Shacl

SHACL Skill

Design, implement, and validate RDF data structures with the Shapes Constraint Language. This skill embodies the practical wisdom that shapes provide a fundamentally different way of thinking about data than classes—they describe structural expectations and patterns without requiring formal hierarchies.

Core Philosophy (Cagle's Principles)

"OWL is not going away, but there is definitely a shift occurring within the semantic community around the use of SHAPES, rather than classes and properties."

Key Insights:

1. Shapes, not classes: "A shape is, at its core, simply a pattern that is common to one or more nodes in a graph"
2. SHACL wraps SPARQL: "Learn SPARQL. SHACL can be thought of as a dedicated wrapper around SPARQL queries and filters"
3. Validation, not inference: SHACL validates existing data; OWL prevents invalid data through inference
4. Beyond validation: SHACL provides "structural information, including data groupings, ordering, and interfaces"
5. The future is shapes: "SHACL will eventually be foundational not just for RDF, but at the broader level of data design and management"

Guide Router

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SHACL vs OWL: The Paradigm Shift

Why the Shift?

"Reasoners are disappearing from newer knowledge graph systems, partly because there is no real demand and partly because SPARQL and SPARQL Update are more targeted in their capabilities." — Cagle

Philosophical Differences

OWL (Open World):

# No children stated doesn't mean childless
ex:John a ex:Person .
# John might have children we don't know about
# OWL CANNOT conclude John is childless

SHACL (Closed World by Default):

ex:PersonShape a sh:NodeShape ;
    sh:targetClass ex:Person ;
    sh:property [
        sh:path ex:hasChild ;
        sh:maxCount 0 ;
        sh:message "Person has no children recorded"
    ] .
# SHACL CAN validate that John has no children in the data

SHACL Quick Reference

Core Vocabulary

@prefix sh: <http://www.w3.org/ns/shacl#> .
@prefix ex: <http://example.org/> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

Basic Node Shape

ex:PersonShape a sh:NodeShape ;
    sh:targetClass ex:Person ;           # What to validate
    sh:property ex:NamePropertyShape ;   # Property constraints
    sh:property ex:EmailPropertyShape .

Basic Property Shape

ex:NamePropertyShape a sh:PropertyShape ;
    sh:path ex:name ;                    # Property to constrain
    sh:minCount 1 ;                      # At least one
    sh:maxCount 1 ;                      # At most one
    sh:datatype xsd:string ;             # Must be string
    sh:message "Person must have exactly one name" .

Inline Property Shapes

ex:PersonShape a sh:NodeShape ;
    sh:targetClass ex:Person ;
    sh:property [
        sh:path ex:name ;
        sh:minCount 1 ;
        sh:datatype xsd:string
    ] ;
    sh:property [
        sh:path ex:email ;
        sh:pattern "^[^@]+@[^@]+\\.[^@]+$" ;
        sh:severity sh:Warning
    ] .

Constraint Components Summary

Value Type Constraints

Cardinality Constraints

Value Range Constraints

String Constraints

Value Constraints

Logical Constraints

Shape-Based Constraints

Property Paths

SHACL supports SPARQL-style property paths:

# Predicate path (direct property)
sh:path ex:name

# Sequence path (chain of properties)
sh:path (ex:parent ex:name)

# Alternative path
sh:path [sh:alternativePath (rdfs:label skos:prefLabel)]

# Inverse path
sh:path [sh:inversePath ex:parent]

# Zero or more
sh:path [sh:zeroOrMorePath ex:parent]

# One or more
sh:path [sh:oneOrMorePath ex:parent]

# Zero or one
sh:path [sh:zeroOrOnePath ex:middleName]

Target Types

Class-Based Targets

ex:PersonShape a sh:NodeShape ;
    sh:targetClass ex:Person .

Node Targets

ex:SpecificShape a sh:NodeShape ;
    sh:targetNode ex:ImportantResource .

Property-Based Targets

# Subjects of property
ex:AuthorShape a sh:NodeShape ;
    sh:targetSubjectsOf ex:wrote .

# Objects of property
ex:BookShape a sh:NodeShape ;
    sh:targetObjectsOf ex:wrote .

Implicit Class Targets

# Shape that is also a class
ex:Person a sh:NodeShape, rdfs:Class ;
    sh:property [...] .

Severity Levels

sh:Violation  # Critical failure (default)
sh:Warning    # Non-critical alert
sh:Info       # Informational message

Usage:

ex:EmailShape a sh:PropertyShape ;
    sh:path ex:email ;
    sh:pattern "^[^@]+@[^@]+$" ;
    sh:severity sh:Warning ;
    sh:message "Email format appears invalid" .

Complete Example

@prefix sh: <http://www.w3.org/ns/shacl#> .
@prefix ex: <http://example.org/> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .

ex:EmployeeShape a sh:NodeShape ;
    sh:targetClass ex:Employee ;

    # Required employee ID
    sh:property [
        sh:path ex:employeeId ;
        sh:name "employeeId" ;
        sh:description "Unique employee identifier" ;
        sh:minCount 1 ;
        sh:maxCount 1 ;
        sh:datatype xsd:string ;
        sh:pattern "^EMP[0-9]{6}$" ;
        sh:severity sh:Violation ;
        sh:message "Employee must have exactly one valid ID (format: EMP######)"
    ] ;

    # Required organization
    sh:property [
        sh:path ex:worksFor ;
        sh:name "worksFor" ;
        sh:minCount 1 ;
        sh:class ex:Organization ;
        sh:severity sh:Violation ;
        sh:message "Employee must work for at least one organization"
    ] ;

    # Optional manager (at most one)
    sh:property [
        sh:path ex:hasManager ;
        sh:name "manager" ;
        sh:maxCount 1 ;
        sh:class ex:Employee ;
        sh:severity sh:Warning ;
        sh:message "Employee should have at most one manager"
    ] .

SHACL Use Cases (Cagle's Framework)

1. Data Quality Assurance: Validate data pipelines before storage
2. Classification: Categorize unstructured data into shapes
3. Structural Definition: Define expected data entry structures
4. Functional Metadata: Bridge object-oriented and graph systems
5. UI Generation: Generate forms and interfaces from shapes
6. API Contracts: Define request/response structures
7. AI Integration: Describe query parameters for LLMs
8. Documentation: Self-documenting requirements specifications

Resources

W3C Specifications

• SHACL Core
• SHACL Advanced Features
• SHACL Use Cases and Requirements

Books

• Semantic Web for the Working Ontologist by Allemang, Hendler, Gandon (3rd ed.)
• Learning SPARQL by Bob DuCharme (O'Reilly)

Kurt Cagle's Work

• The Ontologist — Substack newsletter
• The Cagle Report — Enterprise data and AI
• Validating ANYTHING With SHACL
• SHACL and Taxonomies
• SHACL for User Interfaces
• Using SHACL and AI for Creating Queries