Allow \ to accept an array of JSON Pointers for multiple inheritance

[clemensv]

Summary

Currently, the $extends keyword only accepts a single JSON Pointer to an abstract type (Section 3.10.2):

Value: A JSON Pointer to an abstract type.

This proposal suggests extending $extends to also accept an array of JSON Pointers, enabling multiple inheritance from several abstract base types.

Motivation

When converting from other schema languages (e.g., CDDL with its unwrap operator ~), it's common to compose types from multiple base groups:

base-header = (
    version: uint,
    timestamp: uint
)

metadata = (
    created: uint,
    modified: uint
)

full-record = {
    ~base-header,
    ~metadata,
    data: bstr
}

Currently, JSON Structure cannot directly represent this pattern. The converter must either:

1. Only use the first base type and lose information
2. Inline all properties from additional base types (losing the inheritance relationship)

Proposed Change

Allow $extends to accept either:

• A single JSON Pointer (current behavior): "$extends": "#/definitions/BaseType"
• An array of JSON Pointers: "$extends": ["#/definitions/BaseType1", "#/definitions/BaseType2"]

Example:

{
  "type": "object",
  "name": "FullRecord",
  "$extends": ["#/definitions/BaseHeader", "#/definitions/Metadata"],
  "properties": {
    "data": { "type": "binary" }
  }
}

Collision Resolution

When multiple base types define properties with the same name, a resolution rule is needed. Suggested approach:

1. First-wins: Properties from earlier types in the array take precedence (simple, predictable)
2. Last-wins: Properties from later types override earlier ones (similar to many OOP languages)
3. Error: Require explicit redefinition in the extending type to resolve conflicts

Recommendation: First-wins for simplicity and consistency with the existing union type matching rule ("the first matching type").

Compatibility

This is a backward-compatible extension:

• Existing schemas with single $extends values continue to work unchanged
• Only schemas explicitly using array syntax would leverage the new behavior

Related

This issue was identified while implementing CDDL-to-JSON-Structure conversion in avrotize.