title: What does unnamed parameters must be written with the empty name '_' mean?
body: <p>I'm learning Swift from a book and we are using Playgrounds to build out a class.  I received an error that reads: unnamed parameters must be written with the empty name '_'.</p>
  
  <p>I understand an underscore in Swift means "to ignore" but if I add an underscore followed by a space then I receive the error: Parameter requires an explicit type which is fairly easy to understand, meaning that a parameter must be declared as a certain type. :)</p>
  
  <p>I'd like to know exactly what the error "unnamed parameters must be written with the empty name '_'" is trying to say in layman terms because its not making much sense to a noob like me.</p>
  
  <p>Here is the code from the playground up to this point: </p>
  
  <pre><code>//: Playground - noun: a place where people can play
  
  import UIKit
  
  var str = "Hello, playground"
  
  func fahrenheitToCelsius(fahrenheitValue: Double)-&gt; Double {
  
      var result: Double
  
      result = (((fahrenheitValue - 32) * 5) / 9)
  
      return result
  }
  
  var x = fahrenheitToCelsius(fahrenheitValue: 15.3)
  
  print(x)
  
  
  
  
  
  class Door{
  
      var opened: Bool = false
      var locked: Bool = false
      let width: Int = 32
      let height: Int = 72
      let weight: Int = 10
      let color: String = "Red"
  
      //behaviors
      func open(_ Void)-&gt;String{
          opened = true
          return "C-r-r-e-e-a-k-k-k...the door is open!"
      }
  
      func close(_ Void)-&gt;String{
          opened = false
          return "C-r-r-e-e-a-k-k-k...the door is closed!"
      }
  
      func lock(_ Void)-&gt;String{
          locked = true
          return "C-l-i-c-c-c-k-k...the door is locked!"
      }
  
      func unlock(_ Void)-&gt;String{
          locked = false
          return "C-l-i-c-c-c-k-k...the door is unlocked!"
      }
  
  }
  </code></pre>