Provide a DSL to derive both Schema and JSON instances

[fizruk]

It is currently impossible to derive ToSchema, ToJSON or FromJSON from any other class.
However, as @kosmikus has pointed out during HaskellXtogether it should be possible to provide a new DSL to derive all of those (and maybe more?).

I personally rely mostly on Generic-based instances and that just works, but the problem of mismatching instances still persists for hand-written instances (which do happen on occasion). So a unifying DSL would indeed resolve that issue.

Some potential problems:

• what about other common classes, such as ToHttpApiData/FromHttpApiData, ToForm/FromForm?
• what about other interfaces, such as ToRow/FromRow (from postgresql-simple), CSV, XML, etc.?
• unifying DSL might degrade JSON encoding/decoding performance, should we care?

What would you like to use a unifying DSL for? Any ideas are welcome!

[kosmikus]

See my BOB 2017 talk (slides, video) for some more background on this (although unrelated to Swagger).