114 lines
22 KiB
HTML
114 lines
22 KiB
HTML
<!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 `Context` trait in crate `anyhow`."><meta name="keywords" content="rust, rustlang, rust-lang, Context"><title>anyhow::Context - 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><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="shortcut icon" href="../favicon.ico"><style type="text/css">#crate-search{background-image:url("../down-arrow.svg");}</style></head><body class="rustdoc trait"><!--[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='../anyhow/index.html'><div class='logo-container'><img src='../rust-logo.png' alt='logo'></div></a><p class='location'>Trait Context</p><div class="sidebar-elems"><div class="block items"><a class="sidebar-title" href="#required-methods">Required Methods</a><div class="sidebar-links"><a href="#tymethod.context">context</a><a href="#tymethod.with_context">with_context</a></div><a class="sidebar-title" href="#foreign-impls">Implementations on Foreign Types</a><div class="sidebar-links"><a href="#impl-Context%3CT%2C%20Infallible%3E-for-Option%3CT%3E">Option<T></a><a href="#impl-Context%3CT%2C%20E%3E-for-Result%3CT%2C%20E%3E">Result<T, E></a></div><a class="sidebar-title" href="#implementors">Implementors</a></div><p class='location'><a href='index.html'>anyhow</a></p><script>window.sidebarCurrent = {name: 'Context', ty: 'trait', 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"><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><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='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/anyhow/lib.rs.html#573-585' title='goto source code'>[src]</a></span><span class='in-band'>Trait <a href='index.html'>anyhow</a>::<wbr><a class="trait" href=''>Context</a></span></h1><div class="docblock type-decl hidden-by-usual-hider"><pre class='rust trait'>pub trait Context<T, E>: Sealed {
|
||
fn <a href='#tymethod.context' class='fnname'>context</a><C>(self, context: C) -> <a class="type" href="../anyhow/type.Result.html" title="type anyhow::Result">Result</a><T, <a class="struct" href="../anyhow/struct.Error.html" title="struct anyhow::Error">Error</a>><br> <span class="where">where<br> C: <a class="trait" href="https://doc.rust-lang.org/nightly/core/fmt/trait.Display.html" title="trait core::fmt::Display">Display</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static</span>;
|
||
<div class='item-spacer'></div> fn <a href='#tymethod.with_context' class='fnname'>with_context</a><C, F>(self, f: F) -> <a class="type" href="../anyhow/type.Result.html" title="type anyhow::Result">Result</a><T, <a class="struct" href="../anyhow/struct.Error.html" title="struct anyhow::Error">Error</a>><br> <span class="where">where<br> C: <a class="trait" href="https://doc.rust-lang.org/nightly/core/fmt/trait.Display.html" title="trait core::fmt::Display">Display</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static,<br> F: <a class="trait" href="https://doc.rust-lang.org/nightly/core/ops/function/trait.FnOnce.html" title="trait core::ops::function::FnOnce">FnOnce</a>() -> C</span>;
|
||
}</pre></div><div class='docblock'><p>Provides the <code>context</code> method for <code>Result</code>.</p>
|
||
<p>This trait is sealed and cannot be implemented for types outside of
|
||
<code>anyhow</code>.</p>
|
||
<br>
|
||
<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
|
||
<div class="example-wrap"><pre class="rust rust-example-rendered">
|
||
<span class="kw">use</span> <span class="ident">anyhow</span>::{<span class="ident">Context</span>, <span class="prelude-ty">Result</span>};
|
||
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">fs</span>;
|
||
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">path</span>::<span class="ident">PathBuf</span>;
|
||
|
||
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">ImportantThing</span> {
|
||
<span class="ident">path</span>: <span class="ident">PathBuf</span>,
|
||
}
|
||
|
||
<span class="kw">impl</span> <span class="ident">ImportantThing</span> {
|
||
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">detach</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-</span><span class="op">></span> <span class="prelude-ty">Result</span><span class="op"><</span>()<span class="op">></span> {...}
|
||
}
|
||
|
||
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">do_it</span>(<span class="kw-2">mut</span> <span class="ident">it</span>: <span class="ident">ImportantThing</span>) <span class="op">-</span><span class="op">></span> <span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">Vec</span><span class="op"><</span><span class="ident">u8</span><span class="op">></span><span class="op">></span> {
