Templates: export SHACL shapes from templates for external validation

[tkuhn]

Summary

Generate a SHACL shape from a nanopublication template, so that nanopublications created from that template can be validated by any standard SHACL engine (rdf4j-shacl, pySHACL, TopBraid …) — independently of nanodash, and producing a standard sh:ValidationReport.

This is Part B of the design doc docs/shacl-alignment.md.

Is it structurally possible?

Yes — as a lossy projection over a well-defined subset, not an isomorphism. The generative-vs-descriptive mismatch is not the blocker for validation export (validation never needs SHACL to construct anything). The existing inward bridge (Template.processShaclTemplate()) already delineates the exportable subset — this is essentially that correspondence table run backwards.

Projects faithfully into core SHACL (most production templates)

• Fixed-subject, fixed-predicate, placeholder-object triples → sh:PropertyShape.
• Fixed triples (constant S/P/O) → sh:hasValue / sh:targetClass.
• Optional / required / repeatable → sh:minCount / sh:maxCount.
• Inline restricted choice → sh:in.
• A placeholder reused as the subject of several triples → property shapes on the same NodeShape; anchored groups → sh:node / sh:qualifiedValueShape.

Needs SHACL-AF (sh:sparql) — possible but non-core

• Predicate-position placeholders (?s ?relation ?o): sh:path cannot be a variable.
• Arbitrary cross-placeholder value-equality joins (beyond sh:equals).
• Live value sources (nt:possibleValuesFromApi): snapshot into sh:in, or SPARQL target.

Dropped (not data-validity concerns)

nt:hasNanopubLabelPattern, nt:hasTag, nt:AdvancedStatement, minting roles, nt:hasDefaultProvenance, nt:hasRequiredPubinfoElement.

Semantic caveats

1. Looser than generation. SHACL (open-world) only checks required triples are present and constraints hold; it does not reject extra triples. A generated shape means "conformant to the template's constraints," not "produced by template T" (the latter is the nt:wasCreatedFromTemplate pubinfo triple).
2. Targetability varies. sh:targetClass works when the template introduces a typed resource; otherwise fall back to sh:targetSubjectsOf/sh:targetObjectsOf. sh:targetNode is unusable (minted IRI unknown in advance).

Suggested scope

Document each generated shape as a validation projection, not a round-trip. Suggest shipping core-only shapes first (drop the SPARQL-needing constructs and log what was dropped), add SHACL-AF later. Consider publishing shapes as nanopublications keyed via dct:isVersionOf / nt:wasCreatedFromTemplate.

See docs/shacl-alignment.md for the full design.