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<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Creates a `TryStream` from a seed and a closure returning a `TryFuture`."><title>try_unfold in futures_util::stream - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-46f98efaafac5295.ttf.woff2,FiraSans-Regular-018c141bf0843ffd.woff2,FiraSans-Medium-8f9a781e4970d388.woff2,SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2,SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../../static.files/rustdoc-b0742ba02757f159.css"><meta name="rustdoc-vars" data-root-path="../../" data-static-root-path="../../static.files/" data-current-crate="futures_util" data-themes="" data-resource-suffix="" data-rustdoc-version="1.83.0 (90b35a623 2024-11-26)" data-channel="1.83.0" data-search-js="search-f0d225181b97f9a4.js" data-settings-js="settings-805db61a62df4bd2.js" ><script src="../../static.files/storage-1d39b6787ed640ff.js"></script><script defer src="sidebar-items.js"></script><script defer src="../../static.files/main-f070b9041d14864c.js"></script><noscript><link rel="stylesheet" href="../../static.files/noscript-0111fcff984fae8f.css"></noscript><link rel="alternate icon" type="image/png" href="../../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../../static.files/favicon-2c020d218678b618.svg"></head><body class="rustdoc fn"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle" title="show sidebar"></button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../../futures_util/index.html">futures_<wbr>util</a><span class="version">0.3.31</span></h2></div><div class="sidebar-elems"><section id="rustdoc-toc"><h2 class="location"><a href="#">try_<wbr>unfold</a></h2><h3><a href="#">Sections</a></h3><ul class="block top-toc"><li><a href="#example" title="Example">Example</a></li></ul></section><div id="rustdoc-modnav"><h2><a href="index.html">In futures_<wbr>util::<wbr>stream</a></h2></div></div></nav><div class="sidebar-resizer"></div><main><div class="width-limiter"><rustdoc-search></rustdoc-search><section id="main-content" class="content"><div class="main-heading"><span class="rustdoc-breadcrumbs"><a href="../index.html">futures_util</a>::<wbr><a href="index.html">stream</a></span><h1>Function <span class="fn">try_unfold</span><button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><rustdoc-toolbar></rustdoc-toolbar><span class="sub-heading"><a class="src" href="../../src/futures_util/stream/try_stream/try_unfold.rs.html#59-65">source</a> </span></div><pre class="rust item-decl"><code>pub fn try_unfold<T, F, Fut, Item>(init: T, f: F) -> <a class="struct" href="struct.TryUnfold.html" title="struct futures_util::stream::TryUnfold">TryUnfold</a><T, F, Fut><div class="where">where
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F: <a class="trait" href="https://doc.rust-lang.org/1.83.0/core/ops/function/trait.FnMut.html" title="trait core::ops::function::FnMut">FnMut</a>(T) -> Fut,
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Fut: <a class="trait" href="../future/trait.TryFuture.html" title="trait futures_util::future::TryFuture">TryFuture</a><Ok = <a class="enum" href="https://doc.rust-lang.org/1.83.0/core/option/enum.Option.html" title="enum core::option::Option">Option</a><<a class="primitive" href="https://doc.rust-lang.org/1.83.0/core/primitive.tuple.html">(Item, T)</a>>>,</div></code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Creates a <code>TryStream</code> from a seed and a closure returning a <code>TryFuture</code>.</p>
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<p>This function is the dual for the <code>TryStream::try_fold()</code> adapter: while
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<code>TryStream::try_fold()</code> reduces a <code>TryStream</code> to one single value,
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<code>try_unfold()</code> creates a <code>TryStream</code> from a seed value.</p>
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<p><code>try_unfold()</code> will call the provided closure with the provided seed, then
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wait for the returned <code>TryFuture</code> to complete with <code>(a, b)</code>. It will then
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yield the value <code>a</code>, and use <code>b</code> as the next internal state.</p>
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<p>If the closure returns <code>None</code> instead of <code>Some(TryFuture)</code>, then the
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<code>try_unfold()</code> will stop producing items and return <code>Poll::Ready(None)</code> in
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future calls to <code>poll()</code>.</p>
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<p>In case of error generated by the returned <code>TryFuture</code>, the error will be
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returned by the <code>TryStream</code>. The <code>TryStream</code> will then yield
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<code>Poll::Ready(None)</code> in future calls to <code>poll()</code>.</p>
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<p>This function can typically be used when wanting to go from the “world of
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futures” to the “world of streams”: the provided closure can build a
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<code>TryFuture</code> using other library functions working on futures, and
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<code>try_unfold()</code> will turn it into a <code>TryStream</code> by repeating the operation.</p>
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<h2 id="example"><a class="doc-anchor" href="#example">§</a>Example</h2>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>futures::stream::{<span class="self">self</span>, TryStreamExt};
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<span class="kw">let </span>stream = stream::try_unfold(<span class="number">0</span>, |state| <span class="kw">async move </span>{
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<span class="kw">if </span>state < <span class="number">0 </span>{
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<span class="kw">return </span><span class="prelude-val">Err</span>(SomeError);
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}
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<span class="kw">if </span>state <= <span class="number">2 </span>{
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<span class="kw">let </span>next_state = state + <span class="number">1</span>;
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<span class="kw">let </span>yielded = state * <span class="number">2</span>;
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<span class="prelude-val">Ok</span>(<span class="prelude-val">Some</span>((yielded, next_state)))
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} <span class="kw">else </span>{
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<span class="prelude-val">Ok</span>(<span class="prelude-val">None</span>)
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}
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});
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<span class="kw">let </span>result: <span class="prelude-ty">Result</span><Vec<i32>, <span class="kw">_</span>> = stream.try_collect().<span class="kw">await</span>;
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<span class="macro">assert_eq!</span>(result, <span class="prelude-val">Ok</span>(<span class="macro">vec!</span>[<span class="number">0</span>, <span class="number">2</span>, <span class="number">4</span>]));</code></pre></div>
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</div></details></section></div></main></body></html>
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