Author name cluster
Ian Stark
Possible papers associated with this exact author name in Arrow. This page groups case-insensitive exact name matches and is not a full identity disambiguation profile.
Possible papers
4TCS Journal 2008 Journal Article
Free-algebra models for the π -calculus
- Ian Stark
The finite π -calculus has an explicit set-theoretic functor-category model that is known to be fully abstract for strong late bisimulation congruence. We characterize this as the initial free algebra for an appropriate set of operations and equations in the enriched Lawvere theories of Plotkin and Power. Thus we obtain a novel algebraic description for models of the π -calculus, and validate an existing construction as the universal such model. The algebraic operations are intuitive, covering name creation, communication of names over channels, and nondeterministic choice; the equations then combine these features in a modular fashion. We work in an enriched setting, over a “possible worlds” category of sets indexed by available names. This expands significantly on the classical notion of algebraic theories: we can specify operations that act only on fresh names, or have arities that vary as processes evolve. Based on our algebraic theory of π we describe a category of models for the π -calculus, and show that they all preserve bisimulation congruence. We develop a direct construction of free models in this category; and generalise previous results to prove that all free-algebra models are fully abstract. We show how local modifications to the theory can give alternative models for π I and the early π -calculus. From the theory of π we also obtain a Moggi-style computational monad, suitable for a programming language semantics of mobile communicating systems. This addresses the challenging area of correctly combining computational monads: in this case those for concurrency, name generation, and communication.
CSL Conference 2004 Conference Paper
A Dependent Type Theory with Names and Binding
- Ulrich Schöpp
- Ian Stark
Abstract We consider the problem of providing formal support for working with abstract syntax involving variable binders. Gabbay and Pitts have shown in their work on Fraenkel-Mostowski (FM) set theory how to address this through first-class names: in this paper we present a dependent type theory for programming and reasoning with such names. Our development is based on a categorical axiomatisation of names, with freshness as its central notion. An associated adjunction captures constructions known from FM theory: the freshness quantifier И, name-binding, and unique choice of fresh names. The Schanuel topos — the category underlying FM set theory — is an instance of this axiomatisation. Working from the categorical structure, we define a dependent type theory which it models. This uses bunches to integrate the monoidal structure corresponding to freshness, from which we define novel multiplicative dependent products ∏ * and sums ∑ *, as well as a propositions-as-types generalisation H of the freshness quantifier.
MFCS Conference 1993 Invited Paper
Observable Properties of Higher Order Functions that Dynamically Create Local Names, or What's new?
- Andrew M. Pitts
- Ian Stark
Abstract The research reported in this paper is concerned with the problem of reasoning about properties of higher order functions involving state. It is motivated by the desire to identify what, if any, are the difficulties created purely by locality of state, independent of other properties such as side-effects, exceptional termination and non-termination due to recursion. We consider a simple language (equivalent to a fragment of Standard ML) of typed, higher order functions that can dynamically create fresh names; names are created with local scope, can be tested for equality and can be passed around via function application, but that is all. Despite the extreme simplicity of the language and its operational semantics, the observable properties of such functions are shown to be very subtle. A notion of ‘logical relation’ is introduced which incorporates a version of representation independence for local names. We show how to use it to establish observational equivalences. The method is shown to be complete (and decidable) for expressions of first order types, but incomplete at higher types.