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09:00-10:40 Session 70D
Location: Maths LT1
Species, Profunctors and Taylor Expansion Weighted by SMCC--A Unified Framework for Modelling Nondeterministic, Probabilistic and Quantum Programs--

ABSTRACT. Motivated by a tight connection between Joyal's combinatorial species and quantitative models of linear logic, this paper introduces \emph{weighted generalised species} (or \emph{weighted profunctors}), where weights are morphisms of a given symmetric monoidal closed category (SMCC). For each SMCC W, we show that the category of W-weighted profunctors is a Lafont category, a categorical model of linear logic with exponential. As a model of programming languages, the construction of this paper gives a unified framework that induces adequate models of nondeterministic, probabilistic, algebraic and quantum programming languages by appropriately choosing the weight SMCC.

Logical paradoxes in quantum computation

ABSTRACT. The precise features of quantum theory enabling quantum computational advantage are unclear. Contextuality has emerged as a promising hypothesis: e.g. the magic states needed to practically achieve quantum computation are contextual.

Strong contextuality, as defined by Abramsky-Brandenburger, is an extremal form of contextuality describing systems that exhibit logically paradoxical behaviour.

After introducing number-theoretic techniques for constructing exotic quantum paradoxes, we give large families of strongly contextual magic states that are optimal in the sense that they enable deterministic injection of gates of the Clifford hierarchy. We thereby bolster a refinement of the resource theory of contextuality that emphasises the computational power of logical paradoxes.

A Complete Axiomatisation of the ZX-Calculus for Clifford+T Quantum Mechanics

ABSTRACT. We introduce the first complete and approximatively universal diagrammatic language for quantum mechanics. We make the ZX-Calculus, a diagrammatic language introduced by Coecke and Duncan, complete for the so-called Clifford+T quantum mechanics by adding two new axioms to the language. The completeness of the ZX-Calculus for Clifford+T quantum mechanics was one of the main open questions in categorical quantum mechanics. We prove the completeness of the Clifford+T ZX-Calculus using the recently studied ZW-Calculus, a calculus dealing with integer matrices. We also prove that this fragment of the ZX-Calculus represents exactly all the matrices over some finite dimensional extension of the ring of dyadic rationals.

Two complete axiomatisations of pure-state qubit quantum computing
SPEAKER: Kang Feng Ng

ABSTRACT. Categorical quantum mechanics places finite-dimensional quantum theory in the context of compact closed categories, with an emphasis on diagrammatic reasoning. In this framework, two equational diagrammatic calculi have been proposed for pure-state qubit quantum computing: the ZW calculus, developed by Coecke, Kissinger and the first author for the purpose of qubit entanglement classification, and the ZX calculus, introduced by Coecke and Duncan to give an abstract description of complementary observables. Neither calculus, however, provided a complete axiomatisation of their model.

In this paper, we present extended versions of ZW and ZX, and show their completeness for pure-state qubit theory, thus solving two major open problems in categorical quantum mechanics. First, we extend the original ZW calculus to represent states and linear maps with coefficients in an arbitrary commutative ring, and prove completeness by a strategy that rewrites all diagrams into a normal form. We then extend the language and axioms of the original ZX calculus, and show their completeness for pure-state qubit theory through a translation between ZX and ZW specialised to the field of complex numbers. This translation expands the one used by Jeandel, Perdrix, and Vilmart to derive an axiomatisation of the approximately universal Clifford+T fragment; restricting the field of complex numbers to a suitable subring, we obtain an alternative axiomatisation of the same theory.

Diagrammatic Reasoning beyond Clifford+T Quantum Mechanics

ABSTRACT. The ZX-Calculus is a graphical language for diagrammatic reasoning in quantum mechanics and quantum information theory. An axiomatisation has recently been proven to be complete for an approximatively universal fragment of quantum mechanics, the so-called Clifford+T fragment. We focus here on the expressive power of this axiomatisation beyond Clifford+T Quantum mechanics. We consider the full pure qubit quantum mechanics, and mainly prove two results: (i) First, the axiomatisation for Clifford+T quantum mechanics is also complete for all equations involving some kind of linear diagrams. The linearity of the diagrams reflects the phase group structure, an essential feature of the ZX-calculus. In particular all the axioms of the ZX-calculus are involving linear diagrams. (ii) We also show that the axiomatisation for Clifford+T is not complete in general but can be completed by adding a single (non linear) axiom, providing a simpler axiomatisation of the ZX-calculus for pure quantum mechanics than the one recently introduced by Ng&Wang.

