kishikawakatsumi

extension String.StringInterpolation { mutating func appendInterpolation(_ val: Any?, default defaultValue: String) { if let val = val { appendInterpolation(val) } else { defaultValue.write(to: &self) } } } let value: Int? = nil print("The value is: \(value, default: "<nil>")")

Version:

swift-DEVELOPMENT-SNAPSHOT-2018-12-15-a

Output:

The value is: <nil>

Error: