module Network.CloudSeeder.Monads.Environment
  ( MonadEnvironment(..)
  ) where

import Prelude hiding (readFile)

import Control.Monad.Except (ExceptT)
import Control.Monad.Logger (LoggingT)
import Control.Monad.Reader (ReaderT)
import Control.Monad.State (StateT)
import Control.Monad.Trans (MonadTrans, lift)
import Control.Monad.Writer (WriterT)

import qualified Data.Text as T
import qualified System.Environment as IO

-- | A class of monads that can access environment variables
class Monad m => MonadEnvironment m where
  getEnv :: T.Text -> m (Maybe T.Text)

  default getEnv :: (MonadTrans t, MonadEnvironment m', m ~ t m') => T.Text -> m (Maybe T.Text)
  getEnv = lift . getEnv

instance MonadEnvironment IO where
  getEnv = fmap (fmap T.pack) . IO.lookupEnv . T.unpack

instance MonadEnvironment m => MonadEnvironment (ExceptT e m)
instance MonadEnvironment m => MonadEnvironment (LoggingT m)
instance MonadEnvironment m => MonadEnvironment (ReaderT r m)
instance MonadEnvironment m => MonadEnvironment (StateT s m)
instance (MonadEnvironment m, Monoid w) => MonadEnvironment (WriterT w m)