Haskell monadic typeclass

main.hs

module Main where

import Control.Monad.Identity

-- boostrap the conditions leading to 2 different implementations of the same business logic
main :: IO ()
main = do
  -- use a monad instance as 1st parameter in order to choose the implementation
  expectedCalledWithMaybeInstance <- putStrLn $ getFirstName Nothing "heyhey"
  expectedCalledWithIOInstance <- putStrLn $ getFirstName (Identity 0) "hoho"
  putStrLn "done"

class UserGetter a where
  getUserByName :: a -> String -> Either String String

instance UserGetter (Identity a) where
  getUserByName _ str = Left $ "userFirstName " ++ str

instance UserGetter (Maybe a) where
  getUserByName _ str = Right $ "otherUserFirstName " ++ str

-- business logic without any knowledge of the underlying implementation
getFirstName :: (UserGetter m) => m -> String -> String
getFirstName ctx name = do
  let firstName = getUserByName ctx name
  case firstName of
    Left str -> "called from Identity " ++ str
    Right str -> "called from Maybe " ++ str