Range parameterized types: use-site variance without the existential questions
Article No.: 2, Pages 1 - 6
Abstract
Use-site variance approaches such as Java wildcards allows to flexibly derive many co- and contravariant types from one generic class definition. Safety is achieved by restricting the access to members of the parameterized types, but the definition of proper access rules is complicated by the possible nesting of types. Existing approaches for use-site variance relate parameterized classes to bounded existential types and employ a conversion of the type parameters to fresh type variables in order to type an operation. This technique only indirectly explains the operations of a parameterized class and confronts the programmer with type variables which are introduced by the compiler for the purpose of type checking.
In this paper, we propose a more straightforward, uniform and powerful mechanism to systematically address the access restrictions of parameterized types with use-site variance. Parameterized classes receive two type values for each type parameter, and the operations may be determined by simply substituting these values in covariant resp. contravariant positions. The two type values may be interpreted as the bounds of a type range, and we define a safe and flexible variant subtype relation based on range containment. With the exception of wildcard capture, our approach supports all of the common features of other use-site variance models.
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- Range parameterized types: use-site variance without the existential questions
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Published: 06 July 2009
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