307 lines
15 KiB
Plaintext
Generated
307 lines
15 KiB
Plaintext
Generated
<!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';
|
|
}
|
|
|
|
|
|
if (e.key == "ArrowRight") {
|
|
window.location.href = 'goroutines';
|
|
}
|
|
|
|
}
|
|
</script>
|
|
<body>
|
|
<div class="example" id="errors">
|
|
<h2><a href="./">Go by Example</a>: Errors</h2>
|
|
|
|
<table>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
<p>In Go it’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’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>
|
|
|
|
</td>
|
|
<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">"errors"</span>
|
|
<span class="s">"fmt"</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">"can't work with 42"</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’s possible to use custom types as <code>error</code>s by
|
|
implementing the <code>Error()</code> method on them. Here’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">"%d - %s"</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>&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">&</span><span class="nx">argError</span><span class="p">{</span><span class="nx">arg</span><span class="p">,</span> <span class="s">"can't work with it"</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">"f1 failed:"</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">"f1 worked:"</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">"f2 failed:"</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">"f2 worked:"</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’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't work with 42</span>
|
|
<span class="go">f2 worked: 10</span>
|
|
<span class="go">f2 failed: 42 - can't work with it</span>
|
|
<span class="go">42</span>
|
|
<span class="go">can'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>
|
|
<script>
|
|
var codeLines = [];
|
|
codeLines.push('');codeLines.push('package main\u000A');codeLines.push('import (\u000A \"errors\"\u000A \"fmt\"\u000A)\u000A');codeLines.push('func f1(arg int) (int, error) {\u000A if arg \x3D\x3D 42 {\u000A');codeLines.push(' return -1, errors.New(\"can\'t work with 42\")\u000A');codeLines.push(' }\u000A');codeLines.push(' return arg + 3, nil\u000A}\u000A');codeLines.push('type argError struct {\u000A arg int\u000A prob string\u000A}\u000A');codeLines.push('func (e *argError) Error() string {\u000A return fmt.Sprintf(\"%d - %s\", e.arg, e.prob)\u000A}\u000A');codeLines.push('func f2(arg int) (int, error) {\u000A if arg \x3D\x3D 42 {\u000A');codeLines.push(' return -1, \x26argError{arg, \"can\'t work with it\"}\u000A }\u000A return arg + 3, nil\u000A}\u000A');codeLines.push('func main() {\u000A');codeLines.push(' for _, i :\x3D range []int{7, 42} {\u000A if r, e :\x3D f1(i); e !\x3D nil {\u000A fmt.Println(\"f1 failed:\", e)\u000A } else {\u000A fmt.Println(\"f1 worked:\", r)\u000A }\u000A }\u000A for _, i :\x3D range []int{7, 42} {\u000A if r, e :\x3D f2(i); e !\x3D nil {\u000A fmt.Println(\"f2 failed:\", e)\u000A } else {\u000A fmt.Println(\"f2 worked:\", r)\u000A }\u000A }\u000A');codeLines.push(' _, e :\x3D f2(42)\u000A if ae, ok :\x3D e.(*argError); ok {\u000A fmt.Println(ae.arg)\u000A fmt.Println(ae.prob)\u000A }\u000A}\u000A');codeLines.push('');codeLines.push('');
|
|
</script>
|
|
<script src="site.js" async></script>
|
|
</body>
|
|
</html>
|