reduceArray.swift

func reduceArray<A,T>(
      accFunc f: (A, T) -> A
    , value v: [T]
    , startValue sV: A
) -> A {

    if v.count == 0 {
        return sV;
    }

    let firstValue: T = v.first!;
    let newStartValue = f(sV, firstValue);

    let result =  reduceArray(
          accFunc: f
        , value: Array(v.dropFirst())
        , startValue: newStartValue
    );

    return result;
}

let numbers = [1, 2, 3, 4]
func sumN(a: Int, b: Int) -> Int { return a + b }
let numberSum: Int = reduceArray(
      accFunc: sumN
    , value: numbers
    , startValue: 0
);

print(numberSum);