common-lisp Functions as first class values Implementing reverse and revappend
Example
Common Lisp already has a reverse function, but if it didn't, then it could be implemented easily using reduce. Given a list like
(1 2 3) === (cons 1 (cons 2 (cons 3 '())))
the reversed list is
(cons 3 (cons 2 (cons 1 '()))) === (3 2 1)
That may not be an obvious use of reduce, but if we have a "reversed cons" function, say xcons, such that
(xcons 1 2) === (2 . 1)
Then
(xcons (xcons (xcons () 1) 2) 3)
which is a reduction.
(reduce (lambda (x y)
(cons y x))
'(1 2 3 4)
:initial-value '())
;=> (4 3 2 1)
Common Lisp has another useful function, revappend, which is a combination of reverse and append. Conceptually, it reverses a list and appends it to some tail:
(revappend '(3 2 1) '(4 5 6))
;;=> (1 2 3 4 5 6)
This can also be implemented with reduce. It fact, it's the same as the implementation of reverse above, except that the initial-value would need to be (4 5 6) instead of the empty list.
(reduce (lambda (x y)
(cons y x))
'(3 2 1)
:initial-value '(4 5 6))
;=> (1 2 3 4 5 6)
