| <!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 `Stream` from a seed and a closure returning a `Future`."><meta name="keywords" content="rust, rustlang, rust-lang, unfold"><title>unfold in futures::prelude::stream - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../../../normalize.css"><link rel="stylesheet" href="../../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../../../ayu.css" disabled><link rel="stylesheet" href="../../../dark.css" disabled><link rel="stylesheet" href="../../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../../storage.js"></script><script defer src="sidebar-items.js"></script><script defer src="../../../main.js"></script><noscript><link rel="stylesheet" href="../../../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../../../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../../../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../../../favicon.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">☰</button><a class="sidebar-logo" href="../../../futures/index.html"><div class="logo-container"><img class="rust-logo" src="../../../rust-logo.svg" alt="logo"></div></a><h2></h2></nav><nav class="sidebar"><a class="sidebar-logo" href="../../../futures/index.html"><div class="logo-container"><img class="rust-logo" src="../../../rust-logo.svg" alt="logo"></div></a><div class="sidebar-elems"><h2><a href="index.html">In futures::prelude::stream</a></h2></div></nav><main><div class="width-limiter"><nav class="sub"><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1 class="fqn">Function <a href="../../index.html">futures</a>::<wbr><a href="../index.html">prelude</a>::<wbr><a href="index.html">stream</a>::<wbr><a class="fn" href="#">unfold</a><button id="copy-path" onclick="copy_path(this)" title="Copy item path to clipboard"><img src="../../../clipboard.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="srclink" href="../../../src/futures_util/stream/unfold.rs.html#50-53">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><div class="item-decl"><pre class="rust fn"><code>pub fn unfold<T, F, Fut, Item>(init: T, f: F) -> <a class="struct" href="../../stream/struct.Unfold.html" title="struct futures::stream::Unfold">Unfold</a><T, F, Fut><span class="where fmt-newline">where<br> F: FnMut(T) -> Fut,<br> Fut: <a class="trait" href="../../future/trait.Future.html" title="trait futures::future::Future">Future</a><Output = Option<(Item, T)>>,</span></code></pre></div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Creates a <code>Stream</code> from a seed and a closure returning a <code>Future</code>.</p> |
| <p>This function is the dual for the <code>Stream::fold()</code> adapter: while |
| <code>Stream::fold()</code> reduces a <code>Stream</code> to one single value, <code>unfold()</code> creates a |
| <code>Stream</code> from a seed value.</p> |
| <p><code>unfold()</code> will call the provided closure with the provided seed, then wait |
| for the returned <code>Future</code> to complete with <code>(a, b)</code>. It will then yield the |
| value <code>a</code>, and use <code>b</code> as the next internal state.</p> |
| <p>If the closure returns <code>None</code> instead of <code>Some(Future)</code>, then the <code>unfold()</code> |
| will stop producing items and return <code>Poll::Ready(None)</code> in future |
| calls to <code>poll()</code>.</p> |
| <p>This function can typically be used when wanting to go from the “world of |
| futures” to the “world of streams”: the provided closure can build a |
| <code>Future</code> using other library functions working on futures, and <code>unfold()</code> |
| will turn it into a <code>Stream</code> by repeating the operation.</p> |
| <h2 id="example"><a href="#example">Example</a></h2> |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>futures::stream::{<span class="self">self</span>, StreamExt}; |
| |
| <span class="kw">let </span>stream = stream::unfold(<span class="number">0</span>, |state| <span class="kw">async move </span>{ |
| <span class="kw">if </span>state <= <span class="number">2 </span>{ |
| <span class="kw">let </span>next_state = state + <span class="number">1</span>; |
| <span class="kw">let </span>yielded = state * <span class="number">2</span>; |
| <span class="prelude-val">Some</span>((yielded, next_state)) |
| } <span class="kw">else </span>{ |
| <span class="prelude-val">None |
| </span>} |
| }); |
| |
| <span class="kw">let </span>result = stream.collect::<Vec<i32>>().<span class="kw">await</span>; |
| <span class="macro">assert_eq!</span>(result, <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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