title: F#: String.Join and |> operator
body: <p>In F# interactive, I can use <code>String.Join("+", ["a"; "b"])</code> successfully, but</p>
  
  <pre><code>["a"; "b"] |&gt; String.Join "+"
  </code></pre>
  
  <p>produces an error:</p>
  
  <pre><code>Script1.fsx(79,15): error FS0001: This expression was expected to have type
      string list -&gt; 'a    
  but here has type
      string
  </code></pre>
  
  <p>How do I use <code>String.Join</code> passing a collection using pipeline?</p>
  
  <p>P.S. The same problem is with <code>lines |&gt; File.WriteAllLines "filename.txt"</code></p>