Hole.swift

struct Hole<T> {
    static var value: T {
        fatalError("Unfilled hole of type `\(String(describing: T.self))`")
    }
}

enum Result<T, E: Error> {
    case success(T)
    case failure(E)

    func map<U>(_ tranform: (T) -> U) -> Result<U, E> {
        switch self {
        case let .success(value):
            return .success(tranform(value))

        case let .failure(error):
            return .failure(error)
        }
    }
}

func getResult() -> Result<String, NSError> {
    return Result
        .success(3)
        .map { value in
            Hole.value // quick help shows you need to return String
        }
}

getResult() // fatal error: Unfilled hole of type `String`

// Could also use a free function, but quick help isn't quite as nice since it shows a function signature instead of the exact type
func hole<T>() -> T {
    fatalError("Unfilled hole of type `\(String(describing: T.self))`")
}