<!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 `result` mod in crate `std`."><meta name="keywords" content="rust, rustlang, rust-lang, result"><title>std::result - 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><link rel="shortcut icon" href="https://doc.rust-lang.org/favicon.ico"></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='../../std/index.html'><img src='https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png' alt='logo' width='100'></a><p class='location'>Module result</p><div class="sidebar-elems"><div class="block items"><ul><li><a href="#structs">Structs</a></li><li><a href="#enums">Enums</a></li></ul></div><p class='location'><a href='../index.html'>std</a></p><script>window.sidebarCurrent = {name: 'result', ty: 'mod', 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'>Module <a href='../index.html'>std</a>::<wbr><a class="mod" href=''>result</a></span><span class='out-of-band'><span class='since' title='Stable since Rust version 1.0.0'>1.0.0</span><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/core/lib.rs.html#201' title='goto source code'>[src]</a></span></h1><div class='docblock'><p>Error handling with the <code>Result</code> type.</p>
<p><a href="enum.Result.html"><code>Result<T, E></code></a> is the type used for returning and propagating
errors. It is an enum with the variants, <a href="enum.Result.html#variant.Ok"><code>Ok(T)</code></a>, representing
success and containing a value, and <a href="enum.Result.html#variant.Err"><code>Err(E)</code></a>, representing error
and containing an error value.</p>
<pre class="rust rust-example-rendered">
<span class="kw">enum</span> <span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">E</span><span class="op">></span> {
<span class="prelude-val">Ok</span>(<span class="ident">T</span>),
<span class="prelude-val">Err</span>(<span class="ident">E</span>),
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0A%23%5Ballow(dead_code)%5D%0Aenum%20Result%3CT%2C%20E%3E%20%7B%0A%20%20%20Ok(T)%2C%0A%20%20%20Err(E)%2C%0A%7D%0A%7D">Run</a></pre>
<p>Functions return <a href="enum.Result.html"><code>Result</code></a> whenever errors are expected and
recoverable. In the <code>std</code> crate, <a href="enum.Result.html"><code>Result</code></a> is most prominently used
for <a href="../../std/io/index.html">I/O</a>.</p>
<p>A simple function returning <a href="enum.Result.html"><code>Result</code></a> might be
defined and used like so:</p>
<pre class="rust rust-example-rendered">
<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Debug</span>)]</span>
<span class="kw">enum</span> <span class="ident">Version</span> { <span class="ident">Version1</span>, <span class="ident">Version2</span> }
<span class="kw">fn</span> <span class="ident">parse_version</span>(<span class="ident">header</span>: <span class="kw-2">&</span>[<span class="ident">u8</span>]) <span class="op">-></span> <span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">Version</span>, <span class="kw-2">&</span><span class="lifetime">'static</span> <span class="ident">str</span><span class="op">></span> {
<span class="kw">match</span> <span class="ident">header</span>.<span class="ident">get</span>(<span class="number">0</span>) {
<span class="prelude-val">None</span> <span class="op">=></span> <span class="prelude-val">Err</span>(<span class="string">"invalid header length"</span>),
<span class="prelude-val">Some</span>(<span class="kw-2">&</span><span class="number">1</span>) <span class="op">=></span> <span class="prelude-val">Ok</span>(<span class="ident">Version</span>::<span class="ident">Version1</span>),
