PromiseKit 6 example with Xcode 9.2 and Swift 4.1

PromiseKitExample.swift

import Foundation
import PromiseKit

struct YoRepository {
    func fetchDataWithPromise() -> Promise<String> {
        return Promise { seal in
            DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + .milliseconds(500)) {
                guard true else {
                    // failure
                    seal.reject(MyError.unknown)
                    return
                }

                // success
                seal.fulfill("yo! fulfill called")
            }
        }
    }
}

struct YoUseCase {
    func fetchYo() {
        firstly {
            YoRepository().fetchDataWithPromise()
        }.done { result in
            print(result)
        }.catch { error in
            print("some kind of error getting yo! -> \(error)")
        }
    }

}

enum MyError: Error {
    case unknown
}