swift - Convert generic free function into Array extension

Question

I wrote some code to perform run length encoding and decoding. I have my encoding function as a method in an extension to Array, but I can't make the decoding in a similar fashion. Is this possible? I can't find any ways of introducing new generic types into extensions.

func runLengthDecode<T: Equatable>(_ runLengthEncoding: [(element: T, count: Int)]) -> [T] {
    return runLengthEncoding.flatMap{ repeatElement($0.element, count: $0.count)}
}

I wish this function were a method on Array, as well. Something along the lines of:

extension Array<T> where Element == (element: T, count: Int) {
    func runLengthDecode() -> [T] {
        return self.flatMap{ repeatElement($0.element, count: $0.count)}
    }
}

Answer 1

Instead of constraining the array extension move the constraint to your method:

extension Array {
    func runLengthDecode<T: Equatable>() -> [T] where Element == (element: T, count: Int)  {
        return flatMap{ repeatElement($0.element, count: $0.count) }
    }
}

or simply

extension Array {
    func runLengthDecode<T: Equatable>() -> [T] where Element == (element: T, count: Int)  {
        return flatMap(repeatElement)
    }
}