<span class="prelude-val">Some</span>(<span class="kw-2">&</span><span class="number">2</span>) <span class="op">=></span> <span class="prelude-val">Ok</span>(<span class="ident">Version</span>::<span class="ident">Version2</span>),
<span class="prelude-val">Some</span>(<span class="kw">_</span>) <span class="op">=></span> <span class="prelude-val">Err</span>(<span class="string">"invalid version"</span>),
}
}
<span class="kw">let</span> <span class="ident">version</span> <span class="op">=</span> <span class="ident">parse_version</span>(<span class="kw-2">&</span>[<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>, <span class="number">4</span>]);
<span class="kw">match</span> <span class="ident">version</span> {
<span class="prelude-val">Ok</span>(<span class="ident">v</span>) <span class="op">=></span> <span class="macro">println</span><span class="macro">!</span>(<span class="string">"working with version: {:?}"</span>, <span class="ident">v</span>),
<span class="prelude-val">Err</span>(<span class="ident">e</span>) <span class="op">=></span> <span class="macro">println</span><span class="macro">!</span>(<span class="string">"error parsing header: {:?}"</span>, <span class="ident">e</span>),
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0A%23%5Bderive(Debug)%5D%0Aenum%20Version%20%7B%20Version1%2C%20Version2%20%7D%0A%0Afn%20parse_version(header%3A%20%26%5Bu8%5D)%20-%3E%20Result%3CVersion%2C%20%26'static%20str%3E%20%7B%0A%20%20%20%20match%20header.get(0)%20%7B%0A%20%20%20%20%20%20%20%20None%20%3D%3E%20Err(%22invalid%20header%20length%22)%2C%0A%20%20%20%20%20%20%20%20Some(%261)%20%3D%3E%20Ok(Version%3A%3AVersion1)%2C%0A%20%20%20%20%20%20%20%20Some(%262)%20%3D%3E%20Ok(Version%3A%3AVersion2)%2C%0A%20%20%20%20%20%20%20%20Some(_)%20%3D%3E%20Err(%22invalid%20version%22)%2C%0A%20%20%20%20%7D%0A%7D%0A%0Alet%20version%20%3D%20parse_version(%26%5B1%2C%202%2C%203%2C%204%5D)%3B%0Amatch%20version%20%7B%0A%20%20%20%20Ok(v)%20%3D%3E%20println!(%22working%20with%20version%3A%20%7B%3A%3F%7D%22%2C%20v)%2C%0A%20%20%20%20Err(e)%20%3D%3E%20println!(%22error%20parsing%20header%3A%20%7B%3A%3F%7D%22%2C%20e)%2C%0A%7D%0A%7D">Run</a></pre>
<p>Pattern matching on <a href="enum.Result.html"><code>Result</code></a>s is clear and straightforward for
simple cases, but <a href="enum.Result.html"><code>Result</code></a> comes with some convenience methods
that make working with it more succinct.</p>
<pre class="rust rust-example-rendered">
<span class="kw">let</span> <span class="ident">good_result</span>: <span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">i32</span>, <span class="ident">i32</span><span class="op">></span> <span class="op">=</span> <span class="prelude-val">Ok</span>(<span class="number">10</span>);
<span class="kw">let</span> <span class="ident">bad_result</span>: <span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">i32</span>, <span class="ident">i32</span><span class="op">></span> <span class="op">=</span> <span class="prelude-val">Err</span>(<span class="number">10</span>);
<span class="comment">// The `is_ok` and `is_err` methods do what they say.</span>
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">good_result</span>.<span class="ident">is_ok</span>() <span class="op">&&</span> <span class="op">!</span><span class="ident">good_result</span>.<span class="ident">is_err</span>());
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">bad_result</span>.<span class="ident">is_err</span>() <span class="op">&&</span> <span class="op">!</span><span class="ident">bad_result</span>.<span class="ident">is_ok</span>());
<span class="comment">// `map` consumes the `Result` and produces another.</span>
