MaybeT

maybet.hs

lookupEnv :: IO (Maybe String)

lookupEnvT :: MaybeT IO String

foo :: IO (Maybe (String, String))
foo = do
  a <- lookupEnv "yay"
  b <- lookupEnv "yay"
  pure $ do 
    aa <- a
    bb <- b
    pure (aa, bb)

foo :: MaybeT IO (String, String)
foo = do
  a <- lookupEnv "yay"
  b <- lookupEnv "yay"
  pure $ (a, b)      

foo :: MaybeT IO (String, String)
foo = 
  (,) <$> lookupEnv "yay" <*> lookupEnv "yay"