Positional and optional item in array ?

[alfredking1988]

Currently, the JSON Schema seems only to allow rear optional items through additionalItems.

Is it possible to allow positional optional items?

Such as this imaginary schema:

{
  "type": "array",
  "items": [
    {
      // user must provide a nickname
      "type": "string"
    },
    {
      // the age is personal, so it is OPTIONAL
      // "optional": true,
      "type": "number"
    },
    {
      // user must provide a email
      "type": "string"
    }
  ]
}

Given the above imaginary schema, the following values are both valid：
['Tom', 30, '[email protected]']: Tom provides all, including the optional age.
['Alice', '[email protected]']: Alice does not provide the optional age.

Of course, the schema author must ensure the mutually exclusive between the positional-optional item and the following item. In the example above, the positional-optional age's type is number, the email's type is string, they are mutually exclusive, so the schema is unambiguous.

[notEthan]

positions in a json schema items array are fixed to the corresponding numeric index, so you'll need different schemas for different permutations of the indices. something like (yaml):

oneOf:
  - items:
      - {type: string, description: "name"}
      - {type: number, description: "age"}
      - {type: string, description: "email"}
  - items:
      - {type: string, description: "name"}
      - {type: string, description: "email"}

that should use $refs to avoid duplication, but even still, it would rapidly get out of hand if you have multiple optional elements. arrays where elements can appear at varying indices are not a kind of structure that json-schema is all that well-suited to describe.

[handrews]

This is the kind of complex edge case keyword that a few people need but most people do not, that is great for implementing as a (3rd-party) extension keyword. So I'm transferring it to our repository that holds those sorts of ideas.