--                                                               
-- 	 StoreFun.lhs

--          An abstract data type of stores of integers, implemented as functions.			
--                                                                 
--          (c) Simon Thompson, 1998.					


-- An alternative implementation of Store.lhs. Note that although
-- it is equivalent to the list implementation as far as the operations
-- initial, value, update are concerned, it is not possible to compare for
-- equality or to show as a String.

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

-- Var is the type of variables.					

type Var = Char

newtype Store = Sto (Var -> Int) 					
--  
initial :: Store 

initial = Sto (\v -> 0)

value :: Store -> Var -> Int

value (Sto sto) v = sto v

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

update (Sto sto) v n 
  = Sto (\w -> if v==w then n else sto w)