<span class="kw">let</span> <span class="ident">good_result</span>: <span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">i32</span>, <span class="ident">i32</span><span class="op">></span> <span class="op">=</span> <span class="ident">good_result</span>.<span class="ident">map</span>(<span class="op">|</span><span class="ident">i</span><span class="op">|</span> <span class="ident">i</span> <span class="op">+</span> <span class="number">1</span>);
<span class="kw">let</span> <span class="ident">bad_result</span>: <span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">i32</span>, <span class="ident">i32</span><span class="op">></span> <span class="op">=</span> <span class="ident">bad_result</span>.<span class="ident">map</span>(<span class="op">|</span><span class="ident">i</span><span class="op">|</span> <span class="ident">i</span> <span class="op">-</span> <span class="number">1</span>);
<span class="comment">// Use `and_then` to continue the computation.</span>
<span class="kw">let</span> <span class="ident">good_result</span>: <span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">bool</span>, <span class="ident">i32</span><span class="op">></span> <span class="op">=</span> <span class="ident">good_result</span>.<span class="ident">and_then</span>(<span class="op">|</span><span class="ident">i</span><span class="op">|</span> <span class="prelude-val">Ok</span>(<span class="ident">i</span> <span class="op">==</span> <span class="number">11</span>));
<span class="comment">// Use `or_else` to handle the error.</span>
<span class="kw">let</span> <span class="ident">bad_result</span>: <span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">i32</span>, <span class="ident">i32</span><span class="op">></span> <span class="op">=</span> <span class="ident">bad_result</span>.<span class="ident">or_else</span>(<span class="op">|</span><span class="ident">i</span><span class="op">|</span> <span class="prelude-val">Ok</span>(<span class="ident">i</span> <span class="op">+</span> <span class="number">20</span>));
<span class="comment">// Consume the result and return the contents with `unwrap`.</span>
<span class="kw">let</span> <span class="ident">final_awesome_result</span> <span class="op">=</span> <span class="ident">good_result</span>.<span class="ident">unwrap</span>();<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Alet%20good_result%3A%20Result%3Ci32%2C%20i32%3E%20%3D%20Ok(10)%3B%0Alet%20bad_result%3A%20Result%3Ci32%2C%20i32%3E%20%3D%20Err(10)%3B%0A%0A%2F%2F%20The%20%60is_ok%60%20and%20%60is_err%60%20methods%20do%20what%20they%20say.%0Aassert!(good_result.is_ok()%20%26%26%20!good_result.is_err())%3B%0Aassert!(bad_result.is_err()%20%26%26%20!bad_result.is_ok())%3B%0A%0A%2F%2F%20%60map%60%20consumes%20the%20%60Result%60%20and%20produces%20another.%0Alet%20good_result%3A%20Result%3Ci32%2C%20i32%3E%20%3D%20good_result.map(%7Ci%7C%20i%20%2B%201)%3B%0Alet%20bad_result%3A%20Result%3Ci32%2C%20i32%3E%20%3D%20bad_result.map(%7Ci%7C%20i%20-%201)%3B%0A%0A%2F%2F%20Use%20%60and_then%60%20to%20continue%20the%20computation.%0Alet%20good_result%3A%20Result%3Cbool%2C%20i32%3E%20%3D%20good_result.and_then(%7Ci%7C%20Ok(i%20%3D%3D%2011))%3B%0A%0A%2F%2F%20Use%20%60or_else%60%20to%20handle%20the%20error.%0Alet%20bad_result%3A%20Result%3Ci32%2C%20i32%3E%20%3D%20bad_result.or_else(%7Ci%7C%20Ok(i%20%2B%2020))%3B%0A%0A%2F%2F%20Consume%20the%20result%20and%20return%20the%20contents%20with%20%60unwrap%60.%0Alet%20final_awesome_result%20%3D%20good_result.unwrap()%3B%0A%7D">Run</a></pre>
