business logic

main.hs

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}

module Main where

import Control.Monad.Identity
import Control.Monad.Trans.Either

data User = User
  { name :: String
  , firstName :: String
  } deriving (Show)

class Monad m => UserGetter m where
  getName :: Int -> m (Either String String)
  getFirstname :: Int -> m (Either String String)

class Monad m => BusinessLogic m where
  getCompleteUser :: Int -> m (Either String User)

instance (UserGetter m, Monad m) => BusinessLogic m where
  getCompleteUser id =do
          n <- getName id
          case n of
            Left err -> return (Left err)
            Right name -> do
              fn <- getFirstname id
              case fn of
                Left err -> return (Left err)
                Right firstName -> return $ Right (User name firstName)