Code sample #1 from https://medium.com/@johnsundell/type-erasure-using-closures-in-swift-1e6125231f8

Type-erasure-using-closures-in-Swift-0.swift

class ModelLoader<T: Unboxable & Requestable> {
    func load(completionHandler: (Result<T>) -> Void) {
        networkService.loadData(from: T.requestURL) { data in
            do {
                try completionHandler(.success(unbox(data: data)))
            } catch {
                let error = ModelLoadingError.unboxingFailed(error)
                completionHandler(.error(error))
            }
        }
    }
}