<h1 id="results-must-be-used" class="section-header"><a href="#results-must-be-used">Results must be used</a></h1>
<p>A common problem with using return values to indicate errors is
that it is easy to ignore the return value, thus failing to handle
the error. <a href="enum.Result.html"><code>Result</code></a> is annotated with the <code>#[must_use]</code> attribute,
which will cause the compiler to issue a warning when a Result
value is ignored. This makes <a href="enum.Result.html"><code>Result</code></a> especially useful with
functions that may encounter errors but don't otherwise return a
useful value.</p>
<p>Consider the <a href="../../std/io/trait.Write.html#method.write_all"><code>write_all</code></a> method defined for I/O types
by the <a href="../../std/io/trait.Write.html"><code>Write</code></a> trait:</p>
<pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">io</span>;
<span class="kw">trait</span> <span class="ident">Write</span> {
<span class="kw">fn</span> <span class="ident">write_all</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>, <span class="ident">bytes</span>: <span class="kw-2">&</span>[<span class="ident">u8</span>]) <span class="op">-></span> <span class="prelude-ty">Result</span><span class="op"><</span>(), <span class="ident">io</span>::<span class="ident">Error</span><span class="op">></span>;
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Ause%20std%3A%3Aio%3B%0A%0Atrait%20Write%20%7B%0A%20%20%20%20fn%20write_all(%26mut%20self%2C%20bytes%3A%20%26%5Bu8%5D)%20-%3E%20Result%3C()%2C%20io%3A%3AError%3E%3B%0A%7D%0A%7D">Run</a></pre>
<p><em>Note: The actual definition of <a href="../../std/io/trait.Write.html"><code>Write</code></a> uses <a href="../../std/io/type.Result.html"><code>io::Result</code></a>, which
is just a synonym for <a href="enum.Result.html"><code>Result</code></a><code><T,</code><a href="../../std/io/struct.Error.html"><code>io::Error</code></a><code>></code>.</em></p>
<p>This method doesn't produce a value, but the write may
fail. It's crucial to handle the error case, and <em>not</em> write
something like this:</p>
<pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">fs</span>::<span class="ident">File</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">io</span>::<span class="ident">prelude</span>::<span class="kw-2">*</span>;
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">file</span> <span class="op">=</span> <span class="ident">File</span>::<span class="ident">create</span>(<span class="string">"valuable_data.txt"</span>).<span class="ident">unwrap</span>();
<span class="comment">// If `write_all` errors, then we'll never know, because the return</span>
<span class="comment">// value is ignored.</span>
<span class="ident">file</span>.<span class="ident">write_all</span>(<span class="string">b"important message"</span>);<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0A%23!%5Ballow(unused_must_use)%5D%20%2F%2F%20%5Co%2F%0Afn%20main()%20%7B%0Ause%20std%3A%3Afs%3A%3AFile%3B%0Ause%20std%3A%3Aio%3A%3Aprelude%3A%3A*%3B%0A%0Alet%20mut%20file%20%3D%20File%3A%3Acreate(%22valuable_data.txt%22).unwrap()%3B%0A%2F%2F%20If%20%60write_all%60%20errors%2C%20then%20we'll%20never%20know%2C%20because%20the%20return%0A%2F%2F%20value%20is%20ignored.%0Afile.write_all(b%22important%20message%22)%3B%0A%7D">Run</a></pre>
<p>If you <em>do</em> write that in Rust, the compiler will give you a
warning (by default, controlled by the <code>unused_must_use</code> lint).</p>
<p>You might instead, if you don't want to handle the error, simply
assert success with <a href="enum.Result.html#method.expect"><code>expect</code></a>. This will panic if the
