Haskell Clean Architecture

Pseudo.hs

-----------------------------
-- Domain.hs

type SessionId = Text
type UserId = Text
type User = Text

class (Monad m) => UserRepo m where
  getUserById :: UserId -> m User

class (Monad m) => SessionRepo m where
  getUserIdBySession :: SessionId -> m UserId

getUser :: (UserRepo m, SessionRepo m)
        => SessionId -> m User
getUser sId = getUserIdBySession sId 
          >>= getUserById

-----------------------------
-- Routes.hs

import qualified Domain

routes :: (Domain.UserRepo m, Domain.SessionRepo m) => m ()
routes =
  get "/user" $ do
    sId <- parseSessionFromCookiesSomehow
    user <- Domain.getUser sId
    displayUserSomeHow user

-----------------------------
-- Redis.hs

import qualified Domain

acquireConnection :: IO Connection
acquireConnection = ...

getUserById :: (Reader Connection m) 
            => Domain.UserId -> m Domain.User
getUserById = ...

getUserIdBySession :: (Reader Connection m) 
                   => Domain.Session -> m Domain.UserId
getUserIdBySession = ...

-----------------------------
-- Main.hs

import qualified Domain
import qualified Redis
import qualified Routes

newtype App a = App 
  { unApp :: ReaderT Connection IO a
  } deriving ( Applicative, Functor, Monad
             , MonadReader Connection, MonadIO
             )

instance Domain.UserRepo App where
  getUserById = Redis.getUserById

instance Domain.SessionRepo App where
  getUserIdBySession = Redis.getUserIdBySession

main = do
  conn <- Redis.acquireConnection
  flip runReaderT conn . unApp $ Routes.routes

Wonderful example! Just like @reouno said, a compiling instance would be nice.

@arnemileswinter You might be interested in https://github.com/eckyputrady/haskell-scotty-realworld-example-app that uses the similar structure as this gist