License: Creative Commons Attribution 3.0 Unported license (CC BY 3.0)
When quoting this document, please refer to the following
DOI: 10.4230/LIPIcs.ECOOP.2020.25
URN: urn:nbn:de:0030-drops-131821
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Nieto, Abel ; Zhao, Yaoyu ; Lhoták, Ondřej ; Chang, Angela ; Pu, Justin

Scala with Explicit Nulls

LIPIcs-ECOOP-2020-25.pdf (0.7 MB)


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.

BibTeX - Entry

  author =	{Abel Nieto and Yaoyu Zhao and Ondřej Lhot{\'a}k and Angela Chang and Justin Pu},
  title =	{{Scala with Explicit Nulls}},
  booktitle =	{34th European Conference on Object-Oriented Programming (ECOOP 2020)},
  pages =	{25:1--25:26},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-154-2},
  ISSN =	{1868-8969},
  year =	{2020},
  volume =	{166},
  editor =	{Robert Hirschfeld and Tobias Pape},
  publisher =	{Schloss Dagstuhl--Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{},
  URN =		{urn:nbn:de:0030-drops-131821},
  doi =		{10.4230/LIPIcs.ECOOP.2020.25},
  annote =	{Keywords: Scala, Java, nullability, language interoperability, type systems}

Keywords: Scala, Java, nullability, language interoperability, type systems
Collection: 34th European Conference on Object-Oriented Programming (ECOOP 2020)
Issue Date: 2020
Date of publication: 06.11.2020
Supplementary Material: ECOOP 2020 Artifact Evaluation approved artifact available at

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