--                                                               
-- 	Store.lhs

--         An abstract data type of stores of integers, implemented as
--         a list of pairs of variables and values.			
--                                                                 
--         (c) Simon Thompson, 1998.					


module Store 
   ( Store, 
     initial,     -- Store
     value,       -- Store -> Var -> Int
     update       -- Store -> Var -> Int -> Store
    ) where

-- Var is the type of variables.					

type Var = Char

-- The implementation is given by a newtype declaration, with one
-- constructor, taking an argument of type [ (Int,Var) ].

data Store = Sto [ (Int,Var) ] 

instance Eq Store where 
  (Sto sto1) == (Sto sto2) = (sto1 == sto2)					

instance Show Store where
  showsPrec n (Sto sto) = showsPrec n sto					
--  
initial :: Store 

initial = Sto []

value  :: Store -> Var -> Int

value (Sto []) v         = 0
value (Sto ((n,w):sto)) v 
  | v==w            = n
  | otherwise       = value (Sto sto) v

update  :: Store -> Var -> Int -> Store

update (Sto sto) v n = Sto ((n,v):sto)