09:00-10:40 Session 70E
Location: Maths LT3
Quantitative Behavioural Reasoning for Higher-order Effectful Programs: Applicative Distances

ABSTRACT. This paper studies the quantitative refinement of Absramsky's applicative similarity and bisimilarity in the context of a generalisation of Fuzz, a call-by-value $\lambda$-calculus with a linear type system that can express program sensitivity, enriched with algebraic operations \emph{\`a la} Plotkin and Power. To do so a general, abstract framework for studying behavioural relations taking values over quantales is defined according to Lawvere's analysis of generalised metric spaces. Barr's notion of relator (or lax extension) is then extended to quantale-valued relations adapting and extending results from the field of monoidal topology. Abstract notions of quantale-valued effectful applicative similarity and bisimilarity are then defined and proved to be a compatible generalised metric (in the sense of Lawvere) and pseudometric, respectively, under mild conditions.

A Generalized Modality for Recursion

ABSTRACT. Nakano's *later* modality allows types to express that the output of a function does not immediately depend on its input, and thus that computing its fixpoint is safe. This idea, guarded recursion, has proved useful in various contexts, from functional programming with infinite data structures to formulations of step-indexing internal to type theory. Categorical models have revealed that the later modality corresponds in essence to a simple reindexing of the discrete time scale.

Unfortunately, existing guarded type theories suffer from significant limitations for programming purposes. These limitations stem from the fact that the later modality is not expressive enough to capture precise input-output dependencies of functions. As a consequence, guarded type theories reject many productive definitions. Combining insights from guarded type theories and synchronous programming languages, we propose a new modality for guarded recursion. % This modality can apply to a type any sufficiently well-behaved reindexing of the time scale. We call such reindexings *time warps*. Several modalities from the literature, including later, correspond to fixed time warps, and thus arise as special cases of ours.

We integrate our modality into a typed lambda-calculus. We equip this calculus with an operational semantics, as well as an adequate denotational semantics in the topos of trees, a standard categorical model for guarded recursion. Building on top of categorical ideas, we describe an abstract type-checking algorithm whose completeness entails the coherence of both semantics.

A functional interpretation with state

ABSTRACT. We present a new variant of Goedel's functional interpretation in which extracted programs, rather than being pure terms of system T, interact with a global state. The purpose of the state is to store relevant information about the underlying mathematical environment. Because the validity of extracted programs can depend on the validity of the state, this offers us an alternative way of dealing with the contraction problem. Furthermore, this new formulation of the functional interpretation gives us a clear semantic insight into the computational content of proofs, and provides us with a way of improving the efficiency of extracted programs.

The Geometry of Computation-Graph Abstraction

ABSTRACT. The popular library TensorFlow (TF) has familiarised the mainstream of machine-learning community with programming language concepts such as dataflow computing and automatic differentiation. Additionally, it has introduced some genuinely new syntactic and semantic programming concepts. In this paper we study one such new concept. It is a subtle but important feature of TensorFlow, the ability to extract and manipulate the state of a computation graph. This feature allows the convenient specification of parameterised models by freeing the programmer of much of the bureaucratic burden of managing them, while still permitting the use of generic, model-independent, search and optimisation algorithms. We study this new language feature, which we call `graph abstraction' in the context of the call-by-value lambda calculus, using the recently developed Dynamic Geometry of Interaction formalism. We give a simple type system guaranteeing the safety of graph abstraction, and we also show the safety of critical language properties such as garbage collection and the beta law. The semantic model also suggests that the feature could be implemented in a general-purpose functional language reasonably efficiently.

