Fixpoint Calculus Research Articles

Semantics for variational Quantum programming

We consider a programming language that can manipulate both classical and quantum information. Our language is type-safe and designed for variational quantum programming, which is a hybrid classical-quantum computational paradigm. The classical subsystem of the language is the Probabilistic FixPoint Calculus (PFPC), which is a lambda calculus with mixed-variance recursive types, term recursion and probabilistic choice. The quantum subsystem is a first-order linear type system that can manipulate quantum information. The two subsystems are related by mixed classical/quantum terms that specify how classical probabilistic effects are induced by quantum measurements, and conversely, how classical (probabilistic) programs can influence the quantum dynamics. We also describe a sound and computationally adequate denotational semantics for the language. Classical probabilistic effects are interpreted using a recently-described commutative probabilistic monad on DCPO. Quantum effects and resources are interpreted in a category of von Neumann algebras that we show is enriched over (continuous) domains. This strong sense of enrichment allows us to develop novel semantic methods that we use to interpret the relationship between the quantum and classical probabilistic effects. By doing so we provide a very detailed denotational analysis that relates domain-theoretic models of classical probabilistic programming to models of quantum programming.

Quantum CPOs

We introduce the monoidal closed category qCPO of quantum cpos, whose objects are "quantized" analogs of omega-complete partial orders (cpos). The category qCPO is enriched over the category CPO of cpos, and contains both CPO, and the opposite of the category FdAlg of finite-dimensional von Neumann algebras as monoidal subcategories. We use qCPO to construct a sound model for the quantum programming language Proto-Quipper-M (PQM) extended with term recursion, as well as a sound and computationally adequate model for the Linear/Non-Linear Fixpoint Calculus (LNL-FPC), which is both an extension of the Fixpoint Calculus (FPC) with linear types, and an extension of a circuit-free fragment of PQM that includes recursive types.

Automata and fixpoints for asynchronous hyperproperties

Hyperproperties have received increasing attention in the last decade due to their importance e.g. for security analyses. Past approaches have focussed on synchronous analyses, i.e. techniques in which different paths are compared lockstepwise. In this paper, we systematically study asynchronous analyses for hyperproperties by introducing both a novel automata model (Alternating Asynchronous Parity Automata) and the temporal fixpoint calculus H µ , the first fixpoint calculus that can systematically express hyperproperties in an asynchronous manner and at the same time subsumes the existing logic HyperLTL. We show that the expressive power of both models coincides over fixed path assignments. The high expressive power of both models is evidenced by the fact that decision problems of interest are highly undecidable, i.e. not even arithmetical. As a remedy, we propose approximative analyses for both models that also induce natural decidable fragments.

Mixed linear and non-linear recursive types

We describe a type system with mixed linear and non-linear recursive types called LNL-FPC (the linear/non-linear fixpoint calculus). The type system supports linear typing which enhances the safety properties of programs, but also supports non-linear typing as well which makes the type system more convenient for programming. Just like in FPC, we show that LNL-FPC supports type-level recursion which in turn induces term-level recursion. We also provide sound and computationally adequate categorical models for LNL-FPC which describe the categorical structure of the substructural operations of Intuitionistic Linear Logic at all non-linear types, including the recursive ones. In order to do so, we describe a new technique for solving recursive domain equations within the category CPO by constructing the solutions over pre-embeddings. The type system also enjoys implicit weakening and contraction rules which we are able to model by identifying the canonical comonoid structure of all non-linear types. We also show that the requirements of our abstract model are reasonable by constructing a large class of concrete models that have found applications not only in classical functional programming, but also in emerging programming paradigms that incorporate linear types, such as quantum programming and circuit description programming languages.

LNL-FPC: The Linear/Non-linear Fixpoint Calculus

We describe a type system with mixed linear and non-linear recursive types called LNL-FPC (the linear/non-linear fixpoint calculus). The type system supports linear typing, which enhances the safety properties of programs, but also supports non-linear typing as well, which makes the type system more convenient for programming. Just as in FPC, we show that LNL-FPC supports type-level recursion, which in turn induces term-level recursion. We also provide sound and computationally adequate categorical models for LNL-FPC that describe the categorical structure of the substructural operations of Intuitionistic Linear Logic at all non-linear types, including the recursive ones. In order to do so, we describe a new technique for solving recursive domain equations within cartesian categories by constructing the solutions over pre-embeddings. The type system also enjoys implicit weakening and contraction rules that we are able to model by identifying the canonical comonoid structure of all non-linear types. We also show that the requirements of our abstract model are reasonable by constructing a large class of concrete models that have found applications not only in classical functional programming, but also in emerging programming paradigms that incorporate linear types, such as quantum programming and circuit description programming languages.

Denotational semantics of recursive types in synthetic guarded domain theory

Just like any other branch of mathematics, denotational semantics of programming languages should be formalised in type theory, but adapting traditional domain theoretic semantics, as originally formulated in classical set theory to type theory has proven challenging. This paper is part of a project on formulating denotational semantics in type theories with guarded recursion. This should have the benefit of not only giving simpler semantics and proofs of properties such as adequacy, but also hopefully in the future to scale to languages with advanced features, such as general references, outside the reach of traditional domain theoretic techniques.Working inGuarded Dependent Type Theory(GDTT), we develop denotational semantics for Fixed Point Calculus (FPC), the simply typed lambda calculus extended with recursive types, modelling the recursive types of FPC using the guarded recursive types ofGDTT. We prove soundness and computational adequacy of the model inGDTTusing a logical relation between syntax and semantics constructed also using guarded recursive types. The denotational semantics is intensional in the sense that it counts the number of unfold-fold reductions needed to compute the value of a term, but we construct a relation relating the denotations of extensionally equal terms, i.e., pairs of terms that compute the same value in a different number of steps. Finally, we show how the denotational semantics of terms can be executed inside type theory and prove that executing the denotation of a boolean term computes the same value as the operational semantics of FPC.

