401&quot;,
&quot;type&quot;: &quot;github&quot;
},
&quot;original&quot;: {
&quot;owner&quot;: &quot;numtide&quot;,
&quot;repo&quot;: &quot;flake-utils&quot;,
&quot;type&quot;: &quot;github&quot;
}
},
&quot;nixpkgs&quot;: {
&quot;locked&quot;: {
&quot;lastModified&quot;: 1681665000,
&quot;narHash&quot;: &quot;sha256-hDGTR59wC3qrQZFxVi2U3vTY+r02+Okbq080hO1C4Nk=&quot;,
&quot;owner&quot;: &quot;NixOS&quot;,
&quot;repo&quot;: &quot;nixpkgs&quot;,
&quot;rev&quot;: &quot;3a6205d9f79fe526be03d8c465403b118ca4cf37&quot;,
&quot;type&quot;: &quot;github&quot;
},
&quot;original&quot;: {
&quot;owner&quot;: &quot;NixOS&quot;,
&quot;repo&quot;: &quot;nixpkgs&quot;,
&quot;type&quot;: &quot;github&quot;
}
},
&quot;root&quot;: {
&quot;inputs&quot;: {
&quot;flake-utils&quot;: &quot;flake-utils&quot;,
&quot;nixpkgs&quot;: &quot;nixpkgs&quot;
}
. . .</code></pre>
</div>
</div>
<div class="paragraph">
<p>If <code>flake.nix</code> seemed intimidating, then this file looks like an
invocation for Cthulhu. The good news is that this file is automatically
generated; you never need to write it. It contains information about all
of the dependencies for the flake, including where they came from, the
exact version/revision, and hash. This lockfile <em>uniquely</em> specifies all
flake dependencies, (e.g., version number, branch, revision, hash), so
that <em>anyone, anywhere, any time, can re-create the exact same
environment that the original developer used.</em></p>
</div>
<div class="paragraph">
<p>No more complaints of &#8220;but it works on my machine!&#8221;. That is the
benefit of using flakes.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_flake_structure">5. Flake structure</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The basic structure of a flake is shown below.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="nix">{
description = <em>package description</em>
inputs = <em>dependencies</em>
outputs = <em>what the flake produces</em>
nixConfig = <em>advanced configuration options</em>
}</code></pre>
</div>
</div>
<div class="sect2">
<h3 id="_description">5.1. Description</h3>
<div class="paragraph">
<p>The <code>description</code> part is self-explanatory; it’s just a string. You
probably won’t need <code>nixConfig</code> unless you’re doing something fancy. I’m
going to focus on what goes into the <code>inputs</code> and <code>outputs</code> sections,
and highlight some of the things I found confusing when I began using flakes.</p>
</div>
</div>
<div class="sect2">
<h3 id="_inputs">5.2. Inputs</h3>
<div class="paragraph">
<p>This section specifies the dependencies of a flake. It’s an <em>attribute
set</em>; it maps keys to values.</p>
</div>
<div class="paragraph">
<p>To ensure that a build is reproducible, the build step runs in a <em>pure</em>
environment with no network access. Therefore, any external dependencies
must be specified in the &#8220;inputs&#8221; section so they can be fetched in
advance (before we enter the pure environment).</p>
</div>
<div class="paragraph">
<p>Each entry in this section maps an input name to a <em>flake reference</em>.
This commonly takes the following form.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">NAME.url = URL-LIKE-EXPRESSION</pre>
</div>
</div>
<div class="paragraph">
<p>As a first example of a flake reference, all (almost all?) flakes depend on &#8220;nixpkgs&#8221;,
which is a large Git repository of programs and libraries that are
pre-packaged for Nix. We can write that as</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="nix">nixpkgs.url = "github:NixOS/nixpkgs/nixos-<em>version</em>";</code></pre>
</div>
</div>
<div class="paragraph">
<p>where <em>version</em> is replaced with the version number that you used to build
the package, e.g. <code>22.11</code>. Information about the latest nixpkgs releases
is available at <a href="https://status.nixos.org/" class="bare">https://status.nixos.org/</a>. You can also write the entry
without the version number</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="nix"><span></span>nixpkgs<span class="tok-o">.</span><span class="tok-ss">url =</span> <span class="tok-s2">&quot;github:NixOS/nixpkgs/nixos&quot;</span><span class="tok-p">;</span></code></pre>
</div>
</div>
<div class="paragraph">
<p>or more simply,</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="nix"><span></span>nixpkgs<span class="tok-o">.</span><span class="tok-ss">url =</span> <span class="tok-s2">&quot;nixpkgs&quot;</span><span class="tok-p">;</span></code></pre>
</div>
</div>
<div class="paragraph">
<p>You might be concerned that omitting the version number would make the
build non-reproducible. If someone else builds the flake, could they end
up with a different version of nixpkgs? No! remember that the lockfile
(<code>flake.lock</code>) <em>uniquely</em> specifies all flake inputs.</p>
</div>
<div class="paragraph">
<p>Git and Mercurial repositories are the most common type of flake
reference, as in the examples below.</p>
</div>
<div class="dlist">
<dl>
<dt class="hdlist1">A Git repository</dt>
<dd>
<p><code>git+https://github.com/NixOS/patchelf</code></p>
</dd>
<dt class="hdlist1">A specific branch of a Git repository</dt>
<dd>
<p><code>git+https://github.com/NixOS/patchelf?ref=master</code></p>
</dd>
<dt class="hdlist1">A specific revision of a Git repository</dt>
<dd>
<p><br>
<code>git+https://github.com/NixOS/patchelf?ref=master&amp;rev=f34751b88bd07d7f44f5cd3200fb4122bf916c7e</code></p>
</dd>
<dt class="hdlist1">A tarball</dt>
<dd>
<p><code><a href="https://github.com/NixOS/patchelf/archive/master.tar.gz" class="bare">https://github.com/NixOS/patchelf/archive/master.tar.gz</a></code></p>
</dd>
</dl>
</div>
<div class="paragraph">
<p>You can find more examples of flake references in the
<a href="https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-flake.html#examples">Nix
Reference Manual</a>.</p>
</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<i class="fa icon-note" title="Note"></i>
</td>
<td class="content">
<div class="paragraph">
<p>Although you probably won’t need to use it, there is another syntax for
flake references that you might encounter. This example</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="nix"><span></span>inputs<span class="tok-o">.</span><span class="tok-nb">import</span><span class="tok-ss">-cargo =</span> <span class="tok-p">{</span>
<span class="tok-ss">type =</span> <span class="tok-s2">&quot;github&quot;</span><span class="tok-p">;</span>
<span class="tok-ss">owner =</span> <span class="tok-s2">&quot;edolstra&quot;</span><span class="tok-p">;</span>
<span class="tok-ss">repo =</span> <span class="tok-s2">&quot;import-cargo&quot;</span><span class="tok-p">;</span>
<span class="tok-p">};</span></code></pre>
</div>
</div>
<div class="paragraph">
<p>is equivalent to</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="nix"><span></span>inputs<span class="tok-o">.</span><span class="tok-nb">import</span><span class="tok-err">-</span>cargo<span class="tok-o">.</span><span class="tok-ss">url =</span> <span class="tok-s2">&quot;github:edolstra/import-cargo&quot;</span><span class="tok-p">;</span></code></pre>
</div>
</div>
</td>
</tr>
</table>
</div>
<div class="paragraph">
<p>Each of the <code>inputs</code> is fetched, evaluated and passed to the <code>outputs</code>
function as a set of attributes with the same name as the corresponding
input.</p>
</div>
</div>
<div class="sect2">
<h3 id="_outputs">5.3. Outputs</h3>
<div class="paragraph">
<p>This section is a function that essentially returns the recipe for
building the flake.</p>
</div>
<div class="paragraph">
<p>We said above that <code>inputs</code> are passed to the <code>outputs</code>, so we need to
list them as parameters. This example references the <code>import-cargo</code>
dependency defined in the previous example.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="nix">outputs = { self, nixpkgs, import-cargo }: {
<mark><em>definitions for outputs</em></mark>
};</code></pre>
</div>
</div>
<div class="paragraph">
<p>So what actually goes in the highlighted section?
That depends on the programming languages your software is written in,
the build system you use, and more. There are Nix functions and tools
that can simplify much of this, and new, easier-to-use ones are released
regularly. We’ll look at some of these in the next section.</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_a_generic_flake">6. A generic flake</h2>
<div class="sectionbody">
<div class="paragraph">
<p>The previous section presented a very high-level view of flakes,
focusing on the basic structure. In this section, we will add a bit more
detail.</p>
</div>
<div class="paragraph">
<p>Flakes are written in the Nix programming language, which is a
functional language. As with most programming languages, there are many
ways to achieve the same result. Below is an example you can follow when
writing your own flakes. I’ll explain the example in some detail.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="pygments highlight nowrap"><code>{
description = "<mark><em>brief package description</em></mark>";
inputs = {
<span class="highlight01">nixpkgs</span>.url = "github:NixOS/nixpkgs";
<span class="highlight02">flake-utils</span>.url = "github:numtide/flake-utils";
<span class="highlight03">...<em>other dependencies</em>...</span> ❶
};
outputs = { self, <span class="highlight01">nixpkgs</span>, <span class="highlight02">flake-utils</span>, <span class="highlight03">...<em>other dependencies</em>...</span> ❷ }:
flake-utils.lib.eachDefaultSystem (system: ❸
let
pkgs = import nixpkgs { inherit system; };
python = pkgs.python3;
in
{
devShells = rec {
default = pkgs.mkShell {
packages = [ <mark><em>packages needed for development shell</em></mark>; ❹ ]))
];
};
packages = rec {
<span class="highlight04">myPackageName</span> = <mark><em>package definition</em></mark>; ❺
default = <span class="highlight04">myPackageName</span>;
};
apps = rec {
<span class="highlight04">myPackageName</span> = flake-utils.lib.mkApp { drv = self.packages.${system}.<span class="highlight04">myPackageName</span>; };
default = <span class="highlight04">myPackageName</span>;
};
}
);
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>We discussed how to specify flake inputs <code>❶</code> in the previous section, so
this part of the flake should be familiar. Remember also that any
dependencies in the input section should also be listed at the beginning
of the outputs section <code>❷</code>.</p>
</div>
<div class="paragraph">
<p>Now it’s time to look at the content of the output section. If we want
the package to be available for multiple systems (e.g.,
&#8220;x86_64-linux&#8221;, &#8220;aarch64-linux&#8221;, &#8220;x86_64-darwin&#8221;, and
&#8220;aarch64-darwin&#8221;), we need to define the output for each of those
systems. Often the definitions are identical, apart from the name of the
system. The eachDefaultSystem function <code>❸</code> provided by flake-utils allows
us to write a single definition using a variable for the system name.
The function then iterates over all default systems to generate the
outputs for each one.</p>
</div>
<div class="paragraph">
<p>The <code>devShells</code> variable specifies the environment that should be
available when doing development on the package. If you don’t need a
special development environment, you can omit this section. At <code>❹</code> you
would list any tools (e.g., compilers and language-specific build tools)
you want to have available in a development shell. If the compiler needs
access to language-specific packages, there are Nix functions to assist
with that. These functions are very language-specific, and not always
well-documented. We will see examples for some languages later in the
tutorial. In general, I recommend that you do a web search for
&#8220;nix language&#8221;, and try to find resources that were written or updated
recently.</p>
</div>
<div class="paragraph">
<p>The <code>packages</code> variable defines the packages that this flake provides.
The package definition <code>❺</code> depends on the programming languages your
software is written in, the build system you use, and more. There are
Nix functions and tools that can simplify much of this, and new,
easier-to-use ones are released regularly. Again, I recommend that you
do a web search for &#8220;nix language&#8221;, and try to find resources that
were written or updated recently.</p>
</div>
<div class="paragraph">
<p>The <code>apps</code> variable identifies any applications provided by the flake.
In particular, it identifies the default executable ❻ that <code>nix run</code>
will run if you don’t specify an app.</p>
</div>
<div class="paragraph">
<p>Below is a list of some functions that are commonly used in
this section.</p>
</div>
<div class="dlist">
<dl>
<dt class="hdlist1">General-purpose</dt>
<dd>
<p>The standard environment provides <code>mkDerivation</code>, which is especially
useful for the typical <code>./configure; make; make install</code> scenario.
It’s customisable.</p>
</dd>
<dt class="hdlist1">Python</dt>
<dd>
<p><code>buildPythonApplication</code>, <code>buildPythonPackage</code>.</p>
</dd>
<dt class="hdlist1">Haskell</dt>
<dd>
<p><code>mkDerivation</code> (Haskell version, which is a wrapper around the
standard environment version), <code>developPackage</code>, <code>callCabal2Nix</code>.</p>
</dd>
</dl>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_another_look_at_hello_flake">7. Another look at hello-flake</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Now that we have a better understanding of the structure of <code>flake.nix</code>,
let’s have a look at the one we saw earlier, in the <code>hello-flake</code> repo.
