LISP cons in python
WARNING AHEAD: The material below may not be practical!
Actually, cons
needs not to be primitive in Lisp, you can build it with λ.
See Use of lambda for cons/car/cdr definition in SICP for details. In Python, it is translated to:
def cons(x, y):
return lambda pair: pair(x, y)
def car(pair):
return pair(lambda p, q: p)
def cdr(pair):
return pair(lambda p, q: q)
Now, car(cons("a", "b"))
should give you 'a'
.
How is that? Prefix Scheme :)
Obviously, you can start building list using cdr
recursion. You can define nil
to be the empty pair in Python.
def nil(): return ()
Note that you must bind variable using =
in Python. Am I right? Since it may mutate the variable, I'd rather define constant function.
Of course, this is not Pythonic but Lispy, not so practical yet elegant.
Exercise: Implement the List Library http://srfi.schemers.org/srfi-1/srfi-1.html of Scheme in Python. Just kidding :)
In Python, it's more typical to use the array-based list
class than Lisp-style linked lists. But it's not too hard to convert between them:
def cons(seq):
result = None
for item in reversed(seq):
result = (item, result)
return result
def iter_cons(seq):
while seq is not None:
car, cdr = seq
yield car
seq = cdr
>>> cons([1, 2, 3, 4, 5, 6])
(1, (2, (3, (4, (5, (6, None))))))
>>> iter_cons(_)
<generator object uncons at 0x00000000024D7090>
>>> list(_)
[1, 2, 3, 4, 5, 6]
Note that Python's lists are implemented as vectors, not as linked lists. You could do lst.insert(0, val)
, but that operation is O(n).
If you want a data structure that behaves more like a linked list, try using a Deque.