Using Session Types for Reasoning About Boundedness in the Pi-Calculus

The classes of depth-bounded and name-bounded processes are fragments of the pi-calculus for which some of the decision problems that are undecidable for the full calculus become decidable. P is depth-bounded at level k if every reduction sequence for P contains successor processes with at most k active nested restrictions. P is name-bounded at level k if every reduction sequence for P contains successor processes with at most k active bound names. Membership of these classes of processes is undecidable. In this paper we use binary session types to decise two type systems that give a sound characterization of the properties: If a process is well-typed in our first system, it is depth-bounded. If a process is well-typed in our second, more restrictive type system, it will also be name-bounded.Comment: In Proceedings EXPRESS/SOS 2017, arXiv:1709.0004

SnS can be modally characterized

AbstractWe show that a modal mu-calculus with label set {1,…,n} can define the Rabin recognizable tree languages up to an equivalence similar to the observational equivalence of Milner

Context-Free Session Types for Applied Pi-Calculus

We present a binary session type system using context-free session types to a version of the applied pi-calculus of Abadi et. al. where only base terms, constants and channels can be sent. Session types resemble process terms from BPA and we use a version of bisimulation equivalence to characterize type equivalence. We present a quotiented type system defined on type equivalence classes for which type equivalence is built into the type system. Both type systems satisfy general soundness properties; this is established by an appeal to a generic session type system for psi-calculi.Comment: In Proceedings EXPRESS/SOS 2018, arXiv:1808.0807

Studerendes oplevelse af reorganisering af problem-baseret læring på Aalborg Universitet

På Aalborg Universitet skete der i 2010 en omorganisering af stu-dieordningerne på Det Teknisk-Naturvidenskabelige Fakultet. Blandt ændringerne var, at de traditionelle projekter fyldte mindre, en større adskillelse af den formelle sammenhæng mellem kurser og projekter og en opdeling af kurserne i blokke på 5 ECTS. Vi sammenlignede erfaringerne med både den nye og den gamle model hos 10.-semesterstuderende på tre uddannelser og konkluderede, at de adspurgte studerende lagde stor vægt på projektarbejdet både i den gamle og den nye model, men at der var en signifikant lavere oplevelse af sammenhæng på et semester nu ift. før. De studerende prioriterede nu kursuseksamen højere til sidst i semesteret. Overordnet viste undersøgelsen også, at forholdet mellem det fagorienterede og det problemorienterede i de to PBL-modeller, og PBL-modeller generelt, ofte ikke var som modellerne overordnet gav udtryk for. Eksempelvis havde den nye model flere fagorienterede kurser og var derfor fra én synsvinkel et skridt mod en mere fagorienteret PBL-model; på den anden side havde de studerende i den nye model tidsmæssigt stadig mest fokus på det problemorienterede projekt og de fagorienterede kurser indeholdt PBL-lignende miniprojekter. De studerendes adfærd i en ændret undervisningsmodel var således mere kompleks, end man umiddelbart kunne antage. In 2010 Aalborg University reorganized the curriculum at the Faculty of Engineering and Science. The new curriculum scheduled less time for group projects and there was a formal separation between project and course work in each semester. The courses were also divided into 5 ECTS blocks. We compared student experiences of the old and new curriculums for three study programmes. Our results reveal that whereas in the old curriculum, students would prioritize the projects, with the new curriculum there was more focus on course work and linked examinations and the students found each semester more fragmented than before. This led us to question whether the new model had more subject-oriented courses, which would suggest a move towards a subject based PBL (problem based learning) model, or whether the students still spent most time on projects because the subject based courses contained PBL mini projects. Generally the study showed that the relationship between subject and problem solving in the two curricula models was not always obvious, and that the students’ behaviour in a changed teaching model was more complex than one might at first anticipate

On the Complexity of Deciding Behavioural Equivalences and Preorders. A Survey

This paper gives an overview of the computational complexity of all the equivalences in the linear/branching time hierarchy [vG90a] and the preordersin the corresponding hierarchy of preorders. We consider finite state or regular processes as well as infinite-state BPA [BK84b] processes. A distinction, which turns out to be important in the finite-state processes, is that of simulation-like equivalences/preorders vs. trace-like equivalencesand preorders. Here we survey various known complexity results for these relations. For regular processes, all simulation-like equivalences and preorders are decidable in polynomial time whereas all trace-like equivalences and preorders are PSPACE-Complete. We also consider interesting specialclasses of regular processes such as deterministic, determinate, unary, locally unary, and tree-like processes and survey the known complexity results inthese special cases. For infinite-state processes the results are quite different. For the class of context-free processes or BPA processes any preorder or equivalence beyond bisimulation is undecidable but bisimulation equivalence is polynomial timedecidable for normed BPA processes and is known to be elementarily decidable in the general case. For the class of BPP processes, all preorders and equivalences apart from bisimilarity are undecidable. However, bisimilarityis decidable in this case and is known to be decidable in polynomial time for normed BPP processes

A Tool for Describing and Checking Natural Semantics Definitions of Programming Languages

Many universities have courses and projects revolving around compiler or interpreter implementation as part of their degree programmes in computer science. In such teaching activities, tool support can be highly beneficial. While there are already several tools for assisting with development of the front end of compilers, tool support tapers off towards the back end, or requires more background experience than is expected of undergraduate students. Structural operational semantics is a useful and mathematically simple formalism for specifying the behaviour of programs and a specification lends itself well to implementation; in particular big-step or natural semantics is often a useful and simple approach. However, many students struggle with learning the notation and often come up with ill-defined and meaningless attempts at defining a structural operational semantics. A survey shows that students working on programming language projects feel that tool support is lacking and would be useful. Many of these problems encountered when developing a semantic definition are similar to problems encountered in programming, in particular ones that are essentially the result of type errors. We present a pedagogical metalanguage based on natural semantics, and its implementation, as an attempt to marry two notions: a syntax similar to textbook notation for natural semantics on the one hand, and automatic verification of some correctness properties on the other by means of a strong type discipline. The metalanguage and the tool provide the facilities for writing and executing specifications as a form of programming. The user can check that the specification is not meaningless as well as execute programs, if the specification makes sense.Comment: In Proceedings FROM 2022, arXiv:2209.0920

Bisimulations for Asynchronous Mobile Processes

Within the past few years there has been renewed interest in thestudy of value-passing process calculi as a consequence of the emergence of the pi-calculus. Here, [MPW89] have determined two variants of the notion of bisimulation, late and early bisimilarity. Most recently [San93] has proposed the new notion of open bisimulation equivalence. In this paper we consider Plain LAL, a mobile process calculus which differs from the pi-calculus in the sense that the communication of data values happens asynchronously. The surprising result is that in the presence of asynchrony, the open, late and early bisimulation equivalences coincide - this in contrast to the pi-calculus where they are distinct. The result allows us to formulate a common equational theory which is sound and complete for finite terms of Plain LAL