# appendInterpolation(_:default:) Interpolates the given optional value’s textual representation, or the specified default string, into the string literal being created. ``` mutating func appendInterpolation(_ value: T?, default: String) where T : CustomStringConvertible, T : TextOutputStreamable ``` ## Parameters `value` The value to include in a string interpolation, if non-`nil`. `default` The string to include if `value` is `nil`. ## Discussion You don’t need to call this method directly. It’s used by the compiler when interpreting string interpolations where you provide a `default` parameter. For example, the following code implicitly calls this method, using the value of the `default` parameter when `value` is `nil`: ``` var age: Int? = 48 print("Your age is \(age, default: "unknown")") // Prints: Your age is 48 age = nil print("Your age is \(age, default: "unknown")") // Prints: Your age is unknown ``` --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)