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Oleg Butuzov 646c8b39da bugfix (mmcgrana/gobyexample#294) sha1 sum calculator 
This bugfix implements correct way to calculate source sha1 hash, before it will changed,
by propagation of unchanged sources.

This commit will also include regenerated static files and *.hash files.
2019-10-22 14:27:02 +03:00

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<!DOCTYPE html>
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<title>Go by Example: Errors</title>
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<div class="example" id="errors">
<h2><a href="./">Go by Example</a>: Errors</h2>
<table>
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<td class="docs">
<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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<td class="docs">
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<td class="code leading">
<a href="http://play.golang.org/p/NiJOpCPO3L0"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>
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<td class="docs">
</td>
<td class="code leading">
<div class="highlight"><pre><span class="kn">import</span> <span class="p">(</span>
<span class="s">&quot;errors&quot;</span>
<span class="s">&quot;fmt&quot;</span>
<span class="p">)</span>
</pre></div>
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</tr>
<tr>
<td class="docs">
<p>By convention, errors are the last return value and
have type <code>error</code>, a built-in interface.</p>
</td>
<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>
</td>
</tr>
<tr>
<td class="docs">
<p><code>errors.New</code> constructs a basic <code>error</code> value
with the given error message.</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="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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<td class="docs">
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="p">}</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>A <code>nil</code> value in the error position indicates that
there was no error.</p>
</td>
<td class="code leading">
<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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<td class="docs">
<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>
</td>
<td class="code leading">
<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>
</td>
</tr>
<tr>
<td class="docs">
</td>
<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 class="docs">
</td>
<td class="code leading">
<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>
</td>
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<td class="docs">
<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>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
</pre></div>
</td>
</tr>
<tr>
<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>
</td>
</tr>
<tr>
<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>
</td>
<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>
</td>
</tr>
</table>
<table>
<tr>
<td class="docs">
</td>
<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>
</td>
</tr>
<tr>
<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>
</td>
<td class="code empty">
</td>
</tr>
</table>
<p class="next">
Next example: <a href="goroutines">Goroutines</a>.
</p>
<p class="footer">
by <a href="https://markmcgranaghan.com">Mark McGranaghan</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/errors">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
</p>
</div>
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