Weak looking-ahead and its application in computer-integrated process planning

Constraint logic programming has been shown to be a very useful tool for knowledge representation and problem-solving in different areas. Finite Domain extensions of PROLOG together with efficient consistency techniques such as forward-checking and looking-ahead make it possible to solve many discrete combinatorial problems within a short development time. In this paper we present the weak looking-ahead strategy (WLA), a new consistency technique on finite domains combining the computational efficiency of forward-checking with the pruning power of looking-ahead. Moreover, incorporating weak looking-ahead into PROLOG\u27s SLD resolution gives a sound and complete inference rule whereas standard looking-ahead itself has been shown to be incomplete. Finally, we will show how to use weak looking-ahead in a real-world application to obtain an early search-space pruning while avoiding the control overhead involved by standard looking-ahead

Model Checking a Temporal Logic via Program Verification

openThe thesis explores the possibility of viewing Model Checking as an instance of program verification in order to allow for the reuse of the vast theory and toolset of Abstract Interpretation in the setting of Model Checking. Model Checking is a formal verification technique used to analyse the correctness of software systems, based on a representation of the system as a formal model, such as a finite-state machine or a transition system, and on a representation of the properties it must satisfy as temporal logic formulae. On the other hand, Abstract Interpretation is a program analysis method, based on the idea of extracting properties of programs by (over-)approximating their semantics over a so-called abstract domain, typically a complete lattice, whose elements represent program properties. The thesis focuses on ACTL, the universal fragment of the temporal logic CTL, which can describe properties of executions which are universally quantified. It shows how properties expressed in ACTL can be mapped into programs written in a suitable programming language, whose semantics consists of counterexamples to the validity of the formula. Then such a program is analysed by Abstract Interpretation over some abstract domain, exploiting the idea of local completeness as put forward in some recent work, combining lower- and under-approximations.The thesis explores the possibility of viewing Model Checking as an instance of program verification in order to allow for the reuse of the vast theory and toolset of Abstract Interpretation in the setting of Model Checking. Model Checking is a formal verification technique used to analyse the correctness of software systems, based on a representation of the system as a formal model, such as a finite-state machine or a transition system, and on a representation of the properties it must satisfy as temporal logic formulae. On the other hand, Abstract Interpretation is a program analysis method, based on the idea of extracting properties of programs by (over-)approximating their semantics over a so-called abstract domain, typically a complete lattice, whose elements represent program properties. The thesis focuses on ACTL, the universal fragment of the temporal logic CTL, which can describe properties of executions which are universally quantified. It shows how properties expressed in ACTL can be mapped into programs written in a suitable programming language, whose semantics consists of counterexamples to the validity of the formula. Then such a program is analysed by Abstract Interpretation over some abstract domain, exploiting the idea of local completeness as put forward in some recent work, combining lower- and under-approximations

Design and implementation of a finite domain constraint logic programming system based on PROLOG with coroutining

Many problems in different areas such as Operations Research, Hardware Design, and Artificial Intelligence can be regarded as constraint solving problems (CSPs). Logic programming offers a convenient way of representing CSPs due to its relational, declarative and nondeterministic form. Unfortunately, standard logic programming languages such as PROLOG tend to be inefficient for solving CSPs, since what could be called constraints in PROLOG is used only in a passive a posteriori manner, leading to symptoms such as late recognition of failure, unnecessary and unintelligent backtracking and multiple computation of the same solutions. There have been intensive research efforts in order to remedy this. One of them, which has caught increasing attention over the past few years, is the Constraint Logic Programming approach: By integrating a domain concept for logic variables and consistency techniques such as forward-checking or looking-ahead into PROLOG, the search space can be restricted in an a priori manner. Thus, a more efficient control strategy can be achieved, preserving the 'clean' dual PROLOG semantics. In this issue, I will present a horizontal compilation approach towards a CLP system maintaining constraints whose variables are ranging over finite domains. Horizontal compilations often referred to as optimizing transformation techniques in other context. A PROLOG system providing a delay mechanism is used in order to achieve the control behaviour described above. The major subtasks of my work are -Design and integration of a domain concept into logic programming, which allows direct access to and manipulation of possible values of logic variables. -Thorough implementation of a forward-checking control strategy in SEPIA. -Design and prototypical implementation of a looking-ahead algorithm. -Summary of the main theoretical results underlying to domains and consistency techniques in logic programming. -Consideration and prototypical implementation of first-fail heuristics. -Embedding these topics into a preprocessor, which transforms FIDO programs into SEPIA programs realizing the advanced control strategies. The general framework of this work is the FIDO lab within the ARC-TEC project, which explores several approaches towards integrating finite domain consistency techniques into logic programming

