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a purely functional...blog?
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<div class="mt-2">2024-05-25</div>
<div class="mt-1 text-sm">
(last updated: 2024-05-25T12:00:00Z)
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</header>
<main class="post mt-4"><p>Welcome! This is the first post on <em>conditional finality</em> and also one that tests all
of the features.</p>
<p><img
alt="conditional finality"
src="./images/conditional-finality.png"
/></p>
<blockquote>
<p>A monad is just a monoid in the category of endofunctors, what’s the problem?</p>
</blockquote>
<h2 id="haskell">haskell?</h2>
<p>This entire blog is generated with <a href="https://jaspervdj.be/hakyll/">hakyll</a>. It’s
a library for generating static sites for Haskell, a purely functional
programming language. It’s a <em>library</em> because it doesn’t come with as many
batteries included as tools like Hugo or Astro. You set up most of the site
yourself by calling the library from Haskell.</p>
<p>Here’s a brief excerpt:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="ot">main ::</span> <span class="dt">IO</span> ()</span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a>main <span class="ot">=</span> hakyllWith config <span class="op">$</span> <span class="kw">do</span></span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a> forM_</span>
<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a> [ <span class="st">&quot;CNAME&quot;</span></span>
<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a> , <span class="st">&quot;favicon.ico&quot;</span></span>
<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a> , <span class="st">&quot;robots.txt&quot;</span></span>
<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a> , <span class="st">&quot;_config.yml&quot;</span></span>
<span id="cb1-8"><a href="#cb1-8" aria-hidden="true" tabindex="-1"></a> , <span class="st">&quot;images/*&quot;</span></span>
<span id="cb1-9"><a href="#cb1-9" aria-hidden="true" tabindex="-1"></a> , <span class="st">&quot;out/*&quot;</span></span>
<span id="cb1-10"><a href="#cb1-10" aria-hidden="true" tabindex="-1"></a> , <span class="st">&quot;fonts/*&quot;</span></span>
<span id="cb1-11"><a href="#cb1-11" aria-hidden="true" tabindex="-1"></a> ]</span>
<span id="cb1-12"><a href="#cb1-12" aria-hidden="true" tabindex="-1"></a> <span class="op">$</span> \f <span class="ot">-&gt;</span> match f <span class="op">$</span> <span class="kw">do</span></span>
<span id="cb1-13"><a href="#cb1-13" aria-hidden="true" tabindex="-1"></a> route idRoute</span>
<span id="cb1-14"><a href="#cb1-14" aria-hidden="true" tabindex="-1"></a> compile copyFileCompiler</span></code></pre></div>
<p>The code highlighting is also generated by hakyll.</p>
<hr />
<h2 id="why">why?</h2>
<p>Haskell is a purely functional language with no mutable state. Its syntax
actually makes it pretty elegant for declaring routes and “rendering” pipelines.</p>
<ol>
<li>Haskell is cool.</li>
<li>It comes with enough features that I don’t feel like I have to build
everything from scratch.</li>
<li>It comes with Pandoc, a Haskell library for converting between markdown
formats. It’s probably more powerful than anything you could do in <code>nodejs</code>.
It renders all of the markdown to HTML as well as the math.
<ol>
<li>It supports KaTeX as well as MathML. I’m a little disappointed with the
KaTeX though. It doesn’t directly render it, but simply injects the KaTeX
files and renders it client-side.</li>
</ol></li>
</ol>
<h3 id="speaking-of-math">speaking of math</h3>
<p>We can have math inline, like so:
<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msubsup><mo>∫</mo><mrow><mo>−</mo><mi>∞</mi></mrow><mi>∞</mi></msubsup><mspace width="0.167em"></mspace><msup><mi>e</mi><mrow><mo>−</mo><msup><mi>x</mi><mn>2</mn></msup></mrow></msup><mspace width="0.167em"></mspace><mi>d</mi><mi>x</mi><mo>=</mo><msqrt><mi>π</mi></msqrt></mrow><annotation encoding="application/x-tex">\int_{-\infty}^\infty \, e^{-x^2}\,dx = \sqrt{\pi}</annotation></semantics></math>. This site ships semantic
MathML math with its HTML, and the MathJax script to the client.</p>
<p>It’d be nice if MathML could just be used and supported across all browsers, but
unfortunately we still aren’t quite there yet. Firefox is the only one where
everything looks 80% of the way to LaTeX. On Safari and Chrome, even simple
equations like <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><msqrt><mi>π</mi></msqrt><annotation encoding="application/x-tex">\sqrt{\pi}</annotation></semantics></math> render improperly.</p>
<p>Pros of MathML:</p>
<ul>
<li>A little more accessible</li>
<li>Can be rendered without additional stylesheets. I just installed the Latin
Modern font, but this isn’t even really necessary</li>
<li>Built-in to most browsers (#UseThePlatform)</li>
</ul>
<p>Cons:</p>
<ul>
<li>Isn’t fully standardized. Might look different on different browsers</li>
<li>Rendering quality isn’t as good as KaTeX</li>
</ul>
<p>This site has MathJax render all of the math so it looks nice and standardized
across browsers, but the math still displays regardless (like say if MathJax
couldn’t load due to slow network) because of MathML. Best of both worlds.</p>
<p>Let’s try it now. Here’s a simple theorem:</p>
<p><math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msup><mi>a</mi><mi>n</mi></msup><mo>+</mo><msup><mi>b</mi><mi>n</mi></msup><mo>≠</mo><msup><mi>c</mi><mi>n</mi></msup><mspace width="0.167em"></mspace><mo>∀</mo><mspace width="0.167em"></mspace><mrow><mo stretchy="true" form="prefix">{</mo><mi>a</mi><mo>,</mo><mspace width="0.167em"></mspace><mi>b</mi><mo>,</mo><mspace width="0.167em"></mspace><mi>c</mi><mo stretchy="true" form="postfix">}</mo></mrow><mo>∈</mo><mstyle mathvariant="double-struck"><mi>ℤ</mi></mstyle><mo>∧</mo><mi>n</mi><mo>≥</mo><mn>3</mn></mrow><annotation encoding="application/x-tex">
a^n + b^n \ne c^n \, \forall\,\left\{ a,\,b,\,c \right\} \in \mathbb{Z} \land n \ge 3
</annotation></semantics></math></p>
<p>The proof is trivial and will be left as an exercise to the reader.</p>
<h2 id="seems-a-little-overengineered">seems a little overengineered</h2>
<p>Probably is. Not as much as the old one, though.</p></main>
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<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
<title>conditional finality</title>
<link href="https://blog.youwen.dev/atom.xml" rel="self" />
<link href="https://blog.youwen.dev" />
<id>https://blog.youwen.dev/atom.xml</id>
<author>
<name>Youwen Wu</name>
<email>youwenw@gmail.com</email>
</author>
<updated>2024-12-28T00:00:00Z</updated>
<entry>
<title>Nix automatic hash updates made easy</title>
<link href="https://blog.youwen.dev/nix-automatic-hash-updates-made-easy.html" />
<id>https://blog.youwen.dev/nix-automatic-hash-updates-made-easy.html</id>
<published>2024-12-28T00:00:00Z</published>
<updated>2024-12-28T00:00:00Z</updated>
<summary type="html"><![CDATA[<article>
<header>
<h1 class="text-4xl">
<a href="./nix-automatic-hash-updates-made-easy.html">Nix automatic hash updates made easy</a>
</h1>
<p
class="mb-1 mt-2 italic font-light text-lg text-accent-light dark:text-accent-dark"
>
keep your flakes up to date
</p>
<div class="mt-2">2024-12-28</div>
<div class="mt-1 text-sm">
</div>
</header>
<main class="post mt-4"><p>Nix users often create flakes to package software out of tree, like this <a href="https://github.com/youwen5/zen-browser-flake">Zen
Browser flake</a> I’ve been
maintaining. Keeping them up to date is a hassle though, since you have to
update the Subresource Integrity (SRI) hashes that Nix uses to ensure
reproducibility.</p>
<p>Here’s a neat method I’ve been using to cleanly handle automatic hash updates.
I use <a href="https://www.nushell.sh/">Nushell</a> to easily work with data, prefetch
some hashes, and put it all in a JSON file that can be read by Nix at build
time.</p>
<p>First, let’s create a file called <code>update.nu</code>. At the top, place this shebang:</p>
<pre class="nu"><code>#!/usr/bin/env -S nix shell nixpkgs#nushell --command nu</code></pre>
<p>This will execute the script in a Nushell environment, which is fetched by Nix.</p>
<h2 id="get-the-up-to-date-urls">Get the up to date URLs</h2>
<p>We need to obtain the latest version of whatever software we want to update.
