When you extend a generic type, you do not provide a type parameter list as part of the extension's definition. Instead, the type parameter list from the original type definition is available within the body of the extension, and the original type parameter names are used to refer to the type parameters from the original definition.

The following example extends the generic Stack type to add a read-only computed property called topItem, which returns the top item on the stack without popping it from the stack:

extension Stack {
    var topItem: T? {
        return items.isEmpty ? nil : items[items.count - 1]
    }
}

The topItem property returns an optional value of type T. If the stack is empty, topItem returns nil; if the stack is not empty, topItem returns the final item in the items array.

Note that this extension does not define a type parameter list. Instead, the Stack type's existing type parameter name, T, is used within the extension to indicate the optional type of the topItem computed property.

The topItem computed property can now be used with any Stack instance to access and query its top item without removing it:

if let topItem = stackOfStrings.topItem {
    println("The top item on the stack is \(topItem).")
}
// prints "the top item on the stack is tres."