> import Prelude hiding (div,return,fmap)
> import qualified Prelude

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

> return :: a -> Either a e
> return x = Left x

> exception :: e -> Either a e
> exception x = Right x

> extend :: (a -> Either a e) -> Either a e -> Either a e
> extend f = join . (fmap f)

> fmap :: (a -> b) -> Either a e -> Either b e
> fmap f (Right e) = Right e
> fmap f (Left x) = Left (f x)

> join :: Either (Either a e) e -> Either a e
> join (Right e) = Right e
> join (Left (Right e)) = Right e
> join (Left (Left x)) = Left x

 handle :: (a -> a -> Either a e) -> Either a e -> Either a e -> Either a e
 handle f x y = extend (\x' -> extend (\y' -> f x' y') y) x

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

> sub :: Int -> Int -> Either Int String
> sub x y = return (x - y)

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

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

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

x = Left 3

> x = add 1 2

y = Left 17

> y = extend (\a -> add a 5) (mul 3 4)

z = Right "Exception: Divide by zero"

> z = div 1 0

w = Right "Exception: Divide by zero"

> w = extend (\a -> add 1 a) (div 1 0)

v = Left 42

> v = extend (\a -> extend (\b -> mul a b) (add 2 4)) (add 4 3)