-- Implementation of a mapping, a set of (key,value) pairs.
-- Sergio Antoy
-- Tue Jul 17 08:32:17 PDT 2001

-- The store is a list of pairs where the first element is the
-- key and the second is the mapped value.
-- The representation is a list.  If an ordering were available for
-- the keys, a more efficient tree representation would be possible.

-- The initial store is an empty mapping.

-- Getting a mapping fails if and only if the key is not in the store.
-- Consider instead returning a default value, e.g., 0.
-- The value of a key is obtained by instantiating an argument of the call.

-- Setting a mapping either adds a (key,value) pair to the store
-- or updates the value of a key already in the store.
-- Setting a mapping always returns a new mapping.

init_store = []
get a b ((x,y):z) | a == x    = b =:= y
                  | otherwise = get a b z
set a b []        = [(a,b)]
set a b ((x,y):z) | a == x    = (x,b) : z
                  | otherwise = (x,y) : set a b z