Add some commentary

src/Path/Internal.hs

@@ -1,3 +1,4 @@
+{-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE DeriveGeneric #-}
 {-# LANGUAGE DeriveDataTypeable #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
@@ -12,8 +13,37 @@ import Data.Data
 import GHC.Generics
 
 -- | Path of some base and type.
+--
+-- Internally is a string. The string can be of two formats only:
+--
+-- 1. File format: @file.txt@, @foo\/bar.txt@, @\/foo\/bar.txt@
+-- 2. Directory format: @foo\/@, @\/foo\/bar\/@
+--
+-- All directories end in a trailing separator. There are no duplicate
+-- path separators @\/\/@, no @..@, no @.\/@, no @~\/@, etc.
 newtype Path b t = Path FilePath
-  deriving (Eq,Ord,Typeable,Data,Generic)
+  deriving (Typeable,Generic)
 
+-- | String equality.
+--
+-- The following property holds:
+--
+-- @show x == show y ≡ x == y@
+instance Eq (Path b t) where
+  (==) (Path x) (Path y) = x == y
+
+-- | String ordering.
+--
+-- The following property holds:
+--
+-- @show x \`compare\` show y ≡ x \`compare\` y@
+instance Ord (Path b t) where
+  compare (Path x) (Path y) = compare x y
+
+-- | Same as 'Path.toFilePath'.
+--
+-- The following property holds:
+--
+-- @x == y ≡ show x == show y@
 instance Show (Path b t) where
   show (Path x) = show x