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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="API documentation for the Rust `try_unfold` fn in crate `futures_util`."><meta name="keywords" content="rust, rustlang, rust-lang, try_unfold"><title>futures_util::stream::try_unfold - Rust</title><link rel="stylesheet" type="text/css" href="../../normalize.css"><link rel="stylesheet" type="text/css" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../../light.css" id="themeStyle"><link rel="stylesheet" type="text/css" href="../../dark.css" disabled ><link rel="stylesheet" type="text/css" href="../../ayu.css" disabled ><script id="default-settings"></script><script src="../../storage.js"></script><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="icon" type="image/svg+xml" href="../../favicon.svg">
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<link rel="alternate icon" type="image/png" href="../../favicon-32x32.png"><style type="text/css">#crate-search{background-image:url("../../down-arrow.svg");}</style></head><body class="rustdoc fn"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../../futures_util/index.html'><div class='logo-container rust-logo'><img src='../../rust-logo.png' alt='logo'></div></a><div class="sidebar-elems"><p class="location"><a href="../index.html">futures_util</a>::<wbr><a href="index.html">stream</a></p><div id="sidebar-vars" data-name="try_unfold" data-ty="fn" data-relpath=""></div><script defer src="sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu"><img src="../../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices" role="menu"></div></div><script src="../../theme.js"></script><nav class="sub"><form class="search-form"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" disabled autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><button type="button" class="help-button">?</button>
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<a id="settings-menu" href="../../settings.html"><img src="../../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class="fqn"><span class="in-band">Function <a href="../index.html">futures_util</a>::<wbr><a href="index.html">stream</a>::<wbr><a class="fn" href="">try_unfold</a></span><span class="out-of-band"><span id="render-detail"><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span><a class="srclink" href="../../src/futures_util/stream/try_stream/try_unfold.rs.html#59-69" title="goto source code">[src]</a></span></h1><pre class="rust fn">pub fn try_unfold<T, F, Fut, Item>(init: T, f: F) -> <a class="struct" href="../../futures_util/stream/struct.TryUnfold.html" title="struct futures_util::stream::TryUnfold">TryUnfold</a><T, F, Fut> <span class="where fmt-newline">where<br> F: <a class="trait" href="https://doc.rust-lang.org/nightly/core/ops/function/trait.FnMut.html" title="trait core::ops::function::FnMut">FnMut</a>(T) -> Fut,<br> Fut: <a class="trait" href="../../futures_util/future/trait.TryFuture.html" title="trait futures_util::future::TryFuture">TryFuture</a><Ok = <a class="enum" href="https://doc.rust-lang.org/nightly/core/option/enum.Option.html" title="enum core::option::Option">Option</a><<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.tuple.html">(</a>Item, T<a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.tuple.html">)</a>>>, </span></pre><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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<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">futures</span>::<span class="ident">stream</span>::{<span class="self">self</span>, <span class="ident">TryStreamExt</span>};
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<span class="kw">let</span> <span class="ident">stream</span> <span class="op">=</span> <span class="ident">stream</span>::<span class="ident">try_unfold</span>(<span class="number">0</span>, <span class="op">|</span><span class="ident">state</span><span class="op">|</span> <span class="kw">async</span> <span class="kw">move</span> {
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<span class="kw">if</span> <span class="ident">state</span> <span class="op"><</span> <span class="number">0</span> {
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<span class="kw">return</span> <span class="prelude-val">Err</span>(<span class="ident">SomeError</span>);
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}
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<span class="kw">if</span> <span class="ident">state</span> <span class="op"><</span><span class="op">=</span> <span class="number">2</span> {
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<span class="kw">let</span> <span class="ident">next_state</span> <span class="op">=</span> <span class="ident">state</span> <span class="op">+</span> <span class="number">1</span>;
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<span class="kw">let</span> <span class="ident">yielded</span> <span class="op">=</span> <span class="ident">state</span> <span class="op">*</span> <span class="number">2</span>;
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<span class="prelude-val">Ok</span>(<span class="prelude-val">Some</span>((<span class="ident">yielded</span>, <span class="ident">next_state</span>)))
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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> <span class="ident">result</span>: <span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">Vec</span><span class="op"><</span><span class="ident">i32</span><span class="op">></span>, <span class="kw">_</span><span class="op">></span> <span class="op">=</span> <span class="ident">stream</span>.<span class="ident">try_collect</span>().<span class="kw">await</span>;
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">result</span>, <span class="prelude-val">Ok</span>(<span class="macro">vec</span><span class="macro">!</span>[<span class="number">0</span>, <span class="number">2</span>, <span class="number">4</span>]));</pre></div>
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