Cabal-2.2.0.1: doc/users-guide/nix-integration.html
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<li class="toctree-l1 current"><a class="current reference internal" href="#">6. Nix Integration</a><ul>
<li class="toctree-l2"><a class="reference internal" href="#enabling-nix-integration">6.1. Enabling Nix Integration</a></li>
<li class="toctree-l2"><a class="reference internal" href="#creating-nix-expressions">6.2. Creating Nix Expressions</a></li>
<li class="toctree-l2"><a class="reference internal" href="#nix-expression-evaluation">6.3. Nix Expression Evaluation</a></li>
<li class="toctree-l2"><a class="reference internal" href="#further-reading">6.4. Further Reading</a></li>
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<div class="section" id="nix-integration">
<h1>6. Nix Integration<a class="headerlink" href="#nix-integration" title="Permalink to this headline">¶</a></h1>
<p><a class="reference external" href="http://nixos.org/nix/">Nix</a> is a package manager popular with some Haskell developers due to its focus on reliability and reproducibility. <code class="docutils literal"><span class="pre">cabal</span></code> now has the ability to integrate with Nix for dependency management during local package development.</p>
<div class="section" id="enabling-nix-integration">
<h2>6.1. Enabling Nix Integration<a class="headerlink" href="#enabling-nix-integration" title="Permalink to this headline">¶</a></h2>
<p>To enable Nix integration, simply pass the <code class="docutils literal"><span class="pre">--enable-nix</span></code> global option when you call <code class="docutils literal"><span class="pre">cabal</span></code>. To use this option everywhere, edit your <code class="docutils literal"><span class="pre">$HOME/.cabal/config</span></code> file to include:</p>
<div class="highlight-cabal"><div class="highlight"><pre><span></span><span class="k">nix</span><span class="p">:</span> True
</pre></div>
</div>
<p>If the package (which must be locally unpacked) provides a <code class="docutils literal"><span class="pre">shell.nix</span></code> or <code class="docutils literal"><span class="pre">default.nix</span></code> file, this flag will cause <code class="docutils literal"><span class="pre">cabal</span></code> to run most commands through <code class="docutils literal"><span class="pre">nix-shell</span></code>. If both expressions are present, <code class="docutils literal"><span class="pre">shell.nix</span></code> is preferred. The following commands are affected:</p>
<ul class="simple">
<li><code class="docutils literal"><span class="pre">cabal</span> <span class="pre">configure</span></code></li>
<li><code class="docutils literal"><span class="pre">cabal</span> <span class="pre">build</span></code></li>
<li><code class="docutils literal"><span class="pre">cabal</span> <span class="pre">repl</span></code></li>
<li><code class="docutils literal"><span class="pre">cabal</span> <span class="pre">install</span></code> (only if installing into a sandbox)</li>
<li><code class="docutils literal"><span class="pre">cabal</span> <span class="pre">haddock</span></code></li>
<li><code class="docutils literal"><span class="pre">cabal</span> <span class="pre">freeze</span></code></li>
<li><code class="docutils literal"><span class="pre">cabal</span> <span class="pre">gen-bounds</span></code></li>
<li><code class="docutils literal"><span class="pre">cabal</span> <span class="pre">run</span></code></li>
</ul>
<p>If the package does not provide an expression, <code class="docutils literal"><span class="pre">cabal</span></code> runs normally.</p>
</div>
<div class="section" id="creating-nix-expressions">
<h2>6.2. Creating Nix Expressions<a class="headerlink" href="#creating-nix-expressions" title="Permalink to this headline">¶</a></h2>
<p>The Nix package manager is based on a lazy, pure, functional programming language; packages are defined by expressions in this language. The fastest way to create a Nix expression for a Cabal package is with the <a class="reference external" href="https://github.com/NixOS/cabal2nix">cabal2nix</a> tool. To create a <code class="docutils literal"><span class="pre">shell.nix</span></code> expression for the package in the current directory, run this command:</p>
<div class="highlight-console"><div class="highlight"><pre><span></span><span class="gp">$</span> cabal2nix --shell ./. >shell.nix
</pre></div>
</div>
</div>
<div class="section" id="nix-expression-evaluation">
<h2>6.3. Nix Expression Evaluation<a class="headerlink" href="#nix-expression-evaluation" title="Permalink to this headline">¶</a></h2>
<p>(This section describes for advanced users how Nix expressions are evaluated.)</p>
<p>First, the Nix expression (<code class="docutils literal"><span class="pre">shell.nix</span></code> or <code class="docutils literal"><span class="pre">default.nix</span></code>) is instantiated with <code class="docutils literal"><span class="pre">nix-instantiate</span></code>. The <code class="docutils literal"><span class="pre">--add-root</span></code> and <code class="docutils literal"><span class="pre">--indirect</span></code> options are used to create an indirect root in the Cabal build directory, preventing Nix from garbage collecting the derivation while in use. The <code class="docutils literal"><span class="pre">IN_NIX_SHELL</span></code> environment variable is set so that <code class="docutils literal"><span class="pre">builtins.getEnv</span></code> works as it would in <code class="docutils literal"><span class="pre">nix-shell</span></code>.</p>
<p>Next, the commands above are run through <code class="docutils literal"><span class="pre">nix-shell</span></code> using the instantiated derivation. Again, <code class="docutils literal"><span class="pre">--add-root</span></code> and <code class="docutils literal"><span class="pre">--indirect</span></code> are used to prevent Nix from garbage collecting the packages in the environment. The child <code class="docutils literal"><span class="pre">cabal</span></code> process reads the <code class="docutils literal"><span class="pre">CABAL_IN_NIX_SHELL</span></code> environment variable to prevent it from spawning additional child shells.</p>
</div>
<div class="section" id="further-reading">
<h2>6.4. Further Reading<a class="headerlink" href="#further-reading" title="Permalink to this headline">¶</a></h2>
<p>The <a class="reference external" href="http://nixos.org/nix/manual/#chap-writing-nix-expressions">Nix manual</a> provides further instructions for writing Nix expressions. The <a class="reference external" href="http://nixos.org/nixpkgs/manual/#users-guide-to-the-haskell-infrastructure">Nixpkgs manual</a> describes the infrastructure provided for Haskell packages.</p>
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