write fails, providing a marginally useful message indicating why:</p>
<pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">fs</span>::<span class="ident">File</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">io</span>::<span class="ident">prelude</span>::<span class="kw-2">*</span>;
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">file</span> <span class="op">=</span> <span class="ident">File</span>::<span class="ident">create</span>(<span class="string">"valuable_data.txt"</span>).<span class="ident">unwrap</span>();
<span class="ident">file</span>.<span class="ident">write_all</span>(<span class="string">b"important message"</span>).<span class="ident">expect</span>(<span class="string">"failed to write message"</span>);<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Ause%20std%3A%3Afs%3A%3AFile%3B%0Ause%20std%3A%3Aio%3A%3Aprelude%3A%3A*%3B%0A%0Alet%20mut%20file%20%3D%20File%3A%3Acreate(%22valuable_data.txt%22).unwrap()%3B%0Afile.write_all(b%22important%20message%22).expect(%22failed%20to%20write%20message%22)%3B%0A%7D">Run</a></pre>
<p>You might also simply assert success:</p>
<pre class="rust rust-example-rendered">
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">file</span>.<span class="ident">write_all</span>(<span class="string">b"important message"</span>).<span class="ident">is_ok</span>());<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Ause%20std%3A%3Afs%3A%3AFile%3B%0Ause%20std%3A%3Aio%3A%3Aprelude%3A%3A*%3B%0Alet%20mut%20file%20%3D%20File%3A%3Acreate(%22valuable_data.txt%22).unwrap()%3B%0Aassert!(file.write_all(b%22important%20message%22).is_ok())%3B%0A%7D">Run</a></pre>
<p>Or propagate the error up the call stack with <a href="../../std/macro.try.html"><code>?</code></a>:</p>
<pre class="rust rust-example-rendered">
<span class="kw">fn</span> <span class="ident">write_message</span>() <span class="op">-></span> <span class="ident">io</span>::<span class="prelude-ty">Result</span><span class="op"><</span>()<span class="op">></span> {
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">file</span> <span class="op">=</span> <span class="ident">File</span>::<span class="ident">create</span>(<span class="string">"valuable_data.txt"</span>)<span class="question-mark">?</span>;
<span class="ident">file</span>.<span class="ident">write_all</span>(<span class="string">b"important message"</span>)<span class="question-mark">?</span>;
<span class="prelude-val">Ok</span>(())
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Ause%20std%3A%3Afs%3A%3AFile%3B%0Ause%20std%3A%3Aio%3A%3Aprelude%3A%3A*%3B%0Ause%20std%3A%3Aio%3B%0A%23%5Ballow(dead_code)%5D%0Afn%20write_message()%20-%3E%20io%3A%3AResult%3C()%3E%20%7B%0A%20%20%20%20let%20mut%20file%20%3D%20File%3A%3Acreate(%22valuable_data.txt%22)%3F%3B%0A%20%20%20%20file.write_all(b%22important%20message%22)%3F%3B%0A%20%20%20%20Ok(())%0A%7D%0A%7D">Run</a></pre>
<h1 id="the-question-mark-operator-" class="section-header"><a href="#the-question-mark-operator-">The question mark operator, <code>?</code></a></h1>
<p>When writing code that calls many functions that return the
<a href="enum.Result.html"><code>Result</code></a> type, the error handling can be tedious. The question mark
operator, <a href="../../std/macro.try.html"><code>?</code></a>, hides some of the boilerplate of propagating errors
up the call stack.</p>
<p>It replaces this:</p>
<pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">fs</span>::<span class="ident">File</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">io</span>::<span class="ident">prelude</span>::<span class="kw-2">*</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">io</span>;
