306 lines
20 KiB
Plaintext
Generated
306 lines
20 KiB
Plaintext
Generated
<!DOCTYPE html>
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<title>Go by Example: Range over Iterators</title>
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<div class="example" id="range-over-iterators">
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<h2><a href="./">Go by Example</a>: Range over Iterators</h2>
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<table>
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<td class="docs">
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<p>Starting with version 1.23, Go has added support for
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<a href="https://go.dev/blog/range-functions">iterators</a>,
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which lets us range over pretty much anything!</p>
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<td class="docs">
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<a href="https://go.dev/play/p/JsdFcZac4E-"><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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<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">"fmt"</span>
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</span></span><span class="line"><span class="cl"> <span class="s">"iter"</span>
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</span></span><span class="line"><span class="cl"> <span class="s">"slices"</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 class="docs">
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<p>Let’s look at the <code>List</code> type from the
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<a href="generics">previous example</a> again. In that example
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we had an <code>AllElements</code> method that returned a slice
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of all elements in the list. With Go iterators, we
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can do it better - as shown below.</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">List</span><span class="p">[</span><span class="nx">T</span> <span class="nx">any</span><span class="p">]</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">head</span><span class="p">,</span> <span class="nx">tail</span> <span class="o">*</span><span class="nx">element</span><span class="p">[</span><span class="nx">T</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 class="docs">
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<pre class="chroma"><code><span class="line"><span class="cl"><span class="kd">type</span> <span class="nx">element</span><span class="p">[</span><span class="nx">T</span> <span class="nx">any</span><span class="p">]</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">next</span> <span class="o">*</span><span class="nx">element</span><span class="p">[</span><span class="nx">T</span><span class="p">]</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">val</span> <span class="nx">T</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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<pre class="chroma"><code><span class="line"><span class="cl"><span class="kd">func</span> <span class="p">(</span><span class="nx">lst</span> <span class="o">*</span><span class="nx">List</span><span class="p">[</span><span class="nx">T</span><span class="p">])</span> <span class="nf">Push</span><span class="p">(</span><span class="nx">v</span> <span class="nx">T</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">lst</span><span class="p">.</span><span class="nx">tail</span> <span class="o">==</span> <span class="kc">nil</span> <span class="p">{</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">lst</span><span class="p">.</span><span class="nx">head</span> <span class="p">=</span> <span class="o">&</span><span class="nx">element</span><span class="p">[</span><span class="nx">T</span><span class="p">]{</span><span class="nx">val</span><span class="p">:</span> <span class="nx">v</span><span class="p">}</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">lst</span><span class="p">.</span><span class="nx">tail</span> <span class="p">=</span> <span class="nx">lst</span><span class="p">.</span><span class="nx">head</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">lst</span><span class="p">.</span><span class="nx">tail</span><span class="p">.</span><span class="nx">next</span> <span class="p">=</span> <span class="o">&</span><span class="nx">element</span><span class="p">[</span><span class="nx">T</span><span class="p">]{</span><span class="nx">val</span><span class="p">:</span> <span class="nx">v</span><span class="p">}</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">lst</span><span class="p">.</span><span class="nx">tail</span> <span class="p">=</span> <span class="nx">lst</span><span class="p">.</span><span class="nx">tail</span><span class="p">.</span><span class="nx">next</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 class="docs">
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<p>All returns an <em>iterator</em>, which in Go is a function
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with a <a href="https://pkg.go.dev/iter#Seq">special signature</a>.</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">lst</span> <span class="o">*</span><span class="nx">List</span><span class="p">[</span><span class="nx">T</span><span class="p">])</span> <span class="nf">All</span><span class="p">()</span> <span class="nx">iter</span><span class="p">.</span><span class="nx">Seq</span><span class="p">[</span><span class="nx">T</span><span class="p">]</span> <span class="p">{</span>
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</span></span><span class="line"><span class="cl"> <span class="k">return</span> <span class="kd">func</span><span class="p">(</span><span class="nx">yield</span> <span class="kd">func</span><span class="p">(</span><span class="nx">T</span><span class="p">)</span> <span class="kt">bool</span><span class="p">)</span> <span class="p">{</span></span></span></code></pre>
