Abstract Types

Some articles on abstract type, type, abstract, abstract types:

Abstract Type
... In programming languages, an abstract type, unlike Concrete class is a type in a nominative type system which cannot be instantiated ... (However, it may have concrete subtypes that do have instances.) An abstract type may have no implementation, or an incomplete implementation ... It may include abstract methods or abstract properties that are shared by its subtypes ...
Use of Abstract Types
... Abstract types are an important feature in statically typed OO languages ... typed languages have no equivalent feature (although the use of duck typing makes abstract types unnecessary) however traits are found in some modern dynamically-typed languages ... classes (have no subtypes), or else be abstract ...

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