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<div class="example" id="errors">
<h2><a href="./">Go by Example</a>: Errors</h2>
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<p>In Go it&rsquo;s idiomatic to communicate errors via an
explicit, separate return value. This contrasts with
the exceptions used in languages like Java and Ruby and
the overloaded single result / error value sometimes
used in C. Go&rsquo;s approach makes it easy to see which
functions return errors and to handle them using the
same language constructs employed for any other,
non-error tasks.</p>
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<a href="http://play.golang.org/p/BmDQXkPPTk"><img title="Run code" src="play.png" class="run" /></a>
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>
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<div class="highlight"><pre><span class="kn">import</span> <span class="s">&quot;errors&quot;</span>
<span class="kn">import</span> <span class="s">&quot;fmt&quot;</span>
</pre></div>
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<p>By convention, errors are the last return value and
have type <code>error</code>, a built-in interface.</p>
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<td class="code leading">
<div class="highlight"><pre><span class="kd">func</span> <span class="nx">f1</span><span class="p">(</span><span class="nx">arg</span> <span class="kt">int</span><span class="p">)</span> <span class="p">(</span><span class="kt">int</span><span class="p">,</span> <span class="kt">error</span><span class="p">)</span> <span class="p">{</span>
<span class="k">if</span> <span class="nx">arg</span> <span class="o">==</span> <span class="mi">42</span> <span class="p">{</span>
</pre></div>
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<p><code>errors.New</code> constructs a basic <code>error</code> value
with the given error message.</p>
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<div class="highlight"><pre> <span class="k">return</span> <span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="nx">errors</span><span class="p">.</span><span class="nx">New</span><span class="p">(</span><span class="s">&quot;can&#39;t work with 42&quot;</span><span class="p">)</span>
</pre></div>
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<div class="highlight"><pre> <span class="p">}</span>
</pre></div>
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<p>A nil value in the error position indicates that
there was no error.</p>
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<div class="highlight"><pre> <span class="k">return</span> <span class="nx">arg</span> <span class="o">+</span> <span class="mi">3</span><span class="p">,</span> <span class="kc">nil</span>
<span class="p">}</span>
</pre></div>
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<p>It&rsquo;s possible to use custom types as <code>error</code>s by
implementing the <code>Error()</code> method on them. Here&rsquo;s a
variant on the example above that uses a custom type
to explicitly represent an argument error.</p>
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<div class="highlight"><pre><span class="kd">type</span> <span class="nx">argError</span> <span class="kd">struct</span> <span class="p">{</span>
<span class="nx">arg</span> <span class="kt">int</span>
<span class="nx">prob</span> <span class="kt">string</span>
<span class="p">}</span>
</pre></div>
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<td class="docs">
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<td class="code leading">
<div class="highlight"><pre><span class="kd">func</span> <span class="p">(</span><span class="nx">e</span> <span class="o">*</span><span class="nx">argError</span><span class="p">)</span> <span class="nx">Error</span><span class="p">()</span> <span class="kt">string</span> <span class="p">{</span>
<span class="k">return</span> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Sprintf</span><span class="p">(</span><span class="s">&quot;%d - %s&quot;</span><span class="p">,</span> <span class="nx">e</span><span class="p">.</span><span class="nx">arg</span><span class="p">,</span> <span class="nx">e</span><span class="p">.</span><span class="nx">prob</span><span class="p">)</span>
<span class="p">}</span>
</pre></div>
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</td>
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">f2</span><span class="p">(</span><span class="nx">arg</span> <span class="kt">int</span><span class="p">)</span> <span class="p">(</span><span class="kt">int</span><span class="p">,</span> <span class="kt">error</span><span class="p">)</span> <span class="p">{</span>
<span class="k">if</span> <span class="nx">arg</span> <span class="o">==</span> <span class="mi">42</span> <span class="p">{</span>
</pre></div>
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<p>In this case we use <code>&amp;argError</code> syntax to build
a new struct, supplying values for the two
fields <code>arg</code> and <code>prob</code>.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="k">return</span> <span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="o">&amp;</span><span class="nx">argError</span><span class="p">{</span><span class="nx">arg</span><span class="p">,</span> <span class="s">&quot;can&#39;t work with it&quot;</span><span class="p">}</span>
