title: What is the purpose of tilde character ~ in YAML?
body: <p>I have looked into YAML official documentation and I have also gone through some tutorials. </p>
  
  <p>I am working with Symfony and I like adding configurations with YAML. But I can't find any explanation for this tilde character <code>~</code> in YAML.</p>
  
  <p>For example, see the code below:</p>
  
  <pre><code>firewalls:
      # ...
      main:
          anonymous: ~
          http_basic: ~
  </code></pre>
  
  <p>Sometimes, I removed this ~ character and I did not see any effect on logic. So what is the purpose of this tide character in YAML?</p>