Sam Staton

Relating coalgebraic notions of bisimulation

A labelled transition system can be understood as a coalgebra for a particular endofunctor on the category of sets. Generalizing, we are led to consider coalgebras for arbitrary endofunctors on arbitrary categories.

Comparing operational models of name-passing process calculi

We study three operational models of name-passing process calculi: coalgebras on (pre)sheaves, indexed labelled transition systems, and history dependent automata. The coalgebraic model is considered both for presheaves over the category of finite sets and injections, and for its subcategory of atomic sheaves known as the Schanuel topos. Each coalgebra induces an indexed labelled transition system. Such transition systems are characterised, relating the coalgebraic approach to an existing model of name-passing. Further, we consider internal labelled transition systems within the sheaf topos, and axiomatise a class that is in precise correspondence with the coalgebraic and the indexed labelled transition system models. By establishing and exploiting the equivalence of the Schanuel topos with a category of named-sets, these internal labelled transition systems are also related to the theory of history dependent automata.

A Congruence Rule Format for Name-Passing Process Calculi from Mathematical Structural Operational Semantics

We introduce a mathematical structural operational semantics that yields a congruence result for bisimilarity and is suitable for investigating rule formats for name-passing systems. Indeed, we instantiate this general abstract model theory in a framework of nominal sets and extract from it a GSOS-like rule format for name-passing process calculi for which the associated notion of behavioural equivalence - given by a form of open bisimilarity - is a congruence

Semantics for probabilistic programming: higher-order functions, continuous distributions, and soft constraints

We study the semantic foundation of expressive probabilistic programming languages, that support higher-order functions, continuous distributions, and soft constraints (such as Anglican, Church, and Venture). We define a metalanguage (an idealised version of Anglican) for probabilistic computation with the above features, develop both operational and denotational semantics, and prove soundness, adequacy, and termination. This involves measure theory, stochastic labelled transition systems, and functor categories, but admits intuitive computational readings, one of which views sampled random variables as dynamically allocated read-only variables. We apply our semantics to validate nontrivial equations underlying the correctness of certain compiler optimisations and inference algorithms such as sequential Monte Carlo simulation. The language enables defining probability distributions on higher-order functions, and we study their properties.Categories and Subject Descriptors CR-number [D.3]: Programming languages

A convenient category for higher-order probability theory

Higher-order probabilistic programming languages allow programmers to write sophisticated models in machine learning and statistics in a succinct and structured way, but step outside the standard measure-theoretic formalization of probability theory. Programs may use both higher-order functions and continuous distributions, or even define a probability distribution on functions. But standard probability theory does not handle higher-order functions well: the category of measurable spaces is not cartesian closed. Here we introduce quasi-Borel spaces. We show that these spaces: form a new formalization of probability theory replacing measurable spaces; form a cartesian closed category and so support higher-order functions; form a well-pointed category and so support good proof principles for equational reasoning; and support continuous probability distributions. We demonstrate the use of quasi-Borel spaces for higher-order functions and probability by: showing that a well-known construction of probability theory involving random functions gains a cleaner expression; and generalizing de Finettis theorem, that is a crucial theorem in probability theory, to quasi-Borel spaces.

Algebraic Effects, Linearity, and Quantum Programming Languages

We develop a new framework of algebraic theories with linear parameters, and use it to analyze the equational reasoning principles of quantum computing and quantum programming languages. We use the framework as follows: we present a new elementary algebraic theory of quantum computation, built from unitary gates and measurement; we provide a completeness theorem or the elementary algebraic theory by relating it with a model from operator algebra; we extract an equational theory for a quantum programming language from the algebraic theory; we compare quantum computation with other local notions of computation by investigating variations on the algebraic theory.

A congruence rule format for name-passing process calculi

Abstract We introduce a GSOS-like rule format for name-passing process calculi. Specifications in this format correspond to theories in nominal logic. The intended models of such specifications arise by initiality from a general categorical model theory. For operational semantics given in this rule format, a natural behavioural equivalence—a form of open bisimilarity—is a congruence.

General Structural Operational Semantics through Categorical Logic

Certain principles are fundamental to operational semantics, regardless of the languages or idioms involved. Such principles include rule-based definitions and proof techniques for congruence results. We formulate these principles in the general context of categorical logic. From this general formulation we recover precise results for particular language idioms by interpreting the logic in particular categories. For instance, results for first-order calculi, such as CCS, arise from considering the general results in the category of sets. Results for languages involving substitution and name generation, such as the pi-calculus, arise from considering the general results in categories of sheaves and group actions. As an extended example, we develop a tyft/tyxt-like rule format for open bisimulation in the pi-calculus.

Linearly-used state in models of call-by-value

We investigate the phenomenon that every monad is a linear state monad. We do this by studying a fully-complete state-passing translation from an impure call-by-value language to a new linear type theory: the enriched call-by-value calculus. The results are not specific to store, but can be applied to any computational effect expressible using algebraic operations, even to effects that are not usually thought of as stateful. There is a bijective correspondence between generic effects in the source language and state access operations in the enriched call-byvalue calculus. From the perspective of categorical models, the enriched call-by-value calculus suggests a refinement of the traditional Kleisli models of effectful call-by-value languages. The new models can be understood as enriched adjunctions.

Instances of Computational Effects: An Algebraic Perspective

We investigate the connections between computational effects, algebraic theories, and monads on functor categories. We develop a syntactic framework with variable binding that allows us to describe equations between programs while taking into account the idea that there may be different instances of a particular computational effect. We use our framework to give a general account of several notions of computation that had previously been analyzed in terms of monads on presheaf categories: the analysis of local store by Plotkin and Power; the analysis of restriction by Pitts; and the analysis of the pi calculus by Stark.