Hi Café. I'm pondering over an old chestnut that I'd forgotten about until recently. I'd like to be able to express schemas for general recursive datatypes (excluding functions and possibly encoding bounds) and have an interactive constructor for that datatype generated automatically. Originally this idea came about as an XML/JS framework for creating advanced form inputs on the client-side, however, I'm interested in how this could be implemented idiomatically in Haskell. I'm guessing that the best way to go about this would be to capture the declaration in template Haskell, having a passthrough for the declaration itself, and also generating a representation that can be used for introspection and the creation of the interactive component. Ideally I'd like to integrate this concept into one of the web-frameworks such as Yesod so that complicated data-types can be constructed interactively on the client-side using (generated) javascript in order to get instantaneous feedback about validity of fields, etc, and avoid page reloads and even ajax communication. The details of the web-side of things would probably be quite messy, but if the Haskell side can be made general enough then this can be considered a separate problem :-) Has anyone had experience with trying to solve this, or a similar issue? What approach did you take? Did you use Template-Haskell, or something else? Thanks!