# Special Festschrift Issue in Honor of Furio Honsell

Editors: Marina Lenisa and Marino Miculan

This Special Issue collects a selection of the papers presented at the Symposium held in Udine on July 22, 2018 in honour of Furio Honsell, on the occasion of his 60th birthday.

Many of Furio's coauthors, collaborators, close colleagues, and former students gathered for the symposium. The presentations were on subjects related to Furio's many technical contributions and they were a tribute to his lasting impact on the field. This special issue collects the revised and expanded versions of papers submitted to the Festschrift. The papers were reviewed by Davide Ancona, Franco Barbanera, Pietro Di Gianantonio, Bruno Dinis, Thomas Hildebrandt, Atsushi Igarashi, Michele Loreti, Franco Parlamento, Alberto Policriti, Michael Rathjen, Jakob Rehof, Paula Severi, Ana Sokolova, Sam Staton, Pawel Urzyczyn, Daniele Varacca. We thank the reviewers for their feedback to the authors that helped them to improve the papers.

We thank the University of Udine, the Department of Mathematics, Computer Science and Physics, and the PRID ENCASE project for logistic and financial support in organizing the symposium in the stunning location of Palazzo Garzolini di Toppo-Wassermann.

Finally, we thank Lars Birkedal and Stefan Milius, Editor-in-Chief and Executive Editor of Logical Methods in Computer Science, for accepting to publish this Special Issue and for their support during its preparation.

Marina Lenisa, Marino Miculan
July 2019, Festschrift Guest Editors

### 1. A topological interpretation of three Leibnizian principles within the functional extensions

Three philosophical principles are often quoted in connection with Leibniz: "objects sharing the same properties are the same object" (Identity of indiscernibles), "everything can possibly exist, unless it yields contradiction" (Possibility as consistency), and "the ideal elements correctly determine the real things" (Transfer). Here we give a precise logico-mathematical formulation of these principles within the framework of the Functional Extensions, mathematical structures that generalize at once compactifications, completions, and elementary extensions of models. In this context, the above Leibnizian principles appear as topological or algebraic properties, namely: a property of separation, a property of compactness, and a property of directeness, respectively. Abiding by this interpretation, we obtain the somehow surprising conclusion that these Leibnizian principles may be fulfilled in pairs, but not all three together.

### 2. Inhabitation for Non-idempotent Intersection Types

The inhabitation problem for intersection types in the lambda-calculus is known to be undecidable. We study the problem in the case of non-idempotent intersection, considering several type assignment systems, which characterize the solvable or the strongly normalizing lambda-terms. We prove the decidability of the inhabitation problem for all the systems considered, by providing sound and complete inhabitation algorithms for them.

### 3. Java & Lambda: a Featherweight Story

We present FJ&$\lambda$, a new core calculus that extends Featherweight Java (FJ) with interfaces, supporting multiple inheritance in a restricted form, $\lambda$-expressions, and intersection types. Our main goal is to formalise how lambdas and intersection types are grafted on Java 8, by studying their properties in a formal setting. We show how intersection types play a significant role in several cases, in particular in the typecast of a $\lambda$-expression and in the typing of conditional expressions. We also embody interface \emph{default methods} in FJ&$\lambda$, since they increase the dynamism of $\lambda$-expressions, by allowing these methods to be called on $\lambda$-expressions. The crucial point in Java 8 and in our calculus is that $\lambda$-expressions can have various types according to the context requirements (target types): indeed, Java code does not compile when $\lambda$-expressions come without target types. In particular, in the operational semantics we must record target types by decorating $\lambda$-expressions, otherwise they would be lost in the runtime expressions. We prove the subject reduction property and progress for the resulting calculus, and we give a type inference algorithm that returns the type of a given program if it is well typed. The design of FJ&$\lambda$ has been driven by the aim of making it a subset of Java 8, while preserving the elegance and compactness of FJ. Indeed, FJ&$\lambda$ programs are typed and […]

### 4. Free complete Wasserstein algebras

We present an algebraic account of the Wasserstein distances $W_p$ on complete metric spaces, for $p \geq 1$. This is part of a program of a quantitative algebraic theory of effects in programming languages. In particular, we give axioms, parametric in $p$, for algebras over metric spaces equipped with probabilistic choice operations. The axioms say that the operations form a barycentric algebra and that the metric satisfies a property typical of the Wasserstein distance $W_p$. We show that the free complete such algebra over a complete metric space is that of the Radon probability measures with finite moments of order $p$, equipped with the Wasserstein distance as metric and with the usual binary convex sums as operations.

### 5. Event Structures for Petri nets with Persistence

Event structures are a well-accepted model of concurrency. In a seminal paper by Nielsen, Plotkin and Winskel, they are used to establish a bridge between the theory of domains and the approach to concurrency proposed by Petri. A basic role is played by an unfolding construction that maps (safe) Petri nets into a subclass of event structures, called prime event structures, where each event has a uniquely determined set of causes. Prime event structures, in turn, can be identified with their domain of configurations. At a categorical level, this is nicely formalised by Winskel as a chain of coreflections. Contrary to prime event structures, general event structures allow for the presence of disjunctive causes, i.e., events can be enabled by distinct minimal sets of events. In this paper, we extend the connection between Petri nets and event structures in order to include disjunctive causes. In particular, we show that, at the level of nets, disjunctive causes are well accounted for by persistent places. These are places where tokens, once generated, can be used several times without being consumed and where multiple tokens are interpreted collectively, i.e., their histories are inessential. Generalising the work on ordinary nets, Petri nets with persistence are related to a new subclass of general event structures, called locally connected, by means of a chain of coreflections relying on an unfolding construction.

### 6. Applicable Mathematics in a Minimal Computational Theory of Sets

In previous papers on this project a general static logical framework for formalizing and mechanizing set theories of different strength was suggested, and the power of some predicatively acceptable theories in that framework was explored. In this work we first improve that framework by enriching it with means for coherently extending by definitions its theories, without destroying its static nature or violating any of the principles on which it is based. Then we turn to investigate within the enriched framework the power of the minimal (predicatively acceptable) theory in it that proves the existence of infinite sets. We show that that theory is a computational theory, in the sense that every element of its minimal transitive model is denoted by some of its closed terms. (That model happens to be the second universe in Jensen's hierarchy.) Then we show that already this minimal theory suffices for developing very large portions (if not all) of scientifically applicable mathematics. This requires treating the collection of real numbers as a proper class, that is: a unary predicate which can be introduced in the theory by the static extension method described in the first part of the paper.

### 7. Reasoning about effects: from lists to cyber-physical agents

Theories for reasoning about programs with effects initially focused on basic manipulation of lists and other mutable data. The next challenge was to consider higher-order programming, adding functions as first class objects to mutable data. Reasoning about actors added the challenge of dealing with distributed open systems of entities interacting asynchronously. The advent of cyber-physical agents introduces the need to consider uncertainty, faults, physical as well as logical effects. In addition cyber-physical agents have sensors and actuators giving rise to a much richer class of effects with broader scope: think of self-driving cars, autonomous drones, or smart medical devices. This paper gives a retrospective on reasoning about effects highlighting key principles and techniques and closing with challenges for future work.