lookupEnv / MaybeT

Main.hs

module Main where

import Control.Monad.Trans.Class (lift)
import Control.Monad.Trans.Maybe (MaybeT(..), runMaybeT)
import Data.Maybe (Maybe(..), fromMaybe)
import System.Environment (lookupEnv)

data ConnectInfo = ConnectInfo { host :: String, user :: String, pass :: String }
  deriving Show

defaultConnectInfo :: ConnectInfo
defaultConnectInfo = ConnectInfo "localhost" "user" "pass"

connectInfo :: IO (Maybe ConnectInfo)
connectInfo = runMaybeT $ do
  host <- MaybeT $ lookupEnv "PGHOST"
  user <- MaybeT $ lookupEnv "PGUSER"
  pass <- MaybeT $ lookupEnv "PGPASS"
  return $ ConnectInfo host user pass

main :: IO ()
main = do
  x <- connectInfo
  putStrLn $ show $ fromMaybe defaultConnectInfo x