If I'm not wrong, it's already possible to write constraints with required args, but then they can only be loaded via annotations, and not via yaml/xml, isn't it?

Relying on getNumberOfRequiredParameters feels fragile. I think this won't work as nicely as we'd like in practice. Can't we think of another check to replace the call? Eg add AbstractConstraint and use an instanceof check against it?

If I'm not wrong, it's already possible to write constraints with required args, but then they can only be loaded via annotations, and not via yaml/xml, isn't it?

yes

Relying on getNumberOfRequiredParameters feels fragile. I think this won't work as nicely as we'd like in practice. Can't we think of another check to replace the call? Eg add AbstractConstraint and use an instanceof check against it?

And what that AbstractConstraint would give us, except an empty marker? Constraint is already abstract :) From my side I don't see what wouldn't work because it's now just determining if the constructor requires something that are mandatory and properly instantiating it instead of making the old assumption that we are instantiating it with single argument.

And what that AbstractConstraint would give us, except an empty marker?

Exactly that: a marker to express that some constraints opt-in to get their arguments by a named constructor.

From my side I don't see what wouldn't work because it's now just determining if the constructor requires something that are mandatory and properly instantiating it instead of making the old assumption that we are instantiating it with single argument.

The next logical step would be to allow constructing constraints using named arguments, even if no arguments are required.

Exactly that: a marker to express that some constraints opt-in to get their arguments by a named constructor.

Maybe then an attribute could be used for this? because if I'd extend an existing constraint it wouldn't work

That'd work for me also!

That'd work for me also!

Thank you, updated.

Thank you @norkunas.