EXPRESSing Session Types

To celebrate the 30th edition of EXPRESS and the 20th edition of SOS we overview how session types can be expressed in a type theory for the standard π-calculus by means of a suitable encoding.The encoding allows one to reuse results about the π-calculus in the context of session-based communications, thus deepening the understanding of sessions and reducing redundancies in their theoretical foundations.Perhaps surprisingly, the encoding has practical implications as well, by enabling refined forms of deadlock analysis as well as allowing session type inference by means of a conventional type inference algorithm.1 Origins of EXPRESS: some personal memories This year marks an important milestone in the history of the EXPRESS/SOS workshop series.Before joining their destinies in 2012, the two workshops EXPRESS and SOS had been running on their own since 1994 and 2004, respectively.Hence, the EXPRESS/SOS'23 workshop in Antwerp will constitute the 30th edition of EXPRESS and the 20th edition of SOS.Two of us (Ilaria Castellani and Davide Sangiorgi) were personally involved in the very first edition of EXPRESS in 1998, and indeed, they may be said to have carried the workshop to the baptismal font, together with Robert de Simone and Catuscia Palamidessi.Let us recall some facts and personal memories.The EXPRESS workshops were originally held as meetings of the European project EXPRESS, a Network of Excellence within the Human Capital and Mobility programme, dedicated to expressiveness issues in Concurrency Theory.This NoE, which lasted from January 1994 till December 1997, gathered researchers from several European countries and was particularly fruitful in supporting young researchers' mobility across different sites.The first three workshops of the NoE were held in Amsterdam (1994), Tarquinia (1995), and Dagstuhl (1996).The fourth and final workshop was held in Santa Margherita Ligure (1997).It was co-chaired by Catuscia Palamidessi and Joachim Parrow, and stood out as a distinctive event, open to external participants and organised as a conference with a call for papers.A few months after this workshop, in the first half of 1998, the co-chairs of the forthcoming CONCUR'98 conference in Nice, Robert de Simone and Davide Sangiorgi, were wondering about endowing CONCUR with a satellite event (such events were still unusual at the time) in order to enhance its attractiveness.Moreover, Davide was sharing offices with Ilaria, who had been the NoE responsible for the site of Sophia Antipolis and was also part of the organising committee of CONCUR'98.It was so, during informal discussions, that the idea of launching EXPRESS as a stand-alone event affiliated with CONCUR was conceived, in order to preserve the heritage of the NoE and give it a continuation.Thus the first edition of EXPRESS, jointly chaired by Catuscia and Ilaria, took place in Nice in 1998, as the first and unique satellite event of CONCUR.However, EXPRESS did not remain a lonely satellite for too long, as other workshops were to join the orbit of CONCUR in the following years (INFINITY, YR-CONCUR, SecCo, TRENDS, . . .), including SOS in 2004.The workshop EXPRESS'98 turned out