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<section id='main' class="content trait">
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<h1 class='fqn'><span class='in-band'>Trait <a href='index.html'>iron</a>::<wbr><a class='trait' href=''>Error</a></span><span class='out-of-band'><span id='render-detail'>
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<a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">
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[<span class='inner'>−</span>]
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</a>
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</span><a id='src-11' class='srclink' href='../error/trait.Error.html?gotosrc=11' title='goto source code'>[src]</a></span></h1>
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<pre class='rust trait'>pub trait Error: <a class='trait' href='https://doc.rust-lang.org/nightly/core/marker/trait.Send.html' title='core::marker::Send'>Send</a> + <a class='trait' href='https://doc.rust-lang.org/nightly/std/error/trait.Error.html' title='std::error::Error'>Error</a> + <a class='trait' href='https://doc.rust-lang.org/nightly/core/fmt/trait.Debug.html' title='core::fmt::Debug'>Debug</a> + <a class='trait' href='../typeable/trait.Typeable.html' title='typeable::Typeable'>Typeable</a> { }</pre><div class='docblock'><p>An extension to std::error::Error which provides dynamic downcasting of
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errors for use in highly generic contexts.</p>
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<h2 id='when-to-use-this-trait' class='section-header'><a href='#when-to-use-this-trait'>When to use this trait</a></h2>
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<p>In the vast majority of cases, a library-specific <code>enum</code> should be used
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for cases where there can be many different types of errors. This has
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the benefit of being very performant and benefiting from all sorts
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of static checking at both the instantiation site and the handling
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site of the error.</p>
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<p>In other cases, being generic over <code>std::error::Error</code> may be correct
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- usually for logging errors or in other places where an error is
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used as <em>input</em>.</p>
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<p>Now, a motivating example for this trait, which doesn't fall under
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either of these cases:</p>
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<p>Imagine we are creating a simple web middleware for verifying incoming
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HTTP requests. It will take in many different user-defined <code>Verifier</code>s
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and will call them one after the other, rejecting the request on any
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error.</p>
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<p>The first step would be to write a <code>Verifier</code> trait:</p>
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<pre class='rust rust-example-rendered'>
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<span class='kw'>pub</span> <span class='kw'>trait</span> <span class='ident'>Verifier</span> {
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<span class='doccomment'>/// Verify the request, yielding an error if the request is invalid.</span>
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<span class='kw'>fn</span> <span class='ident'>verify</span>(<span class='kw-2'>&</span><span class='ident'>Request</span>) <span class='op'>-></span> <span class='prelude-ty'>Result</span><span class='op'><</span>(), ???<span class='op'>></span>;
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}</pre>
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<p>A problem quickly arises - what type do we use for the <code>Err</code> case? We
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cannot use a concrete type since each <code>Verifier</code> may wish to throw
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any number of different errors, and we cannot use a generic since
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the type is chosen by the implementor, not the caller, and it cannot
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be a generic on the trait since we will want to store many <code>Verifier</code>s
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together.</p>
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<p>Enter: <code>Box<error::Error></code>, a type which can be used to represent
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any <code>std::error::Error</code> with the sufficient bounds, and can <em>also</em>
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be handled later by downcasting it to the right error using either
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<code>.downcast</code> or the <code>match_error!</code> macro. This type can be used to meet
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the needs of consumers like <code>Verifier</code>, but should not be used in cases
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where enums or generics are better suited.</p>
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</div><h2 id='methods'>Methods</h2><h3 class='impl'><span class='in-band'><code>impl <a class='trait' href='../iron/error/trait.Error.html' title='iron::error::Error'>Error</a> + 'static</code></span><span class='out-of-band'></span></h3>
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<div class='impl-items'><h4 id='method.is' class='method'><code>fn <a href='#method.is' class='fnname'>is</a><E>(&self) -> <a class='primitive' href='https://doc.rust-lang.org/nightly/std/primitive.bool.html'>bool</a> <span class='where'>where E: <a class='trait' href='../iron/error/trait.Error.html' title='iron::error::Error'>Error</a></span></code></h4>
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<div class='docblock'><p>Is this <code>Error</code> object of type <code>E</code>?</p>
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</div><h4 id='method.downcast' class='method'><code>fn <a href='#method.downcast' class='fnname'>downcast</a><E>(&self) -> <a class='enum' href='https://doc.rust-lang.org/nightly/core/option/enum.Option.html' title='core::option::Option'>Option</a><&E> <span class='where'>where E: <a class='trait' href='../iron/error/trait.Error.html' title='iron::error::Error'>Error</a></span></code></h4>
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<div class='docblock'><p>If this error is <code>E</code>, downcast this error to <code>E</code>, by reference.</p>
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</div></div><h3 class='impl'><span class='in-band'><code>impl <a class='trait' href='../iron/error/trait.Error.html' title='iron::error::Error'>Error</a> + 'static + <a class='trait' href='https://doc.rust-lang.org/nightly/core/marker/trait.Send.html' title='core::marker::Send'>Send</a></code></span><span class='out-of-band'></span></h3>
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<div class='impl-items'><h4 id='method.is-1' class='method'><code>fn <a href='#method.is-1' class='fnname'>is</a><E>(&self) -> <a class='primitive' href='https://doc.rust-lang.org/nightly/std/primitive.bool.html'>bool</a> <span class='where'>where E: <a class='trait' href='https://doc.rust-lang.org/nightly/core/marker/trait.Send.html' title='core::marker::Send'>Send</a> + <a class='trait' href='../iron/error/trait.Error.html' title='iron::error::Error'>Error</a></span></code></h4>
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<div class='docblock'><p>Is this <code>Error + Send</code> object of type <code>E</code>?</p>
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</div><h4 id='method.downcast-1' class='method'><code>fn <a href='#method.downcast-1' class='fnname'>downcast</a><E>(&self) -> <a class='enum' href='https://doc.rust-lang.org/nightly/core/option/enum.Option.html' title='core::option::Option'>Option</a><&E> <span class='where'>where E: <a class='trait' href='https://doc.rust-lang.org/nightly/core/marker/trait.Send.html' title='core::marker::Send'>Send</a> + <a class='trait' href='../iron/error/trait.Error.html' title='iron::error::Error'>Error</a></span></code></h4>
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<div class='docblock'><p>If this error is <code>E</code>, downcast this error to <code>E</code>, by reference.</p>
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</div></div>
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<h2 id='implementors'>Implementors</h2>
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<ul class='item-list' id='implementors-list'>
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<li><code>impl<S> Error for S <span class='where'>where S: <a class='trait' href='https://doc.rust-lang.org/nightly/core/marker/trait.Send.html' title='core::marker::Send'>Send</a> + <a class='trait' href='https://doc.rust-lang.org/nightly/std/error/trait.Error.html' title='std::error::Error'>Error</a> + <a class='trait' href='https://doc.rust-lang.org/nightly/core/fmt/trait.Debug.html' title='core::fmt::Debug'>Debug</a> + <a class='trait' href='../typeable/trait.Typeable.html' title='typeable::Typeable'>Typeable</a></span></code></li>
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</ul><script type="text/javascript" async
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