<span class="p">}</span>
<span class="k">return</span> <span class="nx">arg</span> <span class="o">+</span> <span class="mi">3</span><span class="p">,</span> <span class="kc">nil</span>
<span class="p">}</span>
</pre></div>
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<td class="docs">
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
</pre></div>
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<td class="docs">
<p>The two loops below test out each of our
error-returning functions. Note that the use of an
inline error check on the <code>if</code> line is a common
idiom in Go code.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="k">for</span> <span class="nx">_</span><span class="p">,</span> <span class="nx">i</span> <span class="o">:=</span> <span class="k">range</span> <span class="p">[]</span><span class="kt">int</span><span class="p">{</span><span class="mi">7</span><span class="p">,</span> <span class="mi">42</span><span class="p">}</span> <span class="p">{</span>
<span class="k">if</span> <span class="nx">r</span><span class="p">,</span> <span class="nx">e</span> <span class="o">:=</span> <span class="nx">f1</span><span class="p">(</span><span class="nx">i</span><span class="p">);</span> <span class="nx">e</span> <span class="o">!=</span> <span class="kc">nil</span> <span class="p">{</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;f1 failed:&quot;</span><span class="p">,</span> <span class="nx">e</span><span class="p">)</span>
<span class="p">}</span> <span class="k">else</span> <span class="p">{</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;f1 worked:&quot;</span><span class="p">,</span> <span class="nx">r</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="k">for</span> <span class="nx">_</span><span class="p">,</span> <span class="nx">i</span> <span class="o">:=</span> <span class="k">range</span> <span class="p">[]</span><span class="kt">int</span><span class="p">{</span><span class="mi">7</span><span class="p">,</span> <span class="mi">42</span><span class="p">}</span> <span class="p">{</span>
<span class="k">if</span> <span class="nx">r</span><span class="p">,</span> <span class="nx">e</span> <span class="o">:=</span> <span class="nx">f2</span><span class="p">(</span><span class="nx">i</span><span class="p">);</span> <span class="nx">e</span> <span class="o">!=</span> <span class="kc">nil</span> <span class="p">{</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;f2 failed:&quot;</span><span class="p">,</span> <span class="nx">e</span><span class="p">)</span>
<span class="p">}</span> <span class="k">else</span> <span class="p">{</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;f2 worked:&quot;</span><span class="p">,</span> <span class="nx">r</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
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<td class="docs">
<p>If you want to programmatically use the data in
a custom error, you&rsquo;ll need to get the error as an
instance of the custom error type via type
assertion.</p>
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<td class="code">
<div class="highlight"><pre> <span class="nx">_</span><span class="p">,</span> <span class="nx">e</span> <span class="o">:=</span> <span class="nx">f2</span><span class="p">(</span><span class="mi">42</span><span class="p">)</span>
<span class="k">if</span> <span class="nx">ae</span><span class="p">,</span> <span class="nx">ok</span> <span class="o">:=</span> <span class="nx">e</span><span class="p">.(</span><span class="o">*</span><span class="nx">argError</span><span class="p">);</span> <span class="nx">ok</span> <span class="p">{</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">ae</span><span class="p">.</span><span class="nx">arg</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">ae</span><span class="p">.</span><span class="nx">prob</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
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<td class="docs">
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<td class="code leading">
<div class="highlight"><pre><span class="gp">$</span> go run errors.go
<span class="go">f1 worked: 10</span>
<span class="go">f1 failed: can&#39;t work with 42</span>
<span class="go">f2 worked: 10</span>
<span class="go">f2 failed: 42 - can&#39;t work with it</span>
<span class="go">42</span>
<span class="go">can&#39;t work with it</span>
</pre></div>
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<td class="docs">
<p>See this <a href="http://blog.golang.org/2011/07/error-handling-and-go.html">great post</a>
on the Go blog for more on error handling.</p>
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<p class="next">
Next example: <a href="goroutines">Goroutines</a>.
</p>
<p class="footer">
by <a href="https://twitter.com/mmcgrana">@mmcgrana</a> | <a href="mailto:mmcgrana@gmail.com">feedback</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/errors">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
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