You Need Subtyping

Ever since Stephen Dolan’s 2016 thesis Algebraic Subtyping showed how to combine type inference and subtyping, I’ve been developing increasingly sophisticated programming languages based on those ideas, first IntercalScript in 2019, then CubiML in 2020, PolySubML in 2025, and with my next language already in the planning stages. I’ve long thought that subtyping is the next great evolution in programming language design and that it is a critical feature for new programming languages. However, existing programming languages have little or no subtyping, and there is a general lack of awareness about subtyping in the programming community. Therefore, in this post, I will explain what subtyping is and why it is so important. What is subtyping? If you remember the fad for Object Oriented Programming, “subtyping” might call to mind classes and inheritance hierarchies. However, subtyping is a far more basic and general notion than that. We say that type X is a subtype of type Y in a langua ... Read full article.