let numbers = [1,2,3,4,5]
var result = numbers.reduce (0, {
print("firstNumber: \($0) + secondNumber: \($1)")
return $0 + $1
})
print(result)
// firstNumber: 0 + secondNumber: 1
// firstNumber: 1 + secondNumber: 2
// firstNumber: 3 + secondNumber: 3
// firstNumber: 6 + secondNumber: 4
// firstNumber: 10 + secondNumber: 5
// 15