Abstract

Two different ways of defining ad-hoc polymorphic operations commonly occur in programming languages. With the first form polymorphic operations are defined inductively on the structure of types while with the second form polymorphic operations are defined for specific sets of types. In intensional type analysis operations are defined by induction on the structure of types. Therefore no new cases are necessary for user-defined types, because these types are equivalent to their underlying structure. However, intensional type analysis is “closed ” to extension, as the behavior of the operations cannot be differentiated for the new types, thus destroying the distinctions that these types are designed to express. Haskell type classes on the other hand define polymorphic operations for sets of types. Operations defined by class instances are considered “open”—the programmer can add instances for new types without modifying existing code. However, the operations must be extended with specialized code for each new type, and it may be tedious or even impossible to add extensions that apply to a large universe of new types. Both approaches have their benefits, so it is important to let programmers decide which is most appropriate for their needs. In this paper, we define a language that supports both forms of ad-hoc polymorphism, using the same basic constructs.

Citations