SHACL is a W3C-proposed schema language for expressing structural constraints on RDF graphs. Recent work on formalizing this language has revealed a striking relationship to description logics. SHACL expressions can use three fundamental features that are not so common in description logics. These features are equality tests; disjointness tests; and closure constraints. Moreover, SHACL is peculiar in allowing only a restricted form of expressions (so-called targets) on the left-hand side of inclusion constraints. The goal of this paper is to obtain a clear picture of the impact and expressiveness of these features and restrictions. We show that each of the four features is primitive: using the feature, one can express boolean queries that are not expressible without using the feature. We also show that the restriction that SHACL imposes on allowed targets is inessential, as long as closure constraints are not used. In addition, we show that enriching SHACL with "full" versions of equality tests, or disjointness tests, results in a strictly more powerful language.
翻译:SHACL是一种表达对RDF图表结构性限制的W3C拟议计划语言。最近关于正式化该语言的工作揭示了与描述逻辑的惊人关系。 SHACL的表达方式可以使用三个在描述逻辑中不太常见的基本特征。这些特征是平等测试、脱节测试和封闭限制。此外,SHACL在包容限制的左侧只允许有限形式的表达形式(所谓的目标)是特别的。本文的目的是清楚了解这些特征和限制的影响和表现。我们表明,这四个特征都是原始的:使用特征,可以表达布林恩询问,而无需使用特征是无法表达的。我们还表明,只要不使用关闭限制,SHACL对允许的目标施加的限制是不必要的。此外,我们显示,以“完整”版本的平等测试或脱节性测试来丰富SHACLL的“完整”版本或脱节性测试,其结果都是非常有力的语言。