006/104: printenv

src/printenv.hs

@@ -0,0 +1,25 @@
+{-# LANGUAGE NoImplicitPrelude #-}
+
+import Prelude            (IO, String, ($), (.), (++), (/=), (==), (||),
+                           filter, head, map, mapM_, null, otherwise, putStr,
+                           putStrLn, tail)
+import Control.Monad      ((>>=))
+import Data.Maybe         (fromMaybe)
+import System.Environment (getArgs, getEnvironment, lookupEnv)
+import System.IO.Unsafe   (unsafePerformIO)
+
+env :: String -> String
+env = fromMaybe [] . unsafePerformIO . lookupEnv
+
+printenvs :: (String -> IO ()) -> [String] -> IO ()
+printenvs fx []   = mapM_ fx [k ++ "=" ++ v | (k, v) <- unsafePerformIO getEnvironment]
+printenvs fx vars = mapM_ fx (filter (/= []) $ map env vars)
+
+printenv :: [String] -> IO ()
+printenv str | null str  = printenvs putStrLn []
+             | head str  == "--null" || head str == "-0" = printenvs putStr (tail str)
+             | otherwise = printenvs putStrLn str
+
+main :: IO ()
+main = getArgs >>= printenv
+