System/FilePath/Windows.hs

    {-# LANGUAGE PatternGuards #-}

    -- This template expects CPP definitions for:
    --     MODULE_NAME = Posix | Windows
    --     IS_WINDOWS  = False | True

    -- |
    -- Module      :  System.FilePath.MODULE_NAME
    -- Copyright   :  (c) Neil Mitchell 2005-2014
    -- License     :  BSD3
    --
    -- Maintainer  :  ndmitchell@gmail.com
    -- Stability   :  stable
    -- Portability :  portable
    --
    -- A library for 'FilePath' manipulations, using MODULE_NAME style paths on
    -- all platforms. Importing "System.FilePath" is usually better.
    --
    -- Given the example 'FilePath': @\/directory\/file.ext@
    --
    -- We can use the following functions to extract pieces.
    --
    -- * 'takeFileName' gives @\"file.ext\"@
    --
    -- * 'takeDirectory' gives @\"\/directory\"@
    --
    -- * 'takeExtension' gives @\".ext\"@
    --
    -- * 'dropExtension' gives @\"\/directory\/file\"@
    --
    -- * 'takeBaseName' gives @\"file\"@
    --
    -- And we could have built an equivalent path with the following expressions:
    --
    -- * @\"\/directory\" '</>' \"file.ext\"@.
    --
    -- * @\"\/directory\/file" '<.>' \"ext\"@.
    --
    -- * @\"\/directory\/file.txt" '-<.>' \"ext\"@.
    --
    -- Each function in this module is documented with several examples,
    -- which are also used as tests.
    --
    -- Here are a few examples of using the @filepath@ functions together:
    --
    -- /Example 1:/ Find the possible locations of a Haskell module @Test@ imported from module @Main@:
    --
    -- @['replaceFileName' path_to_main \"Test\" '<.>' ext | ext <- [\"hs\",\"lhs\"] ]@
    --
    -- /Example 2:/ Download a file from @url@ and save it to disk:
    --
    -- @do let file = 'makeValid' url
    --   System.Directory.createDirectoryIfMissing True ('takeDirectory' file)@
    --
    -- /Example 3:/ Compile a Haskell file, putting the @.hi@ file under @interface@:
    --
    -- @'takeDirectory' file '</>' \"interface\" '</>' ('takeFileName' file '-<.>' \"hi\")@
    --
    -- References:
    -- [1] <http://msdn.microsoft.com/en-us/library/windows/desktop/aa365247.aspx Naming Files, Paths and Namespaces> (Microsoft MSDN)
    module System.FilePath.Windows
        (
        -- * Separator predicates
        FilePath,
        pathSeparator, pathSeparators, isPathSeparator,
        searchPathSeparator, isSearchPathSeparator,
        extSeparator, isExtSeparator,

        -- * @$PATH@ methods
        splitSearchPath, getSearchPath,

        -- * Extension functions
        splitExtension,
        takeExtension, replaceExtension, (-<.>), dropExtension, addExtension, hasExtension, (<.>),
        splitExtensions, dropExtensions, takeExtensions, replaceExtensions, isExtensionOf,
        stripExtension,

        -- * Filename\/directory functions
        splitFileName,
        takeFileName, replaceFileName, dropFileName,
        takeBaseName, replaceBaseName,
        takeDirectory, replaceDirectory,
        combine, (</>),
        splitPath, joinPath, splitDirectories,

        -- * Drive functions
        splitDrive, joinDrive,
        takeDrive, hasDrive, dropDrive, isDrive,

        -- * Trailing slash functions
        hasTrailingPathSeparator,
        addTrailingPathSeparator,
        dropTrailingPathSeparator,

        -- * File name manipulations
        normalise, equalFilePath,
        makeRelative,
        isRelative, isAbsolute,
        isValid, makeValid
        )
        where

    import Data.Char(toLower, toUpper, isAsciiLower, isAsciiUpper)
    import Data.Maybe(isJust)
    import Data.List(stripPrefix, isSuffixOf)

    import System.Environment(getEnv)

    infixr 7  <.>, -<.>
    infixr 5  </>

    ---------------------------------------------------------------------
    -- Platform Abstraction Methods (private)

    -- | Is the operating system Unix or Linux like
    isPosix :: Bool
    isPosix = not isWindows

    -- | Is the operating system Windows like
    isWindows :: Bool
    isWindows = True

    ---------------------------------------------------------------------
    -- The basic functions

    -- | The character that separates directories. In the case where more than
    --   one character is possible, 'pathSeparator' is the \'ideal\' one.
    --
    -- > Windows: pathSeparator == '\\'
    -- > Posix:   pathSeparator ==  '/'
    -- > isPathSeparator pathSeparator
    pathSeparator :: Char
    pathSeparator = if isWindows then '\\' else '/'

    -- | The list of all possible separators.
    --
    -- > Windows: pathSeparators == ['\\', '/']
    -- > Posix:   pathSeparators == ['/']
    -- > pathSeparator `elem` pathSeparators
    pathSeparators :: [Char]
    pathSeparators = if isWindows then "\\/" else "/"

    -- | Rather than using @(== 'pathSeparator')@, use this. Test if something
    --   is a path separator.
    --
    -- > isPathSeparator a == (a `elem` pathSeparators)
    isPathSeparator :: Char -> Bool
    isPathSeparator '/' = True
    isPathSeparator '\\' = isWindows
    isPathSeparator _ = False

    -- | The character that is used to separate the entries in the $PATH environment variable.
    --
    -- > Windows: searchPathSeparator == ';'
    -- > Posix:   searchPathSeparator == ':'
    searchPathSeparator :: Char
    searchPathSeparator = if isWindows then ';' else ':'

    -- | Is the character a file separator?
    --
    -- > isSearchPathSeparator a == (a == searchPathSeparator)
    isSearchPathSeparator :: Char -> Bool
    isSearchPathSeparator = (== searchPathSeparator)

    -- | File extension character
    --
    -- > extSeparator == '.'
    extSeparator :: Char
    extSeparator = '.'