<span class="kw">struct</span> <span class="ident">Info</span> {
<span class="ident">name</span>: <span class="ident">String</span>,
<span class="ident">age</span>: <span class="ident">i32</span>,
<span class="ident">rating</span>: <span class="ident">i32</span>,
}
<span class="kw">fn</span> <span class="ident">write_info</span>(<span class="ident">info</span>: <span class="kw-2">&</span><span class="ident">Info</span>) <span class="op">-></span> <span class="ident">io</span>::<span class="prelude-ty">Result</span><span class="op"><</span>()<span class="op">></span> {
<span class="comment">// Early return on error</span>
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">file</span> <span class="op">=</span> <span class="kw">match</span> <span class="ident">File</span>::<span class="ident">create</span>(<span class="string">"my_best_friends.txt"</span>) {
<span class="prelude-val">Err</span>(<span class="ident">e</span>) <span class="op">=></span> <span class="kw">return</span> <span class="prelude-val">Err</span>(<span class="ident">e</span>),
<span class="prelude-val">Ok</span>(<span class="ident">f</span>) <span class="op">=></span> <span class="ident">f</span>,
};
<span class="kw">if</span> <span class="kw">let</span> <span class="prelude-val">Err</span>(<span class="ident">e</span>) <span class="op">=</span> <span class="ident">file</span>.<span class="ident">write_all</span>(<span class="macro">format</span><span class="macro">!</span>(<span class="string">"name: {}\n"</span>, <span class="ident">info</span>.<span class="ident">name</span>).<span class="ident">as_bytes</span>()) {
<span class="kw">return</span> <span class="prelude-val">Err</span>(<span class="ident">e</span>)
}
<span class="kw">if</span> <span class="kw">let</span> <span class="prelude-val">Err</span>(<span class="ident">e</span>) <span class="op">=</span> <span class="ident">file</span>.<span class="ident">write_all</span>(<span class="macro">format</span><span class="macro">!</span>(<span class="string">"age: {}\n"</span>, <span class="ident">info</span>.<span class="ident">age</span>).<span class="ident">as_bytes</span>()) {
<span class="kw">return</span> <span class="prelude-val">Err</span>(<span class="ident">e</span>)
}
<span class="kw">if</span> <span class="kw">let</span> <span class="prelude-val">Err</span>(<span class="ident">e</span>) <span class="op">=</span> <span class="ident">file</span>.<span class="ident">write_all</span>(<span class="macro">format</span><span class="macro">!</span>(<span class="string">"rating: {}\n"</span>, <span class="ident">info</span>.<span class="ident">rating</span>).<span class="ident">as_bytes</span>()) {
<span class="kw">return</span> <span class="prelude-val">Err</span>(<span class="ident">e</span>)
}
<span class="prelude-val">Ok</span>(())
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0A%23!%5Ballow(dead_code)%5D%0Afn%20main()%20%7B%0Ause%20std%3A%3Afs%3A%3AFile%3B%0Ause%20std%3A%3Aio%3A%3Aprelude%3A%3A*%3B%0Ause%20std%3A%3Aio%3B%0A%0Astruct%20Info%20%7B%0A%20%20%20%20name%3A%20String%2C%0A%20%20%20%20age%3A%20i32%2C%0A%20%20%20%20rating%3A%20i32%2C%0A%7D%0A%0Afn%20write_info(info%3A%20%26Info)%20-%3E%20io%3A%3AResult%3C()%3E%20%7B%0A%20%20%20%20%2F%2F%20Early%20return%20on%20error%0A%20%20%20%20let%20mut%20file%20%3D%20match%20File%3A%3Acreate(%22my_best_friends.txt%22)%20%7B%0A%20%20%20%20%20%20%20%20%20%20%20Err(e)%20%3D%3E%20return%20Err(e)%2C%0A%20%20%20%20%20%20%20%20%20%20%20Ok(f)%20%3D%3E%20f%2C%0A%20%20%20%20%7D%3B%0A%20%20%20%20if%20let%20Err(e)%20%3D%20file.write_all(format!(%22name%3A%20%7B%7D%5Cn%22%2C%20info.name).as_bytes())%20%7B%0A%20%20%20%20%20%20%20%20return%20Err(e)%0A%20%20%20%20%7D%0A%20%20%20%20if%20let%20Err(e)%20%3D%20file.write_all(format!(%22age%3A%20%7B%7D%5Cn%22%2C%20info.age).as_bytes())%20%7B%0A%20%20%20%20%20%20%20%20return%20Err(e)%0A%20%20%20%20%7D%0A%20%20%20%20if%20let%20Err(e)%20%3D%20file.write_all(format!(%22rating%3A%20%7B%7D%5Cn%22%2C%20info.rating).as_bytes())%20%7B%0A%20%20%20%20%20%20%20%20return%20Err(e)%0A%20%20%20%20%7D%0A%20%20%20%20Ok(())%0A%7D%0A%7D">Run</a></pre>
