enum Frank {
    static func start(port: Int = 8080) {
        // ... steps 1 to 8 from sample code above ...
    }
}

typealias ResponseBodyProvider = () -> String
private var routedGET: [String: ResponseBodyProvider] = [:]

func get(_ path: String, bodyProvider: @escaping ResponseBodyProvider) {
    routedGET[path] = bodyProvider
}

private func routeRequest(_ request: Request) throws -> String? {
    guard let bodyProvider = routedGET[request.path] else {
        throw NoRouteError() // hypothetical error
    }
    return bodyProvider()
}