Array+combine.swift

extension Array {
    func combine(_ numberOfElements: Int) -> [[Element]] {
        guard numberOfElements > 0 else { return [.init()] }
        guard let first = first else { return [] }
        
        return Array(dropFirst())
            .combine(numberOfElements - 1)
            .map { [first] + $0 } +
            Array(dropFirst())
            .combine(numberOfElements)
    }
}

let array = ["a", "b", "c", "d"]
let combined = array.combine(2)
let pairs = [["a", "b"], ["a", "c"], ["a", "d"], ["b", "c"], ["b", "d"], ["c", "d"]]
assert(pairs == combined)