|
||
<span class="ident">it</span>.<span class="ident">detach</span>().<span class="ident">context</span>(<span class="string">"Failed to detach the important thing"</span>)<span class="question-mark">?</span>;
|
||
|
||
<span class="kw">let</span> <span class="ident">path</span> <span class="op">=</span> <span class="kw-2">&</span><span class="ident">it</span>.<span class="ident">path</span>;
|
||
<span class="kw">let</span> <span class="ident">content</span> <span class="op">=</span> <span class="ident">fs</span>::<span class="ident">read</span>(<span class="ident">path</span>)
|
||
.<span class="ident">with_context</span>(<span class="op">|</span><span class="op">|</span> <span class="macro">format</span><span class="macro">!</span>(<span class="string">"Failed to read instrs from {}"</span>, <span class="ident">path</span>.<span class="ident">display</span>()))<span class="question-mark">?</span>;
|
||
|
||
<span class="prelude-val">Ok</span>(<span class="ident">content</span>)
|
||
}</pre></div>
|
||
<p>When printed, the outermost context would be printed first and the lower
|
||
level underlying causes would be enumerated below.</p>
|
||
<pre><code class="language-console">Error: Failed to read instrs from ./path/to/instrs.json
|
||
|
||
Caused by:
|
||
No such file or directory (os error 2)
|
||
</code></pre>
|
||
<br>
|
||
<h1 id="effect-on-downcasting" class="section-header"><a href="#effect-on-downcasting">Effect on downcasting</a></h1>
|
||
<p>After attaching context of type <code>C</code> onto an error of type <code>E</code>, the resulting
|
||
<code>anyhow::Error</code> may be downcast to <code>C</code> <strong>or</strong> to <code>E</code>.</p>
|
||
<p>That is, in codebases that rely on downcasting, Anyhow's context supports
|
||
both of the following use cases:</p>
|
||
<ul>
|
||
<li>
|
||
<p><strong>Attaching context whose type is insignificant onto errors whose type
|
||
is used in downcasts.</strong></p>
|
||
<p>In other error libraries whose context is not designed this way, it can
|
||
be risky to introduce context to existing code because new context might
|
||
break existing working downcasts. In Anyhow, any downcast that worked
|
||
before adding context will continue to work after you add a context, so
|
||
you should freely add human-readable context to errors wherever it would
|
||
be helpful.</p>
|
||
|
||
<div class="example-wrap"><pre class="rust rust-example-rendered">
|
||
<span class="kw">use</span> <span class="ident">anyhow</span>::{<span class="ident">Context</span>, <span class="prelude-ty">Result</span>};
|
||
|
||
<span class="kw">fn</span> <span class="ident">do_it</span>() <span class="op">-</span><span class="op">></span> <span class="prelude-ty">Result</span><span class="op"><</span>()<span class="op">></span> {
|
||
<span class="ident">helper</span>().<span class="ident">context</span>(<span class="string">"Failed to complete the work"</span>)<span class="question-mark">?</span>;
|
||
...
|
||
}
|
||
|
||
<span class="kw">fn</span> <span class="ident">main</span>() {
|
||
<span class="kw">let</span> <span class="ident">err</span> <span class="op">=</span> <span class="ident">do_it</span>().<span class="ident">unwrap_err</span>();
|
||
<span class="kw">if</span> <span class="kw">let</span> <span class="prelude-val">Some</span>(<span class="ident">e</span>) <span class="op">=</span> <span class="ident">err</span>.<span class="ident">downcast_ref</span>::<span class="op"><</span><span class="ident">SuspiciousError</span><span class="op">></span>() {
|
||
<span class="comment">// If helper() returned SuspiciousError, this downcast will</span>
|
||
<span class="comment">// correctly succeed even with the context in between.</span>
|
||
}
|
||
}</pre></div>
|
||
</li>
|
||
<li>
|
||
<p><strong>Attaching context whose type is used in downcasts onto errors whose
|
||
type is insignificant.</strong></p>
|
||
<p>Some codebases prefer to use machine-readable context to categorize
|
||
lower level errors in a way that will be actionable to higher levels of
|
||
the application.</p>
|
||
|
||
<div class="example-wrap"><pre class="rust rust-example-rendered">
|
||
<span class="kw">use</span> <span class="ident">anyhow</span>::{<span class="ident">Context</span>, <span class="prelude-ty">Result</span>};
|
||
|
||
<span class="kw">fn</span> <span class="ident">do_it</span>() <span class="op">-</span><span class="op">></span> <span class="prelude-ty">Result</span><span class="op"><</span>()<span class="op">></span> {
|
||
<span class="ident">helper</span>().<span class="ident">context</span>(<span class="ident">HelperFailed</span>)<span class="question-mark">?</span>;
|
||
...
