Do I understand processing dynamic scope correctly?

[flaksp]

Hello!

I'm trying to understand dynamic scope and references better. I want to be sure I understand it correctly using following official test case as an example:

{
  "description": "A $dynamicRef with intermediate scopes that don't include a matching $dynamicAnchor does not affect dynamic scope resolution",
  "schema": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "$id": "https://test.json-schema.org/root",
    "$ref": "intermediate-scope",
    "$defs": {
      "foo": {
        "$dynamicAnchor": "items",
        "type": "string"
      },
      "intermediate-scope": {
        "$id": "intermediate-scope",
        "$ref": "list"
      },
      "list": {
        "$id": "list",
        "type": "array",
        "items": {
          "$dynamicRef": "#items"
        },
        "$defs": {
          "items": {
            "$comment": "This is only needed to satisfy the bookending requirement",
            "$dynamicAnchor": "items"
          }
        }
      }
    }
  },
  "tests": [
    {
      "description": "An array of strings is valid",
      "data": [
        "foo",
        "bar"
      ],
      "valid": true
    },
    {
      "description": "An array containing non-strings is invalid",
      "data": [
        "foo",
        42
      ],
      "valid": false
    }
  ]
}

My implementation starts processing the document step-by-step, which produces following stack (inspired by "JSON Schema as a Framework" slides, part 2):

1. Processing schema root:
   • Schema location: https://test.json-schema.org/root#
   • Evaluation path: (empty)
2. It meets $ref keyword:
   • Schema location: https://test.json-schema.org/intermediate-scope#
   • Evaluation path: /$ref
3. It meets $ref keyword again:
   • Schema location: https://test.json-schema.org/list#
   • Evaluation path: /$ref/$ref
4. It goes into items:
   • Schema location: https://test.json-schema.org/list#/items
   • Evaluation path: /$ref/$ref/items

After that, it meets $dynamicRef keyword with #items value.

As I understand from the slides, my implementation should go back down the stack:

1. Schema location is https://test.json-schema.org/list#/items. There is a schema https://test.json-schema.org/list#items defined by $dynamicAnchor ✅
2. Schema location is https://test.json-schema.org/list#. There is a schema https://test.json-schema.org/list#items defined by $dynamicAnchor ✅
3. Schema location is https://test.json-schema.org/intermediate-scope#. There is NO schema https://test.json-schema.org/intermediate-scope#items ⛔️
4. Schema location is https://test.json-schema.org/root#. There is a schema https://test.json-schema.org/root#items defined by $dynamicAnchor ✅

Stack is over. The last dynamic schema in the stack it found is https://test.json-schema.org/root#items, that's why it should be picked. It is correct?

[handrews]

Yes, you want the outermost (first stack frame/dynamic scope encountered) match, and that is the .../root#items

[flaksp]

How will it behave if $dynamicRef contains not just plain fragment, but something different, e.g. URI reference or JSON Pointer (or both)? I don't see such test cases 🤔

[handrews]

$dynamicRef to something that does not include a plain name fragment works just like $ref, as does one to a URI that includes a plain name fragment defined by $anchor instead of by $dynamicAnchor.

If you have something like "$dynamicRef": "/schemas/foo#bar" and bar in /schemas/foo was defined by $dynamicAnchor, all this affects is the starting point. So when you start unwinding the stack, you start at <whatever base URI>/schemas/foo#bar. You would do this if your current schema resource doesn't have the right $dynamicAnchor and you want to target another schema resource as the fallback resolution (meaning that if there's nothing else matching in the stack, it falls back to that starting point).