How to get the summation of diagonal lines using higher-order functions?

To get the first sum, you want the i'th element of the i'th row:

let firstDiag = array.enumerated().map { $1[$0] }.reduce(0, +)

To get the second sum, you want the same thing, but the columns reversed:

let secondDiag = array.enumerated().map { $1.reversed()[$0] }.reduce(0, +)