Consider adding property paths to ShEx

[labra]

In some cases, it may be interesting to be able to define something like:

<GrandParent> { :child/:child IRI } 

which instead of using a single predicate in the triple constraint, uses a property path similar to SPARQL property paths.

At this moment, the previous example can be defined as

<grandParent> { :child { :child IRI } }

[ericprud]

[reopened - apologies for the click-o]

I believe that property paths are a generalization of nested properties, e.g.:

<PatientShape> {
  <name> {
    <given> LITERAL;
    <family> LITERAL
  }+
}

could be represented (with some loss of cardinality specificity) as

<PatientShape> {
  <name>/<given> LITERAL+;
  <name>/<family> LITERAL+
}

This uses the property path to traverse to a value which is then constrained by a NodeConstraint.

Two challenges arise:

• cardinality; If we use a traditional SPARQL property paths interpretation, <name>/<given> LITERAL+ could mean that there are 1+ <name>s, each with a single <given> property, a single <name> with 1+ <given> properties, or 1+ <name>s, each with 1+ <given>s. If we allow SPARQL property path cardinalities, we need to figure out what <name>+/<given>+ LITERAL+ means.

• CLOSEDness: what's it mean if there's a to close e.g. <PatientShape> CLOSED { <name>/<given> LITERAL+ }; do we reject e.g. `<patient1> <name> [ <given> "Alice"; <somethingElse> true ] .

Translations

ShEx nested shape	SPARQL property path
:child { :child .* }*	:child/:child
:child { :child . }	nested SELECT
- outside expressivity -	:child*/:child

[ericprud]

Note that for many use cases involving a property path and a cardinality, the SHACL behavior is pathological; it UNIQUEs the pair of starting node and reached node. For example, a use case like making sure that only one check was written for a particular invoice (sh:path (:paymentCheck :amount); sh:maxCount 1) will not detect multiple checks written for the same amount.

[andrawaag]

Move it to a feature request/discussion in the context of a 2.2 or x release