Michael Hanus

Kiel University | CAU · Department of Computer Science

Logic programming is based on defining relations. Functions are often considered as syntactic sugar which can be transformed into predicates so that their logic is not used for computational purposes. In this paper, we present a method to use functions to improve the operational behavior of logic programs without loosing the flexibility of logic pr...

Logic programming is a flexible programming paradigm due to the use of predicates without a fixed data flow. To extend logic languages with the compact notation of functional programming, there are various proposals to map evaluable functions into predicates in order to stay in the logic programming framework. Since amalgamated functional logic lan...

Logic programming is a flexible programming paradigm due to the use of predicates without a fixed data flow. To extend logic languages with the compact notation of functional programming, there are various proposals to map evaluable functions into predicates in order to stay in the logic programming framework. Since amalgamated functional logic lan...

Pull-tabbing is an evaluation technique for functional logic programs which computes all non-deterministic results in a single graph structure. Pull-tab steps are local graph transformations to move non-deterministic choices towards the root of an expression. Pull-tabbing is independent of a search strategy so that different strategies (depth-first...

Web interfaces are an important part of many applications but their implementation is full of pitfalls due to the client/server nature of web programming. This paper presents a lightweight approach to web programming based on a standard infrastructure, in particular, the common CGI protocol between client and server. No specific additions are neces...

This volume constitutes the pre-proceedings of the 28th International Workshop on Functional and Logic Programming (WFLP 2020), organized by the University of Bologna, Italy, as part of Bologna Federated Conference on Programming Languages 2020. The international Workshop on Functional and (constraint) Logic Programming (WFLP) aims at bringing toge...

Pull-tabbing is an evaluation technique for functional logic programs which computes all non-deterministic results in a single graph structure. Pull-tab steps are local graph transformations to move non-deterministic choices towards the root of an expression. Pull-tabbing is independent of a search strategy so that different strategies (depth-first...

Functional logic languages can solve equations over user-defined data and functions. Thus, the definition of an appropriate meaning of equality has a long history in these languages, ranging from reflexive equality in early equational logic languages to strict equality in contemporary functional logic languages like Curry. With the introduction of...

FlatCurry is a well-established intermediate representation of Curry programs used in compilers that translate Curry code into Prolog or Haskell code. Some FlatCurry constructs have no direct translation into imperative code. These constructs must be each handled differently when translating Curry code into, e.g., C, Open image in new window or Pyt...

Checking the semantic equivalence of operations is an important task in software development. For instance, regression testing is a routine task performed when software systems are developed and improved, and software package managers require the equivalence of operations in different versions of a package within the same major number version. A so...

FlatCurry is a well-established intermediate representation of Curry programs used in compilers that translate Curry code into Prolog and Haskell code. Some FlatCurry constructs have no direct translation into imperative code. These constructs must be each handled differently when translating Curry code into C, C++ and Python code. We introduce a n...

Functional logic languages can solve equations over user-defined data and functions. Thus, the definition of an appropriate meaning of equality has a long history in these languages, ranging from reflexive equality in early equational logic languages to strict equality in contemporary functional logic languages like Curry. With the introduction of...

Set functions are a feature of functional logic programming to encapsulate all results of a non-deterministic computation in a single data structure. Given a function f of a functional logic program written in Curry, we describe a technique to synthesize the definition of the set function of f. The definition produced by our technique is based on s...

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...

Set functions are a feature of functional logic programming to encapsulate all results of a non-deterministic computation in a single data structure. Given a function $f$ of a functional logic program written in Curry, we describe a technique to synthesize the definition of the set function of $f$. The definition produced by our technique is based...

Static type systems are usually not sufficient to express all requirements on function calls. Hence, contracts with pre- and postconditions can be used to express more complex constraints on operations. Contracts can be checked at run time to ensure that operations are only invoked with reasonable arguments and return intended results. Although suc...

Checking the semantic equivalence of operations is an important task in software development. For instance, regression testing is a routine task when software systems are developed and improved, and software package managers require the equivalence of operations in different versions of a package within the same major version. In order to support a...

This book constitutes revised selected papers from the 21st International Conference on Applications of Declarative Programming and Knowledge Management, INAP 2017, the 31st Workshop on Logic Programming, WLP 2017, and the 25th Workshop on Functional and (Constraint) Logic Programming, WFLP 2017. The 12 full papers presented in this volume were car...

