Incorrect Monoid instance

[sgf-dma]

Hi.

Monoid instance of FilePath is incorrect:

> let x = F.decodeString "/a/b" in x `mappend` mempty == x
False

It seems, the reason is append (lib/Filesystem/Path.hs):

171 append :: FilePath -> FilePath -> FilePath
172 append x y = cased where
173         cased = case pathRoot y of
174                 Just RootPosix -> y
175                 Just RootWindowsVolume{} -> y
176                 Just RootWindowsCurrentVolume -> case pathRoot x of
177                         Just RootWindowsVolume{} -> y { pathRoot = pathRoot x }
178                         _ -> y
179                 Just RootWindowsUnc{} -> y
180                 Just RootWindowsDoubleQMark -> y
181                 Nothing -> xy
182         xy = y
183                 { pathRoot = pathRoot x
184                 , pathDirectories = directories
185                 }
186         directories = xDirectories ++ pathDirectories y
187         xDirectories = (pathDirectories x ++) $ if null (filename x)
188                 then []
189                 else [filenameChunk x]
190

in xDirectories, which adds filename of x to the directories of resulting
FilePath. Here is the result with modified Show instance:

> decodeString posix  "/a/b/c"
FilePath {pathRoot = Just RootPosix, pathDirectories = ["a","b"], pathBasename = Just "c", pathExtensions = []}

> decodeString posix  "/a/b/c" `mappend` empty
FilePath {pathRoot = Just RootPosix, pathDirectories = ["a","b","c"], pathBasename = Nothing, pathExtensions = []}