> import Prelude hiding (div)
> import qualified Prelude

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

> add :: Int -> Int -> Int
> add x y = x + y

> sub :: Int -> Int -> Int
> sub x y = x - y

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

> mul :: Int -> Int -> Int
> mul x y = x * y

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

x = Left 3

> x = add 1 2

y = Left 17

> y = add (mul 3 4) 5

z = Right "Exception: Divide by zero"

> z = div 1 0

Uncommenting the following causes a type error:    

    Couldn't match expected type `Int'
           against inferred type `Either String Int'
    In the second argument of `add', namely `(div 1 0)'

 w = add 1 (div 1 0)