Path-Based Program Repair

We propose a path-based approach to program repair for imperative programs. Our repair framework takes as input a faulty program, a logic specification that is refuted, and a hint where the fault may be located. An iterative abstraction refinement loop is then used to repair the program: in each iteration, the faulty program part is re-synthesized considering a symbolic counterexample, where the control-flow is kept concrete but the data-flow is symbolic. The appeal of the idea is two-fold: 1) the approach lazily considers candidate repairs and 2) the repairs are directly derived from the logic specification. In contrast to prior work, our approach is complete for programs with finitely many control-flow paths, i.e., the program is repaired if and only if it can be repaired at the specified fault location. Initial results for small programs indicate that the approach is useful for debugging programs in practice.Comment: In Proceedings FESCA 2015, arXiv:1503.0437

Checking-in on Network Functions

When programming network functions, changes within a packet tend to have consequences---side effects which must be accounted for by network programmers or administrators via arbitrary logic and an innate understanding of dependencies. Examples of this include updating checksums when a packet's contents has been modified or adjusting a payload length field of a IPv6 header if another header is added or updated within a packet. While static-typing captures interface specifications and how packet contents should behave, it does not enforce precise invariants around runtime dependencies like the examples above. Instead, during the design phase of network functions, programmers should be given an easier way to specify checks up front, all without having to account for and keep track of these consequences at each and every step during the development cycle. In keeping with this view, we present a unique approach for adding and generating both static checks and dynamic contracts for specifying and checking packet processing operations. We develop our technique within an existing framework called NetBricks and demonstrate how our approach simplifies and checks common dependent packet and header processing logic that other systems take for granted, all without adding much overhead during development.Comment: ANRW 2019 ~ https://irtf.org/anrw/2019/program.htm

Inference with Constrained Hidden Markov Models in PRISM

A Hidden Markov Model (HMM) is a common statistical model which is widely used for analysis of biological sequence data and other sequential phenomena. In the present paper we show how HMMs can be extended with side-constraints and present constraint solving techniques for efficient inference. Defining HMMs with side-constraints in Constraint Logic Programming have advantages in terms of more compact expression and pruning opportunities during inference. We present a PRISM-based framework for extending HMMs with side-constraints and show how well-known constraints such as cardinality and all different are integrated. We experimentally validate our approach on the biologically motivated problem of global pairwise alignment

Combining Forward and Backward Abstract Interpretation of Horn Clauses

Alternation of forward and backward analyses is a standard technique in abstract interpretation of programs, which is in particular useful when we wish to prove unreachability of some undesired program states. The current state-of-the-art technique for combining forward (bottom-up, in logic programming terms) and backward (top-down) abstract interpretation of Horn clauses is query-answer transformation. It transforms a system of Horn clauses, such that standard forward analysis can propagate constraints both forward, and backward from a goal. Query-answer transformation is effective, but has issues that we wish to address. For that, we introduce a new backward collecting semantics, which is suitable for alternating forward and backward abstract interpretation of Horn clauses. We show how the alternation can be used to prove unreachability of the goal and how every subsequent run of an analysis yields a refined model of the system. Experimentally, we observe that combining forward and backward analyses is important for analysing systems that encode questions about reachability in C programs. In particular, the combination that follows our new semantics improves the precision of our own abstract interpreter, including when compared to a forward analysis of a query-answer-transformed system.Comment: Francesco Ranzato. 24th International Static Analysis Symposium (SAS), Aug 2017, New York City, United States. Springer, Static Analysi

Model Checking Linear Logic Specifications

The overall goal of this paper is to investigate the theoretical foundations of algorithmic verification techniques for first order linear logic specifications. The fragment of linear logic we consider in this paper is based on the linear logic programming language called LO enriched with universally quantified goal formulas. Although LO was originally introduced as a theoretical foundation for extensions of logic programming languages, it can also be viewed as a very general language to specify a wide range of infinite-state concurrent systems. Our approach is based on the relation between backward reachability and provability highlighted in our previous work on propositional LO programs. Following this line of research, we define here a general framework for the bottom-up evaluation of first order linear logic specifications. The evaluation procedure is based on an effective fixpoint operator working on a symbolic representation of infinite collections of first order linear logic formulas. The theory of well quasi-orderings can be used to provide sufficient conditions for the termination of the evaluation of non trivial fragments of first order linear logic.Comment: 53 pages, 12 figures "Under consideration for publication in Theory and Practice of Logic Programming