lisp in python

lisp.py

class Null:
    def __repr__(self):
        return '()'
null = Null()

class cons:
    def __init__(self, car, cdr):
        self.car = car
        self.cdr = cdr
    
    # pls don't use
    def __iter__(self):
        c = self
        while c != null:
            yield c.car
            c = c.cdr
            
    def __repr__(self):
        if isinstance(self.cdr, cons):
            r = 'list(' + ', '.join(self) + ')'
            return r
        return 'cons({}, {})'.format(self.car, self.cdr)

def car(c):
    return c.car
def cdr(c):
    return c.cdr

def list(*args):
    if len(args) == 0:
        return null
    return cons(args[0], list(args[1:]))

def if_(cond, true, false):
    if cond:
        return true
    else:
        return false
    
def nullq(c):
    return c == null

##############################
##### PURITY BEGINS HERE #####
##############################

length = lambda l:
    if_(nullq(l), 0,
        length(cdr(l)) + 1)
    
index = lambda l, i:
    if_(i == 0, car(l), index(cdr(l), i - 1))