Section: New Results

Efficiently Subtyping Union Types

Participant : Francesco Zappa Nardelli.

Julia is a programming language recently designed at MIT to support the needs of the scientific community. Julia occupies a unique position in the design landscape, it is a dynamic language with no type system, yet it has a surprisingly rich set of types and type annotations used to specify multimethod dispatch. The types that can be expressed in function signatures include parametric union types, covariant tuple types, parametric user-defined types with single inheritance, invariant type application, and finally types and values can be reified to appear in signatures. In 2017 with Vitek we started a research project to study the design and the pragmatic use of the Julia language, and formalised the Julia subtyping algorithm. In 2018 we have pursued this study, and we have proved correct the clever and space efficient algorithm relied upon by the Julia runtime. This has been published in [17].