Structural Subtyping and the Notion of Power Type

Many statically and dynamically typed languages attempt to achieve flexibility in their type discipline by some notion of subtyping. Subtyping relaxes the requirement that functions take arguments of a given type, by allowing arguments of any subtype of that type to be given. Such functions can accept arguments of types which will only be defined in the future; the open-ended character of this situation is important in large systems where it is essential to be able to extend existing facilities without modifying them.