more tests.

test/Dir.hs

@@ -1,7 +1,9 @@
 module Dir where
 
-import System.Directory
 import Control.Exception as E
+import Data.List (isPrefixOf)
+import System.Directory
+import System.FilePath (addTrailingPathSeparator)
 
 withDirectory_ :: FilePath -> IO a -> IO a
 withDirectory_ dir action = bracket getCurrentDirectory
@@ -12,3 +14,11 @@ withDirectory :: FilePath -> (FilePath -> IO a) -> IO a
 withDirectory dir action = bracket getCurrentDirectory
                                    setCurrentDirectory
                                    (\d -> setCurrentDirectory dir >> action d)
+
+toRelativeDir :: FilePath -> FilePath -> FilePath
+toRelativeDir dir file
+  | dir' `isPrefixOf` file = drop len file
+  | otherwise              = file
+  where
+    dir' = addTrailingPathSeparator dir
+    len = length dir'