Servant (A web api framework) and GDP (Ghosts of Departed Proofs) is here combined to allow for quite expressive API declarations.
This is achieved by parsing captured API-input as named variables, making the named contexts span the entire request.
This in turn, makes it possible to express a lot of domain knowledge/requirements  in the Servant API type.
For an example of how this can look like check out https://github.com/mtonnberg/gdp-demo