    -- | Is the character an extension character?
    --
    -- > isExtSeparator a == (a == extSeparator)
    isExtSeparator :: Char -> Bool
    isExtSeparator = (== extSeparator)

    ---------------------------------------------------------------------
    -- Path methods (environment $PATH)

    -- | Take a string, split it on the 'searchPathSeparator' character.
    --   Blank items are ignored on Windows, and converted to @.@ on Posix.
    --   On Windows path elements are stripped of quotes.
    --
    --   Follows the recommendations in
    --   <http://www.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html>
    --
    -- > Posix:   splitSearchPath "File1:File2:File3"  == ["File1","File2","File3"]
    -- > Posix:   splitSearchPath "File1::File2:File3" == ["File1",".","File2","File3"]
    -- > Windows: splitSearchPath "File1;File2;File3"  == ["File1","File2","File3"]
    -- > Windows: splitSearchPath "File1;;File2;File3" == ["File1","File2","File3"]
    -- > Windows: splitSearchPath "File1;\"File2\";File3" == ["File1","File2","File3"]
    splitSearchPath :: String -> [FilePath]
    splitSearchPath = f
        where
        f xs = case break isSearchPathSeparator xs of
               (pre, []    ) -> g pre
               (pre, _:post) -> g pre ++ f post

        g "" = ["." | isPosix]
        g ('\"':x@(_:_)) | isWindows && last x == '\"' = [init x]
        g x = [x]

    -- | Get a list of 'FilePath's in the $PATH variable.
    getSearchPath :: IO [FilePath]
    getSearchPath = fmap splitSearchPath (getEnv "PATH")

    ---------------------------------------------------------------------
    -- Extension methods

    -- | Split on the extension. 'addExtension' is the inverse.
    --
    -- > splitExtension "/directory/path.ext" == ("/directory/path",".ext")
    -- > uncurry (++) (splitExtension x) == x
    -- > Valid x => uncurry addExtension (splitExtension x) == x
    -- > splitExtension "file.txt" == ("file",".txt")
    -- > splitExtension "file" == ("file","")
    -- > splitExtension "file/file.txt" == ("file/file",".txt")
    -- > splitExtension "file.txt/boris" == ("file.txt/boris","")
    -- > splitExtension "file.txt/boris.ext" == ("file.txt/boris",".ext")
    -- > splitExtension "file/path.txt.bob.fred" == ("file/path.txt.bob",".fred")
    -- > splitExtension "file/path.txt/" == ("file/path.txt/","")
    splitExtension :: FilePath -> (String, String)
    splitExtension x = case nameDot of
                           "" -> (x,"")
                           _ -> (dir ++ init nameDot, extSeparator : ext)
        where
            (dir,file) = splitFileName_ x
            (nameDot,ext) = breakEnd isExtSeparator file

    -- | Get the extension of a file, returns @\"\"@ for no extension, @.ext@ otherwise.
    --
    -- > takeExtension "/directory/path.ext" == ".ext"
    -- > takeExtension x == snd (splitExtension x)
    -- > Valid x => takeExtension (addExtension x "ext") == ".ext"
    -- > Valid x => takeExtension (replaceExtension x "ext") == ".ext"
    takeExtension :: FilePath -> String
    takeExtension = snd . splitExtension

    -- | Remove the current extension and add another, equivalent to 'replaceExtension'.
    --
    -- > "/directory/path.txt" -<.> "ext" == "/directory/path.ext"
    -- > "/directory/path.txt" -<.> ".ext" == "/directory/path.ext"
    -- > "foo.o" -<.> "c" == "foo.c"
    (-<.>) :: FilePath -> String -> FilePath
    (-<.>) = replaceExtension

    -- | Set the extension of a file, overwriting one if already present, equivalent to '-<.>'.
    --
    -- > replaceExtension "/directory/path.txt" "ext" == "/directory/path.ext"
    -- > replaceExtension "/directory/path.txt" ".ext" == "/directory/path.ext"
    -- > replaceExtension "file.txt" ".bob" == "file.bob"
    -- > replaceExtension "file.txt" "bob" == "file.bob"
    -- > replaceExtension "file" ".bob" == "file.bob"
    -- > replaceExtension "file.txt" "" == "file"
    -- > replaceExtension "file.fred.bob" "txt" == "file.fred.txt"
    -- > replaceExtension x y == addExtension (dropExtension x) y
    replaceExtension :: FilePath -> String -> FilePath
    replaceExtension x y = dropExtension x <.> y

    -- | Add an extension, even if there is already one there, equivalent to 'addExtension'.
    --
    -- > "/directory/path" <.> "ext" == "/directory/path.ext"
    -- > "/directory/path" <.> ".ext" == "/directory/path.ext"
    (<.>) :: FilePath -> String -> FilePath
    (<.>) = addExtension

    -- | Remove last extension, and the \".\" preceding it.
    --
    -- > dropExtension "/directory/path.ext" == "/directory/path"
    -- > dropExtension x == fst (splitExtension x)
    dropExtension :: FilePath -> FilePath
    dropExtension = fst . splitExtension

    -- | Add an extension, even if there is already one there, equivalent to '<.>'.
    --
    -- > addExtension "/directory/path" "ext" == "/directory/path.ext"
    -- > addExtension "file.txt" "bib" == "file.txt.bib"
    -- > addExtension "file." ".bib" == "file..bib"
    -- > addExtension "file" ".bib" == "file.bib"
    -- > addExtension "/" "x" == "/.x"
    -- > addExtension x "" == x
    -- > Valid x => takeFileName (addExtension (addTrailingPathSeparator x) "ext") == ".ext"
    -- > Windows: addExtension "\\\\share" ".txt" == "\\\\share\\.txt"
    addExtension :: FilePath -> String -> FilePath
    addExtension file "" = file
    addExtension file xs@(x:_) = joinDrive a res
        where
            res = if isExtSeparator x then b ++ xs
                  else b ++ [extSeparator] ++ xs

