title: How to join an enumerated char list in Elixir
body: <p>I'm writing a function that converts <em>RNA</em> to <em>DNA</em>. The input comes in as a char list: <code>'ATCG'</code> so I loop through each letter and use a map to get the converted letter. Simple. </p>
  
  <p>The problem is after enumerating the character list I can't merge it back to a char list. Enum.join returns a string, <code>to_charlist</code> just returns a list of the character points, and I don't see any other functions that could help.</p>
  
  <p>Here is my code:</p>
  
  <pre><code>def to_rna(dna) do
    dna_to_rna = %{
     'G' =&gt; 'C',
     'C' =&gt; 'G',
     'T' =&gt; 'A',
     'A' =&gt; 'U'
    }
    Enum.map(dna, fn(letter) -&gt;
      Map.get(dna_to_rna, [letter])
    end)    
  end
  </code></pre>
  
  <p>This outputs a list of characters:  <code>['U', 'G', 'C']</code>. How do I convert this list into a charlist: <code>'UGC'</code>?</p>