<!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 `loop_fn` fn in crate `futures`."><meta name="keywords" content="rust, rustlang, rust-lang, loop_fn"><title>futures::future::loop_fn - 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="../../dark.css"><link rel="stylesheet" type="text/css" href="../../light.css" id="themeStyle"><script src="../../storage.js"></script></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><div class="sidebar-elems"><p class='location'><a href='../index.html'>futures</a>::<wbr><a href='index.html'>future</a></p><script>window.sidebarCurrent = {name: 'loop_fn', ty: 'fn', relpath: ''};</script><script defer src="sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../../theme.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><input class="search-input" name="search" autocomplete="off" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><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</a>::<wbr><a href='index.html'>future</a>::<wbr><a class="fn" href=''>loop_fn</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/future/loop_fn.rs.html#74-82' title='goto source code'>[src]</a></span></h1><pre class='rust fn'>pub fn loop_fn<S, T, A, F>(initial_state: S, func: F) -> <a class="struct" href="../../futures/future/struct.LoopFn.html" title="struct futures::future::LoopFn">LoopFn</a><A, F> <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>(S) -> A,<br> A: <a class="trait" href="../../futures/future/trait.IntoFuture.html" title="trait futures::future::IntoFuture">IntoFuture</a><Item = <a class="enum" href="../../futures/future/enum.Loop.html" title="enum futures::future::Loop">Loop</a><T, S>>, </span></pre><div class='docblock'><p>Creates a new future implementing a tail-recursive loop.</p>
<p>The loop function is immediately called with <code>initial_state</code> and should
return a value that can be converted to a future. On successful completion,
this future should output a <code>Loop<T, S></code> to indicate the status of the
loop.</p>
<p><code>Loop::Break(T)</code> halts the loop and completes the future with output <code>T</code>.</p>
<p><code>Loop::Continue(S)</code> reinvokes the loop function with state <code>S</code>. The returned
future will be subsequently polled for a new <code>Loop<T, S></code> value.</p>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
<pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">futures</span>::<span class="ident">future</span>::{<span class="ident">ok</span>, <span class="ident">loop_fn</span>, <span class="ident">Future</span>, <span class="ident">FutureResult</span>, <span class="ident">Loop</span>};
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">io</span>::<span class="ident">Error</span>;
<span class="kw">struct</span> <span class="ident">Client</span> {
<span class="ident">ping_count</span>: <span class="ident">u8</span>,
}
<span class="kw">impl</span> <span class="ident">Client</span> {
<span class="kw">fn</span> <span class="ident">new</span>() <span class="op">-></span> <span class="self">Self</span> {
<span class="ident">Client</span> { <span class="ident">ping_count</span>: <span class="number">0</span> }
}
<span class="kw">fn</span> <span class="ident">send_ping</span>(<span class="self">self</span>) <span class="op">-></span> <span class="ident">FutureResult</span><span class="op"><</span><span class="self">Self</span>, <span class="ident">Error</span><span class="op">></span> {
<span class="ident">ok</span>(<span class="ident">Client</span> { <span class="ident">ping_count</span>: <span class="self">self</span>.<span class="ident">ping_count</span> <span class="op">+</span> <span class="number">1</span> })
}
<span class="kw">fn</span> <span class="ident">receive_pong</span>(<span class="self">self</span>) <span class="op">-></span> <span class="ident">FutureResult</span><span class="op"><</span>(<span class="self">Self</span>, <span class="ident">bool</span>), <span class="ident">Error</span><span class="op">></span> {
<span class="kw">let</span> <span class="ident">done</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">ping_count</span> <span class="op">>=</span> <span class="number">5</span>;
<span class="ident">ok</span>((<span class="self">self</span>, <span class="ident">done</span>))
}
}
<span class="kw">let</span> <span class="ident">ping_til_done</span> <span class="op">=</span> <span class="ident">loop_fn</span>(<span class="ident">Client</span>::<span class="ident">new</span>(), <span class="op">|</span><span class="ident">client</span><span class="op">|</span> {
<span class="ident">client</span>.<span class="ident">send_ping</span>()
.<span class="ident">and_then</span>(<span class="op">|</span><span class="ident">client</span><span class="op">|</span> <span class="ident">client</span>.<span class="ident">receive_pong</span>())
.<span class="ident">and_then</span>(<span class="op">|</span>(<span class="ident">client</span>, <span class="ident">done</span>)<span class="op">|</span> {
<span class="kw">if</span> <span class="ident">done</span> {
<span class="prelude-val">Ok</span>(<span class="ident">Loop</span>::<span class="ident">Break</span>(<span class="ident">client</span>))
} <span class="kw">else</span> {
<span class="prelude-val">Ok</span>(<span class="ident">Loop</span>::<span class="ident">Continue</span>(<span class="ident">client</span>))
}
})
});</pre>
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