A & (B | C) would produce only A in the schema

[kanitw]

From vega/vega-lite#4440 (comment):

export type TopLevelFacetedUnitSpec = TopLevel<FacetedExtendedUnitSpec> & DataMixins;`

TopLevel<S extends BaseSpec> = S &
  TopLevelProperties & {
...
}

export type FacetedExtendedUnitSpec = ExtendedUnitSpec<FacetMapping<Field>>;

export type ExtendedUnitSpec<
  /** Extra Encoding */
  EE = EmptyObject
> = NormalizedUnitSpec<EE> | CompositeUnitSpec<EE>;

[kanitw]

It seems like TopLevelFacetedUnitSpec currently only gets things from TopLevel, but nothing from FacetedExtendedUnitSpec.

Perhaps, the NormalizedUnitSpec<EE> | CompositeUnitSpec<EE> part is something the schema isn't smart about.

[kanitw]

If I have to guess, the generator doesn't use allOf, but rather create a more complex object, but isn't good at converting A & (B | C) into (A & B) | (A & C) yet.

[kanitw]

I think using allOf will reduce the JSON schema size in many places as well.

For example, in Vega-Lite, we have many things that share common parts such as:

• Encoding and EncodingWithFacet
• *Spec and TopLevel*Spec

Since we already reproduce merged schema for each of these pairs, the output schema can be quite huge.

[domoritz]

export interface A {
    a: number;
}


export interface B {
    b: number;
}

export interface C {
    c: number;
}

export type MyType = A & (B | C);

Produces

{
  "$ref": "#/definitions/MyType",
  "$schema": "http://json-schema.org/draft-06/schema#",
  "definitions": {
    "C": {
      "additionalProperties": false,
      "properties": {
        "c": {
          "type": "number"
        }
      },
      "required": [
        "c"
      ],
      "type": "object"
    },
    "MyType": {
      "additionalProperties": false,
      "properties": {
        "a": {
          "type": "number"
        }
      },
      "required": [
        "a"
      ],
      "type": "object"
    }
  }

[domoritz]

allOf is actually incorrect. The issue is that allOf requires that all definitions match. However, we make heavy use of additionalProperties: false, which means that none of the definitions match.

{
  "$ref": "#/definitions/BC",
  "$schema": "http://json-schema.org/draft-06/schema#",
  "definitions": {
    "B": {
      "additionalProperties": false,
      "properties": {
        "b": {
          "type": "number"
        },
        "x": {
          "type": "number"
        }
      },
      "type": "object"
    },
    "BC": {
      "allOf": [
        {
          "$ref": "#/definitions/B"
        },
        {
          "$ref": "#/definitions/C"
        }
      ]
    },
    "C": {
      "additionalProperties": false,
      "properties": {
        "c": {
          "type": "number"
        },
        "x": {
          "type": "number"
        }
      },
      "type": "object"
    }
  }
}

We actually discussed this issue back on 2017 at #4