Add hoistClient to servant-client

[alpmestan]

It's defined in terms of this new typeclass method of HasClient in servant-client-core:

class RunClient m => HasClient m api where
  [...]
  hoistClientMonad
    :: Proxy m
    -> Proxy api
    -> (forall x. mon x -> mon' x)
    -> Client mon api
    -> Client mon' api

Note that we don't expect source clients to live in the http client backend's m monad, so that we can say start from ClientM, go to some custom monad App, and finally land back in IO, using two successive calls to hoistClientMonad, or one that composes the transformations. This just seems like the right choice, because it's the most flexible option.

Feedback welcome!

[jkarni]

I'm about to be away from the internet for ten days, and am happy with this. One question though: isn't it already possible to do what hoist does simply by defining a new RunClient instance for the monad?

[alpmestan]

@jkarni Maybe, yeah, except that a typeclass instance has to be provided for each target monad, and I think you're pretty much stuck with just one transformation per monad (you'd define the instance for TargetMonad as transformations over the instance of SourceMonad) so it's static. While with hoistClient you get to pick the transformation, and it can be dynamic: partially applying some arguments from some context first and finally transforming the return type of clients.

Not sure whether you were suggesting to refactor the PR in terms of RunClient instead or not, but I'm failing to see the advantages so I'm thinking probably not and that you were just curious. =) (or I might be overlooking something?)

[alpmestan]

No other feedback? Should we merge?