            (a,b) = splitDrive file

    -- | Does the given filename have an extension?
    --
    -- > hasExtension "/directory/path.ext" == True
    -- > hasExtension "/directory/path" == False
    -- > null (takeExtension x) == not (hasExtension x)
    hasExtension :: FilePath -> Bool
    hasExtension = any isExtSeparator . takeFileName

    -- | Does the given filename have the specified extension?
    --
    -- > "png" `isExtensionOf` "/directory/file.png" == True
    -- > ".png" `isExtensionOf` "/directory/file.png" == True
    -- > ".tar.gz" `isExtensionOf` "bar/foo.tar.gz" == True
    -- > "ar.gz" `isExtensionOf` "bar/foo.tar.gz" == False
    -- > "png" `isExtensionOf` "/directory/file.png.jpg" == False
    -- > "csv/table.csv" `isExtensionOf` "/data/csv/table.csv" == False
    isExtensionOf :: String -> FilePath -> Bool
    isExtensionOf ext@('.':_) = isSuffixOf ext . takeExtensions
    isExtensionOf ext         = isSuffixOf ('.':ext) . takeExtensions

    -- | Drop the given extension from a FilePath, and the @\".\"@ preceding it.
    --   Returns 'Nothing' if the FilePath does not have the given extension, or
    --   'Just' and the part before the extension if it does.
    --
    --   This function can be more predictable than 'dropExtensions', especially if the filename
    --   might itself contain @.@ characters.
    --
    -- > stripExtension "hs.o" "foo.x.hs.o" == Just "foo.x"
    -- > stripExtension "hi.o" "foo.x.hs.o" == Nothing
    -- > dropExtension x == fromJust (stripExtension (takeExtension x) x)
    -- > dropExtensions x == fromJust (stripExtension (takeExtensions x) x)
    -- > stripExtension ".c.d" "a.b.c.d"  == Just "a.b"
    -- > stripExtension ".c.d" "a.b..c.d" == Just "a.b."
    -- > stripExtension "baz"  "foo.bar"  == Nothing
    -- > stripExtension "bar"  "foobar"   == Nothing
    -- > stripExtension ""     x          == Just x
    stripExtension :: String -> FilePath -> Maybe FilePath
    stripExtension []        path = Just path
    stripExtension ext@(x:_) path = stripSuffix dotExt path
        where dotExt = if isExtSeparator x then ext else '.':ext

    -- | Split on all extensions.
    --
    -- > splitExtensions "/directory/path.ext" == ("/directory/path",".ext")
    -- > splitExtensions "file.tar.gz" == ("file",".tar.gz")
    -- > uncurry (++) (splitExtensions x) == x
    -- > Valid x => uncurry addExtension (splitExtensions x) == x
    -- > splitExtensions "file.tar.gz" == ("file",".tar.gz")
    splitExtensions :: FilePath -> (FilePath, String)
    splitExtensions x = (a ++ c, d)
        where
            (a,b) = splitFileName_ x
            (c,d) = break isExtSeparator b

    -- | Drop all extensions.
    --
    -- > dropExtensions "/directory/path.ext" == "/directory/path"
    -- > dropExtensions "file.tar.gz" == "file"
    -- > not $ hasExtension $ dropExtensions x
    -- > not $ any isExtSeparator $ takeFileName $ dropExtensions x
    dropExtensions :: FilePath -> FilePath
    dropExtensions = fst . splitExtensions

    -- | Get all extensions.
    --
    -- > takeExtensions "/directory/path.ext" == ".ext"
    -- > takeExtensions "file.tar.gz" == ".tar.gz"
    takeExtensions :: FilePath -> String
    takeExtensions = snd . splitExtensions

    -- | Replace all extensions of a file with a new extension. Note
    --   that 'replaceExtension' and 'addExtension' both work for adding
    --   multiple extensions, so only required when you need to drop
    --   all extensions first.
    --
    -- > replaceExtensions "file.fred.bob" "txt" == "file.txt"
    -- > replaceExtensions "file.fred.bob" "tar.gz" == "file.tar.gz"
    replaceExtensions :: FilePath -> String -> FilePath
    replaceExtensions x y = dropExtensions x <.> y

    ---------------------------------------------------------------------
    -- Drive methods

    -- | Is the given character a valid drive letter?
    -- only a-z and A-Z are letters, not isAlpha which is more unicodey
    isLetter :: Char -> Bool
    isLetter x = isAsciiLower x || isAsciiUpper x

    -- | Split a path into a drive and a path.
    --   On Posix, \/ is a Drive.
    --
    -- > uncurry (++) (splitDrive x) == x
    -- > Windows: splitDrive "file" == ("","file")
    -- > Windows: splitDrive "c:/file" == ("c:/","file")
    -- > Windows: splitDrive "c:\\file" == ("c:\\","file")
    -- > Windows: splitDrive "\\\\shared\\test" == ("\\\\shared\\","test")
    -- > Windows: splitDrive "\\\\shared" == ("\\\\shared","")
    -- > Windows: splitDrive "\\\\?\\UNC\\shared\\file" == ("\\\\?\\UNC\\shared\\","file")
    -- > Windows: splitDrive "\\\\?\\UNCshared\\file" == ("\\\\?\\","UNCshared\\file")
    -- > Windows: splitDrive "\\\\?\\d:\\file" == ("\\\\?\\d:\\","file")
    -- > Windows: splitDrive "/d" == ("","/d")
    -- > Posix:   splitDrive "/test" == ("/","test")
    -- > Posix:   splitDrive "//test" == ("//","test")
    -- > Posix:   splitDrive "test/file" == ("","test/file")
    -- > Posix:   splitDrive "file" == ("","file")
    splitDrive :: FilePath -> (FilePath, FilePath)
    splitDrive x | isPosix = span (== '/') x
    splitDrive x | Just y <- readDriveLetter x = y
    splitDrive x | Just y <- readDriveUNC x = y
    splitDrive x | Just y <- readDriveShare x = y
    splitDrive x = ("",x)

    addSlash :: FilePath -> FilePath -> (FilePath, FilePath)
    addSlash a xs = (a++c,d)
        where (c,d) = span isPathSeparator xs

