https://prule.github.io/pauls-blog/tags/domain-modelling/Recent content in Domain-Modelling on Pauls BlogHugoen-usFri, 22 May 2026 12:00:00 +1000https://prule.github.io/pauls-blog/post/coding/2026/kotlin-value-objects/Fri, 22 May 2026 12:00:00 +1000https://prule.github.io/pauls-blog/post/coding/2026/kotlin-value-objects/<p>Use value objects. Primitives leak implementation details, enable invalid states, and produce meaningless function signatures. A <code>UserId</code> is not an <code>Int</code>. A <code>Money</code> is not a <code>Double</code>. The type system is your first line of defence — use it.</p> <h2 id="the-problem-with-primitives">The Problem With Primitives</h2> <div class="highlight"><pre tabindex="0" class="chroma"><code class="language-kotlin" data-lang="kotlin"><span class="line"><span class="cl"><span class="k">fun</span> <span class="nf">transfer</span><span class="p">(</span><span class="n">from</span><span class="p">:</span> <span class="n">Int</span><span class="p">,</span> <span class="n">to</span><span class="p">:</span> <span class="n">Int</span><span class="p">,</span> <span class="n">amount</span><span class="p">:</span> <span class="n">Double</span><span class="p">)</span> <span class="p">{</span> <span class="o">..</span><span class="p">.</span> <span class="p">}</span> </span></span></code></pre></div><p>Nothing stops a caller passing the arguments in the wrong order. Nothing prevents a negative amount. Nothing distinguishes a user ID from an account ID. The compiler is silent. The bug ships.</p>