Soft Linear Logic Research Articles

Bounded Combinatory Logic and lower complexity

We introduce a stratified version of Combinatory Logic1 in which there are two classes of terms called player and opponent such that the class of player terms is strictly contained in the class of opponent terms. We show that the system characterizes Polynomial Time in a similar way to Soft Linear Logic. With the addition of explicit “lazy” products to the player terms and various notions of distributivity, we obtain a characterization of Polynomial Space. This paper is an expanded version of the abstract presented at DICE 2013.

On session types and polynomial time

We show how systems of session types can enforce interactions to take bounded time for all typable processes. The type system we propose is based on Lafont's soft linear logic and is strongly inspired by recent works about session types as intuitionistic linear logic formulas. Our main result is the existence, for every typable process, of a polynomial bound on the length of reduction sequences starting from it and on the size of its reducts.

Light logics and higher-order processes

We show that the techniques for resource control that have been developed by the so-calledlight logicscan be fruitfully applied also to process algebras. In particular, we present a restriction of higher-order π-calculus inspired by soft linear logic. We prove that any soft process terminates in polynomial time. We argue that the class of soft processes may be naturally enlarged so that interesting processes are expressible, still maintaining the polynomial bound on executions.

An Implicit Characterization of PSPACE

We present a type system for an extension of lambda calculus with a conditional construction, named STA B , that characterizes the PSPACE class. This system is obtained by extending STA, a type assignment for lambda-calculus inspired by Lafont’s Soft Linear Logic and characterizing the PTIME class. We extend STA by means of a ground type and terms for Booleans and conditional. The key issue in the design of the type system is to manage the contexts in the rule for conditional in an additive way. Thanks to this rule, we are able to program polynomial time Alternating Turing Machines. From the well-known result APTIME = PSPACE, it follows that STA B is complete for PSPACE. Conversely, inspired by the simulation of Alternating Turing machines by means of Deterministic Turing machine, we introduce a call-by-name evaluation machine with two memory devices in order to evaluate programs in polynomial space. As far as we know, this is the first characterization of PSPACE that is based on lambda calculus and light logics.

Soft Session Types

We show how systems of session types can enforce interactions to be bounded for all typable processes. The type system we propose is based on Lafont's soft linear logic and is strongly inspired by recent works about session types as intuitionistic linear logic formulas. Our main result is the existence, for every typable process, of a polynomial bound on the length of any reduction sequence starting from it and on the size of any of its reducts.

Light Logics and Higher-Order Processes

We show that the techniques for resource control that have been developed in the so-called "light logics" can be fruitfully applied also to process algebras. In particular, we present a restriction of Higher-Order pi-calculus inspired by Soft Linear Logic. We prove that any soft process terminates in polynomial time. We argue that the class of soft processes may be naturally enlarged so that interesting processes are expressible, still maintaining the polynomial bound on executions.

Context semantics, linear logic, and computational complexity

We show that context semantics can be fruitfully applied to the quantitative analysis of proof normalization in linear logic. In particular, context semantics lets us define the weight of a proof-net as a measure of its inherent complexity: it is both an upper bound to normalization time (modulo a polynomial overhead, independently on the reduction strategy) and a lower bound to the amount of resources needed to compute the normal form. Weights are then exploited in proving strong soundness theorems for various subsystems of linear logic, namely elementary linear logic, soft linear logic, and light linear logic.

Quantum implicit computational complexity

We introduce a quantum lambda calculus inspired by Lafont’s Soft Linear Logic and capturing the polynomial quantum complexity classes EQP, BQP and ZQP. The calculus is based on the “classical control and quantum data” paradigm. This is the first example of a formal system capturing quantum complexity classes in the spirit of implicit computational complexity — it is machine-free and no explicit bound (e.g., polynomials) appears in its syntax.

From light logics to type assignments: a case study

Using Soft Linear Logic (SLL) as case study, we analyze a method for transforming a light logic into a type assignment system for the λ-calculus, inheriting the complexity properties of the logics. Namely the typing assures the strong normalization in a number of steps polynomial in the size of the term, and moreover all polynomial functions can be computed by λ-terms that can be typed in the system. The proposed method is general enough to be used also for other light logics.

Soft Linear Set Theory

A formulation of naïve set theory is given in Lafont’s Soft Linear Logic, a logic with polynomial time cut-elimination. We demonstrate that the provably total functions of this set theory are precisely the PTIME functions. A novelty of this approach is the representation of the unary/binary natural numbers by two distinct sets (the safe naturals and the soft naturals).