|
||
}
|
||
|
||
<span class="kw">fn</span> <span class="ident">main</span>() {
|
||
<span class="kw">let</span> <span class="ident">err</span> <span class="op">=</span> <span class="ident">do_it</span>().<span class="ident">unwrap_err</span>();
|
||
<span class="kw">if</span> <span class="kw">let</span> <span class="prelude-val">Some</span>(<span class="ident">e</span>) <span class="op">=</span> <span class="ident">err</span>.<span class="ident">downcast_ref</span>::<span class="op"><</span><span class="ident">HelperFailed</span><span class="op">></span>() {
|
||
<span class="comment">// If helper failed, this downcast will succeed because</span>
|
||
<span class="comment">// HelperFailed is the context that has been attached to</span>
|
||
<span class="comment">// that error.</span>
|
||
}
|
||
}</pre></div>
|
||
</li>
|
||
</ul>
|
||
</div>
|
||
<h2 id='required-methods' class='small-section-header'>Required methods<a href='#required-methods' class='anchor'></a></h2><div class='methods'><h3 id='tymethod.context' class='method'><code>fn <a href='#tymethod.context' class='fnname'>context</a><C>(self, context: C) -> <a class="type" href="../anyhow/type.Result.html" title="type anyhow::Result">Result</a><T, <a class="struct" href="../anyhow/struct.Error.html" title="struct anyhow::Error">Error</a>> <span class="where fmt-newline">where<br> C: <a class="trait" href="https://doc.rust-lang.org/nightly/core/fmt/trait.Display.html" title="trait core::fmt::Display">Display</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static, </span></code></h3><div class='docblock'><p>Wrap the error value with additional context.</p>
|
||
</div><h3 id='tymethod.with_context' class='method'><code>fn <a href='#tymethod.with_context' class='fnname'>with_context</a><C, F>(self, f: F) -> <a class="type" href="../anyhow/type.Result.html" title="type anyhow::Result">Result</a><T, <a class="struct" href="../anyhow/struct.Error.html" title="struct anyhow::Error">Error</a>> <span class="where fmt-newline">where<br> C: <a class="trait" href="https://doc.rust-lang.org/nightly/core/fmt/trait.Display.html" title="trait core::fmt::Display">Display</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static,<br> F: <a class="trait" href="https://doc.rust-lang.org/nightly/core/ops/function/trait.FnOnce.html" title="trait core::ops::function::FnOnce">FnOnce</a>() -> C, </span></code></h3><div class='docblock'><p>Wrap the error value with additional context that is evaluated lazily
|
||
only once an error does occur.</p>
|
||
</div></div><span class='loading-content'>Loading content...</span>
|
||
<h2 id='foreign-impls' class='small-section-header'>Implementations on Foreign Types<a href='#foreign-impls' class='anchor'></a></h2><h3 id='impl-Context%3CT%2C%20E%3E-for-Result%3CT%2C%20E%3E' class='impl'><code class='in-band'>impl<T, E> <a class="trait" href="../anyhow/trait.Context.html" title="trait anyhow::Context">Context</a><T, E> for <a class="enum" href="https://doc.rust-lang.org/nightly/core/result/enum.Result.html" title="enum core::result::Result">Result</a><T, E> <span class="where fmt-newline">where<br> E: StdError + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static, </span></code><a href='#impl-Context%3CT%2C%20E%3E-for-Result%3CT%2C%20E%3E' class='anchor'></a><a class='srclink' href='../src/anyhow/context.rs.html#42-60' title='goto source code'>[src]</a></h3><div class='impl-items'><h4 id='method.context' class="method hidden"><code>fn <a href='#method.context' class='fnname'>context</a><C>(self, context: C) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/result/enum.Result.html" title="enum core::result::Result">Result</a><T, <a class="struct" href="../anyhow/struct.Error.html" title="struct anyhow::Error">Error</a>> <span class="where fmt-newline">where<br> C: <a class="trait" href="https://doc.rust-lang.org/nightly/core/fmt/trait.Display.html" title="trait core::fmt::Display">Display</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static, </span></code><a class='srclink' href='../src/anyhow/context.rs.html#46-51' title='goto source code'>[src]</a></h4><h4 id='method.with_context' class="method hidden"><code>fn <a href='#method.with_context' class='fnname'>with_context</a><C, F>(self, context: F) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/result/enum.Result.html" title="enum core::result::Result">Result</a><T, <a class="struct" href="../anyhow/struct.Error.html" title="struct