If you compare this flake definition to the colour-coded template
presented in the previous section, most of it should look familiar.</p>
</div>
<div class="literalblock">
<div class="title">flake.nix</div>
<div class="content">
<pre class="nowrap">{
description = "a very simple and friendly flake";
inputs = {
nixpkgs.url = "github:NixOS/nixpkgs";
flake-utils.url = "github:numtide/flake-utils";
};
outputs = { self, nixpkgs, flake-utils }:
flake-utils.lib.eachDefaultSystem (system:
let
pkgs = import nixpkgs { inherit system; };
in
{
packages = rec {
hello =
. . .
<em>SOME UNFAMILIAR STUFF</em>
. . .
};
default = hello;
};
apps = rec {
hello = flake-utils.lib.mkApp { drv = self.packages.${system}.hello; };
default = hello;
};
}
);
}</pre>
</div>
</div>
<div class="paragraph">
<p>This <code>flake.nix</code> doesn’t have a <code>devShells</code> section, because development
on the current version doesn’t require anything beyond
the &#8220;bare bones&#8221; linux commands. Later we will add a feature that requires
additional development tools.</p>
</div>
<div class="paragraph">
<p>Now let’s look at the section I labeled <code>SOME UNFAMILIAR STUFF</code> and
see what it does.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap"> packages = rec {
hello = pkgs.stdenv.mkDerivation rec { ❶
name = "hello-flake";
src = ./.; ❷
unpackPhase = "true";
buildPhase = ":";
installPhase =
''
mkdir -p $out/bin ❸
cp $src/hello-flake $out/bin/hello-flake ❹
chmod +x $out/bin/hello-flake ❺
'';
};</pre>
</div>
</div>
<div class="paragraph">
<p>This flake uses <code>mkDerivation</code> <code>❶</code> which is a very useful
general-purpose package builder provided by the Nix standard
environment. It’s especially useful for the typical
<code>./configure; make; make install</code> scenario, but for this flake we don’t
even need that.</p>
</div>
<div class="paragraph">
<p>The <code>name</code> variable is the name of the flake, as it would appear in a
package listing if we were to add it to Nixpkgs or another package
collection. The <code>src</code> variable <code>❷</code> supplies the location of the source
files, relative to <code>flake.nix</code>. When a flake is accessed for the first
time, the repository contents are fetched in the form of a tarball. The
<code>unpackPhase</code> variable indicates that we do want the tarball to be
unpacked.</p>
</div>
<div class="paragraph">
<p>The <code>buildPhase</code> variable is a sequence of Linux commands to build the
package. Typically, building a package requires compiling the source
code. However, that’s not required for a simple shell script. So
<code>buildPhase</code> consists of a single command, <code>:</code>,
which is a no-op or &#8220;do nothing&#8221; command.</p>
</div>
<div class="paragraph">
<p>The <code>installPhase</code> variable is a sequence of Linux commands that will do
the actual installation. In this case, we create a directory <code>❸</code> for the
installation, copy the <code>hello-flake</code> script there <code>❹</code>, and make the
script executable <code>❺</code>. The environment variable <code>$src</code> refers to the
source directory, which we specified earlier <code>❷</code>.</p>
</div>
<div class="paragraph">
<p>Earlier we said that the build step runs in a pure environment to ensure
that builds are reproducible. This means no Internet access; indeed no
access to any files outside the build directory. During the build and
install phases, the only commands available are those provided by the
Nix standard environment and the external dependencies identified in the
<code>inputs</code> section of the flake.</p>
</div>
<div class="paragraph">
<p>I’ve mentioned the Nix standard environment before, but I didn’t explain
what it is. The standard environment, or <code>stdenv</code>, refers to the
functionality that is available during the build and install phases of a
Nix package (or flake). It includes the commands listed
below<sup class="footnote">[<a id="_footnoteref_2" class="footnote" href="#_footnotedef_2" title="View footnote.">2</a>]</sup>.</p>
</div>
<div class="ulist">
<ul>
<li>
<p>The GNU C Compiler, configured with C and C++ support.</p>
</li>
<li>
<p>GNU coreutils (contains a few dozen standard Unix commands).</p>
</li>
<li>
<p>GNU findutils (contains find).</p>
</li>
<li>
<p>GNU diffutils (contains diff, cmp).</p>
</li>
<li>
<p>GNU sed.</p>
</li>
<li>
<p>GNU grep.</p>
</li>
<li>
<p>GNU awk.</p>
</li>
<li>
<p>GNU tar.</p>
</li>
<li>
<p>gzip, bzip2 and xz.</p>
</li>
<li>
<p>GNU Make.</p>
</li>
<li>
<p>Bash.</p>
</li>
<li>
<p>The patch command.</p>
</li>
<li>
<p>On Linux, stdenv also includes the patchelf utility.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>Only a few environment variables are available. The most interesting
ones are listed below.</p>
</div>
<div class="ulist">
<ul>
<li>
<p><code>$name</code> is the package name.</p>
</li>
<li>
<p><code>$src</code> refers to the source directory.</p>
</li>
<li>
<p><code>$out</code> is the path to the location in the Nix store where the package
will be added.</p>
</li>
<li>
<p><code>$system</code> is the system that the package is being built for.</p>
</li>
<li>
<p><code>$PWD</code> and <code>$TMP</code> both point to a temporary build directories</p>
</li>
<li>
<p><code>$HOME</code> and <code>$PATH</code> point to nonexistent directories, so the build
cannot rely on them.</p>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_modifying_the_flake">8. Modifying the flake</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="_the_nix_development_shell">8.1. The Nix development shell</h3>
<div class="paragraph">
<p>Let’s make a simple modification to the script. This will give you an
opportunity to check your understanding of flakes.</p>
</div>
<div class="paragraph">
<p>The first step is to enter a development shell.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ nix develop</pre>
</div>
</div>
<div class="paragraph">
<p>The <code>flake.nix</code> file specifies all of the tools that are needed during
development of the package. The <code>nix develop</code> command puts us in a shell
with those tools. As it turns out, we didn’t need any extra tools
(beyond the standard environment) for development yet, but that’s
usually not the case. Also, we will soon need another tool.</p>
</div>
<div class="paragraph">
<p>A development environment only allows you to <em>develop</em> the package.
Don’t expect the package <em>outputs</em> (e.g. executables) to be available
until you build them. However, our script doesn’t need to be compiled,
so can’t we just run it?</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ hello-flake
bash: line 16: hello-flake: command not found</pre>
</div>
</div>
<div class="paragraph">
<p>That worked before; why isn’t it working now? Earlier we used
<code>nix shell</code> to enter a <em>runtime</em> environment where <code>hello-flake</code> was
available and on the <code>$PATH</code>. This time we entered a <em>development</em>
environment using the <code>nix develop</code> command. Since the flake hasn’t been
built yet, the executable won’t be on the <code>$PATH</code>. We can, however, run
it by specifying the path to the script.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ ./hello-flake
Hello from your flake!</pre>
</div>
</div>
<div class="paragraph">
<p>We can also build the flake using the <code>nix build</code> command, which places
the build outputs in a directory called <code>result</code>.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ nix build
$ result/bin/hello-flake
Hello from your flake!</pre>
</div>
</div>
<div class="paragraph">
<p>Rather than typing the full path to the executable, it’s more convenient
to use <code>nix run</code>.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ nix run
Hello from your flake!</pre>
</div>
</div>
<div class="paragraph">
<p>Here’s a summary of the more common Nix commands.</p>
</div>
<table class="tableblock frame-all grid-all stretch">
<colgroup>
<col style="width: 17%;">
<col style="width: 83%;">
</colgroup>
<thead>
<tr>
<th class="tableblock halign-left valign-top">command</th>
<th class="tableblock halign-left valign-top">Action</th>
</tr>
</thead>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>nix develop</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Enters a <em>development</em> shell with all the required
development tools (e.g. compilers and linkers) available (as specified
by <code>flake.nix</code>).</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>nix shell</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Enters a <em>runtime</em> shell where the flake’s executables are
available on the <code>$PATH</code>.</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>nix build</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Builds the flake and puts the output in a directory called
<code>result</code>.</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>nix run</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Runs the flake’s default executable, rebuilding the package
first if needed. Specifically, it runs the version in the Nix store, not
the version in <code>result</code>.</p></td>
</tr>
</tbody>
</table>
</div>
<div class="sect2">
<h3 id="_introducing_a_dependency">8.2. Introducing a dependency</h3>
<div class="paragraph">
<p>Now we’re ready to make the flake a little more interesting.
Instead of using the <code>echo</code> command in the script, we can use the Linux <code>cowsay</code>
command.
Here&#8217;s the <code>hello-flake</code> file, with the modified line highlighted.</p>
</div>
<div class="listingblock">
<div class="title">hello-flake</div>
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="nix"><span></span><span class="tok-c1">#!/usr/bin/env sh</span>
cowsay <span class="tok-s2">&quot;Hello from your flake!&quot;</span></code></pre>
</div>
</div>
<div class="paragraph">
<p>Let’s test the modified script.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ ./hello-flake
./hello-flake: line 3: cowsay: command not found</pre>
</div>
</div>
<div class="paragraph">
<p>What went wrong? Remember that we are in a <em>development</em> shell. Since
<code>flake.nix</code> didn’t define the <code>devShells</code> variable, the development
shell only includes the Nix standard environment. In particular, the
<code>cowsay</code> command is not available.</p>
</div>
<div class="paragraph">
<p>To fix the problem, we can modify <code>flake.nix</code>.
We don’t need to add <code>cowsay</code> to the <code>inputs</code> section because it’s included in <code>nixpkgs</code>,
which is already an input.
However, we also want it to be available in a develoment shell.
The highlighted modifications below will accomplish that.</p>
</div>
<div class="listingblock">
<div class="title">flake.nix</div>
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="nix"><span></span><span class="tok-p">{</span>
<span class="tok-c1"># See https://github.com/mhwombat/nix-for-numbskulls/blob/main/flakes.md</span>
<span class="tok-c1"># for a brief overview of what each section in a flake should or can contain.</span>
<span class="tok-ss">description =</span> <span class="tok-s2">&quot;a very simple and friendly flake&quot;</span><span class="tok-p">;</span>
<span class="tok-ss">inputs =</span> <span class="tok-p">{</span>
nixpkgs<span class="tok-o">.</span><span class="tok-ss">url =</span> <span class="tok-s2">&quot;github:NixOS/nixpkgs&quot;</span><span class="tok-p">;</span>
flake-utils<span class="tok-o">.</span><span class="tok-ss">url =</span> <span class="tok-s2">&quot;github:numtide/flake-utils&quot;</span><span class="tok-p">;</span>
<span class="tok-p">};</span>
<span class="tok-ss">outputs =</span> <span class="tok-p">{</span> self<span class="tok-p">,</span> nixpkgs<span class="tok-p">,</span> flake-utils <span class="tok-p">}:</span>
flake-utils<span class="tok-o">.</span>lib<span class="tok-o">.</span>eachDefaultSystem <span class="tok-p">(</span>system<span class="tok-p">:</span>
<span class="tok-k">let</span>
<span class="tok-ss">pkgs =</span> <span class="tok-nb">import</span> nixpkgs <span class="tok-p">{</span> <span class="tok-k">inherit</span> system<span class="tok-p">;</span> <span class="tok-p">};</span>
<span class="tok-k">in</span>
<span class="tok-p">{</span>
<span class="hll"> <span class="tok-ss">devShells =</span> <span class="tok-k">rec</span> <span class="tok-p">{</span>
</span><span class="hll"> <span class="tok-ss">default =</span> pkgs<span class="tok-o">.</span>mkShell <span class="tok-p">{</span>
</span><span class="hll"> <span class="tok-ss">packages =</span> <span class="tok-p">[</span> pkgs<span class="tok-o">.</span>cowsay <span class="tok-p">];</span>
</span><span class="hll"> <span class="tok-p">};</span>
</span><span class="hll"> <span class="tok-p">};</span>
</span>
<span class="tok-ss">packages =</span> <span class="tok-k">rec</span> <span class="tok-p">{</span>
<span class="tok-ss">hello =</span> pkgs<span class="tok-o">.</span>stdenv<span class="tok-o">.</span>mkDerivation <span class="tok-k">rec</span> <span class="tok-p">{</span>
<span class="tok-ss">name =</span> <span class="tok-s2">&quot;hello-flake&quot;</span><span class="tok-p">;</span>
<span class="tok-ss">src =</span> <span class="tok-o">.</span><span class="tok-l">/.</span><span class="tok-p">;</span>
<span class="tok-ss">unpackPhase =</span> <span class="tok-s2">&quot;true&quot;</span><span class="tok-p">;</span>
<span class="tok-ss">buildPhase =</span> <span class="tok-s2">&quot;:&quot;</span><span class="tok-p">;</span>
<span class="tok-ss">installPhase =</span>
<span class="tok-s1">&#39;&#39;</span>
<span class="tok-s1"> mkdir -p $out/bin</span>
<span class="tok-s1"> cp $src/hello-flake $out/bin/hello-flake</span>
<span class="tok-s1"> chmod +x $out/bin/hello-flake</span>
<span class="tok-s1"> &#39;&#39;</span><span class="tok-p">;</span>
<span class="tok-p">};</span>
<span class="tok-ss">default =</span> hello<span class="tok-p">;</span>
<span class="tok-p">};</span>
<span class="tok-ss">apps =</span> <span class="tok-k">rec</span> <span class="tok-p">{</span>
<span class="tok-ss">hello =</span> flake-utils<span class="tok-o">.</span>lib<span class="tok-o">.</span>mkApp <span class="tok-p">{</span> <span class="tok-ss">drv =</span> self<span class="tok-o">.</span>packages<span class="tok-o">.</span><span class="tok-err">$</span><span class="tok-p">{</span>system<span class="tok-p">}</span><span class="tok-o">.</span>hello<span class="tok-p">;</span> <span class="tok-p">};</span>
<span class="tok-ss">default =</span> hello<span class="tok-p">;</span>
<span class="tok-p">};</span>
<span class="tok-p">}</span>
<span class="tok-p">);</span>
<span class="tok-p">}</span></code></pre>
</div>
</div>
<div class="paragraph">
<p>Now we restart the development shell and see that the <code>cowsay</code> command is
available and the script works. Because we’ve updated source files
but haven’t <code>git commit</code>ed the new version, we get a warning message
about it being &#8220;dirty&#8221;. It’s just a warning, though; the script runs
correctly.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ nix develop
warning: Git tree '/home/amy/codeberg/nix-book/source/modify-hello-flake/hello-flake' is dirty
$ which cowsay # is it available now?
