Short Answer: Parametric Polymorphism (although various implementations provide various subsets).
Generics (or Templates in C++) refer to the ability to parameterize types and functions with types. This is useful for parameterized classes and polymorphic functions as found in languages such as Ada, C++, Eiffel, and etc., although these are "syntactic" or restricted forms [Cardelli 85]. Generics are orthogonal to inheritance, since types (and classes) may be generically parameterized. Generics provide for reusability in programming languages. An example is a Stack with a generically parameterized base type. This allows a single Stack class to provide many instantiations such as a Stack of ints, a Stack of any fundamental or user defined type, or even a Stack of Stacks of ... Another example is a polymorphic sort function taking a base type with a comparison operator. The function can be called with any type (containing a comparison operator). See [Booch 87b] for several examples in Ada and [Stroustrup xx] and [Murray 93] for examples in C++.
While generics have many advantages, typical limitations include a static nature, which is an advantage for strong typechecking but a potential disadvantage when causing dynamic compilation (leading to a time/space efficiency tradeoff), and sources can cause inlining and create source code dependencies and expand code size (unlike a single-body or "true" parametrically polymorphic implementation. Generics can also be viewed as a special case of type variables.
Functions are typically generic in statically-typed parametrically-polymorphic languages. One such popular functional language is ML, in which all functions are generic. Russell and Haskel are more modern variants (references are forthcoming, however see APPENDIX E).
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