"Which" in Haskell

which.hs

module Main where

import System.Directory
import System.Environment
import System.Info

firstJust :: [Maybe a] -> Maybe a
firstJust [] = Nothing
firstJust (x:xs) = case x of
                     Nothing -> firstJust xs
                     Just y -> Just y

main :: IO ()
main = do
  args <- getArgs
  path <- getEnv "PATH"
  let search = head args
      auxExtensions = if os == "mingw32"
                      then ["", ".cmd", ".bat"]
                      else [""]
  exes <- mapM findExecutable $ map (search++) auxExtensions
  let exe = firstJust exes
  progName <- getProgName
  case exe of
    Nothing -> putStrLn (progName ++ ": no " ++ search ++ " in (" ++ path ++ ")")
    Just path -> putStrLn path