/nix/store/gfi27h4y5n4aralcxrc0377p8mjb1cvb-cowsay-3.7.0/bin/cowsay
$ ./hello-flake
________________________
&lt; Hello from your flake! &gt;
------------------------
\ ^__^
\ (oo)\_______
(__)\ )\/\
||----w |
|| ||</pre>
</div>
</div>
<div class="paragraph">
<p>Alternatively, we could use <code>nix run</code>.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ nix run
warning: Git tree '/home/amy/codeberg/nix-book/source/modify-hello-flake/hello-flake' is dirty
________________________
&lt; Hello from your flake! &gt;
------------------------
\ ^__^
\ (oo)\_______
(__)\ )\/\
||----w |
|| ||</pre>
</div>
</div>
<div class="paragraph">
<p>Note, however, that <code>nix run</code> rebuilt the package in the Nix store and
ran <em>that</em>. It did not alter the copy in the <code>result</code> directory, as
we’ll see next.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ cat result/bin/hello-flake
#!/nix/store/zlf0f88vj30sc7567b80l52d19pbdmy2-bash-5.2-p15/bin/sh
echo "Hello from your flake!"</pre>
</div>
</div>
<div class="paragraph">
<p>If we want to update the version in <code>result</code>, we need <code>nix build</code> again.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ nix build
warning: Git tree '/home/amy/codeberg/nix-book/source/modify-hello-flake/hello-flake' is dirty
$ cat result/bin/hello-flake
#!/nix/store/zlf0f88vj30sc7567b80l52d19pbdmy2-bash-5.2-p15/bin/sh
cowsay "Hello from your flake!"</pre>
</div>
</div>
<div class="paragraph">
<p>Let’s <code>git commit</code> the changes and verify that the warning goes away. We
don’t need to <code>git push</code> the changes until we’re ready to share them.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ git commit hello-flake flake.nix -m 'added bovine feature'
[main eb4d08c] added bovine feature
2 files changed, 7 insertions(+), 1 deletion(-)
$ nix run
________________________
&lt; Hello from your flake! &gt;
------------------------
\ ^__^
\ (oo)\_______
(__)\ )\/\
||----w |
|| ||</pre>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_development_workflows">8.3. Development workflows</h3>
<div class="paragraph">
<p>If you’re getting confused about when to use the different commands,
it’s because there’s more than one way to use Nix. I tend to think of it
as two different development workflows.</p>
</div>
<div class="paragraph">
<p>My usual, <em>high-level workflow</em> is quite simple.</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p><code>nix run</code> to re-build (if necessary) and run the executable.</p>
</li>
<li>
<p>Fix any problems in <code>flake.nix</code> or the source code.</p>
</li>
<li>
<p>Repeat until the package works properly.</p>
</li>
</ol>
</div>
<div class="paragraph">
<p>In the high-level workflow, I don’t use a development shell because I
don’t need to directly invoke development tools such as compilers and
linkers. Nix invokes them for me according to the output definition in
<code>flake.nix</code>.</p>
</div>
<div class="paragraph">
<p>Occasionally I want to work at a lower level, and invoke compiler,
linkers, etc. directly. Perhaps want to work on one component without
rebuilding the entire package. Or perhaps I’m confused by some error
message, so I want to temporarily bypass Nix and work directly with
the compiler. In this case I temporarily switch to a <em>low-level
workflow</em>.</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p><code>nix develop</code> to enter a development shell with any development tools
I need (e.g. compilers, linkers, documentation generators).</p>
</li>
<li>
<p>Directly invoke tools such as compilers.</p>
</li>
<li>
<p>Fix any problems in <code>flake.nix</code> or the source code.</p>
</li>
<li>
<p>Directly invoke the executable. Note that the location of the
executable depends on the development tools – It probably isn’t
<code>result</code>!</p>
</li>
<li>
<p>Repeat until the package works properly.</p>
</li>
</ol>
</div>
<div class="paragraph">
<p>I generally only use <code>nix build</code> if I just want to build the package but
not execute anything (perhaps it’s just a library).</p>
</div>
</div>
<div class="sect2">
<h3 id="_this_all_seems_like_a_hassle">8.4. This all seems like a hassle!</h3>
<div class="paragraph">
<p>It is a bit annoying to modify <code>flake.nix</code> and ether rebuild or reload
the development environment every time you need another tool. However,
this Nix way of doing things ensures that all of your dependencies, down
to the exact versions, are captured in <code>flake.lock</code>, and that anyone
else will be able to reproduce the development environment.</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_a_new_flake_from_scratch">9. A new flake from scratch</h2>
<div class="sectionbody">
<div class="paragraph">
<p>At last we are ready to create a flake from scratch!
The sections in this chapter are very similar;
read the one for your language of choice.
If you&#8217;re interested in a language that I haven&#8217;t covered, feel free to suggest it by creating an
<a href="https://codeberg.org/mhwombat/nix-book/issues">issue</a>.</p>
</div>
<div class="sect2">
<h3 id="_haskell">9.1. Haskell</h3>
<div class="paragraph">
<p>Start with an empty directory and create a git repository.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ mkdir hello-haskell
$ cd hello-haskell
$ git init
Initialized empty Git repository in /home/amy/codeberg/nix-book/source/new-flake/haskell-flake/hello-haskell/.git/</pre>
</div>
</div>
<div class="sect3">
<h4 id="_a_simple_haskell_program">9.1.1. A simple Haskell program</h4>
<div class="paragraph">
<p>Next, we’ll create a simple Haskell program.</p>
</div>
<div class="listingblock">
<div class="title">Main.hs</div>
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="haskell"><div class="lineno"><table class="linenotable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span>
<span class="normal">2</span>
<span class="normal">3</span>
<span class="normal">4</span>
<span class="normal">5</span>
<span class="normal">6</span>
<span class="normal">7</span></pre></div></td><td class="code"><div><pre><span></span><span class="tok-kr">import</span><span class="tok-w"> </span><span class="tok-nn">Network.HostName</span>
<span class="tok-nf">main</span><span class="tok-w"> </span><span class="tok-ow">::</span><span class="tok-w"> </span><span class="tok-kt">IO</span><span class="tok-w"> </span><span class="tok-nb">()</span>
<span class="tok-nf">main</span><span class="tok-w"> </span><span class="tok-ow">=</span><span class="tok-w"> </span><span class="tok-kr">do</span>
<span class="tok-w"> </span><span class="tok-n">putStrLn</span><span class="tok-w"> </span><span class="tok-s">&quot;Hello from Haskell inside a Nix flake!&quot;</span>
<span class="tok-w"> </span><span class="tok-n">h</span><span class="tok-w"> </span><span class="tok-ow">&lt;-</span><span class="tok-w"> </span><span class="tok-n">getHostName</span>
<span class="tok-w"> </span><span class="tok-n">putStrLn</span><span class="tok-w"> </span><span class="tok-o">$</span><span class="tok-w"> </span><span class="tok-s">&quot;Your hostname is: &quot;</span><span class="tok-w"> </span><span class="tok-o">++</span><span class="tok-w"> </span><span class="tok-n">h</span>
</pre></div></td></tr></table></div></code></pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="_optional_testing_before_packaging">9.1.2. (Optional) Testing before packaging</h4>
<div class="paragraph">
<p>Before we package the program, let’s verify that it runs. We’re going to
need a Haskell compiler. By now you’ve probably figured out that we can write a
<code>flake.nix</code> and define a development shell that includes Haskell. We’ll
do that shortly, but first I want to show you a handy shortcut. We can
lauch a <em>temporary</em> shell with any Nix packages we want. This is
convenient when you just want to try out some new software and you’re
not sure if you’ll use it again. It’s also convenient when you’re not
ready to write <code>flake.nix</code> (perhaps you’re not sure what tools and
packages you need), and you want to experiment a bit first.</p>
</div>
<div class="paragraph">
<p>The command to enter a temporary shell is</p>
</div>
<div class="paragraph">
<p><code>nix-shell -p <em>packages</em></code></p>
</div>
<div class="paragraph">
<p>If there are multiple packages, they should be separated by spaces.</p>
</div>
<div class="admonitionblock important">
<table>
<tr>
<td class="icon">
<i class="fa icon-important" title="Important"></i>
</td>
<td class="content">
<div class="paragraph">
<p>The command used here is <code>nix-shell</code> with a hyphen, not <code>nix shell</code>
with a space; those are two different commands. In fact there are
hyphenated and non-hyphenated versions of many Nix commands, and yes,
it’s confusing. The non-hyphenated commands were introduced when support
for flakes was added to Nix. I predict that eventually all hyphenated
commands will be replaced with non-hyphenated versions. Until then, a
useful rule of thumb is that non-hyphenated commands are for for working
directly with flakes; hyphenated commands are for everything else.</p>
</div>
</td>
</tr>
</table>
</div>
<div class="sect4">
<h5 id="_some_unsuitable_shells">Some unsuitable shells</h5>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<i class="fa icon-note" title="Note"></i>
</td>
<td class="content">
<div class="paragraph">
<p>In this section, we will try commands that fail in subtle ways.
Examining these failures will give you a much better understanding of Haskell development with Nix,
and help you avoid (or at least diagnose) similar problems in future.
If you&#8217;re impatient, you can skip to the next section to see the right way to do it.
You can come back to this section later to learn more.</p>
</div>
</td>
</tr>
</table>
</div>
<div class="paragraph">
<p>Let’s enter a shell with the Glasgow Haskell Compiler ("ghc") and try to run the program.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ nix-shell -p ghc
$ runghc Main.hs
Main.hs:1:1: error:
Could not find module ‘Network.HostName’
Use -v (or `:set -v` in ghci) to see a list of the files searched for.
|
1 | import Network.HostName
| ^^^^^^^^^^^^^^^^^^^^^^^</pre>
</div>
</div>
<div class="paragraph">
<p>The error message tells us that we need the module <code>Network.HostName</code>.
That module is provided by the Haskell package called <code>hostname</code>.
Let&#8217;s exit that shell and try again, this time adding the <code>hostname</code> package.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ exit
$ nix-shell -p "[ghc hostname]"
$ runghc Main.hs
Main.hs:1:1: error:
Could not find module ‘Network.HostName’
Use -v (or `:set -v` in ghci) to see a list of the files searched for.
|
1 | import Network.HostName
| ^^^^^^^^^^^^^^^^^^^^^^^</pre>
</div>
</div>
<div class="paragraph">
<p>That reason that failed is that we asked for the wrong package.
The Nix package <code>hostname</code> isn&#8217;t the Haskell package we wanted,
it&#8217;s a different package entirely (an alias for <code>hostname-net-tools</code>.)
The package we want is in the <em>package set</em> called <code>haskellPackages</code>, so we can refer to it as <code>haskellPackages.hostname</code>.</p>
</div>
<div class="paragraph">
<p>Let&#8217;s try that again, with the correct package.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ exit
$ nix-shell -p "[ghc haskellPackages.hostname]"
$ runghc Main.hs
Main.hs:1:1: error:
Could not find module ‘Network.HostName’
Use -v (or `:set -v` in ghci) to see a list of the files searched for.
|
1 | import Network.HostName
| ^^^^^^^^^^^^^^^^^^^^^^^</pre>
</div>
</div>
<div class="paragraph">
<p>Now what&#8217;s wrong?
