SumWithBlock.swift

extension Sequence {
    func sum<T>(_ transform: (Element) -> T) -> T where T: AdditiveArithmetic {
        var sum = T.zero
        for element in self {
            sum += transform(element)
        }
        return sum
    }
}

extension Sequence where Element: AdditiveArithmetic {
    func sum() -> Element {
        return sum { $0 }
    }
}

struct House {
    let occupants: Int
}

let houses = [
    House(occupants: 3),
    House(occupants: 13),
    House(occupants: 5),
    House(occupants: 1),
]

print(houses.sum { $0.occupants })