Python and Lisp : car and cdr

[Terry Reedy]

On 6/19/2011 9:24 AM, Steven D'Aprano wrote:

> No. Each cell in a Lisp-style linked list has exactly two elements, and
> in Python are usually implemented as nested tuples:
>
> (head, tail)  # Annoyingly, this is also known as (car, cdr).
>
> where head is the data value and tail is either another Lisp-style list
> or a marker for empty (such as the empty tuple () or None).
>
> So a one-element linked list might be given as:
>
> (42, None)
>
> A two element list:  (42, (43, None))
> Three element list:  (42, (43, (44, None)))
>
> and so forth. So while you could harmlessly use a slice L[1:], there is
> no point, since L[1:] will have at most a single element.

It should be noted that the head element of any 'list' can also be a 
'list' (as with Python lists),

t = { { (1,None), (2,(3,None)) ), ( (4,(5,None)), (6,None) ) )

so that the structure is actually a tree, which is a much more general 
data structure than a true sequence of atoms. But TREP (for 
tree-processing) is not as catchy as LISP (for list processing).

-- 
Terry Jan Reedy