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Monday 3.Sept. Tuesday 4.Sept. Wednesday 5.Sept Thursday 6.Sept

Monday, 3. September

Registration [Location: 2nd floor, Hörsaalzentrum]

The registration desk will be opened from 8:30 a.m. It is located in the second floor of the building while the conference room (lecture hall HZ 10) is in the third floor of the building.

PPDP: Contributed Talks #1 [Chair: Magnus Madsen  • Location: HZ 10 (third floor, Hörsaalzentrum)]
Cosette: Symbolic Execution for JavaScript
José Fragoso Santos, Petar Maksimović, Théotime Grohens, Julian Dolby and Philippa Gardner

We present a new framework for trustworthy, compositional symbolic execution of JavaScript programs. Its aim is to provide general-purpose symbolic analysis and assist developers in the testing of their code: the developer writes symbolic tests for which we provide concrete counter-models. We follow a new, general methodology for designing compositional program analyses for dynamic languages and prove that the underlying symbolic execution is sound and does not generate false positives. We establish additional trust by using the theory to precisely guide the implementation and by thorough testing. We apply our framework to whole-program symbolic testing of real-world JavaScript libraries and compositional debugging of separation logic specifications of JavaScript programs.

Verse: An EDSL for cryptographic primitives
Abhishek Dang and Piyush Kurur

Cryptographic primitives need high-speed implementations that are also resistant to side channel attacks. The absolute control over instructions and registers that such implementations demand makes assembly language programming a necessity. In this article, we describe Verse, a typed low-level language embedded in Coq designed specifically to generate assembly language programs for cryptographic primitives. Despite being a low-level language, the programming experience is markedly high-level: • The type system of Verse is rich enough to even prevent errors in array indexing and endian conversion. • Being embedded in Coq, we have at our disposal Gallina, the underlying functional programming language, as a macro assembler for code generation, and Ltac, the tactic language, as an automation tool for proof obligations inherent to our type system. We also provide a generic framework to formulate semantic aspects of Verse. This framework has value beyond providing an interpretation of Verse in Coq. We demonstrate this versatility by using it to localise uninitialised/clobbered variable use, and arithmetic overflows.

Coffee Break [Location: 2nd floor, Hörsaalzentrum]
PPDP: Honour and Memory of Martin Hofmann [Chair: Philippa Gardner  • Location: HZ 10 (third floor, Hörsaalzentrum)]

We sadly note that one of our pc members and friends, Martin Hofmann, died in January during a mountain trip close to Nikko, Japan. He is missed dearly. The session is a brief remembrance on his behalf.

Remembering Martin Hofmann: Semantic Equivalence Checking for HHVM Bytecode (Invited Talk)
Nick Benton

We describe a semantic differencing tool used to compare the bytecode generated by two different compilers for Hack/PHP at Facebook. The tool is a prover for a simple relational logic for low-level code and is used in testing, allowing developers to focus on semantically significant differences between the outputs of the old and new compilers.

PPDP: Contributed Talks #2 [Chair: Philippa Gardner  • Location: HZ 10 (third floor, Hörsaalzentrum)]
Nondeterministic Manifest Contracts
Yuki Nishida and Atsushi Igarashi

We study a manifest contract system-a typed calculus of higher-order contracts where contracts are tightly integrated into a refinement type system-for a functional language with nondeterministic choice. The extension is not trivial, especially in the presence of dependent function types, because a naive extension would lead to inconsistent type equivalence, which makes contract information in refinement types meaningless. To solve the problem, we propose a new kind of nondeterministic choice called coordinated choice, in which each occurrence of a choice operator is given a name and choices of the same name coordinately take the same branch. We introduce the notion of orthant that helps both intuitive understanding and the development of formal semantics of the new choice. We formalize a manifest contract system λ^H||Φ using the coordinated choice and show its basic properties of progress, type preservation, and contract satisfaction, the last of which states correctness of contracts in refinement types.

An Internalist Approach to Correct-by-Construction Compilers
Alberto Pardo, Emmanuel Gunther, Miguel Pagano and Marcos Viera

In this paper we present a methodology to organize the construction of a correct compiler, taking advantage of the power of full dependently type systems. The basic idea consists of decorating the abstract syntax of languages with their semantics, allowing to express the correctness of the compiler at type level. We show our methodology in a first small example and then explore how it can be promoted to more realistic languages, realizing that our internalistic approach is feasible for defining a correct-by-construction compiler from an imperative language with general recursion to a stack based intermediate language. We also show how this methodology can be combined with the externalist approach, compiling from the intermediate language to an assembly-like low level code and separately proving its correctness.

Lunch Break [Location: Mensa Casino]
PPDP: Invited Talk [Chair: Peter Thiemann  • Location: HZ 10 (third floor, Hörsaalzentrum)]
Calculating Distributions (Invited Talk)
Chung-Chieh Shan

The ways we reason about probability distributions and explore their applications have been naturally shifting: away from thoughtful proving with definitions using first principles, and towards mechanical calculation with expressions using derived principles. This talk reviews three useful operations on distributions that we have started to express using equational derivations and even to automate as program transformations. These operations are (1) to recognize a density function as belonging to a known distribution family, (2) to eliminate an unused random variable by summation or integration, and (3) to disintegrate a joint measure into a marginal and a conditional measure. It is thus promising to support probabilistic reasoning by drawing techniques from both programming languages and computer algebra. Ongoing challenges include how to handle a wide variety of container data types and generating programs, and how human guidance should interact with machine assistance.

