ThrowingOptionalUnwrap.swift

struct Person {
    let firstName: String
    let lastName: String
    let ethnicity: String?
}

// Using throwable optional unwrapping

extension Person {
    init(firstName: String?, lastName: String?, ethnicity: String?) throws {
        self.firstName = try firstName.unwrapped(withName: "firstName")
        self.lastName = try lastName.unwrapped(withName: "lastName")
        self.ethnicity = ethnicity
    }
}

// Using regular optional unwrapping

extension Person {
    init(firstName: String?, lastName: String?) throws {
        guard let fName = firstName else {
            throw MissingValueError(valueName: "fisrtName")
        }
        guard let lName = lastName else {
            throw MissingValueError(valueName: "lastName")
        }
        
        self.firstName = fName
        self.lastName = lName
        self.ethnicity = nil // just so the above init is distinct from this one
    }
}

// ----------

struct MissingValueError: Error, CustomStringConvertible {
    let valueName: String
    var description: String {
        return "Missing value for \(valueName)"
    }
}

extension Optional {
    
    func unwrapped(withName name: String? = nil) throws -> Wrapped {
        switch self {
        case .some(let value):
            return value
        case .none:
            let name = name ?? String(describing: Wrapped.self)
            throw MissingValueError(valueName: name)
        }
    }
    
}