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</td>
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<td class="docs">
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<p>The iterator function takes another function as
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a parameter, called <code>yield</code> by convention (but
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the name can be arbitrary). It will call <code>yield</code> for
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every element we want to iterate over, and note <code>yield</code>’s
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return value for a potential early termination.</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">for</span> <span class="nx">e</span> <span class="o">:=</span> <span class="nx">lst</span><span class="p">.</span><span class="nx">head</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">e</span> <span class="p">=</span> <span class="nx">e</span><span class="p">.</span><span class="nx">next</span> <span class="p">{</span>
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</span></span><span class="line"><span class="cl"> <span class="k">if</span> <span class="p">!</span><span class="nf">yield</span><span class="p">(</span><span class="nx">e</span><span class="p">.</span><span class="nx">val</span><span class="p">)</span> <span class="p">{</span>
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</span></span><span class="line"><span class="cl"> <span class="k">return</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>
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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 class="docs">
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<p>Iteration doesn’t require an underlying data structure,
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and doesn’t even have to be finite! Here’s a function
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returning an iterator over Fibonacci numbers: it keeps
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running as long as <code>yield</code> keeps returning <code>true</code>.</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="nf">genFib</span><span class="p">()</span> <span class="nx">iter</span><span class="p">.</span><span class="nx">Seq</span><span class="p">[</span><span class="kt">int</span><span class="p">]</span> <span class="p">{</span>
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</span></span><span class="line"><span class="cl"> <span class="k">return</span> <span class="kd">func</span><span class="p">(</span><span class="nx">yield</span> <span class="kd">func</span><span class="p">(</span><span class="kt">int</span><span class="p">)</span> <span class="kt">bool</span><span class="p">)</span> <span class="p">{</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">a</span><span class="p">,</span> <span class="nx">b</span> <span class="o">:=</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">1</span></span></span></code></pre>
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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">for</span> <span class="p">{</span>
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</span></span><span class="line"><span class="cl"> <span class="k">if</span> <span class="p">!</span><span class="nf">yield</span><span class="p">(</span><span class="nx">a</span><span class="p">)</span> <span class="p">{</span>
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</span></span><span class="line"><span class="cl"> <span class="k">return</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="nx">a</span><span class="p">,</span> <span class="nx">b</span> <span class="p">=</span> <span class="nx">b</span><span class="p">,</span> <span class="nx">a</span><span class="o">+</span><span class="nx">b</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>
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</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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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>
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</span></span><span class="line"><span class="cl"> <span class="nx">lst</span> <span class="o">:=</span> <span class="nx">List</span><span class="p">[</span><span class="kt">int</span><span class="p">]{}</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">lst</span><span class="p">.</span><span class="nf">Push</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">lst</span><span class="p">.</span><span class="nf">Push</span><span class="p">(</span><span class="mi">13</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">lst</span><span class="p">.</span><span class="nf">Push</span><span class="p">(</span><span class="mi">23</span><span class="p">)</span></span></span></code></pre>
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<td class="docs">
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<p>Since <code>List.All</code> returns an iterator, we can use it
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in a regular <code>range</code> loop.</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">for</span> <span class="nx">e</span> <span class="o">:=</span> <span class="k">range</span> <span class="nx">lst</span><span class="p">.</span><span class="nf">All</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">e</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 class="docs">
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<p>Packages like <a href="https://pkg.go.dev/slices">slices</a> have
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a number of useful functions to work with iterators.