Coffee Break [Location: 2nd floor, Hörsaalzentrum]
PPDP: Most Influential Paper 10-year Award and Contributed Talks #3 [Chair: Anton Setzer  • Location: HZ 10 (third floor, Hörsaalzentrum)]
Most influential paper 10-year award: Programming with proofs and explicit contexts
Brigitte Pientka and Joshua Dunfield
On Intersection Types and Probabilistic Lambda Calculi
Flavien Breuvart and Ugo Dal Lago

We define two intersection type systems for the pure, untyped, probabilistic lambda-calculus, and prove that type derivations precisely reflect the probability of convergence of the underlying term. We first define a simple system of oracle intersection types in which derivations are annotated by binary strings and the probability of termination can be computed by combining all the different possible annotations. Although inevitable due to recursion theoretic limitations, the fact that (potentially) infinitely many derivations need to be considered is of course an issue when seeing types as a verification methodology. We thus develop a more complex system: the monadic intersection type system. In this second system, the probability of termination of a term is shown to be the least upper bound of the weights of its type derivations.

Sequential and Parallel Improvements in a Concurrent Functional Programming Language
Manfred Schmidt-Schauss, David Sabel, and Nils Dallmeyer

We propose a model for measuring the runtime of concurrent programs by the minimal number of evaluation steps. The focus of this paper are improvements, which are program transformations that improve this number in every context, where we distinguish between sequential and parallel improvements, for one or more processors, respectively. We apply the methods to CHF, a model of Concurrent Haskell extended by futures allowing declarative implementations of concurrent programs. The language CHF is a typed higher-order functional language with concurrent threads, monadic IO and MVars as synchronizing variables. We show that all deterministic reduction rules and several further useful program transformations are sequential and parallel improvements.

PPDP and LOPSTR Reception (Welcome Drink and Snacks) [Location: Cafe Sturm und Drang (ground floor, Hörsaalzentrum)]

Tuesday, 4. September

Registration [Location: 2nd floor, Hörsaalzentrum]

The registration desk will be opened from 8:30 a.m. It is located in the second floor of the building while the conference rooms (lecture halls HZ 10 and HZ 11) are in the third floor of the building.
Today, the registration desk closes at 14:45 to prepare the social event.

PPDP & LOPSTR: Joint Invited Talk [Chair: Peter Thiemann  • Location: HZ 10 (third floor, Hörsaalzentrum)]
Formal methods for JavaScript (Invited Talk)
Philippa Gardner

We present a novel, unified approach to the development of compositional symbolic execution tools, which bridges the gap between traditional symbolic execution and compositional program reasoning based on separation logic. We apply our approach to JavaScript, providing support for full verification, whole-program symbolic testing, and automatic compositional testing based on bi-abduction.

Coffee Break [Location: 2nd floor, Hörsaalzentrum]
PPDP: Contributed Talks #4 [Chair: Annette Bienusa • Location: HZ 10 (third floor, Hörsaalzentrum)]
Certified Compilation of Financial Contracts
Danil Annenkov and Martin Elsman

We present an extension to a certified financial contract management system that allows for templated declarative financial contracts and for integration with financial stochastic models through verified compilation into so-called payoff expressions. Such expressions readily allow for determining the value of a contract in a given evaluation context, such as contexts created for stochastic simulations. The templating mechanism is useful both at the contract specification level, for writing generic reusable contracts, and for reuse of code that, without the templating mechanism, needs to be recompiled for different evaluation contexts. We report on the effect of using the certified system in the context of a GPGPU-based Monte Carlo simulation engine for pricing various over-the-counter (OTC) financial contracts. The full contract-management system, including the payoff-language compilation, is verified in the Coq proof assistant and certified Haskell code is extracted from our Coq development along with Futhark code for use in a data-parallel pricing engine.

Two-Phase Dynamic Analysis of Message-Passing Go Programs based on Vector Clocks
Martin Sulzmann and Kai Stadtmüller

Understanding the run-time behavior of concurrent programs is a challenging task. A popular approach is to establish a happens-before relation via vector clocks. Thus, we can identify bugs and performance bottlenecks, for example, by checking if two conflicting events may happen concurrently. We employ a two-phase method to derive vector clock information for a wide range of concurrency features that includes all of the message-passing features in Go. The first instrumentation phase yields a run-time trace that contains all events that took place. The second trace replay phase carried out offline infers vector clock information. Trace replay operates on thread-local traces. Thus, we can observe behavior that might result from some alternative schedule. Our approach is not tied to any specific language. We have built a prototype for the Go programming language and provide empirical evidence of the usefulness of our method.

