Swift Programming: The Big Nerd Ranch Guide

Type Constraint where Clauses

Write a new method that takes every element of an array and pushes it onto a stack.

Listing 22.17 Pushing items from an array onto a stack

...
struct Stack<Element>: Sequence {
    ...
    mutating func pushAll(_ array: [Element]) {
        for item in array {
            self.push(item)
        }
    }
}

...
for value in myStack {
    print("for-in loop: got \(value)")
}

myStack.pushAll([1, 2, 3])
for value in myStack {
    print("after pushing: got \(value)")
}

pushAll(_:) is useful, but it is not as general as it could be. You now know that any type that conforms to Sequence can be used in a for-in loop, so why should this method require an array? It should be able to accept any kind of sequence – even another Stack, now that Stack conforms ...