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09:00-10:30 Session 46: FLoC Plenary Lecture: Peter O'Hearn
Location: Maths LT1
Continuous Reasoning: Scaling the Impact of Formal Methods

ABSTRACT. Formal reasoning about programs is one of the oldest and most fundamental research directions in computer science. It has also been one of the most elusive. There has been a tremendous amount of valuable research in formal  methods, but rarely have formal reasoning techniques been deployed as part of the development process of large industrial codebases.


This talk describes work in continuous reasoning, where formal reasoning about a (changing) codebase is done in a fashion which mirrors the iterative, continuous model of software development that is increasingly practiced in industry. We suggest that advances in continuous reasoning will allow formal reasoning to scale to more programs, and more programmers. We describe our experience using continuous reasoning with large, rapidly changing codebases at Facebook, and we describe open problems and directions for research for the scientific community.


This a paper with the same title accompanying this talk appears in the LICS’18 proceedings.

10:30-11:00Coffee Break
11:00-12:30 Session 47B: Linear Logic
Proof nets for bi-intuitionistic linear logic

ABSTRACT. Bi-Intuitionistic Linear Logic (BILL) is an extension of Intuitionistic Linear Logic with a par, dual to the tensor, and subtraction, dual to linear implication. It is the logic of categories with a monoidal closed and a monoidal co-closed structure that are related by linear distributivity, a strength of the tensor over the par. It conservatively extends Full Intuitionistic Linear Logic (FILL), which includes only the par.

We give proof nets for the multiplicative, unit-free fragment MBILL-. Correctness is by local rewriting in the style of Danos contractibility. This rewrite relation yields sequentialization into a relational sequent calculus that extends the existing one for FILL. We give a second, geometric correctness condition via Danos-Regnier switching, and demonstrate composition both inductively and as a one-off global operation.

Unique perfect matchings and proof nets

ABSTRACT. This paper establishes a bridge between linear logic and mainstream graph theory, building previous work by Retoré (2003). We show that the problem of correctness for MLL+Mix proof nets is equivalent to the problem of uniqueness of a perfect matching. By applying matching theory, we obtain new results for MLL+Mix proof nets: a linear-time correctness criterion, a quasi-linear sequentialization algorithm, and a characterization of the sub-polynomial complexity of the correctness problem. We also use graph algorithms to compute the dependency relation of Bagnol et al. (2015) and the kingdom ordering of Bellin (1997), and relate them to the notion of blossom which is central to combinatorial maximum matching algorithms.

Lifting Coalgebra Modalities and IMELL Model Structure to Eilenberg-Moore Categories

ABSTRACT. A categorical model of the multiplicative and exponential fragments of intuitionistic linear logic (IMELL), known as a linear category, is a symmetric monoidal closed category with a monoidal coalgebra modality (also known as a linear exponential comonad). Inspired by R. Blute and P. Scott's work on categories of modules of Hopf algebras as models of linear logic, we study Eilenberg-Moore categories of monads as models of IMELL. We define an IMELL lifting monad on a linear category as a Hopf monad -- in the Bruguieres, Lack, and Virelizier sense --  with a mixed distributive law over the monoidal coalgebra modality. As our main result, we show that the linear category structure lifts to Eilenberg-Moore categories of IMELL lifting monads. We explain how monoids in the Eilenberg-Moore of the monoidal coalgebra modality can induce IMELL lifting monads and provide sources for such monoids. Along the way, we also define mixed distributive laws of bimonads over coalgebra modalities and lifting differential category structure to Eilenberg-Moore categories of exponential lifting monads. 

