Resource Calculus Research Articles

Finitary Simulation of Infinitary $\beta$-Reduction via Taylor Expansion, and Applications

Originating in Girard's Linear logic, Ehrhard and Regnier's Taylor expansion of $\lambda$-terms has been broadly used as a tool to approximate the terms of several variants of the $\lambda$-calculus. Many results arise from a Commutation theorem relating the normal form of the Taylor expansion of a term to its B\"ohm tree. This led us to consider extending this formalism to the infinitary $\lambda$-calculus, since the $\Lambda_{\infty}^{001}$ version of this calculus has B\"ohm trees as normal forms and seems to be the ideal framework to reformulate the Commutation theorem. We give a (co-)inductive presentation of $\Lambda_{\infty}^{001}$. We define a Taylor expansion on this calculus, and state that the infinitary $\beta$-reduction can be simulated through this Taylor expansion. The target language is the usual resource calculus, and in particular the resource reduction remains finite, confluent and terminating. Finally, we state the generalised Commutation theorem and use our results to provide simple proofs of some normalisation and confluence properties in the infinitary $\lambda$-calculus.

Normalization by Evaluation for the Lambek Calculus

The syntactic calculus of Lambek is a deductive system for the multiplicative fragment of intuitionistic non-commutative linear logic. As a fine-grained calculus of resources, it has many applications, mostly in formal computational investigations of natural languages. This paper introduces a calculus of beta-eta-long normal forms for derivations in the Lambek calculus with multiplicative unit and conjunction among its logical connectives. Reduction to normal form follows the normalization by evaluation (NbE) strategy: (i) evaluate a derivation in a Kripke model of Lambek calculus; (ii) extract normal forms from the obtained semantic values. The implementation of the NbE algorithm requires the presence of a strong monad in the Kripke interpretation of positive formulae, in analogy with the extension of intuitionistic propositional logic with falsity and disjunction. The NbE algorithm can also be instantiated with minor variations to calculi for related substructural logics, such as multiplicative and dual intuitionistic linear logic (MILL and DILL).

Taylor Expansion, Finiteness and Strategies

We examine some recent methods introduced to extend Ehrhard and Regnier's result on Taylor expansion: infinite linear combinations of approximants of a lambda-term can be normalized while keeping all coefficients finite. The methods considered allow to extend this result to non-uniform calculi; we show that when focusing on precise reduction strategies, such as Call-By-Value, Call-By-Need, PCF or variants of Call-By-Push-Value, the extension of Ehrhard and Regnier's finiteness result can hold or not, depending on the structure of the original calculus.In particular, we introduce a resource calculus for Call-By-Need, and show that the finiteness result about its Taylor expansion can be derived from our Call-By-Value considerations. We also introduce a resource calculus for a presentation of PCF with an explicit fixpoint construction, and show how it interferes with the finiteness result. We examine then Ehrhard and Guerrieri's Bang Calculus which enjoys some Call-By-Push-Value features in a slightly different presentation.

Diagrammatic algebra: from linear to concurrent systems

We introduce the resource calculus, a string diagrammatic language for concurrent systems. Significantly, it uses the same syntax and operational semantics as the signal flow calculus --- an algebraic formalism for signal flow graphs, which is a combinatorial model of computation of interest in control theory. Indeed, our approach stems from the simple but fruitful observation that, by replacing real numbers (modelling signals) with natural numbers (modelling resources) in the operational semantics, concurrent behaviour patterns emerge. The resource calculus is canonical: we equip it and its stateful extension with equational theories that characterise the underlying space of definable behaviours---a convex algebraic universe of additive relations---via isomorphisms of categories. Finally, we demonstrate that our calculus is sufficiently expressive to capture behaviour definable by classical Petri nets.

Standardization in resource lambda-calculus

The resource calculus is an extension of the lambda-calculus allowing to model resource consumption. It is intrinsically non-deterministic and has two general notions of reduction - one parallel, preserving all the possible results as a formal sum, and one non-deterministic, performing an exclusive choice at every step. We prove that the non-deterministic reduction enjoys a notion of standardization, which is the natural extension with respect to the similar one in classical lambda-calculus. The full parallel reduction only enjoys a weaker notion of standardization instead. The result allows an operational characterization of may-solvability, which has been introduced and already characterized (from the syntactical and logical points of view) by Pagani and Ronchi Della Rocca.

Constructing differential categories and deconstructing categories of games

Differential categories were introduced by Blute, Cockett and Seely to axiomatize categorically Ehrhard and Regnierʼs syntactic differential operator. We present an abstract construction that takes a symmetric monoidal category and yields a differential category, and show how this construction may be applied to categories of games. In one instance, we recover the category previously used to give a fully abstract model of a nondeterministic imperative language. The construction exposes the differential structure already present in this model, and shows how the differential combinator may be encoded in the imperative language. The second instance corresponds to a new Cartesian differential category of games. We give a model of a simply-typed resource calculus, Resource PCF, in this category and show that it possesses the finite definability property. Comparison with a semantics based on Bucciarelli, Ehrhard and Manzonettoʼs relational model reveals that the latter also possesses this property and is fully abstract.

Full Abstraction for the Resource Lambda Calculus with Tests, through Taylor Expansion

We study the semantics of a resource-sensitive extension of the lambda calculus in a canonical reflexive object of a category of sets and relations, a relational version of Scott's original model of the pure lambda calculus. This calculus is related to Boudol's resource calculus and is derived from Ehrhard and Regnier's differential extension of Linear Logic and of the lambda calculus. We extend it with new constructions, to be understood as implementing a very simple exception mechanism, and with a "must" parallel composition. These new operations allow to associate a context of this calculus with any point of the model and to prove full abstraction for the finite sub-calculus where ordinary lambda calculus application is not allowed. The result is then extended to the full calculus by means of a Taylor Expansion formula. As an intermediate result we prove that the exception mechanism is not essential in the finite sub-calculus.

