Extra.hs

{-# LANGUAGE CPP #-}
{-# OPTIONS_GHC -fno-warn-duplicate-exports #-}

-- | Extra functions for "System.Environment". All these functions are available in later GHC versions,
--   but this code works all the way back to GHC 7.2.
module System.Environment.Extra(
    module System.Environment,
    getExecutablePath, lookupEnv
    ) where

import System.Environment

#if __GLASGOW_HASKELL__ < 706
import Control.Exception.Extra
import System.IO.Error
import Data.Functor

-- | Alias for 'getProgName' in GHC 7.4 and below, otherwise
--   returns the absolute pathname of the current executable.
getExecutablePath :: IO FilePath
getExecutablePath = getProgName

-- | Return the value of the environment variable var, or Nothing if there is no such value.
lookupEnv :: String -> IO (Maybe String)
lookupEnv x = catchBool isDoesNotExistError (Just <$> getEnv x) (const $ return Nothing)
#endif