Programming Languages and Systems

This open access book constitutes the proceedings of the 30th European Symposium on Programming, ESOP 2021, which was held during March 27 until April 1, 2021, as part of the European Joint Conferences on Theory and Practice of Software, ETAPS 2021. The conference was planned to take place in Luxembourg and changed to an online format due to the COVID-19 pandemic. The 24 papers included in this volume were carefully reviewed and selected from 79 submissions. They deal with fundamental issues in the specification, design, analysis, and implementation of programming languages and systems

A Theory of Typestate-Oriented Programming

Engineers in many disciplines use state machines to reason about system changes, and many object-oriented libraries require their clients to follow state machine protocols. No existing language, however, has native support for state machines, and programmers often lose productivity and introduce errors when trying to understand and follow interaction protocols. The Plaid language extends the object paradigm with explicit states and state transitions, in order to better model object state transitions. In this paper, we present P laidcore, a core calculus for Plaid, which uses states and permissions to statically guarantee that clients use object protocols correctly