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Uttarayan Mondal
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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="A Futures channel-like utility to signal when a value is wanted."><meta name="keywords" content="rust, rustlang, rust-lang, want"><title>want - 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 mod"><!--[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='../want/index.html'><div class='logo-container rust-logo'><img src='../rust-logo.png' alt='logo'></div></a><p class="location">Crate want</p><div class="block version"><p>Version 0.3.0</p></div><div class="sidebar-elems"><a id="all-types" href="all.html"><p>See all want's items</p></a><div class="block items"><ul><li><a href="#structs">Structs</a></li><li><a href="#functions">Functions</a></li></ul></div><p class="location"></p><div id="sidebar-vars" data-name="want" data-ty="mod" data-relpath="../"></div></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">Crate <a class="mod" href="">want</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/want/lib.rs.html#1-585" title="goto source code">[src]</a></span></h1><div class="docblock"><p>A Futures channel-like utility to signal when a value is wanted.</p>
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<p>Futures are supposed to be lazy, and only starting work if <code>Future::poll</code>
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is called. The same is true of <code>Stream</code>s, but when using a channel as
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a <code>Stream</code>, it can be hard to know if the receiver is ready for the next
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value.</p>
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<p>Put another way, given a <code>(tx, rx)</code> from <code>futures::sync::mpsc::channel()</code>,
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how can the sender (<code>tx</code>) know when the receiver (<code>rx</code>) actually wants more
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work to be produced? Just because there is room in the channel buffer
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doesn’t mean the work would be used by the receiver.</p>
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<p>This is where something like <code>want</code> comes in. Added to a channel, you can
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make sure that the <code>tx</code> only creates the message and sends it when the <code>rx</code>
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has <code>poll()</code> for it, and the buffer was empty.</p>
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<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
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<pre><code class="language-nightly"># //#![feature(async_await)]
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extern crate want;
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# fn spawn<T>(_t: T) {}
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# fn we_still_want_message() -> bool { true }
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# fn mpsc_channel() -> (Tx, Rx) { (Tx, Rx) }
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# struct Tx;
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# impl Tx { fn send<T>(&mut self, _: T) {} }
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# struct Rx;
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# impl Rx { async fn recv(&mut self) -> Option<Expensive> { Some(Expensive) } }
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// Some message that is expensive to produce.
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struct Expensive;
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// Some futures-aware MPSC channel...
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let (mut tx, mut rx) = mpsc_channel();
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// And our `want` channel!
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let (mut gv, mut tk) = want::new();
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// Our receiving task...
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spawn(async move {
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// Maybe something comes up that prevents us from ever
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// using the expensive message.
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//
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// Without `want`, the "send" task may have started to
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// produce the expensive message even though we wouldn't
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// be able to use it.
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if !we_still_want_message() {
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return;
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}
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// But we can use it! So tell the `want` channel.
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tk.want();
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match rx.recv().await {
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Some(_msg) => println!("got a message"),
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None => println!("DONE"),
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}
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});
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// Our sending task
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spawn(async move {
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// It's expensive to create a new message, so we wait until the
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// receiving end truly *wants* the message.
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if let Err(_closed) = gv.want().await {
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// Looks like they will never want it...
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return;
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}
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// They want it, let's go!
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tx.send(Expensive);
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});
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# fn main() {}
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</code></pre>
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</div><h2 id="structs" class="section-header"><a href="#structs">Structs</a></h2>
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<table><tr class="module-item"><td><a class="struct" href="struct.Closed.html" title="want::Closed struct">Closed</a></td><td class="docblock-short"><p>The <code>Taker</code> has canceled its interest in a value.</p>
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</td></tr><tr class="module-item"><td><a class="struct" href="struct.Giver.html" title="want::Giver struct">Giver</a></td><td class="docblock-short"><p>An entity that gives a value when wanted.</p>
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</td></tr><tr class="module-item"><td><a class="struct" href="struct.SharedGiver.html" title="want::SharedGiver struct">SharedGiver</a></td><td class="docblock-short"><p>A cloneable <code>Giver</code>.</p>
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</td></tr><tr class="module-item"><td><a class="struct" href="struct.Taker.html" title="want::Taker struct">Taker</a></td><td class="docblock-short"><p>An entity that wants a value.</p>
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</td></tr></table><h2 id="functions" class="section-header"><a href="#functions">Functions</a></h2>
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<table><tr class="module-item"><td><a class="fn" href="fn.new.html" title="want::new fn">new</a></td><td class="docblock-short"><p>Create a new <code>want</code> channel.</p>
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</td></tr></table></section><section id="search" class="content hidden"></section><section class="footer"></section><div id="rustdoc-vars" data-root-path="../" data-current-crate="want"></div>
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