Word type supports Underflow/Overflow error

BWord.hs

module Data.BWord(BWord(..),fromBWord) where

newtype BWord = BWord Integer
              deriving (Eq,Ord,Show)

fromBWord :: BWord -> Integer
fromBWord (BWord x) = x

minBound' :: Integer
minBound' = fromIntegral (minBound::Word)

maxBound' :: Integer
maxBound' = fromIntegral (maxBound::Word)

instance Bounded BWord where
    minBound = fromInteger minBound'
    maxBound = fromInteger maxBound'

mkBWord :: Integer -> BWord
mkBWord n
    | n < minBound' = error "Underflow"
    | maxBound' < n = error "Overflow"
    | otherwise = BWord n

instance Num BWord where
  BWord x + BWord y = mkBWord $ x + y
  BWord x * BWord y = mkBWord $ x * y
  BWord x - BWord y = mkBWord $ x - y
  abs x = x
  signum (BWord 0) = 0
  signum (BWord _) = 1
  fromInteger = mkBWord


test1 = (maxBound::BWord)
test2 = (maxBound::BWord) +1
test3 = (maxBound::BWord) -1 +1
test4 = (maxBound::BWord) +1 -1
test5 = (maxBound::BWord) -2 +1
test6 = (maxBound::BWord) +1 -2

test7 = (minBound::BWord)
test8 = (minBound::BWord) -1
test9 = (minBound::BWord) +1 -1
test10 = (minBound::BWord) -1 +1
test11 = (minBound::BWord) +2 -1
test12 = (minBound::BWord) -1 +2