The syntax we used in the <code>nix-shell</code> command above is fine, but it doesn&#8217;t make the package <em>available to GHC</em>!</p>
</div>
</div>
<div class="sect4">
<h5 id="_a_suitable_shell_for_a_quick_test">A suitable shell for a quick test</h5>
<div class="paragraph">
<p>Consider the Haskell "pandoc" package, which provides both an executable (the Nix package <code>pandoc</code>)
and a library (the Nix package <code>haskellPackages.pandoc</code>).
There are several different shells we could create involving both Pandoc and GHC,
and it&#8217;s important to understand the differences between them.</p>
</div>
<table class="tableblock frame-all grid-all stretch">
<colgroup>
<col style="width: 50%;">
<col style="width: 50%;">
</colgroup>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>nix-shell -p "[ghc pandoc]"</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Makes the Pandoc <em>executable</em> available at the command line, but the <em>library</em> won&#8217;t be visible to GHC.</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>nix-shell -p "haskellPackages.ghcWithPackages (pkgs: with pkgs; [ pandoc ])"</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Makes the Pandoc <em>library</em> visible to GHC, but we won&#8217;t be able to run the <em>executable</em>.</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>nix-shell -p "[pandoc (haskellPackages.ghcWithPackages (pkgs: with pkgs; [ pandoc ]))]"</code></p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Makes the Pandoc <em>executable</em> available at the command line, and the <em>library</em> visible to GHC.</p></td>
</tr>
</tbody>
</table>
<div class="paragraph">
<p>Now we can create a shell that can run the program.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ nix-shell -p "haskellPackages.ghcWithPackages (pkgs: with pkgs; [ hostname ])"
$ runghc Main.hs
Hello from Haskell inside a Nix flake!
Your hostname is: wombat11k</pre>
</div>
</div>
<div class="paragraph">
<p>Success! Now we know the program works.</p>
</div>
</div>
</div>
<div class="sect3">
<h4 id="_the_cabal_file">9.1.3. The cabal file</h4>
<div class="paragraph">
<p>It&#8217;s time to write a Cabal file for this program.
This is just an ordinary Cabal file; we don&#8217;t need to do anything special for Nix.</p>
</div>
<div class="listingblock">
<div class="title">hello-flake-haskell.cabal</div>
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="cabal"><div class="lineno"><table class="linenotable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal"> 1</span>
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<span class="normal">13</span>
<span class="normal">14</span>
<span class="normal">15</span>
<span class="normal">16</span>
<span class="normal">17</span>
<span class="normal">18</span>
<span class="normal">19</span>
<span class="normal">20</span>
<span class="normal">21</span>
<span class="normal">22</span>
<span class="normal">23</span>
<span class="normal">24</span>
<span class="normal">25</span>
<span class="normal">26</span></pre></div></td><td class="code"><div><pre><span></span>cabal-version: 3.0
name: hello-flake-haskell
version: 1.0.0
synopsis: A simple demonstration using a Nix flake to package a Haskell program that prints a greeting.
description:
For more information and a tutorial on how to use this package,
please see the README at &lt;https://codeberg.org/mhwombat/hello-flake-haskell#readme&gt;.
homepage: https://codeberg.org/mhwombat/hello-flake-haskell
bug-reports: https://codeberg.org/mhwombat/hello-flake-haskell/issues
license: GPL-3.0-only
license-file: LICENSE
author: Amy de Buitléir
maintainer: amy@nualeargais.ie
copyright: (c) 2023 Amy de Buitléir
category: Text
build-type: Simple
executable hello-flake-haskell
main-is: Main.hs
build-depends:
base,
hostname
-- NOTE: Best practice is to specify version constraints for the packages we depend on.
-- However, I&#39;m confident that this package will only be used as a Nix flake.
-- Nix will automatically ensure that anyone running this program is using the
-- same library versions that I used to build it.
</pre></div></td></tr></table></div></code></pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="_optional_building_and_running_with_cabal_install">9.1.4. (Optional) Building and running with cabal-install</h4>
<div class="paragraph">
<p>At this point, I would normally write <code>flake.nix</code> and use Nix to build the program.
I&#8217;ll cover that in the next section.
However, it&#8217;s useful to know how to build the package manually in a Nix envronment,
without using a Nix flake.
When you&#8217;re new to Nix, this can help you differentiate between problems in your flake definition
and problems in your Cabal file.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ cabal build
sh: line 35: cabal: command not found</pre>
</div>
</div>
<div class="paragraph">
<p>Aha! We need <code>cabal-install</code> in our shell.
Rather than launch another shell-within-a-shell, let&#8217;s exit create a new one.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ exit
$ nix-shell -p "[ cabal-install (haskellPackages.ghcWithPackages (pkgs: with pkgs; [ hostname ]))]"
$ cabal build
Warning: The package list for 'hackage.haskell.org' is 21 days old.
Run 'cabal update' to get the latest list of available packages.
Resolving dependencies...
Build profile: -w ghc-9.4.8 -O1
In order, the following will be built (use -v for more details):
- hello-flake-haskell-1.0.0 (exe:hello-flake-haskell) (first run)
Configuring executable 'hello-flake-haskell' for hello-flake-haskell-1.0.0..
Warning: Packages using 'cabal-version: &gt;= 1.10' and before 'cabal-version:
3.4' must specify the 'default-language' field for each component (e.g.
Haskell98 or Haskell2010). If a component uses different languages in
different modules then list the other ones in the 'other-languages' field.
Warning: The 'license-file' field refers to the file 'LICENSE' which does not
exist.
Preprocessing executable 'hello-flake-haskell' for hello-flake-haskell-1.0.0..
Building executable 'hello-flake-haskell' for hello-flake-haskell-1.0.0..
[1 of 1] Compiling Main ( Main.hs, /home/amy/codeberg/nix-book/source/new-flake/haskell-flake/hello-haskell/dist-newstyle/build/x86_64-linux/ghc-9.4.8/hello-flake-haskell-1.0.0/x/hello-flake-haskell/build/hello-flake-haskell/hello-flake-haskell-tmp/Main.o )
[2 of 2] Linking /home/amy/codeberg/nix-book/source/new-flake/haskell-flake/hello-haskell/dist-newstyle/build/x86_64-linux/ghc-9.4.8/hello-flake-haskell-1.0.0/x/hello-flake-haskell/build/hello-flake-haskell/hello-flake-haskell
$ cabal run
Hello from Haskell inside a Nix flake!
Your hostname is: wombat11k
$ exit</pre>
</div>
</div>
<div class="paragraph">
<p>After a lot of output messages, the build succeeds and the program runs.</p>
</div>
</div>
<div class="sect3">
<h4 id="_the_nix_flake">9.1.5. The Nix flake</h4>
<div class="paragraph">
<p>Now we should write <code>flake.nix</code>. We already know how to write most of
the flake from the examples we did earlier. The two parts that would be
different are the development shell and the package builder.</p>
</div>
<div class="paragraph">
<p>However, there&#8217;s a simpler way, using <code>haskell-flake</code>.</p>
</div>
<div class="listingblock">
<div class="title">flake.nix</div>
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="nix"><div class="lineno"><table class="linenotable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal"> 1</span>
<span class="normal"> 2</span>
<span class="normal"> 3</span>
<span class="normal"> 4</span>
<span class="normal"> 5</span>
<span class="normal"> 6</span>
<span class="normal"> 7</span>
<span class="normal"> 8</span>
<span class="normal"> 9</span>
<span class="normal">10</span>
<span class="normal">11</span>
<span class="normal">12</span>
<span class="normal">13</span>
<span class="normal">14</span>
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<span class="normal">20</span>
<span class="normal">21</span>
<span class="normal">22</span>
<span class="normal">23</span>
<span class="normal">24</span></pre></div></td><td class="code"><div><pre><span></span><span class="tok-p">{</span>
<span class="tok-ss">description =</span> <span class="tok-s2">&quot;a flake using Haskell&quot;</span><span class="tok-p">;</span>
<span class="tok-c1"># This example uses haskell-flake to make things simpler.</span>
<span class="tok-c1"># See https://haskell.flake.page/ for more information and examples.</span>
<span class="tok-ss">inputs =</span> <span class="tok-p">{</span>
nixpkgs<span class="tok-o">.</span><span class="tok-ss">url =</span> <span class="tok-s2">&quot;github:nixos/nixpkgs/nixpkgs-unstable&quot;</span><span class="tok-p">;</span>
flake-parts<span class="tok-o">.</span><span class="tok-ss">url =</span> <span class="tok-s2">&quot;github:hercules-ci/flake-parts&quot;</span><span class="tok-p">;</span>
haskell-flake<span class="tok-o">.</span><span class="tok-ss">url =</span> <span class="tok-s2">&quot;github:srid/haskell-flake&quot;</span><span class="tok-p">;</span>
<span class="tok-p">};</span>
<span class="tok-ss">outputs =</span> inputs<span class="tok-p">@{</span> self<span class="tok-p">,</span> nixpkgs<span class="tok-p">,</span> flake-parts<span class="tok-p">,</span> <span class="tok-o">...</span> <span class="tok-p">}:</span>
flake-parts<span class="tok-o">.</span>lib<span class="tok-o">.</span>mkFlake <span class="tok-p">{</span> <span class="tok-k">inherit</span> inputs<span class="tok-p">;</span> <span class="tok-p">}</span> <span class="tok-p">{</span>
<span class="tok-ss">systems =</span> nixpkgs<span class="tok-o">.</span>lib<span class="tok-o">.</span>systems<span class="tok-o">.</span>flakeExposed<span class="tok-p">;</span>
<span class="tok-ss">imports =</span> <span class="tok-p">[</span> inputs<span class="tok-o">.</span>haskell-flake<span class="tok-o">.</span>flakeModule <span class="tok-p">];</span>
<span class="tok-ss">perSystem =</span> <span class="tok-p">{</span> self&#39;<span class="tok-p">,</span> pkgs<span class="tok-p">,</span> <span class="tok-o">...</span> <span class="tok-p">}:</span> <span class="tok-p">{</span>
haskellProjects<span class="tok-o">.</span><span class="tok-ss">default =</span> <span class="tok-p">{};</span>
<span class="tok-c1"># haskell-flake doesn&#39;t set the default package, but you can do it here.</span>
packages<span class="tok-o">.</span><span class="tok-ss">default =</span> self&#39;<span class="tok-o">.</span>packages<span class="tok-o">.</span>hello-flake-haskell<span class="tok-p">;</span>
<span class="tok-p">};</span>
<span class="tok-p">};</span>
<span class="tok-p">}</span>
</pre></div></td></tr></table></div></code></pre>
</div>
</div>
<div class="paragraph">
<p>The above definition will work for most of your haskell projects;
simply change the <code>description</code> and the package name in <code>packages.default</code>.
Let’s try out the new flake.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ nix run
warning: Git tree '/home/amy/codeberg/nix-book/source/new-flake/haskell-flake/hello-haskell' is dirty
error: getting status of '/nix/store/0ccnxa25whszw7mgbgyzdm4nqc0zwnm8-source/flake.nix': No such file or directory</pre>
</div>
</div>
<div class="paragraph">
<p>Why can’t it find <code>flake.nix</code>? Nix flakes only &#8220;see&#8221; files that are
part of the repository. We need to add all of the important files to the
repo before building or running the flake.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ git add flake.nix hello-flake-haskell.cabal Main.hs
$ nix run
warning: Git tree '/home/amy/codeberg/nix-book/source/new-flake/haskell-flake/hello-haskell' is dirty
warning: creating lock file '/home/amy/codeberg/nix-book/source/new-flake/haskell-flake/hello-haskell/flake.lock'
warning: Git tree '/home/amy/codeberg/nix-book/source/new-flake/haskell-flake/hello-haskell' is dirty
these 2 derivations will be built:
/nix/store/5ar7vhd4nz8wbqrsgaxqzkh6b4ggvsrv-source-hello-flake-haskell-sdist.tar.gz.drv
/nix/store/hi70w0gzjfj213r0xhhva7n617hfa378-hello-flake-haskell-1.0.0.drv
building '/nix/store/5ar7vhd4nz8wbqrsgaxqzkh6b4ggvsrv-source-hello-flake-haskell-sdist.tar.gz.drv'...
error: builder for '/nix/store/5ar7vhd4nz8wbqrsgaxqzkh6b4ggvsrv-source-hello-flake-haskell-sdist.tar.gz.drv' failed with exit code 1;
last 7 log lines:
&gt; unpacking source archive /nix/store/fdxvjgdpsfjrkanzbx43g1yxf2b1lp4b-source-hello-flake-haskell
&gt; source root is source-hello-flake-haskell
&gt; Config file path source is default config file.
