> That points to a deficiency in the “composition over inheritance” aphorism: those aren’t the only two games in town. If you have procedures as first-class types (like blocks in Smalltalk, or lambdas in many languages), then you might prefer those over composition or inheritance.
First-class procedures/functions are a form of composition. Requiring a function type behaves like requiring an interface/class type with only one method. (In languages like F#, `Func<_,_>` is literally defined internally as an abstract class with one method, `Invoke`, although there are other mechanisms to auto-inline lambdas when enough static information is available to do so.) In either case, you can place it into a field of an object or data structure, or pass it directly as an argument to a function/method.
First-class procedures/functions are a form of composition. Requiring a function type behaves like requiring an interface/class type with only one method. (In languages like F#, `Func<_,_>` is literally defined internally as an abstract class with one method, `Invoke`, although there are other mechanisms to auto-inline lambdas when enough static information is available to do so.) In either case, you can place it into a field of an object or data structure, or pass it directly as an argument to a function/method.