    -- See [1].
    -- "\\?\D:\<path>" or "\\?\UNC\<server>\<share>"
    readDriveUNC :: FilePath -> Maybe (FilePath, FilePath)
    readDriveUNC (s1:s2:'?':s3:xs) | all isPathSeparator [s1,s2,s3] =
        case map toUpper xs of
            ('U':'N':'C':s4:_) | isPathSeparator s4 ->
                let (a,b) = readDriveShareName (drop 4 xs)
                in Just (s1:s2:'?':s3:take 4 xs ++ a, b)
            _ -> case readDriveLetter xs of
                     -- Extended-length path.
                     Just (a,b) -> Just (s1:s2:'?':s3:a,b)
                     Nothing -> Nothing
    readDriveUNC _ = Nothing

    {- c:\ -}
    readDriveLetter :: String -> Maybe (FilePath, FilePath)
    readDriveLetter (x:':':y:xs) | isLetter x && isPathSeparator y = Just $ addSlash [x,':'] (y:xs)
    readDriveLetter (x:':':xs) | isLetter x = Just ([x,':'], xs)
    readDriveLetter _ = Nothing

    {- \\sharename\ -}
    readDriveShare :: String -> Maybe (FilePath, FilePath)
    readDriveShare (s1:s2:xs) | isPathSeparator s1 && isPathSeparator s2 =
            Just (s1:s2:a,b)
        where (a,b) = readDriveShareName xs
    readDriveShare _ = Nothing

    {- assume you have already seen \\ -}
    {- share\bob -> "share\", "bob" -}
    readDriveShareName :: String -> (FilePath, FilePath)
    readDriveShareName name = addSlash a b
        where (a,b) = break isPathSeparator name

    -- | Join a drive and the rest of the path.
    --
    -- > Valid x => uncurry joinDrive (splitDrive x) == x
    -- > Windows: joinDrive "C:" "foo" == "C:foo"
    -- > Windows: joinDrive "C:\\" "bar" == "C:\\bar"
    -- > Windows: joinDrive "\\\\share" "foo" == "\\\\share\\foo"
    -- > Windows: joinDrive "/:" "foo" == "/:\\foo"
    joinDrive :: FilePath -> FilePath -> FilePath
    joinDrive = combineAlways

    -- | Get the drive from a filepath.
    --
    -- > takeDrive x == fst (splitDrive x)
    takeDrive :: FilePath -> FilePath
    takeDrive = fst . splitDrive

    -- | Delete the drive, if it exists.
    --
    -- > dropDrive x == snd (splitDrive x)
    dropDrive :: FilePath -> FilePath
    dropDrive = snd . splitDrive

    -- | Does a path have a drive.
    --
    -- > not (hasDrive x) == null (takeDrive x)
    -- > Posix:   hasDrive "/foo" == True
    -- > Windows: hasDrive "C:\\foo" == True
    -- > Windows: hasDrive "C:foo" == True
    -- >          hasDrive "foo" == False
    -- >          hasDrive "" == False
    hasDrive :: FilePath -> Bool
    hasDrive = not . null . takeDrive

    -- | Is an element a drive
    --
    -- > Posix:   isDrive "/" == True
    -- > Posix:   isDrive "/foo" == False
    -- > Windows: isDrive "C:\\" == True
    -- > Windows: isDrive "C:\\foo" == False
    -- >          isDrive "" == False
    isDrive :: FilePath -> Bool
    isDrive x = not (null x) && null (dropDrive x)

    ---------------------------------------------------------------------
    -- Operations on a filepath, as a list of directories

    -- | Split a filename into directory and file. '</>' is the inverse.
    --   The first component will often end with a trailing slash.
    --
    -- > splitFileName "/directory/file.ext" == ("/directory/","file.ext")
    -- > Valid x => uncurry (</>) (splitFileName x) == x || fst (splitFileName x) == "./"
    -- > Valid x => isValid (fst (splitFileName x))
    -- > splitFileName "file/bob.txt" == ("file/", "bob.txt")
    -- > splitFileName "file/" == ("file/", "")
    -- > splitFileName "bob" == ("./", "bob")
    -- > Posix:   splitFileName "/" == ("/","")
    -- > Windows: splitFileName "c:" == ("c:","")
    splitFileName :: FilePath -> (String, String)
    splitFileName x = (if null dir then "./" else dir, name)
        where
            (dir, name) = splitFileName_ x

    -- version of splitFileName where, if the FilePath has no directory
    -- component, the returned directory is "" rather than "./".  This
    -- is used in cases where we are going to combine the returned
    -- directory to make a valid FilePath, and having a "./" appear would
    -- look strange and upset simple equality properties.  See
    -- e.g. replaceFileName.
    splitFileName_ :: FilePath -> (String, String)
    splitFileName_ x = (drv ++ dir, file)
        where
            (drv,pth) = splitDrive x
            (dir,file) = breakEnd isPathSeparator pth

    -- | Set the filename.
    --
    -- > replaceFileName "/directory/other.txt" "file.ext" == "/directory/file.ext"
    -- > Valid x => replaceFileName x (takeFileName x) == x
    replaceFileName :: FilePath -> String -> FilePath
    replaceFileName x y = a </> y where (a,_) = splitFileName_ x

    -- | Drop the filename. Unlike 'takeDirectory', this function will leave
    --   a trailing path separator on the directory.
    --
    -- > dropFileName "/directory/file.ext" == "/directory/"
    -- > dropFileName x == fst (splitFileName x)
    dropFileName :: FilePath -> FilePath
    dropFileName = fst . splitFileName

