module Store where

import List((\\))

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

empty :: Store a b
empty = Store []    

get :: Eq a=> Store a b-> a-> Maybe b
get (Store s) a = get' s where
  get' []         = Nothing
  get' ((b, v):s) =
    if a == b then Just v else get' s

upd :: Eq a=> Store a b-> a-> b-> Store a b
upd (Store s) a v = Store (upd' s) where
  upd' []         = [(a, v)]
  upd' ((b, w):s) = 
    if a == b then (a, v):s 
    else (b,w): upd' s  

instance (Show a, Show b)=> Show (Store a b) where
  show (Store s) = show s

instance (Eq a, Eq b)=> Eq (Store a b) where
  Store s1 == Store s2 = 
    null (s1 \\ s2) && null (s1 \\ s2)