In this case, I’ll use GitHub releases as my source of truth.</p>
<p>You can use the GitHub API to fetch metadata about all the releases of a repository.</p>
<pre><code>https://api.github.com/repos/($repo)/releases</code></pre>
<p>Roughly speaking, the raw JSON returned by the GitHub releases API looks something like:</p>
<pre><code>[
{tag_name: &quot;foo&quot;, prerelease: false, ...},
{tag_name: &quot;bar&quot;, prerelease: true, ...},
{tag_name: &quot;foobar&quot;, prerelease: false, ...},
]
</code></pre>
<p>Note that the ordering of the objects in the array is chronological.</p>
<blockquote>
<p>Even if you aren’t using GitHub releases, as long as there is a reliable way to
programmatically fetch the latest download URLs of whatever software you’re
packaging, you can adapt this approach for your specific case.</p>
</blockquote>
<p>We use Nushell’s <code>http get</code> to make a network request. Nushell will
automatically detect and parse the JSON reponse into a Nushell table.</p>
<p>In my case, Zen Browser frequently publishes prerelease “twilight” builds which
we don’t want to update to. So, we ignore any releases tagged “twilight” or
marked “prerelease” by filtering them out with the <code>where</code> selector.</p>
<p>Finally, we retrieve the tag name of the item at the first index, which would
be the latest release (since the JSON array was chronologically sorted).</p>
<pre class="nu"><code>#!/usr/bin/env -S nix shell nixpkgs#nushell --command nu
# get the latest tag of the latest release that isn&#39;t a prerelease
def get_latest_release [repo: string] {
try {
http get $&quot;https://api.github.com/repos/($repo)/releases&quot;
| where prerelease == false
| where tag_name != &quot;twilight&quot;
| get tag_name
| get 0
} catch { |err| $&quot;Failed to fetch latest release, aborting: ($err.msg)&quot; }
}</code></pre>
<h2 id="prefetching-sri-hashes">Prefetching SRI hashes</h2>
<p>Now that we have the latest tags, we can easily obtain the latest download URLs, which are of the form:</p>
<pre><code>https://github.com/zen-browser/desktop/releases/download/$tag/zen.linux-x86_64.tar.bz2
https://github.com/zen-browser/desktop/releases/download/$tag/zen.aarch64-x86_64.tar.bz2</code></pre>
<p>However, we still need the corresponding SRI hashes to pass to Nix.</p>
<div class="sourceCode" id="cb6"><pre class="sourceCode nix"><code class="sourceCode nix"><span id="cb6-1"><a href="#cb6-1" aria-hidden="true" tabindex="-1"></a>src = fetchurl <span class="op">{</span></span>
<span id="cb6-2"><a href="#cb6-2" aria-hidden="true" tabindex="-1"></a> <span class="va">url</span> <span class="op">=</span> <span class="st">&quot;https://github.com/zen-browser/desktop/releases/download/1.0.2-b.5/zen.linux-x86_64.tar.bz2&quot;</span><span class="op">;</span></span>
<span id="cb6-3"><a href="#cb6-3" aria-hidden="true" tabindex="-1"></a> <span class="va">hash</span> <span class="op">=</span> <span class="st">&quot;sha256-00000000000000000000000000000000000000000000&quot;</span><span class="op">;</span></span>
<span id="cb6-4"><a href="#cb6-4" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span></code></pre></div>
<p>The easiest way to obtain these new hashes is to update the URL and then set
the hash property to an empty string (<code>""</code>). Nix will spit out an hash mismatch
error with the correct hash. However, this is inconvenient for automated
command line scripting.</p>
<p>The Nix documentation mentions
<a href="https://nix.dev/manual/nix/2.18/command-ref/nix-prefetch-url">nix-prefetch-url</a>
as a way to obtain these hashes, but as usual, it doesn’t work quite right and
has also been replaced by a more powerful but underdocumented experimental
feature instead.</p>
<p>The <a href="https://nix.dev/manual/nix/2.18/command-ref/new-cli/nix3-store-prefetch-file">nix store
prefetch-file</a>
command does what <code>nix-prefetch-url</code> is supposed to do, but handles the caveats
that lead to the wrong hash being produced automatically.</p>
<p>Let’s write a Nushell function that outputs the SRI hash of the given URL. We
tell <code>prefetch-file</code> to output structured JSON that we can parse.</p>
<p>Since Nushell <em>is</em> a shell, we can directly invoke shell commands like usual,
and then process their output with pipes.</p>
<pre class="nu"><code>def get_nix_hash [url: string] {
nix store prefetch-file --hash-type sha256 --json $url | from json | get hash
}</code></pre>
<p>Cool! Now <code>get_nix_hash</code> can give us SRI hashes that look like this:</p>
<pre><code>sha256-K3zTCLdvg/VYQNsfeohw65Ghk8FAjhOl8hXU6REO4/s=</code></pre>
<h2 id="putting-it-all-together">Putting it all together</h2>
<p>Now that we’re able to fetch the latest release, obtain the download URLs, and
compute their SRI hashes, we have all the information we need to make an
automated update. However, these URLs are typically hardcoded in our Nix
expressions. The question remains as to how to update these values.</p>
<p>A common way I’ve seen updates performed is using something like <code>sed</code> to
modify the Nix expressions in place. However, there’s actually a more
maintainable and easy to understand approach.</p>
<p>Let’s have our Nushell script generate the URLs and hashes and place them in a
JSON file! Then, we’ll be able to read the JSON file from Nix and obtain the
URL and hash.</p>
<pre class="nu"><code>def generate_sources [] {
let tag = get_latest_release &quot;zen-browser/desktop&quot;
let prev_sources = open ./sources.json
if $tag == $prev_sources.version {
# everything up to date
return $tag
}
# generate the download URLs with the new tag
let x86_64_url = $&quot;https://github.com/zen-browser/desktop/releases/download/($tag)/zen.linux-x86_64.tar.bz2&quot;
let aarch64_url = $&quot;https://github.com/zen-browser/desktop/releases/download/($tag)/zen.linux-aarch64.tar.bz2&quot;
# create a Nushell record that maps cleanly to JSON
let sources = {
# add a version field as well for convenience
version: $tag
x86_64-linux: {
url: $x86_64_url
hash: (get_nix_hash $x86_64_url)
}
aarch64-linux: {
url: $aarch64_url
hash: (get_nix_hash $aarch64_url)
}
}
echo $sources | save --force &quot;sources.json&quot;
return $tag
}</code></pre>
<p>Running this script with</p>
<div class="sourceCode" id="cb10"><pre class="sourceCode bash"><code class="sourceCode bash"><span id="cb10-1"><a href="#cb10-1" aria-hidden="true" tabindex="-1"></a><span class="fu">chmod</span> +x ./update.nu</span>
<span id="cb10-2"><a href="#cb10-2" aria-hidden="true" tabindex="-1"></a><span class="ex">./update.nu</span></span></code></pre></div>
<p>gives us the file <code>sources.json</code>:</p>
<div class="sourceCode" id="cb11"><pre class="sourceCode json"><code class="sourceCode json"><span id="cb11-1"><a href="#cb11-1" aria-hidden="true" tabindex="-1"></a><span class="fu">{</span></span>
<span id="cb11-2"><a href="#cb11-2" aria-hidden="true" tabindex="-1"></a> <span class="dt">&quot;version&quot;</span><span class="fu">:</span> <span class="st">&quot;1.0.2-b.5&quot;</span><span class="fu">,</span></span>
<span id="cb11-3"><a href="#cb11-3" aria-hidden="true" tabindex="-1"></a> <span class="dt">&quot;x86_64-linux&quot;</span><span class="fu">:</span> <span class="fu">{</span></span>
<span id="cb11-4"><a href="#cb11-4" aria-hidden="true" tabindex="-1"></a> <span class="dt">&quot;url&quot;</span><span class="fu">:</span> <span class="st">&quot;https://github.com/zen-browser/desktop/releases/download/1.0.2-b.5/zen.linux-x86_64.tar.bz2&quot;</span><span class="fu">,</span></span>
<span id="cb11-5"><a href="#cb11-5" aria-hidden="true" tabindex="-1"></a> <span class="dt">&quot;hash&quot;</span><span class="fu">:</span> <span class="st">&quot;sha256-K3zTCLdvg/VYQNsfeohw65Ghk8FAjhOl8hXU6REO4/s=&quot;</span></span>
<span id="cb11-6"><a href="#cb11-6" aria-hidden="true" tabindex="-1"></a> <span class="fu">},</span></span>
<span id="cb11-7"><a href="#cb11-7" aria-hidden="true" tabindex="-1"></a> <span class="dt">&quot;aarch64-linux&quot;</span><span class="fu">:</span> <span class="fu">{</span></span>
<span id="cb11-8"><a href="#cb11-8" aria-hidden="true" tabindex="-1"></a> <span class="dt">&quot;url&quot;</span><span class="fu">:</span> <span class="st">&quot;https://github.com/zen-browser/desktop/releases/download/1.0.2-b.5/zen.linux-aarch64.tar.bz2&quot;</span><span class="fu">,</span></span>
<span id="cb11-9"><a href="#cb11-9" aria-hidden="true" tabindex="-1"></a> <span class="dt">&quot;hash&quot;</span><span class="fu">:</span> <span class="st">&quot;sha256-NwIYylGal2QoWhWKtMhMkAAJQ6iNHfQOBZaxTXgvxAk=&quot;</span></span>
<span id="cb11-10"><a href="#cb11-10" aria-hidden="true" tabindex="-1"></a> <span class="fu">}</span></span>
<span id="cb11-11"><a href="#cb11-11" aria-hidden="true" tabindex="-1"></a><span class="fu">}</span></span></code></pre></div>
<p>Now, let’s read this from Nix. My file organization looks like the following:</p>
<pre><code>./
| flake.nix
| zen-browser-unwrapped.nix
| ...other files...</code></pre>
<p><code>zen-browser-unwrapped.nix</code> contains the derivation for Zen Browser. Let’s add
<code>version</code>, <code>url</code>, and <code>hash</code> to its inputs:</p>
<div class="sourceCode" id="cb13"><pre class="sourceCode nix"><code class="sourceCode nix"><span id="cb13-1"><a href="#cb13-1" aria-hidden="true" tabindex="-1"></a><span class="op">{</span></span>
<span id="cb13-2"><a href="#cb13-2" aria-hidden="true" tabindex="-1"></a> <span class="va">stdenv</span><span class="op">,</span></span>
<span id="cb13-3"><a href="#cb13-3" aria-hidden="true" tabindex="-1"></a> <span class="va">fetchurl</span><span class="op">,</span></span>
<span id="cb13-4"><a href="#cb13-4" aria-hidden="true" tabindex="-1"></a> <span class="co"># add these below</span></span>
<span id="cb13-5"><a href="#cb13-5" aria-hidden="true" tabindex="-1"></a> <span class="va">version</span><span class="op">,</span></span>
<span id="cb13-6"><a href="#cb13-6" aria-hidden="true" tabindex="-1"></a> <span class="va">url</span><span class="op">,</span></span>
<span id="cb13-7"><a href="#cb13-7" aria-hidden="true" tabindex="-1"></a> <span class="va">hash</span><span class="op">,</span></span>
<span id="cb13-8"><a href="#cb13-8" aria-hidden="true" tabindex="-1"></a> <span class="op">...</span></span>
<span id="cb13-9"><a href="#cb13-9" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>:</span>
<span id="cb13-10"><a href="#cb13-10" aria-hidden="true" tabindex="-1"></a>stdenv.mkDerivation <span class="op">{</span></span>
<span id="cb13-11"><a href="#cb13-11" aria-hidden="true" tabindex="-1"></a> <span class="co"># inherit version from inputs</span></span>
<span id="cb13-12"><a href="#cb13-12" aria-hidden="true" tabindex="-1"></a> <span class="kw">inherit</span> version<span class="op">;</span></span>
<span id="cb13-13"><a href="#cb13-13" aria-hidden="true" tabindex="-1"></a> <span class="va">pname</span> <span class="op">=</span> <span class="st">&quot;zen-browser-unwrapped&quot;</span><span class="op">;</span></span>
