[Haskell-beginners] Re: overflow safe Word types

[Maciej Piechotka]

On Fri, 2010-03-26 at 14:23 -0400, Ashish Agarwal wrote:
> Is there an alternative implementation of the types Word8, Word16,
> etc. that disallow overflow? For example, currently:
> 
> 
> Prelude Word> (fromInteger 256) :: Word8
> 0
> 
> 
> I'd like a type whose constructors disallow this.
> 
> 

safeConvert :: (Integral a, Integral b) :: a -> Maybe b
safeConvert x | fromIntegral y == x = Just $! y
              | otherwise           = Nothing
              where y = fromIntegral x

You can write:

newtype Safe a = Safe a deriving Eq

instance Show a => Show (Safe a) where
	show (Safe x) = show x

instance Integral a => Num (Safe a) where
	(Safe x) + (Safe y) = Safe $! x + y
	(Safe x) * (Safe y) = Safe $! x * y
	(Safe x) - (Safe y) = Safe $! x - y
	negate     (Safe y) = Safe $! negate y
	abs        (Safe y) = Safe $! abs y
	signum     (Safe y) = Safe $! signum y
	fromInteger x       =
	  Safe $! fromMaybe (error "overflow") (safeConvert x)

...

if you want

Regards
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