Static type systems are usually not sufficient to express all requirements on function calls. Hence, contracts with pre- and postconditions can be used to express more complex constraints on operations. Contracts can be checked at run time to ensure that operations are only invoked with reasonable arguments and return intended results. Although suc...

We present CurryCheck, a tool to automate the testing of programs written in the functional logic programming language Curry. CurryCheck executes unit tests as well as property tests which are parameterized over one or more arguments. CurryCheck tests properties by systematically enumerating test cases so that, for smaller finite domains, CurryChec...

Functional logic programming languages support non-deterministic search and a flexible use of defined operations by applying them to unknown values. The use of these features has the risk that equal values might be computed several times or I/O computations could fail due to non-deterministic subcomputations. To detect such problems at compile time...

We investigate proving properties of Curry programs using Agda. First, we address the functional correctness of Curry functions that, apart from some syntactic and semantic differences, are in the intersection of the two languages. Second, we use Agda to model non-deterministic functions with two distinct and competitive approaches incorporating th...

We investigate proving properties of Curry programs using Agda. First, we address the functional correctness of Curry functions that, apart from some syntactic and semantic differences, are in the intersection of the two languages. Second, we use Agda to model non-deterministic functions with two distinct and competitive approaches incorporating th...

We present an extension of the declarative programming language Curry to support the access to data stored in relational databases via SQL. Since Curry is statically typed, our emphasis on this SQL integration is on type safety. Our extension respects the type system of Curry so that run-time errors due to ill-typed data are avoided. This is obtain...

Although functional as well as logic languages use equality to discriminate between logically different cases, the operational meaning of equality is different in such languages. Functional languages reduce equational expressions to their Boolean values, True or False, logic languages use unification to check the validity only and fail otherwise. C...

We present CurryCheck, a tool to automate the testing of programs written in the functional logic programming language Curry. CurryCheck executes unit tests as well as property tests which are parameterized over one or more arguments. In the latter case, CurryCheck tests these properties by systematically enumerating test cases so that, for smaller...

In functional logic programs, rules are applicable independently of textual order, i.e., any rule can potentially be used to evaluate an expression. This is similar to logic languages and contrary to functional languages, e.g., Haskell enforces a strict sequential interpretation of rules. However, in some situations it is convenient to express alte...

In functional logic programs, rules are applicable independently of textual order, i.e., any rule can potentially be used to evaluate an expression. This is similar to logic languages and contrary to functional languages, e.g., Haskell enforces a strict sequential interpretation of rules. However, in some situations it is convenient to express alte...

Although functional as well as logic languages use equality to discriminate between logically different cases, the operational meaning of equality is different in such languages. Functional languages reduce equational expressions to their Boolean values, True or False, logic languages use unification to check the validity only and fail otherwise. C...

Constraint Handling Rules (CHR) is a rule-based language to specify application-oriented constraint solvers. CHR requires a host language that provides the basic constraints used in a CHR program. In this paper, we argue that an integrated functional logic language like Curry is an appropriate host language for CHR since it supports a natural formu...

We present the design, implementation, and application of a system, called CASS, for the analysis of functional logic programs. The system is generic so that various kinds of analyses (e.g., groundness, non-determinism, demanded arguments) can be easily integrated. In order to analyze larger applications consisting of dozens or hundreds of modules,...

We present a partial evaluator for functional logic programs written in Curry. In contrast to previous approaches to the partial evaluation of functional logic programs, we take into account the features used in contemporary Curry programs, in particular, non-deterministic operations and recursive let expressions. For this purpose, we base our part...

Curry is a successful, general-purpose, functional logic programming language that predefines a singleton type Success explicitly to support its logic component. We take the likely-controversial position that without Success Curry would be as much logic or more. We draw a short history and motivation for the existence of this type and justify why i...

Encapsulated search is a key feature of (functional) logic languages. It allows the programmer to access and process different results of a non-deterministic computation within a program. Unfortunately, due to advanced operational features (lazy evaluation, partial values, infinite structures), there is no straightforward definition of the semantic...

We present a system, called CASS, for the analysis of functional logic programs. The system is generic so that various kinds of analyses (e.g., groundness, non-determinism, demanded arguments) can be easily integrated. In order to analyze larger applications consisting of dozens or hundreds of modules, CASS supports a modular and incremental analys...