<span id="cb13-14"><a href="#cb13-14" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb13-15"><a href="#cb13-15" aria-hidden="true" tabindex="-1"></a> <span class="va">src</span> <span class="op">=</span> fetchurl <span class="op">{</span></span>
<span id="cb13-16"><a href="#cb13-16" aria-hidden="true" tabindex="-1"></a> <span class="co"># inherit the URL and hash we obtain from the inputs</span></span>
<span id="cb13-17"><a href="#cb13-17" aria-hidden="true" tabindex="-1"></a> <span class="kw">inherit</span> url hash<span class="op">;</span></span>
<span id="cb13-18"><a href="#cb13-18" aria-hidden="true" tabindex="-1"></a> <span class="op">};</span></span>
<span id="cb13-19"><a href="#cb13-19" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<p>Then in <code>flake.nix</code>, let’s provide the derivation with the data from <code>sources.json</code>:</p>
<div class="sourceCode" id="cb14"><pre class="sourceCode nix"><code class="sourceCode nix"><span id="cb14-1"><a href="#cb14-1" aria-hidden="true" tabindex="-1"></a><span class="kw">let</span></span>
<span id="cb14-2"><a href="#cb14-2" aria-hidden="true" tabindex="-1"></a> <span class="va">supportedSystems</span> <span class="op">=</span> <span class="op">[</span></span>
<span id="cb14-3"><a href="#cb14-3" aria-hidden="true" tabindex="-1"></a> <span class="st">&quot;x86_64-linux&quot;</span></span>
<span id="cb14-4"><a href="#cb14-4" aria-hidden="true" tabindex="-1"></a> <span class="st">&quot;aarch64-linux&quot;</span></span>
<span id="cb14-5"><a href="#cb14-5" aria-hidden="true" tabindex="-1"></a> <span class="op">];</span></span>
<span id="cb14-6"><a href="#cb14-6" aria-hidden="true" tabindex="-1"></a> <span class="va">forAllSystems</span> <span class="op">=</span> nixpkgs.lib.genAttrs supportedSystems<span class="op">;</span></span>
<span id="cb14-7"><a href="#cb14-7" aria-hidden="true" tabindex="-1"></a><span class="kw">in</span></span>
<span id="cb14-8"><a href="#cb14-8" aria-hidden="true" tabindex="-1"></a><span class="op">{</span></span>
<span id="cb14-9"><a href="#cb14-9" aria-hidden="true" tabindex="-1"></a> <span class="co"># rest of file omitted for simplicity</span></span>
<span id="cb14-10"><a href="#cb14-10" aria-hidden="true" tabindex="-1"></a> <span class="va">packages</span> <span class="op">=</span> forAllSystems <span class="op">(</span></span>
<span id="cb14-11"><a href="#cb14-11" aria-hidden="true" tabindex="-1"></a> <span class="va">system</span><span class="op">:</span></span>
<span id="cb14-12"><a href="#cb14-12" aria-hidden="true" tabindex="-1"></a> <span class="kw">let</span></span>
<span id="cb14-13"><a href="#cb14-13" aria-hidden="true" tabindex="-1"></a> <span class="va">pkgs</span> <span class="op">=</span> <span class="bu">import</span> nixpkgs <span class="op">{</span> <span class="kw">inherit</span> system<span class="op">;</span> <span class="op">};</span></span>
<span id="cb14-14"><a href="#cb14-14" aria-hidden="true" tabindex="-1"></a> <span class="co"># parse sources.json into a Nix attrset</span></span>
<span id="cb14-15"><a href="#cb14-15" aria-hidden="true" tabindex="-1"></a> <span class="va">sources</span> <span class="op">=</span> <span class="bu">builtins</span>.fromJSON <span class="op">(</span><span class="bu">builtins</span>.readFile <span class="ss">./sources.json</span><span class="op">);</span></span>
<span id="cb14-16"><a href="#cb14-16" aria-hidden="true" tabindex="-1"></a> <span class="kw">in</span></span>
<span id="cb14-17"><a href="#cb14-17" aria-hidden="true" tabindex="-1"></a> <span class="kw">rec</span> <span class="op">{</span></span>
<span id="cb14-18"><a href="#cb14-18" aria-hidden="true" tabindex="-1"></a> <span class="va">zen-browser-unwrapped</span> <span class="op">=</span> pkgs.callPackage <span class="ss">./zen-browser-unwrapped.nix</span> <span class="op">{</span></span>
<span id="cb14-19"><a href="#cb14-19" aria-hidden="true" tabindex="-1"></a> <span class="kw">inherit</span> <span class="op">(</span>sources.$<span class="op">{</span><span class="va">system</span><span class="op">})</span> hash url<span class="op">;</span></span>
<span id="cb14-20"><a href="#cb14-20" aria-hidden="true" tabindex="-1"></a> <span class="kw">inherit</span> <span class="op">(</span>sources<span class="op">)</span> version<span class="op">;</span></span>
<span id="cb14-21"><a href="#cb14-21" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb14-22"><a href="#cb14-22" aria-hidden="true" tabindex="-1"></a> <span class="co"># if the above is difficult to understand, it is equivalent to the following:</span></span>
<span id="cb14-23"><a href="#cb14-23" aria-hidden="true" tabindex="-1"></a> <span class="va">hash</span> <span class="op">=</span> sources.$<span class="op">{</span><span class="va">system</span><span class="op">}</span>.hash<span class="op">;</span></span>
<span id="cb14-24"><a href="#cb14-24" aria-hidden="true" tabindex="-1"></a> <span class="va">url</span> <span class="op">=</span> sources.$<span class="op">{</span><span class="va">system</span><span class="op">}</span>.url<span class="op">;</span></span>
<span id="cb14-25"><a href="#cb14-25" aria-hidden="true" tabindex="-1"></a> <span class="va">version</span> <span class="op">=</span> sources.version<span class="op">;</span></span>
<span id="cb14-26"><a href="#cb14-26" aria-hidden="true" tabindex="-1"></a> <span class="op">};</span></span>
<span id="cb14-27"><a href="#cb14-27" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<p>Now, running <code>nix build .#zen-browser-unwrapped</code> will be able to use the hashes
and URLs from <code>sources.json</code> to build the package!</p>
<h2 id="automating-it-in-ci">Automating it in CI</h2>
<p>We now have a script that can automatically fetch releases and generate hashes
and URLs, as well as a way for Nix to use the outputted JSON to build
derivations. All that’s left is to fully automate it using CI!</p>
<p>We are going to use GitHub actions for this, as it’s free and easy and you’re
probably already hosting on GitHub.</p>
<p>Ensure you’ve set up actions for your repo and given it sufficient permissions.</p>
<p>We’re gonna run it on a cron timer that checks for updates at 8 PM PST every day.</p>
<p>We use DeterminateSystems’ actions to help set up Nix. Then, we simply run our
update script. Since we made the script return the tag it fetched, we can store
it in a variable and then use it in our commit message.</p>
<pre><code>name: Update to latest version, and update flake inputs
on:
schedule:
- cron: &quot;0 4 * * *&quot;
workflow_dispatch:
jobs:
update:
name: Update flake inputs and browser
runs-on: ubuntu-latest
steps:
- name: Checkout Repository
uses: actions/checkout@v4
- name: Check flake inputs
uses: DeterminateSystems/flake-checker-action@v4
- name: Install Nix
uses: DeterminateSystems/nix-installer-action@main
- name: Set up magic Nix cache
uses: DeterminateSystems/magic-nix-cache-action@main
- name: Check for update and perform update
run: |
git config --global user.name &quot;github-actions[bot]&quot;
git config --global user.email &quot;github-actions[bot]@users.noreply.github.com&quot;
chmod +x ./update.nu
export ZEN_LATEST_VER=&quot;$(./update.nu)&quot;
git add -A
git commit -m &quot;github-actions: update to $ZEN_LATEST_VER&quot; || echo &quot;Latest version is $ZEN_LATEST_VER, no updates found&quot;
nix flake update --commit-lock-file
git push</code></pre>
<p>Now, our repository will automatically check for and perform updates every day!</p></main>
</article>
]]></summary>
</entry>
<entry>
<title>a haskellian blog</title>
<link href="https://blog.youwen.dev/a-haskellian-blog.html" />
<id>https://blog.youwen.dev/a-haskellian-blog.html</id>
<published>2024-05-25T00:00:00Z</published>
<updated>2024-05-25T12:00:00Z</updated>
<summary type="html"><![CDATA[<article>
<header>
<h1 class="text-4xl">
<a href="./a-haskellian-blog.html">a haskellian blog</a>
</h1>
<p
class="mb-1 mt-2 italic font-light text-lg text-accent-light dark:text-accent-dark"
>
a purely functional...blog?
</p>
<div class="mt-2">2024-05-25</div>
<div class="mt-1 text-sm">
(last updated: 2024-05-25T12:00:00Z)
</div>
</header>
<main class="post mt-4"><p>Welcome! This is the first post on <em>conditional finality</em> and also one that tests all
of the features.</p>
<p><img
alt="conditional finality"
src="./images/conditional-finality.png"
/></p>
<blockquote>
<p>A monad is just a monoid in the category of endofunctors, what’s the problem?</p>
</blockquote>
<h2 id="haskell">haskell?</h2>
<p>This entire blog is generated with <a href="https://jaspervdj.be/hakyll/">hakyll</a>. It’s
a library for generating static sites for Haskell, a purely functional
programming language. It’s a <em>library</em> because it doesn’t come with as many
batteries included as tools like Hugo or Astro. You set up most of the site
yourself by calling the library from Haskell.</p>
<p>Here’s a brief excerpt:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="ot">main ::</span> <span class="dt">IO</span> ()</span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a>main <span class="ot">=</span> hakyllWith config <span class="op">$</span> <span class="kw">do</span></span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a> forM_</span>
<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a> [ <span class="st">&quot;CNAME&quot;</span></span>
<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a> , <span class="st">&quot;favicon.ico&quot;</span></span>
<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a> , <span class="st">&quot;robots.txt&quot;</span></span>
<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a> , <span class="st">&quot;_config.yml&quot;</span></span>
<span id="cb1-8"><a href="#cb1-8" aria-hidden="true" tabindex="-1"></a> , <span class="st">&quot;images/*&quot;</span></span>
<span id="cb1-9"><a href="#cb1-9" aria-hidden="true" tabindex="-1"></a> , <span class="st">&quot;out/*&quot;</span></span>
<span id="cb1-10"><a href="#cb1-10" aria-hidden="true" tabindex="-1"></a> , <span class="st">&quot;fonts/*&quot;</span></span>
<span id="cb1-11"><a href="#cb1-11" aria-hidden="true" tabindex="-1"></a> ]</span>
<span id="cb1-12"><a href="#cb1-12" aria-hidden="true" tabindex="-1"></a> <span class="op">$</span> \f <span class="ot">-&gt;</span> match f <span class="op">$</span> <span class="kw">do</span></span>
<span id="cb1-13"><a href="#cb1-13" aria-hidden="true" tabindex="-1"></a> route idRoute</span>
<span id="cb1-14"><a href="#cb1-14" aria-hidden="true" tabindex="-1"></a> compile copyFileCompiler</span></code></pre></div>
<p>The code highlighting is also generated by hakyll.</p>
<hr />
<h2 id="why">why?</h2>
<p>Haskell is a purely functional language with no mutable state. Its syntax
actually makes it pretty elegant for declaring routes and “rendering” pipelines.</p>
<ol>
<li>Haskell is cool.</li>
<li>It comes with enough features that I don’t feel like I have to build
everything from scratch.</li>
<li>It comes with Pandoc, a Haskell library for converting between markdown
formats. It’s probably more powerful than anything you could do in <code>nodejs</code>.
