[lmm]

HATEOAS, discoverable apis, and the overengineered gunk that has today hijacked the name of REST (ironic, when much of the point of the original REST was to be not SOAP) remain pipe dreams. You can't write an API that clients that don't know your API can use, and I suspect it will take at least a hundred years until that becomes possible.

Certainly XML schema doesn't let you accomplish this. All I've ever seen it accomplish is telling you that a document doesn't conform to the schema, functionality that you can trivially achieve in JSON any number of ways (e.g. an API version field in the data).

There's no point trying to write a schema system without a use case that it can solve, and I've never seen such a thing.