Xtext is an open-source framework for implementing external, textual domain-specific languages (DSLs). So far, most DSLs implemented with Xtext and similar tools focus on structural aspects such as service specifications and entities. Because behavioral aspects are significantly more complicated to implement, they are often delegated to general-pur...

Xtext is an open-source framework for implementing external, textual domain-specific languages (DSLs). So far, most DSLs implemented with Xtext and similar tools focus on structural aspects such as service specifications and entities. Because behavioral aspects are significantly more complicated to implement, they are often delegated to general-pur...

The expressive power of functional logic languages supports high-level specifications as well as efficient implementations of problems in the same language. If specifications are executable, they can be used both as initial prototypical implementations and as contracts for checking the reliable execution of implementations intended to satisfy the s...

Functional logic languages combine lazy (demand-driven) evaluation strategies from functional programming with non-deterministic computations from logic programming. The lazy evaluation of non-deterministic subexpressions results in a demand-driven exploration of the search space: if the value of some subexpression is not required, the complete sea...

KiCS2 is a new system to compile functional logic programs of the source language Curry into purely functional Haskell programs. The implementation is based on the idea to represent the search space as a data structure and logic variables as operations that generate their values. This has the advantage that one can apply various, and in particular,...

In this paper we present our first steps towards a new system to compile functional logic programs of the source language Curry into purely functional Haskell programs. Our implementation is based on the idea to represent non-deterministic results as values of the data types corresponding to the results. This enables the application of various sea...

Patterns distill successful experience in solving common software problems. We introduce a handful of new software design patterns for functional logic languages. Some patterns are motivated by the evolution of the paradigm in the last 10 years. Following usual approaches, for each pattern we propose a name and we describe its intent, applicabilit...

We describe a framework to support the implementation of web-based systems intended to manipulate data stored in relational databases. Since the conceptual model of a relational database is often specified as an entity-relationship (ER) model, we propose to use the ER model to generate a complete implementation in the declarative programming langua...

In this article we show how functional logic programming techniques can be used to construct a bidirectional transformation between structured process models of the business process modeling notation (BPMN) and executable models of the business process execution language (BPEL). We specify the abstract syntax of structured process models by a conte...

In order to give application programs access to data stored in the web in semistructured formats, in particular, in XML format, we propose a domain-specific language for declarative processing such data. Our language is embedded in the functional logic programming language Curry and offers powerful matching constructs that enable a declarative desc...

The evolution of programming languages is the stepwise introduction of abstractions hiding the underlying computer hardware and the details of program execution. Assembly languages introduce mnemonic instructions and symbolic labels for hiding machine codes and addresses. Fortran introduces arrays and expressions in standard mathematical notation f...

We present a high-level transformation scheme to translate lazy functional logic programs into pure Haskell programs. This transformation is based on a recent proposal to efficiently implement lazy non-deterministic computations in Haskell in a monadic style. We build on this work and define a systematic method to transform lazy functional logic pr...

Assertions or contracts are an important technique to improve the quality of software. Thus, assertions are also desirable for functional logic programming. Unfortunately, there is no established meaning of assertions in languages with a demand-driven evaluation strategy. Strict assertions are immediately checked but may influence the behavior of p...

We propose a novel approach to encapsulate non-deterministic computations in functional logic programs. Our approach is based on set functions that return the set of all the results of a correspond- ing ordinary operation. A characteristic feature of our approach is the complete separation between a usually-non-deterministic oper- ation and its pos...

This paper proposes a declarative description of user inter- faces that abstracts from low-level implementation details. In particular, the user interfaces specified in our framework,are executable as graph- ical user interfaces on single local host computers as well as web,user interfaces via web standard browsers. Thus, our approach combines the...

This book constitutes the thoroughly refereed post-conference proceedings of the 18th International Symposium on Logic-Based Program Synthesis and Transformation, LOPSTR 2008, held in Valencia, Spain, during July 17-18, 2008. The 11 revised full papers presented together with one invited talk were carefully reviewed and selected for inclusion in th...

This paper presents a new program analysis framework to approximate call patterns and their results in functional logic computations. We consider programs containing non- strict, nondeterministic operations in order to make the analysis applicable to modern functional logic languages like Curry or TOY. For this purpose, we present a new xpoint char...

This paper presents an environment to support high-level database programming in the multi-paradigm declarative programming language Curry. We define an application programming interface (API) that abstracts from the concrete database access methods. The API sup- ports transactions and exploits Curry's type system to ensure a strict separation betw...

