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38 lines
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<title>assert — Ludic</title>
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<meta name="description" content="Abort with a located message when an invariant is false.">
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<header class="nav"><div class="wrap nav-in"><a class="brand" href="index.html"><span class="logo">L</span> Ludic</a><button class="nav-toggle" aria-label="Toggle menu" aria-expanded="false">☰</button><nav class="nav-links"><a href="index.html">Home</a><a href="api.html">API Reference</a><a class="nav-cta" href="https://git.workshopsoft.io/workshopsoft/ludic">Source ↗</a></nav></div></header>
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<div class="crumbs"><a href="api.html">API Reference</a> <span>›</span> <a href="api.html#builtins">Builtin functions</a> <span>›</span> <span class="here">assert</span></div>
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<span class="kind-badge kind-builtin">function</span>
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<h1 id="top">assert</h1>
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<code class="sig">assert(cond: bool, msg: string)</code>
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<div class="desc"><p>When <code>cond</code> is false, prints <code>file:line: assertion failed: <msg></code> to standard error and aborts (exit code 1); when it is true, execution continues. It is the guarded form of <a href="fn-panic"><code>panic</code></a> — for the programmer-bug cases the language should catch loudly rather than let corrupt the world: an index that must be in range, a value that must be non-negative, a state that must hold before a step. The location is baked in at compile time, so a failure points at the exact assertion. Assertions document and enforce the invariants a system relies on; keep them for "this must be true" checks, not for recoverable runtime conditions.</p></div>
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<div class="params"><h2>Parameters</h2><div class="param" id="param-cond"><code class="pname">cond</code><span class="pdesc">the invariant that must hold (a bool)</span></div><div class="param" id="param-msg"><code class="pname">msg</code><span class="pdesc">a message describing the invariant (a string)</span></div></div>
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<div class="examples"><h2>Example</h2><pre data-lang="ludic">program Demo {
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function withdraw(balance: int, amount: int) -> int {
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assert(amount >= 0, "amount must be non-negative")
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assert(amount <= balance, "cannot overdraw")
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return balance - amount
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}
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entry {
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print(withdraw(100, 30)) # 70
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print(withdraw(100, 250)) # aborts: Demo.ludic:4: assertion failed: cannot overdraw
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}
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}</pre></div>
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