It renders all of the markdown to HTML as well as the math.
<ol>
<li>It supports KaTeX as well as MathML. I’m a little disappointed with the
KaTeX though. It doesn’t directly render it, but simply injects the KaTeX
files and renders it client-side.</li>
</ol></li>
</ol>
<h3 id="speaking-of-math">speaking of math</h3>
<p>We can have math inline, like so:
<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msubsup><mo>∫</mo><mrow><mo>−</mo><mi>∞</mi></mrow><mi>∞</mi></msubsup><mspace width="0.167em"></mspace><msup><mi>e</mi><mrow><mo>−</mo><msup><mi>x</mi><mn>2</mn></msup></mrow></msup><mspace width="0.167em"></mspace><mi>d</mi><mi>x</mi><mo>=</mo><msqrt><mi>π</mi></msqrt></mrow><annotation encoding="application/x-tex">\int_{-\infty}^\infty \, e^{-x^2}\,dx = \sqrt{\pi}</annotation></semantics></math>. This site ships semantic
MathML math with its HTML, and the MathJax script to the client.</p>
<p>It’d be nice if MathML could just be used and supported across all browsers, but
unfortunately we still aren’t quite there yet. Firefox is the only one where
everything looks 80% of the way to LaTeX. On Safari and Chrome, even simple
equations like <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><msqrt><mi>π</mi></msqrt><annotation encoding="application/x-tex">\sqrt{\pi}</annotation></semantics></math> render improperly.</p>
<p>Pros of MathML:</p>
<ul>
<li>A little more accessible</li>
<li>Can be rendered without additional stylesheets. I just installed the Latin
Modern font, but this isn’t even really necessary</li>
<li>Built-in to most browsers (#UseThePlatform)</li>
</ul>
<p>Cons:</p>
<ul>
<li>Isn’t fully standardized. Might look different on different browsers</li>
<li>Rendering quality isn’t as good as KaTeX</li>
</ul>
<p>This site has MathJax render all of the math so it looks nice and standardized
across browsers, but the math still displays regardless (like say if MathJax
couldn’t load due to slow network) because of MathML. Best of both worlds.</p>
<p>Let’s try it now. Here’s a simple theorem:</p>
<p><math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msup><mi>a</mi><mi>n</mi></msup><mo>+</mo><msup><mi>b</mi><mi>n</mi></msup><mo>≠</mo><msup><mi>c</mi><mi>n</mi></msup><mspace width="0.167em"></mspace><mo>∀</mo><mspace width="0.167em"></mspace><mrow><mo stretchy="true" form="prefix">{</mo><mi>a</mi><mo>,</mo><mspace width="0.167em"></mspace><mi>b</mi><mo>,</mo><mspace width="0.167em"></mspace><mi>c</mi><mo stretchy="true" form="postfix">}</mo></mrow><mo>∈</mo><mstyle mathvariant="double-struck"><mi>ℤ</mi></mstyle><mo>∧</mo><mi>n</mi><mo>≥</mo><mn>3</mn></mrow><annotation encoding="application/x-tex">
a^n + b^n \ne c^n \, \forall\,\left\{ a,\,b,\,c \right\} \in \mathbb{Z} \land n \ge 3
</annotation></semantics></math></p>
<p>The proof is trivial and will be left as an exercise to the reader.</p>
<h2 id="seems-a-little-overengineered">seems a little overengineered</h2>
<p>Probably is. Not as much as the old one, though.</p></main>
</article>
]]></summary>
</entry>
</feed>

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<main class="post mt-4"><p>Nix users often create flakes to package software out of tree, like this <a href="https://github.com/youwen5/zen-browser-flake">Zen
Browser flake</a> I’ve been
maintaining. Keeping them up to date is a hassle though, since you have to
update the Subresource Integrity (SRI) hashes that Nix uses to ensure
reproducibility.</p>
<p>Here’s a neat method I’ve been using to cleanly handle automatic hash updates.
I use <a href="https://www.nushell.sh/">Nushell</a> to easily work with data, prefetch
some hashes, and put it all in a JSON file that can be read by Nix at build
time.</p>
<p>First, let’s create a file called <code>update.nu</code>. At the top, place this shebang:</p>
<pre class="nu"><code>#!/usr/bin/env -S nix shell nixpkgs#nushell --command nu</code></pre>
<p>This will execute the script in a Nushell environment, which is fetched by Nix.</p>
<h2 id="get-the-up-to-date-urls">Get the up to date URLs</h2>
<p>We need to obtain the latest version of whatever software we want to update.
In this case, I’ll use GitHub releases as my source of truth.</p>
<p>You can use the GitHub API to fetch metadata about all the releases of a repository.</p>
<pre><code>https://api.github.com/repos/($repo)/releases</code></pre>
<p>Roughly speaking, the raw JSON returned by the GitHub releases API looks something like:</p>
<pre><code>[
{tag_name: &quot;foo&quot;, prerelease: false, ...},
{tag_name: &quot;bar&quot;, prerelease: true, ...},
{tag_name: &quot;foobar&quot;, prerelease: false, ...},
]
</code></pre>
<p>Note that the ordering of the objects in the array is chronological.</p>
<blockquote>
<p>Even if you aren’t using GitHub releases, as long as there is a reliable way to
programmatically fetch the latest download URLs of whatever software you’re
packaging, you can adapt this approach for your specific case.</p>
</blockquote>
<p>We use Nushell’s <code>http get</code> to make a network request. Nushell will
automatically detect and parse the JSON reponse into a Nushell table.</p>
<p>In my case, Zen Browser frequently publishes prerelease “twilight” builds which
we don’t want to update to. So, we ignore any releases tagged “twilight” or
marked “prerelease” by filtering them out with the <code>where</code> selector.</p>
<p>Finally, we retrieve the tag name of the item at the first index, which would
be the latest release (since the JSON array was chronologically sorted).</p>
<pre class="nu"><code>#!/usr/bin/env -S nix shell nixpkgs#nushell --command nu
# get the latest tag of the latest release that isn&#39;t a prerelease
def get_latest_release [repo: string] {
try {
http get $&quot;https://api.github.com/repos/($repo)/releases&quot;
| where prerelease == false
| where tag_name != &quot;twilight&quot;
| get tag_name
| get 0
} catch { |err| $&quot;Failed to fetch latest release, aborting: ($err.msg)&quot; }
}</code></pre>
<h2 id="prefetching-sri-hashes">Prefetching SRI hashes</h2>
<p>Now that we have the latest tags, we can easily obtain the latest download URLs, which are of the form:</p>
<pre><code>https://github.com/zen-browser/desktop/releases/download/$tag/zen.linux-x86_64.tar.bz2
https://github.com/zen-browser/desktop/releases/download/$tag/zen.aarch64-x86_64.tar.bz2</code></pre>
<p>However, we still need the corresponding SRI hashes to pass to Nix.</p>
<div class="sourceCode" id="cb6"><pre class="sourceCode nix"><code class="sourceCode nix"><span id="cb6-1"><a href="#cb6-1" aria-hidden="true" tabindex="-1"></a>src = fetchurl <span class="op">{</span></span>
<span id="cb6-2"><a href="#cb6-2" aria-hidden="true" tabindex="-1"></a> <span class="va">url</span> <span class="op">=</span> <span class="st">&quot;https://github.com/zen-browser/desktop/releases/download/1.0.2-b.5/zen.linux-x86_64.tar.bz2&quot;</span><span class="op">;</span></span>
<span id="cb6-3"><a href="#cb6-3" aria-hidden="true" tabindex="-1"></a> <span class="va">hash</span> <span class="op">=</span> <span class="st">&quot;sha256-00000000000000000000000000000000000000000000&quot;</span><span class="op">;</span></span>
<span id="cb6-4"><a href="#cb6-4" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span></code></pre></div>
<p>The easiest way to obtain these new hashes is to update the URL and then set
the hash property to an empty string (<code>""</code>). Nix will spit out an hash mismatch
error with the correct hash. However, this is inconvenient for automated
command line scripting.</p>
<p>The Nix documentation mentions
<a href="https://nix.dev/manual/nix/2.18/command-ref/nix-prefetch-url">nix-prefetch-url</a>
as a way to obtain these hashes, but as usual, it doesn’t work quite right and
has also been replaced by a more powerful but underdocumented experimental
feature instead.</p>
<p>The <a href="https://nix.dev/manual/nix/2.18/command-ref/new-cli/nix3-store-prefetch-file">nix store
prefetch-file</a>
command does what <code>nix-prefetch-url</code> is supposed to do, but handles the caveats
that lead to the wrong hash being produced automatically.</p>
<p>Let’s write a Nushell function that outputs the SRI hash of the given URL. We
tell <code>prefetch-file</code> to output structured JSON that we can parse.</p>
<p>Since Nushell <em>is</em> a shell, we can directly invoke shell commands like usual,
and then process their output with pipes.</p>
<pre class="nu"><code>def get_nix_hash [url: string] {
nix store prefetch-file --hash-type sha256 --json $url | from json | get hash
}</code></pre>
<p>Cool! Now <code>get_nix_hash</code> can give us SRI hashes that look like this:</p>
<pre><code>sha256-K3zTCLdvg/VYQNsfeohw65Ghk8FAjhOl8hXU6REO4/s=</code></pre>
<h2 id="putting-it-all-together">Putting it all together</h2>
<p>Now that we’re able to fetch the latest release, obtain the download URLs, and
compute their SRI hashes, we have all the information we need to make an
automated update. However, these URLs are typically hardcoded in our Nix
expressions. The question remains as to how to update these values.</p>
<p>A common way I’ve seen updates performed is using something like <code>sed</code> to
modify the Nix expressions in place. However, there’s actually a more
maintainable and easy to understand approach.</p>
<p>Let’s have our Nushell script generate the URLs and hashes and place them in a
JSON file! Then, we’ll be able to read the JSON file from Nix and obtain the
URL and hash.</p>
<pre class="nu"><code>def generate_sources [] {
let tag = get_latest_release &quot;zen-browser/desktop&quot;
let prev_sources = open ./sources.json
if $tag == $prev_sources.version {
# everything up to date
return $tag
}
# generate the download URLs with the new tag
let x86_64_url = $&quot;https://github.com/zen-browser/desktop/releases/download/($tag)/zen.linux-x86_64.tar.bz2&quot;
let aarch64_url = $&quot;https://github.com/zen-browser/desktop/releases/download/($tag)/zen.linux-aarch64.tar.bz2&quot;
