199 lines
11 KiB
Plaintext
Generated
199 lines
11 KiB
Plaintext
Generated
<!DOCTYPE html>
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<html>
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<head>
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<meta charset="utf-8">
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<title>Go by Example: Custom Errors</title>
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<div class="example" id="custom-errors">
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<h2><a href="./">Go by Example</a>: Custom Errors</h2>
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<table>
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<tr>
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<td class="docs">
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<p>It’s possible to use custom types as <code>error</code>s by
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implementing the <code>Error()</code> method on them. Here’s a
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variant on the example above that uses a custom type
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to explicitly represent an argument error.</p>
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</td>
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<td class="code empty leading">
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<a href="https://go.dev/play/p/qKKTjmc6SuB"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
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<pre class="chroma"><code><span class="line"><span class="cl"><span class="kn">package</span> <span class="nx">main</span></span></span></code></pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<pre class="chroma"><code><span class="line"><span class="cl"><span class="kn">import</span> <span class="p">(</span>
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</span></span><span class="line"><span class="cl"> <span class="s">"errors"</span>
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</span></span><span class="line"><span class="cl"> <span class="s">"fmt"</span>
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</span></span><span class="line"><span class="cl"><span class="p">)</span></span></span></code></pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>A custom error type usually has the suffix “Error”.</p>
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</td>
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<td class="code leading">
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<pre class="chroma"><code><span class="line"><span class="cl"><span class="kd">type</span> <span class="nx">argError</span> <span class="kd">struct</span> <span class="p">{</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">arg</span> <span class="kt">int</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">message</span> <span class="kt">string</span>
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</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Adding this <code>Error</code> method makes <code>argError</code> implement
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the <code>error</code> interface.</p>
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</td>
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<td class="code leading">
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<pre class="chroma"><code><span class="line"><span class="cl"><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="nf">Error</span><span class="p">()</span> <span class="kt">string</span> <span class="p">{</span>
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</span></span><span class="line"><span class="cl"> <span class="k">return</span> <span class="nx">fmt</span><span class="p">.</span><span class="nf">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">message</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<pre class="chroma"><code><span class="line"><span class="cl"><span class="kd">func</span> <span class="nf">f</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>
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</span></span><span class="line"><span class="cl"> <span class="k">if</span> <span class="nx">arg</span> <span class="o">==</span> <span class="mi">42</span> <span class="p">{</span></span></span></code></pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Return our custom error.</p>
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</td>
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<td class="code leading">
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<pre class="chroma"><code><span class="line"><span class="cl"> <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>
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</span></span><span class="line"><span class="cl"> <span class="p">}</span>
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</span></span><span class="line"><span class="cl"> <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>
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</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<pre class="chroma"><code><span class="line"><span class="cl"><span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span></span></span></code></pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p><code>errors.As</code> is a more advanced version of <code>errors.Is</code>.
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It checks that a given error (or any error in its chain)
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matches a specific error type and converts to a value
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of that type, returning <code>true</code>. If there’s no match, it
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returns <code>false</code>.</p>
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</td>
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<td class="code">
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<pre class="chroma"><code><span class="line"><span class="cl"> <span class="nx">_</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nf">f</span><span class="p">(</span><span class="mi">42</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="kd">var</span> <span class="nx">ae</span> <span class="o">*</span><span class="nx">argError</span>
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</span></span><span class="line"><span class="cl"> <span class="k">if</span> <span class="nx">errors</span><span class="p">.</span><span class="nf">As</span><span class="p">(</span><span class="nx">err</span><span class="p">,</span> <span class="o">&</span><span class="nx">ae</span><span class="p">)</span> <span class="p">{</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="nx">ae</span><span class="p">.</span><span class="nx">arg</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="nx">ae</span><span class="p">.</span><span class="nx">message</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="p">}</span> <span class="k">else</span> <span class="p">{</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"err doesn't match argError"</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="p">}</span>
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</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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</td>
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</tr>
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</table>
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<table>
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<tr>
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<td class="docs">
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</td>
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<td class="code">
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<pre class="chroma"><code><span class="line"><span class="cl"><span class="gp">$</span> go run custom-errors.go
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</span></span><span class="line"><span class="cl"><span class="go">42
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</span></span></span><span class="line"><span class="cl"><span class="go">can't work with it</span></span></span></code></pre>
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</td>
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</tr>
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</table>
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<p class="next">
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Next example: <a href="goroutines" rel="next">Goroutines</a>.
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</p>
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<p class="footer">
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Based on
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<em><a href="https://gobyexample.com/">Go by Example</a></em>
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by <a href="https://markmcgranaghan.com">Mark McGranaghan</a>
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and <a href="https://eli.thegreenplace.net">Eli Bendersky</a>,
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licensed under
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<a href="https://creativecommons.org/licenses/by/3.0/">CC BY 3.0</a>.
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<a href="https://git.inkch.xyz/inkch/gobyexample/commit/7123d715f43f2880034db62116a476c01499cae9">
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Style modifications by inkch
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</a>.
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<a href="https://github.com/mmcgrana/gobyexample">Original source</a>.
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</p>
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</div>
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<script>
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var codeLines = [];
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codeLines.push('');codeLines.push('package main\u000A');codeLines.push('import (\u000A \"errors\"\u000A \"fmt\"\u000A)\u000A');codeLines.push('type argError struct {\u000A arg int\u000A message string\u000A}\u000A');codeLines.push('func (e *argError) Error() string {\u000A return fmt.Sprintf(\"%d - %s\", e.arg, e.message)\u000A}\u000A');codeLines.push('func f(arg int) (int, error) {\u000A if arg \u003D\u003D 42 {\u000A');codeLines.push(' return -1, \u0026argError{arg, \"can\'t work with it\"}\u000A }\u000A return arg + 3, nil\u000A}\u000A');codeLines.push('func main() {\u000A');codeLines.push(' _, err :\u003D f(42)\u000A var ae *argError\u000A if errors.As(err, \u0026ae) {\u000A fmt.Println(ae.arg)\u000A fmt.Println(ae.message)\u000A } else {\u000A fmt.Println(\"err doesn\'t match argError\")\u000A }\u000A}\u000A');codeLines.push('');
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