# Run:
#    python List.py
#
# Python doesn't have a linked list type built in, the way it has
# arrays (which are called lists), dictionaries, and tuples.  So,
# let's build our own.

# Make a new linked list.  The r must be emptyList or another cons
# value.
def cons(f, r):
    # We choose to implement lists internally with tuples.
    # We could have made a class, used dictionaries, or arrays
    return (f, r)

def first(L):
    # This is how to dereference the first value in a tuple
    return L[0]

def rest(L):
    return L[1]

def isLL(L):
    return (L == None) or (type(L) == type(()))


# Some trivial examples:
print first(cons(1, None))
print rest(cons(1, None))

L = cons("Hello", cons("World", None))
print first(L)
print first(rest(L))