Abstract
Variance is concerned with the interplay of parametric polymorphism (i.e., templates, generics) and subtyping. The study of variance gives answers to the question of when an instantiation of a generic class can be a subtype of another. In this work, we combine the mechanisms of use-site variance (as in Java) and definition-site variance (as in Scala and C#) in a single type system, based on Java. This allows maximum flexibility in both the specification and use of generic types, thus increasing the reusability of code. Our VarJ calculus achieves a safe synergy of def-site and use-site variance, while supporting the full complexities of the Java realization of variance, including F-bounded polymorphism and wildcard capture. We show that the interaction of these features with definition-site variance is non-trivial and offer a full proof of soundness—the first in the literature for an approach combining variance mechanisms.
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Altidor, J., Reichenbach, C., Smaragdakis, Y. (2012). Java Wildcards Meet Definition-Site Variance. In: Noble, J. (eds) ECOOP 2012 – Object-Oriented Programming. ECOOP 2012. Lecture Notes in Computer Science, vol 7313. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-31057-7_23
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DOI: https://doi.org/10.1007/978-3-642-31057-7_23
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