<p>With this:</p>
<pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">fs</span>::<span class="ident">File</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">io</span>::<span class="ident">prelude</span>::<span class="kw-2">*</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">io</span>;
<span class="kw">struct</span> <span class="ident">Info</span> {
<span class="ident">name</span>: <span class="ident">String</span>,
<span class="ident">age</span>: <span class="ident">i32</span>,
<span class="ident">rating</span>: <span class="ident">i32</span>,
}
<span class="kw">fn</span> <span class="ident">write_info</span>(<span class="ident">info</span>: <span class="kw-2">&</span><span class="ident">Info</span>) <span class="op">-></span> <span class="ident">io</span>::<span class="prelude-ty">Result</span><span class="op"><</span>()<span class="op">></span> {
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">file</span> <span class="op">=</span> <span class="ident">File</span>::<span class="ident">create</span>(<span class="string">"my_best_friends.txt"</span>)<span class="question-mark">?</span>;
<span class="comment">// Early return on error</span>
<span class="ident">file</span>.<span class="ident">write_all</span>(<span class="macro">format</span><span class="macro">!</span>(<span class="string">"name: {}\n"</span>, <span class="ident">info</span>.<span class="ident">name</span>).<span class="ident">as_bytes</span>())<span class="question-mark">?</span>;
<span class="ident">file</span>.<span class="ident">write_all</span>(<span class="macro">format</span><span class="macro">!</span>(<span class="string">"age: {}\n"</span>, <span class="ident">info</span>.<span class="ident">age</span>).<span class="ident">as_bytes</span>())<span class="question-mark">?</span>;
<span class="ident">file</span>.<span class="ident">write_all</span>(<span class="macro">format</span><span class="macro">!</span>(<span class="string">"rating: {}\n"</span>, <span class="ident">info</span>.<span class="ident">rating</span>).<span class="ident">as_bytes</span>())<span class="question-mark">?</span>;
<span class="prelude-val">Ok</span>(())
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0A%23!%5Ballow(dead_code)%5D%0Afn%20main()%20%7B%0Ause%20std%3A%3Afs%3A%3AFile%3B%0Ause%20std%3A%3Aio%3A%3Aprelude%3A%3A*%3B%0Ause%20std%3A%3Aio%3B%0A%0Astruct%20Info%20%7B%0A%20%20%20%20name%3A%20String%2C%0A%20%20%20%20age%3A%20i32%2C%0A%20%20%20%20rating%3A%20i32%2C%0A%7D%0A%0Afn%20write_info(info%3A%20%26Info)%20-%3E%20io%3A%3AResult%3C()%3E%20%7B%0A%20%20%20%20let%20mut%20file%20%3D%20File%3A%3Acreate(%22my_best_friends.txt%22)%3F%3B%0A%20%20%20%20%2F%2F%20Early%20return%20on%20error%0A%20%20%20%20file.write_all(format!(%22name%3A%20%7B%7D%5Cn%22%2C%20info.name).as_bytes())%3F%3B%0A%20%20%20%20file.write_all(format!(%22age%3A%20%7B%7D%5Cn%22%2C%20info.age).as_bytes())%3F%3B%0A%20%20%20%20file.write_all(format!(%22rating%3A%20%7B%7D%5Cn%22%2C%20info.rating).as_bytes())%3F%3B%0A%20%20%20%20Ok(())%0A%7D%0A%7D">Run</a></pre>
<p><em>It's much nicer!</em></p>
<p>Ending the expression with <a href="../../std/macro.try.html"><code>?</code></a> will result in the unwrapped
