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W3C提出的用于表达RDF图结构约束的模式语言。近期对该语言形式化的研究揭示了其与描述逻辑之间显著的联系。SHACL表达式可使用三种在描述逻辑中并不常见的核心特征，即全等性测试、不相交性测试与闭合约束。此外，SHACL的特殊之处在于仅允许在包含约束的左侧使用受限的表达式形式（即所谓的“目标”）。本文旨在明确这些特征与约束的影响及表现力。我们证明上述四个特征均具有原子性：使用该特征可表达若不使用则无法表达的布尔查询。同时证明，只要不使用闭合约束，SHACL对允许目标的限制并非本质性的。此外，我们表明，为SHACL增加“完整”版本的全等性测试或不相交性测试，将产生严格意义上更强大的语言。