SICP Exercise 2.17: Last Pair

Define a procedure last-pair that returns the list that contains only the last element of a given (nonempty) list:

> (last-pair (list 23 72 149 34))
(34)

We know that we can identify the last pair in a list by the cdr of the pair being nil, and returning this pair will give us the result we need. All we need to do is to walk through the list recursively, in a similar manner to the list operations defined in the book, and stop when we reach such a pair.

Here's my implementation:

(define (last-pair l)
  (let ((tail (cdr l)))
     (if (null? tail)
         l
         (last-pair tail))))

And here it is in action:

> (last-pair (list 23 72 149 34))
(34)
> (last-pair (list 1 2 3 4))
(4)
> (last-pair (list 4 3 2 1))
(1)