    -- | Get the file name.
    --
    -- > takeFileName "/directory/file.ext" == "file.ext"
    -- > takeFileName "test/" == ""
    -- > takeFileName x `isSuffixOf` x
    -- > takeFileName x == snd (splitFileName x)
    -- > Valid x => takeFileName (replaceFileName x "fred") == "fred"
    -- > Valid x => takeFileName (x </> "fred") == "fred"
    -- > Valid x => isRelative (takeFileName x)
    takeFileName :: FilePath -> FilePath
    takeFileName = snd . splitFileName

    -- | Get the base name, without an extension or path.
    --
    -- > takeBaseName "/directory/file.ext" == "file"
    -- > takeBaseName "file/test.txt" == "test"
    -- > takeBaseName "dave.ext" == "dave"
    -- > takeBaseName "" == ""
    -- > takeBaseName "test" == "test"
    -- > takeBaseName (addTrailingPathSeparator x) == ""
    -- > takeBaseName "file/file.tar.gz" == "file.tar"
    takeBaseName :: FilePath -> String
    takeBaseName = dropExtension . takeFileName

    -- | Set the base name.
    --
    -- > replaceBaseName "/directory/other.ext" "file" == "/directory/file.ext"
    -- > replaceBaseName "file/test.txt" "bob" == "file/bob.txt"
    -- > replaceBaseName "fred" "bill" == "bill"
    -- > replaceBaseName "/dave/fred/bob.gz.tar" "new" == "/dave/fred/new.tar"
    -- > Valid x => replaceBaseName x (takeBaseName x) == x
    replaceBaseName :: FilePath -> String -> FilePath
    replaceBaseName pth nam = combineAlways a (nam <.> ext)
        where
            (a,b) = splitFileName_ pth
            ext = takeExtension b

    -- | Is an item either a directory or the last character a path separator?
    --
    -- > hasTrailingPathSeparator "test" == False
    -- > hasTrailingPathSeparator "test/" == True
    hasTrailingPathSeparator :: FilePath -> Bool
    hasTrailingPathSeparator "" = False
    hasTrailingPathSeparator x = isPathSeparator (last x)

    hasLeadingPathSeparator :: FilePath -> Bool
    hasLeadingPathSeparator "" = False
    hasLeadingPathSeparator x = isPathSeparator (head x)

    -- | Add a trailing file path separator if one is not already present.
    --
    -- > hasTrailingPathSeparator (addTrailingPathSeparator x)
    -- > hasTrailingPathSeparator x ==> addTrailingPathSeparator x == x
    -- > Posix:    addTrailingPathSeparator "test/rest" == "test/rest/"
    addTrailingPathSeparator :: FilePath -> FilePath
    addTrailingPathSeparator x = if hasTrailingPathSeparator x then x else x ++ [pathSeparator]

    -- | Remove any trailing path separators
    --
    -- > dropTrailingPathSeparator "file/test/" == "file/test"
    -- >           dropTrailingPathSeparator "/" == "/"
    -- > Windows:  dropTrailingPathSeparator "\\" == "\\"
    -- > Posix:    not (hasTrailingPathSeparator (dropTrailingPathSeparator x)) || isDrive x
    dropTrailingPathSeparator :: FilePath -> FilePath
    dropTrailingPathSeparator x =
        if hasTrailingPathSeparator x && not (isDrive x)
        then let x' = dropWhileEnd isPathSeparator x
             in if null x' then [last x] else x'
        else x

    -- | Get the directory name, move up one level.
    --
    -- >           takeDirectory "/directory/other.ext" == "/directory"
    -- >           takeDirectory x `isPrefixOf` x || takeDirectory x == "."
    -- >           takeDirectory "foo" == "."
    -- >           takeDirectory "/" == "/"
    -- >           takeDirectory "/foo" == "/"
    -- >           takeDirectory "/foo/bar/baz" == "/foo/bar"
    -- >           takeDirectory "/foo/bar/baz/" == "/foo/bar/baz"
    -- >           takeDirectory "foo/bar/baz" == "foo/bar"
    -- > Windows:  takeDirectory "foo\\bar" == "foo"
    -- > Windows:  takeDirectory "foo\\bar\\\\" == "foo\\bar"
    -- > Windows:  takeDirectory "C:\\" == "C:\\"
    takeDirectory :: FilePath -> FilePath
    takeDirectory = dropTrailingPathSeparator . dropFileName

    -- | Set the directory, keeping the filename the same.
    --
    -- > replaceDirectory "root/file.ext" "/directory/" == "/directory/file.ext"
    -- > Valid x => replaceDirectory x (takeDirectory x) `equalFilePath` x
    replaceDirectory :: FilePath -> String -> FilePath
    replaceDirectory x dir = combineAlways dir (takeFileName x)

    -- | An alias for '</>'.
    combine :: FilePath -> FilePath -> FilePath
    combine a b | hasLeadingPathSeparator b || hasDrive b = b
                | otherwise = combineAlways a b

    -- | Combine two paths, assuming rhs is NOT absolute.
    combineAlways :: FilePath -> FilePath -> FilePath
    combineAlways a b | null a = b
                      | null b = a
                      | hasTrailingPathSeparator a = a ++ b
                      | otherwise = case a of
                          [a1,':'] | isWindows && isLetter a1 -> a ++ b
                          _ -> a ++ [pathSeparator] ++ b

