Hands-On Design Patterns with Swift

Conditional conformance

Conditional conformance is a feature that lets you provide conformance or extensions to existing types on the condition that a set of requirements is fulfilled.

Let's define a simple protocol:

protocol Summable {    var sum: Int { get }}

The previous protocol defines that any type that is Summable should expose a sum property of the Int type.

We can retrofit this type on Arrays of Int types as we know how to add integers together to produce a sum:

extension Array: Summable where Element == Int {    var sum: Int {        return self.reduce(0) { $0 + $1 }    }}[1,2,3,4].sum == 10

It's all good, but now we also want to make sums of all number types. In Swift, there is a base protocol, called Numeric, that is shared by all number types. ...