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<h1 class='fqn'><span class='in-band'>Macro <a href='index.html'>core</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'>
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</span><a class='srclink' href='../src/core/macros.rs.html#329-336' title='goto source code'>[src]</a></span></h1>
<pre class="rust macro">
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">try</span> {
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">expr</span>:<span class="ident">expr</span>) <span class="op">=></span> { ... };
}</pre>
<div class='docblock'><p>Helper macro for reducing boilerplate code for matching <code>Result</code> together
with converting downstream errors.</p>
<p>Prefer using <code>?</code> syntax to <code>try!</code>. <code>?</code> is built in to the language and is
more succinct than <code>try!</code>. It is the standard method for error propagation.</p>
<p><code>try!</code> matches the given <code>Result</code>. 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 <code>Result</code>.</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="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="macro">println</span><span class="macro">!</span>(<span class="string">"I wrote to the file"</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="macro">println</span><span class="macro">!</span>(<span class="string">"I wrote to the file"</span>);
<span class="prelude-val">Ok</span>(())
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=fn%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%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%20println!(%22I%20wrote%20to%20the%20file%22)%3B%0A%20%20%20%20Ok(())%0A%7D%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%20println!(%22I%20wrote%20to%20the%20file%22)%3B%0A%20%20%20%20Ok(())%0A%7D%0A%7D">Run</a></pre>
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