Communicating finite-state machines are a fundamental, well-studied model of finite-state processes that communicate via unbounded first-in first-out channels. We show that they are expressively equivalent to existential MSO logic with two first-order variables and the order relation.

Abstract datatypes can be typed conveniently using existential types. Even though it is a powerful abstraction mechanism, current type inference systems based on Hindley-Milner do not allow existential types as first-class citizens – every existential type must be explicitly declared, packed, and unpacked using a data constructor. We present an extension of theMLF type system with first-class e...

We integrate existential types into a strongly typed C-like language. In particular, we show how a bad combination of existential types, mutation, and aliasing can cause a subtle violation of type safety. We explore two independent ways to strengthen the type system to restore safety. One restricts the mutation of existential packages. The other restricts the types of aliases of existential pac...

Existential risks are those that threaten the entire future of humanity. Many theories of value imply that even relatively small reductions in net existential risk have enormous expected value. Despite their importance, issues surrounding human-extinction risks and related hazards remain poorly understood. In this article, I clarify the concept of existential risk and develop an improved classi...