title: What does "{ _ in }" mean in Swift?
body: <p>I found some code when I read a book about CoreData in Swift. I am confused about the meaning of the piece of code below. What's the meaning when declaring the closure like <code>configurationBlock: NSFetchRequest -&gt; () = { _ in }</code>. especially the meaning of { _ in }.</p>
  
  <pre><code>public static func fetchInContext(context: NSManagedObjectContext, @noescape configurationBlock: NSFetchRequest -&gt; () = { _ in }) -&gt; [Self] {
      let request = NSFetchRequest(entityName: Self.entityName)
      configurationBlock(request)
      guard let result = try! context.executeFetchRequest(request) as? [Self] else { fatalError("Fetched objects have wrong type") }
      return result
  }
  </code></pre>