Sum Swift extension, including sum by property.

Sequence+Sum.swift

extension Sequence where Element: AdditiveArithmetic {
    func sum() -> Element {
        reduce(.zero, +)
    }
}

extension Sequence {
    func sum<T: AdditiveArithmetic>(_ keyPath: KeyPath<Element, T>) -> T {
        map { $0[keyPath: keyPath] }.sum()
    }

    func compactSum<T: AdditiveArithmetic>(_ keyPath: KeyPath<Element, T?>) -> T {
        map { $0[keyPath: keyPath] }.compactMap { $0 }.sum()
    }

    func sum<T: AdditiveArithmetic>(_ transform: (Element) throws -> T) rethrows -> T {
        try map(transform).sum()
    }

    func compactSum<T: AdditiveArithmetic>(_ transform: (Element) throws -> T?) rethrows -> T {
        try map(transform).compactMap( { $0 }).sum()
    }

}