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<ol class="chapter"><li><a href="0_0_introduction.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><a href="1_0_background_information.html"><strong aria-hidden="true">2.</strong> Some background information</a></li><li><ol class="section"><li><a href="1_1_trait_objects.html"><strong aria-hidden="true">2.1.</strong> Trait objects and fat pointers</a></li><li><a href="1_2_generators_pin.html"><strong aria-hidden="true">2.2.</strong> Generators and Pin</a></li><li><a href="1_3_pin.html"><strong aria-hidden="true">2.3.</strong> Pin</a></li></ol></li><li><a href="2_0_future_example.html"><strong aria-hidden="true">3.</strong> The main example</a></li><li><a href="2_1_concurrent_futures.html"><strong aria-hidden="true">4.</strong> Bonus 1: concurrent futures</a></li></ol>
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<h1 class="menu-title">Futures Explained in 200 Lines of Rust</h1>
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<main>
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<h1><a class="header" href="#futures-explained-in-200-lines-of-rust" id="futures-explained-in-200-lines-of-rust">Futures Explained in 200 Lines of Rust</a></h1>
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<p>This book aims to explain <code>Futures</code> in Rust using an example driven approach.</p>
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<p>The goal is to get a better understanding of <code>Futures</code> by implementing a toy
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<code>Reactor</code>, a very simple <code>Executor</code> and our own <code>Futures</code>. </p>
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<p>We'll start off solving a small problem without <code>Futures</code>, <code>Wakers</code> or async/await
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and then gradually adapt our example so it implements all these concepts, and
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can be solved using the executor provided by both <code>tokio</code> and <code>async_str</code>.</p>
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<p>In the end I've made some reader excercises you can do if you want to fix some
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of the most glaring ommissions and shortcuts we took and create a slightly better
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example yourself.</p>
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<h2><a class="header" href="#what-does-this-book-give-you-that-isnt-covered-elsewhere" id="what-does-this-book-give-you-that-isnt-covered-elsewhere">What does this book give you that isn't covered elsewhere?</a></h2>
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<p>That's a valid question. There are many good resources and examples already. First
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of all, this book will point you to some background information that I have found
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very valuable to get an understanding of concurrent programming in general.</p>
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<p>I find that many discussions arise, not because <code>Futures</code> is a hard concept to
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grasp, but that concurrent programming is a hard concept in general.</p>
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<p>Secondly, I've always found small runnable examples very exiting to learn from. It's
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all code that you can download, play with and learn from.</p>
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<h2><a class="header" href="#what-well-do-and-not" id="what-well-do-and-not">What we'll do and not</a></h2>
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<p><strong>We'll:</strong></p>
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<ul>
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<li>Implement our own <code>Futures</code> and get to know the <code>Reactor/Executor</code> pattern</li>
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<li>Implement our own waker and learn why it's a bit foreign compared to other types</li>
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<li>Talk a bit about runtime complexity and what to keep in mind when writing async Rust.</li>
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<li>Make sure all examples can be run on the playground</li>
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<li>Not rely on any helpers or libraries, but try to face the complexity and learn</li>
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</ul>
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<p><strong>We'll not:</strong></p>
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<ul>
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<li>Talk about how futures are implemented in Rust the language, the state machine and so on</li>
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<li>Explain how the different runtimes differ, however, you'll hopefully be a bit
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better off if you read this before you go research them</li>
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<li>Explain concurrent programming, but I will supply sources</li>
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</ul>
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<p>I do want to explore Rusts internal implementation but that will be for a later
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book.</p>
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<h2><a class="header" href="#credits-and-thanks" id="credits-and-thanks">Credits and thanks</a></h2>
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<p>I'll like to take the chance of thanking the people behind <code>mio</code>, <code>tokio</code>,
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<code>async_std</code>, <code>Futures</code>, <code>libc</code>, <code>crossbeam</code> and many other libraries which so
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much is built upon. Reading and exploring some of this code is nothing less than
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impressive.</p>
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<h2><a class="header" href="#why-is-futures-in-rust-hard-to-understand" id="why-is-futures-in-rust-hard-to-understand">Why is <code>Futures</code> in Rust hard to understand</a></h2>
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<p>Well, I think it has to do with several things:</p>
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<ol>
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<li>
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<p>Futures has a very interesting implementation, compiling down to a state
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machine using generators to suspend and resume execution. In a language such as
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Rust this is pretty hard to do ergonomically and safely. You are exposed to some
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if this complexity when working with futures and want to understand them, not
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only learn how to use them.</p>
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</li>
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<li>
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<p>Rust doesn't provide a runtime. That means you'll actually have to choose one
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yourself and actually know what a <code>Reactor</code> and an <code>Executor</code> is. While not
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too difficult, you need to make more choices than you need in GO and other
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languages designed with a concurrent programming in mind and ships with a
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runtime.</p>
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</li>
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<li>
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<p>Futures exist in two versions, Futures 1.0 and Futures 3.0. Futures 1.0 was
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known to have some issues regarding ergonomics. Turns out that modelling
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async coding after <code>Promises</code> in JavaScript can turn in to extremely long errors
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and type signatures with a type system as Rust.</p>
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</li>
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</ol>
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<p>Futures 3.0 are not compatible with Futures 1.0 without performing some work.</p>
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<ol start="4">
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<li>Async await syntax was recently stabilized</li>
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</ol>
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<p>what we'll
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really do is to stub out a <code>Reactor</code>, and <code>Executor</code> and implement</p>
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