Swift Functional Programming - Second Edition

The contains method

This Tree is far from complete and needs lots of functionality. For instance, we may need to check whether the Tree contains a specific value. To be able to do this, we need to add the following method to our Tree:

static func contains(_ key: Element, tree: Tree<Element>) -> Bool {     switch tree {     case .leaf(let element):         return key == element     case node(let lhs, let rhs):         return contains(key, tree: lhs) || contains(key, tree: rhs)     } } 

We can test the contains method as follows:

let isFound = Tree.contains("First", tree: functionalTree)  print(isFound) // prints true