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<ol class="chapter"><li class="expanded affix "><a href="index.html">chainerror</a></li><li class="expanded "><a href="tutorial1.html"><strong aria-hidden="true">1.</strong> Simple String Errors</a></li><li class="expanded "><a href="tutorial2.html"><strong aria-hidden="true">2.</strong> Simple Chained String Errors</a></li><li class="expanded "><a href="tutorial3.html"><strong aria-hidden="true">3.</strong> Mapping Errors</a></li><li class="expanded "><a href="tutorial4.html"><strong aria-hidden="true">4.</strong> Saving coding chars</a></li><li class="expanded "><a href="tutorial5.html"><strong aria-hidden="true">5.</strong> The source() of Errors</a></li><li class="expanded "><a href="tutorial6.html"><strong aria-hidden="true">6.</strong> Downcast the Errors</a></li><li class="expanded "><a href="tutorial7.html"><strong aria-hidden="true">7.</strong> The root cause of all Errors</a></li><li class="expanded "><a href="tutorial8.html"><strong aria-hidden="true">8.</strong> Finding an Error cause</a></li><li class="expanded "><a href="tutorial9.html" class="active"><strong aria-hidden="true">9.</strong> Selective Error Handling</a></li><li class="expanded "><a href="tutorial10.html"><strong aria-hidden="true">10.</strong> ErrorKind to the rescue</a></li><li class="expanded "><a href="tutorial11.html"><strong aria-hidden="true">11.</strong> Debug for the ErrorKind</a></li><li class="expanded "><a href="tutorial12.html"><strong aria-hidden="true">12.</strong> Deref for the ErrorKind</a></li><li class="expanded "><a href="tutorial13.html"><strong aria-hidden="true">13.</strong> Writing a library</a></li><li class="expanded "><a href="tutorial14.html"><strong aria-hidden="true">14.</strong> Going back to std</a></li><li class="expanded affix "><a href="end.html">The End</a></li></ol>
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<main>
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<h1><a class="header" href="#selective-error-handling" id="selective-error-handling">Selective Error Handling</a></h1>
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<p>What about functions returning different Error types?</p>
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<p>In this example <code>func1()</code> can return either <code>Func1ErrorFunc2</code> or <code>Func1ErrorIO</code>.</p>
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<p>We might want to <code>match</code> on <code>func1()</code> with something like:</p>
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<pre><code class="language-rust ignore">fn main() -> Result<(), Box<Error + Send + Sync>> {
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match func1() {
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Err(e) if let Some(s) = e.downcast_chain_ref::<Func1ErrorIO>() =>
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eprintln!("Func1ErrorIO:\n{:?}", s),
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Err(e) if let Some(s) = e.downcast_chain_ref::<Func1ErrorFunc2>() =>
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eprintln!("Func1ErrorFunc2:\n{:?}", s),
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Ok(_) => {},
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}
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Ok(())
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}
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</code></pre>
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<p>but this is not valid rust code, so we end up doing it the hard way.
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In the next chapter, we will see, how to solve this more elegantly.</p>
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<pre><pre class="playpen"><code class="language-rust">use chainerror::*;
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use std::error::Error;
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use std::io;
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use std::result::Result;
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fn do_some_io() -> Result<(), Box<Error + Send + Sync>> {
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Err(io::Error::from(io::ErrorKind::NotFound))?;
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Ok(())
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}
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derive_str_cherr!(Func2Error);
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fn func2() -> Result<(), Box<Error + Send + Sync>> {
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let filename = "foo.txt";
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do_some_io().map_err(mstrerr!(Func2Error, "Error reading '{}'", filename))?;
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Ok(())
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}
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derive_str_cherr!(Func1ErrorFunc2);
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derive_str_cherr!(Func1ErrorIO);
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fn func1() -> Result<(), Box<Error + Send + Sync>> {
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func2().map_err(mstrerr!(Func1ErrorFunc2, "func1 error calling func2"))?;
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let filename = "bar.txt";
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do_some_io().map_err(mstrerr!(Func1ErrorIO, "Error reading '{}'", filename))?;
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Ok(())
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}
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fn main() -> Result<(), Box<Error + Send + Sync>> {
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if let Err(e) = func1() {
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if let Some(s) = e.downcast_ref::<ChainError<Func1ErrorIO>>() {
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eprintln!("Func1ErrorIO:\n{:?}", s);
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}
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if let Some(s) = e.downcast_chain_ref::<Func1ErrorFunc2>() {
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eprintln!("Func1ErrorFunc2:\n{:?}", s);
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}
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}
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Ok(())
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}
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<span class="boring">#[allow(dead_code)]
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</span><span class="boring">mod chainerror {
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</span>{{#includecomment ../src/lib.rs}}
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<span class="boring">}
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</span></code></pre></pre>
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