Add a None constructor for Octets.

Return Octet.None when an octet is created from an invalid Int.

diff --git a/src/Octet.hs b/src/Octet.hs
index f8af4df8c90ec51f6db9bac93e92a1d1abc7c79f..e0a4bf655c1ece922a37d5fcdc8f2634957a1a3d 100644 (file)
--- a/src/Octet.hs
+++ b/src/Octet.hs
@@ -5,14 +5,14 @@ import Bit
 -- An Octet consists of eight bits. For our purposes, the most
 -- significant bit will come "first." That is, b1 is in the 2^7
 -- place while b8 is in the 2^0 place.
-data Octet = Octet { b1 :: Bit,
-                     b2 :: Bit,
-                     b3 :: Bit,
-                     b4 :: Bit,
-                     b5 :: Bit,
-                     b6 :: Bit,
-                     b7 :: Bit,
-                     b8 :: Bit }
+data Octet = None | Octet { b1 :: Bit,
+                            b2 :: Bit,
+                            b3 :: Bit,
+                            b4 :: Bit,
+                            b5 :: Bit,
+                            b6 :: Bit,
+                            b7 :: Bit,
+                            b8 :: Bit }
            deriving (Eq, Show)
 
 -- Convert each bit to its integer value, and multiply by the
@@ -30,10 +30,10 @@ octet_to_int x =
     0   * (bit_to_int (b8 x))
 
 
--- Supply an integer greater than 255 at your own risk.
+
 octet_from_int :: Int -> Octet
 octet_from_int x
-    | (x < 0) = min_octet 
+    | (x < 0) || (x > 255) = Octet.None
     | otherwise = (Octet a1 a2 a3 a4 a5 a6 a7 a8)
     where
       a1 = if (x `mod` 128) > 0 then One else Zero