# create a Nushell record that maps cleanly to JSON
let sources = {
# add a version field as well for convenience
version: $tag
x86_64-linux: {
url: $x86_64_url
hash: (get_nix_hash $x86_64_url)
}
aarch64-linux: {
url: $aarch64_url
hash: (get_nix_hash $aarch64_url)
}
}
echo $sources | save --force &quot;sources.json&quot;
return $tag
}</code></pre>
<p>Running this script with</p>
<div class="sourceCode" id="cb10"><pre class="sourceCode bash"><code class="sourceCode bash"><span id="cb10-1"><a href="#cb10-1" aria-hidden="true" tabindex="-1"></a><span class="fu">chmod</span> +x ./update.nu</span>
<span id="cb10-2"><a href="#cb10-2" aria-hidden="true" tabindex="-1"></a><span class="ex">./update.nu</span></span></code></pre></div>
<p>gives us the file <code>sources.json</code>:</p>
<div class="sourceCode" id="cb11"><pre class="sourceCode json"><code class="sourceCode json"><span id="cb11-1"><a href="#cb11-1" aria-hidden="true" tabindex="-1"></a><span class="fu">{</span></span>
<span id="cb11-2"><a href="#cb11-2" aria-hidden="true" tabindex="-1"></a> <span class="dt">&quot;version&quot;</span><span class="fu">:</span> <span class="st">&quot;1.0.2-b.5&quot;</span><span class="fu">,</span></span>
<span id="cb11-3"><a href="#cb11-3" aria-hidden="true" tabindex="-1"></a> <span class="dt">&quot;x86_64-linux&quot;</span><span class="fu">:</span> <span class="fu">{</span></span>
<span id="cb11-4"><a href="#cb11-4" aria-hidden="true" tabindex="-1"></a> <span class="dt">&quot;url&quot;</span><span class="fu">:</span> <span class="st">&quot;https://github.com/zen-browser/desktop/releases/download/1.0.2-b.5/zen.linux-x86_64.tar.bz2&quot;</span><span class="fu">,</span></span>
<span id="cb11-5"><a href="#cb11-5" aria-hidden="true" tabindex="-1"></a> <span class="dt">&quot;hash&quot;</span><span class="fu">:</span> <span class="st">&quot;sha256-K3zTCLdvg/VYQNsfeohw65Ghk8FAjhOl8hXU6REO4/s=&quot;</span></span>
<span id="cb11-6"><a href="#cb11-6" aria-hidden="true" tabindex="-1"></a> <span class="fu">},</span></span>
<span id="cb11-7"><a href="#cb11-7" aria-hidden="true" tabindex="-1"></a> <span class="dt">&quot;aarch64-linux&quot;</span><span class="fu">:</span> <span class="fu">{</span></span>
<span id="cb11-8"><a href="#cb11-8" aria-hidden="true" tabindex="-1"></a> <span class="dt">&quot;url&quot;</span><span class="fu">:</span> <span class="st">&quot;https://github.com/zen-browser/desktop/releases/download/1.0.2-b.5/zen.linux-aarch64.tar.bz2&quot;</span><span class="fu">,</span></span>
<span id="cb11-9"><a href="#cb11-9" aria-hidden="true" tabindex="-1"></a> <span class="dt">&quot;hash&quot;</span><span class="fu">:</span> <span class="st">&quot;sha256-NwIYylGal2QoWhWKtMhMkAAJQ6iNHfQOBZaxTXgvxAk=&quot;</span></span>
<span id="cb11-10"><a href="#cb11-10" aria-hidden="true" tabindex="-1"></a> <span class="fu">}</span></span>
<span id="cb11-11"><a href="#cb11-11" aria-hidden="true" tabindex="-1"></a><span class="fu">}</span></span></code></pre></div>
<p>Now, let’s read this from Nix. My file organization looks like the following:</p>
<pre><code>./
| flake.nix
| zen-browser-unwrapped.nix
| ...other files...</code></pre>
<p><code>zen-browser-unwrapped.nix</code> contains the derivation for Zen Browser. Let’s add
<code>version</code>, <code>url</code>, and <code>hash</code> to its inputs:</p>
<div class="sourceCode" id="cb13"><pre class="sourceCode nix"><code class="sourceCode nix"><span id="cb13-1"><a href="#cb13-1" aria-hidden="true" tabindex="-1"></a><span class="op">{</span></span>
<span id="cb13-2"><a href="#cb13-2" aria-hidden="true" tabindex="-1"></a> <span class="va">stdenv</span><span class="op">,</span></span>
<span id="cb13-3"><a href="#cb13-3" aria-hidden="true" tabindex="-1"></a> <span class="va">fetchurl</span><span class="op">,</span></span>
<span id="cb13-4"><a href="#cb13-4" aria-hidden="true" tabindex="-1"></a> <span class="co"># add these below</span></span>
<span id="cb13-5"><a href="#cb13-5" aria-hidden="true" tabindex="-1"></a> <span class="va">version</span><span class="op">,</span></span>
<span id="cb13-6"><a href="#cb13-6" aria-hidden="true" tabindex="-1"></a> <span class="va">url</span><span class="op">,</span></span>
<span id="cb13-7"><a href="#cb13-7" aria-hidden="true" tabindex="-1"></a> <span class="va">hash</span><span class="op">,</span></span>
<span id="cb13-8"><a href="#cb13-8" aria-hidden="true" tabindex="-1"></a> <span class="op">...</span></span>
<span id="cb13-9"><a href="#cb13-9" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>:</span>
<span id="cb13-10"><a href="#cb13-10" aria-hidden="true" tabindex="-1"></a>stdenv.mkDerivation <span class="op">{</span></span>
<span id="cb13-11"><a href="#cb13-11" aria-hidden="true" tabindex="-1"></a> <span class="co"># inherit version from inputs</span></span>
<span id="cb13-12"><a href="#cb13-12" aria-hidden="true" tabindex="-1"></a> <span class="kw">inherit</span> version<span class="op">;</span></span>
<span id="cb13-13"><a href="#cb13-13" aria-hidden="true" tabindex="-1"></a> <span class="va">pname</span> <span class="op">=</span> <span class="st">&quot;zen-browser-unwrapped&quot;</span><span class="op">;</span></span>
<span id="cb13-14"><a href="#cb13-14" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb13-15"><a href="#cb13-15" aria-hidden="true" tabindex="-1"></a> <span class="va">src</span> <span class="op">=</span> fetchurl <span class="op">{</span></span>
<span id="cb13-16"><a href="#cb13-16" aria-hidden="true" tabindex="-1"></a> <span class="co"># inherit the URL and hash we obtain from the inputs</span></span>
<span id="cb13-17"><a href="#cb13-17" aria-hidden="true" tabindex="-1"></a> <span class="kw">inherit</span> url hash<span class="op">;</span></span>
<span id="cb13-18"><a href="#cb13-18" aria-hidden="true" tabindex="-1"></a> <span class="op">};</span></span>
<span id="cb13-19"><a href="#cb13-19" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<p>Then in <code>flake.nix</code>, let’s provide the derivation with the data from <code>sources.json</code>:</p>
<div class="sourceCode" id="cb14"><pre class="sourceCode nix"><code class="sourceCode nix"><span id="cb14-1"><a href="#cb14-1" aria-hidden="true" tabindex="-1"></a><span class="kw">let</span></span>
<span id="cb14-2"><a href="#cb14-2" aria-hidden="true" tabindex="-1"></a> <span class="va">supportedSystems</span> <span class="op">=</span> <span class="op">[</span></span>
<span id="cb14-3"><a href="#cb14-3" aria-hidden="true" tabindex="-1"></a> <span class="st">&quot;x86_64-linux&quot;</span></span>
<span id="cb14-4"><a href="#cb14-4" aria-hidden="true" tabindex="-1"></a> <span class="st">&quot;aarch64-linux&quot;</span></span>
<span id="cb14-5"><a href="#cb14-5" aria-hidden="true" tabindex="-1"></a> <span class="op">];</span></span>
<span id="cb14-6"><a href="#cb14-6" aria-hidden="true" tabindex="-1"></a> <span class="va">forAllSystems</span> <span class="op">=</span> nixpkgs.lib.genAttrs supportedSystems<span class="op">;</span></span>
<span id="cb14-7"><a href="#cb14-7" aria-hidden="true" tabindex="-1"></a><span class="kw">in</span></span>
<span id="cb14-8"><a href="#cb14-8" aria-hidden="true" tabindex="-1"></a><span class="op">{</span></span>
<span id="cb14-9"><a href="#cb14-9" aria-hidden="true" tabindex="-1"></a> <span class="co"># rest of file omitted for simplicity</span></span>
<span id="cb14-10"><a href="#cb14-10" aria-hidden="true" tabindex="-1"></a> <span class="va">packages</span> <span class="op">=</span> forAllSystems <span class="op">(</span></span>
<span id="cb14-11"><a href="#cb14-11" aria-hidden="true" tabindex="-1"></a> <span class="va">system</span><span class="op">:</span></span>
<span id="cb14-12"><a href="#cb14-12" aria-hidden="true" tabindex="-1"></a> <span class="kw">let</span></span>
<span id="cb14-13"><a href="#cb14-13" aria-hidden="true" tabindex="-1"></a> <span class="va">pkgs</span> <span class="op">=</span> <span class="bu">import</span> nixpkgs <span class="op">{</span> <span class="kw">inherit</span> system<span class="op">;</span> <span class="op">};</span></span>
<span id="cb14-14"><a href="#cb14-14" aria-hidden="true" tabindex="-1"></a> <span class="co"># parse sources.json into a Nix attrset</span></span>
<span id="cb14-15"><a href="#cb14-15" aria-hidden="true" tabindex="-1"></a> <span class="va">sources</span> <span class="op">=</span> <span class="bu">builtins</span>.fromJSON <span class="op">(</span><span class="bu">builtins</span>.readFile <span class="ss">./sources.json</span><span class="op">);</span></span>
<span id="cb14-16"><a href="#cb14-16" aria-hidden="true" tabindex="-1"></a> <span class="kw">in</span></span>
<span id="cb14-17"><a href="#cb14-17" aria-hidden="true" tabindex="-1"></a> <span class="kw">rec</span> <span class="op">{</span></span>
<span id="cb14-18"><a href="#cb14-18" aria-hidden="true" tabindex="-1"></a> <span class="va">zen-browser-unwrapped</span> <span class="op">=</span> pkgs.callPackage <span class="ss">./zen-browser-unwrapped.nix</span> <span class="op">{</span></span>
<span id="cb14-19"><a href="#cb14-19" aria-hidden="true" tabindex="-1"></a> <span class="kw">inherit</span> <span class="op">(</span>sources.$<span class="op">{</span><span class="va">system</span><span class="op">})</span> hash url<span class="op">;</span></span>
<span id="cb14-20"><a href="#cb14-20" aria-hidden="true" tabindex="-1"></a> <span class="kw">inherit</span> <span class="op">(</span>sources<span class="op">)</span> version<span class="op">;</span></span>
<span id="cb14-21"><a href="#cb14-21" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb14-22"><a href="#cb14-22" aria-hidden="true" tabindex="-1"></a> <span class="co"># if the above is difficult to understand, it is equivalent to the following:</span></span>
<span id="cb14-23"><a href="#cb14-23" aria-hidden="true" tabindex="-1"></a> <span class="va">hash</span> <span class="op">=</span> sources.$<span class="op">{</span><span class="va">system</span><span class="op">}</span>.hash<span class="op">;</span></span>
