title: What does a question mark mean in Nix when not used in a function definition?
body: <p><sup>I'm aware of <a href="https://stackoverflow.com/questions/56406588/what-does-a-question-mark-mean-in-nix">a similar question</a>, but that refers to <code>?</code> used in <a href="https://nixos.org/manual/nix/stable/#ss-functions" rel="nofollow noreferrer">function definitions with a set pattern</a> (<a href="https://web.archive.org/web/20210412071147/https://nixos.org/manual/nix/stable/#ss-functions" rel="nofollow noreferrer">pinned</a>).</sup></p>
  <p>I looked up the implementation of <code>lib.makeOverridable</code> and came across <a href="https://github.com/NixOS/nixpkgs/blob/d0a243c4babf502463293f289361f82359ff00c8/lib/customisation.nix#L85" rel="nofollow noreferrer">this line</a>:</p>
  <pre class="lang-hs prettyprint-override"><code>  ${if result ? overrideAttrs then &quot;overrideAttrs&quot; else null} = fdrv:
  </code></pre>
  <p>(Also learned that this kind of dynamic assignment is possible, so that's a bonus.)</p>