title: Reference to generic type 'Set' requires arguments in <...> & Generic Parameter 'element' could not be inferred
body: <p>I was working on In-App Purchases and came across two errors. </p>
  
  <p>The first one is <code>private let productIdentifiers: Set</code> It comes up with the error <strong>"Reference to generic type 'Set' requires arguments in &lt;...>"</strong></p>
  
  <p>The second error in my code is <code>private var purchasedProductIdentifiers = Set()</code>  It comes up with the error <strong>"Generic parameter 'Element' could not be inferred"</strong> </p>
  
  <p>Here is most of my code near the two errors:</p>
  
  <pre><code>public class IAPHelper : NSObject  {
  
  private let productIdentifiers: Set
  private var purchasedProductIdentifiers = Set()
  
  private var productsRequest: SKProductsRequest?
  private var productsRequestCompletionHandler: ProductsRequestCompletionHandler?
  
   static let IAPHelperPurchaseNotification =     "IAPHelperPurchaseNotification"
  
  public init(productIds: Set&lt;ProductIdentifier&gt;) {
  self.productIdentifiers = productIds
  
  
  super.init()
   }
  }
  </code></pre>
  
  <p>Help would be nice!!</p>
  
  <p>EDIT: I now get the error '>' is not a postfix unary operator when I change the second error to <code>private var purchasedProductIdentifiers =  Set&lt;ProductIdentifier&gt; = Set()</code></p>