| <!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="Provides the `context` method for `Result`."><meta name="keywords" content="rust, rustlang, rust-lang, Context"><title>Context in anyhow - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../normalize.css"><link rel="stylesheet" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../ayu.css" disabled><link rel="stylesheet" href="../dark.css" disabled><link rel="stylesheet" href="../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../storage.js"></script><script defer src="sidebar-items.js"></script><script defer src="../main.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../favicon.svg"></head><body class="rustdoc trait"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle">☰</button><a class="sidebar-logo" href="../anyhow/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></div></a><h2></h2></nav><nav class="sidebar"><a class="sidebar-logo" href="../anyhow/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></div></a><h2 class="location"><a href="#">Context</a></h2><div class="sidebar-elems"><section><h3><a href="#required-methods">Required Methods</a></h3><ul class="block"><li><a href="#tymethod.context">context</a></li><li><a href="#tymethod.with_context">with_context</a></li></ul><h3><a href="#foreign-impls">Implementations on Foreign Types</a></h3><ul class="block"><li><a href="#impl-Context%3CT%2C%20Infallible%3E-for-Option%3CT%3E">Option<T></a></li><li><a href="#impl-Context%3CT%2C%20E%3E-for-Result%3CT%2C%20E%3E">Result<T, E></a></li></ul><h3><a href="#implementors">Implementors</a></h3></section><h2><a href="index.html">In anyhow</a></h2></div></nav><main><div class="width-limiter"><nav class="sub"><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1 class="fqn">Trait <a href="index.html">anyhow</a>::<wbr><a class="trait" href="#">Context</a><button id="copy-path" onclick="copy_path(this)" title="Copy item path to clipboard"><img src="../clipboard.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="srclink" href="../src/anyhow/lib.rs.html#601-613">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><div class="item-decl"><pre class="rust trait"><code>pub trait Context<T, E>: Sealed { |
| fn <a href="#tymethod.context" class="fnname">context</a><C>(self, context: C) -> <a class="type" href="type.Result.html" title="type anyhow::Result">Result</a><T, <a class="struct" href="struct.Error.html" title="struct anyhow::Error">Error</a>><br> <span class="where">where<br> C: Display + Send + Sync + 'static</span>; |
| <span class="item-spacer"></span> fn <a href="#tymethod.with_context" class="fnname">with_context</a><C, F>(self, f: F) -> <a class="type" href="type.Result.html" title="type anyhow::Result">Result</a><T, <a class="struct" href="struct.Error.html" title="struct anyhow::Error">Error</a>><br> <span class="where">where<br> C: Display + Send + Sync + 'static,<br> F: FnOnce() -> C</span>; |
| }</code></pre></div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><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> |
| <h2 id="example"><a href="#example">Example</a></h2> |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>anyhow::{Context, <span class="prelude-ty">Result</span>}; |
| <span class="kw">use </span>std::fs; |
| <span class="kw">use </span>std::path::PathBuf; |
| |
| <span class="kw">pub struct </span>ImportantThing { |
| path: PathBuf, |
| } |
| |
| <span class="kw">impl </span>ImportantThing { |
| <span class="kw">pub fn </span>detach(<span class="kw-2">&mut </span><span class="self">self</span>) -> <span class="prelude-ty">Result</span><()> {...} |
| } |
| |
| <span class="kw">pub fn </span>do_it(<span class="kw-2">mut </span>it: ImportantThing) -> <span class="prelude-ty">Result</span><Vec<u8>> { |
| it.detach().context(<span class="string">"Failed to detach the important thing"</span>)<span class="question-mark">?</span>; |
| |
| <span class="kw">let </span>path = <span class="kw-2">&</span>it.path; |
| <span class="kw">let </span>content = fs::read(path) |