Transformation of Combinatorial Optimization Problems Written in Extended SQL into Constraint Problems
Genki Sakanashi and Masahiko Sakai

The combinatorial optimization is an important area, which gives one of the best solutions for our real problems. This paper focuses on an SQL style of declarative languages to ease descriptions of combinatorial optimization problems, and provides their solution method powered by state-of-the-art CP/SMT solvers. From the semantic point of view, the search space of a combinatorial problem is given as a finite set of relations, and a solution is a relation in the set. Relations in the search space are filtered by constraints of the problem in similar to the filter-function on lists in functional languages, and the remaining relations are solutions of the problem. According to this notion, we extended Structured Query Language (SQL) by introducing some operations on sets of relations: generating a set of relations, filtering a set of relations according to constraints, and selecting one of the optimum relations with respect to a goal function. Toward an effective implementation, a set of relations is represented as a pair of a relation containing variables with finite domains and constraints on variables. This enables to solve the target problem by CP/SMT solvers. We also give an experimental result on the graph vertex coloring problem.

LOPSTR: Analysis of TRS [Chair: Vincent Nys • Location: HZ 11 (third floor, Hörsaalzentrum)]
Proving Program Properties as First-Order Satisfiability
Salvador Lucas

Program semantics can often be expressed as a (many-sorted) first-order theory S, and program properties as sentences φ which are intended to hold in the canonical model of such a theory, which is often incomputable. Recently, we have shown that properties φ expressed as the existential closure of a boolean combination of atoms can be disproved by just finding a model of S and the negation ¬φ of φ. Furthermore, this idea works quite well in practice due to the existence of powerful tools for the automatic generation of models for (many-sorted) first-order theo- ries. In this paper we extend our previous results to arbitrary properties, expressed as sentences without any special restriction. Consequently, one can prove a program property φ by just finding a model of an appropri- ate theory (including S and possibly something else) and an appropriate first-order formula related to φ. Beyond its possible theoretical interest, we show that our results can also be of practical use in several respects.

Guided Unfoldings for Finding Loops in Standard Term Rewriting
Étienne Payet

In this paper, we reconsider the unfolding-based technique that we have introduced previously for detecting loops in standard term rewriting. We improve it by guiding the unfolding process, using distin- guished positions in the rewrite rules. This results in a depth-first compu- tation of the unfoldings, whereas the original technique was breadth-first. We have implemented this new approach in our tool NTI and compared it to the previous one on a bunch of rewrite systems. The results we get are promising (better times, more successful proofs).

Homeomorphic Embedding modulo Combinations of Associativity and Commutativity Axioms
María Alpuente, Angel Cuenca-Ortega, Santiago Escobar, and José Meseguer

The Homeomorphic Embedding relation has been amply used for defining termination criteria of symbolic methods for program analysis, transformation, and veri- fication. However, homeomorphic embedding has never been investigated in the context of order-sorted rewrite theories that support symbolic execution methods modulo equa- tional axioms. This paper generalizes the symbolic homeomorphic embedding relation to order–sorted rewrite theories that may contain various combinations of associativity and/or commutativity axioms for different binary operators. We systematically measure the performance of increasingly efficient formulations of the homemomorphic embed- ding relation modulo associativity and commutativity axioms. From our experimental results, we conclude that our most efficient version indeed pays off in practice.

Lunch Break [Location: Mensa Casino]
PPDP: Contributed Talks #5 [Chair: Michael Hanus • Location: HZ 10 (third floor, Hörsaalzentrum)]
Automatic reordering for dataflow safety of Datalog
Mistral Contrastin, Dominic Orchard and Andrew Rice

Automatic Reordering for Dataflow Safety of Datalog Abstract: Clauses and subgoals in a Datalog program can be given in any order without affecting program meaning. However, practical applications of the language require the use of built-in or external predicates with particular dataflow requirements. These can be expressed as input or output modes on arguments. We describe a static analysis of moding for Datalog which can transform an ill-moded program into a well-moded program by reordering clause subgoals, satisfying dataflow requirements. We describe an incremental algorithm which efficiently finds a reordering if it exists. This frees the programmer to focus on the declarative specification of their program rather than on the implementation details of external predicates. We prove that our computed reorderings yield well-moded programs (soundness) and that if a program can be made well-moded, we compute a reordering to do so (completeness).

Improving Refutational Completeness of Relational Search via Divergence Test
Dmitri Rozplokhas and Dmitry Boulytchev

We describe a search optimization technique for implementation of relational programming language miniKanren which makes more queries to converge. Specifically, we address the problem of conjunction non-commutativity. Our technique is based on a certain divergence criterion, which we use to trigger a dynamic reordering of conjuncts. We present a formal semantics of a miniKanren-like language and prove that our optimization does not compromise already converging programs, thus, being a proper improvement. We also present the prototype implementation of the improved search and demonstrate its application for a number of realistic specifications.

Web-based Visualisation for Definite Clause Grammars using Prolog Meta-Interpreters
Falco Nogatz, Jona Kalkus and Dietmar Seipel

Definite Clause Grammars (DCGs) are a convenient way to describe lists in Prolog. They are a common mean to specify grammars for natural language processing and to parse formal languages. Despite its long history, tools dedicated to the development and debugging of DCGs are rare. In this paper, we present an interactive, web-based tool to visualise the execution of a DCG. To collect the required information from SWI-Prolog, we discuss several techniques, including meta-interpreters, trace interceptors, and term expansions.

