## ----------------------------------------------------------------
## an implementation of the PAIR interface using Python lists
## ----------------------------------------------------------------

def list_make_pair(a, b): return [a, b]
def list_first(pair):     return pair[0]
def list_second(pair):    return pair[1]

## ----------------------------------------------------------------
## an implementation of the pair interface using Python functions
## ----------------------------------------------------------------

def proc_make_pair(a, b): return lambda p: p(a, b)
def proc_first(pair):     return pair(lambda a, b: a)
def proc_second(pair):    return pair(lambda a, b: b)

## ----------------------------------------------------------------
## run a simple test to demonstrate the different implementations
## ----------------------------------------------------------------

def do_some_pairs(type_str, MAKE_PAIR, FIRST, SECOND):
    print('Using the', type_str, 'implementation of the pair interface')
    pair_1 = MAKE_PAIR(2, 3)
    pair_2 = MAKE_PAIR(1, pair_1)
    x = FIRST(pair_2)
    y = FIRST(SECOND(pair_2))
    z = SECOND(SECOND(pair_2))
    print(x, y, z)

do_some_pairs("list",     list_make_pair, list_first, list_second)
do_some_pairs("function", proc_make_pair, proc_first, proc_second)

## ----------------------------------------------------------------
## We could also implement pairs using Python tuples, dictionaries,
## functions + booleans, functions + symbols, and ...
## ----------------------------------------------------------------