An Intersection of Concerns - Extended Types for Julia

Explore a new subtyping algorithm for Julia that extends the language's type system capabilities in this 31-minute conference talk from JuliaCon Global 2025. Examine the fundamental principles of semantic subtyping that underpin Julia's type system, moving beyond common explanations of type particularities like invariance versus covariance to focus on core semantic subtyping features. Discover how the proposed algorithm trades some complexity of `where T` constructs to enable powerful new type system extensions including intersection types, negation types, sum types, and interface types. Learn about the technical details of this subtyping algorithm through comparisons with similar type system features found in other major programming languages, gaining insights into how these enhancements could expand Julia's type system expressiveness and functionality.