Taming the wildcards

Altidor, John; Huang, Shan; Smaragdakis, Yannis

doi:10.1145/1993498.1993569

Cited by 9 publications

References 14 publications

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Java Wildcards Meet Definition-Site Variance

Altidor

Reichenbach

Smaragdakis

2012

ECOOP 2012 – Object-Oriented Programming

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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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Java Wildcards Meet Definition-Site Variance

Altidor

Reichenbach

Smaragdakis

2012

ECOOP 2012 – Object-Oriented Programming

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Taming the wildcards

Altidor

Huang²,

Smaragdakis

2011

SIGPLAN Not.

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Variance allows the safe integration of parametric and subtype polymorphism. Two flavors of variance, definition-site versus usesite variance, have been studied and have had their merits hotly debated. Definition-site variance (as in Scala and C#) offers simple type-instantiation rules, but causes fractured definitions of naturally invariant classes; Use-site variance (as in Java) offers simplicity in class definitions, yet complex type-instantiation rules that elude most programmers.We present a unifying framework for reasoning about variance. Our framework is quite simple and entirely denotational, that is, it evokes directly the definition of variance with a small core calculus that does not depend on specific type systems. This general framework can have multiple applications to combine the best of both worlds: for instance, it can be used to add use-site variance annotations to the Scala type system. We show one such application in detail: we extend the Java type system with a mechanism that modularly infers the definition-site variance of type parameters, while allowing use-site variance annotations on any type-instantiation.Applying our technique to six Java generic libraries (including the Java core library) shows that 20-58% (depending on the library) of generic definitions are inferred to have single-variance; 8-63% of method signatures can be relaxed through this inference, and up to 91% of existing wildcard annotations are unnecessary and can be elided.

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