> import Prelude hiding (div)
> import qualified Prelude

------------------------------------------------------------
 Language E + div exception + abstracted exception threading
------------------------------------------------------------

> add :: Either Int String -> Either Int String -> Either Int String
> add = exceptionHandle (\x y -> Left (x + y))

> sub :: Either Int String -> Either Int String -> Either Int String
> sub = exceptionHandle (\x y -> Left (x - y))

> div :: Either Int String -> Either Int String -> Either Int String
> div = exceptionHandle (\x y -> if y==0 then Right "Exception: Divide by zero"
>                                else Left (x `Prelude.div` y))

> mul :: Either Int String -> Either Int String -> Either Int String
> mul = exceptionHandle (\x y -> Left (x * y))

> exceptionHandle :: (a -> a -> Either a String)
>                 -> Either a String
>                 -> Either a String
>                 -> Either a String
> exceptionHandle f x y = case x of 
>                 Right e -> Right e
>                 Left x' -> case y of
>                               Right e' -> Right e'
>                               Left y'  -> f x' y'

------------------------------------------------------------
 Examples
------------------------------------------------------------

x = Left 3

> x = add (Left 1) (Left 2)

y = Left 17

> y = add (mul (Left 3) (Left 4)) (Left 5)

z = Right "Exception: Divide by zero"

> z = div (Left 1) (Left 0)

w = Right "Exception: Divide by zero"

> w = add (Left 1) (div (Left 1) (Left 0))