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<a href="." >局部值</a>
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<h1 id="局部值">局部值</h1>
<p>有时我们会需要用一个比较复杂的表达式计算某一个值，并且反复使用之，这时我们把这个复杂表达式赋予一个局部值，然后反复引用该局部值。如果说输入变量相当于函数的入参，输出值相当于函数的返回值，那么局部值就相当于函数内定义的局部变量。</p>
<p>局部值通过locals块定义，例如：</p>
<pre class="language-"><code class="lang-hcl"><span class="token keyword">locals</span> <span class="token punctuation">{</span>
<span class="token property">service_name</span> <span class="token punctuation">=</span> <span class="token string">"forum"</span>
<span class="token property">owner</span> <span class="token punctuation">=</span> <span class="token string">"Community Team"</span>
<span class="token punctuation">}</span>
</code></pre>
<p>一个locals块可以定义多个局部值，也可以定义任意多个locals块。赋给局部值的可以是更复杂的表达式，也可以是其他data、resource的输出、输入变量，甚至是其他的局部值：</p>
<pre class="language-"><code class="lang-hcl"><span class="token keyword">locals</span> <span class="token punctuation">{</span>
<span class="token comment"># Ids for multiple sets of EC2 instances, merged together</span>
<span class="token property">instance_ids</span> <span class="token punctuation">=</span> concat(aws_instance.blue.*.id, aws_instance.green.*.id)
<span class="token punctuation">}</span>
<span class="token keyword">locals</span> <span class="token punctuation">{</span>
<span class="token comment"># Common tags to be assigned to all resources</span>
<span class="token property">common_tags</span> <span class="token punctuation">=</span> <span class="token punctuation">{</span>
<span class="token property">Service</span> <span class="token punctuation">=</span> local.service_name
<span class="token property">Owner</span> <span class="token punctuation">=</span> local.owner
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre>
<p>引用局部值的表达式是local.\<name\>(注意，虽然局部值定义在locals块内，但引用是务必使用<strong>local</strong>而不是locals)，例如：</name\></p>
<pre class="language-"><code class="lang-hcl"><span class="token keyword">resource <span class="token type variable">"aws_instance"</span></span> <span class="token string">"example"</span> <span class="token punctuation">{</span>
<span class="token comment"># ...</span>
<span class="token property">tags</span> <span class="token punctuation">=</span> local.common_tags
<span class="token punctuation">}</span>
</code></pre>
<p>局部值只能在同一模块内的代码中引用。</p>
<p>局部值可以帮助我们避免重复复杂的表达式，提升代码的可读性，但如果过度使用也有可能增加代码的复杂度，使得代码的维护者更难理解所使用的表达式和值。适度使用局部值，仅用于反复引用同一复杂表达式的场景，未来当我们需要修改该表达式时局部值将使得修改变得相当轻松。</p>
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