How to write extensions for collection types

Question

extension Array {
    func sum() -> Int {
        var sum = 0
        for num in self {
            sum += num
        }
        return sum
    }
}

[1,2,3].sum()

This code shows what I would like to do. Though i get an error on the this line: sum += num. The error i get is: Could not find an overload for '+=' that accepts the supplied arguments.

I assume the error has something to do with the fact that Array can contain lots of different types, not just Int, so it's bugging out. But how to fix?

Answer 1

There isn't currently a way to extend only a particular type of Array (Array<Int> in this case). That'd be a great request to file at bugreport.apple.com

In the meantime you can do this (not in an extension):

func sum(ints:Int[]) -> Int {
    return ints.reduce(0, +)
}

Answer 2

All that's needed is an explicit cast to Int:

extension Array {
    func Sum() -> Int {
        var sum = 0

        for num in self {
            sum += (num as Int)
        }

        return sum
    }
}

println([1,2,3].Sum()) //6