Binary Search implementation in Swift

binarySearch.swift

func binarySearch<T: Comparable> (on array: [T], target: T) -> Int?{
    if(array.count <= 1 && array.first != target){
        return nil
    }
    let middle = array.count/2
    if(target == array[middle]){
        return middle
    }else if(target < array[middle]){
        return binarySearch(on: Array(array.dropLast(middle)), target: target)
    }else{
        let result = binarySearch(on: Array(array.dropFirst(middle)), target: target)
        return result != nil ? (middle + result!) : result!
    }
}

let numbers = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67]

binarySearch(on: numbers, target: 67) == numbers.index(of: 67) //true