However, in the frameworks and environments I've worked in, the approaches I've seen have never been DRY. Most of the time there's no plan or pattern - validations are written in the model spec, and validations are written in the form on the view. (Note: Most of my first-hand experience is with Rails, Sinatra, and PHP w/ jQuery)
Mulling it over, it seems like it would not be difficult to create a generator which, given a set of validations (e.g. model name, field(s), condition), could produce both the necessary client-side and server-side material. Alternately, such a tool could take the server-side validations (such as the
validates code in an ActiveRecord model) and generate client-side validations (such as jQuery plugins, which would then be applied to the form.
Obviously, the above is just a "hey I had this idea" musing, and not a formal proposal. This sort of thing is surely more difficult than it seemed when the idea hit me.
That brings me to the question: How would you approach designing a "write once, run on server and client" technique for data validation?
Related subtopics: Do tools like that exist for any particular frameworks or client-server technologies? What are major gotchas or challenges with trying to maintain only one set of validations?