&gt; Config file not found: /build/source-hello-flake-haskell/.config/cabal/config
&gt; Writing default configuration to
&gt; /build/source-hello-flake-haskell/.config/cabal/config
&gt; /build/source-hello-flake-haskell/./LICENSE: withBinaryFile: does not exist (No such file or directory)
For full logs, run 'nix log /nix/store/5ar7vhd4nz8wbqrsgaxqzkh6b4ggvsrv-source-hello-flake-haskell-sdist.tar.gz.drv'.
error: 1 dependencies of derivation '/nix/store/hi70w0gzjfj213r0xhhva7n617hfa378-hello-flake-haskell-1.0.0.drv' failed to build</pre>
</div>
</div>
<div class="paragraph">
<p>We’d like to share this package with others, but first we should do some
cleanup. When the package was built (automatically by the <code>nix run</code>
command), it created a <code>flake.lock</code> file. We need to add this to the
repo, and commit all important files.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ git add flake.lock
$ git commit -a -m 'initial commit'
[master (root-commit) 39ade13] initial commit
4 files changed, 137 insertions(+)
create mode 100644 Main.hs
create mode 100644 flake.lock
create mode 100644 flake.nix
create mode 100644 hello-flake-haskell.cabal</pre>
</div>
</div>
<div class="paragraph">
<p>You can test that your package is properly configured by going to
another directory and running it from there.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ cd ..
$ nix run ./hello-haskell
these 2 derivations will be built:
/nix/store/5ar7vhd4nz8wbqrsgaxqzkh6b4ggvsrv-source-hello-flake-haskell-sdist.tar.gz.drv
/nix/store/hi70w0gzjfj213r0xhhva7n617hfa378-hello-flake-haskell-1.0.0.drv
building '/nix/store/5ar7vhd4nz8wbqrsgaxqzkh6b4ggvsrv-source-hello-flake-haskell-sdist.tar.gz.drv'...
error: builder for '/nix/store/5ar7vhd4nz8wbqrsgaxqzkh6b4ggvsrv-source-hello-flake-haskell-sdist.tar.gz.drv' failed with exit code 1;
last 7 log lines:
&gt; unpacking source archive /nix/store/fdxvjgdpsfjrkanzbx43g1yxf2b1lp4b-source-hello-flake-haskell
&gt; source root is source-hello-flake-haskell
&gt; Config file path source is default config file.
&gt; Config file not found: /build/source-hello-flake-haskell/.config/cabal/config
&gt; Writing default configuration to
&gt; /build/source-hello-flake-haskell/.config/cabal/config
&gt; /build/source-hello-flake-haskell/./LICENSE: withBinaryFile: does not exist (No such file or directory)
For full logs, run 'nix log /nix/store/5ar7vhd4nz8wbqrsgaxqzkh6b4ggvsrv-source-hello-flake-haskell-sdist.tar.gz.drv'.
error: 1 dependencies of derivation '/nix/store/hi70w0gzjfj213r0xhhva7n617hfa378-hello-flake-haskell-1.0.0.drv' failed to build</pre>
</div>
</div>
<div class="paragraph">
<p>If you move the project to a public repo, anyone can run it. Recall from
the beginning of the tutorial that you were able to run <code>hello-flake</code>
directly from my repo with the following command.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">nix run "git+https://codeberg.org/mhwombat/hello-flake"</pre>
</div>
</div>
<div class="paragraph">
<p>Modify the URL accordingly and invite someone else to run your new
Haskell flake.</p>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_python">9.2. Python</h3>
<div class="paragraph">
<p>Start with an empty directory and create a git repository.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ mkdir hello-python
$ cd hello-python
$ git init
Initialized empty Git repository in /home/amy/codeberg/nix-book/source/new-flake/python-flake/hello-python/.git/</pre>
</div>
</div>
<div class="sect3">
<h4 id="_a_simple_python_program">9.2.1. A simple Python program</h4>
<div class="paragraph">
<p>Next, we’ll create a simple Python program.</p>
</div>
<div class="listingblock">
<div class="title">hello.py</div>
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="python"><div class="lineno"><table class="linenotable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span>
<span class="normal">2</span>
<span class="normal">3</span>
<span class="normal">4</span>
<span class="normal">5</span>
<span class="normal">6</span>
<span class="normal">7</span></pre></div></td><td class="code"><div><pre><span></span><span class="tok-ch">#!/usr/bin/env python</span>
<span class="tok-k">def</span> <span class="tok-nf">main</span><span class="tok-p">():</span>
<span class="tok-nb">print</span><span class="tok-p">(</span><span class="tok-s2">&quot;Hello from inside a Python program built with a Nix flake!&quot;</span><span class="tok-p">)</span>
<span class="tok-k">if</span> <span class="tok-vm">__name__</span> <span class="tok-o">==</span> <span class="tok-s2">&quot;__main__&quot;</span><span class="tok-p">:</span>
<span class="tok-n">main</span><span class="tok-p">()</span>
</pre></div></td></tr></table></div></code></pre>
</div>
</div>
<div class="paragraph">
<p>Before we package the program, let’s verify that it runs. We’re going to
need Python. By now you’ve probably figured out that we can write a
<code>flake.nix</code> and define a development shell that includes Python. We’ll
do that shortly, but first I want to show you a handy shortcut. We can
lauch a <em>temporary</em> shell with any Nix packages we want. This is
convenient when you just want to try out some new software and you’re
not sure if you’ll use it again. It’s also convenient when you’re not
ready to write <code>flake.nix</code> (perhaps you’re not sure what tools and
packages you need), and you want to experiment a bit first.</p>
</div>
<div class="paragraph">
<p>The command to enter a temporary shell is</p>
</div>
<div class="paragraph">
<p><code>nix-shell -p <em>packages</em></code></p>
</div>
<div class="paragraph">
<p>If there are multiple packages, they should be separated by spaces.</p>
</div>
<div class="admonitionblock note">
<table>
<tr>
<td class="icon">
<i class="fa icon-note" title="Note"></i>
</td>
<td class="content">
<div class="paragraph">
<p>The command used here is <code>nix-shell</code> with a hyphen, not <code>nix shell</code>
with a space; those are two different commands. In fact there are
hyphenated and non-hyphenated versions of many Nix commands, and yes,
it’s confusing. The non-hyphenated commands were introduced when support
for flakes was added to Nix. I predict that eventually all hyphenated
commands will be replaced with non-hyphenated versions. Until then, a
useful rule of thumb is that non-hyphenated commands are for for working
directly with flakes; hyphenated commands are for everything else.</p>
</div>
</td>
</tr>
</table>
</div>
<div class="paragraph">
<p>Let’s enter a shell with Python so we can test the program.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ nix-shell -p python3
$ python hello.py
Hello from inside a Python program built with a Nix flake!</pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="_a_python_builder">9.2.2. A Python builder</h4>
<div class="paragraph">
<p>Next, create a Python script to build the package. We’ll use Python’s
setuptools, but you can use other build tools. For more information on
setuptools, see the
<a href="https://packaging.python.org/en/latest/guides/distributing-packages-using-setuptools/">Python
Packaging User Guide</a>, especially the section on
<a href="https://packaging.python.org/en/latest/guides/distributing-packages-using-setuptools/#setup-args">setup
args</a>.</p>
</div>
<div class="listingblock">
<div class="title">setup.py</div>
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="python"><div class="lineno"><table class="linenotable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal"> 1</span>
<span class="normal"> 2</span>
<span class="normal"> 3</span>
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<span class="normal"> 5</span>
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<span class="normal"> 7</span>
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<span class="normal"> 9</span>
<span class="normal">10</span>
<span class="normal">11</span>
<span class="normal">12</span></pre></div></td><td class="code"><div><pre><span></span><span class="tok-ch">#!/usr/bin/env python</span>
<span class="tok-kn">from</span> <span class="tok-nn">setuptools</span> <span class="tok-kn">import</span> <span class="tok-n">setup</span>
<span class="tok-n">setup</span><span class="tok-p">(</span>
<span class="tok-n">name</span><span class="tok-o">=</span><span class="tok-s1">&#39;hello-flake-python&#39;</span><span class="tok-p">,</span>
<span class="tok-n">version</span><span class="tok-o">=</span><span class="tok-s1">&#39;0.1.0&#39;</span><span class="tok-p">,</span>
<span class="tok-n">py_modules</span><span class="tok-o">=</span><span class="tok-p">[</span><span class="tok-s1">&#39;hello&#39;</span><span class="tok-p">],</span>
<span class="tok-n">entry_points</span><span class="tok-o">=</span><span class="tok-p">{</span>
<span class="tok-s1">&#39;console_scripts&#39;</span><span class="tok-p">:</span> <span class="tok-p">[</span><span class="tok-s1">&#39;hello-flake-python = hello:main&#39;</span><span class="tok-p">]</span>
<span class="tok-p">},</span>
<span class="tok-p">)</span>
</pre></div></td></tr></table></div></code></pre>
</div>
</div>
<div class="paragraph">
<p>We won’t write <code>flake.nix</code> just yet. First we’ll try building the
package manually.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ python -m build
/nix/store/qp5zys77biz7imbk6yy85q5pdv7qk84j-python3-3.11.6/bin/python: No module named build</pre>
</div>
</div>
<div class="paragraph">
<p>The missing module error happens because we don’t have <code>build</code> available
in the temporary shell. We can fix that by adding &#8220;build&#8221; to the
temporary shell. When you need support for both a language and some of
its packages, it’s best to use one of the Nix functions that are
specific to the programming language and build system. For Python, we
can use the <code>withPackages</code> function.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ nix-shell -p "python3.withPackages (ps: with ps; [ build ])"</pre>
</div>
</div>
<div class="paragraph">
<p>Note that we’re now inside a temporary shell inside the previous
temporary shell! To get back to the original shell, we have to <code>exit</code>
twice. Alternatively, we could have done <code>exit</code> followed by the
<code>nix-shell</code> command.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ python -m build</pre>
</div>
</div>
<div class="paragraph">
<p>After a lot of output messages, the build succeeds.</p>
</div>
</div>
<div class="sect3">
<h4 id="_the_nix_flake_2">9.2.3. The Nix flake</h4>
<div class="paragraph">
<p>Now we should write <code>flake.nix</code>. We already know how to write most of
the flake from the examples we did earlier. The two parts that will be
different are the development shell and the package builder.</p>
</div>
<div class="paragraph">
<p>Let’s start with the development shell. It seems logical to write
something like the following.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap"> devShells = rec {
default = pkgs.mkShell {
packages = [ (python.withPackages (ps: with ps; [ build ])) ];
};
};</pre>
</div>
</div>
<div class="paragraph">
<p>Note that we need the parentheses to prevent <code>python.withPackages</code> and
the argument from being processed as two separate tokens. Suppose we
wanted to work with <code>virtualenv</code> and <code>pip</code> instead of <code>build</code>. We could
write something like the following.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap"> devShells = rec {
default = pkgs.mkShell {
packages = [
# Python plus helper tools
(python.withPackages (ps: with ps; [
virtualenv # Virtualenv
pip # The pip installer
]))
];
};
};</pre>
</div>
</div>
<div class="paragraph">
<p>For the package builder, we can use the <code>buildPythonApplication</code>
function.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap"> packages = rec {
hello = python.pkgs.buildPythonApplication {
name = "hello-flake-python";
buildInputs = with python.pkgs; [ pip ];
src = ./.;
};
default = hello;
};</pre>
</div>
</div>
<div class="paragraph">
<p>If you put all the pieces together, your <code>flake.nix</code> should look
something like this.</p>
</div>
<div class="listingblock">
<div class="title">flake.nix</div>
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="nix"><div class="lineno"><table class="linenotable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal"> 1</span>
<span class="normal"> 2</span>
<span class="normal"> 3</span>
<span class="normal"> 4</span>
<span class="normal"> 5</span>
<span class="normal"> 6</span>
<span class="normal"> 7</span>
<span class="normal"> 8</span>
<span class="normal"> 9</span>
<span class="normal">10</span>
<span class="normal">11</span>
<span class="normal">12</span>
<span class="normal">13</span>
<span class="normal">14</span>
<span class="normal">15</span>
<span class="normal">16</span>
<span class="normal">17</span>
<span class="normal">18</span>
<span class="normal">19</span>
<span class="normal">20</span>
<span class="normal">21</span>
<span class="normal">22</span>
<span class="normal">23</span>
<span class="normal">24</span>
<span class="normal">25</span>
<span class="normal">26</span>
<span class="normal">27</span>
<span class="normal">28</span>
<span class="normal">29</span>
<span class="normal">30</span>
<span class="normal">31</span>
<span class="normal">32</span>
<span class="normal">33</span>
<span class="normal">34</span>
<span class="normal">35</span>
<span class="normal">36</span>
