Type Driven Design Comparison

NonEmptyArray<Element>+mean.swift

// LinkedIn Post: https://www.linkedin.com/posts/lucaswk_creating-rule-enforced-types-is-a-powerful-activity-7167473213868785665-tJRS?utm_source=share&utm_medium=member_desktop

// Ordinary way
extension [Double] {
    var mean: Double? {
        guard !isEmpty else {
            return nil
        }

        guard let sum else {
            return nil
        }

        return sum / Double(count)
    }

    var sum: Double? {
        guard !isEmpty else {
            return nil
        }
        return reduce(.zero, +)
    }
}

// Type-Driven-Design
extension NonEmptyArray<Double> {
    var mean: Double {
        sum / Double(count)
    }
    var sum: Double {
        items.reduce(.zero, +)
    }
}

// Where:
struct NonEmptyArray<Element> {
    let items: [Element]

    init?(_ items: [Element]) {
        guard items.isEmpty else {
            return nil
        }
        self.items = items
    }

    var count: Int {
        items.count
    }
}