Analyzing recursive programs using a fixed-point calculus

We show that recursive programs where variables range over finite domains can be effectively and efficiently analyzed by describing the analysis algorithm using a formula in a fixed-point calculus. In contrast with programming in traditional languages, a fixed-point calculus serves as a high-level programming language to easily, correctly, and succinctly describe model-checking algorithms While there have been declarative high-level formalisms that have been proposed earlier for analysis problems (e.g., Datalog the fixed-point calculus we propose has the salient feature that it also allows algorithmic aspects to be specified. We exhibit two classes of algorithms of symbolic (BDD-based) algorithms written using this framework-- one for checking for errors in sequential recursive Boolean programs, and the other to check for errors reachable within a bounded number of context-switches in a concurrent recursive Boolean program. Our formalization of these otherwise complex algorithms is extremely simple, and spans just a page of fixed-point formulae. Moreover, we implement these algorithms in a tool called Getafix which expresses algorithms as fixed-point formulae and evaluates them efficiently using a symbolic fixed-point solver called Mucke . The resulting model-checking tools are surprisingly efficient and are competitive in performance with mature existing tools that have been fine-tuned for these problems.

A fixpoint calculus for local and global program flows

We define a new fixpoint modal logic, the visibly pushdown μ-calculus (VP-μ), as an extension of the modal μ-calculus. The models of this logic are execution trees of structured programs where the procedure calls and returns are made visible. This new logic can express pushdown specifications on the model that its classical counterpart cannot, and is motivated by recent work on visibly pushdown languages [4]. We show that our logic naturally captures several interesting program specifications in program verification and dataflow analysis. This includes a variety of program specifications such as computing combinations of local and global program flows, pre/post conditions of procedures, security properties involving the context stack, and interprocedural dataflow analysis properties. The logic can capture flow-sensitive and inter-procedural analysis, and it has constructs that allow skipping procedure calls so that local flows in a procedure can also be tracked. The logic generalizes the semantics of the modal μ-calculus by considering summaries instead of nodes as first-class objects, with appropriate constructs for concatenating summaries, and naturally captures the way in which pushdown models are model-checked. The main result of the paper is that the model-checking problem for VP-μ is effectively solvable against pushdown models with no more effort than that required for weaker logics such as CTL. We also investigate the expressive power of the logic VP-μ: we show that it encompasses all properties expressed by a corresponding pushdown temporal logic on linear structures ( caret [2]) as well as by the classical μ-calculus. This makes VP-μ the most expressive known program logic for which algorithmic software model checking is feasible. In fact, the decidability of most known program logics (μ-calculus, temporal logics LTL and CTL, caret , etc.) can be understood by their interpretation in the monadic second-order logic over trees. This is not true for the logic VP-μ, making it a new powerful tractable program logic.

SPVT: An Efficient Verification Tool for Security Protocol

This paper describes the security protocol verifier SPVT developed by Objective-Caml. In SPVT (security protocol verifying tool), the specification language is the π-like calculus extended with three appendixes, the Dolev-Yao model is described with Horn logic rules, the π-like calculus model of security protocol is transformed into the logic program model by abstract rules, the security properties are verified based on the calculus of the logic program’s fixpoint, and the counter-examples on security properties are constructed from the process of the fixpoint calculus and the process of the property verification. The simplified Needham-Schroeder public-key authentication protocol is used to exemplify the automatic verification process of security protocol with SPVT, and the results show the validity of the verifier.

On Improving Backwards Verification of Timed Automata (Extended Abstract)

Verification techniques for Timed Automata [2] built in tools like Kronos [7] are based on the fixpoint calculus of an appropriate operator. In this work, we present different alternatives to calculate that fixpoint, which have direct impact in the number of iterations needed to converge.

A Reduction Result for Circumscribed Semi-Horn Formulas

Circumscription has been perceived as an elegant mathematical technique for modeling nonmonotonic and commonsense reasoning, but difficult to apply in practice due to the use of second-order formulas. One proposal for dealing with the computational problems is to identify classes of first-order formulas whose circumscription can be shown to be equivalent to a first-order formula. In previous work, we presented an algorithm which reduces certain classes of second-order circumscription axioms to logically equivalent first-order formulas. The basis for the algorithm is an elimination lemma due to Ackermann. In this paper, we capitalize on the use of a generalization of Ackermann's Lemma in order to deal with a subclass of universal formulas called semi-Horn formulas. Our results subsume previous results by Kolaitis and Papadimitriou regarding a characterization of circumscribed definite logic programs which are first-order expressible. The method for distinguishing which formulas are reducible is based on a boundedness criterion. The approach we use is to first reduce a circumscribed semi-Horn formula to a fixpoint formula which is reducible if the formula is bounded, otherwise not. In addition to a number of other extensions, we also present a fixpoint calculus which is shown to be sound and complete for bounded fixpoint formulas.

Fixed-point calculus

The aim of this paper is to present a small calculus of extreme fixed points and to show it in action. The fixed-point theorem that was the main incentive for writing this paper is the fusion theorem presented in Section 3. It exploits the calculational properties of Galois connections.