Bounded First-Class Universe Levels in Dependent Type Theory

Abstract: In dependent type theory, being able to refer to a type universe as a term itself increases its expressive power, but requires mechanisms in place to prevent Girard's paradox from introducing logical inconsistency in the presence of type-in-type. The simplest mechanism is a hierarchy of universes indexed by a sequence of levels, typically the naturals. To improve reusability of definitions, they can be made level polymorphic, abstracting over level variables and adding a notion of level expressions. For even more expressive power, level expressions can be made first-class as terms themselves, and level polymorphism is subsumed by dependent functions quantifying over levels. Furthermore, bounded level polymorphism provides more expressivity by being able to explicitly state constraints on level variables. While semantics for first-class levels with constraints are known, syntax and typing rules have not been explicitly written down. Yet pinning down a well-behaved syntax is not trivial; there exist prior type theories with bounded level polymorphism that fail to satisfy subject reduction. In this work, we design an explicit syntax for a type theory with bounded first-class levels, parametrized over arbitrary well-founded sets of levels. We prove the metatheoretic properties of subject reduction, type safety, consistency, and canonicity, entirely mechanized from syntax to semantics in Lean.