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Ebook: Type-Safe Programming for the Semantic Web
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Graph-based data formats are a flexible way of representing data – semantic data models in particular – where the schema is part of the data, and have become more popular and had some commercial success in recent years. Semantic data models are also the basis for the Semantic Web – a Web of data governed by open standards in which computer programs can freely access the data provided.
This book is about checking the correctness of programs that can access semantic data. Although the flexibility of semantic data models is one of their greatest strengths, it can lead programmers to accidentally fail to account for unintuitive edge cases, leading to run-time errors or unintended side-effects during program execution. A program may even run for a long time before such an error occurs and the program crashes. Providing a type system is an established methodology for proving the absence of run-time errors in programs without requiring execution. The book defines type systems that can detect and avoid such run-time errors based on schema languages available for the Semantic Web. Using the Web Ontology Language (OWL) and its theoretic underpinnings i.e. description logics, and the Shapes Constraint Language (SHACL) in particular, the book defines systems that can provide type-safe data access to semantic data graphs. The book is divided into 3 parts: Part I contains an introduction and preliminaries; Part II covers type systems for the Semantic Web; and Part III includes related work and conclusions.
Graph-based data formats are flexible in representing data. In particular semantic data models, where the schema is part of the data, gained traction and commercial success in recent years. Semantic data models are also the basis for the Semantic Web–a Web of data governed by open standards in which computer programs can freely access the provided data. This thesis is concerned with the correctness of programs that access semantic data. While the flexibility of semantic data models is one of their biggest strengths, it can easily lead to programmers accidentally not accounting for unintuitive edge cases. Often, such exceptions surface during program execution as run-time errors or unintended side-effects. Depending on the exact condition, a program may run for a long time before the error occurs and the program crashes.
This thesis defines type systems that can detect and avoid such run-time errors based on schema languages available for the Semantic Web. In particular, this thesis uses the Web Ontology Language (OWL) and its theoretic underpinnings, i. e., description logics, as well as the Shapes Constraint Language (SHACL) to define type systems that provide type-safe data access to semantic data graphs. Providing a safe type system is an established methodology for proving the absence of run-time errors in programs without requiring execution. Both schema languages are based on possible world semantics but differ in the treatment of incomplete knowledge. While OWL allows for modelling incomplete knowledge through an open-world semantics, SHACL relies on a fixed domain and closed-world semantics. We provide the formal underpinnings for type systems based on each of the two schema languages. In particular, we base our notion of types on sets of values which allows us to specify a subtype relation based on subset semantics. In case of description logics, subsumption is a routine problem. For the type system based on SHACL, we are able to translate it into a description logic subsumption problem.