validation: format[Exclusive](Minimum|Maximum)

[epoberezkin]

Originally added as wiki: https://github.com/json-schema/json-schema/wiki/formatMinimum-%28v5-proposal%29

Proposed keywords

• formatMinimum
• formatMaximum
• formatExclusiveMinimum
• formatExclusiveMaximum

Purpose

Currently, numeric values can be constrained using minimum/maximum.

However, non-numeric formatted data cannot have such constraints, even when the format has a clear ordering. These additional keywords would allow for minimum/maximum constraints on such data.

Values

The values of formatMinimum/formatMaximum may be any JSON value. Particular values of format may imply a particular shape of data.

The values of formatExclusiveMinimum/formatExclusiveMaximum are booleans.

Validation

Validation is very similar to minimum/maximum/exclusiveMinimum/exclusiveMaximum. The only differences are:

• All data types are constrained (not just numbers)
• Instead of numerical ordering, an alternative ordering is used, selected based on the value of format.
• Validation of these keywords is completely optional, exactly like format. Support for one of these keywords does not imply partial or complete support for any of the others.

Example

{
    "type": "string",
    "format": "date-time",
    "formatMinimum": "2013-01-01T00:00Z"
}

In this example, the schema specifies that the string should be an ISO 8601 date-time, at some point after New Years 2013 (GMT).

Note that because ISO 8601 date-times can specify a timezone (and timezone differences should be accounted for in comparisons), there is no dictionary-ordering of strings that would correctly enforce this constraint - this constraint can only be expressed in a format-aware way.

Concerns

Using format-specific ordering is deliberately designed to avoid the localisation/"collation" issues that would be presented by string orderings, so we dodge a bullet there.

format values mentioned in the spec should also mention an expected ordering, if applicable.

This still does not provide a canonical way to specify localised dictionary orderings for strings. A format pattern like "text-de" might help, but should we specify that, or leave it up to schema authors/consumers to agree on something?

[handrews]

A date-time-specific version of this was previously filed as #99

[epoberezkin]

@handrews Yes, I also added a comment in #99. This idea seems more generic.

[handrews]

From #99 , we should also consider formatStep.

[epoberezkin]

@handrews or formatMultipleOf to be consistent with numbers.

[handrews]

Since exclusiveMinimum and exclusiveMaximum have since become numeric rather than booleans, I assume formatExclusiveMinimum and formatExclusiveMaximum would not also be values (of the same sort as described by the format) rather than booleans.

If we agree on that, given that no one has spoken against this in the past ten months, I think we can move on to a PR introducing all of these (including formatMultipleOf). Now is the time to speak up if anyone forgot about this :-)

[epoberezkin]

@handrews formatMultipleOf for the next release? It complicates formats more than min/max...

[handrews]

@epoberezkin good point. For "date-time" you would need a time-delta type. I'll write up min/max as a PR and file multipleOf for future consideration.

[handrews]

Given the amount of work needed on hyper-schema, I am not sure I will have time to write a PR for this. Anyone who wants to see this in draft-07 is encouraged to submit a PR.