On the use of type predicates in object-oriented software: the case of Smalltalk

Abstract

Object-orientation relies on polymorphism to express behavioral variants. As opposed to traditional procedural design, explicit type-based conditionals should be avoided. This message is conveyed in introductory material on object orientation, as well as in object-oriented reengineering patterns. Is this principle followed in practice? In other words, are type predicates actually used in object-oriented software, and if so, to which extent?