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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.
220 lines
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220 lines
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<!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: Defer</title>
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<link rel=stylesheet href="site.css">
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</head>
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<script>
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onkeydown = (e) => {
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if (e.key == "ArrowLeft") {
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window.location.href = 'panic';
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if (e.key == "ArrowRight") {
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window.location.href = 'collection-functions';
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<body>
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<div class="example" id="defer">
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<h2><a href="./">Go by Example</a>: Defer</h2>
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<table>
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<tr>
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<td class="docs">
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<p><em>Defer</em> is used to ensure that a function call is
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performed later in a program’s execution, usually for
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purposes of cleanup. <code>defer</code> is often used where e.g.
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<code>ensure</code> and <code>finally</code> would be used in other languages.</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="http://play.golang.org/p/5SDVfc_jxbg"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
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<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
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</pre></div>
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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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<div class="highlight"><pre><span class="kn">import</span> <span class="p">(</span>
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<span class="s">"fmt"</span>
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<span class="s">"os"</span>
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<span class="p">)</span>
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</pre></div>
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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>Suppose we wanted to create a file, write to it,
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and then close when we’re done. Here’s how we could
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do that with <code>defer</code>.</p>
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</td>
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<td class="code leading">
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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>
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</pre></div>
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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>Immediately after getting a file object with
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<code>createFile</code>, we defer the closing of that file
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with <code>closeFile</code>. This will be executed at the end
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of the enclosing function (<code>main</code>), after
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<code>writeFile</code> has finished.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre> <span class="nx">f</span> <span class="o">:=</span> <span class="nx">createFile</span><span class="p">(</span><span class="s">"/tmp/defer.txt"</span><span class="p">)</span>
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<span class="k">defer</span> <span class="nx">closeFile</span><span class="p">(</span><span class="nx">f</span><span class="p">)</span>
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<span class="nx">writeFile</span><span class="p">(</span><span class="nx">f</span><span class="p">)</span>
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<span class="p">}</span>
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</pre></div>
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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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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">createFile</span><span class="p">(</span><span class="nx">p</span> <span class="kt">string</span><span class="p">)</span> <span class="o">*</span><span class="nx">os</span><span class="p">.</span><span class="nx">File</span> <span class="p">{</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"creating"</span><span class="p">)</span>
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<span class="nx">f</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">os</span><span class="p">.</span><span class="nx">Create</span><span class="p">(</span><span class="nx">p</span><span class="p">)</span>
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<span class="k">if</span> <span class="nx">err</span> <span class="o">!=</span> <span class="kc">nil</span> <span class="p">{</span>
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<span class="nb">panic</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
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<span class="p">}</span>
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<span class="k">return</span> <span class="nx">f</span>
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<span class="p">}</span>
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</pre></div>
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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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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">writeFile</span><span class="p">(</span><span class="nx">f</span> <span class="o">*</span><span class="nx">os</span><span class="p">.</span><span class="nx">File</span><span class="p">)</span> <span class="p">{</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"writing"</span><span class="p">)</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Fprintln</span><span class="p">(</span><span class="nx">f</span><span class="p">,</span> <span class="s">"data"</span><span class="p">)</span>
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</pre></div>
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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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<div class="highlight"><pre><span class="p">}</span>
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</pre></div>
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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>It’s important to check for errors when closing a
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file, even in a deferred function.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">closeFile</span><span class="p">(</span><span class="nx">f</span> <span class="o">*</span><span class="nx">os</span><span class="p">.</span><span class="nx">File</span><span class="p">)</span> <span class="p">{</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"closing"</span><span class="p">)</span>
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<span class="nx">err</span> <span class="o">:=</span> <span class="nx">f</span><span class="p">.</span><span class="nx">Close</span><span class="p">()</span>
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</pre></div>
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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">
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<div class="highlight"><pre> <span class="k">if</span> <span class="nx">err</span> <span class="o">!=</span> <span class="kc">nil</span> <span class="p">{</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Fprintf</span><span class="p">(</span><span class="nx">os</span><span class="p">.</span><span class="nx">Stderr</span><span class="p">,</span> <span class="s">"error: %v\n"</span><span class="p">,</span> <span class="nx">err</span><span class="p">)</span>
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<span class="nx">os</span><span class="p">.</span><span class="nx">Exit</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
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<span class="p">}</span>
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<span class="p">}</span>
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</pre></div>
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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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<p>Running the program confirms that the file is closed
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after being written.</p>
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</td>
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<td class="code">
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<div class="highlight"><pre><span class="gp">$</span> go run defer.go
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<span class="go">creating</span>
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<span class="go">writing</span>
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<span class="go">closing</span>
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</pre></div>
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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="collection-functions">Collection Functions</a>.
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</p>
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<p class="footer">
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by <a href="https://markmcgranaghan.com">Mark McGranaghan</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/defer">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</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 \"fmt\"\u000A \"os\"\u000A)\u000A');codeLines.push('func main() {\u000A');codeLines.push(' f :\u003D createFile(\"/tmp/defer.txt\")\u000A defer closeFile(f)\u000A writeFile(f)\u000A}\u000A');codeLines.push('func createFile(p string) *os.File {\u000A fmt.Println(\"creating\")\u000A f, err :\u003D os.Create(p)\u000A if err !\u003D nil {\u000A panic(err)\u000A }\u000A return f\u000A}\u000A');codeLines.push('func writeFile(f *os.File) {\u000A fmt.Println(\"writing\")\u000A fmt.Fprintln(f, \"data\")\u000A');codeLines.push('}\u000A');codeLines.push('func closeFile(f *os.File) {\u000A fmt.Println(\"closing\")\u000A err :\u003D f.Close()\u000A');codeLines.push(' if err !\u003D nil {\u000A fmt.Fprintf(os.Stderr, \"error: %v\\n\", err)\u000A os.Exit(1)\u000A }\u000A}\u000A');codeLines.push('');
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