## Cons cells (python edition)

Came across a puzzle asking about implementing Lisp's car and cdr functions in Python. Turned out to be pretty fun and concise:

``````# cons.py

# `cons`struct cons cell that holds pairs of values/pointers to values
def cons(a, b):
def pair(f):
return f(a, b)
return pair

# return the first half (`c`ontents of `a`ddress `r`egister) of cons cell
def car(f):
return f(lambda a, b: a)

# return second first half (`c`ontents of `d`ecrement `r`egister) of cons cell
def cdr(f):
return f(lambda a, b: b)

# return cons cell as a `dotted pair` string
def dotted(cons_cell, s=None):
s = [] if s is None else s

for v in ['(', car(cons_cell), ' . ', cdr(cons_cell), ')']:
if callable(v):
dotted(v, s)
else:
s.append('nil' if v is None else str(v))

return ''.join(s)

if __name__ == '__main__':
print(dotted(cons(1, 2)))
print(dotted(cons(1, cons(2, cons(3, None)))))
print(dotted(cons(cons(1, 2), cons(3, 4))))
``````
``````\$ python cons.py
(1 . 2)
(1 . (2 . (3 . nil)))
((1 . 2) . (3 . 4))
``````