    -- | Combine two paths with a path separator.
    --   If the second path starts with a path separator or a drive letter, then it returns the second.
    --   The intention is that @readFile (dir '</>' file)@ will access the same file as
    --   @setCurrentDirectory dir; readFile file@.
    --
    -- > Posix:   "/directory" </> "file.ext" == "/directory/file.ext"
    -- > Windows: "/directory" </> "file.ext" == "/directory\\file.ext"
    -- >          "directory" </> "/file.ext" == "/file.ext"
    -- > Valid x => (takeDirectory x </> takeFileName x) `equalFilePath` x
    --
    --   Combined:
    --
    -- > Posix:   "/" </> "test" == "/test"
    -- > Posix:   "home" </> "bob" == "home/bob"
    -- > Posix:   "x:" </> "foo" == "x:/foo"
    -- > Windows: "C:\\foo" </> "bar" == "C:\\foo\\bar"
    -- > Windows: "home" </> "bob" == "home\\bob"
    --
    --   Not combined:
    --
    -- > Posix:   "home" </> "/bob" == "/bob"
    -- > Windows: "home" </> "C:\\bob" == "C:\\bob"
    --
    --   Not combined (tricky):
    --
    --   On Windows, if a filepath starts with a single slash, it is relative to the
    --   root of the current drive. In [1], this is (confusingly) referred to as an
    --   absolute path.
    --   The current behavior of '</>' is to never combine these forms.
    --
    -- > Windows: "home" </> "/bob" == "/bob"
    -- > Windows: "home" </> "\\bob" == "\\bob"
    -- > Windows: "C:\\home" </> "\\bob" == "\\bob"
    --
    --   On Windows, from [1]: "If a file name begins with only a disk designator
    --   but not the backslash after the colon, it is interpreted as a relative path
    --   to the current directory on the drive with the specified letter."
    --   The current behavior of '</>' is to never combine these forms.
    --
    -- > Windows: "D:\\foo" </> "C:bar" == "C:bar"
    -- > Windows: "C:\\foo" </> "C:bar" == "C:bar"
    (</>) :: FilePath -> FilePath -> FilePath
    (</>) = combine

    -- | Split a path by the directory separator.
    --
    -- > splitPath "/directory/file.ext" == ["/","directory/","file.ext"]
    -- > concat (splitPath x) == x
    -- > splitPath "test//item/" == ["test//","item/"]
    -- > splitPath "test/item/file" == ["test/","item/","file"]
    -- > splitPath "" == []
    -- > Windows: splitPath "c:\\test\\path" == ["c:\\","test\\","path"]
    -- > Posix:   splitPath "/file/test" == ["/","file/","test"]
    splitPath :: FilePath -> [FilePath]
    splitPath x = [drive | drive /= ""] ++ f path
        where
            (drive,path) = splitDrive x

            f "" = []
            f y = (a++c) : f d
                where
                    (a,b) = break isPathSeparator y
                    (c,d) = span  isPathSeparator b

    -- | Just as 'splitPath', but don't add the trailing slashes to each element.
    --
    -- >          splitDirectories "/directory/file.ext" == ["/","directory","file.ext"]
    -- >          splitDirectories "test/file" == ["test","file"]
    -- >          splitDirectories "/test/file" == ["/","test","file"]
    -- > Windows: splitDirectories "C:\\test\\file" == ["C:\\", "test", "file"]
    -- >          Valid x => joinPath (splitDirectories x) `equalFilePath` x
    -- >          splitDirectories "" == []
    -- > Windows: splitDirectories "C:\\test\\\\\\file" == ["C:\\", "test", "file"]
    -- >          splitDirectories "/test///file" == ["/","test","file"]
    splitDirectories :: FilePath -> [FilePath]
    splitDirectories = map dropTrailingPathSeparator . splitPath

    -- | Join path elements back together.
    --
    -- > joinPath a == foldr (</>) "" a
    -- > joinPath ["/","directory/","file.ext"] == "/directory/file.ext"
    -- > Valid x => joinPath (splitPath x) == x
    -- > joinPath [] == ""
    -- > Posix: joinPath ["test","file","path"] == "test/file/path"
    joinPath :: [FilePath] -> FilePath
    -- Note that this definition on c:\\c:\\, join then split will give c:\\.
    joinPath = foldr combine ""

    ---------------------------------------------------------------------
    -- File name manipulators

    -- | Equality of two 'FilePath's.
    --   If you call @System.Directory.canonicalizePath@
    --   first this has a much better chance of working.
    --   Note that this doesn't follow symlinks or DOSNAM~1s.
    --
    -- Similar to 'normalise', this does not expand @".."@, because of symlinks.
    --
    -- >          x == y ==> equalFilePath x y
    -- >          normalise x == normalise y ==> equalFilePath x y
    -- >          equalFilePath "foo" "foo/"
    -- >          not (equalFilePath "/a/../c" "/c")
    -- >          not (equalFilePath "foo" "/foo")
    -- > Posix:   not (equalFilePath "foo" "FOO")
    -- > Windows: equalFilePath "foo" "FOO"
    -- > Windows: not (equalFilePath "C:" "C:/")
    equalFilePath :: FilePath -> FilePath -> Bool
    equalFilePath a b = f a == f b
        where
            f x | isWindows = dropTrailingPathSeparator $ map toLower $ normalise x
                | otherwise = dropTrailingPathSeparator $ normalise x

    -- | Contract a filename, based on a relative path. Note that the resulting path
    --   will never introduce @..@ paths, as the presence of symlinks means @..\/b@
    --   may not reach @a\/b@ if it starts from @a\/c@. For a worked example see
    --   <http://neilmitchell.blogspot.co.uk/2015/10/filepaths-are-subtle-symlinks-are-hard.html this blog post>.
    --
    --   The corresponding @makeAbsolute@ function can be found in
    --   @System.Directory@.
    --
    -- >          makeRelative "/directory" "/directory/file.ext" == "file.ext"
    -- >          Valid x => makeRelative (takeDirectory x) x `equalFilePath` takeFileName x
    -- >          makeRelative x x == "."
    -- >          Valid x y => equalFilePath x y || (isRelative x && makeRelative y x == x) || equalFilePath (y </> makeRelative y x) x
    -- > Windows: makeRelative "C:\\Home" "c:\\home\\bob" == "bob"
    -- > Windows: makeRelative "C:\\Home" "c:/home/bob" == "bob"
    -- > Windows: makeRelative "C:\\Home" "D:\\Home\\Bob" == "D:\\Home\\Bob"
    -- > Windows: makeRelative "C:\\Home" "C:Home\\Bob" == "C:Home\\Bob"
    -- > Windows: makeRelative "/Home" "/home/bob" == "bob"
    -- > Windows: makeRelative "/" "//" == "//"
    -- > Posix:   makeRelative "/Home" "/home/bob" == "/home/bob"
    -- > Posix:   makeRelative "/home/" "/home/bob/foo/bar" == "bob/foo/bar"
    -- > Posix:   makeRelative "/fred" "bob" == "bob"
    -- > Posix:   makeRelative "/file/test" "/file/test/fred" == "fred"
    -- > Posix:   makeRelative "/file/test" "/file/test/fred/" == "fred/"
    -- > Posix:   makeRelative "some/path" "some/path/a/b/c" == "a/b/c"
    makeRelative :: FilePath -> FilePath -> FilePath
    makeRelative root path
     | equalFilePath root path = "."
     | takeAbs root /= takeAbs path = path
     | otherwise = f (dropAbs root) (dropAbs path)
        where
            f "" y = dropWhile isPathSeparator y
            f x y = let (x1,x2) = g x
                        (y1,y2) = g y
                    in if equalFilePath x1 y1 then f x2 y2 else path