LOPSTR: Around Logic-Based distributed/concurrent programming [Chair: Salvador Lucas • Location: HZ 11 (third floor, Hörsaalzentrum)]
Multiparty Classical Choreographies
Marco Carbone, Luís Cruz-Filipe, Fabrizio Montesi, and Agata Murawska

We present Multiparty Classical Choreographies (MCC), a language model where global descriptions of communicating systems (choreographies) implement typed multiparty sessions. Typing is achieved by generalising classical linear logic to judgements that explicitly record parallelism by means of hypersequents. Our approach unifies different lines of work on choreographies and processes with multiparty sessions, as well as their connection to linear logic. Thus, results developed in one context are carried over to the others. Key novelties of MCC include support for server invocation in choreographies, as well as logic-driven compilation of choreographies with replicated processes.

Correct-by-construction Process Composition Using Classical Linear Logic Inference
Petros Papapanagiotou and Jacques Fleuriot

The need for rigorous process composition is encountered in many situations pertaining to the development and analysis of complex systems. We discuss the use of Classical Linear Logic (CLL) for correct- by-construction resource-based process composition, with guaranteed deadlock freedom, systematic resource accounting, and concurrent execution. We introduce algorithms to automate the necessary inference steps for binary compositions of processes in parallel, conditionally, and in sequence. We combine decision procedures and heuristics to achieve intuitive and practically useful compositions in an applied setting.

Confluence of CHR revisited: invariants and modulo equivalence
Henning Christiansen and Maja H. Kirkeby

Abstract simulation of one transition system by another is introduced as a means to simulate a potentially infinite class of similar transition sequences within a single transition sequence. This is useful for proving confluence under invariants of a given system, as it may reduce the number of proof cases to consider from infinity to a finite number. The classical confluence results for Constraint Handling Rules (CHR) can be explained in this way, using CHR as a simulation of itself. Using an abstract simulation based on a ground representation, we extend these results to include confluence under invariant and modulo equivalence, which have not been done in a satisfactory way before.

Break (No Coffee)

Use this break to walk from the conference location to the start of the city tour (Old Opera Frankfurt).

  • A guided walk will start at 15:15h from Campus Westend. Here is a Google Map with a route description.
  • An alternative is to use the bus no. 64 in direction "Frankfurt (Main) Baseler Platz ". It departs at 15:25 h from "Bremer Straße" and arrives at 15:30h on "Alte Oper" (Old Opera). Get off the bus on the third stop. In this case the following Google Map may be helpful.
The following schematic map may be helpful to find the start of the guided city walk and the dinner location:

Social Event: Guided two hour city walk through the city centre of Frankfurt. [Location: The tour starts in front of the Old Opera Frankfurt]

This city walk through the old town centre (Römerberg) focuses on various aspects of Frankfurt's colourful history, including its role as electoral site of German kings and host city for the coronation ceremonies of Holy Roman emperors. Frankfurt's role as a free trade city and site of Germany's first national assembly, held right here at St Paul's Church in 1848, is also highlighted. The tour covers an abundance of significant events within the smallest of geographical areas.

Conference Dinner [Location: Vinum Frankfurt]

Wednesday, 5. September

Registration [Location: 2nd floor, Hörsaalzentrum]

The registration desk will be opened from 8:30 a.m. It is located in the second floor of the building while the conference rooms (lecture halls HZ 10 and HZ 11) are in the third floor of the building.

PPDP & LOPSTR: Joint Invited Talk [Chair: Peter Stuckey  • Location: HZ 10 (third floor, Hörsaalzentrum)]
Constrained Horn Clauses for Verification (Invited Talk)
Jorge Navas

Developing scalable software verification tools is a very difficult task. First, due to the undecidability of the verification problem, these tools, must be highly tuned and engineered to provide reasonable efficiency and precision trade-offs. Second, different programming languages come with very diverse assortments of syntactic and semantic features. Third, the diverse encoding of the verification problem makes the integration with other powerful solvers and verifiers difficult. This talk presents SeaHorn — an open source automated Constrained Horn clause-based reasoning framework. SeaHorn combines advanced automated solving techniques based on Satisfiability Modulo Theory (SMT) and Abstract Interpretation. SeaHorn is built on top of LLVM using its front-end(s) to deal with the idiosyncrasies of the syntax and it highly benefits from LLVM optimizations to reduce the verification effort. SeaHorn uses Constrained Horn clauses (CHC) which are a uniform way to formally represent a broad variety of transition systems while allowing many encoding styles of verification conditions. Moreover, the recent popularity of CHC as an intermediate language for verification engines makes it possible to interface SeaHorn with a variety of new and emerging tools. All of these features make SeaHorn a versatile and highly customizable tool which allows researchers to easily build or experiment with new verification techniques.

