Yes, as in Haskell type classes. Or Rust traits. Basically, anything that acts like an interface (defines a contract) but allows external definition. So you're

Yes, as in Haskell type classes. Or Rust traits. Basically, anything that acts like an interface (defines a contract) but allows external definition. So you're not stuck wrapping your Integer in a ShowableInteger just to have an Integer that implements the Show interface.