module Store1(Store, initial, value, update) where

initial :: Store a b
value   :: Eq a=> Store a b-> a-> Maybe b
update  :: Eq a=> Store a b-> a-> b-> Store a b

data Store a b = Store [(a, b)]

                       deriving Show    

initial  = Store []  

value (Store ls) a = case filter ((a ==).fst) ls of
                    (_, x):_ -> Just x
                    []       -> Nothing      

update (Store ls) a b  = Store ((a, b): ls)