Coffee Break [Location: 2nd floor, Hörsaalzentrum]
PPDP: Contributed Talks #6 [Chair: Alberto Pardo • Location: HZ 10 (third floor, Hörsaalzentrum)]
Declarative GUIs: Simple, Consistent, and Verified
Stephan Adelsberger, Anton Setzer and Eric Walkingshaw

Graphical user interfaces (GUIs) are ubiquitous in real-world software and a notorious source of bugs that are difficult to catch through software testing. Model checking has been used to prove the absence of certain kinds of bugs, but model checking works on an abstract model of the GUI application, which might be inconsistent with its implementation. We present a library for developing directly verified, state-dependent GUI applications in the dependently typed programming language Agda. In the library, the type of a GUI's controller depends on a specification of the GUI itself, statically enforcing consistency between them. Arbitrary properties can be defined and proved in terms of user interactions and state transitions. Our library connects to a custom-built Haskell back-end for declarative vector-based GUI elements. Compared to an earlier version of our library built on an existing imperative GUI framework, the more declarative back-end supports simpler definitions and proofs. As a practical application of our library to a safety-critical domain, we present a case study developed in cooperation with the Medical University of Vienna. The case study implements a healthcare process for prescribing anticoagulants, which is highly error-prone when followed manually. Our implementation generates GUIs from an abstract description of a data-aware business process, making our approach easy to reuse and adapt to other safety-critical processes. We prove medically relevant safety properties about the executable GUI application, such as that given certain inputs, certain states must or must not be reached.

Combinatorics of Explicit Substitutions
Maciej Bendkowski and Pierre Lescanne

lambda-upsilon is an extension of the lambda-calculus which internalises the calculus of substitutions. In the current paper, we investigate the combinatorial properties of lambda-upsilon focusing on the quantitative aspects of substitution resolution. We exhibit an unexpected correspondence between the counting sequence for lambda-upsilon terms and famous Catalan numbers. As a by-product, we establish effective sampling schemes for random lambda-upsilon terms. We show that typical lambda-upsilon terms represent, in a strong sense, non-strict computations in the classic lambda-calculus. Moreover, typically almost all substitutions are in fact suspended, i.e., unevaluated, under closures. Consequently, we argue that lambda-upsilon is an intrinsically non-strict calculus of explicit substitutions. Finally, we investigate the distribution of various redexes governing the substitution resolution in lambda-upsilon and investigate the quantitative contribution of various substitution primitives.

Schematic Polymorphism in the Abella Proof Assistant
Gopalan Nadathur and Yuting Wang

The Abella interactive theorem prover has proven to be an effective vehicle for reasoning about relational specifications. However, the system has a limitation that arises from the fact that it is based on a simply typed logic: formalizations that are identical except in the respect that they apply to different types have to be repeated at each type. We develop an approach that overcomes this limitation while preserving the logical underpinnings of the system. In this approach object constructors, formulas and other relevant logical notions are allowed to be parameterized by types, with the interpretation that they stand for the (infinite) collection of corresponding constructs that are obtained by instantiating the type parameters. The proof structures that we consider for formulas that are schematized in this fashion are limited to ones whose type instances are valid proofs in the simply typed logic. We develop schematic proof rules that ensure this property, a task that is complicated by the fact that type information influences the notion of unification that plays a key role in the logic. Our ideas, which have been implemented in an updated version of the system, accommodate schematic polymorphism both in the core logic of Abella and in the executable specification logic that it embeds. Authors:

LOPSTR: Analysis of LP [Chair: Michael Hanus • Location: HZ 11 (third floor, Hörsaalzentrum)]
Compiling Control as Offline Conjunctive Partial Deduction
Vincent Nys and Danny De Schreye

We present a new approach to a technique known as compiling control, whose aim is to compile away special mechanisms for non-standard atom selection in logic programs. It has previously been conjectured that compiling control could be implemented as an instance of the first Futamura projection, in which an interpreter is specialized for an input program. However, the exact nature of such an interpreter and of the required technique for specialization were never specified. In this work, we propose a Prolog meta-interpreter which applies the desired non-standard selection rule and which is amenable to specialization using offline partial deduction. After the initial analysis phase of compiling control, we collect annotations to specialize the interpreter using the Logen system for offline partial deduction. We also show that the result of the specialization is equivalent to the program obtained using the traditional approach to compiling control. In this way, we simplify the synthesis step.

Predicate Specialization for Definitional Higher-order Logic Programs
Antonis Troumpoukis and Angelos Charalambidis

Higher-order logic programming is an interesting extension of traditional logic programming that allows predicates to appear as arguments and variables to be used where predicates typically occur. Higher-order characteristics are indeed desirable but on the other hand they are also usually more expensive to support. In this paper we propose a program specialization technique based on partial evaluation that can be applied to a modest but useful class of higher-order logic programs and can transform them into first-order programs without introducing additional data structures. The resulting first-order programs can be executed by conventional logic programming interpreters and benefit from other optimizations that might offer. We provide an implementation and experimental results that suggest the efficiency of the transformation.

An Assertion language for slicing Constraint Logic Languages
Moreno Falaschi and Carlos Olarte

Constraint Logic Programming (CLP) is a language scheme for combining two declarative paradigms: constraint solving and logic programming. Concurrent Constraint Programming (CCP) is a declarative model for concurrency where agents interact by telling and asking constraints in a shared store. In a previous paper, we have developed a framework for dynamic slicing of CCP where the user first identifies that a (partial) computation is wrong. Then, she marks (selects) some parts of the final state corresponding to the data (constraints) and processes that she wants to study more deeply. An automatic process of slicing begins, and the partial computation is depurated by removing irrelevant information. In this paper we give two major contributions. First, we extend the framework to CLP, thus generalizing the previous work. Second, we provide an assertion language suitable for both, CCP and CLP, which allows the user to specify some properties of the computations in her program. If a state in a computation does not satisfy an assertion then some wrong information is identified and an automatic slicing process can start. This way we make one step further towards automatizing the slicing process. We show that our framework can be integrated with the previous semi-automatic one, giving the user more choices and flexi- bility. We show by means of examples and experiments the usefulness of our approach.

