import ParseLib

class Eq l => Domain d l | d -> l where
         (<<) :: d -> d -> d
         empty :: d
class Domain e l => Environment e l t | e -> l t where
         get :: e -> l -> Maybe t
         attribute :: l -> t -> e

class Eq' a where
	(=?=) :: a -> a -> Bool

newtype MyLabel = MyLabel String deriving Eq

instance Eq' MyLabel where
	l =?= l' = l == l'

data BinTreeEnv l t = 
         EmptyEnv | 
         AttributeLeaf l t | 
         Union (BinTreeEnv l t) (BinTreeEnv l t)
instance (Eq l, Eq' l) => Domain (BinTreeEnv l t) l where
         EmptyEnv << d = d
         d << EmptyEnv = d
         d << d' = Union d d'
         empty = EmptyEnv
instance (Eq l, Eq' l) => Environment (BinTreeEnv l t) l t where
        get EmptyEnv l = Nothing
        get (AttributeLeaf l t) l' = 
                                         ----------------------------
                if l =?= l' then Just t  -- can't replace =?= with == 
                                         ----------------------------
                else Nothing
        get (Union d d') l = error "!??"
        get (Union d d') l =
                 case get d' l of
                        Nothing -> get d l
                        t -> t
        attribute l t = AttributeLeaf l t

type Env1 = BinTreeEnv MyLabel Integer