System.FilePath.Posix

filepath-1.4.0.0: Library for manipulating FilePaths in a cross platform way.

Copyright | (c) Neil Mitchell 2005-2014
License | BSD3
Maintainer | ndmitchell@gmail.com
Stability | stable
Portability | portable
Safe Haskell | Safe
Language | Haskell98

System.FilePath.Posix

Contents

- Separator predicates
- `$PATH` methods
- Extension functions
- Filename/directory functions
- Drive functions
- Trailing slash functions
- File name manipulations

Description

A library for `FilePath` manipulations, using Posix style paths on all platforms. Importing System.FilePath is usually better.

Given the eample `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.IO.createDirectoryIfMissing True (takeDirectory file)

Example 3: Compile a Haskell file, putting the `.hi` file under `interface`:

    takeDirectory file </> "interface" </> (takeFileName file -<.> "hi")

References: [1] Naming Files, Paths and Namespaces (Microsoft MSDN)

Synopsis

- type FilePath = String
- pathSeparator :: Char
- pathSeparators :: [Char]
- isPathSeparator :: Char -> Bool
- searchPathSeparator :: Char
- isSearchPathSeparator :: Char -> Bool
- extSeparator :: Char
- isExtSeparator :: Char -> Bool
- splitSearchPath :: String -> [FilePath]
- getSearchPath :: IO [FilePath]
- splitExtension :: FilePath -> (String, String)
- takeExtension :: FilePath -> String
- replaceExtension :: FilePath -> String -> FilePath
- (-<.>) :: FilePath -> String -> FilePath
- dropExtension :: FilePath -> FilePath
- addExtension :: FilePath -> String -> FilePath
- hasExtension :: FilePath -> Bool
- (<.>) :: FilePath -> String -> FilePath
- splitExtensions :: FilePath -> (FilePath, String)
- dropExtensions :: FilePath -> FilePath
- takeExtensions :: FilePath -> String
- splitFileName :: FilePath -> (String, String)
- takeFileName :: FilePath -> FilePath
- replaceFileName :: FilePath -> String -> FilePath
- dropFileName :: FilePath -> FilePath
- takeBaseName :: FilePath -> String
- replaceBaseName :: FilePath -> String -> FilePath
- takeDirectory :: FilePath -> FilePath
- replaceDirectory :: FilePath -> String -> FilePath
- combine :: FilePath -> FilePath -> FilePath
- (</>) :: FilePath -> FilePath -> FilePath
- splitPath :: FilePath -> [FilePath]
- joinPath :: [FilePath] -> FilePath
- splitDirectories :: FilePath -> [FilePath]
- splitDrive :: FilePath -> (FilePath, FilePath)
- joinDrive :: FilePath -> FilePath -> FilePath
- takeDrive :: FilePath -> FilePath
- hasDrive :: FilePath -> Bool
- dropDrive :: FilePath -> FilePath
- isDrive :: FilePath -> Bool
- hasTrailingPathSeparator :: FilePath -> Bool
- addTrailingPathSeparator :: FilePath -> FilePath
- dropTrailingPathSeparator :: FilePath -> FilePath
- normalise :: FilePath -> FilePath
- equalFilePath :: FilePath -> FilePath -> Bool
- makeRelative :: FilePath -> FilePath -> FilePath
- isRelative :: FilePath -> Bool
- isAbsolute :: FilePath -> Bool
- isValid :: FilePath -> Bool
- makeValid :: FilePath -> FilePath

# Separator predicates

type FilePath = String Source

File and directory names are values of type `String`, whose precise meaning is operating system dependent. Files can be opened, yielding a handle which can then be used to operate on the contents of that file.

pathSeparator :: Char

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

pathSeparators :: [Char]

The list of all possible separators.

    Windows: pathSeparators == ['\\', '/']
    Posix:   pathSeparators == ['/']
    pathSeparator `elem` pathSeparators

isPathSeparator :: Char -> Bool

Rather than using `(== `pathSeparator`)`, use this. Test if something is a path separator.

    isPathSeparator a == (a `elem` pathSeparators)

searchPathSeparator :: Char

The character that is used to separate the entries in the $PATH environment variable.

    Windows: searchPathSeparator == ';'
    Posix:   searchPathSeparator == ':'

isSearchPathSeparator :: Char -> Bool

Is the character a file separator?

    isSearchPathSeparator a == (a == searchPathSeparator)

extSeparator :: Char

File extension character

    extSeparator == '.'

isExtSeparator :: Char -> Bool

Is the character an extension character?

    isExtSeparator a == (a == extSeparator)

# `$PATH` methods

splitSearchPath :: String -> [FilePath]

Take a string, split it on the `searchPathSeparator` character.

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"]

getSearchPath :: IO [FilePath]

Get a list of `FilePath`s in the $PATH variable.

# Extension functions

splitExtension :: FilePath -> (String, String)

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/","")

takeExtension :: FilePath -> String

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"

replaceExtension :: FilePath -> String -> FilePath

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

(-<.>) :: FilePath -> String -> FilePath infixr 7

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"

dropExtension :: FilePath -> FilePath

Remove last extension, and the "." preceding it.

    dropExtension "/directory/path.ext" == "/directory/path"
    dropExtension x == fst (splitExtension x)

addExtension :: FilePath -> String -> FilePath

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"
    Valid x => takeFileName (addExtension (addTrailingPathSeparator x) "ext") == ".ext"
    Windows: addExtension "\\\\share" ".txt" == "\\\\share\\.txt"

hasExtension :: FilePath -> Bool

Does the given filename have an extension?

    hasExtension "/directory/path.ext" == True
    hasExtension "/directory/path" == False
    null (takeExtension x) == not (hasExtension x)