Lunch Break [Location: Mensa Casino]
PPDP: Contributed Talks #7 [Chair: Dietmar Seipel • Location: HZ 10 (third floor, Hörsaalzentrum)]
Implicit Parameters for Logic Programming
Magnus Madsen and Ondřej Lhoták

Implicit parameters allow programmers to omit certain arguments from function calls and have them automatically inferred by the compiler based on their types. At every call site, the compiler determines the values of the implicit parameters based on their declared types and the bindings currently in implicit scope. The programmer controls this mechanism in two ways: by adding bindings to the implicit scope, or by explicitly providing the implicit parameters for the function call. Implicit parameters are known from functional and object-oriented languages such as Haskell and Scala. In recent years, more languages have added support for implicit parameters, including Agda, Coq, and Idris. Implicit parameters have played an impressive role as the foundation for a broad range of language features such as type classes, capability and effect systems, software transactional memory, macros, and more. In this paper, we propose a design of implicit parameters for typed Horn clause based logic programming languages, such as Datalog and Prolog. We illustrate the usefulness of implicit parameters and show how they support logic programming in the large. We explore some of the differences that arise between implicit parameters in functional languages and in logic languages.

Verifying Fail-Free Declarative Programs
Michael Hanus

Failed computations are a frequent problem in software system development. Some failures have external reasons (e.g., missing files) that can be caught by exception handlers. Many other failures have internal reasons, such as calling a partially defined operation with unintended arguments. In order to avoid the latter kind of failures, one can try to analyze the program at compile time for potential occurrences of these failures at run time. In this paper we present an approach to verify the absence of such failures in functional logic programs. Since programming with failures is a typical technique in logic programming, we are not interested to abandon partially defined operations at all. Instead, we want to verify conditions which ensure that operations can be executed without running into a failure. For this purpose, we propose to annotate operations with non-fail conditions that are verified at compile time with an SMT solver. For successfully verified programs, it is ensured that computations never fail provided that the non-fail condition of the main operation is satisfied.

Static Performance Guarantees for Programs with Run-time Checks
Maximiliano Klemen, Nataliia Stulova, Pedro Lopez-Garcia, Jose F. Morales and Manuel V. Hermenegildo

Instrumenting programs for performing run-time checking of properties, such as regular shapes, is a common and useful technique that helps programmers detect incorrect program behaviors. This is specially true in dynamic languages such as Prolog. However, such run-time checks inevitably introduce run-time overhead (in execution time, memory, energy, etc.). Several approaches have been proposed for reducing this overhead, such as eliminating the checks that can statically be proved to always succeed, and/or optimizing the way in which the (remaining) checks are performed. However, there are cases in which it is not possible to remove all checks statically (e.g., open libraries which must check their interfaces, complex properties, unknown code, etc.) and in which, even after optimizations, these remaining checks may still introduce an unacceptable level of overhead. It is thus important for programmers to be able to determine the additional cost due to the run-time checks and compare it to some notion of admissible cost. The common practice used for estimating run-time checking overhead is profiling, which is not exhaustive by nature. Instead, we propose a method that uses static analysis to estimate such overhead, with the advantage that the estimations are functions parameterized by input data sizes. Unlike profiling, this approach can provide guarantees for all possible execution traces, and allows assessing how the overhead grows as the size of the input grows. Our method also extends an existing assertion verification framework to express “admissible” overheads, and statically and automatically checks whether the instru- mented program conforms with such specifications. Finally, we present an experimental evaluation of our approach that suggests that our method is feasible and promising.

LOPSTR: Invited Tutorial #1 [Chair: Moreno Falaschi • Location: HZ 11 (third floor, Hörsaalzentrum)]
The VeriMAP System for program transformation and verification (Invited Tutorial)
Fabio Fioravanti

Constrained Horn Clauses (CHC) are becoming very popular for representing programs and verification problems, and several tools have been developed for checking their satisfiability.
In this tutorial we will survey recent work on satisfiability-preserving transformation techniques for CHC and we will show how the VeriMAP system can be used effectively to
(i) generate CHC verification conditions from the programming language semantics,
(ii) prove safety properties of imperative programs manipulating integers and arrays,
(iii) prove relational program properties, such as program equivalence and non-interference,
(iv) check the satisfiability of CHC with inductively-defined data structures (e.g. lists and trees),
(v) prove safety and controllability properties of time-aware business processes.

