Keywords minContains/maxContains

[epoberezkin]

These keywords will complement the existing keyword contains to define minimum and/or maximum number of items that are valid against the schema in contains keyword (in the same schema object).

Example:

{
  "$comment": "validate that the array contains >= 2 and <= 4 boolean true values",
  "type": "array",
  "items": {"type": "boolean"},
  "contains": {"const": true},
  "minContains": 2,
  "maxContains": 4
}

Meta-schema:

{
  "properties": {
    "minContains": {"type": "number", "minimum": 0, "default": 1},
    "maxContains": {"type": "number"}
  },
  "dependencies": {
    "minContains": ["contains"],
    "maxContains": ["contains"]
  }
}

"minContains": 0 would allow limiting the maximum number of matching elements without requiring one matching element.

[handrews]

Removed the [proposal] prefix as I've been removing all of them since we have control over the labels now. Proposals are labeled with "Enhancement".

[handrews]

This came up in https://stackoverflow.com/questions/47348506/json-schema-require-a-specific-array-element

[handrews]

This proposal seems pretty reasonable to me. Any objections? I'm going to tentatively put it in the draft-08 milestone. Anyone who feels like submitting a PR is welcome to, otherwise I'll get around to it eventually.