The Type Soundness Theorem That You Really Want to Prove - From Syntactic to Semantic Approaches

Watch a Milner Award lecture from POPL'18 where Derek Dreyer from MPI-SWS presents a compelling argument for shifting focus from syntactic to semantic type soundness in programming language research. Explore the evolution and limitations of type systems theory, starting with Robin Milner's 1978 proposal through Wright and Felleisen's syntactic approach in the 1990s. Learn why traditional syntactic type soundness proves insufficient for practical applications, particularly in handling unsafe language features and ensuring data abstraction. Discover how modern semantic type soundness, built on step-indexed Kripke logical relations and higher-order concurrent separation logic, offers more comprehensive guarantees about type system effectiveness. Understand how this advanced approach enables machine-checked proofs that are both more practical and engaging than conventional syntactic methods, while maintaining the original vision of Milner's semantic formulation.