<span id="cb14-24"><a href="#cb14-24" aria-hidden="true" tabindex="-1"></a> <span class="va">url</span> <span class="op">=</span> sources.$<span class="op">{</span><span class="va">system</span><span class="op">}</span>.url<span class="op">;</span></span>
<span id="cb14-25"><a href="#cb14-25" aria-hidden="true" tabindex="-1"></a> <span class="va">version</span> <span class="op">=</span> sources.version<span class="op">;</span></span>
<span id="cb14-26"><a href="#cb14-26" aria-hidden="true" tabindex="-1"></a> <span class="op">};</span></span>
<span id="cb14-27"><a href="#cb14-27" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<p>Now, running <code>nix build .#zen-browser-unwrapped</code> will be able to use the hashes
and URLs from <code>sources.json</code> to build the package!</p>
<h2 id="automating-it-in-ci">Automating it in CI</h2>
<p>We now have a script that can automatically fetch releases and generate hashes
and URLs, as well as a way for Nix to use the outputted JSON to build
derivations. All that’s left is to fully automate it using CI!</p>
<p>We are going to use GitHub actions for this, as it’s free and easy and you’re
probably already hosting on GitHub.</p>
<p>Ensure you’ve set up actions for your repo and given it sufficient permissions.</p>
<p>We’re gonna run it on a cron timer that checks for updates at 8 PM PST every day.</p>
<p>We use DeterminateSystems’ actions to help set up Nix. Then, we simply run our
update script. Since we made the script return the tag it fetched, we can store
it in a variable and then use it in our commit message.</p>
<pre><code>name: Update to latest version, and update flake inputs
on:
schedule:
- cron: &quot;0 4 * * *&quot;
workflow_dispatch:
jobs:
update:
name: Update flake inputs and browser
runs-on: ubuntu-latest
steps:
- name: Checkout Repository
uses: actions/checkout@v4
- name: Check flake inputs
uses: DeterminateSystems/flake-checker-action@v4
- name: Install Nix
uses: DeterminateSystems/nix-installer-action@main
- name: Set up magic Nix cache
uses: DeterminateSystems/magic-nix-cache-action@main
- name: Check for update and perform update
run: |
git config --global user.name &quot;github-actions[bot]&quot;
git config --global user.email &quot;github-actions[bot]@users.noreply.github.com&quot;
chmod +x ./update.nu
export ZEN_LATEST_VER=&quot;$(./update.nu)&quot;
git add -A
git commit -m &quot;github-actions: update to $ZEN_LATEST_VER&quot; || echo &quot;Latest version is $ZEN_LATEST_VER, no updates found&quot;
nix flake update --commit-lock-file
git push</code></pre>
<p>Now, our repository will automatically check for and perform updates every day!</p></main>
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<title>conditional finality</title>
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<item>
<title>Nix automatic hash updates made easy</title>
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<h1 class="text-4xl">
<a href="./nix-automatic-hash-updates-made-easy.html">Nix automatic hash updates made easy</a>
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<p
class="mb-1 mt-2 italic font-light text-lg text-accent-light dark:text-accent-dark"
>
keep your flakes up to date
</p>
<div class="mt-2">2024-12-28</div>
<div class="mt-1 text-sm">
</div>
</header>
<main class="post mt-4"><p>Nix users often create flakes to package software out of tree, like this <a href="https://github.com/youwen5/zen-browser-flake">Zen
Browser flake</a> I’ve been
maintaining. Keeping them up to date is a hassle though, since you have to
update the Subresource Integrity (SRI) hashes that Nix uses to ensure
reproducibility.</p>
<p>Here’s a neat method I’ve been using to cleanly handle automatic hash updates.
I use <a href="https://www.nushell.sh/">Nushell</a> to easily work with data, prefetch
some hashes, and put it all in a JSON file that can be read by Nix at build
time.</p>
<p>First, let’s create a file called <code>update.nu</code>. At the top, place this shebang:</p>
<pre class="nu"><code>#!/usr/bin/env -S nix shell nixpkgs#nushell --command nu</code></pre>
<p>This will execute the script in a Nushell environment, which is fetched by Nix.</p>
<h2 id="get-the-up-to-date-urls">Get the up to date URLs</h2>
<p>We need to obtain the latest version of whatever software we want to update.
In this case, I’ll use GitHub releases as my source of truth.</p>
<p>You can use the GitHub API to fetch metadata about all the releases of a repository.</p>
<pre><code>https://api.github.com/repos/($repo)/releases</code></pre>
<p>Roughly speaking, the raw JSON returned by the GitHub releases API looks something like:</p>
<pre><code>[
{tag_name: &quot;foo&quot;, prerelease: false, ...},
{tag_name: &quot;bar&quot;, prerelease: true, ...},
{tag_name: &quot;foobar&quot;, prerelease: false, ...},
]
</code></pre>
<p>Note that the ordering of the objects in the array is chronological.</p>
<blockquote>
<p>Even if you aren’t using GitHub releases, as long as there is a reliable way to
programmatically fetch the latest download URLs of whatever software you’re
packaging, you can adapt this approach for your specific case.</p>
</blockquote>
<p>We use Nushell’s <code>http get</code> to make a network request. Nushell will
automatically detect and parse the JSON reponse into a Nushell table.</p>
<p>In my case, Zen Browser frequently publishes prerelease “twilight” builds which
we don’t want to update to. So, we ignore any releases tagged “twilight” or
marked “prerelease” by filtering them out with the <code>where</code> selector.</p>
<p>Finally, we retrieve the tag name of the item at the first index, which would
be the latest release (since the JSON array was chronologically sorted).</p>
<pre class="nu"><code>#!/usr/bin/env -S nix shell nixpkgs#nushell --command nu
# get the latest tag of the latest release that isn&#39;t a prerelease
def get_latest_release [repo: string] {
try {
http get $&quot;https://api.github.com/repos/($repo)/releases&quot;
| where prerelease == false
| where tag_name != &quot;twilight&quot;
| get tag_name
| get 0
} catch { |err| $&quot;Failed to fetch latest release, aborting: ($err.msg)&quot; }
}</code></pre>
<h2 id="prefetching-sri-hashes">Prefetching SRI hashes</h2>
<p>Now that we have the latest tags, we can easily obtain the latest download URLs, which are of the form:</p>
<pre><code>https://github.com/zen-browser/desktop/releases/download/$tag/zen.linux-x86_64.tar.bz2
https://github.com/zen-browser/desktop/releases/download/$tag/zen.aarch64-x86_64.tar.bz2</code></pre>
<p>However, we still need the corresponding SRI hashes to pass to Nix.</p>
<div class="sourceCode" id="cb6"><pre class="sourceCode nix"><code class="sourceCode nix"><span id="cb6-1"><a href="#cb6-1" aria-hidden="true" tabindex="-1"></a>src = fetchurl <span class="op">{</span></span>
<span id="cb6-2"><a href="#cb6-2" aria-hidden="true" tabindex="-1"></a> <span class="va">url</span> <span class="op">=</span> <span class="st">&quot;https://github.com/zen-browser/desktop/releases/download/1.0.2-b.5/zen.linux-x86_64.tar.bz2&quot;</span><span class="op">;</span></span>
<span id="cb6-3"><a href="#cb6-3" aria-hidden="true" tabindex="-1"></a> <span class="va">hash</span> <span class="op">=</span> <span class="st">&quot;sha256-00000000000000000000000000000000000000000000&quot;</span><span class="op">;</span></span>
<span id="cb6-4"><a href="#cb6-4" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>;</span></code></pre></div>
<p>The easiest way to obtain these new hashes is to update the URL and then set
the hash property to an empty string (<code>""</code>). Nix will spit out an hash mismatch
error with the correct hash. However, this is inconvenient for automated
command line scripting.</p>
<p>The Nix documentation mentions
<a href="https://nix.dev/manual/nix/2.18/command-ref/nix-prefetch-url">nix-prefetch-url</a>
as a way to obtain these hashes, but as usual, it doesn’t work quite right and
has also been replaced by a more powerful but underdocumented experimental
feature instead.</p>
<p>The <a href="https://nix.dev/manual/nix/2.18/command-ref/new-cli/nix3-store-prefetch-file">nix store
prefetch-file</a>
command does what <code>nix-prefetch-url</code> is supposed to do, but handles the caveats
that lead to the wrong hash being produced automatically.</p>
<p>Let’s write a Nushell function that outputs the SRI hash of the given URL. We
tell <code>prefetch-file</code> to output structured JSON that we can parse.</p>
<p>Since Nushell <em>is</em> a shell, we can directly invoke shell commands like usual,
and then process their output with pipes.</p>
<pre class="nu"><code>def get_nix_hash [url: string] {
nix store prefetch-file --hash-type sha256 --json $url | from json | get hash
}</code></pre>
<p>Cool! Now <code>get_nix_hash</code> can give us SRI hashes that look like this:</p>
<pre><code>sha256-K3zTCLdvg/VYQNsfeohw65Ghk8FAjhOl8hXU6REO4/s=</code></pre>
<h2 id="putting-it-all-together">Putting it all together</h2>
<p>Now that we’re able to fetch the latest release, obtain the download URLs, and
compute their SRI hashes, we have all the information we need to make an
automated update. However, these URLs are typically hardcoded in our Nix
expressions. The question remains as to how to update these values.</p>
<p>A common way I’ve seen updates performed is using something like <code>sed</code> to
modify the Nix expressions in place. However, there’s actually a more
maintainable and easy to understand approach.</p>
<p>Let’s have our Nushell script generate the URLs and hashes and place them in a
JSON file! Then, we’ll be able to read the JSON file from Nix and obtain the
URL and hash.</p>
<pre class="nu"><code>def generate_sources [] {
let tag = get_latest_release &quot;zen-browser/desktop&quot;
let prev_sources = open ./sources.json
if $tag == $prev_sources.version {
# everything up to date
return $tag
}
