storeEntity    :: (Entity a) => a -> IO ()
retrieveEntity :: (Entity a) => Integer -> IO a
publishEntity  :: (Entity a) => a -> IO ()

main = do
    let user1 = User 1 "Thomas" "Meier" "tm@meier.com"
    storeEntity user1
    user2 <- retrieveEntity 1 :: IO User -- how to avoid this type annotation?
    publishEntity user2

{-# LANGUAGE DeriveGeneric, DeriveAnyClass #-}
module Example where
import GHC.Generics
import Data.Aeson

-- | Entity type class
class (ToJSON e, FromJSON e, Eq e, Show e) => Entity e where
    getId :: e -> Integer

-- | a user entity
data User = User {
      userId    :: Integer
    , firstName :: String
    , lastName  :: String
    , email     :: String
} deriving (Show, Eq, Generic, ToJSON, FromJSON)

instance Entity User where
    getId = userId


-- | load persistent entity of type a and identified by id
retrieveEntity :: (Entity a) => Integer -> IO a
retrieveEntity id = do
    -- compute file path based on id
    let jsonFileName = getPath id
    -- parse entity from JSON file
    eitherEntity <- eitherDecodeFileStrict jsonFileName
    case eitherEntity of
        Left msg -> fail msg
        Right e  -> return e

-- | store persistent entity of type a to a json file
storeEntity :: (Entity a) => a -> IO ()
storeEntity entity = do
    -- compute file path based on entity id
    let jsonFileName = getPath (getId entity)
    -- serialize entity as JSON and write to file
    encodeFile jsonFileName entity

-- | compute path of data file based on id
getPath :: Integer -> String
getPath id = ".stack-work/" ++ show id ++ ".json"

publishEntity :: (Entity a) => a -> IO ()
publishEntity = print

main = do
    let user1 = User 1 "Thomas" "Meier" "tm@meier.com"
    storeEntity user1
    user2 <- retrieveEntity 1 :: IO User
    print user2