Add type-safe synonyms of ‘toFilePath’

This helps to “double check” programmers' assumptions about what kind of
path he is converting into ‘FilePath’. Without these synonyms it's
possible to silently convert wrong type of path into ‘FilePath’.

src/Path.hs

@@ -31,6 +31,10 @@ module Path
   ,dirname
   -- * Conversion
   ,toFilePath
+  ,fromAbsDir
+  ,fromRelDir
+  ,fromAbsFile
+  ,fromRelFile
   )
   where
 
@@ -203,6 +207,22 @@ mkRelFile s =
 toFilePath :: Path b t -> FilePath
 toFilePath (Path l) = l
 
+-- | Convert absolute path to directory to 'FilePath' type.
+fromAbsDir :: Path Abs Dir -> FilePath
+fromAbsDir = toFilePath
+
+-- | Convert relative path to directory to 'FilePath' type.
+fromRelDir :: Path Rel Dir -> FilePath
+fromRelDir = toFilePath
+
+-- | Convert absolute path to file to 'FilePath' type.
+fromAbsFile :: Path Abs File -> FilePath
+fromAbsFile = toFilePath
+
+-- | Convert relative path to file to 'FilePath' type.
+fromRelFile :: Path Rel File -> FilePath
+fromRelFile = toFilePath
+
 --------------------------------------------------------------------------------
 -- Operations