An Expressive, Scalable Type Theory for Certi ed Code

We present the type theory LTT, intended to form a basis for typed target languages, providing an internal notion of logical proposition and proof. The inclusion of explicit proofs allows the type system to guarantee properties that would otherwise be incompatible with decidable type checking. LTT also provides linear facilities for tracking ephemeral properties that hold only for certain program states. Our type theory allows for re-use of typechecking software by casting a variety of type systems within a single language. We provide additional re-use with a framework for modular development of operational semantics. This framework allows independent type systems and their operational semantics to be joined together, automatically inheriting the type safety properties of those individual systems. This material is based on work supported in part by ARPA grant F-19628-95-C-0050 and NSF grant CCR9984812. Any opinions, ndings, and conclusions or recommendations in this publication are those of the authors and do not re ect the views of these agencies.