            g x = (dropWhile isPathSeparator a, dropWhile isPathSeparator b)
                where (a,b) = break isPathSeparator $ dropWhile isPathSeparator x

            -- on windows, need to drop '/' which is kind of absolute, but not a drive
            dropAbs x | hasLeadingPathSeparator x && not (hasDrive x) = tail x
            dropAbs x = dropDrive x

            takeAbs x | hasLeadingPathSeparator x && not (hasDrive x) = [pathSeparator]
            takeAbs x = map (\y -> if isPathSeparator y then pathSeparator else toLower y) $ takeDrive x

    -- | Normalise a file
    --
    -- * \/\/ outside of the drive can be made blank
    --
    -- * \/ -> 'pathSeparator'
    --
    -- * .\/ -> \"\"
    --
    -- Does not remove @".."@, because of symlinks.
    --
    -- > Posix:   normalise "/file/\\test////" == "/file/\\test/"
    -- > Posix:   normalise "/file/./test" == "/file/test"
    -- > Posix:   normalise "/test/file/../bob/fred/" == "/test/file/../bob/fred/"
    -- > Posix:   normalise "../bob/fred/" == "../bob/fred/"
    -- > Posix:   normalise "/a/../c" == "/a/../c"
    -- > Posix:   normalise "./bob/fred/" == "bob/fred/"
    -- > Windows: normalise "c:\\file/bob\\" == "C:\\file\\bob\\"
    -- > Windows: normalise "c:\\" == "C:\\"
    -- > Windows: normalise "C:.\\" == "C:"
    -- > Windows: normalise "\\\\server\\test" == "\\\\server\\test"
    -- > Windows: normalise "//server/test" == "\\\\server\\test"
    -- > Windows: normalise "c:/file" == "C:\\file"
    -- > Windows: normalise "/file" == "\\file"
    -- > Windows: normalise "\\" == "\\"
    -- > Windows: normalise "/./" == "\\"
    -- >          normalise "." == "."
    -- > Posix:   normalise "./" == "./"
    -- > Posix:   normalise "./." == "./"
    -- > Posix:   normalise "/./" == "/"
    -- > Posix:   normalise "/" == "/"
    -- > Posix:   normalise "bob/fred/." == "bob/fred/"
    -- > Posix:   normalise "//home" == "/home"
    normalise :: FilePath -> FilePath
    normalise path = result ++ [pathSeparator | addPathSeparator]
        where
            (drv,pth) = splitDrive path
            result = joinDrive' (normaliseDrive drv) (f pth)

            joinDrive' "" "" = "."
            joinDrive' d p = joinDrive d p

            addPathSeparator = isDirPath pth
                && not (hasTrailingPathSeparator result)
                && not (isRelativeDrive drv)

            isDirPath xs = hasTrailingPathSeparator xs
                || not (null xs) && last xs == '.' && hasTrailingPathSeparator (init xs)

            f = joinPath . dropDots . propSep . splitDirectories

            propSep (x:xs) | all isPathSeparator x = [pathSeparator] : xs
                           | otherwise = x : xs
            propSep [] = []

            dropDots = filter ("." /=)

    normaliseDrive :: FilePath -> FilePath
    normaliseDrive "" = ""
    normaliseDrive _ | isPosix = [pathSeparator]
    normaliseDrive drive = if isJust $ readDriveLetter x2
                           then map toUpper x2
                           else x2
        where
            x2 = map repSlash drive

            repSlash x = if isPathSeparator x then pathSeparator else x

    -- Information for validity functions on Windows. See [1].
    isBadCharacter :: Char -> Bool
    isBadCharacter x = x >= '\0' && x <= '\31' || x `elem` ":*?><|\""

    badElements :: [FilePath]
    badElements =
        ["CON","PRN","AUX","NUL","CLOCK$"
        ,"COM1","COM2","COM3","COM4","COM5","COM6","COM7","COM8","COM9"
        ,"LPT1","LPT2","LPT3","LPT4","LPT5","LPT6","LPT7","LPT8","LPT9"]

    -- | Is a FilePath valid, i.e. could you create a file like it? This function checks for invalid names,
    --   and invalid characters, but does not check if length limits are exceeded, as these are typically
    --   filesystem dependent.
    --
    -- >          isValid "" == False
    -- >          isValid "\0" == False
    -- > Posix:   isValid "/random_ path:*" == True
    -- > Posix:   isValid x == not (null x)
    -- > Windows: isValid "c:\\test" == True
    -- > Windows: isValid "c:\\test:of_test" == False
    -- > Windows: isValid "test*" == False
    -- > Windows: isValid "c:\\test\\nul" == False
    -- > Windows: isValid "c:\\test\\prn.txt" == False
    -- > Windows: isValid "c:\\nul\\file" == False
    -- > Windows: isValid "\\\\" == False
    -- > Windows: isValid "\\\\\\foo" == False
    -- > Windows: isValid "\\\\?\\D:file" == False
    -- > Windows: isValid "foo\tbar" == False
    -- > Windows: isValid "nul .txt" == False
    -- > Windows: isValid " nul.txt" == True
    isValid :: FilePath -> Bool
    isValid "" = False
    isValid x | '\0' `elem` x = False
    isValid _ | isPosix = True
    isValid path =
            not (any isBadCharacter x2) &&
            not (any f $ splitDirectories x2) &&
            not (isJust (readDriveShare x1) && all isPathSeparator x1) &&
            not (isJust (readDriveUNC x1) && not (hasTrailingPathSeparator x1))
        where
            (x1,x2) = splitDrive path
            f x = map toUpper (dropWhileEnd (== ' ') $ dropExtensions x) `elem` badElements

