{-# OPTIONS -XGADTs #-}

data Option a where
   None :: Option a
   Some :: a -> Option a

ret :: a -> Option a
ret x = Some x

letb :: Option a -> (a -> Option b) -> Option b
letb None _ = None
letb (Some x) f = f x

raise :: Option a
raise = None

instance Monad Option where
   return = ret
   x >>= f = letb x f

data Tree a where
    Leaf :: a -> Tree a
    Node :: Tree a -> Tree a -> Tree a

sumpos :: Tree Integer -> Option Integer
sumpos (Leaf x1) = if x1 >= 0 
                   then return x1 
                   else raise
sumpos (Node n1 n2) = do s1 <- sumpos n1
                         s2 <- sumpos n2
                         return (s1 + s2)

sumpos2 :: Tree Integer -> Option Integer
sumpos2 (Leaf x1) = if x1 >= 0 
                    then Some x1 
                    else None
sumpos2 (Node n1 n2) = case sumpos n1 of
                         None -> None
                         Some s1 -> case sumpos n2 of
                                     None -> None
                                     Some s2 -> Some (s1 + s2)


instance Show a => Show (Option a) where 
    show None = "None"
    show (Some x) = "Some(" ++ (show x) ++ ")"

t1 = (Node (Leaf 12) (Node (Leaf (-13)) (Leaf 13)))
t2 = (Node (Leaf 12) (Node (Leaf 17) (Leaf 13)))