Soft Linear Logic and Polynomial Complexity Classes

We describe some results inspired to Lafont's Soft Linear Logic (SLL) which is a subsystem of second-order linear logic with restricted rules for exponentials, correct and complete for polynomial time computations. SLL is the basis for the design of type assignment systems for lambda-calculus, characterizing the complexity classes PTIME, PSPACE and NPTIME. PTIME is characterized by a type assignments system where types are a proper subset of SLL formulae. The characterization consists in the fact that a well typed term can be reduced to normal form by a number of beta-reductions polynomial in its lenght, and moreover all polynomial time functions can be computed by well typed terms. PSPACE is characterized by a type assignment system obtained from the previous one, by extending the set of types by a type for booleans, and the lambda-calculus by two boolean constants and a conditional constructor. The system assigns types to terms in such a way that the evaluation of programs (closed terms of type boolean) can be performed carefully in polynomial space. Moreover all polynomial space decision problems can be computed by terms typable in this system. In order to characterize NPTIME we extend the lambda-calculus by a nondeterministic choice operator, and the system by a rule for dealing with this new term constructor.

A logical account of pspace

We propose a characterization of PSPACE by means of atype assignment for an extension of lambda calculus with a conditional construction. The type assignment STA B is an extension of STA, a type assignment for lambda-calculus inspired by Lafont's Soft Linear Logic. We extend STA by means of a ground type and terms for booleans. The key point is that the elimination rule for booleans is managed in an additive way. Thus, we are able to program polynomial time Alternating Turing Machines. Conversely, we introduce a call-by-name evaluation machine in order tocompute programs in polynomial space. As far as we know, this is the first characterization of PSPACE which is based on lambda calculusand light logics.

Towards a theory of resource: an approach based on soft exponentials

To express fine-grained resource-sensitive reasoning, a temporal soft linear logic (TSLL) is introduced as an extension of both Girard's (propositional classical) linear logic (CLL) and Lafont's (propositional classical) soft linear logic (SLL). It is known that the linear exponential operator in CLL can express a specific infinitely reusable resource, i.e. it is reusable not only for any number, but also many times. In contrast, the soft exponential operator in SLL, which is a weak version of the linear exponential operator, can express a specific usable resource, i.e. it is usable in any number, but only once (i.e. it is consumed after use). In TSLL, the resource operators (i.e. linear and soft exponentials) and some temporal operators are combined based on the interpretation that “time” is regarded as a “resource”. The completeness theorem (with respect to phase semantics) for TSLL and the cut-elimination and decidability theorems for some subsystems of TSLL are proved as the main results of this paper. A decidable subsystem, called a bounded soft linear logic (BSLL), which has a restricted soft exponential operator, can represent a specific finitely usable resource.

Combining Soft Linear Logic and Spatio-temporal Operators

A new logic, spatio-temporal soft linear logic, which has soft linear exponential, linear exponential and spatio-temporal operators, is introduced as a sequent calculus. A Petri net interpretation and an informational interpretation for this logic are given by using simple complete semantics. By using this logic and these interpretations, various kinds of spatio-temporal and resource-sensitive reasoning can be expressed.

Phase semantics and decidability of elementary affine logic

Light, elementary and soft linear logics are formal systems derived from Linear Logic, enjoying remarkable normalization properties. In this paper, we prove decidability of Elementary Affine Logic, EAL . The result is obtained by semantical means, first defining a class of phase models for EAL and then proving soundness and (strong) completeness, following Okada's technique. Phase models for Light Affine Logic and Soft Linear Logic are also defined and shown complete.

Soft linear logic and polynomial time

We present a subsystem of second-order linear logic with restricted rules for exponentials so that proofs correspond to polynomial time algorithms, and vice versa.

Phase semantics and decidability of elementary affine logic

Light, elementary and soft linear logics are formal systems derived from Linear Logic, enjoying remarkable normalization properties. In this paper, we prove decidability of Elementary Affine Logic, EAL . The result is obtained by semantical means, first defining a class of phase models for EAL and then proving soundness and (strong) completeness, following Okada's technique. Phase models for Light Affine Logic and Soft Linear Logic are also defined and shown complete.

Soft linear logic and polynomial time

We present a subsystem of second-order linear logic with restricted rules for exponentials so that proofs correspond to polynomial time algorithms, and vice versa.