<span class="normal">37</span>
<span class="normal">38</span>
<span class="normal">39</span>
<span class="normal">40</span>
<span class="normal">41</span>
<span class="normal">42</span>
<span class="normal">43</span>
<span class="normal">44</span>
<span class="normal">45</span>
<span class="normal">46</span>
<span class="normal">47</span>
<span class="normal">48</span></pre></div></td><td class="code"><div><pre><span></span><span class="tok-p">{</span>
<span class="tok-c1"># See https://github.com/mhwombat/nix-for-numbskulls/blob/main/flakes.md</span>
<span class="tok-c1"># for a brief overview of what each section in a flake should or can contain.</span>
<span class="tok-ss">description =</span> <span class="tok-s2">&quot;a very simple and friendly flake written in Python&quot;</span><span class="tok-p">;</span>
<span class="tok-ss">inputs =</span> <span class="tok-p">{</span>
nixpkgs<span class="tok-o">.</span><span class="tok-ss">url =</span> <span class="tok-s2">&quot;github:NixOS/nixpkgs&quot;</span><span class="tok-p">;</span>
flake-utils<span class="tok-o">.</span><span class="tok-ss">url =</span> <span class="tok-s2">&quot;github:numtide/flake-utils&quot;</span><span class="tok-p">;</span>
<span class="tok-p">};</span>
<span class="tok-ss">outputs =</span> <span class="tok-p">{</span> self<span class="tok-p">,</span> nixpkgs<span class="tok-p">,</span> flake-utils <span class="tok-p">}:</span>
flake-utils<span class="tok-o">.</span>lib<span class="tok-o">.</span>eachDefaultSystem <span class="tok-p">(</span>system<span class="tok-p">:</span>
<span class="tok-k">let</span>
<span class="tok-ss">pkgs =</span> <span class="tok-nb">import</span> nixpkgs <span class="tok-p">{</span> <span class="tok-k">inherit</span> system<span class="tok-p">;</span> <span class="tok-p">};</span>
<span class="tok-ss">python =</span> pkgs<span class="tok-o">.</span>python3<span class="tok-p">;</span>
<span class="tok-k">in</span>
<span class="tok-p">{</span>
<span class="tok-ss">devShells =</span> <span class="tok-k">rec</span> <span class="tok-p">{</span>
<span class="tok-ss">default =</span> pkgs<span class="tok-o">.</span>mkShell <span class="tok-p">{</span>
<span class="tok-ss">packages =</span> <span class="tok-p">[</span>
<span class="tok-c1"># Python plus helper tools</span>
<span class="tok-p">(</span>python<span class="tok-o">.</span>withPackages <span class="tok-p">(</span>ps<span class="tok-p">:</span> <span class="tok-k">with</span> ps<span class="tok-p">;</span> <span class="tok-p">[</span>
virtualenv <span class="tok-c1"># Virtualenv</span>
pip <span class="tok-c1"># The pip installer</span>
<span class="tok-p">]))</span>
<span class="tok-p">];</span>
<span class="tok-p">};</span>
<span class="tok-p">};</span>
<span class="tok-ss">packages =</span> <span class="tok-k">rec</span> <span class="tok-p">{</span>
<span class="tok-ss">hello =</span> python<span class="tok-o">.</span>pkgs<span class="tok-o">.</span>buildPythonApplication <span class="tok-p">{</span>
<span class="tok-ss">name =</span> <span class="tok-s2">&quot;hello-flake-python&quot;</span><span class="tok-p">;</span>
<span class="tok-ss">buildInputs =</span> <span class="tok-k">with</span> python<span class="tok-o">.</span>pkgs<span class="tok-p">;</span> <span class="tok-p">[</span> pip <span class="tok-p">];</span>
<span class="tok-ss">src =</span> <span class="tok-o">.</span><span class="tok-l">/.</span><span class="tok-p">;</span>
<span class="tok-p">};</span>
<span class="tok-ss">default =</span> hello<span class="tok-p">;</span>
<span class="tok-p">};</span>
<span class="tok-ss">apps =</span> <span class="tok-k">rec</span> <span class="tok-p">{</span>
<span class="tok-ss">hello =</span> flake-utils<span class="tok-o">.</span>lib<span class="tok-o">.</span>mkApp <span class="tok-p">{</span> <span class="tok-ss">drv =</span> self<span class="tok-o">.</span>packages<span class="tok-o">.</span><span class="tok-err">$</span><span class="tok-p">{</span>system<span class="tok-p">}</span><span class="tok-o">.</span>hello<span class="tok-p">;</span> <span class="tok-p">};</span>
<span class="tok-ss">default =</span> hello<span class="tok-p">;</span>
<span class="tok-p">};</span>
<span class="tok-p">}</span>
<span class="tok-p">);</span>
<span class="tok-p">}</span>
</pre></div></td></tr></table></div></code></pre>
</div>
</div>
<div class="paragraph">
<p>Let’s try out the new flake.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ nix run
warning: Git tree '/home/amy/codeberg/nix-book/source/new-flake/python-flake/hello-python' is dirty
error: getting status of '/nix/store/0ccnxa25whszw7mgbgyzdm4nqc0zwnm8-source/flake.nix': No such file or directory</pre>
</div>
</div>
<div class="paragraph">
<p>Why can’t it find <code>flake.nix</code>? Nix flakes only &#8220;see&#8221; files that are
part of the repository. We need to add all of the important files to the
repo before building or running the flake.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ git add flake.nix setup.py hello.py
$ nix run
warning: Git tree '/home/amy/codeberg/nix-book/source/new-flake/python-flake/hello-python' is dirty
warning: creating lock file '/home/amy/codeberg/nix-book/source/new-flake/python-flake/hello-python/flake.lock'
warning: Git tree '/home/amy/codeberg/nix-book/source/new-flake/python-flake/hello-python' is dirty
this derivation will be built:
/nix/store/dh97mik5m481j14am9dgy8jx9ppv5b6d-hello-flake-python.drv
building '/nix/store/dh97mik5m481j14am9dgy8jx9ppv5b6d-hello-flake-python.drv'...
Hello from inside a Python program built with a Nix flake!</pre>
</div>
</div>
<div class="paragraph">
<p>We’d like to share this package with others, but first we should do some
cleanup. When the package was built (automatically by the <code>nix run</code>
command), it created a <code>flake.lock</code> file. We need to add this to the
repo, and commit all important files.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ git add flake.lock
$ git commit -a -m 'initial commit'
[master (root-commit) 6bec7c3] initial commit
4 files changed, 127 insertions(+)
create mode 100644 flake.lock
create mode 100644 flake.nix
create mode 100644 hello.py
create mode 100644 setup.py</pre>
</div>
</div>
<div class="paragraph">
<p>You can test that your package is properly configured by going to
another directory and running it from there.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ cd ..
$ nix run ./hello-python
Hello from inside a Python program built with a Nix flake!</pre>
</div>
</div>
<div class="paragraph">
<p>If you move the project to a public repo, anyone can run it. Recall from
the beginning of the tutorial that you were able to run <code>hello-flake</code>
directly from my repo with the following command.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">nix run "git+https://codeberg.org/mhwombat/hello-flake"</pre>
</div>
</div>
<div class="paragraph">
<p>Modify the URL accordingly and invite someone else to run your new
Python flake.</p>
</div>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_recipes">10. Recipes</h2>
<div class="sectionbody">
<div class="paragraph">
<p>This chapter provides examples of how to use Nix in a variety of scanarios.
Multiple types of recipes are provided are provided for some scenarios;
comparing the different recipes will help you better understand Nix.</p>
</div>
<div class="ulist">
<ul>
<li>
<p>An <em>"ad hoc" shell</em>
is useful when you want to quickly create an environment
for a one-off task.</p>
</li>
<li>
<p>A <em>traditional nix shell</em>
is useful when you want to define an environment that you will
use more than once.</p>
</li>
<li>
<p><em>Nix flakes</em>
are the recommended approach for development projects.</p>
</li>
<li>
<p>You can use <code>nix-shell</code> to run scripts in arbitrary languages, providing
the necessary dependencies. This is particularly convenient for
standalone scripts because you don’t need to create a repo and write a
separate <code>flake.nix</code>.
The script should start with two <em>"shebang"</em> (<code>#!</code>) commands. The first
should invoke <code>nix-shell</code>. The second should declares the script
interpreter and any dependencies.</p>
</li>
</ul>
</div>
<div class="sect2">
<h3 id="_access_to_a_top_level_package_from_the_nixpkgsnixos_repo">10.1. Access to a top-level package from the Nixpkgs/NixOS repo</h3>
<div class="paragraph">
<p>Ex: Access two packages from nixpkgs:
hello and cowsay.</p>
</div>
<div class="sect3">
<h4 id="_from_the_command_line">10.1.1. From the command line</h4>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ nix-shell -p "[hello cowsay]"
$ hello
Hello, world!
$ cowsay "moo"
_____
&lt; moo &gt;
-----
\ ^__^
\ (oo)\_______
(__)\ )\/\
||----w |
|| ||</pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="_in_shell_nix">10.1.2. In <code>shell.nix</code></h4>
<div class="listingblock">
<div class="title">shell.nix</div>
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="nix"><div class="lineno"><table class="linenotable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span>
<span class="normal">2</span>
<span class="normal">3</span>
<span class="normal">4</span>
<span class="normal">5</span>
<span class="normal">6</span>
<span class="normal">7</span></pre></div></td><td class="code"><div><pre><span></span><span class="tok-k">with</span> <span class="tok-p">(</span><span class="tok-nb">import</span> <span class="tok-l">&lt;nixpkgs&gt;</span> <span class="tok-p">{});</span>
mkShell <span class="tok-p">{</span>
<span class="tok-ss">buildInputs =</span> <span class="tok-p">[</span>
hello
cowsay
<span class="tok-p">];</span>
<span class="tok-p">}</span>
</pre></div></td></tr></table></div></code></pre>
</div>
</div>
<div class="paragraph">
<p>Here’s a demonstration using the shell.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ nix-shell
$ hello
Hello, world!
$ cowsay "moo"
_____
&lt; moo &gt;
-----
\ ^__^
\ (oo)\_______
(__)\ )\/\
||----w |
|| ||</pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="_in_a_bash_script">10.1.3. In a Bash script</h4>
<div class="listingblock">
<div class="title">Script</div>
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="bash"><div class="lineno"><table class="linenotable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span>
<span class="normal">2</span>
<span class="normal">3</span>
<span class="normal">4</span></pre></div></td><td class="code"><div><pre><span></span><span class="tok-ch">#! /usr/bin/env nix-shell</span>
<span class="tok-c1">#! nix-shell -i bash -p &quot;[hello cowsay]&quot;</span>
hello
cowsay<span class="tok-w"> </span><span class="tok-s2">&quot;Pretty cool, huh?&quot;</span>
</pre></div></td></tr></table></div></code></pre>
</div>
</div>
<div class="literalblock">
<div class="title">Output</div>
<div class="content">
<pre class="nowrap">Hello, world!
___________________
&lt; Pretty cool, huh? &gt;
-------------------
\ ^__^
\ (oo)\_______
(__)\ )\/\
||----w |
|| ||</pre>
</div>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_access_to_a_package_defined_in_a_remote_git_repo">10.2. Access to a package defined in a remote git repo</h3>
<div class="paragraph">
<p>Ex: Access a package called <code>hello-nix</code>,
which is defined in a remote git repo on codeberg.
To use a package from GitHub, GitLab, or any other public platform,
modify the URL.</p>
</div>
<div class="sect3">
<h4 id="_in_shell_nix_2">10.2.1. In <code>shell.nix</code></h4>
<div class="listingblock">
<div class="title">shell.nix</div>
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="nix"><div class="lineno"><table class="linenotable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal"> 1</span>
<span class="normal"> 2</span>
<span class="normal"> 3</span>
<span class="normal"> 4</span>
<span class="normal"> 5</span>
<span class="normal"> 6</span>
<span class="normal"> 7</span>
<span class="normal"> 8</span>
<span class="normal"> 9</span>
<span class="normal">10</span></pre></div></td><td class="code"><div><pre><span></span><span class="tok-k">with</span> <span class="tok-p">(</span><span class="tok-nb">import</span> <span class="tok-l">&lt;nixpkgs&gt;</span> <span class="tok-p">{});</span>
<span class="tok-k">let</span>
<span class="tok-ss">hello-nix =</span> <span class="tok-nb">import</span> <span class="tok-p">(</span><span class="tok-nb">builtins</span><span class="tok-o">.</span>fetchGit <span class="tok-p">{</span>
<span class="tok-ss">url =</span> <span class="tok-s2">&quot;https://codeberg.org/mhwombat/hello-nix&quot;</span><span class="tok-p">;</span>
<span class="tok-ss">rev =</span> <span class="tok-s2">&quot;aa2c87f8b89578b069b09fdb2be30a0c9d8a77d8&quot;</span><span class="tok-p">;</span>
<span class="tok-p">});</span>
<span class="tok-k">in</span>
mkShell <span class="tok-p">{</span>
<span class="tok-ss">buildInputs =</span> <span class="tok-p">[</span> hello-nix <span class="tok-p">];</span>
<span class="tok-p">}</span>
</pre></div></td></tr></table></div></code></pre>
</div>
</div>
<div class="paragraph">
<p>Here’s a demonstration using the shell.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ nix-shell
$ hello-nix
Hello from your nix package!</pre>
</div>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_access_to_a_flake_defined_in_a_remote_git_repo">10.3. Access to a flake defined in a remote git repo</h3>
<div class="paragraph">
<p>Ex: Access a flake called <code>hello-flake</code>,
which is defined in a remote git repo on codeberg.