Degrees of Relatedness - A Unified Framework for Parametricity, Irrelevance, Ad Hoc Polymorphism, Intersections, Unions and Algebra in Dependent Type Theory
SPEAKER: Andreas Nuyts

ABSTRACT. Dependent type theory allows us to write programs and to prove properties about those programs in the same language. However, some properties do not require much proof, as they are evident from a program's implementation, e.g. if a polymorphic program is not ad hoc but relationally parametric, then we get parametricity theorems for free. If we want to safely shortcut proofs by relying on the evident good behaviour of a program, then we need a type-level guarantee that the program is indeed well-behaved. This can be achieved by annotating function types with a modality describing the behaviour of functions.

We consider a dependent type system with modalities for relational parametricity, irrelevance (i.e. type-checking time erasability of an argument) and ad hoc polymorphism. The interplay of three modalities and dependent types raises a number of questions. For example: If a term depends on a variable with a given modality, then how should its type depend on it? Are all modalities always applicable, e.g. should we consider parametric functions from the booleans to the naturals? Do we need any further modalities in order to properly reason about these three?

We develop a type system, based on a denotational presheaf model, that answers these questions. The core idea is to equip every type with a number of relations --- just equality for small types, but more for larger types --- and to describe function behaviour by saying how functions act on those relations. The system has modality-aware equality judgements (ignoring irrelevant parts) and modality-aware proving operators (for proving free theorems) which are even self-applicable. It also supports sized types, some form of intersection and union types, and parametric quantification over algebraic structures. We prove soundness in a denotational presheaf model.

10:30-11:00Coffee Break
11:00-12:00 Session 74D: LICS Invited Talk: Val Tannen
Location: Maths LT1
Provenance Analysis for First-order Model Checking

ABSTRACT. Is a given finite structure a model of a given first-order sentence? The provenance analysis of this question determines how its answer depends on the atomic facts that determine the structure. Provenance questions like this one have emerged in databases, scientific workflows, networks, formal verification, and other areas.

In joint work with Erich Grädel (RWTH Aachen University), done during the Logical Structures in Computation Program at the Simons Institute, we extend the semiring provenance framework, developed in databases, to the first-order model checking problem. This provides a non-standard semantics for first-order logic that refines logical truth to values in commutative semirings: a semiring of provenance polynomials, the Viterbi semiring of confidence scores, access control semirings, etc. We discuss applications and extensions of this framework.

12:00-12:40 Session 75A
Location: Maths LT1
On computability and tractability for infinite sets

ABSTRACT. We propose a definition for computable functions on hereditarily definable sets. Such sets are possibly infinite data structures that can be defined using a fixed underlying logical structure, such as (N,=). We show that, under suitable assumptions on the underlying structure, a programming language called definable while programs captures exactly the computable functions. Next, we introduce a complexity class called fixed-dimension polynomial time, which intuitively speaking describes polynomial computation on hereditarily definable sets. We show that this complexity class contains all functions computed by definable while programs with suitably defined resource bounds. Proving the converse inclusion would prove that Choiceless Polynomial Time with Counting captures order-invariant polynomial time on finite graphs.

Definable Ellipsoid Method, Sums-of-Squares Proofs, and the Isomorphism Problem

ABSTRACT. The ellipsoid method is an algorithm that solves the (weak) feasibility and linear optimization problems for convex sets by making oracle calls to their (weak) separation problem. We observe that the previously known method to show that this reduction can be done in fixed-point logic with counting (FPC) for linear and semidefinite programs applies to any family of explicitly bounded convex sets. We use this observation to show that the exact feasibility problem for semidefinite programs is expressible in the infinitary version of FPC. As a corollary we get that, for the isomorphism problem, the Lasserre/Sums-of-Squares semidefinite programming hierarchy of relaxations collapses to the Sherali-Adams linear programming hierarchy, up to a small loss in the degree.