success (<a href="enum.Result.html#variant.Ok"><code>Ok</code></a>) value, unless the result is <a href="enum.Result.html#variant.Err"><code>Err</code></a>, in which case
<a href="enum.Result.html#variant.Err"><code>Err</code></a> is returned early from the enclosing function.</p>
<p><a href="../../std/macro.try.html"><code>?</code></a> can only be used in functions that return <a href="enum.Result.html"><code>Result</code></a> because of the
early return of <a href="enum.Result.html#variant.Err"><code>Err</code></a> that it provides.</p>
</div><h2 id='structs' class='section-header'><a href="#structs">Structs</a></h2>
<table>
<tr class=' module-item'>
<td><a class="struct" href="struct.IntoIter.html"
title='struct std::result::IntoIter'>IntoIter</a></td>
<td class='docblock-short'>
<p>An iterator over the value in a <a href="enum.Result.html#variant.Ok"><code>Ok</code></a> variant of a <a href="enum.Result.html"><code>Result</code></a>.</p>
</td>
</tr>
<tr class=' module-item'>
<td><a class="struct" href="struct.Iter.html"
title='struct std::result::Iter'>Iter</a></td>
<td class='docblock-short'>
<p>An iterator over a reference to the <a href="enum.Result.html#variant.Ok"><code>Ok</code></a> variant of a <a href="enum.Result.html"><code>Result</code></a>.</p>
</td>
</tr>
<tr class=' module-item'>
<td><a class="struct" href="struct.IterMut.html"
title='struct std::result::IterMut'>IterMut</a></td>
<td class='docblock-short'>
<p>An iterator over a mutable reference to the <a href="enum.Result.html#variant.Ok"><code>Ok</code></a> variant of a <a href="enum.Result.html"><code>Result</code></a>.</p>
</td>
</tr></table><h2 id='enums' class='section-header'><a href="#enums">Enums</a></h2>
<table>
<tr class=' module-item'>
<td><a class="enum" href="enum.Result.html"
title='enum std::result::Result'>Result</a></td>
<td class='docblock-short'>
<p><code>Result</code> is a type that represents either success (<a href="enum.Result.html#variant.Ok"><code>Ok</code></a>) or failure (<a href="enum.Result.html#variant.Err"><code>Err</code></a>).</p>
</td>
</tr></table></section><section id="search" class="content hidden"></section><section class="footer"></section><aside id="help" class="hidden"><div><h1 class="hidden">Help</h1><div class="shortcuts"><h2>Keyboard Shortcuts</h2><dl><dt><kbd>?</kbd></dt><dd>Show this help dialog</dd><dt><kbd>S</kbd></dt><dd>Focus the search field</dd><dt><kbd>↑</kbd></dt><dd>Move up in search results</dd><dt><kbd>↓</kbd></dt><dd>Move down in search results</dd><dt><kbd>↹</kbd></dt><dd>Switch tab</dd><dt><kbd>⏎</kbd></dt><dd>Go to active search result</dd><dt><kbd>+</kbd></dt><dd>Expand all sections</dd><dt><kbd>-</kbd></dt><dd>Collapse all sections</dd></dl></div><div class="infos"><h2>Search Tricks</h2><p>Prefix searches with a type followed by a colon (e.g. <code>fn:</code>) to restrict the search to a given type.</p><p>Accepted types are: <code>fn</code>, <code>mod</code>, <code>struct</code>, <code>enum</code>, <code>trait</code>, <code>type</code>, <code>macro</code>, and <code>const</code>.</p><p>Search functions by type signature (e.g. <code>vec -> usize</code> or <code>* -> vec</code>)</p><p>Search multiple things at once by splitting your query with comma (e.g. <code>str,u8</code> or <code>String,struct:Vec,test</code>)</p></div></div></aside><script>window.rootPath = "../../";window.currentCrate = "std";</script><script src="../../aliases.js"></script><script src="../../main.js"></script><script defer src="../../search-index.js"></script></body></html>
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