Use the derived instance for Ord and add a test case.

There was a bug in my implementation of Ord for Octets that was
pointed out by Michael McKibben. As a result, the octets corresponding
to (for example) 2 and 4 would compare incorrectly. This is fixed by
simply adopting the derived Ord instance for Octets. Why didn't I do
that in the first place?

With the bug fixed, I've added a QuickCheck property to ensure that
the Ord instances for Octet and Int agree when the Int is between 0
and 255.

diff --git a/src/Octet.hs b/src/Octet.hs
index 34f9d4f821e61997ccbdb2f1438c10a06c71b49a..affce396f79a403941c9e0dda7ff9dfa19151d99 100644 (file)
--- a/src/Octet.hs
+++ b/src/Octet.hs
@@ -30,7 +30,7 @@ data Octet =
           b6 :: Bit,
           b7 :: Bit,
           b8 :: Bit }
-    deriving (Eq)
+    deriving (Eq, Ord)
 
 
 instance Show Octet where
@@ -83,19 +83,6 @@ instance Maskable Octet where
   apply_mask oct _ _ = oct
 
 
-instance Ord Octet where
-  (Octet x1 x2 x3 x4 x5 x6 x7 x8) <= (Octet y1 y2 y3 y4 y5 y6 y7 y8)
-    | x1 > y1 = False
-    | x2 > y2 = False
-    | x3 > y3 = False
-    | x4 > y4 = False
-    | x5 > y5 = False
-    | x6 > y6 = False
-    | x7 > y7 = False
-    | x8 > y8 = False
-    | otherwise = True
-
-
 instance Bounded Octet where
   -- | The octet with the least possible value.
   minBound =
@@ -163,7 +150,8 @@ octet_properties =
   testGroup
     "Octet Properties "
     [ prop_from_enum_to_enum_inverses,
-      prop_read_show_inverses ]
+      prop_read_show_inverses,
+      prop_ord_instances_agree ]
 
 -- QuickCheck properties
 prop_from_enum_to_enum_inverses :: TestTree
@@ -188,6 +176,20 @@ prop_read_show_inverses =
         x' :: Int
         x' = read $ show oct
 
+-- | Ensure that the Ord instance for Octets agrees with the Ord
+--   instance for Int (i.e. that numerical comparisons work).
+prop_ord_instances_agree :: TestTree
+prop_ord_instances_agree =
+  testProperty "the Octet and Int Ord instances agree" prop
+  where
+    prop :: Int -> Int -> Property
+    prop x y = (0 <= x) && (x <= 255) && (0 <= y) && (y <= 255) ==> ord == ord'
+      where
+        ord = (x <= y)
+
+        oct1 = toEnum x :: Octet
+        oct2 = toEnum y :: Octet
+        ord' = (oct1 <= oct2)
 
 -- HUnit Tests
 test_octet_to_enum1 :: TestTree