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gobyexample/public/non-blocking-channel-operations
Hana 9e216da9ef go.mod: add go.mod and move pygments to third_party 
After go1.16, go will use module mode by default,
even when the repository is checked out under GOPATH
or in a one-off directory. Add go.mod, go.sum to keep
this repo buildable without opting out of the module
mode.

> go mod init github.com/mmcgrana/gobyexample
> go mod tidy
> go mod vendor

In module mode, the 'vendor' directory is special
and its contents will be actively maintained by the
go command. pygments aren't the dependency the go will
know about, so it will delete the contents from vendor
directory. Move it to `third_party` directory now.

And, vendor the blackfriday package.

Note: the tutorial contents are not affected by the
change in go1.16 because all the examples in this
tutorial ask users to run the go command with the
explicit list of files to be compiled (e.g.
`go run hello-world.go` or `go build command-line-arguments.go`).
When the source list is provided, the go command does
not have to compute the build list and whether it's
running in GOPATH mode or module mode becomes irrelevant.
2021-02-15 16:45:26 -05:00

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<!DOCTYPE html>
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<title>Go by Example: Non-Blocking Channel Operations</title>
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<body>
<div class="example" id="non-blocking-channel-operations">
<h2><a href="./">Go by Example</a>: Non-Blocking Channel Operations</h2>
<table>
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<td class="docs">
<p>Basic sends and receives on channels are blocking.
However, we can use <code>select</code> with a <code>default</code> clause to
implement <em>non-blocking</em> sends, receives, and even
non-blocking multi-way <code>select</code>s.</p>
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<a href="http://play.golang.org/p/TFv6-7OVNVq"><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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<div class="highlight"><pre><span class="kn">import</span> <span class="s">&quot;fmt&quot;</span>
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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>
<span class="nx">messages</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="kt">string</span><span class="p">)</span>
<span class="nx">signals</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="kt">bool</span><span class="p">)</span>
</pre></div>
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<td class="docs">
<p>Here&rsquo;s a non-blocking receive. If a value is
available on <code>messages</code> then <code>select</code> will take
the <code>&lt;-messages</code> <code>case</code> with that value. If not
it will immediately take the <code>default</code> case.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="k">select</span> <span class="p">{</span>
<span class="k">case</span> <span class="nx">msg</span> <span class="o">:=</span> <span class="o">&lt;-</span><span class="nx">messages</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;received message&quot;</span><span class="p">,</span> <span class="nx">msg</span><span class="p">)</span>
<span class="k">default</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;no message received&quot;</span><span class="p">)</span>
<span class="p">}</span>
</pre></div>
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<p>A non-blocking send works similarly. Here <code>msg</code>
cannot be sent to the <code>messages</code> channel, because
the channel has no buffer and there is no receiver.
Therefore the <code>default</code> case is selected.</p>
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<div class="highlight"><pre> <span class="nx">msg</span> <span class="o">:=</span> <span class="s">&quot;hi&quot;</span>
<span class="k">select</span> <span class="p">{</span>
<span class="k">case</span> <span class="nx">messages</span> <span class="o">&lt;-</span> <span class="nx">msg</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;sent message&quot;</span><span class="p">,</span> <span class="nx">msg</span><span class="p">)</span>
<span class="k">default</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;no message sent&quot;</span><span class="p">)</span>
<span class="p">}</span>
</pre></div>
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<td class="docs">
<p>We can use multiple <code>case</code>s above the <code>default</code>
clause to implement a multi-way non-blocking
select. Here we attempt non-blocking receives
on both <code>messages</code> and <code>signals</code>.</p>
</td>
<td class="code">
<div class="highlight"><pre> <span class="k">select</span> <span class="p">{</span>
<span class="k">case</span> <span class="nx">msg</span> <span class="o">:=</span> <span class="o">&lt;-</span><span class="nx">messages</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;received message&quot;</span><span class="p">,</span> <span class="nx">msg</span><span class="p">)</span>
<span class="k">case</span> <span class="nx">sig</span> <span class="o">:=</span> <span class="o">&lt;-</span><span class="nx">signals</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;received signal&quot;</span><span class="p">,</span> <span class="nx">sig</span><span class="p">)</span>
<span class="k">default</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;no activity&quot;</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
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<td class="code">
<div class="highlight"><pre><span class="gp">$</span> go run non-blocking-channel-operations.go
<span class="go">no message received</span>
<span class="go">no message sent</span>
<span class="go">no activity</span>
</pre></div>
</td>
</tr>
</table>
<p class="next">
Next example: <a href="closing-channels">Closing Channels</a>.
</p>
<p class="footer">
by <a href="https://markmcgranaghan.com">Mark McGranaghan</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/non-blocking-channel-operations">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
</p>
</div>
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