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The simplest example of type introspection in Java is the instanceof [1] operator. The instanceof operator determines whether a particular object belongs to a particular class (or a subclass of that class, or a class that implements that interface).
For example, in the GHC standard library, the class IArray expresses a general immutable array interface. In this class, the type class constraint IArray a e means that a is an array type that contains elements of type e. (This restriction on polymorphism is used to implement unboxed array types, for example.)
Roberts (p. 171) gives a related example in Java, using a Class to represent a stack frame. The example given is a solution to the Tower of Hanoi problem wherein a stack simulates polymorphic recursion with a beginning, temporary and ending nested stack substitution structure. [5]
Polymorphism can be distinguished by when the implementation is selected: statically (at compile time) or dynamically (at run time, typically via a virtual function). This is known respectively as static dispatch and dynamic dispatch, and the corresponding forms of polymorphism are accordingly called static polymorphism and dynamic polymorphism.
Duck typing is similar to, but distinct from, structural typing.Structural typing is a static typing system that determines type compatibility and equivalence by a type's structure, whereas duck typing is dynamic and determines type compatibility by only that part of a type's structure that is accessed during runtime.
Impredicative polymorphism (also called first-class polymorphism) is the most powerful form of parametric polymorphism. [1]: 340 In formal logic, a definition is said to be impredicative if it is self-referential; in type theory, it refers to the ability for a type to be in the domain of a quantifier it contains. This allows the instantiation ...
Python sets are very much like mathematical sets, and support operations like set intersection and union. Python also features a frozenset class for immutable sets, see Collection types. Dictionaries (class dict) are mutable mappings tying keys and corresponding values. Python has special syntax to create dictionaries ({key: value})
C++ does not have the keyword super that a subclass can use in Java to invoke the superclass version of a method that it wants to override. Instead, the name of the parent or base class is used followed by the scope resolution operator. For example, the following code presents two classes, the base class Rectangle, and the derived class Box.