This paper proposes an environment to support high-level database programming in a declarative programming language. In order to ensure safe database updates, all access and update operations related to the database are generated from high-level descriptions in the entity- relationship (ER) model. We propose a representation of ER diagrams in the d...

Designing debugging tools for lazy functional programming languages is a complex task which is often solved by expensive tracing of lazy computations. We present a new approach in which the information collected as a trace is reduced considerably (kilobytes instead of megabytes). The idea is to collect a kind of step information for a call-by-value...

Declarative programming languages advocate a program- ming style expressing the properties of problems and their solutions rather than how to compute individual solutions. Depending on the un- derlying formalism to express such properties, one can distinguish differ- ent classes of declarative languages, like functional, logic, or constraint progra...

We propose a framework to construct web-oriented user interfaces in a high-level way by exploiting declarative programming tech- niques. Such user interfaces are intended to manipulate complex data in a type-safe way, i.e., it is ensured that only type-correct data is accepted by the interface, where types can be specified by standard types of a pr...

In this work, we develop a partial evaluation technique for residuating functional logic programs, which generalize the concurrent computation models for logic programs with delays to functional logic programs. We show how to lift the nondeterministic choices from run time to specialization time. We ascertain the conditions under which the original...

Computing with failures is a typical programming technique in functional logic programs. However, there are also situations where a program should not fail (e.g., in a deterministic top-level computation) but the evaluation fails accidentally, e.g., due to missing pattern combinations in an operation defined by pattern matching. In this case, the p...

We present CurryBrowser, a generic analysis environment for the declarative multi-paradigm language Curry. CurryBrowser supports browsing through the program code of an application written in Curry, i.e., the main module and all directly or indirectly imported modules. Each module can be shown in different formats (e.g., source code, interface, int...

Functional logic languages extend purely functional languages with two features: operations defined by overlapping rules and logic variables in both defining rules and expressions to evaluate. In this paper, we show that only one of these features is sufficient in a core language. On the one hand, overlapping rules can be eliminated by introducing...

This paper proposes a new technique for the high-level construction of type-safe web-oriented user interfaces. Our approach is useful to equip applications processing structured data with interfaces to manipulate these data in an efficient and maintainable way. The in- terfaces are web-based, i.e., the data can be manipulated with stan- dard web br...

We present an efficient implementation method for a language that amalgamates functional and logic programming styles. The operational semantics of the language consists of resolution to solve predicates and narrowing and rewriting to evaluate functional expressions. The implementation is based on an extension of the Warren Abstract Machine (WAM)....

Unification in the presence of an equational theory is an important problem in theorem-proving and in the integration of functional and logic programming languages. This paper presents an improvement of the proposed lazy unification methods by incorporating simplification into the unification process. Since simplification is a deterministic computa...

This paper proposes an integration of Constraint Handling Rules (CHR), a rule-based language to specify application-oriented constraint solvers, into the declarative multi-paradigm language Curry. This integration provides a convenient way to specify and use flexible constraint systems in applications implemented in Curry. We propose to represent C...

Functional logic programming languages combine the most important declarative programming paradigms, and attempts to com-bine these paradigms have a long history. The declarative multi-paradigm language Curry is influenced by recent advances in the foundations and implementation of functional logic languages. The development of Curry is an internat...

Information about the nondeterminism behavior of a functional logic program is important for various reasons. For instance, a nondeterministic choice in I/O operations results in a run-time error. Thus, it is desirable to ensure at compile time that a given program is not going to crash in this way. Furthermore, knowledge about nondeterminism can b...

In this paper we present CurryBrowser, a generic analysis environment for the declarative multi-paradigm language Curry. CurryBrowser supports browsing through the implementation of an application written in Curry, i.e., the main module and all directly or indirectly imported modules. Each module can be shown in different formats (e.g., source code...

We propose an extension of functional logic languages that allows the denition of operations with patterns containing other dened operation symbols. Such ìfunction patternsî have many advantages over traditional constructor pat- terns. They allow a direct representation of specications as declarative programs, provide better abstractions of pattern...

Declarative multi-paradigm languages combine the most important features of functional, logic and concurrent programming. The computational model of such integrated languages is usually based on a combination of two different operational principles: narrowing and residuation. This work is motivated by the fact that a precise definition of an operat...

