v6 validation: "constant"

[handrews]

Originally written by @geraintluff at https://github.com/json-schema/json-schema/wiki/constant-(v5-proposal)

Proposed keywords

• constant

Purpose

For ordinary use, this would be equivalent to a single-valued enum, simply tidier.

The only real difference comes in its behaviour with $data-substitution. $data-subsitution would be allowed for this keyword, which means that this keyword is not capable of specifying literal values of the form: {"$data":...}, because these would be interpreted as $data-substitutions.

However, literal values of this form can still be specified using enum, so there is no loss of functionality.

Values

The value of this keyword would be any value - however, it would be subject to $data-substitution.

Validation

Instances are only valid if they are exactly equal to the value of this keyword.

Example

Simple constant

{
    "type": "object",
    "properties": {
        "five": {
            "constant": 5
        }
    }
}

Valid: {}, {"five": 5}
Invalid: {"five": 0}, {"five": "5"}

Using $data to specify equality

{
    "type": "object",
    "properties": {
        "a": {"type": "string"},
        "b": {
            "constant": {"$data": "1/a"}
        }
    },
    "required": ["a", "b"]
}

Valid: {"a": "foo", "b": "foo"}, {"a": "bar", "b": "bar"}
Invalid: {"a": "foo", "b": "bar"}

Concerns

Similarity to enum

Unless $data is being used, the same effect can be obtained using fewer actual characters:

• {"constant":"whatever"} - 23 characters
• {"enum":["whatever"]} - 21 characters

However, when used in combination with $data, it opens up possibilities that are not otherwise available.

[handrews]

I personally find constant to be useful independent of $data as it more clearly expresses the intent of the schema author to require a fixed value, and not an enum that (at the moment) happens to only have one entry.

[HotelDon]

Personally, I get the most use out of constant when combining it with $data, but you bring up an excellent point I hadn't considered with regards to ambiguity when you use enum. While I'm still pushing for $data, I think constant could stand on its own if it really needed to.

[sceutre]

Maybe "const" instead of "constant". Seems more natural/widely spread as a keyword, and trounces the "enum" form in character count ;-)

[epoberezkin]

It is useful, particularly in combination with $data keyword - it allows to validate joins in objects from ORM systems.

[fny]

If you're really concerned about character count, you could always use const:

{"constant":"whatever"} - 23 characters
{"enum":["whatever"]} - 21 characters
{"const":"whatever"} - 20 characters