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ICFP 2020
Sun 23 - Sat 29 August 2020

The metatheory of Scala’s core type system—the \emph{Dependent Object Types (DOT)} calculus—is hard to extend, like the metatheory of other type systems combining subtyping and dependent types. Soundness of important Scala features therefore remains an open problem in theory and in practice. To address some of these problems, we use a \emph{semantics-first} approach to develop a logical relations model for a new version of DOT, called \textbf{guarded DOT (gDOT)}. Our logical relations model makes use of an abstract form of \emph{step-indexing}, as supported by the Iris framework, to model various forms of recursion in gDOT. To demonstrate the expressiveness of gDOT, we show that it handles Scala examples that could not be handled by previous versions of DOT, and prove using our logical relations model that gDOT provides the desired data abstraction. The gDOT type system, its semantic model, its soundness proofs, and all examples in the paper have been mechanized in Coq.