# appendInterpolation(_:formatter:) Appends an optionally-formatted instance of a Foundation type to a string interpolation. ``` mutating func appendInterpolation(_ subject: Subject, formatter: Formatter? = nil) where Subject : ReferenceConvertible ``` ## Parameters `subject` The Foundation object to append. `formatter` A formatter to convert `subject` to a string representation. ## Discussion Don’t call this method directly; it’s used by the compiler when interpreting string interpolations. --- 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)