I agree that that is how a parsing & instantiating framework should be built, and you should be able to hook in at any level -- but the reality is that normally

I agree that that is how a parsing & instantiating framework should be built, and you should be able to hook in at any level -- but the reality is that normally when one consumes such data you are using both functions at the same time - de-serializing. Having list vs. set vs map notations is both syntax (for parsing) and really a form of meta-data, making explicit what promises are made of the data, and therefore hinting at which data-structs you may want to instantiate from the data. I suppose that ideally the meta-data should be a separate, optional statement made about the data, but that's not the M.O. for any common formats.