Custom Swift String Interpolation: Logs Privacy

logs.swift

struct Message {
    enum Privacy {
        case `public`
        case `private`
    }

    var value: String
}

extension Message: CustomStringConvertible {
    var description: String { value }
}

extension Message: ExpressibleByStringLiteral {
    init(stringLiteral value: StringLiteralType) {
        self.init(value: value)
    }
}

extension Message: ExpressibleByStringInterpolation {
    init(stringInterpolation: StringInterpolation) {
        self.init(value: stringInterpolation.string)
    }

    struct StringInterpolation: StringInterpolationProtocol {
        var string = ""

        init(literalCapacity: Int, interpolationCount: Int) {
            string.reserveCapacity(literalCapacity)
        }

        mutating func appendLiteral(_ literal: String) {
            string.append(literal)
        }

        mutating func appendInterpolation<T>(_ value: T, privacy: Privacy = .private) where T: CustomStringConvertible {
            switch privacy {
            case .public:
                string.append(value.description)
            case .private:
                string.append(
                    Array(repeating: "*", count: value.description.count).joined()
                )
            }
        }
    }
}

func log(_ message: Message) {
    print(message)
}

log("Hello, \("world!", privacy: .public)")
log("Hello, \("secret", privacy: .private)")
log("Static strings say whatever they want")