| .with_context(|| <span class="macro">format!</span>(<span class="string">"Failed to read instrs from {}"</span>, path.display()))<span class="question-mark">?</span>; |
| |
| <span class="prelude-val">Ok</span>(content) |
| }</code></pre></div> |
| <p>When printed, the outermost context would be printed first and the lower |
| level underlying causes would be enumerated below.</p> |
| <div class="example-wrap"><pre class="language-console"><code>Error: Failed to read instrs from ./path/to/instrs.json |
| |
| Caused by: |
| No such file or directory (os error 2)</code></pre></div> |
| <p>Refer to the <a href="struct.Error.html#display-representations">Display representations</a> documentation for other forms in |
| which this context chain can be rendered.</p> |
| <br> |
| <h2 id="effect-on-downcasting"><a href="#effect-on-downcasting">Effect on downcasting</a></h2> |
| <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"><code><span class="kw">use </span>anyhow::{Context, <span class="prelude-ty">Result</span>}; |
| |
| <span class="kw">fn </span>do_it() -> <span class="prelude-ty">Result</span><()> { |
| helper().context(<span class="string">"Failed to complete the work"</span>)<span class="question-mark">?</span>; |
| ... |
| } |
| |
| <span class="kw">fn </span>main() { |
| <span class="kw">let </span>err = do_it().unwrap_err(); |
| <span class="kw">if let </span><span class="prelude-val">Some</span>(e) = err.downcast_ref::<SuspiciousError>() { |
| <span class="comment">// If helper() returned SuspiciousError, this downcast will |
| // correctly succeed even with the context in between. |
| </span>} |
| }</code></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"><code><span class="kw">use </span>anyhow::{Context, <span class="prelude-ty">Result</span>}; |
| |
| <span class="kw">fn </span>do_it() -> <span class="prelude-ty">Result</span><()> { |
| helper().context(HelperFailed)<span class="question-mark">?</span>; |
| ... |
| } |
| |
| <span class="kw">fn </span>main() { |
| <span class="kw">let </span>err = do_it().unwrap_err(); |
| <span class="kw">if let </span><span class="prelude-val">Some</span>(e) = err.downcast_ref::<HelperFailed>() { |
| <span class="comment">// If helper failed, this downcast will succeed because |
| // HelperFailed is the context that has been attached to |
| // that error. |
| </span>} |
| }</code></pre></div> |
| </li> |
| </ul> |
| </div></details><h2 id="required-methods" class="small-section-header">Required Methods<a href="#required-methods" class="anchor"></a></h2><div class="methods"><details class="rustdoc-toggle method-toggle" open><summary><section id="tymethod.context" class="method has-srclink"><a class="srclink rightside" href="../src/anyhow/lib.rs.html#603-605">source</a><h4 class="code-header">fn <a href="#tymethod.context" class="fnname">context</a><C>(self, context: C) -> <a class="type" href="type.Result.html" title="type anyhow::Result">Result</a><T, <a class="struct" href="struct.Error.html" title="struct anyhow::Error">Error</a>><span class="where fmt-newline">where<br> C: Display + Send + Sync + 'static,</span></h4></section></summary><div class="docblock"><p>Wrap the error value with additional context.</p> |
| </div></details><details class="rustdoc-toggle method-toggle" open><summary><section id="tymethod.with_context" class="method has-srclink"><a class="srclink rightside" href="../src/anyhow/lib.rs.html#609-612">source</a><h4 class="code-header">fn <a href="#tymethod.with_context" class="fnname">with_context</a><C, F>(self, f: F) -> <a class="type" href="type.Result.html" title="type anyhow::Result">Result</a><T, <a class="struct" href="struct.Error.html" title="struct anyhow::Error">Error</a>><span class="where fmt-newline">where<br> C: Display + Send + Sync + 'static,<br> F: FnOnce() -> C,</span></h4></section></summary><div class="docblock"><p>Wrap the error value with additional context that is evaluated lazily |
| only once an error does occur.</p> |