    -- | Take a FilePath and make it valid; does not change already valid FilePaths.
    --
    -- > isValid (makeValid x)
    -- > isValid x ==> makeValid x == x
    -- > makeValid "" == "_"
    -- > makeValid "file\0name" == "file_name"
    -- > Windows: makeValid "c:\\already\\/valid" == "c:\\already\\/valid"
    -- > Windows: makeValid "c:\\test:of_test" == "c:\\test_of_test"
    -- > Windows: makeValid "test*" == "test_"
    -- > Windows: makeValid "c:\\test\\nul" == "c:\\test\\nul_"
    -- > Windows: makeValid "c:\\test\\prn.txt" == "c:\\test\\prn_.txt"
    -- > Windows: makeValid "c:\\test/prn.txt" == "c:\\test/prn_.txt"
    -- > Windows: makeValid "c:\\nul\\file" == "c:\\nul_\\file"
    -- > Windows: makeValid "\\\\\\foo" == "\\\\drive"
    -- > Windows: makeValid "\\\\?\\D:file" == "\\\\?\\D:\\file"
    -- > Windows: makeValid "nul .txt" == "nul _.txt"
    makeValid :: FilePath -> FilePath
    makeValid "" = "_"
    makeValid path
            | isPosix = map (\x -> if x == '\0' then '_' else x) path
            | isJust (readDriveShare drv) && all isPathSeparator drv = take 2 drv ++ "drive"
            | isJust (readDriveUNC drv) && not (hasTrailingPathSeparator drv) =
                makeValid (drv ++ [pathSeparator] ++ pth)
            | otherwise = joinDrive drv $ validElements $ validChars pth
        where
            (drv,pth) = splitDrive path

            validChars = map f
            f x = if isBadCharacter x then '_' else x

            validElements x = joinPath $ map g $ splitPath x
            g x = h a ++ b
                where (a,b) = break isPathSeparator x
            h x = if map toUpper (dropWhileEnd (== ' ') a) `elem` badElements then a ++ "_" <.> b else x
                where (a,b) = splitExtensions x

    -- | Is a path relative, or is it fixed to the root?
    --
    -- > Windows: isRelative "path\\test" == True
    -- > Windows: isRelative "c:\\test" == False
    -- > Windows: isRelative "c:test" == True
    -- > Windows: isRelative "c:\\" == False
    -- > Windows: isRelative "c:/" == False
    -- > Windows: isRelative "c:" == True
    -- > Windows: isRelative "\\\\foo" == False
    -- > Windows: isRelative "\\\\?\\foo" == False
    -- > Windows: isRelative "\\\\?\\UNC\\foo" == False
    -- > Windows: isRelative "/foo" == True
    -- > Windows: isRelative "\\foo" == True
    -- > Posix:   isRelative "test/path" == True
    -- > Posix:   isRelative "/test" == False
    -- > Posix:   isRelative "/" == False
    --
    -- According to [1]:
    --
    -- * "A UNC name of any format [is never relative]."
    --
    -- * "You cannot use the "\\?\" prefix with a relative path."
    isRelative :: FilePath -> Bool
    isRelative x = null drive || isRelativeDrive drive
        where drive = takeDrive x

    {- c:foo -}
    -- From [1]: "If a file name begins with only a disk designator but not the
    -- backslash after the colon, it is interpreted as a relative path to the
    -- current directory on the drive with the specified letter."
    isRelativeDrive :: String -> Bool
    isRelativeDrive x =
        maybe False (not . hasTrailingPathSeparator . fst) (readDriveLetter x)

    -- | @not . 'isRelative'@
    --
    -- > isAbsolute x == not (isRelative x)
    isAbsolute :: FilePath -> Bool
    isAbsolute = not . isRelative

    -----------------------------------------------------------------------------
    -- dropWhileEnd (>2) [1,2,3,4,1,2,3,4] == [1,2,3,4,1,2])
    -- Note that Data.List.dropWhileEnd is only available in base >= 4.5.
    dropWhileEnd :: (a -> Bool) -> [a] -> [a]
    dropWhileEnd p = reverse . dropWhile p . reverse

    -- takeWhileEnd (>2) [1,2,3,4,1,2,3,4] == [3,4])
    takeWhileEnd :: (a -> Bool) -> [a] -> [a]
    takeWhileEnd p = reverse . takeWhile p . reverse

    -- spanEnd (>2) [1,2,3,4,1,2,3,4] = ([1,2,3,4,1,2], [3,4])
    spanEnd :: (a -> Bool) -> [a] -> ([a], [a])
    spanEnd p xs = (dropWhileEnd p xs, takeWhileEnd p xs)

    -- breakEnd (< 2) [1,2,3,4,1,2,3,4] == ([1,2,3,4,1],[2,3,4])
    breakEnd :: (a -> Bool) -> [a] -> ([a], [a])
    breakEnd p = spanEnd (not . p)

    -- | The stripSuffix function drops the given suffix from a list. It returns
    -- Nothing if the list did not end with the suffix given, or Just the list
    -- before the suffix, if it does.
    stripSuffix :: Eq a => [a] -> [a] -> Maybe [a]
    stripSuffix xs ys = fmap reverse $ stripPrefix (reverse xs) (reverse ys)