12:30-14:00Lunch Break
14:00-15:00 Session 49B: FSCD Invited talk: Peter Selinger
Challenges in quantum programming languages

ABSTRACT. In this talk, I will give an overview of some recent progress and current challenges in the design of quantum programming languages. Unlike classical programs, which can in principle be debugged by stopping the program at critical moments and examining the contents of variables, quantum programs are not amenable to traditional debugging because the state of a quantum system cannot usually be examined in a meaningful way. Therefore, we need other methods for ensuring the correctness of quantum programs, such as formal verification. For this reason, I advocate the use of strongly typed, functional programming languages for quantum computing. As far as functional quantum programming languages are concerned, there is currently a relatively wide gap between theory and practice. On the one hand, we have languages with strong theoretical foundations, such as the quantum lambda calculus, which operate at a relatively low level of abstraction and lack many features that would be useful to practical quantum programmers. On the other hand, we have practical functional quantum programming languages such as Quipper, which is implemented as an embedded language in Haskell, has many high-level features, and has been used in large-scale projects, but lacks a theoretical basis and a strong type system. We have recently attempted to narrow this gap through a family of languages called Proto-Quipper, which are designed to offer Quipper-like features while having sound theoretical foundations. I will give an overview of Quipper and its most useful features, report on the progress we made with formalizing fragments of Quipper, and outline several of the still remaining challenges.

15:00-15:30 Session 50B: Quantum Computing
A diagrammatic axiomatisation of fermionic quantum circuits

ABSTRACT. We introduce the fermionic ZW calculus, a string-diagrammatic language for fermionic quantum computing (FQC). After defining a fermionic circuit model, we present the basic components of the calculus, together with their interpretation, and show how the main physical gates of interest in FQC can be represented in the language. We then list our axioms, and derive some additional equations. We prove that the axioms provide a complete equational axiomatisation of the monoidal category whose objects are quantum systems of finitely many local fermionic modes, with operations that preserve or reverse the parity (number of particles mod 2) of states, and the tensor product, corresponding to the composition of two systems, as monoidal product. We achieve this through a procedure that rewrites any diagram in a normal form. We conclude by showing, as an example, how the statistics of a fermionic Mach-Zehnder interferometer can be calculated in the diagrammatic language.

15:30-16:00Coffee Break
16:00-18:00 Session 51B: Corrado Böhm Memorial
ALGORAND A Truly Distributed Ledger

ABSTRACT. A distributed ledger is a tamperproof sequence of data that can be read and augmented by everyone. Distributed ledgers stand to revolutionize the way a democratic society operates. They secure all kinds of traditional transactions –such as payments, asset transfers, titling– in the exact order in which they occur; and enable totally new transactions ---such as cryptocurrencies and smart contracts. They can remove intermediaries and usher in a new paradigm for trust. As currently implemented, however, distributed ledgers cannot achieve their enormous potential.

Algorand is an alternative, democratic, and efficient distributed ledger. Unlike prior ledgers based on ‘proof of work’, it dispenses with ‘miners’. Indeed, Algorand requires only a negligible amount of computation. Moreover, its transaction history does not ‘fork’ with overwhelming probability: i.e., Algorand guarantees the finality of all transactions.

Finally, Algorand enjoys flexible self-governance. By using its hallmark propose-and-agree process, Algorand can correct its course as necessary or desirable, without any ‘hard forks’.

Corrado Böhm: the white magician in programming and its semantics

ABSTRACT. Several results of Corrado Böhm will be presented  that have made programming more transparant and efficient.

  1.  Self-compiling. In his PhD thesis Corrado carefully presented a program that could translate itself to machine code. This resulted in the boot-strap of computers warming up efficiently.
  2.  Eliminating the go-to. With Giuseppe Jacopini Corrado showed that jumps in programming can be avoided. This resulted in  structured programming.
  3.  The foundation of functional programming. Corrado was one of the first to realize that the computational model of lambda calculus can be used for programming by introducing the CUCH-machine.
  4. Fine-structure of lambda terms.  In a paper in Italian Corrado studied what lambda terms cannot be equated. This resulted in a deep analysis of lambda models.
19:00-21:30 FLoC reception at Ashmolean Museum

FLoC reception at Ashmolean Museum. Drinks and canapés available from 7pm (pre-booking via FLoC registration system required; guests welcome).