Annotation to provide validation features on top of types

[jclark]

We could get quite a lot of mileage from a simple @validate annotation with fields describing the kinds of validation to perform, e.g.

@validate { minInclusive = 1 };
type PositiveInt int;

For numeric types we could follow XML Schema part 2 and allow:

• minInclusive
• maxInclusive
• minExclusive
• maxExclusive

Other obvious things:

• maxLength (for anything with a length, including strings and lists)
• minLength (similarly)
• pattern/regexp (on strings)

A more general and sophisticated way of doing this would be refinement types (#238), but that is orders of magnitude more work.

This doesn't deal with cases where you really want to transform the data as well notably:

• date/time-related types including durations
• strings that represents lists using whitespace or other delimiter to delimit items (as with XML Schema 2 list datatype)

[jclark]

This can also be useful in conjunction with automated data mapping.

@validate is perhaps the wrong word, given that validation is now only one use case. Maybe @constrain would be more neutral.

[sanjiva]

+1.

[srinathperera]

Looking from just data mapping point of view, statistics such as mean, standard deviation etc also useful ( for numerical fields).

Also, another angle is that although user does not provide this, in Choreo, we can trace, measure, and annotate in the background.

[jclark]

This is potentially relevant to GraphQL. Simple types with validation constraints can be exposed as user-defined scalars.

[jclark]

I think we should perhaps do this in a more first-class way using refinement types #238.

[jclark]

@shafreenAnfar mentions JSR 303 https://beanvalidation.org/1.0/spec/

[shafreenAnfar]

Yes. And Bean Validation 2.0 is available under https://beanvalidation.org/2.0/spec/ (formerly JSR 380). I feel Bean validation is an integral part of writing REST APIs with Spring Boot.

The list of built-in constraints can be found here.