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For example, <code>Collect</code> takes any iterator and collects
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all its values into a slice.</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="nx">all</span> <span class="o">:=</span> <span class="nx">slices</span><span class="p">.</span><span class="nf">Collect</span><span class="p">(</span><span class="nx">lst</span><span class="p">.</span><span class="nf">All</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">"all:"</span><span class="p">,</span> <span class="nx">all</span><span class="p">)</span></span></span></code></pre>
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<pre class="chroma"><code><span class="line"><span class="cl"> <span class="k">for</span> <span class="nx">n</span> <span class="o">:=</span> <span class="k">range</span> <span class="nf">genFib</span><span class="p">()</span> <span class="p">{</span></span></span></code></pre>
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</td>
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<td class="docs">
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<p>Once the loop hits <code>break</code> or an early return, the <code>yield</code> function
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passed to the iterator will return <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="k">if</span> <span class="nx">n</span> <span class="o">>=</span> <span class="mi">10</span> <span class="p">{</span>
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</span></span><span class="line"><span class="cl"> <span class="k">break</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="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="nx">n</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 class="code">
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<pre class="chroma"><code><span class="line"><span class="cl"><span class="gp">$</span> go run range-over-iterators.go
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</span></span><span class="line"><span class="cl"><span class="go">10
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</span></span></span><span class="line"><span class="cl"><span class="go">13
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</span></span></span><span class="line"><span class="cl"><span class="go">23
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</span></span></span><span class="line"><span class="cl"><span class="go">all: [10 13 23]
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</span></span></span><span class="line"><span class="cl"><span class="go">1
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</span></span></span><span class="line"><span class="cl"><span class="go">1
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</span></span></span><span class="line"><span class="cl"><span class="go">2
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</span></span></span><span class="line"><span class="cl"><span class="go">3
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</span></span></span><span class="line"><span class="cl"><span class="go">5
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</span></span></span><span class="line"><span class="cl"><span class="go">8</span></span></span></code></pre>
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</table>
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<p class="next">
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Next example: <a href="errors" rel="next">Errors</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/de3b5db09c9b980ba971c708a3fe522606e44572">
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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 \"fmt\"\u000A \"iter\"\u000A \"slices\"\u000A)\u000A');codeLines.push('type List[T any] struct {\u000A head, tail *element[T]\u000A}\u000A');codeLines.push('type element[T any] struct {\u000A next *element[T]\u000A val T\u000A}\u000A');codeLines.push('func (lst *List[T]) Push(v T) {\u000A if lst.tail \u003D\u003D nil {\u000A lst.head \u003D \u0026element[T]{val: v}\u000A lst.tail \u003D lst.head\u000A } else {\u000A lst.tail.next \u003D \u0026element[T]{val: v}\u000A lst.tail \u003D lst.tail.next\u000A }\u000A}\u000A');codeLines.push('func (lst *List[T]) All() iter.Seq[T] {\u000A return func(yield func(T) bool) {\u000A');codeLines.push(' for e :\u003D lst.head; e !\u003D nil; e \u003D e.next {\u000A if !yield(e.val) {\u000A return\u000A }\u000A }\u000A }\u000A}\u000A');codeLines.push('func genFib() iter.Seq[int] {\u000A return func(yield func(int) bool) {\u000A a, b :\u003D 1, 1\u000A');codeLines.push(' for {\u000A if !yield(a) {\u000A return\u000A }\u000A a, b \u003D b, a+b\u000A }\u000A }\u000A}\u000A');codeLines.push('func main() {\u000A lst :\u003D List[int]{}\u000A lst.Push(10)\u000A lst.Push(13)\u000A lst.Push(23)\u000A');codeLines.push(' for e :\u003D range lst.All() {\u000A fmt.Println(e)\u000A }\u000A');codeLines.push(' all :\u003D slices.Collect(lst.All())\u000A fmt.Println(\"all:\", all)\u000A');codeLines.push(' for n :\u003D range genFib() {\u000A');codeLines.push(' if n \u003E\u003D 10 {\u000A break\u000A }\u000A fmt.Println(n)\u000A }\u000A}\u000A');codeLines.push('');
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