{-# LANGUAGE ScopedTypeVariables #-}

module SimplePersistence

( Id

, Entity

, getId

, persist

, retrieve

) where

import Data.Typeable

-- | Identifier for an Entity

type Id = String

-- | The Entity type class provides generic persistence to txt files

class (Show a, Read a, Typeable a) => Entity a where

-- | return the unique Id of the entity. This function must be implemented by type class instances.

getId :: a -> Id

-- | persist an entity of type a and identified by an Id to a file

persist :: a -> IO ()

persist entity = do

-- compute file path based on entity type and id

let fileName = getPath (typeOf entity) (getId entity)

-- serialize entity as JSON and write to file

writeFile fileName (show entity)

-- | load persistent entity of type a and identified by an Id

retrieve :: Id -> IO a

retrieve id = do

-- compute file path based on entity type and id

let fileName = getPath (typeOf (undefined :: a)) id

-- read file content into string

contentString <- readFile fileName

-- parse entity from string

return (read contentString)

-- | compute path of data file

getPath :: TypeRep -> String -> FilePath

getPath tr id = show tr ++ "." ++ id ++ ".txt"