anyhow::Error">Error</a>> <span class="where fmt-newline">where<br> C: <a class="trait" href="https://doc.rust-lang.org/nightly/core/fmt/trait.Display.html" title="trait core::fmt::Display">Display</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static,<br> F: <a class="trait" href="https://doc.rust-lang.org/nightly/core/ops/function/trait.FnOnce.html" title="trait core::ops::function::FnOnce">FnOnce</a>() -> C, </span></code><a class='srclink' href='../src/anyhow/context.rs.html#53-59' title='goto source code'>[src]</a></h4></div><h3 id='impl-Context%3CT%2C%20Infallible%3E-for-Option%3CT%3E' class='impl'><code class='in-band'>impl<T> <a class="trait" href="../anyhow/trait.Context.html" title="trait anyhow::Context">Context</a><T, <a class="enum" href="https://doc.rust-lang.org/nightly/core/convert/enum.Infallible.html" title="enum core::convert::Infallible">Infallible</a>> for <a class="enum" href="https://doc.rust-lang.org/nightly/core/option/enum.Option.html" title="enum core::option::Option">Option</a><T></code><a href='#impl-Context%3CT%2C%20Infallible%3E-for-Option%3CT%3E' class='anchor'></a><a class='srclink' href='../src/anyhow/context.rs.html#82-97' title='goto source code'>[src]</a></h3><div class='docblock'>
|
||
<div class="example-wrap"><pre class="rust rust-example-rendered">
|
||
<span class="kw">use</span> <span class="ident">anyhow</span>::{<span class="ident">Context</span>, <span class="prelude-ty">Result</span>};
|
||
|
||
<span class="kw">fn</span> <span class="ident">maybe_get</span>() <span class="op">-</span><span class="op">></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
|
||
...
|
||
}
|
||
|
||
<span class="kw">fn</span> <span class="ident">demo</span>() <span class="op">-</span><span class="op">></span> <span class="prelude-ty">Result</span><span class="op"><</span>()<span class="op">></span> {
|
||
<span class="kw">let</span> <span class="ident">t</span> <span class="op">=</span> <span class="ident">maybe_get</span>().<span class="ident">context</span>(<span class="string">"there is no T"</span>)<span class="question-mark">?</span>;
|
||
...
|
||
}</pre></div>
|
||
</div><div class='impl-items'><h4 id='method.context-1' class="method hidden"><code>fn <a href='#method.context' class='fnname'>context</a><C>(self, context: C) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/result/enum.Result.html" title="enum core::result::Result">Result</a><T, <a class="struct" href="../anyhow/struct.Error.html" title="struct anyhow::Error">Error</a>> <span class="where fmt-newline">where<br> C: <a class="trait" href="https://doc.rust-lang.org/nightly/core/fmt/trait.Display.html" title="trait core::fmt::Display">Display</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static, </span></code><a class='srclink' href='../src/anyhow/context.rs.html#83-88' title='goto source code'>[src]</a></h4><h4 id='method.with_context-1' class="method hidden"><code>fn <a href='#method.with_context' class='fnname'>with_context</a><C, F>(self, context: F) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/result/enum.Result.html" title="enum core::result::Result">Result</a><T, <a class="struct" href="../anyhow/struct.Error.html" title="struct anyhow::Error">Error</a>> <span class="where fmt-newline">where<br> C: <a class="trait" href="https://doc.rust-lang.org/nightly/core/fmt/trait.Display.html" title="trait core::fmt::Display">Display</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> + <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> + 'static,<br> F: <a class="trait" href="https://doc.rust-lang.org/nightly/core/ops/function/trait.FnOnce.html" title="trait core::ops::function::FnOnce">FnOnce</a>() -> C, </span></code><a class='srclink' href='../src/anyhow/context.rs.html#90-96' title='goto source code'>[src]</a></h4></div><span class='loading-content'>Loading content...</span>
|
||
<h2 id='implementors' class='small-section-header'>Implementors<a href='#implementors' class='anchor'></a></h2><div class='item-list' id='implementors-list'></div><span class='loading-content'>Loading content...</span><script type="text/javascript" src="../implementors/anyhow/trait.Context.js" async></script></section><section id="search" class="content hidden"></section><section class="footer"></section><script>window.rootPath = "../";window.currentCrate = "anyhow";</script><script src="../main.js"></script><script defer src="../search-index.js"></script></body></html> |