Coffee Break [Location: 2nd floor, Hörsaalzentrum]
PPDP: Contributed Talks #8 [Chair: David Sabel • Location: HZ 10 (third floor, Hörsaalzentrum)]
Three Improvements to the Top-Down Solver
Helmut Seidl and Ralf Vogler

The local solver TD is a generic fixpoint engine which explores a given system of equations on demand. It has been successfully applied to the interprocedural analysis of procedural languages. The solver TD gains efficiency by detecting variable dependencies on the fly. This algorithm has been recently extended to deal with widening and narrowing as well. In particular, it has been equipped with an automatic detection of widening and narrowing points. That version, however, is only guaranteed to terminate under two conditions: only finitely many variables are encountered, and all right-hand sides are monotonic. While the first condition is unavoidable, the second limits the applicability of the solver. Another limitation is that the solver maintains the current abstract values of all encountered variables in one data-structure - thus prohibiting interprocedural analyses to scale to larger programs. In the present paper, we therefore extend the top-down solver TD in three ways. First, we indicate how the restriction to monotonic right-hand sides can be lifted. Then we show how the solver can be tuned to store abstract values only when it turns out to be inevitable. Finally, we also show how the solver can be extended to side-effecting equation systems, which have successfully been applied to a seamless combination of context-sensitive with flow-insensitive analyses.

Lazy Abstraction for Higher-Order Program Verification
Taku Terao

This paper proposes a lazy abstraction algorithm for verification of functional programs. The feature of lazy abstraction method is that the predicate abstraction and the model checking are fused, and that abstractions for unreachable configurations are pruned. We define an abstract semantics that characterizes the precision of the lazy abstraction algorithm, and prove the soundness of our verification method and the progress property of our abstraction refinement algorithm. We have implemented a prototype of our method, and confirmed through experiments that total efficiency of verification is improved, compared with previous eager abstraction methods.

An Infrastructure for Combining Domain Knowledge with Automated Theorem Provers
Sylvia Grewe, Sebastian Erdweg, André Pacak and Mira Mezini

Computer science has seen much progress in the area of automated verification in the last decades. Yet, there are many domains where abstract strategies for verifying standard properties are well-understood by domain experts, but still not automated to a satisfactory degree. One example for such a domain are type soundness proofs. Being able to express domain-specific verification strategies using domain-specific terminology and concepts can help to narrow down this gap toward more automated verification. We present the requirements, design, and implementation of a configurable verification infrastructure that allows for expressing domain knowledge about proofs and for interfacing with existing automated theorem provers and solvers to verify individual proof steps. As an application scenario for our infrastructure, we present the development of a standard type soundness proof for a typed subset of SQL.

Pattern Matching and Fixed Points: Resources Types and Strong Call-By-Need
Pablo Barenbaum, Eduardo Bonelli and Kareem Mohamed

Resource types are types that statically quantify some aspect of program execution. They come in various guises; this paper focusses on a manifestation of resource types known as non-idempotent intersection types. We use them to characterize weak normalisation for a type-erased lambda calculus for the Calculus of Inductive Construction (λe), as introduced by Gregoire and Leroy. The λe calculus consists of the lambda calculus together with constructors, pattern matching and a fixed-point operator. The characterization is then used to prove the completeness of a strong call-by-need strategy for λe. This strategy operates on open terms: rather than having evaluation stop when it reaches an abstraction, as in weak call-by-need, it computes strong normal forms by admitting reduction inside the body of abstractions and substitutions. Moreover, argument evaluation is by-need: arguments are evaluated when needed and at most once. Such a notion of reduction is of interest in areas such as partial evaluation and proof-checkers such as Coq.

LOPSTR: Program Analysis [Chair: Maja Kirkeby • Location: HZ 11 (third floor, Hörsaalzentrum)]
Multivariant Assertion-based Guidance in Abstract Interpretation
Isabel Garcia-Contreras, Jose F. Morales, and Manuel V. Hermenegildo

Approximations during program analysis are a necessary evil, as they ensure essential properties, such as analysis soundness and termination, but they also imply that the analysis is not guaranteed to always produce useful results. In such cases it is necessary to have some means for users to provide information to guide analysis and thus to improve precision and/or shorten analysis time. This allows dealing with, e.g., constructs for which the analysis is not complete and loses precision or for which the source is only partially available. We present techniques for supporting within an abstract interpretation framework a rich set of assertions that can deal with multivariance / context-sensitivity, and can handle different run-time semantics for those assertions that cannot be discharged at compile time. We show how the proposed approach can be applied to both improving precision and accelerating convergence, and provide a procedure to decide whether the invariants used to guide the analyzer are checked, incompatible, or neither. We also provide some formal results on the effects of such assertions on the analysis results.

Eliminating Unstable Tests in Floating-Point Programs
Laura Titolo, César Muñoz, Marco A. Feliú, and Mariano M. Moscato

Round-off errors arising from the difference between real numbers and their floating-point representation cause the control flow of conditional floating-point statements to deviate from the ideal flow of the real-number computation. This problem, which is called test instability, may result in a significant difference between the computation of a floating-point program and the expected output in real arithmetic. In this paper, a formally proven program transformation is proposed to detect and correct the effects of unstable tests. The output of this transformation is a floating-point program that is guaranteed to return either the result of the original floating-point program when it can be assured that both its real and its floating-point flows agree or a warning when these flows may diverge. The proposed approach is illustrated with the transformation of the core computation of a polygon containment algorithm developed at NASA that is used in a geofencing system for unmanned aircraft systems.

