title: error: cannot convert value of type '() -> ()' to closure result type 'String' using Swift + PromiseKit
body: <p>I am new to promises in Swift and using PromiseKit to try and create a very simple response in a playground and attempt to use it. I have the following code:</p>
  <pre><code>import UIKit
  import PromiseKit
  
  func foo(_ error: Bool) -&gt; Promise&lt;String&gt; {
    return Promise { fulfill, reject in
      if (!error) {
        fulfill(&quot;foo&quot;)
      } else {
        reject(Error(domain:&quot;&quot;, code:1, userInfo:nil))
      }
    }
  }
  
  foo(true).then { response -&gt; String in {
    print(response)
    }
  }
  </code></pre>
  <p>However I get the following error:</p>
  <pre><code>error: MyPlayground.playground:11:40: error: cannot convert value of type '() -&gt; ()' to closure result type 'String'
  foo(true).then { response -&gt; String in {
  </code></pre>