title: Liquid filter for Ruby arrays with Jekyll
body: <p>I want to write a Jekyll filter, which take a list as a argument, and return a string of a list without redundent elements.</p>
  
  <p>My filter code is:</p>
  
  <pre><code>module Jekyll
      module ArrayToSet
          def array_to_set_string(arr)
              arr.uniq.to_s
          end
      end
  end
  
  Liquid::Template.register_filter(Jekyll::ArrayToSet)
  </code></pre>
  
  <p>The usage code is </p>
  
  <pre><code>{{ [1,2,2,3] | array_to_set_string }}
  </code></pre>
  
  <p>But the complilation <code>jekyll build</code> failed, because the <code>arr</code> in the function is nul.</p>
  
  <p>When I change the input argument as string <code>"[1,2,2,3]"</code>, <code>arr</code> is a normal string. </p>
  
  <p>Does jekyll forbid non-string argument for filter?</p>