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Scala with Explicit Nulls Nieto, Abel Zhao, Yaoyu Lhoták, Ondřej Chang, Angela Pu, Justin Scala Java nullability language interoperability type systems The Scala programming language makes all reference types implicitly nullable. This is a problem, because null references do not support most operations that do make sense on regular objects, leading to runtime errors. In this paper, we present a modification to the Scala type system that makes nullability explicit in the types. Specifically, we make reference types non-nullable by default, while still allowing for nullable types via union types. We have implemented this design for explicit nulls as a fork of the Dotty (Scala 3) compiler. We evaluate our scheme by migrating a number of Scala libraries to use explicit nulls. Finally, we give a denotational semantics of type nullification, the interoperability layer between Java and Scala with explicit nulls. We show a soundness theorem stating that, for variants of System F_ω that model Java and Scala, nullification preserves values of types. Schloss Dagstuhl – Leibniz-Zentrum für Informatik Abel Nieto and Yaoyu Zhao and Ondřej Lhoták and Angela Chang and Justin Pu 2020 Is Part Of LIPIcs, Volume 166, 34th European Conference on Object-Oriented Programming (ECOOP 2020) InProceedings Text doc-type:ResearchArticle publishedVersion application/pdf doi:10.4230/LIPIcs.ECOOP.2020.25 urn:nbn:de:0030-drops-131821 https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ECOOP.2020.25 eng https://creativecommons.org/licenses/by/3.0/legalcode