To use a package from GitHub, GitLab, or any other public platform,
modify the URL.</p>
</div>
<div class="sect3">
<h4 id="_in_shell_nix_3">10.3.1. In <code>shell.nix</code></h4>
<div class="listingblock">
<div class="title">shell.nix</div>
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="nix"><div class="lineno"><table class="linenotable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal"> 1</span>
<span class="normal"> 2</span>
<span class="normal"> 3</span>
<span class="normal"> 4</span>
<span class="normal"> 5</span>
<span class="normal"> 6</span>
<span class="normal"> 7</span>
<span class="normal"> 8</span>
<span class="normal"> 9</span>
<span class="normal">10</span>
<span class="normal">11</span></pre></div></td><td class="code"><div><pre><span></span><span class="tok-k">with</span> <span class="tok-p">(</span><span class="tok-nb">import</span> <span class="tok-l">&lt;nixpkgs&gt;</span> <span class="tok-p">{});</span>
<span class="tok-k">let</span>
<span class="tok-ss">hello-flake =</span> <span class="tok-p">(</span> <span class="tok-nb">builtins</span><span class="tok-o">.</span>getFlake
<span class="tok-l">git+https://codeberg.org/mhwombat/hello-flake?ref=main&amp;rev=3aa43dbe7be878dde7b2bdcbe992fe1705da3150</span>
<span class="tok-p">)</span><span class="tok-o">.</span>packages<span class="tok-o">.</span><span class="tok-err">$</span><span class="tok-p">{</span><span class="tok-nb">builtins</span><span class="tok-o">.</span>currentSystem<span class="tok-p">}</span><span class="tok-o">.</span>default<span class="tok-p">;</span>
<span class="tok-k">in</span>
mkShell <span class="tok-p">{</span>
<span class="tok-ss">buildInputs =</span> <span class="tok-p">[</span>
hello-flake
<span class="tok-p">];</span>
<span class="tok-p">}</span>
</pre></div></td></tr></table></div></code></pre>
</div>
</div>
<div class="paragraph">
<p>Here’s a demonstration using the shell.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ nix-shell
$ hello-flake
Hello from your flake!</pre>
</div>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_access_to_a_haskell_library_package_in_the_nixpkgs_repo_without_a_cabal_file">10.4. Access to a Haskell library package in the nixpkgs repo (without a <code>.cabal</code> file)</h3>
<div class="paragraph">
<p>Occasionally you might want to run a short Haskell program that depends on a Haskell library,
but you don&#8217;t want to bother writing a cabal file.</p>
</div>
<div class="paragraph">
<p>Example: Access the <code>containers</code> package from the <code>haskellPackages</code> set in the nixpkgs repo.</p>
</div>
<div class="sect3">
<h4 id="_in_shell_nix_4">10.4.1. In <code>shell.nix</code></h4>
<div class="listingblock">
<div class="title">shell.nix</div>
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="nix"><div class="lineno"><table class="linenotable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span>
<span class="normal">2</span>
<span class="normal">3</span>
<span class="normal">4</span>
<span class="normal">5</span>
<span class="normal">6</span>
<span class="normal">7</span>
<span class="normal">8</span>
<span class="normal">9</span></pre></div></td><td class="code"><div><pre><span></span><span class="tok-k">with</span> <span class="tok-p">(</span><span class="tok-nb">import</span> <span class="tok-l">&lt;nixpkgs&gt;</span> <span class="tok-p">{});</span>
<span class="tok-k">let</span>
<span class="tok-ss">customGhc =</span> haskellPackages<span class="tok-o">.</span>ghcWithPackages <span class="tok-p">(</span>pkgs<span class="tok-p">:</span> <span class="tok-k">with</span> pkgs<span class="tok-p">;</span> <span class="tok-p">[</span> containers <span class="tok-p">]);</span>
<span class="tok-k">in</span>
mkShell <span class="tok-p">{</span>
<span class="tok-ss">buildInputs =</span> <span class="tok-p">[</span>
customGhc
<span class="tok-p">];</span>
<span class="tok-p">}</span>
</pre></div></td></tr></table></div></code></pre>
</div>
</div>
<div class="paragraph">
<p>Here&#8217;s a short Haskell program that uses it.</p>
</div>
<div class="listingblock">
<div class="title">Main.hs</div>
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="haskell"><div class="lineno"><table class="linenotable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal"> 1</span>
<span class="normal"> 2</span>
<span class="normal"> 3</span>
<span class="normal"> 4</span>
<span class="normal"> 5</span>
<span class="normal"> 6</span>
<span class="normal"> 7</span>
<span class="normal"> 8</span>
<span class="normal"> 9</span>
<span class="normal">10</span>
<span class="normal">11</span></pre></div></td><td class="code"><div><pre><span></span><span class="tok-kr">import</span><span class="tok-w"> </span><span class="tok-nn">Data.Map</span>
<span class="tok-nf">m</span><span class="tok-w"> </span><span class="tok-ow">::</span><span class="tok-w"> </span><span class="tok-kt">Map</span><span class="tok-w"> </span><span class="tok-kt">String</span><span class="tok-w"> </span><span class="tok-kt">Int</span>
<span class="tok-nf">m</span><span class="tok-w"> </span><span class="tok-ow">=</span><span class="tok-w"> </span><span class="tok-n">fromList</span><span class="tok-w"> </span><span class="tok-p">[(</span><span class="tok-s">&quot;cats&quot;</span><span class="tok-p">,</span><span class="tok-w"> </span><span class="tok-mi">3</span><span class="tok-p">),</span><span class="tok-w"> </span><span class="tok-p">(</span><span class="tok-s">&quot;dogs&quot;</span><span class="tok-p">,</span><span class="tok-w"> </span><span class="tok-mi">2</span><span class="tok-p">)]</span>
<span class="tok-nf">main</span><span class="tok-w"> </span><span class="tok-ow">::</span><span class="tok-w"> </span><span class="tok-kt">IO</span><span class="tok-w"> </span><span class="tok-nb">()</span>
<span class="tok-nf">main</span><span class="tok-w"> </span><span class="tok-ow">=</span><span class="tok-w"> </span><span class="tok-kr">do</span>
<span class="tok-w"> </span><span class="tok-kr">let</span><span class="tok-w"> </span><span class="tok-n">cats</span><span class="tok-w"> </span><span class="tok-ow">=</span><span class="tok-w"> </span><span class="tok-n">findWithDefault</span><span class="tok-w"> </span><span class="tok-mi">0</span><span class="tok-w"> </span><span class="tok-s">&quot;cats&quot;</span><span class="tok-w"> </span><span class="tok-n">m</span>
<span class="tok-w"> </span><span class="tok-kr">let</span><span class="tok-w"> </span><span class="tok-n">dogs</span><span class="tok-w"> </span><span class="tok-ow">=</span><span class="tok-w"> </span><span class="tok-n">findWithDefault</span><span class="tok-w"> </span><span class="tok-mi">0</span><span class="tok-w"> </span><span class="tok-s">&quot;dogs&quot;</span><span class="tok-w"> </span><span class="tok-n">m</span>
<span class="tok-w"> </span><span class="tok-kr">let</span><span class="tok-w"> </span><span class="tok-n">zebras</span><span class="tok-w"> </span><span class="tok-ow">=</span><span class="tok-w"> </span><span class="tok-n">findWithDefault</span><span class="tok-w"> </span><span class="tok-mi">0</span><span class="tok-w"> </span><span class="tok-s">&quot;zebras&quot;</span><span class="tok-w"> </span><span class="tok-n">m</span>
<span class="tok-w"> </span><span class="tok-n">print</span><span class="tok-w"> </span><span class="tok-o">$</span><span class="tok-w"> </span><span class="tok-s">&quot;I have &quot;</span><span class="tok-w"> </span><span class="tok-o">++</span><span class="tok-w"> </span><span class="tok-n">show</span><span class="tok-w"> </span><span class="tok-n">cats</span><span class="tok-w"> </span><span class="tok-o">++</span><span class="tok-w"> </span><span class="tok-s">&quot; cats, &quot;</span><span class="tok-w"> </span><span class="tok-o">++</span><span class="tok-w"> </span><span class="tok-n">show</span><span class="tok-w"> </span><span class="tok-n">dogs</span><span class="tok-w"> </span><span class="tok-o">++</span><span class="tok-w"> </span><span class="tok-s">&quot; dogs, and &quot;</span><span class="tok-w"> </span><span class="tok-o">++</span><span class="tok-w"> </span><span class="tok-n">show</span><span class="tok-w"> </span><span class="tok-n">zebras</span><span class="tok-w"> </span><span class="tok-o">++</span><span class="tok-w"> </span><span class="tok-s">&quot; zebras.&quot;</span>
</pre></div></td></tr></table></div></code></pre>
</div>
</div>
<div class="paragraph">
<p>Here&#8217;s a demonstration using the program.</p>
</div>
<div class="literalblock">
<div class="content">
<pre class="nowrap">$ nix-shell
$ runghc Main.hs
"I have 3 cats, 2 dogs, and 0 zebras."</pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="_in_a_haskell_script">10.4.2. In a Haskell script</h4>
<div class="listingblock">
<div class="title">Script</div>
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="haskell"><div class="lineno"><table class="linenotable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal"> 1</span>
<span class="normal"> 2</span>
<span class="normal"> 3</span>
<span class="normal"> 4</span>
<span class="normal"> 5</span>
<span class="normal"> 6</span>
<span class="normal"> 7</span>
<span class="normal"> 8</span>
<span class="normal"> 9</span>
<span class="normal">10</span>
<span class="normal">11</span>
<span class="normal">12</span>
<span class="normal">13</span>
<span class="normal">14</span>
<span class="normal">15</span></pre></div></td><td class="code"><div><pre><span></span><span class="tok-o">#!</span><span class="tok-w"> </span><span class="tok-o">/</span><span class="tok-n">usr</span><span class="tok-o">/</span><span class="tok-n">bin</span><span class="tok-o">/</span><span class="tok-n">env</span><span class="tok-w"> </span><span class="tok-n">nix</span><span class="tok-o">-</span><span class="tok-n">shell</span>
<span class="tok-o">#!</span><span class="tok-w"> </span><span class="tok-n">nix</span><span class="tok-o">-</span><span class="tok-n">shell</span><span class="tok-w"> </span><span class="tok-o">-</span><span class="tok-n">p</span><span class="tok-w"> </span><span class="tok-s">&quot;haskellPackages.ghcWithPackages (p: [p.containers])&quot;</span>
<span class="tok-o">#!</span><span class="tok-w"> </span><span class="tok-n">nix</span><span class="tok-o">-</span><span class="tok-n">shell</span><span class="tok-w"> </span><span class="tok-o">-</span><span class="tok-n">i</span><span class="tok-w"> </span><span class="tok-n">runghc</span>
<span class="tok-kr">import</span><span class="tok-w"> </span><span class="tok-nn">Data.Map</span>
<span class="tok-nf">m</span><span class="tok-w"> </span><span class="tok-ow">::</span><span class="tok-w"> </span><span class="tok-kt">Map</span><span class="tok-w"> </span><span class="tok-kt">String</span><span class="tok-w"> </span><span class="tok-kt">Int</span>
<span class="tok-nf">m</span><span class="tok-w"> </span><span class="tok-ow">=</span><span class="tok-w"> </span><span class="tok-n">fromList</span><span class="tok-w"> </span><span class="tok-p">[(</span><span class="tok-s">&quot;cats&quot;</span><span class="tok-p">,</span><span class="tok-w"> </span><span class="tok-mi">3</span><span class="tok-p">),</span><span class="tok-w"> </span><span class="tok-p">(</span><span class="tok-s">&quot;dogs&quot;</span><span class="tok-p">,</span><span class="tok-w"> </span><span class="tok-mi">2</span><span class="tok-p">)]</span>
<span class="tok-nf">main</span><span class="tok-w"> </span><span class="tok-ow">::</span><span class="tok-w"> </span><span class="tok-kt">IO</span><span class="tok-w"> </span><span class="tok-nb">()</span>
<span class="tok-nf">main</span><span class="tok-w"> </span><span class="tok-ow">=</span><span class="tok-w"> </span><span class="tok-kr">do</span>
<span class="tok-w"> </span><span class="tok-kr">let</span><span class="tok-w"> </span><span class="tok-n">cats</span><span class="tok-w"> </span><span class="tok-ow">=</span><span class="tok-w"> </span><span class="tok-n">findWithDefault</span><span class="tok-w"> </span><span class="tok-mi">0</span><span class="tok-w"> </span><span class="tok-s">&quot;cats&quot;</span><span class="tok-w"> </span><span class="tok-n">m</span>