Many functional logic languages are based on narrowing, a unification-based goal-solving mechanism which subsumes the reduction mechanism of functional languages and the resolution principle of logic languages. Needed narrowing is an optimal evaluation strategy which constitutes the basis of modern (narrowing-based) lazy functional logic languages....

An injective finite mapping is an abstraction common to many programs. We describe the design of an injective finite mapping and its implementation in Curry, a functional logic language. Curry supports the concurrent asynchronous execution of distinct portions of a program. This condition prevents passing from one portion to another a structure con...

We describe the architecture of a virtual machine for executing func- tional logic programming languages. A distinguishing feature of our machine is that it preserves the operational completeness of non-deterministic programs by concurrently executing a pool of independent computations. Each computation executes only root-needed sequential narrowin...

In this work, we introduce a profiling scheme for modern functional logic languages covering notions like laziness, sharing, and non-determinism. Firstly, we instrument a natural (big-step) semantics in order to associate a symbolic cost to each basic operation (e.g., variable updates, function unfoldings, case evaluations). While this cost semanti...

We introduce the theoretical basis for tracing lazy functional logic computations in a declarative multi-paradigm language like Curry. Tracing computations is a difficult task due to the subtleties of the underlying operational semantics which combines laziness and non-determinism. In this work, we define an instrumented operational semantics that...

In this work, we introduce a profiling scheme for modern functional logic languages covering notions like laziness, sharing, and non-determinism. Firstly, we instrument a natural (big-step) semantics in order to associate a symbolic cost to each basic operation (e.g., variable updates, function unfoldings, case evaluations). While this cost semanti...

A lightweight approach to debugging functional logic pro- grams by observations is presented, implemented for the language Curry. The Curry Object Observation System (COOSy) comprises a portable library plus a viewing tool. A programmer can observe data structures and functions by annotating expressions in his program. The possibly partial values o...

One of the key features of the integration of functional and logic lan- guages is the access to non-deterministic computations from the functional part of the program. In order to ensure the determinism of top-level computations in a functional logic program, which is usually a monadic sequence of I/O operations, one has to encapsulate the non-dete...

In this paper we propose a new concept to deal with dynamic predicates in functional logic programs. The definition of a dynamic predicate can change over time, i.e., one can add or remove facts that define this predicate. Our approach is easy to use and has a clear semantics that does not depend on the particular (demand-driven) evaluation strateg...

The growing use of XML in commercial as well as non-commercial domains to transport information poses new challenges to concepts to access this information. Common ways to access parts of a document use XPath-expressions. We provide a transformation of DTDs into a knowledge base in Description Logic. We use reasoning capabilities grounded in descri...

This paper deals with a framework to program autonomous robots in the declarative multi-paradigm language Curry. Our goal is to apply a high-level declarative pro- gramming language for the programming of embedded systems. For this purpose, we use a specialization of Curry called Embedded Curry. We show the basic ideas of our framework and an imple...

We present a new evaluation strategy for functional logic programs described by weakly orthogonal conditional term rewriting systems. Our notion of weakly orthogonal conditional rewrite system extends a notion of Bergstra and Klop and covers a large part of programs defined by conditional equations. Our strategy combines the flexibility of logic pr...

In this paper we present a framework to program autonomous robots in the declarative multi-paradigm language Curry. This is an experiment to use high-level declarative programming languages for the programming of embedded systems. Our programming model is based on a recent proposal to integrate a process-oriented specification language in Curry. We...

Recent proposals for multi-paradigm declarative programming combine the most important features of functional, logic and concurrent programming into a single framework. The operational semantics of these languages is usually based on a combination of narrowing and residuation. In this paper, we introduce a non-standard, residualizing semantics for...

In this paper we present a system, called CurryWeb, to sup- port web-based learning. Characteristic features of this system is open- ness and self-responsible use. Openness means that there is no strong distinction between instructors and students, i.e., every user can learn with the system or add new learning material to it. Self-responsible use m...

Practical declarative multi-paradigm languages combine the main features of functional, logic and concurrent programming (e.g., laziness, sharing, higher-order, logic variables, non-determinism, search strategies). Usually, these languages also include interfaces to external functions as well as to constraint solvers. In this work, we introduce the...

In this work we provide a semantic description of functional logic languages covering notions like laziness, sharing, and non-determinism. Such a semantic description is essential, for instance, to have appropriate language definitions in order to reason about programs and check the correctness of implementations. First, we define a “big-step” sema...