Formalisation of a frame stack semantics for a Java-like language
Aleksy Schubert and Jacek Chrząszcz

We present a Coq formalisation of the small-step operational semantics of Jafun, a small Java-like language with classes. This format of semantics makes it possible to naturally specify and prove invariants that should hold at each computation step. In contrast to the Featherweight Java approach the semantics explicitly manipulates frame stack of method calls. Thanks to that one can express properties of computation that depend on execution of particular methods. On the basis of the semantics, we developed a type system that makes it possible to delineate a notion of a complex value and classify certain methods as extensional functions operating on them. In our formalisation we make a mechanised proof that the operational semantics for the un- typed version of the semantics agrees with the one for the typed one. We discuss different methods to make such formalisation effort and provide experiments that substantiate it.

Thursday, 6. September

Registration [Location: 2nd floor, Hörsaalzentrum]

The registration desk will be opened from 8:30 a.m. It is located in the second floor of the building while the conference room of LOPSTR (lecture hall HZ 10) is located in the third floor of the building and the room SH1.101 is in another building (Seminarhaus).

LOPSTR & WFLP: Joint Invited Talk [Chair: Pedro Lopez  • Location: HZ 10 (third floor, Hörsaalzentrum)]
Experiences in designing scalable static analyses (Invited Talk)
Laure Gonnord

Proving the absence of bugs in a given software (problem which has been known to be intrinsically hard since Turing and Cook) is not the only challenge in software development. Indeed, the ever growing complexity of software increases the need for more trustable optimisations. Solving these two problems (reliability, optimisation) implies the development of safe (without false negative answers) and efficient (wrt memory and time) analyses, yet precise enough (with few false positive answers). In this talk I will present some experiences in the design of scalable static analyses inside compilers, and try to make a synthesis about the general framework we, together with my coauthors, used to develop them. I will also show some experimental evidence of the impact of this work on real-world compilers, as well as future perspective for this area of research. Keywords: Static Analysis, Abstract Interpretation, Compilation, Code Optimisation.

Coffee Break [Location: 2nd floor, Hörsaalzentrum]
LOPSTR: Invited Tutorial #2 [Chair: Fred Mesnard • Location: HZ 10 (third floor, Hörsaalzentrum)]
25 years of Ciao (Invited Tutorial)
Manuel Hermenegildo

Ciao is a logic-based, multi-paradigm programming language which has pioneered over the years many interesting language- and programming environment-related concepts. An example is the notion of programming languages as modular language-building tools rather than closed designs. Another is the idea of dynamic languages that can optionally and gradually offer formal guarantees, which is also a solution for the classic dichotomy between dynamic and static typing: Ciao has many dynamic features (e.g., dynamically typed, dynamic program modification) but includes an assertion language for (optionally) declaring program properties and powerful tools for static inference and static/dynamic checking of such assertions, testing, documentation, etc. We will provide a hands-on overview of these features, concentrating on the novel aspects, the motivations behind their design and implementation, their evolution over time, and, specially, their use. In particular, we will show how the system can be used not only as a programming tool and as a language design tool, but also as a general-purpose program analysis and verification tool, based on the technique of translating program semantics (ranging from source to bytecode, LLVM, or assembly) into Horn-clause representation, and idea which Ciao also introduced early on. Finally, we will present some recent work in areas such as scalability, incrementality, or static vs. dynamic costs, as well as some future plans and ideas.

WFLP: Contributed Talks #1 [Location: SH1.101 (Seminarhaus)]
Transpiling Programming Computable Functions to Answer Set Programs
Ingmar Dasseville and Marc Denecker
Synthesizing Set Functions
Sergio Antoy, Michael Hanus and Finn Teegen
When You Should Use Lists in Haskell (Mostly, You Should Not)
Johannes Waldmann
Runtime verification in Erlang by using contracts
Lars-Åke Fredlund, Julio Mariño, Sergio Pérez and Salvador Tamarit
Lunch Break [Location: Mensa Casino]
WFLP: Contributed Talks #2 [Location: HZ 10 (third floor, Hörsaalzentrum)]
Reference Type Logic Variables in Constraint-logic Object-oriented Programming
Jan C. Dageförde
Functional Federated Learning in Erlang (ffl-erl)
Gregor Ulm, Emil Gustavsson and Mats Jirstrand
Towards a constraint solver for proving confluence with invariant and equivalence of realistic CHR programs
Henning Christiansen and Maja Kirkeby
Code Generation for Higher Inductive Types
Paventhan Vivekanandan
Coffee Break [Location: 2nd floor, Hörsaalzentrum]
WFLP: Contributed Talks #3 [Location: HZ 10 (third floor, Hörsaalzentrum)]
Making Bubbling Practical
Steven Libby and Sergio Antoy
FMS: Functional Programming as a Modelling Language
Ingmar Dasseville and Gerda Janssens
Enhancing POI testing approach through the use of additional information
Sergio Pérez and Salvador Tamarit
Measuring Coverage of Prolog Programs Using Mutation Testing
Alexandros Efraimidis, Joshua Schmidt, Sebastian Krings and Philipp Körner