Recursive shapes

Shape references can form cycles. SHACL 1.0 leaves their meaning undefined; Shifty uses the following policy.

During validation, a cycle can conform if no reachable constraint violation rules it out. For example, a shape may require each person and everyone they know to have a name. Two named people who know each other conform; removing either name causes the relevant checks to fail. This is called greatest fixed-point validation.

Shifty rejects cycles through negation, such as a shape that requires a node to fail that same shape. Qualified cardinality can also introduce a negative dependency. Inspect a rejected schema with:

shifty inspect --stage strata shapes.ttl

Inference derives new facts only from asserted or previously derived facts (a least fixed point). When enabled, it finishes before validation.

Recursive successes can have coinductive evidence leaves. These do not provide a finite set of triples to delete, so deletion-based repair may be blocked.

The recursion design notes explain the algorithms.