The optimizer doesn't make the `call-with-values` free, but the cost isn't huge. This program: https://gist.github.com/3e306257338f2ec44854 shows about 15% slowdown using your versions over using `let-values` (and requiring a specific number of values).
Sam On Thu, Oct 22, 2015 at 11:52 AM Jens Axel Søgaard <[email protected]> wrote: > Hi All, > > This is an old issue, but as far as I know, there is no convenient to > discard extra values in a context where multiple values are received. > > I propose we add first-value, second-value, and, nth-value: > > > (first-value (values 'a 'b 'c)) > 'a > > > (second-value (values 'a 'b 'c 'd)) > 'b > > > (nth-value (values 'a 'b 'c 'd) 2) > 'c > > The most common use-case for me is discarding extra values from for/fold. > > These operations could be implemented like this: > > (define-syntax-rule (first-value expr) > (call-with-values (λ () expr) (λ (a . _) a))) > > (define-syntax-rule (second-value expr) > (call-with-values (λ () expr) (λ (a b . _) b))) > > (define-syntax-rule (nth-value expr n) > (call-with-values (λ () expr) > (λ vs > (unless (>= (length vs) n) > (error 'nth-value (~a "expected at least " n > "values"))) > (list-ref vs n)))) > > However thinking about the efficiency - it feels wrong to use > call-with-values. > Does the optimizer handle call-with-values so well, that first-value and > second-value becomes efficient? > > /Jens Axel > > > > > -- > You received this message because you are subscribed to the Google Groups > "Racket Developers" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To post to this group, send email to [email protected]. > To view this discussion on the web visit > https://groups.google.com/d/msgid/racket-dev/CABefVgy%2Bzfu0N730TYFXtTsk2kOKsOFrHX6v4B%2B43Zm-ZUkuHw%40mail.gmail.com > <https://groups.google.com/d/msgid/racket-dev/CABefVgy%2Bzfu0N730TYFXtTsk2kOKsOFrHX6v4B%2B43Zm-ZUkuHw%40mail.gmail.com?utm_medium=email&utm_source=footer> > . > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "Racket Developers" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-dev/CAK%3DHD%2BZ8GaGrXyFrfzbCWZTmNi9dXZ2eHjG3eGNAetg83GS4ng%40mail.gmail.com. For more options, visit https://groups.google.com/d/optout.