(<.>) :: FilePath -> String -> FilePath infixr 7

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"

splitExtensions :: FilePath -> (FilePath, String)

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")

dropExtensions :: FilePath -> FilePath

Drop all extensions.

    dropExtensions "/directory/path.ext" == "/directory/path"
    dropExtensions "file.tar.gz" == "file"
    not $ hasExtension $ dropExtensions x
    not $ any isExtSeparator $ takeFileName $ dropExtensions x

takeExtensions :: FilePath -> String

Get all extensions.

    takeExtensions "/directory/path.ext" == ".ext"
    takeExtensions "file.tar.gz" == ".tar.gz"

# Filename/directory functions

splitFileName :: FilePath -> (String, String)

Split a filename into directory and file. `combine` 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:","")

takeFileName :: FilePath -> FilePath

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)

replaceFileName :: FilePath -> String -> FilePath

Set the filename.

    replaceFileName "/directory/other.txt" "file.ext" == "/directory/file.ext"
    Valid x => replaceFileName x (takeFileName x) == x

dropFileName :: FilePath -> FilePath

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)

takeBaseName :: FilePath -> String

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"

replaceBaseName :: FilePath -> String -> FilePath

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

takeDirectory :: FilePath -> FilePath

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:\\"

replaceDirectory :: FilePath -> String -> FilePath

Set the directory, keeping the filename the same.

    replaceDirectory "root/file.ext" "/directory/" == "/directory/file.ext"
    Valid x => replaceDirectory x (takeDirectory x) `equalFilePath` x

combine :: FilePath -> FilePath -> FilePath

Combine two paths, if the second path starts with a path separator or a drive letter, then it returns the second.

    Valid x => combine (takeDirectory x) (takeFileName x) `equalFilePath` x

Combined: > Posix: combine "" "test" == "test" > Posix: combine "home" "bob" == "home/bob" > Posix: combine "x:" "foo" == "x:/foo" > Windows: combine C:\\foo "bar" == C:\\foo\\bar > Windows: combine "home" "bob" == "home\bob"

Not combined: > Posix: combine "home" "bob" == "bob" > Windows: combine "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 `combine` is to never combine these forms.

    Windows: combine "home" "/bob" == "/bob"
    Windows: combine "home" "\\bob" == "\\bob"
    Windows: combine "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 `combine` is to never combine these forms.

    Windows: combine "D:\\foo" "C:bar" == "C:bar"
    Windows: combine "C:\\foo" "C:bar" == "C:bar"

(</>) :: FilePath -> FilePath -> FilePath infixr 5

Join two values with a path separator. For examples and caveats see the equivalent function `combine`.

    Posix:   "/directory" </> "file.ext" == "/directory/file.ext"
    Windows: "/directory" </> "file.ext" == "/directory\\file.ext"

splitPath :: FilePath -> [FilePath]

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"]

joinPath :: [FilePath] -> FilePath

Join path elements back together.

    joinPath ["/","directory/","file.ext"] == "/directory/file.ext"
    Valid x => joinPath (splitPath x) == x
    joinPath [] == ""
    Posix: joinPath ["test","file","path"] == "test/file/path"

splitDirectories :: FilePath -> [FilePath]

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"]

# Drive functions

splitDrive :: FilePath -> (FilePath, FilePath)

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")

joinDrive :: FilePath -> FilePath -> FilePath

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"

takeDrive :: FilePath -> FilePath

Get the drive from a filepath.

    takeDrive x == fst (splitDrive x)

hasDrive :: FilePath -> Bool

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

dropDrive :: FilePath -> FilePath

Delete the drive, if it exists.

    dropDrive x == snd (splitDrive x)

isDrive :: FilePath -> Bool

Is an element a drive

    Posix:   isDrive "/" == True
    Posix:   isDrive "/foo" == False
    Windows: isDrive "C:\\" == True
    Windows: isDrive "C:\\foo" == False
             isDrive "" == False

# Trailing slash functions

hasTrailingPathSeparator :: FilePath -> Bool

Is an item either a directory or the last character a path separator?

    hasTrailingPathSeparator "test" == False
    hasTrailingPathSeparator "test/" == True

addTrailingPathSeparator :: FilePath -> FilePath

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/"

dropTrailingPathSeparator :: FilePath -> FilePath

Remove any trailing path separators

    dropTrailingPathSeparator "file/test/" == "file/test"
              dropTrailingPathSeparator "/" == "/"
    Windows:  dropTrailingPathSeparator "\\" == "\\"
    Posix:    not (hasTrailingPathSeparator (dropTrailingPathSeparator x)) || isDrive x

# File name manipulations

normalise :: FilePath -> FilePath

Normalise a file

- // outside of the drive can be made blank
- / -> `pathSeparator`
- ./ -> ""

    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 "./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"

equalFilePath :: FilePath -> FilePath -> Bool

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.

             x == y ==> equalFilePath x y
             normalise x == normalise y ==> equalFilePath x y
             equalFilePath "foo" "foo/"
             not (equalFilePath "foo" "/foo")
    Posix:   not (equalFilePath "foo" "FOO")
    Windows: equalFilePath "foo" "FOO"
    Windows: not (equalFilePath "C:" "C:/")

makeRelative :: FilePath -> FilePath -> FilePath

Contract a filename, based on a relative path.

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"

isRelative :: FilePath -> Bool

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."

isAbsolute :: FilePath -> Bool

    not . isRelative

    isAbsolute x == not (isRelative x)

isValid :: FilePath -> Bool

Is a FilePath valid, i.e. could you create a file like it?

             isValid "" == 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

makeValid :: FilePath -> FilePath

Take a FilePath and make it valid; does not change already valid FilePaths.

    isValid (makeValid x)
    isValid x ==> makeValid x == x
    makeValid "" == "_"
    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"