12:30-14:00Lunch Break
14:00-15:40 Session 76E
Location: Maths LT1
Computable decision making on the reals and other spaces via partiality and nondeterminism

ABSTRACT. Though many safety-critical software systems use floating point to represent real-world input and output, programmers usually have idealized versions in mind that compute with real numbers. Significant deviations from the ideal can cause errors and jeopardize safety. Some programming systems implement exact real arithmetic, which resolves this matter but complicates others, such as decision making. In these systems, it is impossible to compute (total and deterministic) discrete decisions based on connected spaces such as R. We present programming-language semantics based on constructive topology with variants allowing nondeterminism and/or partiality. Either nondeterminism or partiality suffices to allow computable decision making on connected spaces such as R. We then introduce pattern matching on spaces, a language construct for creating programs on spaces, generalizing pattern matching in functional programming, where patterns need not represent decidable predicates and also may overlap or be inexhaustive, giving rise to nondeterminism or partiality, respectively. Nondeterminism and/or partiality also yield formal logics for constructing approximate decision procedures. We implemented these constructs in the Marshall language for exact real arithmetic.

A Theory of Register Monitors

ABSTRACT. The task of a monitor is to watch, at run-time, the execution of a reactive system, and signal the occurrence of an error in the observed sequence of events. While finite-state monitors have been studied extensively, in practice, monitoring software also makes use of unbounded memory. We define a model of automata equipped with integer-valued registers which can execute only a bounded number of instructions between consecutive events, and thus can form the theoretical basis for the study of infinite-state monitors. We classify these {\em register monitors} according to the number $k$ of available registers, and the type of register instructions. In stark contrast to the theory of computability for register machines, we prove that for every $k>1$, monitors with $k+1$ counters (with instruction set $\langle +1,= \rangle$) are strictly more expressive than monitors with $k$ counters: there is a safety $\omega$-language $L_k$ such that the finite prefixes not in $L_k$ can be recognized on-line with $k+1$ but not with $k$ counters. We also show that adder monitors (with instruction set $\langle 1,+,= \rangle$) are strictly more expressive than counter monitors, but are complete for monitoring all computable safety $\omega$-languages for $k=6$. {\em Real-time monitors} are further required to signal the occurrence of an error as soon as it occurs. The expressiveness hierarchy for counter monitors carries over to real-time monitors. We then show that 2 adders cannot simulate 3 counters in real-time. Finally, we show that real-time adder monitors with inequalities are at least as expressive as the model of real-time Turing machines studied in the 1960s.

Efficient Algorithms for Asymptotic Bounds on Termination Time in VASS
SPEAKER: Dominik Velan

ABSTRACT. Vector Addition Systems with States (VASS) provide a well-known and fundamental model for the analysis of concurrent processes, parametrized systems, and are also used as abstract models of programs in resource bound analysis. In this paper we study the problem of obtaining asymptotic bounds on the termination time of a given VASS. In particular, we focus on the practically important case of obtaining polynomial bounds on termination time.

Our main contributions are as follows: First, we present a polynomial-time algorithm for deciding whether a given VASS has a linear asymptotic complexity. We also show that if a complexity of a VASS is not linear, it is at least quadratic. Second, we classify VASS according to quantitative properties of their cycles. We show that certain singularities in these properties are the key reason for non-polynomial asymptotic complexity of VASS. In absence of singularities, we show that the asymptotic complexity is always polynomial and of the form $\Theta(n^k)$, for some integer $k\leq d$. We present a polynomial-time algorithm computing the optimal $k$. For general VASS, the same algorithm, which is based on a complete technique for the construction of ranking functions in VASS, produces a valid lower bound, i.e. a $k$ such that the termination complexity is $\Omega(n^k)$. Our results are based on new insights into the geometry of VASS dynamics, which hold the potential for further applicability to VASS analysis.

