DiazCarrete, 
When building #haskell Servant applications with persistence, a common (?) pattern is to request a database connection from a pool in the callback we pass to "hoistServer" https://hackage.haskell.org/package/servant-server-0.20/docs/Servant-Server.html#v:hoistServer and then pass the connection down to the handler using ReaderT https://github.com/danidiaz/comments-project/blob/3bb720124b31f0a8e351751bdcc6651ed75d9e27/comments/lib/Comments/Runner.hs#L57
It works, but I'm kinda unhappy about it because it forces you to use ReaderT in those intermediate components that lie between the top-level handler and the repository component which actually uses the connection. 😕