<span class="tok-w"> </span><span class="tok-kr">let</span><span class="tok-w"> </span><span class="tok-n">dogs</span><span class="tok-w"> </span><span class="tok-ow">=</span><span class="tok-w"> </span><span class="tok-n">findWithDefault</span><span class="tok-w"> </span><span class="tok-mi">0</span><span class="tok-w"> </span><span class="tok-s">&quot;dogs&quot;</span><span class="tok-w"> </span><span class="tok-n">m</span>
<span class="tok-w"> </span><span class="tok-kr">let</span><span class="tok-w"> </span><span class="tok-n">zebras</span><span class="tok-w"> </span><span class="tok-ow">=</span><span class="tok-w"> </span><span class="tok-n">findWithDefault</span><span class="tok-w"> </span><span class="tok-mi">0</span><span class="tok-w"> </span><span class="tok-s">&quot;zebras&quot;</span><span class="tok-w"> </span><span class="tok-n">m</span>
<span class="tok-w"> </span><span class="tok-n">print</span><span class="tok-w"> </span><span class="tok-o">$</span><span class="tok-w"> </span><span class="tok-s">&quot;I have &quot;</span><span class="tok-w"> </span><span class="tok-o">++</span><span class="tok-w"> </span><span class="tok-n">show</span><span class="tok-w"> </span><span class="tok-n">cats</span><span class="tok-w"> </span><span class="tok-o">++</span><span class="tok-w"> </span><span class="tok-s">&quot; cats, &quot;</span><span class="tok-w"> </span><span class="tok-o">++</span><span class="tok-w"> </span><span class="tok-n">show</span><span class="tok-w"> </span><span class="tok-n">dogs</span><span class="tok-w"> </span><span class="tok-o">++</span><span class="tok-w"> </span><span class="tok-s">&quot; dogs, and &quot;</span><span class="tok-w"> </span><span class="tok-o">++</span><span class="tok-w"> </span><span class="tok-n">show</span><span class="tok-w"> </span><span class="tok-n">zebras</span><span class="tok-w"> </span><span class="tok-o">++</span><span class="tok-w"> </span><span class="tok-s">&quot; zebras.&quot;</span>
</pre></div></td></tr></table></div></code></pre>
</div>
</div>
<div class="literalblock">
<div class="title">Output</div>
<div class="content">
<pre class="nowrap">"I have 3 cats, 2 dogs, and 0 zebras."</pre>
</div>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_access_to_a_haskell_package_on_your_local_computer">10.5. Access to a Haskell package on your local computer</h3>
<div class="paragraph">
<p>Ex: Access three Haskell packages
(<code>pandoc-linear-table</code>, <code>pandoc-logic-proof</code>, and <code>pandoc-columns</code>)
that are on my hard drive.</p>
</div>
<div class="sect3">
<h4 id="_in_shell_nix_5">10.5.1. In <code>shell.nix</code></h4>
<div class="listingblock">
<div class="title">shell.nix</div>
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="nix"><div class="lineno"><table class="linenotable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal"> 1</span>
<span class="normal"> 2</span>
<span class="normal"> 3</span>
<span class="normal"> 4</span>
<span class="normal"> 5</span>
<span class="normal"> 6</span>
<span class="normal"> 7</span>
<span class="normal"> 8</span>
<span class="normal"> 9</span>
<span class="normal">10</span>
<span class="normal">11</span>
<span class="normal">12</span>
<span class="normal">13</span>
<span class="normal">14</span></pre></div></td><td class="code"><div><pre><span></span><span class="tok-k">with</span> <span class="tok-p">(</span><span class="tok-nb">import</span> <span class="tok-l">&lt;nixpkgs&gt;</span> <span class="tok-p">{});</span>
<span class="tok-k">let</span>
<span class="tok-ss">pandoc-linear-table =</span> haskellPackages<span class="tok-o">.</span>callCabal2nix <span class="tok-s2">&quot;pandoc-linear-table&quot;</span> <span class="tok-l">/home/amy/github/pandoc-linear-table</span> <span class="tok-p">{};</span>
<span class="tok-ss">pandoc-logic-proof =</span> haskellPackages<span class="tok-o">.</span>callCabal2nix <span class="tok-s2">&quot;pandoc-logic-proof&quot;</span> <span class="tok-l">/home/amy/github/pandoc-logic-proof</span> <span class="tok-p">{};</span>
<span class="tok-ss">pandoc-columns =</span> haskellPackages<span class="tok-o">.</span>callCabal2nix <span class="tok-s2">&quot;pandoc-columns&quot;</span> <span class="tok-l">/home/amy/github/pandoc-columns</span> <span class="tok-p">{};</span>
<span class="tok-k">in</span>
mkShell <span class="tok-p">{</span>
<span class="tok-ss">buildInputs =</span> <span class="tok-p">[</span>
pandoc
pandoc-linear-table
pandoc-logic-proof
pandoc-columns
<span class="tok-p">];</span>
<span class="tok-p">}</span>
</pre></div></td></tr></table></div></code></pre>
</div>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_access_to_a_haskell_package_on_your_local_computer_with_interdependencies">10.6. Access to a Haskell package on your local computer, with interdependencies</h3>
<div class="paragraph">
<p>Ex: Access four Haskell packages
(<code>pandoc-linear-table</code>, <code>pandoc-logic-proof</code>, <code>pandoc-columns</code> and <code>pandoc-maths-web</code>)
that are on my hard drive.
The fourth package depends on the first three to build.</p>
</div>
<div class="sect3">
<h4 id="_in_shell_nix_6">10.6.1. In <code>shell.nix</code></h4>
<div class="listingblock">
<div class="title">shell.nix</div>
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="nix"><div class="lineno"><table class="linenotable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal"> 1</span>
<span class="normal"> 2</span>
<span class="normal"> 3</span>
<span class="normal"> 4</span>
<span class="normal"> 5</span>
<span class="normal"> 6</span>
<span class="normal"> 7</span>
<span class="normal"> 8</span>
<span class="normal"> 9</span>
<span class="normal">10</span>
<span class="normal">11</span>
<span class="normal">12</span>
<span class="normal">13</span>
<span class="normal">14</span>
<span class="normal">15</span>
<span class="normal">16</span>
<span class="normal">17</span>
<span class="normal">18</span>
<span class="normal">19</span></pre></div></td><td class="code"><div><pre><span></span><span class="tok-k">with</span> <span class="tok-p">(</span><span class="tok-nb">import</span> <span class="tok-l">&lt;nixpkgs&gt;</span> <span class="tok-p">{});</span>
<span class="tok-k">let</span>
<span class="tok-ss">pandoc-linear-table =</span> haskellPackages<span class="tok-o">.</span>callCabal2nix <span class="tok-s2">&quot;pandoc-linear-table&quot;</span> <span class="tok-l">/home/amy/github/pandoc-linear-table</span> <span class="tok-p">{};</span>
<span class="tok-ss">pandoc-logic-proof =</span> haskellPackages<span class="tok-o">.</span>callCabal2nix <span class="tok-s2">&quot;pandoc-logic-proof&quot;</span> <span class="tok-l">/home/amy/github/pandoc-logic-proof</span> <span class="tok-p">{};</span>
<span class="tok-ss">pandoc-columns =</span> haskellPackages<span class="tok-o">.</span>callCabal2nix <span class="tok-s2">&quot;pandoc-columns&quot;</span> <span class="tok-l">/home/amy/github/pandoc-columns</span> <span class="tok-p">{};</span>
<span class="tok-ss">pandoc-maths-web =</span> haskellPackages<span class="tok-o">.</span>callCabal2nix <span class="tok-s2">&quot;pandoc-maths-web&quot;</span> <span class="tok-l">/home/amy/github/pandoc-maths-web</span>
<span class="tok-p">{</span>
<span class="tok-k">inherit</span> pandoc-linear-table pandoc-logic-proof pandoc-columns<span class="tok-p">;</span>
<span class="tok-p">};</span>
<span class="tok-k">in</span>
mkShell <span class="tok-p">{</span>
<span class="tok-ss">buildInputs =</span> <span class="tok-p">[</span>
pandoc
pandoc-linear-table
pandoc-logic-proof
pandoc-columns
pandoc-maths-web
<span class="tok-p">];</span>
<span class="tok-p">}</span>
</pre></div></td></tr></table></div></code></pre>
</div>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_access_to_a_python_library_package_in_the_nixpkgs_repo_without_using_a_python_builder">10.7. Access to a Python library package in the nixpkgs repo (without using a Python builder)</h3>
<div class="paragraph">
<p>Occasionally you might want to run a short Python program that depends on a Python library,
but you don&#8217;t want to bother configuring a builder.</p>
</div>
<div class="paragraph">
<p>Example: Access the <code>html_sanitizer</code> package from the <code>python3nnPackages</code> set in the nixpkgs repo.</p>
</div>
<div class="sect3">
<h4 id="_in_a_python_script">10.7.1. In a Python script</h4>
<div class="listingblock">
<div class="title">Script</div>
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="python"><div class="lineno"><table class="linenotable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal"> 1</span>
<span class="normal"> 2</span>
<span class="normal"> 3</span>
<span class="normal"> 4</span>
<span class="normal"> 5</span>
<span class="normal"> 6</span>
<span class="normal"> 7</span>
<span class="normal"> 8</span>
<span class="normal"> 9</span>
<span class="normal">10</span>
<span class="normal">11</span></pre></div></td><td class="code"><div><pre><span></span><span class="tok-ch">#! /usr/bin/env nix-shell</span>
<span class="tok-c1">#! nix-shell -i python3 -p python3Packages.html-sanitizer</span>
<span class="tok-kn">from</span> <span class="tok-nn">html_sanitizer</span> <span class="tok-kn">import</span> <span class="tok-n">Sanitizer</span>
<span class="tok-n">sanitizer</span> <span class="tok-o">=</span> <span class="tok-n">Sanitizer</span><span class="tok-p">()</span> <span class="tok-c1"># default configuration</span>
<span class="tok-n">original</span><span class="tok-o">=</span><span class="tok-s1">&#39;&lt;span style=&quot;font-weight:bold&quot;&gt;some text&lt;/span&gt;&#39;</span>
<span class="tok-nb">print</span><span class="tok-p">(</span><span class="tok-s1">&#39;original: &#39;</span><span class="tok-p">,</span> <span class="tok-n">original</span><span class="tok-p">)</span>
<span class="tok-n">sanitized</span><span class="tok-o">=</span><span class="tok-n">sanitizer</span><span class="tok-o">.</span><span class="tok-n">sanitize</span><span class="tok-p">(</span><span class="tok-n">original</span><span class="tok-p">)</span>
<span class="tok-nb">print</span><span class="tok-p">(</span><span class="tok-s1">&#39;sanitized: &#39;</span><span class="tok-p">,</span> <span class="tok-n">sanitized</span><span class="tok-p">)</span>
</pre></div></td></tr></table></div></code></pre>
</div>
</div>
<div class="literalblock">
<div class="title">Output</div>
<div class="content">
<pre class="nowrap">original: &lt;span style="font-weight:bold"&gt;some text&lt;/span&gt;
sanitized: &lt;strong&gt;some text&lt;/strong&gt;</pre>
</div>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_set_an_environment_variable">10.8. Set an environment variable</h3>
<div class="paragraph">
<p>Ex: Set the value of the environment variable FOO to &#8220;bar&#8221;</p>
</div>
<div class="sect3">
<h4 id="_in_shell_nix_7">10.8.1. In <code>shell.nix</code></h4>
<div class="listingblock">
<div class="title">shell.nix</div>
<div class="content">
<pre class="pygments highlight nowrap"><code data-lang="nix"><div class="lineno"><table class="linenotable"><tr><td class="linenos"><div class="linenodiv"><pre><span class="normal">1</span>
<span class="normal">2</span>
<span class="normal">3</span>
<span class="normal">4</span>
<span class="normal">5</span>
<span class="normal">6</span></pre></div></td><td class="code"><div><pre><span></span><span class="tok-k">with</span> <span class="tok-p">(</span><span class="tok-nb">import</span> <span class="tok-l">&lt;nixpkgs&gt;</span> <span class="tok-p">{});</span>
mkShell <span class="tok-p">{</span>
<span class="tok-ss">shellHook =</span> <span class="tok-s1">&#39;&#39;</span>
<span class="tok-s1"> export FOO=&quot;bar&quot;</span>
<span class="tok-s1"> &#39;&#39;</span><span class="tok-p">;</span>
<span class="tok-p">}</span>
</pre></div></td></tr></table></div></code></pre>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
<div id="footnotes">
<hr>
<div class="footnote" id="_footnotedef_1">
<a href="#_footnoteref_1">1</a>. REPL is an acronym for (Read-Eval-Print-Loop).
</div>
<div class="footnote" id="_footnotedef_2">
<a href="#_footnoteref_2">2</a>. For more information on the standard environment, see the <a href="https://nixos.org/manual/nixpkgs/stable/#sec-tools-of-stdenv">Nixpkgs manual</a>
</div>
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