A Hybrid, Dynamic Logic for Hybrid-Dynamic Information Flow

ABSTRACT. Information-flow security is important to the safety and privacy of cyber-physical systems (CPSs) across many domains: information leakage can both violate user privacy and provide information that supports further attacks. CPSs face the challenge that information can flow both in discrete cyber channels and in continuous real-valued physical channels ranging from time to physical flow of resources. We call these hybrid information flows and introduce dHL, the first logic for verifying these flows in hybrid-dynamical models of CPSs. We achieve verification of hybrid information flows by extending differential dynamic logic (dL) for hybrid-dynamical systems with hybrid-logical features for explicit representation and relation of program states. By verifying hybrid information flows, we ensure security even under a strong attacker model wherein an attacker can observe time and physical values continuously. We present a Hilbert-style proof calculus for dHL, prove it sound, and compare the expressive power of dHL with dL. We demonstrate dHL's abilities by developing a hybrid system model of the smart electrical grid FREEDM. We verify that the naive model has a previously-unknown information flow vulnerability and verify that a revised model resolves the vulnerability. To the best of our knowledge, this is both the first information flow proof for hybrid information flows and the first for a hybrid-dynamical model. We discuss applications of hybrid information flow to a range of critical systems.

Differential Equation Axiomatization: The Impressive Power of Differential Ghosts

ABSTRACT. We prove the completeness of an axiomatization for differential equation invariants. First, we show that the differential equation axioms in differential dynamic logic are complete for all algebraic invariants. Our proof exploits differential ghosts, which introduce additional variables that can be chosen to evolve freely along new differential equations. Cleverly chosen differential ghosts are the proof-theoretical counterpart of dark matter. They create new hypothetical state, whose relationship to the original state variables satisfies invariants that did not exist before. The reflection of these new invariants in the original system enables its analysis.

We then show that extending the axiomatization with existence and uniqueness axioms makes it complete for all local progress properties, and further extension with a real induction axiom makes it complete for all real arithmetic invariants. This yields a parsimonious axiomatization, which serves as the logical foundation for reasoning about invariants of differential equations. Moreover, our approach is purely axiomatic, and so the axiomatization is suitable for sound implementation in foundational theorem provers.

14:00-15:40 Session 76F
Location: Maths LT3
Probabilistic stable functions on discrete cones are power series.

ABSTRACT. We study the category Cstabm of measurable cones and measurable stable functions—a denotational model of an higher-order language with continuous probabilities and full recursion. We look at Cstabm as a model for discrete probabilities, by showing the existence of a full, faithful and cartesian closed functor which embeds probabilistic coherence spaces—a fully abstract denotational model of an higher language with full recursion and discrete probabilities—into Cstabm. The proof is based on a generalization of Bernstein’s theorem in real analysis allowing to see stable functions between some cones as generalized power series.

Allegories: decidability and graph homomorphisms

ABSTRACT. Allegories were introduced by Freyd and Scedrov; they form a fragment of Tarski’s calculus of relations. We show that their equational theory is decidable by characterising it in terms of a specific class of graph homomorphisms. We actually do so for an extension of allegories which we prove to be conservative, namely allegories with top. This generalisation makes it possible to exploit the correspondence between terms and K4-free graphs, for which isomorphism was known to be finitely axiomatisable.

Syntax and Semantics for Operations with Scopes
SPEAKER: Maciej Piróg

ABSTRACT. Motivated by the problem of separating syntax from semantics in programming with algebraic effects and handlers, we propose a categorical model of abstract syntax with so-called scoped operations. As a building block of a term, a scoped operation is not merely a node in a tree, as it can also encompass a whole part of the term (a scope). Some examples from the area of programming are given by the operation "catch" for handling exceptions, in which the part in the scope is the code that may raise an exception, or the operation "once", which selects a single solution from a nondeterministic computation. A distinctive feature of such operations is their behaviour under program composition, that is, syntactic substitution.

Our model is based on what Ghani et al. call the monad of explicit substitutions, defined using the initial-algebra semantics in the category of endofunctors. We also introduce a new kind of multi-sorted algebras, called scoped algebras, which serve as interpretations of syntax with scopes. In generality, scoped algebras are given in the style of the presheaf formalisation of syntax with binders of Fiore et al. As the main technical result, we show that our monad indeed arises from free objects in the category of scoped algebras.

Importantly, we show that our results are immediately applicable. In particular, we show a Haskell implementation together with some practical, real-life examples.

