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

The list of all possible separators.

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

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

 isPathSeparator a == (a `elem` pathSeparators)

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

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

Is the character a file separator?

 isSearchPathSeparator a == (a == searchPathSeparator)

File extension character

 extSeparator == '.'

Is the character an extension character?

 isExtSeparator a == (a == extSeparator)

Take a string, split it on the searchPathSeparator character.

 Windows: splitSearchPath "File1;File2;File3" == ["File1","File2","File3"]
 Posix:   splitSearchPath "File1:File2:File3" == ["File1","File2","File3"]

Split on the extension. addExtension is the inverse.

 uncurry (++) (splitExtension x) == 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/","")

Get the extension of a file, returns "" for no extension, .ext otherwise.

 takeExtension x == snd (splitExtension x)
 takeExtension (addExtension x "ext") == ".ext"
 takeExtension (replaceExtension x "ext") == ".ext"

Set the extension of a file, overwriting one if already present.

 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"

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

 dropExtension x == fst (splitExtension x)

Add an extension, even if there is already one there. E.g. addExtension "foo.txt" "bat" -> "foo.txt.bat".

 addExtension "file.txt" "bib" == "file.txt.bib"
 addExtension "file." ".bib" == "file..bib"
 addExtension "file" ".bib" == "file.bib"
 addExtension "/" "x" == "/.x"
 takeFileName (addExtension (addTrailingPathSeparator x) "ext") == ".ext"
 Windows: addExtension "\\\\share" ".txt" == "\\\\share\\.txt"

Does the given filename have an extension?

 null (takeExtension x) == not (hasExtension x)

Split on all extensions

 splitExtensions "file.tar.gz" == ("file",".tar.gz")

Drop all extensions

 not $ hasExtension (dropExtensions x)

Split a filename into directory and file. combine is the inverse.

 uncurry (++) (splitFileName x) == x
 uncurry combine (splitFileName (makeValid x)) == (makeValid x)
 splitFileName "file/bob.txt" == ("file/", "bob.txt")
 splitFileName "file/" == ("file/", "")
 splitFileName "bob" == ("", "bob")
 Posix:   splitFileName "/" == ("/","")
 Windows: splitFileName "c:" == ("c:","")

Get the file name.

 takeFileName "test/" == ""
 takeFileName x == snd (splitFileName x)
 takeFileName (replaceFileName x "fred") == "fred"
 takeFileName (combine x "fred") == "fred"
 isRelative (takeFileName (makeValid x))

Set the filename.

 replaceFileName (makeValid x) (takeFileName (makeValid x)) == makeValid x

Drop the filename.

 dropFileName x == fst (splitFileName x)

Get the base name, without an extension or path.

 takeBaseName "file/test.txt" == "test"
 takeBaseName "dave.ext" == "dave"
 takeBaseName "" == ""
 takeBaseName "test" == "test"
 takeBaseName (addTrailingPathSeparator x) == ""
 takeBaseName "file/file.tar.gz" == "file.tar"

Set the base name.

 replaceBaseName "file/test.txt" "bob" == "file/bob.txt"
 replaceBaseName "fred" "bill" == "bill"
 replaceBaseName "/dave/fred/bob.gz.tar" "new" == "/dave/fred/new.tar"
 replaceBaseName x (takeBaseName x) == x

Get the directory name, move up one level.

 Posix:    takeDirectory "/foo/bar/baz" == "/foo/bar"
 Posix:    takeDirectory "/foo/bar/baz/" == "/foo/bar/baz"
 Windows:  takeDirectory "foo\\bar" == "foo"
 Windows:  takeDirectory "foo\\bar\\\\" == "foo\\bar"
 Windows:  takeDirectory "C:\\" == "C:\\"

Set the directory, keeping the filename the same.

 replaceDirectory x (takeDirectory x) `equalFilePath` x

Combine two paths, if the second path isAbsolute, then it returns the second.

 combine (takeDirectory (makeValid x)) (takeFileName (makeValid x)) `equalFilePath` makeValid x
 Posix:   combine "/" "test" == "/test"
 Posix:   combine "home" "bob" == "home/bob"
 Windows: combine "home" "bob" == "home\\bob"

Split a path by the directory separator.

 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"]

Join path elements back together.

 joinPath (splitPath (makeValid x)) == makeValid x

Just as splitPath, but don't add the trailing slashes to each element.

 splitDirectories "test/file" == ["test","file"]
 splitDirectories "/test/file" == ["/","test","file"]
 joinPath (splitDirectories (makeValid x)) `equalFilePath` makeValid x
 splitDirectories "" == []

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

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

Add a trailing file path separator if one is not already present.

 hasTrailingPathSeparator (addTrailingPathSeparator x)
 if hasTrailingPathSeparator x then addTrailingPathSeparator x == x else True
 Posix:    addTrailingPathSeparator "test/rest" == "test/rest/"

Remove any trailing path separators

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

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 "\\\\server\\test" == "\\\\server\\test"
 Windows: normalise "c:/file" == "C:\\file"

Contract a filename, based on a relative path.

 Windows: makeRelative x (x `combine` y) == y || takeDrive x == x
 Posix:   makeRelative x (x `combine` y) == y
 (isRelative x && makeRelative y x == x) || y `combine` makeRelative y x == x
 Windows: makeRelative "C:\\Home" "c:\\home\\bob" == "bob"
 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"

Is a path relative, or is it fixed to the root?

 Windows: isRelative "path\\test" == True
 Windows: isRelative "c:\\test" == False
 Posix:   isRelative "test/path" == True
 Posix:   isRelative "/test" == False

not . isRelative

 isAbsolute x == not (isRelative x)

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

 Posix:   isValid "/random_ path:*" == True
 Posix:   isValid x == True
 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

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

 isValid (makeValid x)
 if isValid x then makeValid x == x else True
 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"