[plt-scheme] Multiple values

I'm not certain that butlast-and-last is an "actual" example.  If you 
stick to lists, then the following works and is much faster, too (using 
MzScheme on a short list, anyway)...

(define (cleaver l)
   (let ((reversed (reverse l)))
     (list (reverse (cdr reversed)) ;; or values
           (car reversed))))
rac



On Feb 20, 2006, at 4:03 PM, hufflen at lifc.univ-fcomte.fr wrote:

>> 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.
>
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