It has been a while since I wrote lisp code. Today, I am going to
write a reduce that returns its intermediate results. The solution I
present here is shorter than the solution presented on StackOverflow.
Clojure already has a function called reductions
that does the same thing, but I wanted to implement my own version in
Common Lisp.
So, here it goes…
(defun reduce-with-intermediates (fn xs &optional carry)
(if (null xs)
(nreverse carry)
(let ((res (if (null carry)
(funcall fn (car xs))
(funcall fn (car xs) (car carry)))))
(reduce-with-intermediates fn (cdr xs) (cons res carry)))))REDUCE-WITH-INTERMEDIATESLet us test it on a simple example:
(reduce-with-intermediates #'+ '(1 2 3 4 5))(1 3 6 10 15)Here is a more complicated but an interesting example:
(reduce-with-intermediates (lambda (x &optional y) (append y (list x))) '(1 2 3 4))((1) (1 2) (1 2 3) (1 2 3 4))