# generate the download URLs with the new tag
let x86_64_url = $&quot;https://github.com/zen-browser/desktop/releases/download/($tag)/zen.linux-x86_64.tar.bz2&quot;
let aarch64_url = $&quot;https://github.com/zen-browser/desktop/releases/download/($tag)/zen.linux-aarch64.tar.bz2&quot;
# create a Nushell record that maps cleanly to JSON
let sources = {
# add a version field as well for convenience
version: $tag
x86_64-linux: {
url: $x86_64_url
hash: (get_nix_hash $x86_64_url)
}
aarch64-linux: {
url: $aarch64_url
hash: (get_nix_hash $aarch64_url)
}
}
echo $sources | save --force &quot;sources.json&quot;
return $tag
}</code></pre>
<p>Running this script with</p>
<div class="sourceCode" id="cb10"><pre class="sourceCode bash"><code class="sourceCode bash"><span id="cb10-1"><a href="#cb10-1" aria-hidden="true" tabindex="-1"></a><span class="fu">chmod</span> +x ./update.nu</span>
<span id="cb10-2"><a href="#cb10-2" aria-hidden="true" tabindex="-1"></a><span class="ex">./update.nu</span></span></code></pre></div>
<p>gives us the file <code>sources.json</code>:</p>
<div class="sourceCode" id="cb11"><pre class="sourceCode json"><code class="sourceCode json"><span id="cb11-1"><a href="#cb11-1" aria-hidden="true" tabindex="-1"></a><span class="fu">{</span></span>
<span id="cb11-2"><a href="#cb11-2" aria-hidden="true" tabindex="-1"></a> <span class="dt">&quot;version&quot;</span><span class="fu">:</span> <span class="st">&quot;1.0.2-b.5&quot;</span><span class="fu">,</span></span>
<span id="cb11-3"><a href="#cb11-3" aria-hidden="true" tabindex="-1"></a> <span class="dt">&quot;x86_64-linux&quot;</span><span class="fu">:</span> <span class="fu">{</span></span>
<span id="cb11-4"><a href="#cb11-4" aria-hidden="true" tabindex="-1"></a> <span class="dt">&quot;url&quot;</span><span class="fu">:</span> <span class="st">&quot;https://github.com/zen-browser/desktop/releases/download/1.0.2-b.5/zen.linux-x86_64.tar.bz2&quot;</span><span class="fu">,</span></span>
<span id="cb11-5"><a href="#cb11-5" aria-hidden="true" tabindex="-1"></a> <span class="dt">&quot;hash&quot;</span><span class="fu">:</span> <span class="st">&quot;sha256-K3zTCLdvg/VYQNsfeohw65Ghk8FAjhOl8hXU6REO4/s=&quot;</span></span>
<span id="cb11-6"><a href="#cb11-6" aria-hidden="true" tabindex="-1"></a> <span class="fu">},</span></span>
<span id="cb11-7"><a href="#cb11-7" aria-hidden="true" tabindex="-1"></a> <span class="dt">&quot;aarch64-linux&quot;</span><span class="fu">:</span> <span class="fu">{</span></span>
<span id="cb11-8"><a href="#cb11-8" aria-hidden="true" tabindex="-1"></a> <span class="dt">&quot;url&quot;</span><span class="fu">:</span> <span class="st">&quot;https://github.com/zen-browser/desktop/releases/download/1.0.2-b.5/zen.linux-aarch64.tar.bz2&quot;</span><span class="fu">,</span></span>
<span id="cb11-9"><a href="#cb11-9" aria-hidden="true" tabindex="-1"></a> <span class="dt">&quot;hash&quot;</span><span class="fu">:</span> <span class="st">&quot;sha256-NwIYylGal2QoWhWKtMhMkAAJQ6iNHfQOBZaxTXgvxAk=&quot;</span></span>
<span id="cb11-10"><a href="#cb11-10" aria-hidden="true" tabindex="-1"></a> <span class="fu">}</span></span>
<span id="cb11-11"><a href="#cb11-11" aria-hidden="true" tabindex="-1"></a><span class="fu">}</span></span></code></pre></div>
<p>Now, let’s read this from Nix. My file organization looks like the following:</p>
<pre><code>./
| flake.nix
| zen-browser-unwrapped.nix
| ...other files...</code></pre>
<p><code>zen-browser-unwrapped.nix</code> contains the derivation for Zen Browser. Let’s add
<code>version</code>, <code>url</code>, and <code>hash</code> to its inputs:</p>
<div class="sourceCode" id="cb13"><pre class="sourceCode nix"><code class="sourceCode nix"><span id="cb13-1"><a href="#cb13-1" aria-hidden="true" tabindex="-1"></a><span class="op">{</span></span>
<span id="cb13-2"><a href="#cb13-2" aria-hidden="true" tabindex="-1"></a> <span class="va">stdenv</span><span class="op">,</span></span>
<span id="cb13-3"><a href="#cb13-3" aria-hidden="true" tabindex="-1"></a> <span class="va">fetchurl</span><span class="op">,</span></span>
<span id="cb13-4"><a href="#cb13-4" aria-hidden="true" tabindex="-1"></a> <span class="co"># add these below</span></span>
<span id="cb13-5"><a href="#cb13-5" aria-hidden="true" tabindex="-1"></a> <span class="va">version</span><span class="op">,</span></span>
<span id="cb13-6"><a href="#cb13-6" aria-hidden="true" tabindex="-1"></a> <span class="va">url</span><span class="op">,</span></span>
<span id="cb13-7"><a href="#cb13-7" aria-hidden="true" tabindex="-1"></a> <span class="va">hash</span><span class="op">,</span></span>
<span id="cb13-8"><a href="#cb13-8" aria-hidden="true" tabindex="-1"></a> <span class="op">...</span></span>
<span id="cb13-9"><a href="#cb13-9" aria-hidden="true" tabindex="-1"></a><span class="op">}</span>:</span>
<span id="cb13-10"><a href="#cb13-10" aria-hidden="true" tabindex="-1"></a>stdenv.mkDerivation <span class="op">{</span></span>
<span id="cb13-11"><a href="#cb13-11" aria-hidden="true" tabindex="-1"></a> <span class="co"># inherit version from inputs</span></span>
<span id="cb13-12"><a href="#cb13-12" aria-hidden="true" tabindex="-1"></a> <span class="kw">inherit</span> version<span class="op">;</span></span>
<span id="cb13-13"><a href="#cb13-13" aria-hidden="true" tabindex="-1"></a> <span class="va">pname</span> <span class="op">=</span> <span class="st">&quot;zen-browser-unwrapped&quot;</span><span class="op">;</span></span>
<span id="cb13-14"><a href="#cb13-14" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb13-15"><a href="#cb13-15" aria-hidden="true" tabindex="-1"></a> <span class="va">src</span> <span class="op">=</span> fetchurl <span class="op">{</span></span>
<span id="cb13-16"><a href="#cb13-16" aria-hidden="true" tabindex="-1"></a> <span class="co"># inherit the URL and hash we obtain from the inputs</span></span>
<span id="cb13-17"><a href="#cb13-17" aria-hidden="true" tabindex="-1"></a> <span class="kw">inherit</span> url hash<span class="op">;</span></span>
<span id="cb13-18"><a href="#cb13-18" aria-hidden="true" tabindex="-1"></a> <span class="op">};</span></span>
<span id="cb13-19"><a href="#cb13-19" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<p>Then in <code>flake.nix</code>, let’s provide the derivation with the data from <code>sources.json</code>:</p>
<div class="sourceCode" id="cb14"><pre class="sourceCode nix"><code class="sourceCode nix"><span id="cb14-1"><a href="#cb14-1" aria-hidden="true" tabindex="-1"></a><span class="kw">let</span></span>
<span id="cb14-2"><a href="#cb14-2" aria-hidden="true" tabindex="-1"></a> <span class="va">supportedSystems</span> <span class="op">=</span> <span class="op">[</span></span>
<span id="cb14-3"><a href="#cb14-3" aria-hidden="true" tabindex="-1"></a> <span class="st">&quot;x86_64-linux&quot;</span></span>
<span id="cb14-4"><a href="#cb14-4" aria-hidden="true" tabindex="-1"></a> <span class="st">&quot;aarch64-linux&quot;</span></span>
<span id="cb14-5"><a href="#cb14-5" aria-hidden="true" tabindex="-1"></a> <span class="op">];</span></span>
<span id="cb14-6"><a href="#cb14-6" aria-hidden="true" tabindex="-1"></a> <span class="va">forAllSystems</span> <span class="op">=</span> nixpkgs.lib.genAttrs supportedSystems<span class="op">;</span></span>
<span id="cb14-7"><a href="#cb14-7" aria-hidden="true" tabindex="-1"></a><span class="kw">in</span></span>
<span id="cb14-8"><a href="#cb14-8" aria-hidden="true" tabindex="-1"></a><span class="op">{</span></span>
<span id="cb14-9"><a href="#cb14-9" aria-hidden="true" tabindex="-1"></a> <span class="co"># rest of file omitted for simplicity</span></span>
<span id="cb14-10"><a href="#cb14-10" aria-hidden="true" tabindex="-1"></a> <span class="va">packages</span> <span class="op">=</span> forAllSystems <span class="op">(</span></span>
<span id="cb14-11"><a href="#cb14-11" aria-hidden="true" tabindex="-1"></a> <span class="va">system</span><span class="op">:</span></span>
<span id="cb14-12"><a href="#cb14-12" aria-hidden="true" tabindex="-1"></a> <span class="kw">let</span></span>
<span id="cb14-13"><a href="#cb14-13" aria-hidden="true" tabindex="-1"></a> <span class="va">pkgs</span> <span class="op">=</span> <span class="bu">import</span> nixpkgs <span class="op">{</span> <span class="kw">inherit</span> system<span class="op">;</span> <span class="op">};</span></span>
<span id="cb14-14"><a href="#cb14-14" aria-hidden="true" tabindex="-1"></a> <span class="co"># parse sources.json into a Nix attrset</span></span>
<span id="cb14-15"><a href="#cb14-15" aria-hidden="true" tabindex="-1"></a> <span class="va">sources</span> <span class="op">=</span> <span class="bu">builtins</span>.fromJSON <span class="op">(</span><span class="bu">builtins</span>.readFile <span class="ss">./sources.json</span><span class="op">);</span></span>
<span id="cb14-16"><a href="#cb14-16" aria-hidden="true" tabindex="-1"></a> <span class="kw">in</span></span>
<span id="cb14-17"><a href="#cb14-17" aria-hidden="true" tabindex="-1"></a> <span class="kw">rec</span> <span class="op">{</span></span>
<span id="cb14-18"><a href="#cb14-18" aria-hidden="true" tabindex="-1"></a> <span class="va">zen-browser-unwrapped</span> <span class="op">=</span> pkgs.callPackage <span class="ss">./zen-browser-unwrapped.nix</span> <span class="op">{</span></span>
<span id="cb14-19"><a href="#cb14-19" aria-hidden="true" tabindex="-1"></a> <span class="kw">inherit</span> <span class="op">(</span>sources.$<span class="op">{</span><span class="va">system</span><span class="op">})</span> hash url<span class="op">;</span></span>
<span id="cb14-20"><a href="#cb14-20" aria-hidden="true" tabindex="-1"></a> <span class="kw">inherit</span> <span class="op">(</span>sources<span class="op">)</span> version<span class="op">;</span></span>
<span id="cb14-21"><a href="#cb14-21" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb14-22"><a href="#cb14-22" aria-hidden="true" tabindex="-1"></a> <span class="co"># if the above is difficult to understand, it is equivalent to the following:</span></span>
