In my experience, validation logic is either trivially represented by a struct (could be a json structure, which is easily shared) or requires some form of I/O.

In my experience, validation logic is either trivially represented by a struct (could be a json structure, which is easily shared) or requires some form of I/O. In the latter case, it is usually easier to just handle this on the server side. On the former case, I agree that a simple validation library has yet to emerge as the One True Way to Validate.