[plt-scheme] Multiple values

> At Mon, 20 Feb 2006 13:06:08 -0800, Gregory Woodhouse wrote:
>(...)

> (...) So, if you were to think of this slightly
> more generally, you might compare this:
>
>   (let ([x (compute-x ...)]
>         [y (compute-y ...)])
>     (+ x y))
>
> to this:
>
>   (let-values ([(x y) (compute-both-x-and-y ...)])
>     (+ x y))

   Yes.  More generally, I often use multiple values because they allow
multiple computation. For an "actual" example, let us consider a
non-empty linear list. Let us assume that we have to get, on the one
hand, the list but its last element, on the other hand, its last
element. You can write two functions for these two purposes, but they
would go along the list twice. Multiple values allow to get the two
results in one pass:

(define (butlast-and-last l)
  ;;  "l" is supposed to be non-empty, returns two values:  a linear list
with
  ;;  all the elements of "l" but the last one, and the last one.
  (let ((first (car l))
        (rest (cdr l)))
    (if (null? rest)
        (values '() first)
        (call-with-values (lambda () (butlast-and-last rest))
          (lambda (l0 last-0) (values (cons first l0) last-0))))))

   Yours sincerely,

J.-M. H.