future.swift

struct Urn {}
struct Track {}

// Return sync but is async :O
func track(for urn: Urn) -> Future<Track> {
    let promise = Promise<Track>()

    let remoteTrack = Track() // Imaginte this is async

    promise.resolve(with: remoteTrack)
    return promise
}


// Client
let future = track(for: Urn())

future.then { result in
    switch result {
    case let .success(track):
        print(track)
    case let .failure(error):
        print(error.localizedDescription)
    }
}