When Subtyping Constraints Liberate A Novel Type Inference Approach for First-Class Polymorphism

Type inference in the presence of first-class or "impredicative" second-order polymorphism a la System F has been an active research area for several decades, with original works dating back to the end of the 80s. Yet, until now many basic problems remain open, such as how to type check expressions like (lambda x. (x 123, x True)) id reliably. We show that a type inference approach based on multi-bounded polymorphism, a form of implicit polymorphic subtyping with multiple lower and upper bounds, can help us resolve most of these problems in a uniquely simple and regular way. We define F-{