17556593
Type assignment and termination of interaction nets
Interaction nets have proved to be a useful tool for the study of computational aspects of various formalisms (e.g. λ-calculus, term rewriting systems), but they are also a programming paradigm in themselves, and this is actually how they were introduced by Lafont. In this paper we consider semi-simple interaction nets as a programming language, and present a type assignment system using intersection types. First we show that interactions preserve types (i.e., the system enjoys subject reduction), and we compare this type assignment system with the intersection systems for λ-calculus and term rewriting systems. Then we define a recursion scheme that ensures termination of all interaction sequences. By relaxing the scheme and using the type assignment system, we derive another sufficient condition for termination of interaction nets. Finally, we show that although the type system based on general intersection types is not decidable, its restriction to rank 2 types is, and we give an algorithm that computes principal types for nets.
6993516
A Theory of Operational Equivalence for Interaction Nets
The notion of contextual equivalence is fundamental in the theory of programming languages. By setting up a notion of bisimilarity, and showing that it coincides with contextual equivalence, one obtains a simple coinductive proof technique for showing that two programs are equivalent in all contexts. In this paper we apply these (now standard) techniques to interactions nets, a graphical programming language characterized by local reduction. This work generalizes previous studies of operational equivalence in interaction nets since it can be applied to untyped systems, thus all systems of interaction nets are captured.
1019046
Implementing Nominal Unification
Nominal matching and unification underly the dynamics of nominal rewriting. Urban, Pitts and Gabbay gave a nominal unification algorithm which finds the most general solution to a nominal matching or unification problem, if one exists. Later the algorithm was extended by Fernandez and Gabbay to deal with name generation and locality. In this paper we describe first a direct implementation of the nominal unification algorithm, including the extensions, in Maude. This implementation is not efficient (it is exponential in time), but we will show that we can obtain a feasible implementation by using termgraphs.
16649053
Coinductive techniques for operational equivalence of interaction nets
In this paper we study a notion of operational equivalence for interaction nets, following the recent success of applying methods based on bisimulation to functional and object oriented programming languages. We set up notions of contextual equivalence and bisimilarity and show that they coincide. A coinduction principle then gives a simple and robust way of showing when two interaction nets are contextually equivalent. We include several examples to demonstrate the usefulness of the approach, in particular for optimizing interaction nets.
9830270
Declarative specification of Web sites with Strudel
Abstract. Strudel is a system for implementing data-intensive Web sites, which typically integrate information from multiple data sources and have complex structure. Strudel's key idea is separating the management of a Web site's data, the specification of its content and structure, and the visual representation of its pages. Strudel provides a declarative query language for specifying a site's content and structure, and a simple template language for specifying a site's HTML representation. This paper contains a comprehensive description of the Strudel system and details the benefits of declarative site specification. We describe our experiences using Strudel in a production application and describe three different, but complementary, systems that extend and improve upon Strudel's original ideas.
14930878
SilkRoute: A framework for publishing relational data in XML
XML is the "lingua franca" for data exchange between interenterprise applications. In this work, we describe SilkRoute, a framework for publishing relational data in XML. In SilkRoute, relational data is published in three steps: the relational tables are presented to the database administrator in a canonical XML view; the database administrator defines in the XQuery query language a public, virtual XML view over the canonical XML view; and an application formulates an XQuery query over the public view. SilkRoute composes the application query with the public-view query, translates the result into SQL, executes this on the relational engine, and assembles the resulting tuple streams into an XML document. This work makes some key contributions to XML query processing. First, it describes an algorithm that translates an XQuery expression into SQL. The translation depends on a query representation that separates the structure of the output XML document from the computation that produces the document's content. The second contribution addresses the optimization problem of how to decompose an XML view over a relational database into an optimal set of SQL queries. We define formally the optimization problem, describe the search space, and propose a greedy, cost-based optimization algorithm, which obtains its cost estimates from the relational engine. Experiments confirm that the algorithm produces queries that are nearly optimal.
10188867
A query language for a Web-site management system
We have designed a system, called STRUDEL, which applies familiar concepts from database management systems, to the process of building web sites. The main motivation for developing STRUDEL is the observation that with current technology, creating and managing large sites is tedious, because a site designer must simultaneously perform (at least) three tasks: (1) choosing what information will be available at the site, (2) organizing that information in individual pages or in graphs of linked pages, and (3) specifying the visual presentation of pages in HTML. Furthermore, since there is no separation between the physical organization of the information underlying a web site and the logical view we have on it, changing or restructuring a site are unwieldy tasks. In STRUDEL, the web site manager can separate the logical view of information available at a web site, the structure of that information in linked pages, and the graphical presentation of pages in HTML. First, the site builder defines independently the data that will be available at the site. This process may require creating an integrated view of data from multiple (external) sources. Second, the site builder defines the structure of the web-site. The structure is defined as a view over the underlying information, and different versions of the site can be defined by specifying multiple views. Finally, the graphical representation of the pages in the web site is specified. This paper describes the query language that lies at the heart of the STRUDEL system. In STRUDEL, we model the da ta at the different levels as graphs. That is, the data in the external sources, the da ta in the integrated view and the web-site itself are modeled as graphs. A graph model is appropriate because site da ta may be derived from multiple sources, such as existing database systems and HTML files. Consequently, our system requires a query language for (1) defining the integrated view of the data, and (2) defining the structure of web sites. An important requirement of our query language is that it be able to construct graphs. Our query processor needs to be able to answer queries tha t involve accessing different da ta sources. Even though we model the sources as containing graphs, we cannot assume they have a uniform representation of graphs. Hence, our query processor needs to adhere to possible limitations on access to data in the graphs, and should be able to exploit additional querying capabilities that an external source may have. We have designed a general framework for processing STRUDEL queries over multiple unstructured data sources, and are designing optimizations that use the capabilities of external sources whenever possible. The purpose of this paper is to describe the syntax and semantics of STRUQL, the query language at the core of STRUDEL. We believe that STRuQL is a language of independent interest, and is useful for other applications involving the management of semistructured data, as well as a view definition language for such data. We discuss the relationship of STRUQL to other languages proposed in the li terature in Section 6: see [Abi97, Bun97].
14181773
Nominal rewriting
Nominal rewriting is based on the observation that if we add support for @a-equivalence to first-order syntax using the nominal-set approach, then systems with binding, including higher-order reduction schemes such as @l-calculus beta-reduction, can be smoothly represented. Nominal rewriting maintains a strict distinction between variables of the object-language (atoms) and of the meta-language (variables or unknowns). Atoms may be bound by a special abstraction operation, but variables cannot be bound, giving the framework a pronounced first-order character, since substitution of terms for variables is not capture-avoiding. We show how good properties of first-order rewriting survive the extension, by giving an efficient rewriting algorithm, a critical pair lemma, and a confluence theorem for orthogonal systems.
6149937
Publishing Relational Data in XML: the SilkRoute Approach
Xyleme is a dynamic warehouse for XML data of the Web supporting query evaluation, change control and data integration. We briefly present our motivations, the general architecture and some aspects of Xyleme. The project we describe here was completed at the end of 2000. A prototype has been implemented. This prototype is now being turned into a product by a start-up company also called Xyleme [14]. Xyleme: a complex tissue of wood cells, functions in conduction and storage ...