Decidable Subtyping of Existential Types for Julia
Article No.: 191, Pages 1091 - 1114
Abstract
Julia is a modern scientific-computing language that relies on multiple dispatch to implement generic libraries. While the language does not have a static type system, method declarations are decorated with expressive type annotations to determine when they are applicable. To find applicable methods, the implementation uses subtyping at run-time. We show that Julia's subtyping is undecidable, and we propose a restriction on types to recover decidability by stratifying types into method signatures over value types---where the former can freely use bounded existential types but the latter are restricted to use-site variance. A corpus analysis suggests that nearly all Julia programs written in practice already conform to this restriction.
Supplementary Material
This material is an extension of the PLDI 2024 paper with a technical appendix.
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Published: 20 June 2024
Published in PACMPL Volume 8, Issue PLDI
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