- From: Peter F. Patel-Schneider <peter.patel-schneider@nuance.com>
- Date: Mon, 15 Aug 2016 15:03:31 -0700
- To: <public-rdf-shapes@w3.org>
Here are a few of the problems with the current public working draft I found during a quick scan of it. * pre-binding SPARQL does not evaluate variables that occur in basic graph patterns. This means that the definition of pre-binding has unusual behaviour. For example, the normative SPARQL definition of sh:class will return validation results for every pair of nodes in the graph such that there is an rdf:type/rdfs:subClass* path from the first to the second. This problem affects many parts of the definition of SHACL. It means that the normative definition of many SHACL constructs is counter to intuitions. This problem is not ameliorated by the caution box in Appendix B. * syntax of SPARQL variables SPARQL treats $ and ? as equivalent so $PATH and ?PATH both refer to the PATH variable. SHACL uses $ as a special marker and includes $ and ? as part of the variable. Would ?PATH be substituted as $PATH is? If a SPARQL query for a SHACL constraint only used ?this would the variable this be pre-bound? * pre-binding optional? "SPARQL variables using the $ marker represent external values that must be pre-bound or substituted in the SPARQL query before execution." "When SPARQL constraints are executed, the validation engine should pre-bind values for these variables." Are some $-marked variables not necessarily pre-bound, counter to the earlier requirement? * $PATH vs other $-prefixed variables The variable PATH is treated specially in SHACL. However, the general description of $ does not specially call out PATH: "SPARQL variables using the $ marker represent external values that must be pre-bound or substituted in the SPARQL query before execution." * $value $value is used in many ASK queries. However the definition of ASK validators does not appear to pre-bind value. * aggregation The prohibition "Furthermore, any query that uses the variable $this in an aggregation is invalid." is vague. It appears to disallow the use of $this in any part of the SPARQL 1.1 aggregation machinery, as the pointer in the sentence is to Section 11 of the SPARQL specification. This would rule out all of the examples of aggregation in the SHACL document. * ASK validators syntax The syntax for ASK queries in SPARQL 1.1 is "ASK" DatasetClause* WhereClause SolutionModifier The syntax for WhereClause is 'WHERE'? GroupGraphPattern The syntax for EXISTS constructs SPARQL 1.1 is 'EXISTS' GroupGraphPattern Stripping the ASK from the beginning of an ASK query does not generally end up with a GroupGraphPattern that can be used as the argument for EXISTS. It appears that the values of sh:ask are never used as ASK queries by SHACL processors. Why then are these of the form of ASK queries? * different levels of SHACL implementation There are several different kinds of SHACL implementations that are hinted at in the document. "SHACL implementations may, but are not required to, support entailment regimes." "Access to the shapes graph is not a requirement for supporting the SHACL Core language." "This sections [sic] defines the built-in SHACL constraint components that MUST be supported by all SHACL validation engines." "Not all SHACL validation engines need to support this variable." "The same support policies as for $shapesGraph apply for this variable." "SPARQL engines with full SHACL support can install a new SPARQL function based on the SPARQL 1.1 Extensible Value Testing mechanism." "SHACL validation engines are not required to support any entailment regimes." "SHACL implementations with full support of the SHACL SPARQL extension mechanism must implement a function sh:hasShape, ...." "A SHACL validation engine MUST implement all constructs in the Core of SHACL (Sections 2, 3, 4). A SHACL engine MAY not implement the other parts of SHACL." "Implementations that cover only the the SHACL Core features are not required to implement these mechanisms or the sh:hasShape function." "SHACL validation engines MAY pre-bind the variable $shapesGraph to provide access to the shapes graph." "A SHACL validation engine MAY use such suggestions to determine which shapes graph to use for validating a data graph." "A SHACL validation engine MAY take this information into account to determine which shapes graph to use for validating a data graph that uses that ontology or vocabulary." There needs to be a section that explicitly defines the different levels of implementation. * order of processing for filters The discussion of how filters are processed appears to be contradictory. First there is: "SHACL validation engines MAY alter the order of the depicted steps as long as the returned validation results are correct." Later there is: "Filter shapes MUST be evaluated before validating the associated shapes or constraints." * $shapesGraph The status of $shapesGraph is unclear: "SPARQL variables using the $ marker represent external values that must be pre-bound or substituted in the SPARQL query before execution." "SHACL validation engines MAY pre-bind the variable $shapesGraph to provide access to the shapes graph." * circular filters What happens if a shape is one of its own filters? * EXISTS and blank nodes The definition of ASK binds the value variable and then uses it inside an EXISTS. The definition of SPARQL provides a counter-intuitive result if this variable is bound to a blank node, resulting in, for example, a sh:class constraint with class ex:C returning no violation for _:d in any data graph containing the triple ex:c rdf:type ex:C . * union operations on data graphs and shapes graphs It is unclear just what the data graph and the shapes graph are. There is wording that both of these cannot be changed. However, there is also wording that various kinds of union operations are to be performed on shapes and data graphs. * It is unclear what is meant by: "The variable $targetNode is assumed to be pre-bound to the given value of sh:targetNode." Is this something that SHACL implementations have to do? There are several occurences of this kind of wording. * MAY is used in 1.5 but defined in 1.6 * "A SHACL engine MAY not implement the other parts of SHACL." reads as if no SHACL engine is allowed to implement any non-core part of SHACL. * "The data graph SHOULD include all the ontology axioms related to the data and especially all the rdfs:subClassOf triples in order for SHACL to correctly identify class targets and validate Core SHACL constraints." Data graphs are just graphs. How thus can SHOULD be applied to them? * "A SHACL validation engine MAY use such suggestions to determine which shapes graph to use for validating a data graph." Can this be done even when an explicit shapes graph is provided to the engine? * "The same mechanism applies for ontologies or vocabularies included in the shapes graph. The ontology or the vocabulary IRI can point to one or more shapes graphs with the predicate sh:shapesGraph. A SHACL validation engine MAY take this information into account to determine which shapes graph to use for validating a data graph that uses that ontology or vocabulary." If there already is a shapes graph in play, why is there any need for a different shapes graph to be used? * "a deep copy of sh:path as its sh:path" What is "deep copy" in this context? * "A filter is a shape in a shapes graph that can be used to limit the nodes that are validated against a given constraint or shape." Are there some filters that cannot be used in this way? Which ones? * "The following table enumerates variables that have special meaning in SPARQL constraints. When SPARQL constraints are executed, the validation engine should pre-bind values for these variables." However, many other variables also need to be pre-bound, such as the variables corresponding to parameters.
Received on Monday, 15 August 2016 22:04:07 UTC