Parametric Polymorphism - Bounded Parametric Polymorphism

Bounded Parametric Polymorphism

In 1985, Luca Cardelli and Peter Wegner recognized the advantages of allowing bounds on the type parameters. Many operations require some knowledge of the data types, but can otherwise work parametrically. For example, to check whether an item is included in a list, we need to compare the items for equality. In Standard ML, type parameters of the form ’’a are restricted so that the equality operation is available, thus the function would have the type ’’a × ’’a list → bool and ’’a can only be a type with defined equality. In Haskell, bounding is achieved by requiring types to belong to a type class; thus the same function has the type in Haskell. In most object-oriented programming languages that support parametric polymorphism, parameters can be constrained to be subtypes of a given type (see Subtype polymorphism and the article on Generic programming).