> import Prelude hiding (div)
> import qualified Prelude

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

> add :: Either Int String -> Either Int String -> Either Int String
> add x y = case x of
>             Right e -> Right e
>             Left x' -> case y of
>                           Right e' -> Right e'
>                           Left y'  -> Left (x' + y')

> sub :: Either Int String -> Either Int String -> Either Int String
> sub x y = case x of
>             Right e -> Right e
>             Left x' -> case y of
>                           Right e' -> Right e'
>                           Left y'  -> Left (x' - y')

> div :: Either Int String -> Either Int String -> Either Int String
> div x y = case x of
>             Right e -> Right e
>             Left x' -> case y of
>                           Right e' -> Right e'
>                           Left 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 x y = case x of
>             Right e -> Right e
>             Left x' -> case y of
>                           Right e' -> Right e'
>                           Left y'  -> Left $ 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))