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gobyexample/public/errors
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>
<html>
<head>
<meta charset="utf-8">
<title>Go by Example: Errors</title>
<link rel=stylesheet href="site.css">
</head>
<script>
onkeydown = (e) => {
if (e.key == "ArrowLeft") {
window.location.href = 'interfaces';
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<body>
<div class="example" id="errors">
<h2><a href="./">Go by Example</a>: Errors</h2>
<table>
<tr>
<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="code empty leading">
</td>
</tr>
<tr>
<td class="docs">
</td>
<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>
</td>
</tr>
<tr>
<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>
</td>
</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>
</td>
</tr>
<tr>
<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>
</td>
</tr>
<tr>
<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>
</td>
</tr>
<tr>
<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>
</tr>
<tr>
<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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