<span id="cb14-23"><a href="#cb14-23" aria-hidden="true" tabindex="-1"></a> <span class="va">hash</span> <span class="op">=</span> sources.$<span class="op">{</span><span class="va">system</span><span class="op">}</span>.hash<span class="op">;</span></span>
<span id="cb14-24"><a href="#cb14-24" aria-hidden="true" tabindex="-1"></a> <span class="va">url</span> <span class="op">=</span> sources.$<span class="op">{</span><span class="va">system</span><span class="op">}</span>.url<span class="op">;</span></span>
<span id="cb14-25"><a href="#cb14-25" aria-hidden="true" tabindex="-1"></a> <span class="va">version</span> <span class="op">=</span> sources.version<span class="op">;</span></span>
<span id="cb14-26"><a href="#cb14-26" aria-hidden="true" tabindex="-1"></a> <span class="op">};</span></span>
<span id="cb14-27"><a href="#cb14-27" aria-hidden="true" tabindex="-1"></a><span class="op">}</span></span></code></pre></div>
<p>Now, running <code>nix build .#zen-browser-unwrapped</code> will be able to use the hashes
and URLs from <code>sources.json</code> to build the package!</p>
<h2 id="automating-it-in-ci">Automating it in CI</h2>
<p>We now have a script that can automatically fetch releases and generate hashes
and URLs, as well as a way for Nix to use the outputted JSON to build
derivations. All that’s left is to fully automate it using CI!</p>
<p>We are going to use GitHub actions for this, as it’s free and easy and you’re
probably already hosting on GitHub.</p>
<p>Ensure you’ve set up actions for your repo and given it sufficient permissions.</p>
<p>We’re gonna run it on a cron timer that checks for updates at 8 PM PST every day.</p>
<p>We use DeterminateSystems’ actions to help set up Nix. Then, we simply run our
update script. Since we made the script return the tag it fetched, we can store
it in a variable and then use it in our commit message.</p>
<pre><code>name: Update to latest version, and update flake inputs
on:
schedule:
- cron: &quot;0 4 * * *&quot;
workflow_dispatch:
jobs:
update:
name: Update flake inputs and browser
runs-on: ubuntu-latest
steps:
- name: Checkout Repository
uses: actions/checkout@v4
- name: Check flake inputs
uses: DeterminateSystems/flake-checker-action@v4
- name: Install Nix
uses: DeterminateSystems/nix-installer-action@main
- name: Set up magic Nix cache
uses: DeterminateSystems/magic-nix-cache-action@main
- name: Check for update and perform update
run: |
git config --global user.name &quot;github-actions[bot]&quot;
git config --global user.email &quot;github-actions[bot]@users.noreply.github.com&quot;
chmod +x ./update.nu
export ZEN_LATEST_VER=&quot;$(./update.nu)&quot;
git add -A
git commit -m &quot;github-actions: update to $ZEN_LATEST_VER&quot; || echo &quot;Latest version is $ZEN_LATEST_VER, no updates found&quot;
nix flake update --commit-lock-file
git push</code></pre>
<p>Now, our repository will automatically check for and perform updates every day!</p></main>
</article>
]]></description>
<pubDate>Sat, 28 Dec 2024 00:00:00 UT</pubDate>
<guid>https://blog.youwen.dev/nix-automatic-hash-updates-made-easy.html</guid>
<dc:creator>Youwen Wu</dc:creator>
</item>
<item>
<title>a haskellian blog</title>
<link>https://blog.youwen.dev/a-haskellian-blog.html</link>
<description><![CDATA[<article>
<header>
<h1 class="text-4xl">
<a href="./a-haskellian-blog.html">a haskellian blog</a>
</h1>
<p
class="mb-1 mt-2 italic font-light text-lg text-accent-light dark:text-accent-dark"
>
a purely functional...blog?
</p>
<div class="mt-2">2024-05-25</div>
<div class="mt-1 text-sm">
(last updated: 2024-05-25T12:00:00Z)
</div>
</header>
<main class="post mt-4"><p>Welcome! This is the first post on <em>conditional finality</em> and also one that tests all
of the features.</p>
<p><img
alt="conditional finality"
src="./images/conditional-finality.png"
/></p>
<blockquote>
<p>A monad is just a monoid in the category of endofunctors, what’s the problem?</p>
</blockquote>
<h2 id="haskell">haskell?</h2>
<p>This entire blog is generated with <a href="https://jaspervdj.be/hakyll/">hakyll</a>. It’s
a library for generating static sites for Haskell, a purely functional
programming language. It’s a <em>library</em> because it doesn’t come with as many
batteries included as tools like Hugo or Astro. You set up most of the site
yourself by calling the library from Haskell.</p>
<p>Here’s a brief excerpt:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="ot">main ::</span> <span class="dt">IO</span> ()</span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a>main <span class="ot">=</span> hakyllWith config <span class="op">$</span> <span class="kw">do</span></span>
<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a> forM_</span>
<span id="cb1-4"><a href="#cb1-4" aria-hidden="true" tabindex="-1"></a> [ <span class="st">&quot;CNAME&quot;</span></span>
<span id="cb1-5"><a href="#cb1-5" aria-hidden="true" tabindex="-1"></a> , <span class="st">&quot;favicon.ico&quot;</span></span>
<span id="cb1-6"><a href="#cb1-6" aria-hidden="true" tabindex="-1"></a> , <span class="st">&quot;robots.txt&quot;</span></span>
<span id="cb1-7"><a href="#cb1-7" aria-hidden="true" tabindex="-1"></a> , <span class="st">&quot;_config.yml&quot;</span></span>
<span id="cb1-8"><a href="#cb1-8" aria-hidden="true" tabindex="-1"></a> , <span class="st">&quot;images/*&quot;</span></span>
<span id="cb1-9"><a href="#cb1-9" aria-hidden="true" tabindex="-1"></a> , <span class="st">&quot;out/*&quot;</span></span>
<span id="cb1-10"><a href="#cb1-10" aria-hidden="true" tabindex="-1"></a> , <span class="st">&quot;fonts/*&quot;</span></span>
<span id="cb1-11"><a href="#cb1-11" aria-hidden="true" tabindex="-1"></a> ]</span>
<span id="cb1-12"><a href="#cb1-12" aria-hidden="true" tabindex="-1"></a> <span class="op">$</span> \f <span class="ot">-&gt;</span> match f <span class="op">$</span> <span class="kw">do</span></span>
<span id="cb1-13"><a href="#cb1-13" aria-hidden="true" tabindex="-1"></a> route idRoute</span>
<span id="cb1-14"><a href="#cb1-14" aria-hidden="true" tabindex="-1"></a> compile copyFileCompiler</span></code></pre></div>
<p>The code highlighting is also generated by hakyll.</p>
<hr />
<h2 id="why">why?</h2>
<p>Haskell is a purely functional language with no mutable state. Its syntax
actually makes it pretty elegant for declaring routes and “rendering” pipelines.</p>
<ol>
<li>Haskell is cool.</li>
<li>It comes with enough features that I don’t feel like I have to build
everything from scratch.</li>
<li>It comes with Pandoc, a Haskell library for converting between markdown
formats. It’s probably more powerful than anything you could do in <code>nodejs</code>.
It renders all of the markdown to HTML as well as the math.
<ol>
<li>It supports KaTeX as well as MathML. I’m a little disappointed with the
KaTeX though. It doesn’t directly render it, but simply injects the KaTeX
files and renders it client-side.</li>
</ol></li>
</ol>
<h3 id="speaking-of-math">speaking of math</h3>
<p>We can have math inline, like so:
<math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msubsup><mo>∫</mo><mrow><mo>−</mo><mi>∞</mi></mrow><mi>∞</mi></msubsup><mspace width="0.167em"></mspace><msup><mi>e</mi><mrow><mo>−</mo><msup><mi>x</mi><mn>2</mn></msup></mrow></msup><mspace width="0.167em"></mspace><mi>d</mi><mi>x</mi><mo>=</mo><msqrt><mi>π</mi></msqrt></mrow><annotation encoding="application/x-tex">\int_{-\infty}^\infty \, e^{-x^2}\,dx = \sqrt{\pi}</annotation></semantics></math>. This site ships semantic
MathML math with its HTML, and the MathJax script to the client.</p>
<p>It’d be nice if MathML could just be used and supported across all browsers, but
unfortunately we still aren’t quite there yet. Firefox is the only one where
everything looks 80% of the way to LaTeX. On Safari and Chrome, even simple
equations like <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><msqrt><mi>π</mi></msqrt><annotation encoding="application/x-tex">\sqrt{\pi}</annotation></semantics></math> render improperly.</p>
<p>Pros of MathML:</p>
<ul>
<li>A little more accessible</li>
<li>Can be rendered without additional stylesheets. I just installed the Latin
Modern font, but this isn’t even really necessary</li>
<li>Built-in to most browsers (#UseThePlatform)</li>
</ul>
<p>Cons:</p>
<ul>
<li>Isn’t fully standardized. Might look different on different browsers</li>
<li>Rendering quality isn’t as good as KaTeX</li>
</ul>
<p>This site has MathJax render all of the math so it looks nice and standardized
across browsers, but the math still displays regardless (like say if MathJax
couldn’t load due to slow network) because of MathML. Best of both worlds.</p>
<p>Let’s try it now. Here’s a simple theorem:</p>
<p><math display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msup><mi>a</mi><mi>n</mi></msup><mo>+</mo><msup><mi>b</mi><mi>n</mi></msup><mo>≠</mo><msup><mi>c</mi><mi>n</mi></msup><mspace width="0.167em"></mspace><mo>∀</mo><mspace width="0.167em"></mspace><mrow><mo stretchy="true" form="prefix">{</mo><mi>a</mi><mo>,</mo><mspace width="0.167em"></mspace><mi>b</mi><mo>,</mo><mspace width="0.167em"></mspace><mi>c</mi><mo stretchy="true" form="postfix">}</mo></mrow><mo>∈</mo><mstyle mathvariant="double-struck"><mi>ℤ</mi></mstyle><mo>∧</mo><mi>n</mi><mo>≥</mo><mn>3</mn></mrow><annotation encoding="application/x-tex">
a^n + b^n \ne c^n \, \forall\,\left\{ a,\,b,\,c \right\} \in \mathbb{Z} \land n \ge 3
</annotation></semantics></math></p>
<p>The proof is trivial and will be left as an exercise to the reader.</p>
<h2 id="seems-a-little-overengineered">seems a little overengineered</h2>
<p>Probably is. Not as much as the old one, though.</p></main>
</article>
]]></description>
<pubDate>Sat, 25 May 2024 00:00:00 UT</pubDate>
<guid>https://blog.youwen.dev/a-haskellian-blog.html</guid>
<dc:creator>Youwen Wu</dc:creator>
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