module EndOfExe where

import qualified System.Directory as D (findExecutable)
import Data.Maybe (isJust,isNothing)
import System.IO.Unsafe (unsafePerformIO)

-- | Function that is used instead of 'System.Info.os' to check whether the executable ends in .exe. The function returns @IO Nothing@ if there is neither @ys@ nor @(ys ++ ".exe")@ names for executables in the 
-- variable @PATH@
endOfExecutable :: String -> IO (Maybe String)
endOfExecutable ys = do
                       xs <- D.findExecutable ys
                       if isJust xs 
                         then return $ fmap (ys ++) (Just "")
                         else do
                                zs <- D.findExecutable (ys ++ ".exe")
                                if isJust zs
                                  then return $ fmap (ys ++) (Just ".exe")
                                  else error ("Please, install the executable " ++ ys ++ " into the directory in the PATH variable!")
                                  
-- | Function to get the proper name of the executable in the system (it must be seen in the directories in the @PATH@ variable). 
-- You can use 'showE' \"nameOfExecutable\" to get @Just \"nameOfExecutable\"@ if it is present on the system. Further you can adopt it to be used 
-- inside the 'System.Process.callCommand' as the name of the executable
showE :: String -> Maybe String
showE xs | null xs = error "No executable specified!"
         | otherwise = let r = unsafePerformIO . endOfExecutable $ xs in 
            if isJust r
              then r
              else Nothing

-- | Function that being given a list of names of executables (without .exe suffix) looks up for them in the specified list order till the first existing occurrence.
-- If there is no such occurrence (the specified executables are not installed in the directories mentioned in the variable @PATH@) then the function returns @Nothing@.
findSysExes :: [String] -> Maybe String
findSysExes xss | null (dropWhile isNothing . map showE $ xss) = Nothing
                | otherwise = head (dropWhile isNothing . map showE $ xss)