[Feature] Constant elements in JSON schemas

[wjn0]

This is a super quick PR as I work toward supporting some large JSON schemas I'm working with. All it does is support the const keyword in JSON schemas, which are a way of hard coding constant elements: https://json-schema.org/understanding-json-schema/reference/const

[hudson-ai]

Nice! Thank you for the contribution 😄

I assume you moved the constant check to be first in order to avoid generating something else if type is specified? Would you mind adding a test to make sure that behavior is working as expected?

Also, what kinds of values can constants take on? Maybe a few more test cases for strings, objects, etc. would be good, as well as a test that consts work as expected when used as object properties.

@riedgar-ms ping for interest

[wjn0]

Happy to!

Yeah, it should come first IMO because consts can optionally be typed, e.g. {"type": "integer", "const": 1} is a valid schema (however, if 1 was instead "1" that would be an invalid schema, so we don't need to do any type checking once there is a const). I added a test to that effect.

Indeed, consts can be anything, so I added a test for each of several types (integers, strings, arrays, and complex objects), along with a test for when a const is a property of an object.

Thanks for looking!

[riedgar-ms]

Thanks!

I don't think we need to check that the value in const matches any type field; we expect the user to feed us a valid JSON schema. I'm just wondering about negative test cases... although given that all this does is call _to_compact_json(), I'm not sure there's a huge value there. WDYT @hudson-ai ?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.19%. Comparing base (325b2c8) to head (1d0547d).

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #886      +/-   ##
==========================================
+ Coverage   56.83%   60.19%   +3.35%     
==========================================
  Files          63       63              
  Lines        4622     4625       +3     
==========================================
+ Hits         2627     2784     +157     
+ Misses       1995     1841     -154     

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[hudson-ai]

@wjn0 thank you for the additional tests!

@riedgar-ms I agree that checking that type matches the const is unnecessary, as well as extensive negative tests. That being said, I think that there is at least one negative test that would be useful here:

If the user passes both a type and a const (presumed to be compatible with each other), we should assert that the const value has "priority". So I think I'd take that last test that has {"type": "integer", "const": 1} and turn it into a negative test that asserts that 2 doesn't match, for example. What do you two think?

[riedgar-ms]

@hudson-ai @wjn0 Agree on that extra negative test, to make sure that the const takes precedence over any type.

[wjn0]

@hudson-ai @riedgar-ms done! Added a (negative) precedence test, hopefully in keeping with the rest of the suite.

[hudson-ai]

Awesome!! Looks good to me.