> import Prelude hiding (div)
> import qualified Prelude

------------------------------------------------------------
 Language E + div exception + monadic exception threading (2)
------------------------------------------------------------

> instance Monad Value where
>     return x = Value x
>     x >>= f = case x of Exception e -> Exception e
>                         Value x     -> f x

> data Value a = Value a | Exception String deriving Show

> add :: Int -> Int -> Value Int
> add x y = return (x + y)

> sub :: Int -> Int -> Value Int
> sub x y = return (x - y)

> div :: Int -> Int -> Value Int
> div x y = if y==0 then Exception "Divide by zero"
>                   else return (x `Prelude.div` y)

> mul :: Int -> Int -> Value Int
> mul x y = return (x * y)

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

x = Value 3

> x = add 1 2

y = Value 17

> y = do a <- mul 3 4
>        add a 5

z = Exception "Exception: Divide by zero"

> z = div 1 0

w = Exception "Exception: Divide by zero"

> w = do a <- div 1 0
>        add 1 a

v = 42

> v = do a <- add 4 3
>        b <- add 2 4
>        mul a b