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--- Library to support operations related to the program execution path,
--- i.e., environment variable `PATH`.
---
--- @author Michael Hanus
--- @version November 2018
------------------------------------------------------------------------------

module System.Path
  ( dirsInPath, fileInPath, getFileInPath )
 where

import System.Directory   ( doesFileExist, getAbsolutePath )
import System.FilePath    ( (</>), searchPathSeparator )
import System.Environment ( getEnv )
import Data.List          ( split )

--- Returns the list of the directories of the environment variable `PATH`.
dirsInPath :: IO [String]
dirsInPath = do
  path <- getEnv "PATH"
  return $ split (== searchPathSeparator) path

--- Checks whether a file exists in one of the directories
--- of the environment variable `PATH`.
fileInPath :: String -> IO Bool
fileInPath file = do
  dirs <- dirsInPath
  or  <$> mapM (doesFileExist . (</> file)) dirs

--- Checks whether a file exists in one of the directories
--- of the environment variable `PATH` and returns its absolute path,
--- otherwise returns `Nothing`.
getFileInPath :: String -> IO (Maybe String)
getFileInPath file = dirsInPath >>= checkPath
 where
  checkPath [] = return Nothing
  checkPath (dir:dirs) = do
    let dirfile = dir </> file
    direx <- doesFileExist dirfile
    if direx then getAbsolutePath dirfile >>= return . Just
             else checkPath dirs

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