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<main>
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<h1 id="the-rhai-expression-engine"><a class="header" href="#the-rhai-expression-engine">The Rhai Expression Engine</a></h1>
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<p>Ewwii uses <a href="https://rhai.rs/">Rhai</a> as its core expression and scripting engine. This means your configuration is more than just static values or simple substitutions—it’s <strong>real code</strong>, and you can use it anywhere dynamic behavior is needed.</p>
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<p>Rhai expressions can:</p>
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<ul>
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<li>Perform logic and branching (<code>if</code>, <code>match</code>, <code>? :</code>)</li>
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<li>Handle mathematical calculations and string operations</li>
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<li>Access nested data from arrays, maps, or JSON</li>
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<li>Run custom functions</li>
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<li>Be used directly in layout definitions, widget attributes, and style rules</li>
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</ul>
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<p>Unlike Yuck, where expressions were embedded in <code>{ ... }</code> or <code>${ ... }</code>, Rhai treats expressions as <strong>native</strong>. They don’t need to be wrapped in special delimiters. If you can write it in Rhai, it just works.</p>
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<h2 id="example"><a class="header" href="#example">Example</a></h2>
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<pre><code class="language-rust ignore">let value = 12 + foo * 10;
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box(#{
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class: "baz"
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orientation: "h",
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}, [
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button(#{
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class: button_active ? "active" : "inactive",
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on_click: "toggle_thing",
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label: `Some math: ${value}`,
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}),
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]);</code></pre>
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<h2 id="core-features"><a class="header" href="#core-features">Core Features</a></h2>
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<p>Rhai supports a wide range of expression capabilities:</p>
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<ul>
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<li><strong>Mathematics</strong>: <code>+</code>, <code>-</code>, <code>*</code>, <code>/</code>, <code>%</code></li>
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<li><strong>Comparisons</strong>: <code>==</code>, <code>!=</code>, <code><</code>, <code>></code>, <code><=</code>, <code>>=</code></li>
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<li><strong>Boolean logic</strong>: <code>&&</code>, <code>||</code>, <code>!</code></li>
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<li><strong>Conditionals</strong>: <code>if/else</code>, ternary (<code>cond ? a : b</code>)</li>
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<li><strong>Regex matching</strong>: <code>=~</code> operator (Rust-style regex)</li>
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<li><strong>Optional access</strong>: <code>?.</code> and <code>?.[index]</code></li>
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<li><strong>Data structures</strong>: maps/arrays (<code>obj.field</code>, <code>arr[3]</code>, <code>map["key"]</code>)</li>
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<li><strong>Function calls</strong>: standard and Ewwii-specific built-ins (see below)</li>
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<li><strong>String interpolation</strong>: <code>`Value is ${value}`</code> (standard Rhai feature)</li>
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</ul>
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<blockquote>
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<p>Note</p>
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<hr />
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<p>Rhai only allows string interpolation only for the strings that are quoted in `` similar to JavaScript.</p>
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<blockquote>
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<p>Learn more about it <a href="https://rhai.rs/book/ref/strings-chars.html?interpolation#string-interpolation">here</a>.</p>
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</blockquote>
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</blockquote>
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<h2 id="common-built-in-functions"><a class="header" href="#common-built-in-functions">Common Built-in Functions</a></h2>
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<p>💡 <em>You may recognize some of these from the old expression system—but now they're just Rhai functions.</em></p>
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<p>Examples include:</p>
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<ul>
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<li><code>round()</code>, <code>floor()</code>, <code>ceil()</code>, <code>powi()</code>, <code>powf()</code></li>
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<li><code>min()</code>, <code>max()</code>, <code>sin()</code>, <code>cos()</code>, etc.</li>
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<li><code>replace()</code>, <code>matches()</code>, <code>captures()</code></li>
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<li><code>strlength()</code>, <code>arraylength()</code>, <code>objectlength()</code></li>
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<li><code>jq()</code> – run jaq-compatible filters on JSON data</li>
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<li><code>get_env()</code> – read environment variables</li>
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<li><code>formattime()</code> – format UNIX timestamps</li>
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<li><code>formatbytes()</code> – format file sizes (IEC or SI)</li>
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</ul>
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<h2 id="dynamic-usage"><a class="header" href="#dynamic-usage">Dynamic Usage</a></h2>
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<p>Because expressions are just Rhai, you can now write real logic inline or break it into reusable functions:</p>
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<pre><code class="language-rust ignore">fn status_text(active) {
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return active ? "enabled" : "disabled";
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}
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label({
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text: `Status: ${status_text(system_active)}`
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});</code></pre>
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<hr />
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<h3 id="tldr"><a class="header" href="#tldr">TL;DR</a></h3>
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<blockquote>
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<p>If you’ve used scripting languages like JavaScript or Lua, you’ll feel at home. Rhai gives you real control—not just interpolation tricks.</p>
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