<!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 `try` macro in crate `std`."><meta name="keywords" content="rust, rustlang, rust-lang, try"><title>std::try - 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 macro"><!--[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><div class="sidebar-elems"><p class='location'><a href='index.html'>std</a></p><script>window.sidebarCurrent = {name: 'try', ty: 'macro', 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'>Macro <a href='index.html'>std</a>::<wbr><a class="macro" href=''>try</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/macros.rs.html#299-307' title='goto source code'>[src]</a></span></h1><div class="docblock type-decl"><pre class="rust macro">
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">try</span> {
( $ <span class="ident">expr</span> : <span class="ident">expr</span> ) <span class="op">=></span> { ... };
( $ <span class="ident">expr</span> : <span class="ident">expr</span> , ) <span class="op">=></span> { ... };
}</pre>
</div><div class='docblock'><p>Helper macro for reducing boilerplate code for matching <code>Result</code> together
with converting downstream errors.</p>
<p>The <code>?</code> operator was added to replace <code>try!</code> and should be used instead.</p>
<p><code>try!</code> matches the given <a href="../std/result/enum.Result.html"><code>Result</code></a>. In case of the <code>Ok</code> variant, the
expression has the value of the wrapped value.</p>
<p>In case of the <code>Err</code> variant, it retrieves the inner error. <code>try!</code> then
performs conversion using <code>From</code>. This provides automatic conversion
between specialized errors and more general ones. The resulting
error is then immediately returned.</p>
<p>Because of the early return, <code>try!</code> can only be used in functions that
return <a href="../std/result/enum.Result.html"><code>Result</code></a>.</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">std</span>::<span class="ident">io</span>;
<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">enum</span> <span class="ident">MyError</span> {
<span class="ident">FileWriteError</span>
}
<span class="kw">impl</span> <span class="ident">From</span><span class="op"><</span><span class="ident">io</span>::<span class="ident">Error</span><span class="op">></span> <span class="kw">for</span> <span class="ident">MyError</span> {
<span class="kw">fn</span> <span class="ident">from</span>(<span class="ident">e</span>: <span class="ident">io</span>::<span class="ident">Error</span>) <span class="op">-></span> <span class="ident">MyError</span> {
<span class="ident">MyError</span>::<span class="ident">FileWriteError</span>
}
}
<span class="comment">// The preferred method of quick returning Errors</span>
<span class="kw">fn</span> <span class="ident">write_to_file_question</span>() <span class="op">-></span> <span class="prelude-ty">Result</span><span class="op"><</span>(), <span class="ident">MyError</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="ident">file</span>.<span class="ident">write_all</span>(<span class="string">b"This is a list of my best friends."</span>)<span class="question-mark">?</span>;
<span class="prelude-val">Ok</span>(())
}
<span class="comment">// The previous method of quick returning Errors</span>
<span class="kw">fn</span> <span class="ident">write_to_file_using_try</span>() <span class="op">-></span> <span class="prelude-ty">Result</span><span class="op"><</span>(), <span class="ident">MyError</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="macro">try</span><span class="macro">!</span>(<span class="ident">File</span>::<span class="ident">create</span>(<span class="string">"my_best_friends.txt"</span>));
<span class="macro">try</span><span class="macro">!</span>(<span class="ident">file</span>.<span class="ident">write_all</span>(<span class="string">b"This is a list of my best friends."</span>));
<span class="prelude-val">Ok</span>(())
}
<span class="comment">// This is equivalent to:</span>
<span class="kw">fn</span> <span class="ident">write_to_file_using_match</span>() <span class="op">-></span> <span class="prelude-ty">Result</span><span class="op"><</span>(), <span class="ident">MyError</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="macro">try</span><span class="macro">!</span>(<span class="ident">File</span>::<span class="ident">create</span>(<span class="string">"my_best_friends.txt"</span>));
<span class="kw">match</span> <span class="ident">file</span>.<span class="ident">write_all</span>(<span class="string">b"This is a list of my best friends."</span>) {
<span class="prelude-val">Ok</span>(<span class="ident">v</span>) <span class="op">=></span> <span class="ident">v</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">From</span>::<span class="ident">from</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%0Afn%20main()%20%7B%0Ause%20std%3A%3Aio%3B%0Ause%20std%3A%3Afs%3A%3AFile%3B%0Ause%20std%3A%3Aio%3A%3Aprelude%3A%3A*%3B%0A%0Aenum%20MyError%20%7B%0A%20%20%20%20FileWriteError%0A%7D%0A%0Aimpl%20From%3Cio%3A%3AError%3E%20for%20MyError%20%7B%0A%20%20%20%20fn%20from(e%3A%20io%3A%3AError)%20-%3E%20MyError%20%7B%0A%20%20%20%20%20%20%20%20MyError%3A%3AFileWriteError%0A%20%20%20%20%7D%0A%7D%0A%0A%2F%2F%20The%20preferred%20method%20of%20quick%20returning%20Errors%0Afn%20write_to_file_question()%20-%3E%20Result%3C()%2C%20MyError%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%20file.write_all(b%22This%20is%20a%20list%20of%20my%20best%20friends.%22)%3F%3B%0A%20%20%20%20Ok(())%0A%7D%0A%0A%2F%2F%20The%20previous%20method%20of%20quick%20returning%20Errors%0Afn%20write_to_file_using_try()%20-%3E%20Result%3C()%2C%20MyError%3E%20%7B%0A%20%20%20%20let%20mut%20file%20%3D%20try!(File%3A%3Acreate(%22my_best_friends.txt%22))%3B%0A%20%20%20%20try!(file.write_all(b%22This%20is%20a%20list%20of%20my%20best%20friends.%22))%3B%0A%20%20%20%20Ok(())%0A%7D%0A%0A%2F%2F%20This%20is%20equivalent%20to%3A%0Afn%20write_to_file_using_match()%20-%3E%20Result%3C()%2C%20MyError%3E%20%7B%0A%20%20%20%20let%20mut%20file%20%3D%20try!(File%3A%3Acreate(%22my_best_friends.txt%22))%3B%0A%20%20%20%20match%20file.write_all(b%22This%20is%20a%20list%20of%20my%20best%20friends.%22)%20%7B%0A%20%20%20%20%20%20%20%20Ok(v)%20%3D%3E%20v%2C%0A%20%20%20%20%20%20%20%20Err(e)%20%3D%3E%20return%20Err(From%3A%3Afrom(e))%2C%0A%20%20%20%20%7D%0A%20%20%20%20Ok(())%0A%7D%0A%7D">Run</a></pre>
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