Rewriting with Frobenius
SPEAKER: Fabio Zanasi

ABSTRACT. Symmetric monoidal categories have become ubiquitous as a formal environment for the analysis of compound systems in a compositional, resource-sensitive manner using the graphical syntax of string diagrams. Recently, reasoning with string diagrams has been implemented concretely via double-pushout (DPO) hypergraph rewriting. The hypergraph representation has the twin advantages of being convenient for mechanisation and of completely absorbing the structural laws of symmetric monoidal categories, leaving just the domain-specific equations explicit in the rewriting system. In many applications across different disciplines (linguistics, concurrency, quantum computation, control theory, ...) the structural component appears to be richer than just the symmetric monoidal structure, as it includes one or more Frobenius algebras. In this work we develop a DPO rewriting formalism which is able to absorb multiple Frobenius structures, thus sensibly simplifying diagrammatic reasoning in the aforementioned applications. As a proof of concept, we use our formalism to describe an algorithm which computes the reduced form of a diagram of the theory of interacting bialgebras using a simple rewrite strategy.

Ribbon tensorial logic

ABSTRACT. We introduce a topologically-aware version of tensorial logic, called ribbon tensorial logic. To every proof of the logic, we associate a ribbon tangle which tracks the flow of tensorial negations inside the proof. The translation is functorial: it is performed by exhibiting a correspondence between the notion of dialogue category in proof theory and the notion of ribbon category in knot theory. Our main result is that the translation is also faithful: two proofs are equal modulo the equational theory of tensorial logic if and only if the associated ribbon tangles are equal up to topological deformation. This ``proof-as-tangle'' theorem may be understood at the same time as a coherence theorem for ribbon dialogue categories, and as a mathematical foundation for topological game semantics.

15:30-16:00Coffee Break
16:00-17:00 Session 78C: LICS Invited Talk: Javier Esparza
Location: Maths LT1
Verification of population protocols

ABSTRACT. Population protocols (Angluin et al., PODC 2004) are a formal model of sensor networks consisting of identical, finite-state mobile devices. When two devices come into the range of each other, they interact and change their state. Computations are infinite sequences of pairwise interactions where the interacting processes are picked by a fair scheduler.

A population protocol is well specified if for every initial configuration of devices, and for every fair computation starting at it, all devices eventually agree on a consensus value that only depends on the initial configuration. If a protocol is well specified, then it is said to compute the predicate that assigns to each initial configuration its consensus value.

In the talk I will report on our recent results on automatic verification of population protocols: Is a given protocol well specified? Does a given protocol compute a given predicate? Does it compute it within a given time? These questions require to check liveness properties of parameterized systems with an arbitrary number of agents, and are very challenging. I show how to attack them using the theory of Vector Addition Systems and SMT technology.

17:00-17:45 Session 81B: Alonzo Church Award Session
Location: Maths LT1
The 2018 Alonzo Church Award for Outstanding Contributions to Logic and Computation
Constraints, Graphs, Algebra, Logic, and Complexity

ABSTRACT. This is the Acceptance Speech for the 2018 Alonzo Church Award for Outstanding Contributions to Logic and Computation, given to Moshe Vardi and Tomás Feder in recognition of their “fundamental contributions to the computational complexity of constraint-satisfaction problems.” Eligible for the award are the authors of a paper or series of papers published within the past 25 years. Their research resulted in the formulation of the Feder-Vardi Dichotomy Conjecture, which culminated in 2017 in two independent proofs, by Andrei Bulatov and Dmitriy Zhuk.

The award was established in 2015 by the ACM Special Interest Group for Logic and Computation, the European Association for Theoretical Computer Science, the European Association for Computer Science Logic and the Kurt Gödel Society.

This year’s award is given for two papers written by Vardi and Feder: “Monotone Monadic SNP and Constraint Satisfaction,” delivered to the ACM Symposium on the Theory of Computing, 1993; and “The Computational Structure of Monotone Monadic SNP and Constraint Satisfaction: A Study through Datalog and Group Theory,” published in the SIAM Journal of Computing in 1998.