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.
b6 :: Bit,
b7 :: Bit,
b8 :: Bit }
b6 :: Bit,
b7 :: Bit,
b8 :: Bit }
instance Show Octet where
instance Show Octet where
-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 =
instance Bounded Octet where
-- | The octet with the least possible value.
minBound =
testGroup
"Octet Properties "
[ prop_from_enum_to_enum_inverses,
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
-- QuickCheck properties
prop_from_enum_to_enum_inverses :: TestTree
x' :: Int
x' = read $ show oct
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
-- HUnit Tests
test_octet_to_enum1 :: TestTree