What is a categorical model of the differential and the resource λ-calculi?

The differential λ-calculus is a paradigmatic functional programming language endowed with a syntactical differentiation operator that allows the application of a program to an argument in a linear way. One of the main features of this language is that it is resource conscious and gives the programmer suitable primitives to handle explicitly the resources used by a program during its execution. The differential operator also allows us to write the full Taylor expansion of a program. Through this expansion, every program can be decomposed into an infinite sum (representing non-deterministic choice) of ‘simpler’ programs that are strictly linear.The aim of this paper is to develop an abstract ‘model theory’ for the untyped differential λ-calculus. In particular, we investigate what form a general categorical definition of a denotational model for this calculus should take. Starting from the work of Blute, Cockett and Seely on differential categories, we develop the notion of a Cartesian closed differential category and prove that linear reflexive objects living in such categories constitute sound and complete models of the untyped differential λ-calculus. We also give sufficient conditions for Cartesian closed differential categories to model the Taylor expansion. This requires that every model living in such categories equates all programs having the same full Taylor expansion.We then provide a concrete example of a Cartesian closed differential category modelling the Taylor expansion, namely the category MRel of sets and relations from finite multisets to sets. We prove that the extensional model of λ-calculus we have recently built in MRel is linear, and is thus also an extensional model of the untyped differential λ-calculus. In the same category, we build a non-extensional model and prove that it is, nevertheless, extensional on its differential part.Finally, we study the relationship between the differential λ-calculus and the resource calculus, which is a functional programming language combining the ideas behind the differential λ-calculus with those behind Boudol's λ-calculus with multiplicities. We define two translation maps between these two calculi and study the properties of these translations. In particular, this analysis shows that the two calculi share the same notion of a model, and thus that the resource calculus can be interpreted by translation into every linear reflexive object living in a Cartesian closed differential category.

A Mobile Collaboration Resource Calculus Model based on Ambient

A Mobile Collaboration Resource Calculus Model based on Ambient

Categorical Models for Simply Typed Resource Calculi

We introduce the notion of differential λ-category as an extension of Blute-Cockett-Seely's differential Cartesian categories. We prove that differential λ-categories can be used to model the simply typed versions of: (i) the differential λ-calculus, a λ-calculus extended with a syntactic derivative operator; (ii) the resource calculus, a non-lazy axiomatisation of Boudol's λ-calculus with multiplicities. Finally, we provide two concrete examples of differential λ-categories, namely, the category MRel of sets and relations, and the category MFin of finiteness spaces and finitary relations.

Linearity, Non-determinism and Solvability

We study the notion of solvability in the resource calculus, an extension of the λ-calculus modelling resource consumption. Since this calculus is non-deterministic, two different notions of solvability arise, one optimistic (angelical, may) and one pessimistic (demoniac, must). We give a syntactical, operational and logical characterization for the may-solvability and only a partial characterization of the must-solvability. Finally, we discuss the open problem of a complete characterization of the must-solvability.

Linearity, Non-determinism and Solvability

We study the notion of solvability in the resource calculus, an extension of the -calculus modelling resource consumption. Since this calculus is non-deterministic, two different notions of solvabi...

Algebra and logic for access control

Abstract The access control problem in computer security is fundamentally concerned with the ability of system entities to see, make use of, or alter various system resources. We provide a mathematical framework for modelling and reasoning about (distributed) systems with access control. This is based on a calculus of resources and processes together with a Hennessy–Milner-style modal logic, based on the connectives of bunched logic, for which an appropriate correspondence theorem obtains. As a consequence we get a consistent account of both operational behaviour and logical reasoning for systems with access control features. In particular, we are able to introduce a process combinator that describes, as a form of concurrent composition, the action of one agent in the role of another, and provide a logical characterization of this operator via a modality ‘says’. We give a range of examples, including analyses of co-signing, roles, and chains of trust, which illustrates the utility of our mathematical framework.

Uniformity and the Taylor expansion of ordinary lambda-terms

We define the complete Taylor expansion of an ordinary lambda-term as an infinite linear combination–with rational coefficients–of terms of a resource calculus similar to Boudol’s lambda-calculus with multiplicities (or with resources). In our resource calculus, all applications are (multi)linear in the algebraic sense, i.e. commute with linear combinations of the function or the argument. We study the collective behaviour of the beta-reducts of the terms occurring in the Taylor expansion of any ordinary lambda-term, using, in a surprisingly crucial way, a uniformity property that they enjoy. As a corollary, we obtain (the main part of) a proof that this Taylor expansion commutes with Böhm tree computation, syntactically.

Types for Access Control in a Calculus of Mobile Resources

This paper presents a type system for the calculus of Mobile Resources (MR) proposed by Godskesen et al. The type system is able to prevent undesirable border-crossing behaviour such as Trojan horses. This is achieved by combining the notion of group with a notion of security policy. Well-typed processes satisfy a safety property which is preserved under reduction. An algorithm is presented which computes the minimal security policy making a process well typed.

A Calculus and logic of resources and processes

Abstract Recent advances in logics for reasoning about resources provide a new approach to compositional reasoning in interacting systems. We present a calculus of resources and processes, based on a development of Milner’s synchronous calculus of communication systems, SCCS, that uses an explicit model of resource. Our calculus models the co-evolution of resources and processes with synchronization constrained by the availability of resources. We provide a logical characterization, analogous to Hennessy–Milner logic’s characterization of bisimulation in CCS, of bisimulation between resource processes which is compositional in the concurrent and local structure of systems.