Session types are a type-based approach to correct message-passing programs. A session type specifies a channel's protocol as sequences of exchanges. Aiming to uncover the essential notions of session-based concurrency, prior work defined minimal session types (MSTs), a formulation of session types without the sequentiality construct, and showed a minimality result : every process typable with standard session types can be transformed into a process typable using MSTs. Such a minimality result was proven for a higher-order session π -calculus, in which values are abstractions (functions from names to processes). In this paper, we study MSTs but now for the session π -calculus, the (first-order) language in which values are names and for which session types have been more widely studied. We first show that a new minimality result can be obtained by composing known results. Then, we develop optimizations of this new minimality result and prove also a dynamic correctness guarantee.

Static analyses based on typestates are important in certifying correctness of code contracts. Such analyses rely on Deterministic Finite Automata (DFAs) to specify properties of an object. We target the analysis of contracts in low-latency environments, where many useful contracts are impractical to codify as DFAs and/or the size of their associated DFAs leads to sub-par performance. To address this bottleneck, we present a lightweight compositional typestate analyzer, based on an expressive specification language that can succinctly specify code contracts. By implementing it in the static analyzer Infer , we demonstrate considerable performance and usability benefits when compared to existing techniques. A central insight is to rely on a sub-class of DFAs whose analysis uses efficient bit-vector operations.

Session types enable the static verification of message-passing programs. A session type specifies a channel’s protocol as sequences of messages. Prior work established a minimality result: every process typable with standard session types can be compiled down to a process typable using minimal session types: session types without the sequencing construct. This result justifies session types in terms of themselves; it holds for a higher-order session π-calculus, where values are abstractions (functions from names to processes).

Session types are a type-based approach to the verification of message-passing programs. They have been much studied as type systems for the pi-calculus and for languages such as Java. A session type specifies what and when should be exchanged through a channel. Central to session-typed languages are constructs in types and processes that specify sequencing in protocols. Here we study minimal session types, session types without sequencing. This is arguably the simplest form of session types. By relying on a core process calculus with sessions and higher-order concurrency (abstraction-passing), we prove that every process typable with standard (non minimal) session types can be compiled down into a process typed with minimal session types. This means that having sequencing constructs in both processes and session types is redundant; only sequentiality in processes is indispensable, as it can precisely codify sequentiality in types. Our developments draw inspiration from work by Parrow on behavior-preserving decompositions of untyped processes. By casting Parrow’s results in the realm of typed processes, our results reveal a conceptually simple formulation of session types and a principled avenue to the integration of session types into languages without sequencing in types.