| </div></details></div><h2 id="foreign-impls" class="small-section-header">Implementations on Foreign Types<a href="#foreign-impls" class="anchor"></a></h2><details class="rustdoc-toggle implementors-toggle"><summary><section id="impl-Context%3CT%2C%20E%3E-for-Result%3CT%2C%20E%3E" class="impl has-srclink"><a class="srclink rightside" href="../src/anyhow/context.rs.html#42-68">source</a><a href="#impl-Context%3CT%2C%20E%3E-for-Result%3CT%2C%20E%3E" class="anchor"></a><h3 class="code-header">impl<T, E> <a class="trait" href="trait.Context.html" title="trait anyhow::Context">Context</a><T, E> for Result<T, E><span class="where fmt-newline">where<br> E: StdError + Send + Sync + 'static,</span></h3></section></summary><div class="impl-items"><section id="method.context" class="method trait-impl has-srclink"><a class="srclink rightside" href="../src/anyhow/context.rs.html#46-56">source</a><a href="#method.context" class="anchor"></a><h4 class="code-header">fn <a href="#tymethod.context" class="fnname">context</a><C>(self, context: C) -> Result<T, <a class="struct" href="struct.Error.html" title="struct anyhow::Error">Error</a>><span class="where fmt-newline">where<br> C: Display + Send + Sync + 'static,</span></h4></section><section id="method.with_context" class="method trait-impl has-srclink"><a class="srclink rightside" href="../src/anyhow/context.rs.html#58-67">source</a><a href="#method.with_context" class="anchor"></a><h4 class="code-header">fn <a href="#tymethod.with_context" class="fnname">with_context</a><C, F>(self, context: F) -> Result<T, <a class="struct" href="struct.Error.html" title="struct anyhow::Error">Error</a>><span class="where fmt-newline">where<br> C: Display + Send + Sync + 'static,<br> F: FnOnce() -> C,</span></h4></section></div></details><details class="rustdoc-toggle implementors-toggle"><summary><section id="impl-Context%3CT%2C%20Infallible%3E-for-Option%3CT%3E" class="impl has-srclink"><a class="srclink rightside" href="../src/anyhow/context.rs.html#90-113">source</a><a href="#impl-Context%3CT%2C%20Infallible%3E-for-Option%3CT%3E" class="anchor"></a><h3 class="code-header">impl<T> <a class="trait" href="trait.Context.html" title="trait anyhow::Context">Context</a><T, Infallible> for Option<T></h3></section></summary><div class="docblock"> |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>anyhow::{Context, <span class="prelude-ty">Result</span>}; |
| |
| <span class="kw">fn </span>maybe_get() -> <span class="prelude-ty">Option</span><T> { |
| ... |
| } |
| |
| <span class="kw">fn </span>demo() -> <span class="prelude-ty">Result</span><()> { |
| <span class="kw">let </span>t = maybe_get().context(<span class="string">"there is no T"</span>)<span class="question-mark">?</span>; |
| ... |
| }</code></pre></div> |
| </div><div class="impl-items"><section id="method.context-1" class="method trait-impl has-srclink"><a class="srclink rightside" href="../src/anyhow/context.rs.html#91-101">source</a><a href="#method.context-1" class="anchor"></a><h4 class="code-header">fn <a href="#tymethod.context" class="fnname">context</a><C>(self, context: C) -> Result<T, <a class="struct" href="struct.Error.html" title="struct anyhow::Error">Error</a>><span class="where fmt-newline">where<br> C: Display + Send + Sync + 'static,</span></h4></section><section id="method.with_context-1" class="method trait-impl has-srclink"><a class="srclink rightside" href="../src/anyhow/context.rs.html#103-112">source</a><a href="#method.with_context-1" class="anchor"></a><h4 class="code-header">fn <a href="#tymethod.with_context" class="fnname">with_context</a><C, F>(self, context: F) -> Result<T, <a class="struct" href="struct.Error.html" title="struct anyhow::Error">Error</a>><span class="where fmt-newline">where<br> C: Display + Send + Sync + 'static,<br> F: FnOnce() -> C,</span></h4></section></div></details><h2 id="implementors" class="small-section-header">Implementors<a href="#implementors" class="anchor"></a></h2><div id="implementors-list"></div><script src="../implementors/anyhow/trait.Context.js" data-ignore-extern-crates="core" async></script></section></div></main><div id="rustdoc-vars" data-root-path="../" data-current-crate="anyhow" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
